WO2017096420A1

WO2017096420A1 - An antenna

Info

Publication number: WO2017096420A1
Application number: PCT/AU2016/051099
Authority: WO
Inventors: Albertus Jacobus Pretorius; Abraham Gert Willem Du Plooy
Original assignee: Licensys Australasia Pty Ltd
Priority date: 2015-12-09
Filing date: 2016-11-16
Publication date: 2017-06-15
Also published as: EP3387703B1; EP3387703A1; AU2016101994A4; AU2016367704A1; TW201733202A; AU2016102459A4; EP3387703A4

Abstract

An antenna for a communication device, and a number of RFID reader configurations incorporating the antenna, are disclosed. The antenna has a structure comprising a circular radiating base plate, a radiating cone, a solid frusto-conical body. The cone has an apex that points towards the center of the circular base plate, and the apex is positioned on or near the base plate on one side of the base plate. The cone opens/expands away from the base plate. The solid frusto-conical body has an encompassing side (i.e. a side that goes all the way around the circumference of the antenna structure) which extends from the base plate to near an edge on the widest point on the cone, and the material of the body substantially fills the space inside the encompassing side and between the base plate and the cone.

Description

AN ANTENNA

CONTENTS

TECHNICAL FIELD 1

BACKGROUND 2

SUMMARY OF THE INVENTION 10

BRIEF DESCRIPTION OF THE DRAWINGS 14

DETAILED DESCRIPTION 15

APPENDIX 49

CLAIMS 76

ABSTRACT 1

APPENDIX FIGURES 1

SPECIFICATION FIGURES 15

TECHNICAL FIELD

[0001] The present invention involves an antenna with a low physical profile and a particular radiation pattern.

[0002] In one particular (albeit non-limiting) example application, the antenna can be placed in or on the surface of a road, a driveway, or the like, and can be used to perform radio- frequency identification (RFID) with RFID capable tags (RFID tags) which are located on the front and/or the back of passing vehicles. In this application (or like applications), the antenna would be a part of (or associated with) a RFID reader which is operable to communicate with RFID tags. Preferably, the RFID tags will be located on (or integrated as part of) the vehicles' license plates. (Or more specifically, for vehicles which have a license plate on the front and the rear, a RFID tag will preferably be placed on (or integrated as part of) one or both of a said vehicle's license plates, or for vehicles which have only one license plate, a RFID tag will preferably be placed on (or integrated as part of) the single license plate).

[0003] Notwithstanding the foregoing, it is to be clearly understood that no particular limitations are to be implied from any of the example applications or uses mentioned above or discussed below. Thus, the antenna could potentially be used in a wide range of other areas and/or applications as well. By way of example, rather than being used in "on-road" or "in-road" applications for detecting RFID tags which are placed on the front and/or back of vehicles (or on the vehicles' license plate(s)), the antenna could instead potentially find use in side and/or overhead placements to read/communicate with RFID tags on vehicles, or on goods or products which are moving past the antenna (e.g. goods or products being carried past the antenna by a machine, or on a conveyor, etc).

[0004] Nevertheless, for convenience, the invention will hereafter be described with reference to, and in the context of, the above application where the antenna communicates with RFID tags which are located on (or integrated as part of) vehicle license plates.

BACKGROUND

[0005] For the purpose of providing a background and introduction to the present invention, reference is hereby made to two earlier patent applications, namely:

^■ International Patent Application No. PCT/AU2015/050161 (hereinafter referred to as "patent application Ί 61 "); and

^■ International Patent Application No. PCT/AU2015/050384 (hereinafter referred to as "patent application '384").

[0006] The entire contents of both earlier patent applications listed above, namely patent application Ί 61 and patent application '384, are hereby incorporated herein by reference. However, in the event of (or to the extent of) any inconsistency or discrepancy between the disclosure in the present specification and the disclosure in either or both of the earlier patent applications listed above, the present specification takes precedence and overrides. Also, the mere incorporation herein of the contents of the earlier patent applications listed above does not mean that any express or implied restrictions or limitations on any inventions disclosed in either of those earlier patent applications, or that any express or implied restrictions or limitations on any other disclosure(s) given therein, necessarily also apply to the present invention.

[0007] Both of patent applications Ί 61 and '384 explain, inter alia, that there are a number of benefits and advantages that can arise from placing an RFID tag on a vehicle (preferably by embedding or integrating the RFID tag in one or both of the vehicle's licence plates) and also from enabling the said RFID tag to be read by an RFID reader, the antenna of which (at least) is placed on or in the road. Patent applications Ί 61 and '384 also explain (for reasons elaborated on therein) that because of the general geometry associated with the placement location of licence plates on vehicles, and with the dimensions (especially the width) of most road lanes, a required read-zone (i.e. the region near the RFID reader antenna inside which the RFID reader is required to be able to communicate with an RFID tag if said tag is within said region) :

^■ is approximately 4 m wide (2 m on either side of the antenna),

^■ occupies the space from about 5 m to about 1 m before the antenna in a given direction (e.g. the direction of travel in a road lane),

^■ occupies the space after the antenna (in the said same direction) from about 1 m to about 5 m after the antenna, and

^■ extends in height, at least within the horizontal zones defined in the preceding bullet points, from between about 0.3 m and about 1 .3 m above ground (road) level.

[0008] Note that the dimensions of the required read-zone given above may not precisely match the required read-zone dimensions discussed in patent applications Ί 61 and '384. Nevertheless both of those earlier patent applications clearly disclose a required read zone which is at least similar to that given above, even if the zone dimensions quoted differ slightly.

[0009] Patent applications Ί 61 and '384 explain one way of achieving a required read-zone such as that just described, namely by using an omnidirectional vertically polarised radiation pattern, and hence by using an antenna that can provide such a radiation pattern. The required read-zone described in the bullet points above is illustrated in Figure 1 . In Figure 1 , the required read-zone is indicated by reference numeral 2. (A similar required read-zone is also depicted in Appendix Figure A19 and described in the associated passages in the Appendix).

[0010] Patent applications Ί 61 and '384 further explain that the radiation pattern 3 of the RFID reader antenna should preferably have a shape that might be described as a "dropped doughnut" or "squashed toroid" - that is, a shape as shown pictorially in Figure 2 (and also in Figure A23). The "hole" 4 (or more technically the "radiation null" 4) which is at/near the centre on top of the "dropped doughnut" (or "squashed toroid") shaped radiation pattern 3 helps, for example, to reduce the blinding effect of high power reflections from the underside of vehicles passing over the top of the antenna. This is explained further in the accompanying Appendix. It should be noted that Figure 2 (and also Figure A23) merely provides an initial visually appreciable illustration of what is meant by the "dropped doughnut" or "squashed toroid" shape that the radiation pattern 3 should have. The reason why the radiation pattern 3 should have this general shape is discussed in further detail in the Appendix.

[0011] Patent applications Ί 61 and '384 also indicate that RFID tag antennas (such as the antennas of RFID tags used on vehicle license plates) typically have a highly directional radiation pattern. More specifically, the radiation pattern of a RFID tag antenna on a vehicle license plate will almost invariably point generally in a direction 6 which is parallel to the licence plate's "face-on" direction, albeit pointing away from the vehicle/plate, as depicted in Figure 3. The direct radiation communication path 8 between the RFID tag antenna on the license plate and the RFID reader antenna therefore has an elevation (i.e. height/vertical) offset 5, and it may also have a directional (horizontal) offset 7, from the plate's face-on direction. Whether or not there is a directional (horizontal) offset 7 depends on the travel path of the vehicle, and in particular whether the RFID tag antenna on the vehicle's licence plate is passing directly over, or to one side of, the antenna.

[0012] Figure 4 is a plan (i.e. "top down") view of a road comprising three road lanes. All three lanes in this example carry vehicles in the same direction, and all three are approximately 4 m wide. There is an RFID reader antenna placed on/in the road in the middle of the centre lane. Figure 4 shows the following superimposed on the three-lane road:

^■ the required read-zone 2 (square regions indicated by diagonal hatching) ;

^■ the omnidirectional radiation pattern 3 of the RFID reader antenna (note that the omnidirectional radiation pattern 3 in Figure 4 is actually the "dropped doughnut" shape shown in e.g. Figure 2, however this has the appearance of (and is represented as) a simple circle in the "top down" view in Figure 4); and

^■ the effective read-zone 9, which in the two-dimensional "top down" view in Figure 4 has a "figure-8" shape (note: the orientation and the "figure 8" shape of the effective read zone 9 - i.e. with two round "lobes" arranged in line with the direction of travel in the centre of the middle lane - arises due to the geometry of the required read zone 2, and the convergence of the "figure 8" lobes near the RFID reader arises due to angle of read issues for the directional RFID tags on the license plates. The shape of the "figure 8" shaped effective read-zone 9 (and the factors that contribute to give it this shape) are therefore not a result of the design/configuration of the RFID reader antenna. This is discussed further in the accompanying Appendix.)

[0013] It should be noted that Figure 4 (and many of the things it depicts and the information it conveys) is quite similar to Appendix Figure A21 . Further understanding of the information conveyed in Figure 4 may therefore also be obtained from the explanations provided in the Appendix with reference to Appendix Figure A21 .

[0014] Patent applications Ί 61 and '384 explain that, even if "two-way" data communication between an RFID reader and the RFID tag on a vehicle (or on/in the vehicle's license plate) is not achieved (such that positive identification of the specific vehicle ID is not achieved via RFID), nevertheless the RFID reader antenna (i.e. the same RFID reader antenna used for positive vehicle identification via RFID) may still be used to detect the presence and also, for example, the speed, etc, of the (non-positively-identified) vehicle. This may be achieved using conventional RADAR (or "RADAR-like" or "one-way" communication) methods. See further on this below.

[0015] Patent application Ί 61 , in particular, also explains that the RFID reader (of which the RFID reader antenna forms part) may also include (or it may even contain within a common housing or structure) other sensors of importance in traffic management. These sensors could be placed on top of the antenna. This would, however, increase the overall height of the antenna. Both patent applications Ί 61 and '384 indicate that antenna height (and minimizing or at least limiting this) is an important design factor, because vehicles must be able to safely drive/pass over the antenna without damage to the vehicle or the RFID antenna and/or reader. For the purposes of the present explanation, given that the height of traditional "cat-eye" type retro-reflective road markers is typically around 25 mm, and given that these are widely approved for use (indeed they are used extensively without causing damage to vehicles travelling on the roads on which they are installed), it shall be assumed, at least for permanent or non-temporary applications, that if the height to which an antenna or other associated RFID reader equipment projects above the road surface is 25 mm or less, this will not pose any danger or risk of damage to vehicles using the road. In other words, although there is no absolute requirement in this regard, it is envisaged that, at least where the present invention is used in permanent or semi-permanent in-road implementations, the height to which the antenna and any other associated RFID reader equipment projects above the road surface should generally be 25 mm or less. Those skilled in the field of antenna design will readily appreciate the significant challenges this creates in terms of designing an antenna capable of providing the required radiation pattern, not to mention also satisfying several other operational requirements that apply in such implementations, as discussed below.

[0016] Patent application '384 discloses certain antenna designs having configurations which are intended to provide a radiation pattern like that shown, for example, in Figure 2. The antenna configurations in patent application '384 are also intended to help overcome a number of challenges associated with the changeable (and often drastically and dynamically changeable) radio frequency (RF) transmission conditions/environment that exist in the vicinity of the antenna, including due to the "near ground effect". Indeed, it is specifically explained in patent application '384 that:

..[t]he "near ground effect" is the ground effect caused by the ground (which is part of planet Earth), or by the surface on which the antenna is mounted, in the immediate vicinity of the antenna (e.g. within about 6 m or about one typical vehicle length from the antenna). This "near ground effect" (i.e. the ground effect from the "near ground") in particular may be highly variable and even dynamically variable (i.e. subject to change with time and/or due to changes in conditions, etc) ...

While discussing the ability of the ... antenna to help compensate/account for the ground effect, and especially the near ground effect, it is useful also to ... emphasise certain other/related points which are important insofar as the ... antenna and its operation in [the presently-considered on/in road] applications are concerned. A first point is that, when an antenna ... is [positioned on/in the road and] used in, for example, a vehicle detection and/or RFID vehicle identification application, the antenna is effectively being used in a way that may be considered generally similar or analogous to an antenna in a RADAR transmitter/sensor. Indeed, ... RADAR essentially involves a radio signal that is first transmitted by a sensor; that radio signal is then reflected by the object to be observed, and the reflected signal is received and interpreted by the sensor (e.g. for the purpose of detecting the presence of the object, and/or its location and/or movement relative to the sensor, etc). In the case of RFID, a signal may be emitted by an RFID reader (which includes an antenna ...), and a "reflected" signal may then be sent back from e.g. an RFID tag on a vehicle, back to the RFID reader. In RFID, both of these signals (i.e. both the signal emitted by the RFID reader and also the "reflected" signal sent back from the RFID tag to the RFID reader) can be modulated to carry information/data (this modulation of data onto the signals is at least part of what distinguishes RFID from traditional RADAR wherein the signals are unmodulated). In other words, in RFID, information can be modulated onto the signal emitted by the RFID reader such that information is sent from the reader to the tag, and similarly information can be modulated onto the signal sent (reflected) by the RFID tag such that information is sent back from the tag to the reader. Where there is this kind of two-way data exchange, and specifically in RFID vehicle identification applications, the exchange of information may be used to perform (and in fact this may be what makes it possible to perform) the [positive] identification (i.e. ID detection/recognition) of a specific vehicle. ...Alternative arrangements or situations may also be possible where the signal emitted by the RFID reader and the "reflected" signal sent back from the RFID tag to the RFID reader, or one of them, is/are unmodulated, such that there is therefore no two-way data exchange like that just described above. However, even in this alternative case where the signal emitted by the RFID reader and/or the "reflected" signal sent back from the RFID tag to the RFID reader is/are unmodulated, nevertheless the signal sent by the RFID tag, which is still received and interpreted by the reader, may still be used for vehicle detection, among other things. Indeed, when such a reflected signal, which is sent (reflected) back from an RFID tag, is received by the reader, this signal (even if it is an unmodulated signal) may immediately signify the presence of a RFID tag (and hence a vehicle) within the read range of the reader (although which specific vehicle it is - i.e. the specific vehicle identity/ID - may not in this case be determinable, at least not from the signal sent by he RFID tag alone). Furthermore, the way the said signal changes with time (i.e. the way the signal which is sent from the RFID tag and received by the reader changes with time, even if it is an unmodulated signal) may be used (interpreted by the reader) to ascertain information about the (non-identified) vehicle in addition to merely its presence. Indeed the location and movement of the vehicle - e.g. its distance or position relative to the reader, its speed (and possibly direction) of travel, etc - may possibly be determined. It will be appreciated that this last unmodulated-signal scenario is somewhat more akin to traditional RADAR [than the two-way data exchange scenario in which positive vehicle identification is achieved using RFID].

Another point that should be emphasised is that, whilst antennas when used in e.g. vehicle detection and/or RFID vehicle identification applications, may be used in a similar or analogous way to traditional RADAR antennas (see above), nevertheless at the same time, the region within which [a RFID reader antenna used in the presently- considered on/in road applications] needs to operate, and the required transmission ranges, radiation pattern shapes, and even the physical position of the antenna (and hence the physical location in which, and from which, the antenna's signal is transmitted) may all be vastly different to antennas used in conventional RADAR. Indeed, for reasons explained in detail [in patent applications Ί61 and '384, RFID reader antennas used in the presently-considered on/in road applications] will often need to be located at ground level, typically on or in the surface of the ground (i.e. on or in the surface of planet Earth) - e.g. on or in the surface of a road. So, the antenna will generally need to be configured to be positioned at (and such that its signal radiation is emitted from) ground level on planet Earth. This is very different to conventional RADAR wherein traditional RADAR antennas are almost always located well above ground level, typically at least 2 wavelengths above the ground (i.e. the height from which a conventional RADAR antenna operates is generally at least twice the wavelength of the RADAR signal it transmits). Accordingly, traditional RADAR antennas are generally not required to accommodate much (if any) change in signal transmission propagation conditions due to the "near ground effect". Rather, for them, the effect on signal transmission propagation caused by planet Earth [and in particular the changing conditions/environment on planet Earth] may often be assumed negligible or at least constant, e.g. regardless of any time and/or position variant changes in weather or ambient conditions or ground conditions etc. This is very different to the [RFID reader antennas used in the presently-considered on/in road applications] which must operate on/in the ground and where the effect on signal transmission propagation caused by the ground [and in particular the changing conditions/environment] on/in which the antenna is located (especially the near ground) can change drastically both between different locations and also dynamically at the same location ...[For example] signal transmission propagation conditions can change drastically with time even at a single location, e.g. with changes in surface conditions due to surface water vs dry, wet soil vs dry in the vicinity, [etc. Signal transmission propagation conditions can also change drastically between different locations due to such things as] the presence or absence of metal or other conductors in the road base, substances of different conductivity like paint or oil on i the road, etc).

Furthermore, traditional RADAR antennas generally have a very focussed/directional radiation pattern intended to transmit over large or very large transmission distances (typically in a broadcast manner). So, not only are conventional RADAR antennas normally positioned well above ground level, but they have narrow focussed/directional radiation patterns and transmit over large distances (i.e. they operate in what is often termed the far field - a.k.a. the Fraunhofer region). In contrast, [the RFID reader antennas used in the presently-considered on/in road applications] may (and typically will) need to transmit over and within a range that is very much closer to the antenna, possibly even within the antenna's radiating near field a.k.a. Fresnel region. Furthermore, antennas in accordance with embodiments of the present invention may (and typically will) need to provide a radiation pattern that is non-focussed, and which extends further in a direction parallel to the plane of the [antenna's] ground plane than it does in a direction perpendicular to the plane of the [antenna's] ground plane [as discussed above and also in patent applications Ί61 and '384]. By way of illustrative example [for an] antenna ... configured to operate with signals of frequency around

1 GHz (and hence with signal wavelengths of about 300 mm), the antenna, which is part of an RFID reader located on/in the road surface, may be used to (so to speak) "radar" detect and/or identify one or more vehicles within a radius of about 5 or 6 m around the antenna, where the RFID tag(s) on the vehicle(s) is/are at or below a height of about

2 m.

[0017] In summary, patent application '384 refers to certain antenna designs (and antenna design methodologies) which are intended to help overcome a number of the issues and challenges just-described, particularly where (modulated and/or unmodulated) RADAR or RADAR-like transmission is the data transfer method used and with the transmitting antenna on the ground and the reflecting antenna within ~6 m and below.

[0018] Also, as has been explained (and as elaborated on more in patent applications Ί 61 and '384), in the context of RF road vehicle detection/identification applications, there are numerous advantages that arise from placing the RFID reader, or at least the antenna thereof, on or in the road surface. However, as has further been explained just above, the placement of the antenna on/in the road surface, especially where the required read range is within 6 m from the antenna, limits (or it may entirely prevent) the use of conventional radar radiation methods in which the Earth in particular is often quantified as (i.e. it is assumed to be) a single RF element which is homogeneous and stable/non-changing/time-invariant (or almost so).

[0019] In one particular disclosure in patent application '384, a periodic slotted ground is proposed as the base of a modified monopole antenna, such as for example an inverted F antenna or variations thereon. The proposed use of a periodic slotted ground for antennas in patent application '384 is intended to (amongst other things) help maintain a small footprint for the antenna. However, experimentation indicates that such antennas with a small-footprint periodic slotted ground may not be capable of adequately accommodating the potentially wide range of "RF" properties at road pavements (i.e. such antennas with a small-footprint periodic slotted ground may not be capable of meeting operational requirements across all of the different and widely and dynamically variable radio frequency transmission conditions/environments found at road pavements). By way of example, this can be particularly so for roads which are, say, close to the coast where the road (including the road surface and also the underlying road base, surrounding/nearby soil, etc) may sometimes be extremely dry and consequently non-conducting because of hot land winds, but which may change rapidly to moist/wet and relatively conductive because of e.g. moisture-laden salt spray from an onshore sea breeze or because of rain, etc.

[0020] Those skilled in the area of antenna design will recognise that whilst conductivity (including, but not limited to, road-surface conductivity) is one of the important parameters which can influence the radiation pattern of an on-road or in-road antenna, it is not the only relevant parameter. For instance, as another example, in road building, a range of different types of aggregates may be used. The way in which these different types of aggregates age, change, bind, compact, etc, over time differs. The numerous potential effects of this (including differing material makeup, density, porosity, surface shape and texture of the road surface, etc) can also significantly affect the radio frequency transmission conditions/environment on the road, which in turn also influences the radiation pattern of the on/in road antenna.

[0021] Furthermore, experimentation indicates that the realistic gain of an antenna having a small-footprint periodic slotted ground design, for example as in patent application '384, possibly may not exceed 1 dBiL, and realistic values may be less than 0 dBiL. (Note: "dBiL" here is the forward gain of the antenna (in decibels - dB) compared with a hypothetical isotropic antenna which uniformly distributes energy in all directions - hence dBi is shorthand for dB(isotropic) - and the "L" in dBiL signifies that linear polarization of the electromagnetic field is assumed). In any case, this low gain of antennas having a small-footprint periodic slotted ground design, like for example certain of the antennas in patent application '384, can consequently create a need to increase power output from the RFID reader (which powers the antenna) to thereby compensate for low antenna gain. However, this increase in power output from the RFID reader may in turn cause, for example, overheating problems, especially for in-road placements because, unlike on-road placements where the reader is aboveground and heat may be able to dissipate into the air, in in-road placements the reader is (at least mostly) located in the ground and is therefore surrounded and insulated on all sides by earth/soil/road base meaning that heat is comparatively trapped and cannot easily dissipate. Therefore, an increase in power output from the RFID reader to compensate for low antenna gain may cause overheating problems especially for in-road RFID reader/antenna placements. An increase in power output from the RFID reader to compensate for the low antenna gain may also have the effect of reducing overall the reader's relative sensitivity.

[0022] In view of the foregoing, it is thought that it would be desirable if there were a method and/or appropriate antenna hardware/apparatus that could accommodate the potentially widely and dynamically variable radio frequency transmission conditions/environment that may exist on a road at different times, or on different roads at different locations at different times, so as to enable an antenna that can be placed on/in a road, or antennas that can be placed on/in roads at different locations, to achieve a desired antenna radiation pattern consistently (or at least with an acceptable degree of consistency) in all conditions at all locations. It may be particularly desirable if the tuning of in-road or on-road antennas could be made (or if it could become) a more "exact science" - that is to say, if antenna tuning could be performed in such a way that the effect on the antenna's radiation pattern resulting from tuning alterations to the size, design, configuration, etc, of the antenna (or of certain parts of the antenna) is much more predictable and reliable and therefore much less reliant on simple "trial and error" tuning. It is also thought that it would be desirable if the effective gain of an in-road or on-road antenna could be increased in comparison with the small-footprint periodic slotted ground antenna designs discussed above, preferably such that the effective antenna gain is around 3 dBiL or better.

[0023] Despite the introductory discussion and background information provided above, it is to be clearly understood that mere reference in this specification, or in the accompanying Appendix, or in any related drawings, to any previous or existing antenna designs, devices, apparatus, products, systems, methods, practices, publications or to any other information, or to any problems or issues, does not constitute an acknowledgement or admission that any of those things, whether individually or in any combination, formed part of the common general knowledge of those skilled in the field, or that they are admissible prior art.

SUMMARY OF THE INVENTION

[0024] In a first form, the invention relates broadly to an antenna for a communication device, the antenna having a structure comprising:

a circular radiating base plate,

a radiating cone, wherein the cone has an apex that points towards the center of the circular base plate, the apex is positioned on or near the base plate on one side of the base plate, and the cone opens/expands away from the base plate, and

a solid frusto-conical body, wherein the body has an encompassing side (i.e. a side that goes all the way around the circumference of the antenna structure) which extends from the base plate to near an edge on the widest point on the cone, and the material of the body substantially fills the space inside the encompassing side and between the base plate and the cone.

[0025] In the said antenna structure, the encompassing side of the body may extend from at or near an outer perimeter of the base plate to near an edge on the widest point on the cone. Also, the distance between the base plate and the widest point on the cone in a direction perpendicular to the base plate (this distance may be considered to be the height of the antenna) may be less (or much less) than the maximum diameter of the antenna (where the maximum diameter of the antenna may, and usually will, be the diameter of the base plate). In other words, it may be that antenna height « antenna diameter. Furthermore, the diameter of the base plate in the antenna structure may be larger than the maximum diameter of the cone (i.e. cone diameter < base plate diameter).

[0026] The base plate and the cone of the antenna structure may be made from a conductive material. The conductive material may be metal such as copper, silver or an appropriate conductive alloy thereof.

[0027] The body of the antenna structure may be made from a dielectric and physically strong material. More specifically, the material from which the body is made may have a permittivity (dielectric constant) of between approximately 3 and approximately 6. In some embodiments, the material from which the body is made may be a soda lime glass.

[0028] The body may be initially formed with a recess or indent therein. The shape of the said recess or indent may also correspond to the shape of the cone of the antenna, and the cone of the antenna may be formed by plating a thin layer of metal onto the surface of the recess or indent.

[0029] The antenna structure may further include a top plate/lid which extends across and partly or fully covers the space which is formed by and within the open cone.

[0030] In embodiments where the base plate is metal, the metal base plate may be approximately 5-10 mm thick and it may be initially formed separately from the body and then affixed on or to the bottom/underside of the body.

[0031] In some embodiments, the antenna may be configured to be used with a signal frequency of 860-940 MHz. In these embodiments, it may be the case that:

^■ on the side of the base plate on which the cone is located, no point on the antenna is further than 25 mm from that side/surface of the base plate in a direction perpendicular to the base plate; and/or

^■ the diameter of the base plate is less than 190mm; and/or

^■ the encompassing side of the body extends from an outer perimeter edge of the base plate to near the edge on the widest point on the cone, and the angle between the encompassing side and the base plate, when taken in a central plane perpendicular to the base plate, is less than 40°, and preferably 33° - 36°.

[0032] In a second form, the invention relates broadly to an RFID reader incorporating an antenna according to the first form of the invention (as described above), wherein the RFID reader is operable to be used in an application involving road vehicle detection and/or identification and wherein at least the antenna (and possibly also other parts of the RFID reader) is operable to be installed in the surface of the road (i.e. "in-road").

[0033] The RFID reader may further include additional electronic components, and these may be (when the RFID reader is in use) mounted below the surface of the road, beneath the antenna. Furthermore, when the RFID reader is installed in the road for use, the base plate of the antenna may be installed horizontally in (or otherwise parallel to) the surface of the road such that an upper surface of the base plate, which is the surface on the side of the base plate which has the cone (and the surface of the base plate to which the body joins), is level (i.e. substantially coplanar) with the road surface, and the antenna's body and cone may project above the upper surface of the base plate and above the level of the road surface.

[0034] When the RFID reader is installed as just described, the antenna may also be surrounded by an at least partially conductive area which is also on or applied to the road surface. If the said partially conductive area surrounding a single antenna is circular, the minimum radius of said partially conductive area may be approximately twice the wavelength (λ) of the signals to be transmitted and/or received by the antenna. Also, the partially conductive area may have a conductivity of approximately 10³ S/m or more.

[0035] In the RFID reader according to the second form of the invention, the antenna may be operable to, in use, generate a radiation pattern having a "dropped doughnut" or "squashed toroid" shape. The antenna (and the radiation pattern generated thereby in this installation) may also be omnidirectional in the azimuth plane. In some particular embodiments, the antenna may be operable with a signal frequency of 860-940 MHz, and where this is the case the elevation range (relative to the azimuth plane) of the critical read zone in the radiation pattern may be from approximately 3° to approximately 30° elevation. Also in these embodiments, in the said radiation pattern, the path of max gain (relative to the azimuth plane) may be at approximately 30° elevation. Furthermore, in these embodiments, in the said radiation pattern, the 3dB beam width may be approximately 40°, extending from approximately 10° to approximately 50° elevation (relative to the azimuth plane), and there may be an effective radiation null at 90° elevation (relative to the azimuth plane). In some particular implementations, the effective read range of the RFID reader may be from approximately 1 m to approximately 6.4 m from the antenna in any direction along the road surface (i.e. in the azimuth plane).

[0036] In a third form, the invention relates broadly to an RFID reader incorporating an antenna according to the first form of the invention (as described above), wherein the RFID reader is operable to be used in an application involving road vehicle detection and/or identification and wherein at least the antenna (and possibly also other parts of the RFID reader) is operable to be installed or mounted in or on a partially conductive structure.

[0037] In this third form of the invention, the partially conductive structure is operable to be placed on the surface of the road ("on-road"), and when the partially conductive structure is placed on the surface of the road with (at least) the antenna installed or mounted therein or thereon, the antenna may be located a distance vertically above the road surface, possibly (or preferably) at or near the top of the partially conductive structure.

[0038] In some embodiments, the partially conductive structure may be substantially frusto- conical in shape. Where this is the case, the angle of slope of the side on the frusto-conical partially conductive structure may substantially match the angle of slope on the side of the antenna's main frusto-conical body.

[0039] In embodiments such as those described in the previous two paragraphs, if the antenna operates with a signal frequency (λ) of 860-940 MHz, the configuration (and particularly the height) of the partially conductive structure should preferably be such that the height of the antenna's base plate, when it is mounted on the partially conductive structure and the partially conductive structure is on the road surface, is not more than ¾λ, and preferably not more than ¼ λ.

[0040] In the RFID reader according to the third form of the invention, the construction and/or configuration of the partially conductive structure, including in relation to its height, the angle of slope of its side, internal construction, and positioning of internal components, may be chosen and/or varied to tune the partially conductive structure so that, when the partially conductive structure is used in conjunction with (at least) the antenna, the radiation pattern has a desired "dropped doughnuf-shape.

[0041] Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings (the Specification Figures) as follows:

[0043] Figure 1 - required read-zone for an in/on road antenna.

[0044] Figure 2 - "dropped doughnut" (or "squashed toroid") shaped antenna radiation pattern.

[0045] Figure 3 - elevation/height and directional/horizontal offsets of the radiation communication path between a license plate's tag and an in/on road antenna, relative to the plate's "face-on" direction.

[0046] Figure 4 - plan view of a three lane road with an RFID reader antenna placed on/in the road in the middle of the centre lane. (Note: the fact that this Figure illustrates only a single RFID reader antenna, located in the centre lane, is for clarity of illustration only. Normally, in practice (at least where the present invention is used in in-road implementations) there will be an RFID reader antenna placed in the middle of each lane).

[0047] Figure 5 - traditional disk cone antenna with the disk and cone each formed from discrete elongate rod elements.

[0048] Figure 6 - traditional disk cone antenna having a solid disk and solid cone.

[0049] Figure 7 - RFID reader (including antenna structure) with a surrounding partially conductive area of radius R at least twice the signal wavelength λ.

[0050] Figure 8 - schematic illustration of a shape corresponding to that of the proposed antenna structure.

[0051] Figure 9 - cross-sectional view of the proposed antenna structure, also incorporating other reader equipment (i.e. in addition to the basic antenna structure), in an inroad application.

[0052] Figure 10 - annotated cross-sectional view of the proposed antenna structure etc in Figure 9.

[0053] Figure 1 1 - cross-sectional view of the proposed antenna structure, also incorporating certain other reader equipment (i.e. in addition to the basic antenna structure), when the antenna structure is mounted on a partially conductive substructure in the form of an on-road cradle for on-road applications.

[0054] Figure 12 - annotated cross-sectional view of the proposed antenna structure etc in Figure 1 1 .

[0055] Figure 13 - possible different shapes/configurations for the partially conductive on- road cradle which forms part of the antenna design/structure for on-road applications/deployments.

[0056] Figure 14 - schematic representation of the possible shape and configuration of the main frusto-cone body of the antenna structure

[0057] Figure 15 - cross-sectional view of a slight alternative or variant on the proposed antenna structure, also incorporating other reader equipment (i.e. in addition to the basic antenna structure), in an in-road application

[0058] Figure 16 - annotated cross-sectional view of the slight alternative or variant antenna structure etc in Figure 15

[0059] Figure 17 - side-on and partially exploded pictorial view of another slight alternative or variant on the proposed antenna structure and other reader equipment intended for use in an in-road application.

[0060] Figure 18 - side-on pictorial view of the slight alternative or variant on the proposed antenna structure etc in Figure 17.

[0061] Figure 19 - Perspective view of the plotted antenna radiation pattern shape and directivity, illustrating that the radiation pattern is omnidirectional in the azimuth (x-y) plane (i.e. in a plane parallel to the road surface if the antenna is in-road, for example)

[0062] Figure 20 - Plot of one side (or one "lobe") of a cross-section of the radiation pattern as plotted in Figure 19, with the cross-section taken in a vertical (x-z) plane that extends through the centre of the radiation pattern in Figure 19, and also illustrating the following in the plane of this cross section: the elevation range of the critical read zone; the elevation of the path of maximum gain; the 3dB beam width; and the radiation null at 90° to the azimuth (x-y) plane

DETAILED DESCRIPTION

[0063] As explained in the Background section above, it is thought that it would be desirable if there were a method and/or appropriate antenna hardware/apparatus that could accommodate the potentially widely and dynamically variable radio frequency transmission conditions/environment that may exist on a road at different times, or on different roads at different locations at different times, so as to enable an antenna that is on/in a road, or antennas that are on/in roads at different locations, to achieve a desired antenna radiation pattern consistently (or at least with an acceptable degree of consistency) in all conditions at all locations. It is thought that it would also be desirable if the tuning of in-road or on-road antennas could be a more "exact science" - that is to say, if tuning of in-road or on-road antennas could be performed in such a way that the effect on the antenna's radiation pattern resulting from tuning alterations to, for example, the size, design, configuration, relative proportions, etc, of the antenna (or of certain parts of the antenna or associated structures or components) could be more predictable and reliable and therefore less reliant on more simple/crude "trial and error" tuning. It is further thought that it would be desirable if the effective gain of an in-road or-road antenna could be increased in comparison with the small- footprint periodic ground antenna designs discussed in the Background section above, preferably such that the effective antenna gain his around 3 dBiL or better.

[0064] The various antenna structure (and associated RFID reader) configurations and designs that are discussed below with reference to the Specification Figures seek to achieve one or more of the general aims above, or at least go some way towards doing so by, in basic terms, adopting an antenna structure that turns a conventional disk-cone (a.k.a. "discone") antenna structure over, and by also surrounding the antenna structure with an at least partially conductive area or placing the antenna structure on an at least partially conductive substructure.

[0065] At this point, it is useful to note that conventional disk-cone antennas are so named due to their distinctive shape. Conventional disk-cone antenna designs consist of a "disk" at the top and a "cone" underneath wherein the cone is oriented "cone-pointing-up" such that the apex of the cone meets at or near the centre of the "disk". Often, conventional disk-cone antennas are formed from a number of straight, elongate elements with some of the elements at the top arranged in a radial manner to define a disk shape, and with other elements pointing downwards and radially outwards from at or near the centre of the disk to thus define the shape of a cone. A conventional disk-cone antenna of this general kind is illustrated in Figure 5. Conventional disk-cone antennas can also be (and have been) made with a solid disk and a solid cone, as shown in Figure 6 for example; however this latter form is rarely used because, in the various other applications in which conventional disk-cone antennas have traditionally found use (which are unrelated and totally different to the present road vehicle detection/identification application using an on/inroad RFID reader antenna), the use of a solid disk and cone greatly increases the weight of the antenna, and it may also increase such things as potential wind loading on the antenna and its mounting, etc. This has traditionally meant that such "solid" disk- cone antennas are often unsuitable for use in all but a very select few applications.

[0066] In any case, at least in simplistic/introductory terms, as part of the present invention an antenna structure is proposed that takes a conventional disk-cone antenna and turns it over - i.e. inverts it compared to the traditional "cone-pointing-up" orientation. It is also proposed to surround the antenna structure with an at least partially conductive area, or to place the antenna structure on an at least partially conductive substructure. (Hereafter, reference to a/the "partially conductive area" should be understood as referring to an area that is either partially conductive or fully conductive (i.e. it can mean either of these - or in other words "partially conductive area" should be understood to mean an area that is at least partially conductive). Similarly, hereafter, reference to a/the "partially conductive substructure" should be understood as referring to substructure that is either partially conductive or fully conductive (i.e. "partially conductive substructure" means a substructure which is at least partially conductive)

[0067] The option of providing a partially conductive area which surrounds the antenna structure is what will generally be done for permanent (and perhaps also for semi-permanent) in-road antenna placements, as discussed further below. The alternative option of placing the antenna structure on a partially conductive substructure is what may be done in temporary on- road antenna placements, as also discussed below.

[0068] When the proposed antenna structure is surrounded by a partially conductive area, as will generally be the case for in-road antenna placements, the partially conductive area may need to have a certain minimum size. This is to help ensure that the partially conductive area adequately shields the antenna structure from the potentially widely and dynamically variable radio frequency influences of the underlying road, other "near ground" effects, etc. By way of example, if the partially conductive area surrounding the antenna structure is shaped as a circle, the partially conductive area may need to have a certain minimum radius. However, the partially conductive area of course need not be circular. Indeed it may take any number of other shapes (or indeed any shape). Of course, for such other non-circular shapes, the size of the partially conductive area should still be sufficient to provide adequate shielding to the antenna structure. Referring to the example where the partially conductive area is circular, the minimum radius of the partially conductive area in this case may need, at least for the particular embodiment(s) of the antenna structure discussed below, to be approximately twice the wavelength (λ) of the signals to be transmitted and/or received by the antenna (i.e. the radius of a circular partially conductive area should be >2λ). An appreciation of this can be obtained from Figure 7.

[0069] Figure 7 illustrates an antenna structure in the centre (this particular antenna structure is discussed further below), and surrounding the antenna structure is a partially conductive area. In Figure 7 the partially conductive area happens to be circular in shape with radius R of >2λ, as discussed immediately above. A number of other dimensions are given for the antenna structure in Figure 7. The particular dimensions of the antenna structure given in Figure 7 apply to an antenna structure of the type described below intended for use with signals having a frequency of approximately 920 MHz. As shown in Figure 7, for example, the maximum outer diameter D of the antenna structure (at its base) is approximately D = 180 mm. Certain other dimensions of this particular example antenna structure are also shown. And whilst not labelled specifically in Figure 7, given that the operating frequency of the antenna is approximately 920 MHz (which corresponds to a signal wavelength λ of 326 mm), therefore the radius R of the circular partially conductive area that surrounds the antenna structure is approximately R > 2 x 326 mm = 652 mm. This is mentioned here simply to give a general indication of scale.

[0070] In order to ensure that the partially conductive area adequately shields the antenna structure from the potentially variable radio frequency influences of the underlying road (and from other "near ground" influences), the partially conductive area (and hence the material or substance from which it is formed) should also (at least when "finished" and ready for use) have a minimum conductivity. Or in other words, the partially conductive area should (when finished/installed and ready for use) have resistivity which is below a certain maximum. For the particular antenna structure(s) proposed herein, and given the antenna power, desired radiation pattern shape, antenna gain, antenna return loss, etc, the partially conductive area (and hence the material/substance from which it is formed) should preferably (when installed, finished and ready for use) have a conductivity of approximately 10³ S/m or more (i.e. the conductivity should preferably be approx. equal to or more than 1000 Siemens per meter). To put this another way, the partially conductive area (and hence the material/substance from which it is formed) should preferably (when finished) have a resistivity below approximately 10^"3 ilm (i.e. the resistivity should preferably be equal to or less than 0.001 ohm meters).

[0071] For the avoidance of doubt, the partially conductive area (which, it will be recalled, is generally what will be used for in-road deployments of the antenna structure) should be applied to the surface of the road around the antenna structure, or around the location where the antenna structure will be placed. The partially conductive area need not necessarily come into direct contact with the antenna structure (or any part of it) when the antenna structure is installed. However, in order to ensure that the partially conductive area adequately shields the antenna structure from the potentially variable radio frequency influences of the underlying road, etc, and to ensure that no significant "near ground" effects are created by the width of road (if any) exposed between the antenna structure and the partially conductive area, in the vicinity of the antenna structure and surrounding the antenna structure there should preferably only be a small space/gap between the innermost portion/edge of the partially conductive area and the outer/perimeter edge of the antenna structure. For example, in the particular embodiment(s) discussed above and also discussed further below in which the antenna structure is configured to operate with a signal frequency of approximately 920 MHz (and hence a signal wavelength λ of approximately 326 mm) the space between the partially conductive area and the outer/perimeter edge of the antenna structure should preferably be less than 5 mm. Note that no such gap is shown in Figure 7.

[0072] The fact that the partially conductive area need not necessarily be in contact with the antenna structure may help to simplify procedures involved in the creation/formation/installation/deployment of the partially conductive area, and also the installation/deployment etc of the antenna structure (regardless of which occurs first), and it may also help to simplify maintenance for both (because replacing or repairing one need not necessarily affect or require replacing or repairing the other).

[0073] In relation to the creation/formation/installation/deployment of the partially conductive area in particular, this should preferably be as economical and non-disruptive as possible, both in terms of the time, cost, complexity, etc, involved in the creation/formation/installation of the partially conductive area itself, and also given that it will usually be necessary to close the road (or at least a section of the road or the lane(s) involved) while this is taking place.

[0074] It was mentioned above that the partially conductive area should have a minimum conductivity (or in other words a resistivity which is below a certain maximum), and it was also mentioned that for the particular antenna structures proposed herein, given the antenna power, desired radiation pattern shape, etc, the conductivity should preferably be approximately 10³ S/m or more. If the conductivity of the partially conductive area is greater than approximately 10⁶ S/m, the partially conductive area may in fact be considered to be "fully" conductive, and this may actually be suitable or even ideal for providing shielding in the present antenna application; however this is certainly not a requirement and embodiments of the invention may still operate effectively with partially conductive areas where the conductivity is considerably less than "fully" conductive.

[0075] A partially conductive area for which the conductivity is greater than approximately 10⁶ S/m (such that the partially conductive area is, in fact, "fully" conductive) could be created if the partially conductive area were to be made from a mesh made solely or mainly of, for example, stainless steel, copper, aluminium or certain other suitably conductive metal alloys, or perhaps from steel wool or metal cloth. However, the practicalities and difficulties associated with applying such a metal mesh to the road surface (at least or especially if the mesh is a separate, stand-alone object and not embedded in or as part of some other object or substance that can be more easily applied to the road) mean that creating the partially conductive area from nothing (or little) more than such a metal alloy mesh may perhaps be less attractive than other possible alternatives (some of which are discussed below). Also, a partially conductive area which is made from nothing (or little) more than a metal mesh may also have certain associated risks/hazards, particularly e.g. if the mesh were to lift off the road surface due to improper or imperfect installation, or as a result of wear and tear, etc. Therefore, whilst the use of a partially conductive area made from nothing (or little) more than a metal alloy mesh could be highly effective in terms of its ability to shield the antenna structure from the potentially variable radio frequency influences of the underlying road (and from other "near ground" influences), and whilst embodiments of the invention could well operate with such a partially conductive area made from a simple metal alloy mesh, nevertheless for practical reasons it is thought that this is less likely to be used (or perhaps it will be used less often) than other possible alternative means for forming the partially conductive area.

[0076] As an alternative, the partially conductive area could instead be formed and applied as, for example, a paint (or as a fluid which is applied to the road in a similar manner to paint), or as an epoxy which is applied to the road, or even as a polymer which can be melted onto the surface of the road. To achieve the required minimum level of conductivity (see above), a conductor or some form of conductive component or substance could be blended or otherwise incorporated into any of these, in an appropriate quantity (in the case of conductive substances), prior to installation.

[0077] Another consideration that may affect the means chosen for forming the partially conductive area is that the surfaces of roads generally expand and contract and change shape somewhat with time. For instance, when a road is loaded as a vehicle wheel presses down thereon as it passes, the road surface will momentarily compress/change shape slightly beneath and due to the pressure imposed by the vehicle wheel. Also, expansion and contraction of the road surface can occur due to temperature fluctuations (e.g. between day and night, or with the change of season, etc). This expanding and contracting and changing of shape, often repeatedly/cyclically, can consequently create cyclic loading/stress and hence fatigue in any structure which is connected or bonded thereto. This may in turn to lead to fatigue-related failure, for example, of any partially conductive area which is provided thereon, especially if the partially conductive area is in the form of a rigid or brittle structure. On the other hand, the partially conductive area will generally be much less susceptible to fatigue if it is formed from a substance which has, or if its structure allows or provides (at least a degree of) resilience, flexibility, "give" or the like.

[0078] With the foregoing in mind, one means for providing the partially conductive area which, it is thought, could be suitable (including because it can provide the required conductivity but also because it may potentially be produced economically, applied to the road with minimum disruption, and provide a degree of resilience once formed) is to use a substance which can be applied as a paint, or as an epoxy infused cloth that can be laid onto the road, or as a polymer that can be melted onto the road, and whichever of these is used, a conductive component/substance possibly in the form of e.g. graphite powder may be incorporated or blended into the paint, epoxy or polymer. Other conductive components/substances (i.e. other than graphite powder) may of course also be used. Nevertheless, referring for instance to a partially conductive area which is formed from an epoxy/graphite blend, as a comparative example of the hardiness of a partially conductive area formed in this way, epoxy/graphite blends are often also used in yacht building for load-bearing structures and surfaces. Also, epoxy/graphite blends can have a conductivity of up to approximately 10⁴ S/m (which it will be noted is easily sufficient for the purposes of the present invention).

[0079] Another means which is thought to be possibly suitable for forming the partially conductive area is to use carbon cloth (which can have a conductivity in excess ofl0⁵ S/m) which is painted or epoxied onto the road surface. Such a carbon cloth may alternatively be embedded in polymer sheets which can themselves be melted onto the road surface. In other applications and industries, such as boat and yacht building and repairs etc, it has been shown that maintenance and repair of carbon cloth layers/surfaces/structures, and similarly maintenance and repair of carbon cloth infused epoxy/polymer layers/surfaces/structures, can be relatively easy, cost and time efficient, and effective, using well-understood processes and techniques (none of which require detailed explanation here).

[0080] The component, substance or element within the partially conductive area, which provides the conductivity therefore, should preferably be close (ideally as near as possible) to the upper surface of the partially conductive area when the partially conductive area is applied/formed/installed on the road. In other words, once the partially conductive area has been applied/formed/installed on the road, within the vertical thickness of the structure of the partially conductive area, the component, substance or element which provides the conductivity should preferably be as near to the top as possible. This is because the nearer the component, substance or element which provides the conductivity is to the upper surface, the better the shielding it will provide to the antenna structure. Of course, this may also often need to be balanced against the need for the component, substance or element which provides the conductivity to be covered so as to protect it from exposure to the elements, damage or wear when vehicles drive over it, etc.

[0081] Yet another means which is thought to be possibly suitable for forming the partially conductive area is to use a form of prefabricated "patch" type product which can be applied to the road. These could be similar to in many ways to, for example, the road repair/modification product produced by South African company A J Broom Road Products (Pty) Ltd and referred to by them as the BRP Road Patch. Hence, the partially conductive area could possibly be created using something similar to the BRP Road Patch; that is to say, the partially conductive area could possibly be created using a prefabricated product that is manufactured on paper (or some other suitable substrate or base material) and onto which a bitumen rubber binder (or some other similar binder) holds bitumen pre-coated aggregate. The prefabricated product thus produced could be supplied in sheets (i.e. prefabricated sheets) which are, say, 10-15 mm thick and dimensioned to suit the intended application (see above in relation to the size of the partially conductive area). Note that, in order to accommodate this 10-15 mm thickness of a partially conductive area formed from such a patch, the thickness of the base plate in the antenna structure may also need to be increased somewhat compared with baseplate size shown and discussed with reference to the specification Figures below.

[0082] Still referring to the possibility of forming a partially conductive area using a prefabricated patch like product, as described above, the particulate/grain/pebble size of the aggregate bound in the bitumen rubber binder may also be selected to suit; for example, in order to be similar to or match the particulate/grain/pebble size of the aggregate in the road onto which the patch is to be applied. The overall colour of a said patch (including, or due to, the colour of the aggregate) may be made (or the aggregate may be blended) to generally match the colour of the road onto which the patch is to be applied, such that the patch appears to simply be a part of the road (i.e. it is indistinguishable from the road) when applied. Alternatively, the patch could be coloured, or it could have markings (e.g. border or edge markings), etc, in order to make the patch clearly visible or easy to visibly differentiate from other parts/areas of the road. This latter may be of use in situations where it is preferable, or especially where there is a requirement, for vehicle operators/drivers to be able to see (and hence so that they can know) when they are about to pass over an area/location containing an antenna that will detect and/or identify their vehicle - this can be important for privacy reasons, and/or for compliance with requirements for transparency in systems used in law enforcement and evidence collection for providing evidence which has been collected in a lawful and non- questionable fashion, etc. The aggregate, and the "particles" that make it up, may also include an appropriate quantity or proportion of particles that are lighter coloured, or reflective, or perhaps which are reflective particularly for light in particular spectral ranges such as the infrared spectrum. These lighter and/or reflective particles are not necessarily intended simply to lighten the overall colour of the patch surface (they may also have this affect to some extent, although they also may not, depending on the way in which and the proportion in which they are incorporated in the aggregate) - rather part of the purpose of including an appropriate quantity or proportion of particles that are lighter coloured, or reflective, or reflective of radiation in certain parts of the spectrum (e.g. infrared in particular) is to help reduce heating and heat retention, and perhaps provide some degree of radiant heat reflection. Reducing heating and heat retention in the partially conductive area (and in the road material beneath it) may often be important for preventing possible heating or overheating of electronics associated and located with the antenna, given that the partially conductive area and the road material beneath it surround the antenna (in these in-road applications).

[0083] A prefabricated patch like that described above may be adhered to the road surface to form the partial conductive area in any suitable way or using any suitable technique. By way of example, such patches may be adhered using cationic emulsion or anionic emulsions.

[0084] In order for a prefabricated patch like that described above to have sufficient conductivity, a conductor or some form of conductive component or substance could be included in the mixture (along with the aggregate, etc) bound within the bitumen rubber binder. Alternatively, an aluminium alloy or other metal conducting mesh could be incorporated into (or as part of the patch) such that the said conductive metal mesh (rather than simply being applied to the road as a standalone mesh) is applied to the road as part of the patch product. As a further alternative, particulate or granular aluminium (or other metal) could actually be included in (i.e. as part of) the aggregate which is pre-coated in bitumen in the initial formation/fabrication of the patch. The patch thus produced would then potentially have the necessary conductivity, by virtue of the aluminium (or other metal) contained in and as part of the aggregate. This may also have the benefit of providing a useful option for the recycling of waste aluminium (or other metal) from other sources.

[0085] It has been mentioned that, at least (or particularly) for applications where the antenna structure is installed "in-road", it is proposed to surround the antenna structure with a partially conductive area. It has also been explained that the partially conductive area should have a certain minimum size, in order to adequately shield the antenna structure. In situations where only a single antenna structure is used (e.g. installed in the road) at a given location, the antenna structure will have its own associated partially conductive area. However, there may be situations where multiple of the antenna structures are used at a given location. To help visualise this, consider Figure 4. Figure 4 actually shows a situation where only a single antenna structure is used at the depicted location - the antenna structure is mounted "in-road" in the middle of the centre lane of the road. However, in other situations, it could be that multiple of the antenna structures are used, e.g. in a line across the road. For instance, there could be situations in which there is an antenna structure mounted in-road in the centre of each lane of the road, such that the antenna structures together form a line across the road. In such situations, the multiple antenna structures need not necessarily each have their own associated partially conductive areas. Instead, a single partially conductive area could be provided and shared by some or all of the antenna structures. As one possibility, a single partially conductive area shared by all of the antenna structures (where the multiple antenna structures form a line across the road) could be provided as a wide strip extending across all lanes (i.e. across the width) of the road. In order to provide sufficient shielding for the respective antenna structures, the width of this partially conductive strip which extends across the road (i.e. the dimension of the strip in a direction parallel to the direction of travel in the road lanes) may need to be approximately 4λ or more (i.e. > four times the wavelength of the signal used by the antenna structures). It should be noted though that, in situations where multiple of the antenna structures are used at a given location, each one (or one or more of them) could still have its own associated (and un-shared) partially conductive area. However, from a practical point of view, the time, cost, effort, etc, associated with installing or creating a separate partially conductive area around each antenna structure may be greater than for installing or creating a single larger partially conductive area (e.g. like the wide strip extending across the road mentioned above) which is shared by some or all of the antenna structures. Another possible benefit is that the said strip could be coloured, or it could have markings (e.g. edge markings extending across the road before and after the antenna structures in the vehicles' direction of travel), or it could have a different surface texture or stone/particle size or the like, etc, in order to make the strip clearly visible (or perhaps audible when driven over), which (like above) may be of use where vehicle operators need to be able to see when they are about to pass over an area/location where their vehicle will be detected and/or identified (or at least know or be alerted when this happens). Also, like above, the strip may incorporate lighter coloured or reflective particles to assist in minimising heating and heat retention, etc.

[0086] Turning now to consider the antenna structure, as has been explained, one of the proposals presented herein is an antenna structure that, in effect (and in basic terms), inverts a conventional disk-cone antenna structure. However, it should be noted that this simple statement also oversimplifies the present invention, and in particular it oversimplifies the proposed antenna structure, because quite apart from being inverted compared to a conventional disk-cone antenna, there are also a number of other important differences between a conventional disk-cone antenna structure and the presently proposed antenna structure. Several of these other differences, including those differences discussed below, are very important.

[0087] A number of the important differences between a conventional disk-cone antenna structure and the presently proposed antenna structure (aside from the basic orientation) relate to relative sizes and proportions of different parts of the antenna structure. For instance, in a conventional disk-cone antenna, the height of the antenna (i.e. the distance in the antenna's axial direction between the disk and the widest point on the cone) is generally greater (often much greater) than the maximum diameter of the antenna (which is generally on the cone). In other words, in a conventional disk-cone antenna, antenna height » antenna diameter. In contrast to this, in the presently proposed antenna structure, the height of the antenna (i.e. the distance in the antenna's axial direction between the disk and the widest point on the cone) is less (generally much less) than the maximum diameter of the antenna (which is on the disk - see below). That is to say, in the presently proposed antenna structure, antenna height « antenna diameter (or equivalently, antenna diameter » antenna height).

[0088] Furthermore, in a conventional disk-cone antenna, the diameter of the antenna's disc is generally smaller than the maximum diameter of the antenna's cone. In other words, in a conventional disk-cone antenna, cone diameter > disk diameter. In contrast, in the presently proposed antenna structure, the diameter of the antenna's disc is larger than the maximum diameter of the cone (i.e. cone diameter < disk diameter).

[0089] It is also important to stress that conventional disk-cone antennas are designed, and they are generally implemented, in applications where the antenna is mounted a considerable distance above the ground (above planet Earth) and transmits omnidirectionally in a broadcast manner. More specifically, traditional disk-cone antennas are invariably mounted at a height above the ground which is much greater than the wavelength (λ) of the signal being transmitted, and in fact a major purpose of the antenna's disk in disk-cone antennas designed for use in such applications is to, in essence, force or direct the antenna's radiation pattern downward towards the earth. (This last is important because of the earth's inherently low conductivity and its other radio propagation influencing properties, which tend to push the radiation pattern of an antenna upwards away from the earth, as is also explained elsewhere). Configurational differences between a conventional disk cone antenna and the presently proposed antenna structure, including (but not limited to) those mentioned above related to relative proportions of the antenna structure, exist in no small part in order to allow the presently proposed antenna structure to operate, and to provide the required radiation pattern, when placed in, on or in very close proximity to the ground (planet Earth).

[0090] Additionally, a point which is extremely important to understand here is that, according to all conventional thinking in the field of antenna design, the very notion of placing any form of disk cone antenna (or indeed any form or variant of dipole or monopole type antenna) in, on or close to the ground goes against all conventional thinking. This simply would not be done, because according to conventional thinking, the use of this kind (or any of these kinds) of antenna in this way just would not work, given that these kinds of antennas have always been designed and intended for use, e.g. in placements far above (i.e. much more than a wavelength above) planet Earth, and for much more far field transmission distances, whereby the effect of planet Earth on radio propagation can be considered much more constant/homogeneous, etc.

[0091] The basic configuration (and in particular aspects of the shape) of an antenna structure that is in accordance with the present invention is illustrated schematically in Figure 8. It is important to note that Figure 8 does not fully depict the antenna structure - there are numerous parts and features of the proposed antenna structure which are not illustrated in Figure 8. Therefore, it is to be understood that Figure 8 is presented merely as a schematic illustration of the overall shape of the proposed antenna structure, or at least of certain important parts/portions thereof.

[0092] Figure 8 shows that the antenna structure has an overall circular-based and upwardly-tapering frusto-conical shape 10. In other words, the general/overall outer shape 10 of the antenna structure is that of a cone with a circular base, but the cone is terminated/truncated/"cut-off" well before it reaches a point/apex, and the termination/"cut-off" is in a plane parallel to the circular base.

[0093] Also, formed/indented and extending vertically downwards into the "cut-off" top of the antenna structure is a comparatively inverted circular cone shaped opening 12. The inverted cone shaped opening 12 tapers inwards from its widest point at the top down to a convergence point 14. (Hence, the convergence point 14 is the lowermost point on the inverted cone shaped opening 12.) Note that the convergence point 14 is located on or very close to the plane of circular base of the main frusto-cone 10 (in fact, in embodiments discussed below, the convergence point 14 and the plane of circular base meet, and this meeting point is the location of the antenna's feed point). Importantly, the inverted cone shaped opening 12 (and more specifically the conducting material (metal) that is provided thereon - see below) is what forms the operative "cone" of the antenna structure - i.e. this is the part of the antenna structure which corresponds generally to the cone portion in a conventional disk-cone antenna - and this "cone" is therefore one of the operative/radiating parts of the proposed antenna. Thus, the inverted cone shaped opening 12 (which in subsequent discussion will be synonymous with the conducting material actually formed or provided thereon to thus create the said cone-shaped radiating element) will hereafter be referred to simply as the "cone". (Note: in Figure 8 the cone is labelled with reference numeral 12, but in later Figures the cone of the antenna structure may have a different label.) It is to be noted from Figure 8 that the orientation of the cone 12 is inverted/upside-down relative to the orientation of the cone portion in a conventional disk cone antenna (see Figure 5 and Figure 6).

[0094] It is also to be noted in Figure 8 that, at its widest point at the top, the circular opening formed by the open top/mouth of the cone 12 is narrower than the outer diameter of the main frusto-cone shape 10 at the height of the "cut-off". Therefore, at least in Figure 8, there is a flat annular (i.e. a horizontal, flat and ring-shaped) rim 16 formed in between the outer diameter at the top of the main frusto-cone shape 10 and the edge of the circular opening formed by the open top/mouth of the cone 12. However, whilst the overall antenna structure shape is shown in Figure 8 with the rim 16, this rim 16 may not be present in the actual antenna structure. For example, there may instead be provided a slight indent or recess extending vertically down into the top of the "cut-off/truncation in the antenna's main frusto-cone shape 10 - see 16a in Figure 14 - the slight indent or recess 16a being for receiving a lid or top plate of the antenna structure. (The lid/top plate is discussed further below.) The uppermost portion of the sloping slide of the frusto-cone shape 10 may in fact provide a wall or lip surrounding the indent/recess 16a, and the said wall or lip may help to locate and retain the lid / top plate of the antenna structure (i.e. it may help to stop the lid from becoming dislodged or sliding off the top of the antenna structure). The holes 17 (again these are visible in Figure 14) may facilitate screwing or bolting the antenna's lid/top plate in place in the recess 16a.

[0095] In the embodiments of the antenna structure described in further detail below (additional details and features of which are depicted in several of the other Figures) the particular antenna structure is one that is operable with a signal of frequency 860-940 MHz, and in terms of the general overall shape and reference numbers shown in Figure 8:

- the vertical height of the antenna structure 10 is 25 mm;

- the outer diameter of the antenna structure 10 at its lowest and widest point (i.e. the diameter of its circular base) is 180 mm;

- the outer diameter of the frusto-cone at the highest/"cut-off"/truncation point (which is the outer diameter of the rim 16 in Figure 8, if present) is approximately 104-1 10 mm (and consequently, if a vertical cross-section were to be taken through the centre of the antenna, the angle between the sloping/encompassing side of the antenna structure 10 and the plane of the base when viewed in said cross section would be around 33°-36°); and

- the inner diameter of the rim 16 (again, if present) is approximately 80 mm

[0096] It may be useful to note that, the height of 25 mm (or a height not exceeding 25 mm) above the road surface, is one that is commonly approved in the regulations/standards governing on-road and road-surface devices in most jurisdictions. Thus, for example, in most countries/jurisdictions, the allowable height for devices like e.g. conventional retro-reflective "cat eyes" and the like, is typically up to 25 mm. These regulations/standards typically also require, particularly for devices like "cat eyes" and the like, that the sides of such devices should be at an angle of elevation of no more than 45° to the plane of the road surface. This requirement (along with the above-mentioned 25 mm height restriction) is to allow the wheels of cars and other road going vehicles to roll over the said devices without an undue jolt or impact. Furthermore, these regulations/standards typically allow for the maximum diameter of such on- road or road-surface devices to be no more than 190 mm (i.e. 190 mm or less). It will be noted that the dimensions and shape parameters of the antenna structure as listed in the previous paragraph conform to these requirements. The significance of the fact that the antenna structure both conforms to these requirements and that it is also operable to provide the radiation pattern discussed herein (i.e. the significance of the fact that it can and does achieve both) will be readily appreciated by those skilled in the field of antenna design and it should not be underestimated.

[0097] It is explained above that the cone 12 depicted in Figure 8 is the part of the antenna structure which corresponds generally to the cone portion in a conventional disk-cone antenna. The proposed antenna structure also has a disk portion/component (again this is a functional/radiating part of the antenna) which corresponds generally to the disk portion in a conventional disk-cone antenna. However, the disk portion/component in the proposed antenna structure is not depicted in Figure 8. It is depicted, though, in several of the other Figures which show that the disk portion/component is a flat circular disk of conductive material (typically metal) having a diameter the same as, and located immediately beneath, the bottom of the main frusto-cone 10. In fact, the disk portion/component will generally be applied or attached to the underside of the main frusto-cone 10.

[0098] It should also be noted that, in situations where the proposed antenna structure is surrounded by a partially conductive area (typically this will be where the antenna structure is installed/deployed in in-road applications), the partially conductive area that surrounds the antenna structure, even though this need not necessarily be in direct contact with any part of the antenna structure, nevertheless functions "in effect" as something of an extension of the antenna structure's actual disk portion/component. In other words, the partially conductive area (even though it is not necessarily connected to the antenna structure) functions as an extension of the antenna structure's disk portion/component in terms of the influence it has on the antenna's overall radiation pattern.

[0099] It has been mentioned that the cone 12, and also the disk portion/component, of the antenna structure are made from a conductive material (typically, although not necessarily or exclusively, metal - see below). However, the main frusto-cone shape 10 of the antenna structure, which may be referred to as the antenna structure's main frusto-cone body 10, is itself made from a strong/structural dielectric material. The material which is thought most likely to be used for this is glass (and there are many different forms or types of glass that may be suitable), although other strong or structural and dielectric materials could also be used. Glass does have the benefit, though, of being transparent, translucent or at least somewhat permissive to penetration by light, which can have advantages - see below. Preferably, the glass or other strong/structural and dielectric material should have a relative permittivity (or dielectric constant) of between approximately 3 and approximate 6. As possible alternatives to glass, other materials which might be used for making the frusto-cone body 10 include, for example, concrete, certain strong/structural/engineering polymers such as nylons and Teflons, certain alumina (although these may not have the benefit of being transparent or otherwise permissive to penetration by light, at least not to the same extent as glass).

[00100] As explained above, the cone 12 and the disk portion/component are functional/radiating parts of the antenna structure. The glass or other strong/structural and dielectric material from which the antenna structure's main frusto-cone body 10 is made (and hence the frusto-cone body 10 itself) is not a radiating part of the antenna; however the main frusto-cone body 10 is still very much a functional part of the antenna because its form, material and associated RF properties (in other words its size, shape, configuration, material and dielectric properties, etc,) significantly affect the radiation pattern of the antenna and specifically contribute to (or assist in) forming the radiation pattern with the desired "dropped doughnut" shape. Therefore, design choices in relation to the size, shape, configuration, dielectric and other material properties, and other aspects of the design of the antenna structure's main frusto- cone body 10 have been made with great care and attention, because (again) even though this body 10 is not a radiating part of the antenna, it is still an important functional part of the antenna because any changes to its design (even slight changes) would (or could) significantly affect the antenna's radiation pattern (and in particular the shape thereof).

[00101] In addition to being functional in the sense of being influential on the overall radiation pattern of the antenna, the main frusto-cone body 10 of the antenna structure is also functional in the sense of being structural - that is, it is one of the primary components which provides the physical supporting structure of the antenna (and gives it its physical strength). This can be understood quite simply. As has been mentioned, components such as the cone 12 are made from a conductive material (typically metal). In fact, the cone, for example, will generally be made from a thin layer or film of metal, perhaps less than a millimetre or only a fraction of a millimetre in thickness. The cone is also elevated relative to (i.e. it is located vertically above) the antenna's disk (and also relative to the surface of the ground/road in an inroad installation for example). Naturally, thin layers or films of metal such as this, especially if elevated/upstanding and unsupported in "free space", can be very flexibly and flimsy. It should also be recognised that, by simple virtue of its location in its intended (especially in-road) application, the proposed antenna structure may often be directly run over by vehicles travelling along the road in/on which the antenna is installed. Clearly, it is essential for the antenna structure to be able to withstand such forces and impacts repeatedly and over a long period of time without damage or affect on the antenna's functioning or performance. It is therefore also equally clear there needs to be something to prevent any otherwise thin/flimsy pieces or layers or films of metal that make up or are comprised in the antenna structure (in particular the cone 12) from simply be crushed/flattened and completely destroyed by such vehicle impacts. It is this function that the main frusto-cone body 10 of the antenna structure helps to provide. In other words, the main frusto-cone body 10 provides a physical structure which not only can withstand such vehicle impacts itself, but it also provides a supporting foundation for other parts of the antenna such as the cone, etc, which themselves could not withstand such impacts but which can withstand such impacts when mounted or formed on (and hence supported on or by) the main frusto-cone body 10.

[00102] An actual antenna structure in accordance with one embodiment of the invention, and an RFID reader 100 of which the antenna structure forms part, will now be discussed in further detail initially with reference to Figure 9. An annotated version of Figure 9 is also given as Figure 10; however for convenience reference will be made to Figure 9 only.

[00103] It is to be noted firstly that Figure 9 is a view of an RFID reader 100 which incorporates the proposed antenna structure as well as other RFID reader equipment. It should also be noted from the outset that Figure 9 depicts a situation where the RFID reader 100 is installed in an "in-road" installation. In other words, at least some parts of the RFID reader 100, and other associated equipment, are located at or below the level of the road surface RS, whereas other parts are located above the level of the road surface RS. And as will be readily appreciated, Figure 9 is a side-on cross-sectional view, and hence parts of the RFID reader 100 as well as other associated equipment which are located both above and below the level of the road surface RS can be seen.

[00104] As just mentioned, the RFID reader 100 in Figure 9 is installed in an "in-road" installation. In fact, prior to the installation of the RFID reader 100 in the road, an appropriately- shaped recess/hole/cavity (hereafter the "cavity" 1 10) must first be first dug, cut, bored or otherwise formed in the road, in order to receive the RFID reader 100 and the associated parts and equipment therein. In fact, there are several distinct portions of the cavity 1 10, each for receiving and accommodating different parts of the RFID reader 100. The first/main portion of the cavity 1 10 is labelled 1 1 1 in Figure 9. This main portion of the cavity is circular/cylindrical and, at least in this particular embodiment, is approximately 120-125 mm wide, and approximately 30-35 mm deep (i.e. it extends approximately 30-35 mm vertically down below the road surface RS). The main portion 1 1 1 of the cavity 1 10 is sized and shaped to receive a cylindrical "cup"-like container component 160 which is made of metal or other heat-conductive material and to which other parts of the RFID reader 100 attach, as discussed below. At/near the top of the main portion 1 1 1 of the cavity 1 10, and in fact extending around the outer perimeter of the main portion 1 1 1 , there is a wider but shallower second portion 1 12 of the cavity 1 10. In other words, the second portion 1 12 of the cavity extends vertically much more shallowly into and below the road surface RS than the main portion 1 1 1 (typically the second portion 1 12 will be only a few millimetres deep), although the diameter of the second portion 1 12 (typically approximately 180 mm) is considerably greater/wider than that of the first portion 1 10. The second portion 1 12 of the cavity receives the outer/perimeter portion of the RFID reader 100; in particular the underside of the base plate 140 (the base plate 140 is discussed below). And finally, optionally, in the bottom of the main portion 1 1 1 , typically at or near the centre thereof, a third portion 1 13 of the cavity 1 10, which in this particular embodiment is in the form of a bore or shaft, extends vertically downwards considerably more deeply than any other part of the cavity 1 10. This optional bore/shaft portion 1 13 (if provided) should be shaped to receive a heat sink 105. In the embodiment depicted in Figure 9, the (optional) heat sink 105 happens to be an elongate (and vertically-oriented) cylindrical rod, made of metal or some other heat-conductive material, which is >50 mm long and about 12 mm in diameter. Of course, the heat sink (and the portion 1 13 of the cavity in which it is received) could take a range of other shapes and sizes. Figure 15 provides an example of an alternative embodiment with a much larger heat sink 205 designed for dissipating much larger amounts of heat, as compared with the embodiment in Figure 9. The larger heat sink 205 in Figure 15 is actually (again) outwardly cylindrical, but it also has a rectangular-box/prism-shaped hollow interior, as indicated in Section H-H in Figure 15. The said interior may be used for housing electronic parts and equipment associated with the RFID reader in that embodiment - see below. In any event, the size and shape of the heat sink may vary depending on the heat dissipation requirements in a given application (e.g. some RFID readers may generate more heat than others, depending on the equipment or amount of power used in the RFID reader, or depending on the equipment's thermal efficiency, and heat sink size and dissipation requirements may also vary according to how warm the ground and ambient environment is, and how much sun exposure there is at the location in question, etc.)

[00105] In Figure 9, the heat sink 105 is attached, mounted or otherwise secured to the underside of the container 160 (there is actually a recess in the underside of the container 160 into which the top of the heat sink 105 is received - screwed in this case). The function of the heat sink 105 (and likewise the heat sink 205) is to receive heat generated by the RFID reader electronics and which is conducted into the heat sink 105 (e.g. from the RFID reader and its electronics and, in the case of Figure 9, via the metal container 160) and to dissipate said heat into the soil/road base surrounding the heat sink. Dissipating heat in this way may often be important as it may help to prevent overheating within the RFID reader (i.e. overheating which might otherwise damage or at least interfere with the proper operation of the RFID reader electronics, etc).

[00106] It should also be noted that, after the cavity 1 10 (including its various portions - see above) have been formed, but before the container 160 and the heat sink 105 are inserted (and also before the rest of the RFID reader 100 is subsequently attached to the container 160), an adhesive 108 is first applied to at least the walls/surfaces of the various portions of the cavity 1 10. Once the adhesive 108 has been applied at least to the walls/surfaces of the cavity 1 10, the container 160 and the heat sink 105 are then inserted so that when the adhesive 108 sets, the container 160 and the heat sink 105 thereby become adhered and secured in their respective portions (1 1 1 and 1 13) of the cavity. Typically the other parts of the RFID reader 100 will also be attached to (screwed into) the container 160 (see below) before the adhesives sets, so that the underside of the base plate 140 also becomes adhered to and secured in the second portion 1 12 of the cavity (i.e. the other parts of the RFID reader also become secured in place by the adhesive when it sets). Thus, in Figure 9, the adhesive 108 entirely (or at least mostly) fills the space between the walls of the cavity and the various parts of the RFID reader. (In other words, the adhesive 108 fills the space beneath the base plate 140 in the portion 1 12 of the cavity, the adhesive 108 also fills the space between the outside of the container 160 and the wall and floor of the cavity in the main portion 1 1 1 of the cavity, and the adhesive 108 further fills the space between the outer surface of the heat sink 105 and the walls and floor of the cavity in the portion 1 13.) Preferably, the adhesive 108 should be one that conducts heat well (to assist in heat dissipation), and it should also be one that is sufficiently strong to adequately secure the various parts of the RFID reader (see above) and prevent them from being easily dislodged, etc, but at the same time it should (preferably) not be so strong as to inhibit or prevent removal and/or replacement of the RFID reader (or parts thereof, e.g. the container or the heat sink) in case this proves necessary for any reason. Also, the adhesive 108, which may be up to a few millimetres thick, may have or provide some inherent flexibility or resilience, and this may in turn provide at least some degree of shock absorbency for when the RFID reader is secured thereby and vehicles drive directly over the top of the antenna structure. By way of example, certain commercial silicon (or silicon-based) and bitumen (or bitumen-based) adhesives may be suitable for use as the adhesive.

[00107] As a possible (non-illustrated) variant to the above (and this may apply to other embodiments discussed below as well), it is possible that different adhesives may be used at different locations. For example, an adhesive which is comparatively weak but also highly effective at heat transfer (i.e. an adhesive which conducts heat well) may be used in the portions 1 1 1 and 1 13, such that heat from the container 160 and from the heat sink 105 is effectively dissipated, yet removal of these sub-road-surface components (if necessary for maintenance or repair etc) is not impeded or made more difficult by an overly strong adhesive bond. At the same time though, a different adhesive with a much greater bond strength may be used, for example, in the portion 1 12 which secures the underside of the antenna structure in the surface of the road.

[00108] Also depicted in Figure 9 is the partially conductive area (discussed above) which, at least in "in-road" deployments, is applied directly to the road surface RS and surrounds the RFID reader 100. In Figure 9, the partially conductive area is indicated by reference numeral 90. Importantly, in the cross-sectional view in Figure 9, the two sides of the Figure are cut off and therefore the full width of the partially conductive area 90 is not necessarily illustrated. In other words, the partially conductive area 90 may (and it typically will) extend further outward away from the RFID reader 100 than is depicted in Figure 9. The partially conductive area 90 may be applied to or otherwise formed on the road surface either before or after the RFID reader 100 and its associated equipment are installed (and secured by adhesive) in the cavity 1 10. In fact, the partially conductive area 90 may even be applied/formed before the cavity 1 10 itself is created. However, it may often be the case that at least the location where the cavity 1 10 will be created must be previously-determined (and typically marked) before the partially-conductive area 90 is formed on the road, so that the partially conductive area can be correctly positioned relative to the cavity 1 10 (into which the RFID reader 100 will be installed) in order to provide adequate shielding to the RFID reader 100 when (and in the location in which) it is subsequently installed.

[00109] In Figure 9, the main frusto-cone body of the RFID reader (recall that this is also a functional part of the antenna structure) is made of glass. Preferably, the glass used is a form of soda lime glass.

[00110] The antenna structure's cone is labelled with reference number 120 in Figure 9. In the embodiment depicted in Figure 9, the cone 120 is preferably formed as a thin layer (less than or a millimetre or even only a fraction of a millimetre thick) of metal such as e.g. copper, silver or an appropriate conductive alloy thereof (other metals or indeed other conductive materials might also be used). The thin metal (or conductive material) layer which forms the cone 120 may be formed on or applied to the glass body in any suitable way, although it is thought that one (if not the most) appropriate means for achieving this, where the conductive material used is metal, may be by plating the metal that forms the cone 120 directly onto the inverted cone shape in the top of the glass body.

[00111] When the antenna structure is in its installed configuration, as shown for example in Figure 9 (Figure 9 being an in-road installation although the same also applies in and on-road installation), the main frusto-cone body, which is made of glass, effectively sits directly on top of the antenna structure's disc portion/component, which takes the form of a base plate 140. Like the cone 120, the base plate 140 is preferably formed of metal such as e.g. copper, silver or an appropriate conductive alloy thereof (other metals or indeed other conductive materials might also be used). The baseplate will, however, typically be relatively thick (compared to other radiating/metal parts of the antenna structure), for example, with a thickness of 5-10 mm. The metal plate which forms the base plate 140 may be applied or connected to the underside of the glass body in any suitable way. In one appropriate means for achieving this, the base plate 140 could be formed initially separately from the glass body and then affixed thereon (e.g. using an adhesive, or some form of mechanical fastening, etc).

[00112] In addition to being a functional radiating component of the antenna, the base plate 140 is also functional in the sense of being structural - that is, it is another of the primary components which provides the physical supporting structure of the antenna (and gives it its physical strength). More specifically, and especially where the base plate 140 is formed as a solid metal plate which is 5-10 mm thick, the baseplate 140 provides a rigid/solid base for the glass frusto-cone body. And because the glass frusto-cone body sits directly on top of the base plate 140, the base plate 140 effectively "underpins" the frusto-cone body (i.e. it supports it from underneath) and thereby prevents the glass body from e.g. unduly deforming or cracking, etc, when a vehicle drives over it.

[00113] In addition, the base plate 140 also incorporates, or it has attached thereto, a mounting block (hereafter the "screw mount") 150. The screw mount 150 projects out relative to the plane of the base plate 140 in the opposite direction to the direction in which the glass frusto-cone body extends from the base plate. Thus, in the installed configuration shown in Figure 9, where the glass frusto-cone sits on top of and effectively points upwards relative to the base plate 140, the screw mount 150 consequently projects vertically downwards from the underside of the base plate 140. The distance by which the screw mount 150 projects out (downward) from the underside of the base plate 140 will typically be around 5 mm-15 mm. The outer diameter of the screw mount 150 is smaller than the diameter of the base plate 140, and indeed the outer diameter of the screw mount 150 is approximately the same as the internal diameter of the wall of the cylindrical cup-shaped container 160. In fact, in embodiments like the one depicted in Figure 9, the outer (vertical) cylindrical wall of the screw mount 150 is threaded, and at least the upper portion on the inside of the wall of the cup-shaped container 160 is also correspondingly threaded. Accordingly, after the adhesive 108 has been applied into the cavity 1 10, and the container 160 and heat sink 105 have been inserted into the cavity, etc, the way in which the antenna structure is attached to the container 160 (the antenna structure being the baseplate 140, the frusto-cone body, the screw mount 150, etc, all of which are already assembled together) is that the said antenna structure is positioned on top of the container 160 and turned so that the screw mount 150 screws into the threaded portion at the top of the container 160. The antenna structure is then "screwed down" sufficiently that the outer perimeter portion on the underside of the base plate 140 becomes received in (and pressed into and secured by) the adhesive 108 which is in the second portion 1 12 of the cavity, as shown in Figure 9.

[00114] The various dimensions of the antenna structure (i.e. the antenna structure which is incorporated in or as part of the RFID reader 100, as just described) in Figure 9 generally correspond to those dimensions described above with reference to Figure 8. For instance, the diameter of the cone 120 is approximately 104 mm, and the height of the cone 120 is 25 mm or less. Also, the base plate (disc) has an outer diameter of approximately 180 mm. However, it is very important to understand that these dimensions (and this applies equally to the explanations and dimensions given above with reference to Figure 8 and to other embodiments of the invention as well) are all given by way of illustrative example only. These particular dimensions apply to an antenna structure (which is incorporated in or as part of the RFID reader 100) that is tuned to operate with a specific signal frequency (approximately 860-940 MHz), as well as with an appropriate partially conductive area/structure and dielectric structure (the main frusto- conical body). It is to be clearly understood that the size, shape, dimensions, etc, of the various parts of the antenna structure (or at least of some of them) would change, for example, for antennas designed to operate at different signal frequencies. Furthermore, as will be readily understood by those skilled in the field of antenna design, the antenna's various dimensions, and also the shape, material used, material thickness, material properties and other such parameters of the antenna and its various parts, may all be varied for the purpose of tuning the antenna to achieve the required radiation pattern based on the signal frequency used and other use case requirements.

[00115] Next it should be noted from Figure 9 that, even when the screw mount 150 is screwed fully into the container 160 (such that the base plate 140 is received in the portion 1 12 of the cavity just below road level RS), there is still a remaining vertical space/gap 155 inside the container 160 in between the underside of the screw mount 150 and the internal base of the container 160. Items or pieces of electronic equipment associated with the RFID reader 100 (or some of them) may be mounted in (i.e. they may reside in) the said gap 155.

[00116] It should also be noted from Figure 9 that the gap 155 is not the only opening or space where electronic components can be accommodated. There is also space for mounting certain components in the space between the upper surface of cone 120 and beneath the RFID reader's "top plate" or "lid" 130. The space below the lid 130 but above the upper surface of the cone 120 is labelled with reference number 135 in Figure 9. The lid 130, in fact, provides a protective covering or barrier over the top of any electronic components that may be located in space 135 (i.e. to prevent them from exposure to the elements or damage from vehicles rolling over the top of the reader, etc). The space 135 may be a particularly useful location in which to house sensors such as, for example sensors for detecting or measuring sound, gas, etc. Other electronic components which might be (or might also be) housed within the space 135 include equipment for providing conventional radar or imaging functions, and equipment for enabling other forms of wireless connectivity (e.g. a Wi-Fi or Bluetooth connection to facilitate such communication between electronic equipment located inside the RFID reader and remote computers or devices). The space 135 could further incorporate, or it may be used to house, a vibration sensor. A vibration sensor could be particularly useful, for example, because sensing vibrations as vehicles pass can be used to determine an "axle count" and/or axle spacing of passing vehicles (e.g. this can in turn enable, even without the use of RFID, the determination of whether the passing vehicle is e.g. a car, or a truck, or a multiply-articulated road train, and this kind of thing can further in turn be useful for monitoring and managing the integrity of the road surface, determining the need for or scheduling maintenance, traffic management, etc).

[00117] It is also discussed elsewhere herein that RFID readers, and this includes readers incorporating the presently-proposed antenna structure, may be used to provide not only "two- way" data exchange but also "one-way" (or RADAR-like) data exchange. It is further explained elsewhere that "one-way" data exchange in particular, may be useful for the purposes of vehicle detection. The presently-proposed RFID reader may make use of this, in particular, because the amount of power required for two-way communication can be much more than for one-way communication. Accordingly, vehicle detection achieved using "one-way" data exchange could be used, for example, to help minimise power consumption by enabling the RFID reader to operate normally in the lower-powered one-way communication mode, and then only switch to the higher-power two-way communication mode (by switching on the RF communication equipment required for this) when a vehicle is actually detected by a one-way data exchange occurrence, and hence only when the need for actual/positive vehicle identification is required. (The duty cycle in the RFID reader equipment will preferably be such that the high power RF communication equipment required for two-way data exchange can be turned on in a matter of milliseconds, so even if a vehicle is only detected when it is, say, 6 m from the antenna, the time delay in switching on the high power RF equipment should not prevent proper vehicle identification via RFID ("two-way" data exchange), especially if the vehicle is moving at normal road speeds.) In addition to saving power, only using the higher power level required for two- way communication when necessary may also significantly help to reduce heat generation and the risk of overheating in the RFID reader.

[00118] As has been mentioned previously, in the embodiment depicted in Figure 9, the antenna structure (when it is screwed onto the container, etc, as discussed above) is incorporated in and as part of the RFID reader 100. For instance, in the RFID reader 100, the cone 120 is the antenna's "cone" (i.e. it is the radiating part which corresponds approximately to the cone of a traditional disk cone antenna), and similarly the base plate 140 is the antenna's "disk" (i.e. it is the radiating part which corresponds approximately to the disc of a traditional disk cone antenna). The feed point of the antenna is actually at the meeting point where the tip/apex of the cone 120 meets the base plate (the disc) 140.

[00119] It has also been mentioned that the antenna's main glass frusto-cone body is not a radiating part of the antenna structure; however the main frusto-cone body is still very much a functional part of the antenna because its form, material and associated RF properties (in other words its size, shape, configuration, material and dielectric properties, etc,) significantly affect the radiation pattern of the antenna, and specifically contribute to (or assist in) forming the radiation pattern with the desired "dropped doughnut" shape. It has further been mentioned that the choice of glass (including or particularly soda lime glass) as the strong and dielectric material from which the main frusto-cone body is made may have the additional benefit of being transparent or translucent or at least somewhat permissive to penetration by light. The reason this may be beneficial is because, included among other electronic parts or components provided in or as part of the RFID reader 100 (e.g. provided in or extending through the disk/base plate 140), there may be one or more components that incorporate lights, LEDs or the like and which, when illuminated, are visible from outside the RFID reader 100 and even from a distance away from the RFID reader (especially at night or in low light conditions). Such lights or LEDs could be used, for example, to provide indications as to the current operational status of the RFID reader 100 or individual parts or functions of it. For instance, as a simple example, a red light/LED could be provided which "turns on" in situations where there is a fault or malfunction or warning associated with the operation of the RFID reader (e.g. where there is a component malfunction, or a power supply failure or disruption, or an "almost empty" battery or backup battery, etc). However, such lights, LEDs or the like which may be contained within (but visible from without) the RFID reader 100 might also be used for a range of other purposes. For example, because the RFID reader 100 in these "in-road" applications is positioned in the surface of the road (i.e. in the surface on which vehicles are travelling and to which the vehicle's drivers are paying close attention), LEDs or lights in the RFID reader may also be used to provide various forms of signalling to vehicles. For example red and green lights could be used for indicating lanes that are open or closed for vehicle travel, or for indicating the permitted direction of travel in a lane (this last might be useful e.g. in places which implement "tidal flow" traffic management which facilitates vehicular travel, within a given lane, in different directions at different times of day, to help accommodate large volumes of traffic flow in one direction or other at different times of day). There could also be other possible uses, for example a flashing light could be used to provide a warning to road users of an upcoming incident or danger further down the road. Or, red, yellow and green signals could be provided in an RFID reader located just before an intersection with traffic lights, and the red, yellow or green lights in the RFID reader could be changed instantaneously/simultaneously and correspondingly with the change in signal at the traffic lights. The illumination of, or light signals emitted from, any lights or LEDs inside the RFID reader could also be visible and detectable to cameras or other imaging devices, for example those located at the side of the road and used for law enforcement or traffic management purposes. It will be appreciated that the possible uses mentioned above for lights, LEDs or the like which may be provided in or as part of the RFID reader are merely examples, and there may be many other uses or applications for this.

[00120] Figure 9 also shows that the RFID reader incorporates a number of tuning screws/struts 190. These tuning screws 190 are actually optional. In other words, they need not necessarily be present or incorporated into the antenna design. In embodiments where tuning screws 190 are present or used, these may help to further support or strengthen the overall structure of the antenna. Furthermore, where tuning screws 190 are present, the number, arrangement, angle, length, thickness, points of contact with antenna radiating elements, etc, of the screws may be selected or varied in order to tune the antenna. In other words, the presence/absence, and if present the configuration and design, of the tuning screws 190 plays a role in the overall tuning of the antenna to achieve the desired radiation pattern. It should also be noted that, in the embodiment in Figure 9 for example, the tuning screws 190 extend through the glass of the frusto-cone cone body and "screw into" the metal base plate 140. It is therefore possible that tightening or loosening the tuning screws 190, or tightening or loosening one or some of them more or less than others, to thereby slightly (even if only minutely) change/distort the shape of the antenna structure by compressing some parts more than others may also be used to perform fine tuning of the antenna, for example final fine tuning at the time of installation.

[00121] The antenna screws 190 (where present) may be hollow, and they may therefore also provide one or more conduits for cables, wires or the like extending between electronic parts and equipment located in the space 155 beneath the screw mount 150 and electronic parts and equipment located in the space 135 above the cone 120 below the lid 130. Holes in the antenna structure's main frusto-cone body, which receive the tuning screws 190 and allow them to pass through the frusto-cone body, are labelled 192 in Figure 14.

[00122] Turning now to Figure 15, this Figure depicts an RFID reader 200 in accordance with a generally similar but slightly different/varying embodiment compared to the embodiment depicted in Figure 9. An annotated version of Figure 15 is also given as Figure 16; however for convenience reference will be made to Figure 15 only. Furthermore, parts, features, and aspects of the design of the RFID reader 200 (and the antenna structure it incorporates) in Figure 15 which are the same or equivalent to corresponding parts, features, etc, of the RFID reader 100 (and its antenna structure) in Figure 9 will not be described. Therefore, the embodiment in Figure 15 will be described mainly only insofar as it differs in material or notable ways from the embodiment in Figure 9.

[00123] One aspect of the design of the RFID reader 200 in Figure 15 that differs from the embodiment in Figure 9 is that the underside of the base plate 240 is provided with a number of downwardly-depending ground-engaging portions 241 . These ground-engaging portions 241 could be provided as a number of discrete "legs" extending down from the main base plate 240 at different radii and different locations around the base plate. Alternatively, these ground- engaging portions 241 could be provided as concentric rings of different radii extending around (and depending downward from) the main base plate 240. However, if the ground engaging portions 241 are provided as concentric rings, the rings may also need to incorporate a number or series of holes or spaces to allow the adhesive 108 to squeeze through and into the spaces between the rings, so that the adhesive can become properly dispersed and distributed beneath the base plate 240 when the RFID reader 200 is pressed down into the adhesive upon installation.

[00124] As shown in Figure 15, one of the ground-engaging rings 241 (or this could equally be a number of ground-engaging legs extending around beneath the base plate 240 at the appropriate radius) is positioned directly beneath the thickest portion of the glass frusto-cone. The reason for locating one (or a series) of ground-engaging portions directly beneath the thickest portion of the glass frusto-cone is because, when a vehicle drives directly over the antenna structure, and especially when the tire is directly on top of the antenna structure, the vehicle's tires will contact (and hence pressure from the weight of the vehicle will press directly down from) the points which are the highest points on the antenna structure. Therefore, placing one (or a series) of ground-engaging portions directly beneath the highest point on the antenna structure may help to bear this weight most effectively, and prevent damage to the antenna structure, or even flexure of the antenna structure which whilst not necessarily damaging may nevertheless affect the antenna's radiation pattern while the shape of the antenna is distorted.

[00125] As also shown in Figure 15, another of the ground-engaging rings 241 (or this could again equally be a number of ground-engaging legs extending around beneath the base plate 240) is positioned directly beneath the outermost circumference of the base plate 240. This may help to prevent the outermost portions of the antenna structure from being pressed down or deflected relative to the more central parts thereof. In addition to providing support for the antenna structure (e.g. when it is loaded by the weight of a vehicle, etc, as just described), the ground-engaging portions 241 also help to correctly position the base plate of the antenna in the vertical direction - specifically by ensuring that the plane of the base plate (or the plane of its upper surface) sits vertically in line with (or in the plane of) the road surface and/or the semi- conductive area on the road surface.

[00126] Another aspect of the design of the RFID reader 200 in Figure 15 that differs from the embodiment in Figure 9 is that, instead of having a screw mount (like the screw mount 150) that screws into a container (like the container 160), the RFID reader 200 in Figure 15 instead has a vibration/shock absorber/buffer 260 near the centre on the underside of the base plate 240, and it is this vibration buffer 260 that screws directly into an internally threaded portion on the top of the heat sink 205. As mentioned above, the heat sink 205 in Figure 15 is much larger than the heat sink 105 in Figure 9 and it is designed for dissipating much larger amounts of heat. As also mentioned above, the large heat sink 205 in Figure 15 is outwardly cylindrical and it has a rectangular-box/prism-shaped hollow interior, as indicated in Section H-H in Figure 15. This hollow interior may be used for housing electronic parts and equipment associated with the RFID reader. [00127] One or more vertical bores or shafts may be provided, extending between the space above the cone and the space below the antenna, in order to provide a conduit for cabling, wiring, etc, running between electronics mounted in the respective spaces. In Figure 15 (and in Figure 9 too), one bore is actually shown extending vertically down from the point where the cone meets the base plate into the space below the antenna (in Figure 15 the latter space is the space inside the heatsink and in Figure 9 it is the space inside the container). However, as those skilled in the field of antenna design will readily appreciate, the particular central bore depicted in both of these Figures is aligned with the central feed point of the antenna, and in order to achieve the required antenna impedance (which may be e.g. 50 Ω and in which case the diameter of the said depicted central bore should be at least approx. 1 mm), this particular central bore which is depicted in Figure 15 and Figure 9 must remain open/empty (i.e. it cannot be used as a conduit for cabling, etc). However, other bores could be provided at other off-axis locations which could be used as conduits.

[00128] In terms of powering the antenna (and the other electronic components incorporated in or associated with the RFID reader) - and this applies to the embodiments in both Figure 9 and Figure 15 as well as other possible embodiments/variants used in "in-road" applications - this may be done in any manner of ways. For example, by using an induction loop, or by connecting one or more current (power) carrying cables directly to the RFID reader structure. Such current (power) carrying cables could be installed in shallow slots or trenches formed in the road (e.g. cut/dug in the road and then covered over after the cable has been laid). Figure 15 shows, as an example, the way such a power cable 270 could extend underneath the antenna and into the space inside the heatsink (where the powered electronics are located).

[00129] Also, communication and data transfer between the RFID reader and other computers or devices which are separate or external from the RFID reader may be achieved, and again, this may be done in any suitable way. Due to the rugged environment and the permanent (or at least semi-permanent) nature of the installation in "in-road" applications, simply connecting a cable (like an ethernet cable or the like) may often not be suitable for achieving data transfer. However, other conventional wireless communication methods (e.g. Wi- Fi, Bluetooth, etc) may be used, or if the RFID reader is powered by a power cable then conventional "data over power" methods may also be used for communicating. Where a wireless communication method is used, e.g. Wi-Fi or Bluetooth, an additional antenna may be required to support this. Such an antenna could be incorporated inside the cone of the RFID reader, or possibly even in the lid of the RFID reader itself (e.g. if the additional antenna used for Wi-Fi or the like were to be a simple slot antenna formed as a slot in the lid).

[00130] Turning now to Figure 17 and Figure 18, these Figures depict (in partly-exploded and assembled form, respectively) an RFID reader 300 in accordance with another embodiment that is generally similar but slightly different/varying compared to the embodiments depicted in Figure 9 and Figure 15. Actually, the embodiment in Figure 17 and Figure 18 is somewhat similar to the embodiment in Figure 15, for example in that it incorporates a large hollow sub- road heatsink 305 inside which much of the RFID reader's electronics are housed, etc. Figure 17 and Figure 18 are perhaps slightly more pictorial (and less schematic) than Figure 9 and Figure 15, and e.g. more of the electronics etc housed inside the heatsink 305 are actually shown. Nevertheless, once again, parts, features, and aspects of the design of the RFID reader 300 (and the antenna structure it incorporates) in Figure 17 and Figure 18 which are the same or equivalent to corresponding parts, features, etc, of the RFID reader (and its antenna structure) in Figure 9 and Figure 15 generally will not be described. Where such features are described or referred to, they will be given like labelling (e.g. the antenna's base plate, which is labelled 140 in Figure 9, is labelled 240 in Figure 15 and 340 in Figure 17 and Figure 18). Also, whilst Figure 17 and Figure 18 depict (to some extent) the electronics housed inside the heatsink 305, a detailed explanation of these electronics, including in relation to the constituent parts/components, their layout, their interconnection and their operation, etc, is not necessary for present purposes.

[00131] Another aspect of the design of the RFID reader 300 in Figure 17 and Figure 18 that is quite similar to the embodiment in Figure 15 is that the RFID reader 300 has a vibration absorber (shock buffer) 360. However, unlike Figure 15 where the vibration absorber 260 screws into an internally threaded portion on the top of the heat sink 205, in Figure 17 and Figure 18, interposed between the vibration absorber 360 and the antenna structure there is an upper spacer component 362, and interposed between the vibration absorber 360 and the heatsink 305 there is a lower spacer component 364.

[00132] As best shown in Figure 17, the vibration absorber 360 itself has three portions, namely an upper portion 361 , a lower portion 363 and a divider portion 365. The upper portion 361 and lower portion 363 are both cylindrical and both have the same outer diameter (which is smaller than the outer diameter of the heatsink 305). The vertical thickness of the upper portion 361 is greater than the vertical thickness of the lower portion 363. The divider portion 365 is vertically between the upper portion 361 and the lower portion 363, and the outer diameter of the divider portion 365 is greater - the divider portion has an outer diameter approximately equal to the outer diameter of the heatsink 305. The divider portion 365 therefore forms, in effect, a thick ring extending circumferentially (horizontally) around the vibration absorber 360 between the upper and lower portions.

[00133] The upper spacer component 362 is shaped as a cylindrical/annular ring. It's outer cylindrical surface matches the size and shape (i.e. the outer diameter) of the outer surface of the heatsink 305. The internal surface of the upper spacer component 362, which is also cylindrical and parallel to its outer surface, has a smaller diameter. The internal diameter of the upper spacer component 362 is actually equal to (or very slightly larger than) the outer diameter on the upper cylindrical portion 361 of the vibration absorber. The vertical thickness of the upper spacer component 362 is also equal to the vertical thickness of the vibration absorber's upper portion 361 . Therefore, when the RFID reader 300 is assembled, as shown e.g. in Figure 18, the upper spacer 362 effectively sits on top of the vibration absorber's divider portion 365, and the upper spacer 362 extends around the circumference of the vibration absorber's upper portion 361 . Thus, when the RFID reader 300 is assembled, the annular upper horizontal surface of the upper spacer component 362 contacts the underside of the antenna's base plate 340, but the upper surface on the vibration absorber's upper portion 361 also engages directly against the underside of the antenna's base plate 340.

[00134] Unlike the upper spacer component 362, the lower spacer component 364 is not really cylindrical/annular. Rather, the lower spacer component 364 is shaped more like a flat circular disc, although it does have a wide shallow recess formed/indented into its upper horizontal surface, and there is also a narrow, axially-located through-bore extending through its full vertical thickness. The shallow recess in the upper horizontal surface of the lower spacer component 364 is actually the same size and shape as (or very slightly larger than) the shape of the vibration absorber's lower portion 363. Therefore, when the RFID reader 300 is assembled as shown e.g. in Figure 18, the vibration absorber 360 effectively sits directly on top of the lower spacer component 364, and the lower portion 363 of the vibration absorber inserts into and fits snugly within the shallow recess in the upper horizontal surface of the lower spacer component 364. At the same time, the underside of the vibration absorber's divider 365 rests directly on top of the annular rim that surrounds the recess on the upper surface of the lower spacer component 364.

[00135] When the RFID reader 300 is assembled as shown in Figure 18 and actually installed, the heatsink 305 (along with the electronics housed therein) will already be inserted into the accommodating cavity below the surface of the road, and in fact the heatsink 305 will be relatively fixedly secured within the said sub-road cavity. Thereafter, the lower spacer component 364, the vibration absorber 360, the upper spacer component 362 and the antenna structure will all be installed on top, in the configuration just described (and as part of the final installation the antenna structure, etc, will also be adhered/secured in place with its base plate 340 level with the road surface, as described above). However, it is important to note that, even after final installation, as a result of the configuration just described, if a vehicle drives over the top of the installed antenna structure, the slight downward displacement of the antenna structure this may cause can be accommodated/absorbed because, even if heatsink 305 (and hence the lower spacer component 364 too which is directly on top of the heatsink) may be unable to deflect due to being fixedly secured in the sub-road cavity, nevertheless when the antenna structure is slightly displaced downwards, (at least) the upper spacer component 362 will still push down on divider portion 365 of the vibration absorber, and because the divider portion 365 (and indeed the entire vibration absorber 360) is formed from a resilient/squashy/vibration absorbent material, consequently the divider portion 365 (at least) will "squash" to accommodate the downward displacement of the antenna structure without displacing the heatsink or causing any damage to it or to any electronics housed therein.

[00136] It was mentioned above that there is a narrow, axially-located through-bore extending through the full vertical thickness of the lower spacer component 364. It should now be noted that there is also an axially-located through-bore extending through the full vertical thickness of the vibration absorber. These axial bores exists to provide conduits for electrical cables which connect and extend between the electronics housed within the heatsink 305 and the feed point of the antenna structure. (Recall that the antenna's feed point is at the point where the antenna's cone and baseplate intersect.) In the partially-exploded view in Figure 17, the coaxial connector used for connecting these cables to the antenna's feed point is visible. It will also be noted that the vertical through bore extending through the thickness of the vibration absorber 360 is larger than the small through bore extending through the thickness of the lower spacer 364. This is because the bore in the vibration absorber 360 must be large enough to accommodate the coaxial connector just mentioned.

[00137] Next, it should be noted that, apart from being partially-exploded vs assembled views, respectively, Figure 17 and Figure 18 actually also differ slightly from one another in one other way. In both of these Figures, there is an end plate 390 located on the bottom end of the main heatsink 305. In Figure 17, this end plate 390 therefore forms the base of the main heatsink 305, and there are no other parts of the RFID reader assembly beneath it. However, in Figure 18, the end plate 390 is not the lowermost part of the assembly. Instead, in Figure 18, there is an additional section (or an extension) of heatsink extending below (i.e. extending more deeply into the ground) below the end plate 390. In the latter case of Figure 18, the end plate 390 therefore forms a connecting plate between the upper (main) section of heatsink 305, and the lower heatsink extension. Actually, in the case of Figure 18 where there is an extension to the heatsink, the heatsink extension does not directly contact the underside of the end plate 390. Rather, interposed between the underside of the end plate 390 and the upper surface of the heatsink extension there is a component that is substantially identical to the lower spacer component 364, although in comparison with the lower spacer component 364, this component is actually installed upside-down between the underside of the end plate 390 and the upper surface of the heatsink extension. In any event, it should be noted that the heatsink extension in Figure 18 is similar to the upper main portion of the heatsink 305 in that it is hollow and could therefore potentially accommodate electronics (although there are no electronic shown therein in Figure 18), and there are also appropriate through bores in the end plate 390, etc, which could (if necessary) provide conduits for any cabling etc required between the cavity inside the main heatsink 305 in the cavity inside the heatsink extension.

[00138] It is explained elsewhere herein and also in the accompanying Appendix that the radiation pattern generated by the RFID reader's antenna in use should preferably have a shape that may be described as a "dropped doughnut" or "squashed toroid" - that is, a shape as shown pictorially in Figure 2 (and also in Figure A23). However, it is also explained above that Figure 2 (and also Figure A23) merely provides a visually appreciable illustration of what shape is meant by "dropped doughnut" or "squashed toroid". On the other hand, having now described the construction of the RFID reader in a number of possible embodiments, and having described the configuration of the antenna structure, it is useful now to define more precisely the technical parameters of the desired radiation pattern. This will therefore now be explained with reference to Figure 19 and Figure 20, which illustrate the antenna's radiation pattern (and parameters thereof) for a signal frequency of 860-940 MHz.

[00139] Figure 19 is a "heat map" style plot of the directivity of the desired antenna radiation pattern. The point to note firstly (and this point is equally well illustrated in, say, Figure 2 and also Figure A23) is that the antenna is omnidirectional in the azimuth (i.e. x-y) plane. That is to say, if the plane of the ground (or the plane of the road surface) on which the antenna is sitting is the x-y (azimuth) plane (i.e. if the x- and y- axes are perpendicular to one another but both run along the surface of the ground/road) and if the z-axis points vertically upwards perpendicular to the x-y (azimuth) plane from the centre of the antenna, then it is true that the amount of energy the antenna emits, and the way the energy intensity varies with elevation (i.e. the way the energy intensity varies with the angle relative to the azimuth plane) is the same in any radially outward direction from the z-axis (i.e. in any x,y direction). The colours in the heat map in Figure 19 illustrate, in effect, the intensity of the antenna's radiation, and the way this varies according to direction. As just mentioned, the antenna is omnidirectional in the azimuth plane, however a better understanding of the way the energy intensity varies with the angle of elevation relative to the azimuth plane can be gained form the cross-sectional view of the radiation pattern in Figure 20.

[00140] Figure 20 illustrates that, in the desired radiation pattern,

^■ the elevation range of the critical read zone is from 3° to 30° elevation;

^■ the path of max gain is at 30° elevation;

^■ the 3dB beam width is 40°, extending from 10° to 50° elevation;

^■ there is a radiation null at 90° elevation.

[00141] Also, although this is not directly depicted in Figure 19 and Figure 20, the effective read range is from 1 m to 6.4m from the antenna. [00142] The particular embodiments discussed above with reference to Figure 9 (and Figure 10), Figure 15 (and Figure 16) and Figure 17 and Figure 18 all involve an RFID reader which incorporates the presently proposed antenna structure and which is configured to be installed in an "in-road" deployment (such in-road deployments typically also require a partially-conductive area associated with the antenna, as discussed above). However, as also explained or alluded to elsewhere herein, there may often also be a need (or there may be situations where it would be beneficial) to be able to use or deploy an RFID reader (which, again, incorporates the presently-proposed antenna structure) for use in vehicle detection and/or identification but where a permanent (or semi-permanent) "in-road" installation of the RFID reader is not possible or required. Therefore, in order to be able to use or deploy an RFID reader (incorporating the proposed antenna structure) in situations where an "in-road" installation is not possible or required, an additional proposal is made whereby the RFID reader can be used or deployed in "on-road" placements. Basically, instead of being installed permanently (or semi-permanently) in the road, this enables the RFID reader to be deployed temporarily or for only a period of time, effectively, by being placed on the road and without the need to apply anything permanently to the road, or to dig holes in the road, or make any other changes whatsoever to the road. Such on-road deployments could be used or required, for example, by law enforcement personnel when setting up temporary roadblocks for performing random vehicle inspections or driver drug/alcohol testing, or during temporary roadworks or maintenance, or when there is a need to create temporary traffic diversions (but still achieve law enforcement functions at the same time), or for a discrete period of time in order to take measurements and gain information (data) about traffic and vehicle flows and the like in a certain location, etc.

[00143] An RFID reader, of which the proposed antenna structure forms part, and which is configured for use in on-road deployments, will now be discussed in further detail with reference to Figure 1 1 . An annotated version of Figure 1 1 is also given as Figure 12; however for convenience reference will be made to Figure 1 1 only.

[00144] It is to be noted firstly that Figure 1 1 is, once again, a view of an RFID reader which incorporates the proposed antenna structure well as other RFID reader equipment. It should also be noted from the outset that, unlike Figure 9 (and Figure 10) and Figure 15 (and Figure 16) which all depict a situation where the RFID reader is installed in an "in-road" installation, Figure 1 1 (and Figure 12) depicts a situation where the RFID reader is used in an "on-road" deployment. In other words, in Figure 1 1 , all parts of the RFID reader, as well as other associated equipment, are located above the level of the road surface RS. And as will be readily appreciated, Figure 1 1 is a side-on cross-sectional view.

[00145] It is also to be noted that the configuration of the RFID reader itself in Figure 1 1 is in almost all respects identical to the RFID reader in Figure 9 (accordingly like parts and features of the RFID reader are referred to using like reference numerals in both Figure 9 and Figure 1 1 ). However, instead of the RFID reader being installed in a cavity 1 10 which is dug or bored into the road (as in Figure 9), the RFID reader in Figure 1 1 is instead installed in the top of an at least partially conductive substructure.

[00146] In the embodiment discussed herein with reference to Figure 1 1 , the partially conductive substructure takes the form of a partially conductive on-road-locatable cradle 300 (hereafter the "on-road cradle" 300). Basically, the on-road cradle 300 can sit directly on the surface of the road, and the RFID reader 100 is received in (and it is mounted to) the top of the on-road cradle 300. The shape of the on-road cradle 300, at least in the depicted embodiment, is frusto-conical. In Figure 1 1 , the angle/slope of the sides of the frusto-conical cradle 300 match the angle of slope of the sides of the RFID reader's main frusto-cone. However, this need not necessarily always be the case, and Figure 13 provides an example where the shape of the on-road cradle 300 (and in particular the angle of slope of its sides) does not match the angle of slope of the sides of the RFID reader's main frusto-cone body.

[00147] The way in which the RFID reader 100 is mounted to the top of the on-road cradle is that a cylindrical recess is provided in the top of the on-road cradle 300, and at least the top portion of the internal wall of the said cylindrical recess is threaded. The threads on the outer vertical wall of the screw mount 150 (see above) screw directly into these threads in the on-road cradle. So, in effect, the cylindrical recess in the top of the on-road cradle 300 is equivalent to those in the container 160 used in the embodiment in-road in Figure 9, and the way in which the RFID reader 100 attaches thereto is the same in both cases, except that there is no adhesive involved when attaching the RFID reader 100 to the on-road cradle 300. When the RFID reader 100 is thus mounted to the on-road cradle 300, the same (or an equivalent) space 155 is left beneath the screw mount 150 where electronic parts and components of or associated with the RFID reader may be located.

[00148] One difference in Figure 1 1 , compared with Figure 9, relates to the size of the heat sink 105. In Figure 9, the heat sink 105 is long and extends into the ground. This is in order to help dissipate heat from the in-road RFID reader into the ground. However, in Figure 1 1 , the heat sink 105 is comparatively much smaller. This is because, in Figure 1 1 , the on-road cradle 300 is itself made from (or made mostly from) metal. Therefore, in Figure 1 1 , much of (or possibly the whole of) the on-road cradle 300 actually also operates as part of (or as an extension of) the heat sink 105. In other words, some or all of the on-road cradle can help to absorb/receive heat generated by the RFID reader or its associated electronics and to dissipate this into the atmosphere (and importantly, in this on-road deployment, the whole structure is located above the road surface, and much of it is exposed to the ambient atmosphere, which helps significantly with heat dissipation). [00149] Another thing to note about the on-road cradle 300 is its important function, in an on- road deployment scenario, in helping to shield the RFID reader 100 (and in particular the antenna structure) from the potentially widely and dynamically variable radio frequency influences of the road, other "near ground" effects, etc. So, in effect, the on-road cradle 300 provides the same shielding function and properties as is provided by the partially conductive area used in in-road deployments. In order that the antenna be adequately shielded from the road, other "near ground" effects, etc, the size and configuration (and particularly the height) of the cradle should be such that the height of the antenna's base plate, when it is mounted on the cradle (and when the cradle is sitting on the road surface) is not more than ¾λ, and preferably not more than ¼ λ (the closer to ¼ λ the better the shielding).

[00150] Given the height of the RFID reader and cradle, it will be understood that (unlike in the in-road embodiments described above where the antenna/reader is located in the road lane (normally in centre thereof)), in these on-road embodiments the cradle on which the reader is mounted will generally be positioned for use between lanes, or to the side of the lane or road. This is because the cradle is simply too high for most vehicles to be able to drive directly over the top of it. The fact that the cradle (and reader) is positioned for use between lanes, or to the side of the lane or road, also means that the required read zone for communicating with the RFID tags on passing vehicle license plates is different. However, given that the situations where such on-road reader deployments will be used will mostly involve temporary road blocks, traffic diversions and the like, it follows that vehicle speeds around such on-road reader deployments will also normally be lower (often much lower) than the speeds involved in normal free-flow traffic. Accordingly, the size of the required read zone (given the much lower vehicle speeds) may be much smaller than for the in-road deployments more often used for normal, free-flow traffic. Thus, the changed read zone due to the location of the cradle/antenna (i.e. between lanes or to the side, rather than in-lane) is unlikely to cause any problems in terms of read performance.

[00151] A further issue to consider is that, in on-road deployments, which will (by their nature) often be temporary or transient, there often will not be any available pre-installed or existing power supply lines to provide power for the RFID reader. Therefore, instead, batteries (typically rechargeable batteries although replaceable batteries might also be used) and associated power supply electronics are provided inside the on-road cradle 300 itself and these are used to power the RFID reader.

[00152] Yet a further point to note (following on from above) is that, because the RFID reader 100 is positioned on/in the top of the on-road cradle 300 and the reader 100 is therefore located vertically much higher than in in-road deployments, and because this means that it may no longer be feasible for vehicles to be able to drive directly over the top of the RFID reader, it therefore follows that, in these on-road deployments, the on-road cradle 300 bearing the RFID reader 100 may be similar to "witches hats" and the like used in traditional road traffic management in that vehicles must drive around between them.

[00153] There are also other ramifications arising from the fact that, in on-road deployments, the RFID reader is located above the road surface. As shown in Figure 13 for example, the height to which the antenna's ground plane (i.e. the base plate 140) is elevated when the RFID reader is mounted to the on-road cradle 300 may be around 75 mm to 85 mm. Given that, at least in the embodiment discussed herein, the signal frequency with which the antenna is configured to operate is approximately 920 MHz (wavelength λ=326 mm), it follows that the height to which the antenna's ground plane is elevated when the reader is mounted to the on- road cradle 300 is approximately λ/4. As those skilled in the art may appreciate, raising the antenna ground plane by this amount (as a proportion of the operating signal wavelength) may have the effect of somewhat reducing the impact or effect of the dynamically variable radio frequency influences of the road, other "near ground" effects, etc, and it may further have the effect of pulling the radiation pattern down (in relation to or in comparison with a free space placement). Nevertheless, the construction and configurations of the on-road cradle 300, including in relation to its height, the angle of slope of its sides, internal construction, positioning of internal components, etc, are chosen or varied in order to, in effect, help tune the on-road cradle 300 so that, when it is used in conjunction with the RFID reader (including the proposed RFID antenna structure) in on-road deployments, the two together (i.e. the RFID reader when used mounted upon the on-road cradle 300) provide the desired radiation pattern (which, as explained above, is a "dropped doughnut"-shape). The different shapes of the on-road cradle depicted in Figure 13 are examples of the way in which the on-road cradle shape may be varied for this purpose.

[00154] In the present specification and claims (if any), the word 'comprising' and its derivatives including 'comprises' and 'comprise' include each of the stated integers but does not exclude the inclusion of one or more further integers.

[00155] Reference throughout this specification to 'one embodiment' or 'an embodiment' means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases 'in one embodiment' or 'in an embodiment' in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

[00156] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims (if any) appropriately interpreted by those skilled in the art.

APPENDIX

This is the Appendix to the patent specification entitled "AN ANTENNA" above. This Appendix provides background information relevant to the invention, and the embodiments and features thereof, described in the specification above. The information in this Appendix reproduces certain information presented in International Patent Application No. PCT/AU2015/050384 (which was also referred to in the specification above).

IMPORTANT: The information in this Appendix is provided only as background information and to help provide context for the invention discussed in the specification above. Therefore, whilst this Appendix accompanies the above specification and the two (i.e. the above specification and this Appendix both including their respective Figures) together form a single disclosure, and whilst the invention discussed in the specification above may incorporate features mentioned in this Appendix (including even if such features are mentioned only in this Appendix), nevertheless nothing in this Appendix is to be interpreted as necessarily limiting or otherwise affecting the scope of the invention discussed in the specification, except if a contrary intention is apparent from the wording used in the specification.

This Appendix will make reference to a number of Appendix Figures as listed below. Note that all Figures referred to in this Appendix are identifiable as relevant to the Appendix (and disguisable from the Specification Figures referred to elsewhere in the specification) by the prefix "A" to the Appendix Figure number. So, for example, the first two Appendix Figures are labelled as Figure Al and Figure A2. Note also that several of the Appendix Figures contain reference numerals identifying particular features or things depicted therein. The way in which specific reference numerals in the Appendix Figures are referred to in this Appendix is that, for example, reference numeral 1 appearing in Figure A8 will be referred to as "8-1", and likewise reference numeral 6 appearing in Figure A21 will be referred to as "21-6", etc.

In the Appendix Figures:

^■ Figure Al, Figure A2 and Figure A3 together help to illustrate the importance of beam width and direction in successfully reading (communicating with) a RFID tag.

^■ Figure A4 is a plot of the radiation pattern (including the 3 dB beam width) for a directional narrow beam antenna.

^■ Figure A5 is a schematic representation of a typical construction of a patch antenna.

^■ Figure A6 illustrates, from the side, the use of a RFID reader/antenna located on an overhead gantry to read a RFID tag on a vehicle windscreen and/or license plate.

^■ Figure A7 illustrates a RFID overhead reader and a RFID side reader scenario, when viewed in a direction opposite to the direction of travel of the vehicles in the depicted lanes of the road.

^■ Figure A8 illustrates the travel path of a windscreen-mounted RFID tag on a vehicle within an overhead RFID reader beam.

^■ Figure A9 illustrates factors that contribute to create non-linear variation of the signal between an overhead RFID antenna and a windscreen-mounted RFID tag.

Figure A10 shows a vehicle license plate mounted within a cavity to protect it from damage.

Figure Al l illustrates the travel path of a vehicle's front and rear license plate within an overhead RFID reader beam.

Figure A12 helps to explain the required (or at least a desirable) beam shape for a RFID reader antenna placed in/on the road.

Figure A13 illustrates, inter alia, the impact of a short following distance between, on the one hand, a classic patch antenna beam shape, and on the other hand, a flat antenna beam shape, as emitted from an in/on-road reader.

Figure A14 illustrates one possible example of a vertically polarised horizontal slotted upright antenna.

Figure A15 illustrates the radiation pattern of the vertically polarised horizontal slotted upright antenna in Figure A 14.

Figure A16 illustrates how the ground effect can effect the direction of maximum gain in a radiation pattern.

Figure A17 illustrates antenna beams which are pushed upwards due to a conductive ground effect.

Figure A18 illustrates the beam shape, in free space, of a hypothetical/idealised upright half-wave dipole antenna.

Figure A19 illustrates the read -zone for a RFID enabled vehicle license plate.

Figure A20 schematically illustrates the orientation of a RFID tag (which is mounted on a vehicle license plate) within the read-zone of an in/on-road RFID reader.

Figure A21 illustrates the effective read-zone for a RFID tag which is on a vehicle license plate when read using a RFID antenna of the kind provided by the invention.

Figure A22 illustrates example uses of the kind of antenna provided by the invention, and the resulting effective read-zone, in different scenarios.

Figure A23 illustrates the desired radiation pattern for an antenna of the kind provided by the invention.

Figure A24 illustrates an antenna in accordance with one possible embodiment of the invention which is suitable for use as an on/in road RFID reader antenna. The antenna has a top loaded monopole configuration with a round and periodically slotted ground plane. This configuration has been found to achieve a radiation pattern as illustrated in Figure A23.

Figure A25 illustrates an inverted F antenna with a square periodically slotted ground plane, which has been found to achieve the radiation pattern shown in Figure A26, and where the direction of maximum gain is up and down the road, somewhat similar to the antenna in Figure A14.

^■ Figure A27 illustrates an antenna in accordance with another possible embodiment of the invention having an inverted F antenna (IF A) configuration with a round, periodically slotted ground plane, and which has been found to achieve a radiation pattern that is somewhat similar to the one depicted in Figure A23. This antenna also has a slot in the top load cap which may provide an additional antenna which may help to allow or facilitate data communications using WiFi. The radiation pattern for this additional/cap antenna is shown in Figure A28.

^■ Figure A29 illustrates possible desirable placements of a RFID reader antenna (or the placement of the device which houses the RFID reader and its antenna) on or in a road surface.

^■ Figure A30 illustrates the approximate general dimensions of the antenna in the particular possible embodiment in Figure A24.

RFID technology, particularly passive backscatter UHF RFID technology, as described by ISO/IEC 18000 part 6, is thought to be suitable for use in vehicle identification. Hence, the term RFID (and like terms) herein includes passive backscatter UHF RFID technology, as described by ISO/IEC 18000 part 6.

By way of further explanation, the present invention may be useful for all types of RFID, and may also be useful for RF communications between vehicles and a roadside unit. In the case of passive backscatter UHF RFID, the benefits may be much particularly significant (even disruptive in nature) in that they may (at least help to) overcome several limitations associated with current commercially available electronic vehicle identification systems that use passive backscatter UHF RFID. For this reason, the term "RFID" herein may be thought of as referring primarily (although not necessarily exclusively) to passive backscatter UHF RFID. ISO/IEC 18000 part 6 type C might be considered to be the current de facto standard for passive backscatter UHF RFID technology.

Passive backscatter RFID is, in fact, similar in some ways to RADAR (the term "RADAR" actually originated as an acronym of RAdio Detection And Ranging). RADAR essentially involves a radio signal transmitted by a sensor that is then reflected by the object to be observed and the reflected signal is interpreted by the sensor. In the case of RFID, the signal emitted by the RFID reader, and the "reflected" signal (i.e. the signal sent from the RFID tag back to the RFID reader), are modulated to carry information from the reader to the tag, and from the tag to the reader, respectively. In some cases, for example ISO/IEC 18000 part 6 type D, it may be that only the tag modulates information on the reflected signal from the tag to the reader. This can be the case when the tag is energised by the signal from the reader.

For RFID generally, the beam shape of a RFID reader (in three-dimensional space) may be defined as the locus of points at which a RFID tag receives enough energy from the reader to switch on and communicate intelligently with the reader. This is generally a sharp, but moving edge due to the nature of digital electronics and electric field propagation. Normally, the beam shape closely follows (or is closely related to) the radiation pattern of the reader antenna. It is for this reason that RFID systems are often designed with only the reader antenna radiation pattern in mid. However, in fact (as will be discussed further below), the ability of a RFID reader to transmit enough energy for a RFID tag to be switched on, for the RFID tag to decode modulated information from the reader (if necessary), and for the RFID tag to then modulate a response on the signal it reflects back to the reader (the "reflected" signal), depends significantly on the radiation pattern, orientation and environment of both the RFID reader antenna and the RFID tag antenna. Tag sensitivity refers to the minimum signal power at a specific locus (in the air) at which a given tag switches on. This sensitivity can be influenced by a number of factors including chip power levels, tag antenna radiation pattern, tag construction and tag orientation.

It should also be appreciated, at this point, that regulatory requirements often place limitations on the maximum energy or power of radio frequency transmissions in a given area (i.e. limitations on the allowable amount of "power in the air"). This often has a major influence on the efficiency of RFID (and like) systems. These power limitations can mean that it is not always possible (or allowable) to simply increase the transmission power (e.g. the power with which a signal is emitted from a RFID reader) in order to increase the range or distance at which a RFID tag can "power up" and communicate intelligently with the RFID reader. It should also be noted that it is generally considered desirable that a RFID reader should be able to read a tag if the tag is "switched on" by the signal from the reader, as in backscatter RFID systems.

For best performance in systems involving communication between a RFID reader and a RFID tag, the reader and tag, respectively, should preferably each be positioned and oriented in the direction of maximum power/sensitivity of the other. Figure Al illustrates the radiation pattern (in free space) of a high gain, directional antenna of a kind typically found in (or used in) current/conventional RFID systems.

Note that in Figure Al, and likewise in Figure A2 and Figure A3, the reader antenna's radiation pattern is shown in two dimensions only. Of course, the reader antenna's radiation pattern actually projects from the reader antenna (which is located on the very left-hand edge in Figure Al ) in three-dimensions. In other words, the radiation pattern in Figure Al, if represented in three dimensions, would actually extend into and out of the page as well. Therefore, it will be understood that Figure Al, Figure A2 and Figure A3 all actually show the antenna radiation pattern in cross - section, in a vertical plane which extends through the centre of the antenna radiation pattern in the direction of maximum gain, with the said plane viewed from one side.

In Figure Al, Figure A2 and Figure A3, the dark red (indicated as 1 -1 in Figure Al) signifies the maximum power, as allowed by the relevant applicable regulations, and at the other end of the spectrum the blue (indicated as 1 -3 in Figure Al) signifies almost zero power at that locus. (Recall that "tag sensitivity" refers to the minimum signal power at a specific locus (in the air) at which a tag switches on.)

Figure A2 and Figure A3 both illustrate a RFID tag positioned in front of the RFID reader of Figure Al . Figure A2 and Figure A3 both show the RFID tag's radiation pattern, represented in both cases by solid black lines, superimposed on the radiation pattern of the reader antenna of Figure Al . In Figure A2, the RFID tag is located further away from the reader than in Figure A3. However, in Figure A3, the RFID tag is oriented at an angle

(approximately 45°) relative to the reader, whereas in Figure A2 the tag is oriented directly "face on" to the reader.

Hence, in Figure A2, the RFID tag's radiation pattern (and hence its beam) points directly towards the reader, whereas in Figure A3 the RFID tag's radiation pattern (and hence its beam) points somewhat upwards toward a point above the reader. As those skilled in the art will appreciate, the tag in Figure A2 is therefore more likely to be read than the tag in Figure A3, even though the tag in Figure A3 is closer to the reader. These Figures also help to illustrate the importance, in terms of read performance, of not just the reader radiation pattern and tag radiation pattern, but also of "angles of read", and furthermore the significance of the antenna's aperture. The meaning and importance of the antenna aperture will be discussed further below.

Focussed (i.e. high gain, directive, narrow beam) antennas have become a de facto standard for RFID use, largely because they are thought to reduce radio noise by focusing the radiation pattern to the area of intended read. Figure A4 plots a typical radiation pattern for such an antenna. In other words, Figure A4 is a plot of the radiation pattern for a focussed antenna. The radiation pattern plot in Figure A4 is, in effect, a representation of the antenna's "directivity" ("directivity" is the way the antenna's gain varies with direction).

In Figure A4, the radiation pattern is plotted in terms of dBi. Those skilled in the art will recognise that, in the field of antenna design, dBi is shorthand for dB(isotropic) and signifies the directional gain of an antenna compared with a hypothetical isotropic (point) antenna which distributes energy uniformly in all directions. The indicated 3 dB beam width in Figure A4 may be considered to be the "aperture" of the antenna. From an RFID perspective, a 3 dB reduction at a specific locus means a 50% reduction in energy (i.e. a 50% reduction in the "energy in the air") at that locus. This means that, even if the distance from the reader remains constant, the amount of energy that is available to switch on a tag located from the reader in a direction on the edge of the reader's 3 dB beam width is only half of the amount which is available to switch on a tag located in the reader's direction of maximum gain (which is at 90° in Figure A4). Furthermore, the amount of energy that is available to switch on a tag located outside the reader antenna aperture (i.e. outside the 3 dB beam width) will be less than half of that which is available in the reader's direction of maximum gain, and with further movement outside (away from) the reader antenna aperture the amount of energy reduces rapidly. Hence, a tag which is located in the direction of (or on the edge of) the reader's 3 dB beam width (i.e. a tag which is on the periphery of the reader antenna aperture), or a tag which is outside the reader antenna aperture, needs to be closer (possibly much closer) to the reader in order to be switched on as compared to a similar tag located in the direction of the reader antenna's maximum gain. Thus, the antenna in Figure A4 is most effective (i.e. its RFID read range is greatest) for a tag located in its aperture (i.e. within the 3 dB beam width).

The significance of the antenna aperture for focused antennas, as described above, is that, generally, unwanted signals (noise) from outside the antenna aperture are (often naturally) filtered from a signal that is reflected from a tag that is in the antenna aperture. It should be noted that Figure A4 actually relates to an antenna design that is a conventional patch or parabolic design, which is one form of focused antenna currently used in e.g. point to point communications and RFID.

In any case, Figure A4 and the associated discussion above help to illustrate generally how focused antennas reduce radio noise by focusing the radiation pattern to the area of intended read, and thus why focused antennas have become a de facto standard for use in conventional RFID systems. However, as discussed below, such focussed (i.e. high gain, directive, narrow beam) antennas may not be well suited for use in the applications considered herein, specifically given the drastic RFID read range reduction away from the direction of maximum gain (and also when considering issues such as the predictability of the orientations of both the reader and the tag on the vehicle/license plate).

Figure A5 is a schematic representation of a typical construction of a patch antenna. Recall that a patch antenna is one conventional form of focused/directional antenna. It is also important to note how the construction of such a conventional patch antenna impacts on the antenna's radiation pattern. The antenna beam (and its overall radiation pattern) points perpendicularly away from the ground plane (i.e. it points vertically upward relative to the ground plane in Figure A5). For UHF patch antennas as currently used in vehicle identification, typically the ground plane has an area of more than 300 mm x 300 mm. At this point it should be noted that, in the context of the present discussion, the "ground plane" of an antenna should not be confused with the "ground" which is part of planet earth.

RFID has become (and is continuing to become) increasingly popular, and its use is becoming more common/widespread, including in the identification of vehicles on the road. Overhead readers on gantries, as depicted in Figure A6 and Figure A7 for example, seem to have become a de facto RFID reader deployment standard for free-flow multi-lane in-traffic vehicle identification. Gantry based RFID reader infrastructure is, however, complex and typically very time consuming and extremely expensive to deploy and maintain. The result is that RFID vehicle identification is predominantly used only in revenue earning and cost saving applications; for example in freight logistics, toll and congestion charging, and the like. These applications are, however, generally loyalty based, meaning that they rely on compliance by users. Such current RFID vehicle identification systems are generally not well suited to cope with, or adapt to, situations where a person takes steps to prevent detection of their vehicle, or to cause misdetection (incorrect identification) of their vehicle. Current RFID vehicle identification systems are also typically of a closed loop nature (which means that all elements in the systems are specified and regulated by a single entity).

The high cost of current RFID infrastructure (in particular, gantry based overhead RFID reader infrastructure) is one of the major factors currently inhibiting wider deployment of RFID, especially for compulsory and/or regulatory identification of vehicles (e.g. for law enforcement purposes) in an open loop manner (as posed to closed loop). As an example of what is meant by open loop, authorities in one European country should preferably be able to read and verify all vehicles (the RFID tags thereon) travelling on that country's roads, including vehicles visiting from other countries and where the RFID tags on other countries' vehicles may have been issued by separate authorities.

A requirement for compulsory RFID tags on all vehicles, complemented by more cost efficient, and easy to deploy and maintain RFID reader infrastructure, may help to provide vehicle movement, identity and demographic information which is currently desperately needed, not just for law enforcement purposes (e.g. for identifying traffic infringements, enforcing vehicle registration, etc), but also for purposes such as effective traffic and road planning, road network operations and management, etc. The information needed for these purposes is currently not readily available (if at all), or where it is available (to some extent) it is often incomplete and obtained from various (often incompatible) sources using expensive and convoluted technologies and methods. The proposals discussed herein may help (or may be used) to provide RFID reader infrastructure which is more cost efficient and easier to deploy and maintain. And this, together with a requirement for RFID tags on all vehicles, may in turn help to facilitate the generation of more complete, accurate and immediate vehicle and traffic movement, identity and demographic information as needed for the range of traffic and transport related purposes discussed above (potentially even with one kind of RFID reader/sensor being suitable for use in multiple different applications).

In normal conditions, bearing in mind radio properties, interference, the need for data loss retries, etc, it is thought that vehicle identification using passive UHF RFID requires approximately 80 ms to reliably exchange 512 bits of identification data. In this regard, 512 bits of data is thought to be enough to identify a vehicle and perform a rudimentary offline verification of that identity. A vehicle travelling at 36 km/h will travel 0.8 m in 80 ms, and a vehicle travelling at 180 km/h will travel 4 m in 80 ms. Therefore, for the purposes of the present discussion, 4 m of vehicle travel will be used as the minimum exposure required to reliably read a RFID tag on a moving vehicle. Those skilled in this area will recognise that this is based on an assumption of a maximum vehicle speed of 180 km/h, which seems reasonable given that vehicles will very rarely (if ever) travel faster than this on public roads.

Given that most current road/traffic related RFID systems utilise overhead (e.g. gantry based) RFID reader mounting, it follows as a consequence that the most typical location for the placement of RFID tags on vehicles (for use in such current overhead reader systems) is in the windscreen of the vehicle (usually on the inside of the windscreen, near the driver's rear-view mirror). Currently, the general thinking in the industry is that the placement of a RFID tag in (or on the inside of) the vehicle windscreen is easy, such that RFID tag installation is typically left for vehicle owners to perform themselves. It is also thought that such "self-installation" allows for sufficiently reliable RFID tag installation with the placement/location/orientation of the tags being sufficiently consistent across all vehicles. However, as discussed further below, this assumption, namely that the placement/location/orientation of RFID tags is consistent across all vehicles and vehicle types, may be incorrect or at least inappropriate. In any case, read ranges of more than 15 meters have been reported for certain current commercial RFID tag/reader systems. These measurements are, however, based on (or taken in) ideal conditions. Realistically, reliable read ranges are often less than 8 meters.

Figure A8 illustrates the travel path 8-3 of windscreen-mounted RFID tags (such as windscreen-mounted RFID tag 8-2) within an overhead RFID reader beam 8-4. The vertical width of the tag travel path 8-3, which extends from approximately 1 m above the ground to approximately 2 m above the ground, exists due to the fact that RFID tags will be positioned at different heights in different vehicle types. For example, a RFID tag installed in the windscreen of a large truck will typically be higher (closer to 2 m) above the ground than a RFID tag installed in the windscreen of a low-slung sports car (which may be closer to 1 m above the ground). It must also be noted that, for different vehicle types, the vehicle windscreen orientation varies from approximately vertical (as found on e.g. trucks and buses) to almost horizontal (as found on vehicles like sports cars). Hence, the orientation of the RFID tag antenna when installed on the inside of different vehicles' windscreens can vary from approximately vertical to almost horizontal (and this is in addition to the possible variation in the height of the RFID tag placement for different vehicle types discussed above). The reason this is important is because of the significant influence relative antenna position and orientation can have on read performance, as discussed above.

In Figure A8, the reader antenna 8-1 is placed 6 m above the road, which is a typical road clearance height. Considering the range of possible windscreen angles (and hence the range of possible RFID tag antenna orientations), the possible variation in tag height within travel path 8-3, and general RFID technology read performance limitations, in the scenario in Figure A8 a minimum read range of approximately 6.5-7 m is required to read a windscreen-mounted RFID tag reliably. This minimum 6.5-7 meter (approx.) read range is depicted in Figure A8 by the shape of the RFID reader antenna's effective beam 8-4, which (in this two-dimensional, cross-sectional representation) has a "70° sector" shape with a radius of 6.5-7 m (in Figure A8 the radius shown slightly less than 7 m). Also, considering that buses and trucks are likely to travel at lower speeds than passenger vehicles, and normally the tag in their windscreens is placed higher, this reduces the effective read range requirement (possibly to below 6 m) for them.

The scenario in Figure A8 is well within the limits of what can be achieved with RFID, given RFID technology read performance limitations and the geometry imposed by the locations of the RFID reader 8-1 and RFID tags 8-2 in Figure A8. The reason why the scenario in Figure A8 is well within the limits of what is possible can be appreciated from the fact that, in Figure A8, the minimum required tag travel path/distance 8-5 (which is 4 m long for reasons discussed above) easily fits within the effective beam shape 8-4 of the reader antenna 8-1. Hence, for vehicles travelling at even the assumed maximum speed (180 km/h), the vehicle's windscreen-mounted RFID tag 8-2 will be within the effective beam shape 8-4 of the RFID reader 8-1 for (more than) enough time to be reliably read.

However, even though current RFID systems (which utilise overhead RFID reader infrastructure) may be capable of reading windscreen mounted RFID tags, like in the scenario depicted in Figure A8, nevertheless the windscreen as a location for vehicle RFID tag placement is thought to be unsuitable, for reasons discussed below, as is the use of overhead readers/gantries.

Stationary measurements support the theoretically superior read performance achieved by windscreen mounting RFID tags where overhead RFID readers are used, as compared to the case where RFID tags are mounted on vehicle license plates and read by the same overhead RFID readers. This is not surprising given that, for overhead RFID reader placements, windscreen mounting the RFID tag places the tag higher and closer to the reader, as compared to mounting the RFID tag on the license plate. In fact, stationary measurements of windscreen mounted RFID tags read by overhead readers indicate a close to 100% read performance where static influences and expenses are negated. However, for existing real RFID operations involving windscreen mounted RFID tags read by overhead readers (i.e. actual real world RFID system implementations with overhead readers reading windscreen mounted RFID tags in traffic applications) read performance is actually less than 98%, and this equates to a surprising and unacceptably low tolerance of (at least) one potentially missed vehicle identification instance in every 50 vehicles. (It will be appreciated that this level of read-reliability is unacceptably low if the system is to be used for, for example, law enforcement such as identification of traffic infringements, enforcement of vehicle registration, etc.) The 98% read- reliability figure above also seems to drop further as vehicle speed and traffic density increase.

As mentioned above, it is thought that there are inherent problems associated with both (i) the use of windscreen- mounted (or likewise headlamp -mounted) RFID tags, and (ii) the use of overhead RFID readers, for the purpose of vehicle identification, especially in open-road free-flow applications for regulatory compliance purposes. For instance, as mentioned above, in relation to (ii), overhead RFID readers, gantry based RFID reader infrastructure is complex, very time consuming and extremely expensive to deploy and maintain (the significance of this problem in particular must not be underestimated.).

In relation to (i), the use of the windscreen (or headlamp) as the location for the placement of a RFID tag on a vehicle, it must be recognised that such placement of a tag (in a windscreen or a headlamp) leads to a number of factors that significantly affect RFID read reliability, including due to the properties of the glass/plastic used in the windscreen/headlamp and also due to vehicle body materials and shape. It is also important that, as alluded to above, RFID tags when installed in windscreens (or headlamps), are mostly installed by unskilled persons, and this results in a high inconsistency of tag placement location/orientation.

For windscreen mounted RFID tags, and also headlamp mounted a RFID tags, metal body parts of a vehicle can deform/distort/complicate the radiation pattern of the RFID tag's antenna. For instance, the vehicle's metal body generally surrounds the RFID tag antenna and tends to generate a mutual -coupling effect that distorts the antenna properties both in radiation features and signal fidelity. And the windscreen/headlamp glass/plastic, due to both its composition and thickness, often displays an uncertain dielectric variance and may even act as a radio shield as a result of tinting and/or hardening. (In worst-case scenarios, these factors may even cause the vehicle overall to operate, in effect, as a Faraday cage surrounding the RFID tag, thus preventing or severely inhibiting communication between the RFID tag and a RFID reader). The use of the windscreen (or headlamp) as the location for the placement of a RFID tag on a vehicle can therefore lead to issues that randomly and unpredictably affect read performance. Actually, there is even more to this, as discussed further below.

Figure A9 illustrates certain factors that contribute to create non-linear variation of the signal between an overhead RFID reader antenna and a windscreen-mounted RFID tag, including as a result of movement of the vehicle. More specifically, Figure A9 illustrates the direct communication path 9-4 between the overhead RFID reader antenna 9-1 and the windscreen-mounted RFID tag 9-2, together with a number of multi-path factors which contribute to create signal nonlinearity associated with the direct communication path.

Firstly, movement of the vehicle towards the overhead gantry on which the reader 9-1 is mounted causes a decrease in the length of the direct communication path 9-4, as indicated by 9-5. The decreasing length of the direct communication path 9-4, resulting in a Doppler shift, in fact, changes as a trigonometric tangent function of the angle of the communication path (the signal path length ~ tan(a) where a is the angle of the direct communication path), and this, in turn, results in a squared tangent Doppler shift for the reflected signal carrier wave. This effect might perhaps be handled, at least to some extent, by using short data packet lengths allowing rapid communication timing synchronisation. Use of such short packet data lengths may be effective when the signal received is predictable in behaviour and of singular source. However, in addition to the non -linear decreasing length 9-5 of the communication path 9-4 and the effects this causes (just described), the metal surfaces and edges of the vehicle body (especially the bonnet) act as near-perfect reflectors causing a multitude of other near -perfect (but slightly out of phase) reflected communication paths 9-3. The multiple reflected communication paths 9-3 (which are inherently unpredictable due to varying vehicle windscreen and body shapes/configurations, and also bearing in mind that each of these reflected paths 9-3 is also subject to communication path length decrease and the issues associated therewith) combine to result in an overall net communication signal to the RFID reader 9-1 that incorporates the multiple variable signals, each having an exponential tangent (i.e. a highly non-linear) Doppler shift. This results in a communication signal with unpredictable signal distortion, but where the distortion is similar in nature to the real communication signal (this can be worse, and more difficult to filter out, than unrelated background noise or the like). This is especially true for RFID tags with limited power and processing capabilities. The occurrence of these highly damaging permutations on the signal is dependent on relative tag -reader orientations, tag placement, vehicle construction, vehicle velocity and other reflectors (e.g. other vehicles) in close proximity. It should be clear that the alleviation or reduction of such non-linear multi-path problems is very difficult to achieve using windscreen/headlamp mounted RFID tags, because many factors such as vehicle body configuration/construction, tag placement, etc, cannot be controlled, and the above problems are especially difficult to alleviate when the vehicle is moving at high velocity.

The alternative placement of a RFID tag on or in a metal plate on a vehicle (for example, and preferably, on/in a license plate of the vehicle) may help to largely avoid the radio influences of the vehicle (like those discussed above). A metal plate on a vehicle (such as the vehicle's license plate) can also have a highly consistent shape/construction (i.e. the shape/configuration of the metal plate will typically vary very little, if at all, from vehicle to vehicle). Such a metal plate (license plate) can also function as a ground plane which largely shields the antenna beam from the reflective effects of the rest of the vehicle structure. This is especially true where the metal plate (preferably a license plate) on/in which the RFID tag is mounted is itself mounted in such a way that a clear line of sight is maintained to the plate (so that there are no (or few) intervening reflectors/reflections between the RFID tag and the RFID reader). Importantly, in most jurisdictions, the applicable governing legislation requires vehicle license plates to be installed in such a way that they can be clearly seen (i.e. such that there is a clear line of sight to the license plate). This thus makes the vehicle license plate a particularly suitable placement location on a vehicle for a RFID tag if the tag is to be reliably read by a RFID reader. (As has been mentioned, for vehicles which have a license plate on the front and the rear, a RFID tag may preferably be placed on one or both of a said vehicle's license plates, and for vehicles which have only one license plate, a RFID tag may preferably be placed on the single license plate.)

Figure A10 shows a licence plate mounted within a cavity (i.e. within the channel section of a metal girder) to protect it from damage. This mounting does not obstruct the reading of the plate by a human, but an overhead RFID reader would likely have problems reading a RFID tag installed on such a plate (due to the shielding effect of the section of the metal girder that extends out above the plate). This is another example problem associated with the use of overhead RFID readers for vehicle identification. (Note that the license plate, and also the vehicle registration information, have been obscured in Figure A10 for privacy reasons.)

Figure Al l illustrates the travel path 11 -3 of a vehicle's license plate, where the license plate has a RFID tag thereon (making it a "RFID plate" 11-2), within an overhead RFID reader beam 11 -4. Figure Al l actually illustrates that a vehicle may have a RFID plate 11 -2 mounted on the front and/or the back thereof. Similar to Figure A8, the vertical width of the tag travel path 11 -3 in Figure Al l, which extends from approximately (just above) ground level to approximately 1 m above the ground, exists due to the fact that RFID plates 11 -2 may be positioned at different heights (i.e. different distances off the ground) on different vehicle types. For example, a RFID plate 11 - 2 installed on a large truck will typically be higher (closer to 1 m) above the ground than a RFID plat 11 -2 installed on a low-slung sports car (which might be as little as 20 cm or less above the surface of the ground).

However, unlike the situation for RFID tags installed in vehicle windscreens/headlamps (where the vehicle windscreen/headlamp orientation can vary from vertical to almost horizontal), vehicle license plates generally display little (if any) variation from vehicle to vehicle in terms of the orientation (angle) at which they are installed. Vehicle license plates are typically required to be installed vertically, such that the plane of the license plate is perpendicular to the direction of travel of the vehicle. Consequently, RFID tags on/in vehicle license plates (hence "RFID plates" like the plates 11-2 in Figure Al l, and the antennas thereof) generally have highly consistent orientation from vehicle to vehicle, even across different vehicle types. Hence, the orientations of the antennas of RFID tags, when the tags are installed on/in vehicle license plates, will generally vary very little, even across different vehicle types. The importance of this should not be underestimated given the significant influence that relative antenna orientation can have on read performance (see above).

The influence of the vehicle body on a license plate tag may be related to the size of the metal (conducting) background to the plate, which may be small for a sedan and larger for a bus, as an example. A larger conducting background may have a general effect of making the antenna aperture more narrow and perpendicular to the plane of the background. This can have an overall negative effect for a gantry placed reader, but a positive effect for an in/on road reader; see Figure A12 and the relevant discussions below.

In Figure Al l, the reader antenna 11 -1 is again placed 6 m above the road. Considering the possible variation in

RFID plate height (tag height) within travel path 11 -3, and given general RFID technology read performance limitations, in the scenario in Figure Al i a minimum read range of around 7.5 m is required to read the plate tag reliably. This minimum 7.5 m (approx.) read range is depicted in Figure Al 1 by the shape of the RFID reader antenna's effective beam 11-4, which (in this two-dimensional, cross-sectional representation) has a "30° sector" shape with a radius of around 7.5 m (the radius of the beam 11 -4 extends to midway between the 7 m and 8 m arcs from the location of reader 11 -1).

The scenario in Figure Al 1 is on the edge (i.e. it is approaching the limit) of what can be achieved with RFID, given RFID technology read performance limitations (within spectrum regulations) and the geometry imposed by the locations of the RFID reader and RFID plates (tags) in Figure Al l . The reason why the scenario in Figure Al l is approaching the limits of what is possible can be appreciated from the fact that, in Figure Al l, the minimum required tag travel path/distance 11 -5 (which must again be at least 4 m long for reasons discussed above) only just fits within the effective beam 11 -4 of the reader antenna 11 -1. Hence, for vehicles travelling at or near the assumed maximum speed (180 km/h), the vehicle's license plate mounted RFID tag (RFID plate) 11 -2 will be within the effective beam 11 -4 of the RFID reader 11 -1 for only just enough time to be reliably read (or possibly, due to the possible influence of communication distorting/inhibiting factors, the vehicle's RFID plate 11-2 may not be within the effective beam 11-4 for quite long enough, in which case a reliable read may not be, or may not always be, possible).

The fact that the scenario in Figure Al l is on the edge of what can be achieved with RFID should not be interpreted as a downside or problem associated with the choice of the vehicle license plate as a placement location for RFID tags on vehicles. On the contrary, as explained above, there are numerous reasons why the vehicle license plate may be a highly advantageous location for the placement of RFID tags on vehicles. Hence, the fact that the scenario in Figure Al l is on the edge of what can be achieved with RFID is mainly an indication of the problems associated with mounting RFID readers in overhead locations (such as on over -road gantries and the like). The difficulties highlighted by the scenario in Figure Al l therefore clearly indicate that an alternative RFID reader placement location is required, especially where the vehicle license plate is chosen as the location for the placement of RFID tags on vehicles. An in/on-road location is thought to be a more preferable placement location for a RFID reader if vehicles' RFID tags are on or part of the vehicle license plate.

Sensors which are positioned on or in the road (e.g. as opposed to located on overhead gantries or the like) have previously been proposed and trialled. However, previously their use has been avoided due to issues associated with, for example, difficulties in achieving safe access for personnel for maintenance of the on-road or in-road sensors, the potential for damage to the integrity of the road surface due to the placement of the sensor in the road, the undesirable necessity for (at least partial) road closures for installation, repair or maintenance of the sensors, etc. In/on-road sensors also need to deal with road vibrations, wheel impact shocks and on road fluids, dirt, contaminants, etc. Nevertheless, it is thought that an appropriate structure for an on-road or in-road sensor, which alleviates or at least reduces these problems (or which can cope with them acceptably), is feasible. For instance, sensor size, format, construction, power provision and communications facilities may be selected and combined in a manner to minimise the impact on the road and the time to install the device. At the same time the design of the sensor may ensure durability and ease of maintenance of the device on/in the road. Note that full details of the overall sensor design (e.g. the design of the sensor unit, its housing, construction, etc) to meet the above requirements are not critical to the present invention, and therefore will not be discussed herein in detail. As mentioned above, an in/on-road location is thought to be a preferable placement location for a RFID reader, especially if vehicles' RFID tags are on or part of the vehicle license plate (which is also thought to be highly preferable). For one thing, where in/on-road RFID readers are used and RFID tags are located on vehicle license plates, the multi-path problem (discussed above with reference to Figure A9) may be largely alleviated since the only real reflectors which might reflect a signal between the in/on-road RFID reader and an on-plate RFID tag are the road itself and other vehicles in an adjacent lane. The road is a weak reflector which tends to scatter the signal (rather than produce the much more problematic near-perfect, but slightly out of phase, reflections typically associated with the vehicle bonnet etc for windscreen mounted tags). And adjacent vehicle multi-path reflections typically display a close to linear Doppler shift which can be filtered relatively easily.

Figure A12 illustrates a desirable radiation pattern, and hence a desirable beam shape 12-4, for a RFID reader antenna 12-1 which is placed in/on the road. More specifically, Figure A12 illustrates a cross-section of the said desirable beam shape 12-4, in a vertical plane which extends parallel to the vehicle's direction of travel and through the centre of the antenna radiation pattern, with the said plane viewed from one side. It will be noted that the beam shape 12-4 is quite low (relative to vehicle height) and long/wide (relative to travel direction). Contrast this with the radiation pattern 13-2 on the right hand side in Figure A13 which is a radiation pattern for a conventional directional (and upward-pointing) patch antenna.

The RFID tag 12-2 is placed in or on the vehicle's front and/or rear license plate resulting in a potential tag travel path 12-3 which is typically the space between about 200 mm and about 1200 mm above the road surface. In other words, depending on the type of vehicle (e.g. car, truck, bus, motorcycle, etc), its license plate, with the RFID tag thereon, will typically pass through this region 12-3 which is approximately 200 mm- 1200 mm above the ground as the vehicle passes the reader. The bulk of the space inside the effective beam 12-4 of the antenna 12-1 in Figure A12 (i.e. the bulk of the space within which the RFID tag on the vehicle license plate will receive sufficient energy to "switch on" and communicate with the RFID reader) is within this plate tag travel path 12-3. This is why this beam shape 12-4 is thought to be highly suitable.

Those skilled in the art will appreciate, from Figure A 12, how an in/on-road placement may also alleviate or at least reduce read issues associated with short following distances, tailgating, etc. In Figure A12, the illustrated gap between the depicted vehicles is 4 m, which corresponds to quite severe "tailgating", especially at moderate -high vehicle speeds. Nevertheless, even with such close vehicle proximities, a given vehicle (and it's RFID plate tag 12- 2) will be within the beam 12-4 and visible to the reader 12-1 for sufficient time to achieve a reliable read.

Whilst an in/on-road location is considered to be a preferable placement location for a RFID reader for the reasons discussed above, and especially if the location of RFID tags on vehicle is on or part of a vehicle license plate (which is also thought to be highly preferable), nevertheless in/on-road RFID readers do also present certain challenges.

Figure A13 illustrates, on the right-hand side thereof, the radiation pattern (and hence beam shape) 13-2 associated with a reader having an upward-pointing conventional patch antenna. Figure A13 also illustrates, on the left hand side, a low antenna radiation pattern (beam shape) 13-3 as emitted from a form of in/on-road reader 13-1 having an antenna like those discussed below. (Note that the radiation pattern/beam shape 13-3 on the left-hand side in Figure A13 is the same as the radiation pattern/beam shape 12-4 illustrated in Figure A12.)

The metal surface under a vehicle can act as a reflector, and it is close to the reader antenna 13-1. This may result in a blinding energy reflection which, in the case of an upward -pointing conventional patch antenna (or any other kind of upward-pointing focused, narrow beam antenna), will be very high (as indicated by the amount of depicted energy within the region 13-6 in Figure A13). An antenna with a radiation pattern which possesses low radiation in the vertical direction, especially directly above the antenna (like the radiation pattern 13-3 on the left in Figure A13) may help to reduce this reflected blinding energy substantially (this is illustrated by the substantially lesser amount of energy within the region 13-5 in Figure A13, as compared with the amount of energy in the region 13-6 above the patch antenna).

Thus, Figure A13 illustrates that even though an in/on-road location is a preferable placement location for a RFID reader, it is also the case that focused, narrow beam antennas (as conventionally used in other RFID applications) may be inappropriate for use in this application, due to the possibility for a blinding reflection from the underside of a vehicle. Accordingly, it would appear that an antenna with a radiation pattern having an overall low, flat shape would be preferable.

A low, flat shaped antenna radiation pattern could possibly be achieved by turning a directional antenna (like the one illustrated in Figure A5) on its side. However, this would result in a physical structure on the road which might be (typically) approximately 300 mm tall and 300 mm wide. Such a physical structure is obviously not feasible for use on the road as it would obstruct traffic and would likely be destroyed by the first vehicle to collide with it (not to mention the damage caused to the vehicle, potential accident injuries, etc). Thus, simply turning a directional antenna on its side in order to achieve the desired radiation pattern may not be an option.

Hence, there would appear to be a need for an antenna with a radiation pattern having an overall low, flat shape and where the antenna's physical construction also has a low profile (i.e. a low physical height) so as to be suitable for use in on-road or in-road applications. Hereafter, reference to a "height-restricted" antenna should be interpreted as a reference to an antenna which meets, at least, this last-mentioned configuration criterion (i.e. a "height-restricted" antenna is a reference to an antenna that has a low profile physical structure, or in other words, a low physical height). Of course, for reasons discussed above (and discussed further below), a height-restricted antenna which also has an overall low, flat shaped radiation pattern may be particularly desirable.

Low-profile antenna structures may be achieved by using, for example, upright slotted antenna designs. Another advantage of these may be that only one antenna is required to create radiation up and down the road. Figure A14 depicts an example of an upright slotted antenna and Figure A15 illustrates its radiation pattern. Importantly, it should be noted that the antenna in Figure A14 is merely one example of an upright slotted antenna, and the term slotted antenna really defines an entire category of antennas. The particular example upright slotted antenna depicted in Figure A14 uses an upright slotted radiator 14-1 which has a generally forward and backward pointing radiation pattern, and the forward and backward pointing portions of the radiation pattern are each (individually) somewhat similar to the radiation pattern of a patch antenna (see Figure A15). A reflector 14-2 is used to push the radiation to one side, and at the same time functions (to some extent) to neutralise the ground effect. However, it is thought that this type of antenna, although simple to construct, may not be suitable for use in the particular applications discussed herein due to its inability to cover the desired read zone. Further explanations relating to the read zone are given below.

Antenna design and wave propagation analysis, including taking into consideration ground effect, have been investigated in the past. However, for the most part, such previous investigations have focused on low-frequency and long-distance communications, and antenna designs suitable for this. Therefore, the ground effect has generally been considered/analysed at a macro scale (e.g. considering the effects soil, water, grass, forest, etc, can have for communication over large distances). However, in the context of the present discussion, which involves the use of RFID (including ultrahigh frequency (UHF) RFID) for vehicle detection in road/traffic applications, communication distances are comparatively much shorter; almost always less than 8 m and typically around 5 or 6 m. Also in the present context, for reasons discussed above, the RFID reader will preferably be located on/in the surface of the road. Accordingly, within the range of communication distances currently under consideration (typically 8 m or less and usually around 5 or 6 m) the ground will always be the surface of a road. In other words, if the RFID reader is placed on/in the road surface, the ground within a communication distance radius from the RFID reader (i.e. within at least 8 m or less and usually around 5 or 6 m) will usually be constituted by the road surface (and normally/often nothing else). This "close ground" or "near ground" has determinable, but highly changeable radio properties. Antenna design taking into consideration such "near ground" effect (i.e. the ground effect caused by this "near ground") has not been widely investigated in the past. It is thought to be preferable that the variation in road dielectric properties in different weather conditions should also be taken into account, as discussed in the Summary of the Invention section above.

Placing a low profile upright antenna on the ground, or just below the ground surface, will result in a changing of the direction and shape of the antenna's radiation pattern. Typically the direction of maximum gain will change from a zero angle (i.e. along the ground surface) to a higher elevation angle. That is, the direction of maximum gain will be directed (at least somewhat) up/away from the surface - see Figure A16 for a far field radiation pattern.

The ground effect arises due to a change in the material(s) which the antenna's radiation propagates through or reflects from. For example, the ground (or more specifically in the present context, the road and its base) demonstrates various dielectric properties (permittivity and conductivity). This is due to the materials used in the road's construction, and also due to moisture, the latter of which is highly variable/changeable and uncontrollable. The typical impact of a conducting ground effect is to push the direction of maximum gain upwards. Figure A17 illustrates, for the situation where a patch antenna is on its side so that its beam extends horizontally, the way the radiation pattern of the patch antenna is pushed upwards because of the ground effect. This effect may be present where, for example, metal reinforcing is present in the road and/or conductive fluids (e.g. water due to recent rainfall) are on or in the road surface. In Figure A 17, the reader antenna 17-1 (a patch antenna pointing horizontally) is placed on the road. The direction of maximum gain 17-3 is pushed up, in this case to -30 degrees. A narrow aperture beam shape, like the one identified as 17-4 in Figure A17, does not provide enough energy in the vehicle's license plate/RFID tag potential travel path 17-2. In other words, for the narrow aperture beam 17-4, there is not enough of the plate tag travel path 17-2 within the beam (and hence the vehicle's RFID tag will not be within the beam for long enough) for a reliable read of the vehicle's RFID tag to be achieved. The beam aperture, it might be thought, could be widened to increase the amount of the plate tag travel path 17-2 that is within the beam. This possibility is shown by the wider beam aperture 17-5 in Figure A17. However, those skilled in the art may realise that this illustrated possibility (i.e. 17-5 in Figure A17) is perhaps idealistic (and perhaps not realistic) because the ground effect would likely actually push such a directional radiation pattern further away from the road (meaning that the amount of the plate tag travel path 17-2 which is within the beam may not actually increase very much).

The way in which the direction of maximum gain of an antenna is pushed upwards by the ground effect can also be of benefit, and embodiments of the present invention may take advantage of this, in relation to issues associated with radio noise and interference. As those skilled in the art will appreciate, the use of multiple radio transmitters (and the antennas thereof) which are operating on the same or similar frequency and located nearby one another can (in general terms) give rise to interference. That is, the transmitters/antennas can interfere with one another (often referred to as "cross talk"). One way to overcome or reduce this problem is to ensure that radio transmitters/antennas that are located near one another operate on different frequencies (or ideally frequencies that are mathematically orthogonal to one another so as to prevent additive or subtractive interference). However, in many applications (like RFID for example) the number of frequencies that are actually available for use (according to the applicable regulations governing spectrum allocation) can be very limited. Also, the frequencies themselves that are actually available may not meet the desirable condition of orthogonality. In such cases, it can instead be necessary for respective transmitters/antennas which are operating on the same/similar frequency to be located sufficiently far away from one another to prevent or sufficiently minimise cross talk. However, the way the direction of maximum gain of an antenna is pushed upwards by the ground effect may help with this problem (at least to some extent). This will be explained with reference to Figure A16. Figure A16, the direction of the reader antenna's maximum gain is labelled 16-1 and it can be seen that this is elevated (approximately 30° in this example) relative to the horizontal ground. On the right-hand side in Figure A 16, the lines 16-2 represent the reader antenna's aperture. The amount of energy from the reader that is available to switch on a tag located from the reader in a direction outside the reader antenna aperture is (as explained above) much less than the amount that is available for a tag at the same distance from the reader but located in a direction inside the reader's aperture, and the further outside the aperture the less energy is available. In fact, in the example in Figure A16, the amount of energy (as depicted by the plotted locus) in the direction approximately along (or just above) the ground is between -20 dB and -30 dB, meaning that the amount of energy from the antenna in this direction is drastically less than in directions that are within the antenna's aperture. What this indicates in practice is that there may be little or no signal (i.e. little or no energy emitting from the reader antenna) along or very close to the ground. Also, because the antenna's direction of maximum gain is elevated upwards "into the sky", there will usually be very little energy reflected back downwards towards the ground (assuming no overhead bodies to cause reflections or like). Taken together, these two factors (i.e. little or no energy along the ground and little energy reflected back downwards) suggest that if multiple reader antennas are placed on/in the ground, then there may in fact be little (if any) interference between them, because one reader antenna is unlikely to receive or be affected much (if at all) by the signal of nearby antennas.

One type of antenna having a low profile physical structure, specifically an example upright slotted antenna, was described above with reference to Figure A14. Another type of antenna is a dipole antenna. As summarised in Wikipedia for example, a conventional dipole antenna consists of two conductive elements such as metal wires or rods, which are usually bilaterally symmetrical. The driving current from the transmitter is applied, and for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the feedline to the transmitter/receiver is connected to one of the conductors. The most common form of dipole antenna, referred to as a half-wave dipole antenna (or simply a half-wave dipole), has two straight rods or wires oriented end to end on the same axis, with the feedline connected to the two adjacent ends. Dipole antennas in general are resonant antennas, meaning that the elements serve as resonators, with standing waves of radio-frequency current flowing back and forth between their ends. As mentioned above, the most common form of dipole antenna is the half -wave dipole, an in a half-wave dipole each of the two rod elements is approximately 1/4 of a wavelength long, so that the whole antenna is a half -wavelength long (hence the name "half -wave" dipole).

Figure A18 illustrates the theoretical radiation pattern of an upright half-wave dipole antenna in free space, with the antenna located at the centre of the depicted radiation pattern. This perfect/around doughnut (or "toroid") shaped radiation pattern provided by a theoretical half -wave dipole antenna seems intuitively suitable for use in the road vehicle identification application presently under consideration, especially if the centre point of the dipole antenna were to be at the ground/road surface (assuming any effects caused by the road and the surface of the road can be ignored). Such an upright dipole antenna with its "doughnut" beam shape would be directionally independent in the plane of the surface of the road. That is to say, if such an upright dipole antenna with a doughnut beam shape were to be used in a RFID reader on a road for identifying passing vehicles having license plate mounted RFID tags, the vehicles' RFID tags would read equally well regardless of what radial direction the vehicles approach the reader from. In other words, regardless of the direction of travel of a given vehicle as it moves over/past the RFID reader (and some vehicles may approach/pass the reader in different directions vis-a-vis others) nevertheless the vehicle RFID tags will be read equally well. This may be beneficial, say, at cross roads where vehicles may pass the antenna from a variety of directions. It may also be beneficial when rapidly deploying the antennas/RFID readers as no alignment of the antenna is required, only appropriate positioning and spacing (the last - spacing - is necessary only where multiple antennae/sensors are used).

The use of an antenna with a toroid shaped radiation pattern, and the benefits/advantages this can provide, may actually be somewhat contrary to conventional thinking. As has been discussed previously, conventional thinking in fields related to radio communications and RFID has generally been that focused/directional antennas are preferable, and that the antenna should be oriented with its radiation pattern (and in particular its direction of maximum gain) pointing in the direction of likely/intended read. Thus, in a vehicle identification application for example, conventional thinking would suggest that a directional antenna should be employed with the antenna's narrow beam/radiation pattern pointed in the direction of vehicle travel (e.g. up and/or down the road). However, continuing to refer to the vehicle identification application example, a significant difficulty is that vehicles often may not travel precisely in line with or along the direction of intended read. For instance, a vehicle may travel to one side or other of the reader, or it may pass the reader at an angle relative to the reader antenna's direction of maximum gain, such that the vehicle (and the associated antenna/tag) does not move directly into or along the reader antenna's direction of maximum gain. In such situations, the antenna/tag on the vehicle must be read from the side and/or at an angle, and as has been explained, the amount of energy required to read the tag in such situations may be higher (compared to an ideal "face on" read), even if the tag is actually quite close to the reader. In this regard, one of the main benefits of a toroid shaped radiation pattern is that it is inherently non -directional (in a horizontal plane). In other words, for an antenna with a toroid shaped radiation pattern, the antenna's maximum power extends in all horizontal directions around the antenna, and this can help to significantly reduce the problems discussed above.

A dipole antenna emits linearly polarised energy/radiation. Consequently, if a dipole antenna is used as the antenna for a RFID reader, this in turn requires any RFID tags which are to be read by the RFID reader (such as RFID tags on vehicle license plates) to "reflect" a signal (or produce a modulated reply/response signal) with the same polarisation. However, in most RFID systems and technologies that have previously been developed (e.g. for use in logistics), polarisation is not predictive or fixed. Reflections also change the direction of polarisation. Therefore, it has previously been considered preferable, in the field of RFID, to use circularly polarised antennas (due to the ability to better cope with unpredictable polarisation) rather than linearly polarised antennas.

However, as explained above, a vehicle license plate (including a RFID tag (and it's antenna) thereon) is highly predictable in terms of its construction, mounting and configuration, and the impact of reflections from surrounding reflective bodies is likely to be low. Accordingly, contrary to the case for previous RFID applications (where circular polarisation was considered preferable), in the road vehicle identification application currently under consideration, linear polarisation (as emitted by dipole antennas) is thought to be potentially suitable. In fact, linear polarisation of the reader and tag antennas, which may preferably be vertical polarisation (see below), may provide additional benefits. One such benefit is that any component of a noise signal having different polarisation (e.g. any component of the noise signal having horizontal polarisation, where the reader and tag antennas are vertically polarised) may be more easily (or even naturally) filtered out. Another potential benefit of linear polarisation is that the efficiency of energy utilisation may be improved (as there may be zero polarisation mismatch, e.g. between the reader and tag).

It is thought to be feasible to provide a RFID vehicle license plate (i.e. a vehicle license plate having mounted thereon (or incorporating) a RFID tag) wherein the RFID tag incorporates a slotted antenna which is linearly (preferably vertically) polarised. In other words, such a tag antenna may emit linearly polarised energy/radiation, and preferably the energy/radiation emitted by the tag antenna may be vertically polarised. The reason vertical polarisation may be preferable is because, if the RFID reader antenna is a kind of dipole antenna oriented upright, the reader antenna will emit vertically polarised energy/radiation. Thus a RFID vehicle license plate wherein the antenna thereon is also vertically polarised may be an appropriate match for a RFID reader incorporating an upright dipole antenna (located at an "on-road" or "inter-road" level).

However, whilst dipole antennas (as a general category of antenna) may well be suitable for use in RFID readers in the present application, a simple half-wave dipole antenna (being the most common form of dipole antenna) may not be the most suitable or ideal. Rather, the form of antenna used may be a type or species of dipole antenna (i.e. the antenna proposed for use in the present context might be said to fall into the general category of dipole antennas or dipole-like antennas), however it may not be merely a simple half-wave dipole antenna. Instead, the antenna may be an adapted form of dipole antenna, or a variation or modification of a traditional dipole (or dipole-like) antenna configuration. Importantly, as well as being an adapted or modified form of dipole antenna, the antenna should also be a "height-restricted" antenna. (Recall that the term "height-restricted" herein refers to an antenna that has a low profile physical structure, or in other words, a low physical height.) Furthermore, for reasons discussed above, the antenna should be configured to provide a low, flat radiation pattern.

Figure A19 illustrates, for one possible scenario, the read-zone for a vehicle equipped with a RFID enabled license plate. The RFID plate travel path in Figure A19 is 4 m wide with the read-zone starting at 5 m before the reader antenna and ending at 5 m beyond the reader antenna (the reader in this instance is located in the centre of the road lane at the marked 0 m point). The space from 1 m before to 1 m beyond the reader antenna is excluded from the read-zone in an attempt to reduce the blinding effect of radiation reflection (discussed above with reference to Figure A13), and also because of angled-read problems that may arise in this region, especially for vehicles (and the plates thereof) which are moving near the side of the lane (rather than down the centre of the lane directly in line with the reader).

Figure A20 is a schematic representation of what is depicted pictorially in Figure A19. Thus, Figure A20 shows the license plate/RFID tag orientation within the read-zone (detect area) of an in/on-road RFID reader. The typical values for the parameters in Figure A20 (as per Figure A 19) are: L = 1 m, Lx = 4 m, Ly = 2 m and 200 mm < h < 1200 mm. Figure A21 illustrates the effective read-zone 21 -5 for a RFID tag 21 -4 located on a vehicle license plate, as read using an in-road RFID reader 21-1 with an adapted/modified and height-restricted form of upright dipole antenna. The required read -zone 21-7, based on the travel path 21 -3 of the vehicle, covers the typical lane width of (2Ly) 4 m and the required 4 m in-beam travel path (Lx). (Hence, the required read zone 21 -7 in Figure A21 corresponds to the read zone/detect area depicted in Figure A19 and Figure A20.) The RFID reader's (wide and flat) "dropped doughnut" shaped radiation pattern (this being a highly preferable shape for the radiation pattern) is represented in Figure A21 by the circle labelled 21 -2, however it will be understood that this beam shape 21 -2 (which is represented as large a circle in Figure A21) is actually a dropped -doughnut-like or squashed -toroid-like radiation pattern preferably having a shape approximating the one shown in Figure A23. In any case, the RFID reader's radiation pattern 21 -2, with a face-on read range of approximately 6 m, combined with the effect of the angle of read

21- 6 on the plate's RFID tag 21-4, results in the illustrated effective beam shape 21 -5 (or effective read zone 21 -5). The effective beam shape (read zone) 21 -5 is the area in which a RFID tag which is on/in a vehicle license plate will receive enough power from the RFID reader 21 -1 to be switched on and effectively reflect a modulated signal. As shown in Figure A21, the effective read zone 21 -5 is roughly "figure 8"-shaped, with the centre of the figure 8 located at the position of the RFID reader 21 -1 and the two lobes of the "figure 8" on either side thereof in the direction of vehicle travel. (It should of course be recalled that the RFID reader's antenna, being an adapted/modified and height-restricted form or variation of dipole antenna 21 -1, is non-directional and therefore the orientation of the "figure 8" shaped effective read zone 21 -5 - i.e. in line with the vehicle's direction of travel - arises due to the geometry of the required read zones 21 -7, and the convergence of the "figure 8" lobes near the reader arises due to angle of read issues. These factors concerning the orientation of the "figure 8" shaped effective read zone 21-5 are therefore not a result of the design/configuration of the antenna 21 -1 itself).

Figure A22 illustrates example uses of a RFID reader equipped with an adapted and height-restricted dipole antenna

22- 1 (and which provides a doughnut shaped radiation pattern, as in Figure A21), or multiple such RFID readers, with the resulting effective read-zone 22-2, in different read scenarios. The potential travel path of a license plate RFID tag 22-3 is indicated (indicated as 22-3 and also coloured blue in Figure A22), based on where a vehicle may physically drive, on each different type of road. All road lanes in these examples are 3 m wide, which is average for many road lanes. A bi-directional (single carriageway) narrow road 22-4 that is approximately 6 m wide can be covered with a single reader which will read vehicles in both directions (this is the example in the top left of Figure A22). This is because the plates will typically be placed in the centre on the front and back of a vehicle, meaning that the 4 m wide read-zone will actually cater for a 6 m wide vehicle travel zone (this is ignoring motorcycles and the like). A road with a shoulder, or a wide shoulder, 22-5 (the presence of the shoulder increases the width of the area in which a vehicle can travel) may however often require two readers (as illustrated in the top -middle example in Figure A22). A four lane single direction road with shoulders 22-6 may require three readers (as illustrated in the lower left example in Figure A22). A road crossing of two narrow roads 22-7 could potentially require only one reader (which is why this is illustrated in the example on the right-hand side in Figure A22); although a crossing of a narrow road with a road having wider shoulders may require two readers.

The example scenarios in Figure A22 help to illustrate that, for example in applications such as law enforcement applications, one or more RFID readers should be deployed on or in the road such that it is not possible (or at least it is very difficult) for a vehicle to avoid detection (i.e. avoid having its license plate RFID tag read by one of the readers). So, for example where the RFID readers are used in law enforcement applications, it should not be possible for a vehicle to easily avoid detection by "driving around" the reader by skirting around (and not entering) the reader's detection zone. On the other hand, there may be situations where a RFID reader is intentionally placed so as to only cover a portion of the road. As one possible example of this, in traffic or road planning applications, it may be important to gather information about the number or density of vehicles travelling in a particular lane (or lanes) of a given road, but not all lanes, e.g. at particular times. In this situation, the RFID reader might be placed so as to only detect vehicles travelling in the lane(s) in question, but so as not to detect vehicles travelling in other lanes.

From the above it will be understood that an antenna which provides a radiation pattern as illustrated in Figure A23 (or similar to this) is desirable for use in RFID readers which are to be used in "on-road" or "in-road" placement locations in vehicle identification applications. Such a radiation pattern concentrates the maximum power in the zone where a RFID tag on a vehicle license plate is most likely to travel, which is typically 8 m or less from the antenna. (This was also explained above with reference to Figure A12.) This radiation pattern is also directionally independent in relation to the travel of the vehicle, with a low level of power directly above the antenna (this last is important for reasons discussed above).

Figure A23 actually shows the calculated radiation pattern of a particular adapted/modified and height-restricted form/variation of dipole antenna (i.e. an antenna which is adapted/reconfigured compared to a conventional half- wave dipole antenna to have a low-profile or low-height physical structure but so as still to provide an overall radiation pattern shaped like a dropped-doughnut as shown), and which is placed in or on the road. Note that this radiation pattern is quite wide and flat (approximately toroidal and similar to the shape of a doughnut that has been dropped flat onto the ground and flattened somewhat). In terms of being height-restricted, the antenna should preferably be small enough - more preferably less than 50 mm tall and less than 300 mm in diameter - so as to be easily installed in or on the surface of a road. This antenna configuration should preferably also be such as to neutralise ground and surface effects which may occur.

It should be clear from the above that a RFID reader, incorporating an adapted form or variant of dipole antenna (being vertically polarised), which is operable to be placed in or directly on the surface of the road, which has a low physical profile, and which meets the radiation pattern shape requirements outlined above, may be highly desirable in the context of the presently discussed application involving reading RFID enabled vehicle license plates.

Those skilled in the art might suspect that a monopole antenna may be able to provide the presently desired radiation pattern. Indeed, a monopole antenna might be said to be an adapted type of (or a variant of or a special case of) dipole antenna. Indeed, as summarised in Wikipedia for example, a monopole antenna is a type of antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over a conductive ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the lower end of the monopole and the ground plane. One side of the antenna feedline is attached to the lower end of the monopole, and the other side is attached to the ground plane (which is often the Earth). This is slightly different to conventional half-wave dipole antennas which, as described above, generally consist of two identical rod conductors, with the signal from the transmitter applied between the two halves of the antenna. Nevertheless, like other conventional dipole antennas, a monopole antenna is a resonant antenna in that the rod functions as a resonator with standing waves of radio-frequency current flowing back and forth between its ends. The most common form of monopole antenna is the quarter-wave monopole, in which the rod length is approximately 1/4 of a wavelength of the radio waves (hence the name quarter- wave monopole). However, whilst those skilled in the art might suspect that a monopole antenna such as a quarter-wave monopole may be able to provide a radiation pattern that is desirable for the vehicle identification applications discussed herein, unfortunately such standard monopole antennas are generally too tall (i.e. they don't meet the "height- restricted" requirement). Also, current knowledge on monopole antennas relates mostly to long distance communications where the bottom half of a conventional vertical dipole is replaced by a conducting ground plane (of sufficient size to approximate an infinite ground plane). This is especially the case for lower frequency, long distances transmission applications, and where earth may be approximated as an infinite conducting ground plane. The ground plane in the kinds of applications discussed herein will, however (as discussed above), generally be close to the antenna and imperfect and changing in nature.

Figure A24 illustrates the configuration of an antenna in accordance one particular embodiment of the invention that is thought to be particularly suitable for use in the vehicle identification applications discussed herein. The antenna in Figure A24 is actually a form of adapted/modified monopole antenna. The parts of this antenna, as labelled in Figure A24, include: a circular ground plane 24-3 (note that the circular ground plane 24-3 is slotted, as discussed below), a dielectric layer 24-4 of the same shape and located immediately beneath the ground plane (note that the inclusion of this dielectric layer 24-4 is optional, albeit preferable, and there may also be a (again optional) layer of conductive material (not shown in Figure A24) on the opposite side of the dielectric layer 24-2 from the ground plane - this layer (if present) will often be non-slotted such that it forms a non-slotted ground shield), an upright cylindrical monopole 24-2 (which in this case is in the shape of a short, squat cylinder) oriented vertically relative to the horizontal ground plane and perpendicular to the centre of the ground plane (note that the lowermost first end of the monopole 24-2 does not actually connect with or contact the ground plane 24-3; rather the monopole 24-2 hangs above the ground plane (suspended from the top load 24-1) such that the lowermost first end of the monopole 24-2 is separated from the upper surface of the ground plane 24-3 by a small gap (this gap is visible in Figure A30), a number (in this case four) of shortening poles 24-5, each extending vertically upwards from the ground plane, and the shortening poles 24-5 are all located at an equal radial distance outward from the monopole (i.e. they are all at an equal distance from the central vertical axis of the antenna) and with equal spacing around the monopole (note that the shortening poles form what might be termed a "bird cage" configuration), and a circular top load plate (a.k.a. simply "top load" or cap) 24-1, which in this embodiment is of slightly lesser diameter than the ground plane 24-3, and which is mounted on top of the shortening poles 24-5 such that the top load 24-1 is parallel to, but spaced vertically above, the ground plane (and as mentioned above the monopole 24-2 is connected to, and effectively "hangs" from, the underside of the top load 24-1).

Note that the dielectric layer 24-4 (if present), the ground plane 24-3, the monopole 24-2 and the top load 24-1 are all mounted such that their circular centres coincide with the antenna's central vertical axis.

In Figure A24, the top load 24-1 is shown transparently. However this is merely so that other components (e.g. the monopole and the shortening poles etc) can be more easily made out. In practice, it is expected that the top load 24-

1 will be made from a conductive material such as hieh-grade conductive copper or the like (and as such the top load 24-1 will likely be opaque). Similarly, other parts of the antenna including the monopole 24-2, the shortening poles 24-5, the ground plane 24-3 and the non-slotted ground shield (if present) will also likely be made from conductive materials, although there is no necessary requirement for them all to be made from the same material. Materials that may be suitable for these conductive parts of the antenna, and the issues associated with the selection of appropriate materials, will be familiar to those skilled in the art and therefore need not be discussed in detail. In any case, the invention is by no means limited to by any particular materials.

However, it should be noted that whereas most of the parts of the antenna will be made from conductive materials, as discussed above, the solid dielectric layer 24-4, which (if present) is located beneath the ground plane, should be made from a suitable solid material having appropriate dielectric properties. Possible candidate materials might include plastics, polymers, ceramics, some metal alloys or oxides, etc. In any case, any solid dielectric material known by those skilled in the art to be suitable (and preferably with ordinary permittivity and zero (or low) conductivity) may be used for the dielectric layer 24-4 (if present). The solid dielectric layer (and the material chosen for the formation of the dielectric layer) may also help to improve the mechanical strength of the antenna overall, for example if the ground plane is formed from a thin (and consequently flexible or insufficiently rigid) layer of conductive copper.

It should also be noted that, although not solid physical parts of the antenna, the space above the ground plane 24-3 and below the top load 24-1, and the space radially to the outside of the antenna between the antenna and a cavity in which the antenna is located (see Figure A30), and in particular the air located in these regions, are also very important as this too has dielectric properties which significantly affect the performance of the antenna.

Electronics associated with the antenna (e.g. electronic chips, circuitry, cabling, etc, which may form part of the RFID reader, its controller, etc) should preferably be mounted (or otherwise located) vertically underneath the ground plane 24-3 (or beneath the dielectric layer 24-4 and/or ground shield (if present)). This is so that these electronics are shielded from the antenna by the ground plane 24-3 (or by the ground plane plus the dielectric layer/ground shield), and so that the antenna is shielded from the electronics. However, in addition, it may also be possible for further electronics and/or a further antenna to be provided. (An additional antenna might be used for non-RFID purposes, for example Wi-Fi communication or the like which is in addition to the main antenna's RFID function.) If present, such further electronics and/or additional antenna may be positioned on top of (or otherwise vertically above) the top load 24-1. If positioned above the top load 24-1, such electronics and/or further antenna may be shielded from the main antenna by the top load 24-1. Note that Figure A24 does not depict any further electronics or additional antenna (it merely depicts the main antenna).

As mentioned above, in the antenna depicted in Figure A24, the circular ground plane 24-3 is actually slotted. More specifically, the ground plane 24-3 contains periodic slots. In other words, the ground plane 24-3 is a periodically slotted ground plane. In this particular embodiment, there are four distinct sets of slots. In each set, there is a number (six) of arcuate slots. Each slot is shaped like a short arc oriented concentrically with the ground plane's circumference. And in each set the respective slots are radially spaced equally from one another, and each set extends radially outwards from the position of one of the respective shortening poles 24-5. In each set, the arcuate length of the individual slots becomes greater as the radial distance from the centre of the antenna increases. Each set of arcuate slots is separated from the adjacent set of slots by a solid, un-slotted portion of the ground plane 24-3. It should also be noted that the slots, in this embodiment, are cut (or otherwise formed to extend) through the thickness of the ground plane, although in other embodiments the slots might be merely indented into the surface of the ground plane without extending through the full thickness of the ground plane. In any case, in the depicted antenna, the slots do not extend into (or at least they do not extend all the way through) the thickness of the dielectric layer 24-4 (although the slots may do in other embodiments). Importantly, the number, the relative shape, the relative size, the relative depth (into the ground plane and/or the dielectric layer), the relative position, etc, of the slots may be varied in order to alter the performance of the antenna (i.e. these things may be varied in order to "tune" the antenna). In fact, the ability to alter the configuration of the slots is one of the important ways in which an antenna of this kind may be tuned. In general terms, the function of the periodic slots in the ground plane is (it is thought) to help ensure uniformity of the antenna's radiation pattern for the desired read zone, and also to help to negate (or minimise) the variable ground effect. The slotted ground plane is also thought to help reduce the antenna return-loss but without requiring undesirably large increases in the ground plane size/dimension. In other words, it is thought that the use of a slotted ground plane may help to reduce return-loss whilst also allowing the ground plane size/dimension to remain sufficiently small compact. The actual way in which the configuration of the slots is varied, and the affect different changes in slot configuration may have in terms of antenna performance, is outside the scope of the present disclosure.

Importantly, the radiation pattern of the antenna in Figure A24 is a highly desirable (possibly near perfect or near ideal) "dropped doughnut" or a "toroid on the ground" shape, as depicted by Figure A23. Accordingly, the particular antenna in Figure A24 provides a radiation pattern of a shape (shown in Figure A2323) which is thought to be highly desirable/beneficial/functionally suited for RFID readers which are to be used in "on-road" or "in-road" placement locations in vehicle identification applications. By way of further explanation, the shape of the antenna radiation pattern depicted in Figure A23 is still generally "toroid" like. However, compared to say the quite- high/near-spherical toroid shape of the radiation pattern in Figure A18, which is the radiation pattern of an ideal half-wave dipole, the shape of the radiation pattern in Figure A23 (which is the radiation pattern for the antenna depicted in Figure A24) is generally lower and flatter. That is, it is slightly "squat" or "squashed" in the vertical direction, and this is actually thought to be desirable/advantageous because it means that the antenna's energy extends generally more in the horizontal plane (in all directions) and less in the vertical direction (which means the antenna's beam may extend further outwards horizontally but there may also be less "blinding" from the underside of vehicles etc due to the comparatively lesser amount of energy directed in the vertical direction). Basically, the shape of the radiation pattern is similar to that labelled 12-4 in Figure A12 and 13-3 in Figure A13, which is thought to be highly desirable/beneficial/functionally suited for RFID readers for reasons discussed above.

As mentioned in passing above, the positioning of the shortening poles 24-5 around the monopole 24-2 forms what might be termed a "bird cage" configuration. Accordingly, the configuration of the antenna in Figure A24 might be termed a birdcage configuration (or the antenna therein might be termed a birdcage antenna).

Importantly, whilst the birdcage antenna in Figure A24 is quite substantially modified/reconfigured, as compared to say a conventional half-wave dipole or quarter-wave monopole for example, nevertheless the birdcage antenna is still a species or kind of dipole (or monopole) antenna. As such, the birdcage antenna in Figure A24, like other conventional dipole (or monopole) antennas, is a resonant antenna. Accordingly, the sizes of the various parts of the birdcage antenna are inherently and necessarily dependent on the frequency of the radio signal with which the antenna is to operate. Or equivalently, it might be said that the sizes of the various parts of the birdcage antenna are inherently and necessarily dependent on the wavelength of the radio signal at the specified operating frequency. (Note: assuming the speed of light (c_SL) is constant at c_SL = 299.79 X 10⁶ (m s), then the wavelength (2) of a radio signal is related to the frequency (/) of the said radio signal by c_SL = fX or 2 = ^CsL J

In the RFID applications described above for which the birdcage antenna in Figure A24 is thought to be highly suited, operating frequencies will generally be in the ultrahigh frequency (UHF) range. By way of example, typical operating frequencies for an antenna such as this might be in the range of, say, 860-960 MHz. In any case, because the size of the various parts of the birdcage antenna must necessarily change depending on the operating frequency with which the antenna is to be used, it is largely meaningless to describe specific sizes for the individual parts of the antenna (as this would, at best, describe an antenna suitable for use with one specific operating frequency only). Instead, it is more useful to describe the size of the various parts of the birdcage antenna with reference to (or as a function of) the wavelength of the radio signal. This is illustrated in Figure A30.

The following table outlines the various dimensions of the antenna, which are depicted in Figure A30, with reference to (or as a function of) the wavelength (2) of the radio signal.

Dimension in Meaning Approximate* Example: approximate* ("starting Figure A30 dimension as a point") dimensions for an antenna function of signal based on a frequency of wavelength (2) /=936.85 MHz and a corresponding wavelength of 2 =320 mm

The outer diameter of the monopole 26.66 mm 24-2.

The distance between the 45.71 mm longitudinal axis of one shortening

pole 24-5 and the longitudinal axis

of an adjacent shortening pole 24-5.

The outer diameter of the top load 106.66 mm plate 24-1.

The outer diameter of the ground 128 mm plane 24-3 (and also of the

dielectric layer 24-4 beneath the

ground plane, if present)

The overall width/diameter of the > 2/2 >160 mm cavity into which the antenna (or

the housing of the RFID reader with

which the antenna is associated)

should fit

The vertical spacing between the 2/16 20 mm upper surface of the ground plane

24-3 and the under surface of the

top load 24-1

Note that, in Figure A30, there are a few dimensions depicted which are not given as a function of signal wavelength. This is because these dimensions often will not, or will not always, vary (like the other dimensions do) as the configuration of the antenna is varied to operate with different signal frequencies. These possibly non-varying dimensions depicted in Figure A30 include: the thickness of the antenna ground plane 24-3 (in Figure A30 this is actually the combined thickness of the antenna ground plane 24-3 and the dielectric layer 24-4) which in this example is ~3 mm,

the diameter of the shortening poles 24-5 which in this example is also ~3 mm,

the -10 mm space between the underside of the antenna (the underside of the antenna is either the underside of the ground plane, or the underside of the dielectric layer if the dielectric layer is present) and the bottom of the cavity into which the RFID reader (of which the antenna forms part) is inserted, and the small (typically - 1 -2 mm) gap between the upper surface of the ground plane 24-3 and the bottom/first end of the monopole 24-2.

Note that the space mentioned in the last bullet point above, namely the -10 mm space between the underside of the antenna and the bottom of the cavity into which the RFID reader is inserted, is the space in which electronics such as the RFID reader controller, etc, will normally be located. Note also that, in Figure A30, there is a black solid line extending down from the base of the monopole into this space beneath the antenna. This solid line may be thought of as representing the feedline connecting the antenna to the controlling electronics. There are a number of other things that should be noted from Figure A30. For instance, as mentioned above, the lower end of the monopole does not actually connect/contact with the slotted ground plane (the slotted ground plane is represented by dashed lines in Figure A30; the dashes are intended to represent the slots). Thus, as depicted in Figure A30, there is a small horizontal gap between the upper surface of the ground plane and the lower end of the monopole. Also, there is a solid black line extending along the bottom/underside of the dielectric material. This is intended to represent the (typically non-slotted) conductive ground shield (which, like the dielectric material, is optional). There is also a box 30-1 illustrated in Figure A30. The vertical side edges of the box 30-1 extend along the side edges of the cavity in which the antenna is located, the lower horizontal side of the box extends along the base of the cavity, and the top of the box extends slightly above the level of the ground. The box 30-1 is intended to represent (the outline of) the housing or casing of a RFID reader. That is, the housing or casing within which the antenna (plus other reader electronics, power source or power connections, etc) are located.

A further important point to note, and one which is also signified by the (*) in the headings of the table above, is that the dimensions listed in the table and in Figure A30, even when given as a function of the signal wavelength, are approximate only. In other words, these approximate dimensions form a starting point from which an appropriate antenna configuration may be obtained for a specific intended operating signal frequency. Therefore, after the specific signal frequency with which an antenna is to operate has been determined, the particular dimensions given above (and illustrated in Figure A30) may be used as a starting point, and thereafter further fine adjustment of the antenna configuration (e.g. by slightly altering one or more of the said dimensions whilst keeping others fixed, amongst other things) may be used to "fine tune" the antenna for maximum performance (return loss, resonance, bandwidth, etc). (This fine tuning may also involve altering the particular configuration of the slots in the ground plane, altering dimensions or design attributes of the antenna that are not related to signal wavelength, etc.) Such "fine tuning" within size, design and other physical constraints is common to most, if not all, antenna design, as those skilled in the art will readily understand. By way of example (and it is to be stressed that this is a purely hypothetical and non-limiting example), for an antenna having a configuration like the one depicted in Figure A24 and Figure A30 and which is to operate with a signal of frequency /=936.85 MHz and wavelength of λ =320 mm (i.e. as per the example in the table above), the actual dimensions of such an antenna after fine tuning might be: a=28 mm; b=45 mm; c=110 mm; d=128 mm; e>170 mm; and f=20 mm. Again, this is a purely hypothetical and non-limiting example. Even though the antenna dimensions which are given with reference to signal wavelength in the table above are approximate only, as has just been explained, it can nevertheless be appreciated from the discussion of the general antenna configuration in Figure A24 and Figure A30 that for signal frequencies between 860-960 MHz, the signal wavelength will be between 348 mm and 312 mm, and even with a wavelength towards the upper end of this range (348 mm), the overall antenna should still fit within a structure, such as the housing of a RFID reader, having a diameter of, say, 180 mm or less and a height of, say, 40 mm or less. In fact, the antenna should fit within a housing of this size and still leave room for the frame and supporting parts of the housing, etc. furthermore, the housing itself (even if fully recessed/buried in the surface of the road, with no part left above the road surface) should not penetrate the road by much (if any) more than 40 mm. This should therefore allow such an RFID reader housing to be placed in the road without compromising the integrity of the road. The gaps between the antenna (or the RFID reader housing) and the road structure generally should not cause the radiation properties of the antenna to change, however, this may influence the antenna return loss. In any case, this minor effect might be compensated for by adjustment to the input power level to help guarantee sufficient power emitted from antenna body.

In comparison with the antenna discussed above with reference to Figure A24 and Figure A30, an "inverted F antenna" (IF A) as depicted in Figure A25 may be a simpler alternative construction to achieve an antenna that has a low profile physical structure and which is able to provide a low, flat radiation pattern. As shown in Figure A25, the IFA therein has an upstanding antenna element (the resonant part of the antenna) which is shaped like a sideways capital letter "F". The single long edge of the "F" is oriented parallel to the antenna's ground plane. The 'T- shaped antenna element also has two "prongs". The prong which would be the lower of the two if the "F" were in a normal upright orientation will be referred to here as the second prong. The other prong, namely the one which would be the upper of the two if the "F" were in a normal orientation, will be referred to here is the first prong. As illustrated in Figure A25, the second prong extends vertically downwards from partway along the long horizontal portion of the "F" and inserts through a small hole in the centre of the antenna's ground plane (note that the second prong inserts through the hole in the ground plane but does not contact with the ground plane). The first prong extends vertically down from one end of the long horizontal portion and connects to the antenna's ground plane slightly to one side of the second prong.

Unlike the ground plane of the antenna in Figure A24 (which is round), the ground plane of the IFA in Figure A25 is rectangular. In Figure A25, even though no dielectric layer or underlying conductive shield layer is depicted, these could optionally be provided. Also, the configuration of the periodic slots in the ground plane differs. In the particular IFA depicted in Figure A25, there are two sets of slots. In each set, there is a number (11) of arcuate slots. Each slot is shaped like an arc, although it will be noted that the slots in this IFA ground plane are shaped like longer arcs than the slots in the ground plane in Figure A24. Having said this, the slots in the ground plane of the IFA in Figure A25 are similar to the slots in the ground plane in Figure A24 in that they are oriented to form concentric arcs centred on the centre of the antenna. And in each set of slots in Figure A25, the respective slots are radially spaced equally from one another, and each set extends radially outwards in a direction perpendicular to the axis of the horizontal portion of the "F". In Figure A25, the arcuate length of the individual slots becomes greater as the radial distance from the centre of the antenna increases, and one set of arcuate slots is separated from the other set of slots, on both sides, by a solid, un-slotted portion of the ground plane. The slots in the IFA ground plane are formed through the thickness of the ground plane. Importantly, as for the antenna depicted in Figure A24 above, the number, the relative shape, the relative size, the relative depth (into/through the ground plane and/or into any underlying dielectric layer), the relative position, etc, of the slots may be varied in order to alter the performance of the IFA (i.e. these things may be varied in order to "tune" the antenna).

One difficulty associated with an IFA is that the asymmetrical configuration of an IFA can result in a non -perfect (in particular non-symmetrical) toroid radiation pattern. In the example in Figure A25, the particular configuration of the periodic slotted ground plane is used to for several reasons including: to help correct the said asymmetry of the radiation pattern, to reduce the size of the ground plane, to manipulate the surface impedance to help ensure a uniform radiation pattern, and to limit static and changing ground effects. It is thought that, in this way, the periodic slotted ground plane may be matched (at least to a suitable extent) to direct the beam of the IFA up and down the road. In Figure A25 the IFA is matched with a rectangular periodic slotted ground plane, etc, as discussed above, and this particular configuration results in a radiation pattern up and down the road as illustrated in Figure A26.

The periodic slotted ground plane of the IFA could possibly also be used/changed to correct/adapt the IFA's radiation pattern to be nearer to the preferred "dropped doughnut" or "toroid on the ground" shape shown in Figure A23, but with such an alternative configuration the IFA would likely still generate substantially more vertically upward energy than the particular top loaded monopole (i.e. "birdcage") antenna depicted in Figure A24. A cap or top load may be used to reduce upwards radiation (as is indeed the case in the birdcage antenna in Figure A24). Such a cap or top load might therefore also be used with an IFA. However, in the case of an IFA, due to the antenna's inherent asymmetry, the cap or top load placement must generally also be asymmetrical, and the antenna's radiation pattern, it has been found, is extremely sensitive to the cap placement. The overall height of the antenna is also increased with the cap. Nevertheless, it is thought to be possible that through appropriate tuning and use of a top load, etc, an IFA configuration may be used to provide a height -restricted antenna which also has an overall low, flat shaped radiation pattern, as desired.

Following on from the above IFA discussion, and as a further possible enhancement thereto, an additional (typically higher frequency) antenna may be integrated into the top load or cap of an IFA. Such an additional antenna may be used to provide data communications from the device (where the device incorporates the main antenna with the "dropped doughnut" radiation pattern used for RFID) to another device such as a controlling device. For example, the additional antenna may be used for Wi-Fi or WAVE, as described in the IEEE 802.11 standard set. Figure A27 illustrates a possible example of a capped IFA with an elongated slot formed in the cap which functions as the additional antenna. In this particular example the slot forms a 2.4 GHz Wi-Fi antenna. The periodic slotted circular ground plane in this example is used to correct for or accommodate the resulting imbalanced radiation pattern. Figure A28 illustrates the Wi-Fi radiation pattern, which may be ideal for an in/on road device to communicate with another device located on the roadside, or on a vehicle or on a pole, etc. The combined use of the RFID antenna and the additional data communications capability that may be provided by the additional antenna may help to reduce deployment and maintenance costs.

As has been discussed at length above, the antenna proposed herein will often be (and should be suitable to be) placed on the ground, or in the ground just below the surface, or at any position in between, as conditions or the application requirements dictate. This is illustrated in Figure A29.

Another possible application for antennas in accordance with the present invention is to place the antenna upside down on the underside of a bridge or a gantry. A conducting reflector (preferably one having a diameter λ) may be mounted on the underside of the overhead structure, such that the reflector becomes mounted between the structure and the antenna on the underside of the structure. A birdcage antenna like the one in Figure A24 for example may thus be mounted beneath and in the centre of this reflector with a standoff between the reflector and the antenna ground plane of ~ λ/16. Such a configuration may thus use the same birdcage antenna as described above but the toroid radiation pattern may be pushed downwards, preferably with the angle of maximum gain at -45°. This change in the radiation pattern (i.e. with the angle of maximum gain pushed down preferably by -45°) may (it is thought) be caused largely by the use of the conducting reflector mentioned above. The conducting reflector may also (it is thought) help to isolate the unpredictable near-field effect caused by the bridge/gantry structure and also help to maintain antenna efficiency. This radiation pattern (i.e. with the angle of maximum gain pushed down preferably by -45°), which may be suitable for use with windscreen tags that are mounted so as to be effectively vertically polarised, may create a RFID beam which is effective for reading windscreen tags, possibly nearly as well as plate tags, in 2 lanes in any direction. It is believed that the vertical polarisation may (even in this "upside down" configuration) alleviate some of the multi-path problems described above. The toroid radiation pattern may also alleviate blinding reflections of the roof of, for example, buses. The small size of the antenna may also be very useful where space is restricted, as under bridges and in tunnels.

Claims

1 . An antenna for a communication device, the antenna having a structure comprising:

a circular radiating base plate,

a solid frusto-conical body, wherein the body has an encompassing side which extends from the base plate to near an edge on the widest point on the cone, and the material of the body substantially fills the space inside the encompassing side and between the base plate and the cone.

2. The antenna as claimed in claim 1 , wherein the encompassing side of the body extends from at or near an outer perimeter of the base plate to near an edge on the widest point on the cone.

3. The antenna as claimed in claim 1 or 2, wherein the distance between the base plate and the widest point on the cone in a direction perpendicular to the base plate is less than the maximum diameter of the antenna.

4. The antenna as claimed in any one of the preceding claims, wherein the diameter of the base plate is larger than the maximum diameter of the cone.

5. The antenna as claimed in any one of the preceding claims, wherein the base plate and the cone are made from a conductive material.

6. The antenna as claimed in claim 5 wherein the conductive material is metal such as copper, silver or an appropriate conductive alloy thereof.

7. The antenna as claimed in any one of the preceding claims, wherein the body is made from a dielectric and physically strong material.

8. The antenna as claimed in claim 7 wherein the material from which the body is made has a permittivity (dielectric constant) of between approximately 3 and approximately 6.

9. The antenna as claimed in claim 7 or 8, wherein the material from which the body is made is a soda lime glass.

10. The antenna as claimed in any one of claims claim 5 to 9, wherein the body is initially formed with a recess or indent therein, the shape of the said recess or indent corresponding to the shape of the cone of the antenna, and the cone of the antenna is formed by plating a thin layer of metal onto the surface of the recess or indent.

1 1 . The antenna as claimed in any one of the preceding claims wherein the antenna structure further includes a top plate/lid which extends across and partly or fully covers the space which is formed by and within the open cone.

12. The antenna as claimed in any one of claims claim 5 to 1 1 , wherein the metal base plate is approximately 5-10 mm thick and is initially formed separately from the body and then affixed on or to the bottom/underside of the body.

13. The antenna as claimed in any one of the preceding claims, wherein the antenna is configured to be used with a signal frequency of 860-940 MHz.

14. The antenna as claimed in claim 13 wherein, on the side of the base plate on which the cone is located, no point on the antenna is further than 25 mm from that side/surface of the base plate in a direction perpendicular to the base plate.

15. The antenna as claimed in claim 13 or 14, wherein the diameter of the base plate is less than 190mm.

16. The antenna as claimed in any one of claims 13 - 15, wherein the encompassing side of the body extends from an outer perimeter edge of the base plate to near the edge on the widest point on the cone, and the angle between the encompassing side and the base plate, when taken in a central plane perpendicular to the base plate, is less than 40°, and preferably 33° - 36°.

17. An RFID reader, incorporating an antenna as claimed in any one of the preceding claims, wherein the RFID reader is operable to be used in an application involving road vehicle detection and/or identification and wherein at least the antenna is operable to be installed in the surface of the road.

18. The RFID reader as claimed in claim 17 wherein the RFID reader further includes additional electronic components which are, in use, mounted below the surface of the road, beneath the antenna.

19. The RFID reader as claimed in claim 17 or 18 wherein, when the RFID reader is installed in the road for use,

the base plate of the antenna is installed horizontally in the surface of the road such that an upper surface of the base plate, which is the surface on the side of the base plate which has the cone, is level with the road surface, and

the antenna's body and cone project above the upper surface of the base plate and above the level of the road surface.

20. The RFID reader as claimed in claim 19 wherein, when so installed, the antenna is also surrounded by an at least partially conductive area which is also on or applied to the road surface.

21 . The RFID reader as claimed in claim 20 wherein, if the partially conductive area surrounding a single antenna is circular, the minimum radius of said partially conductive area is approximately twice the wavelength (λ) of the signals to be transmitted and/or received by the antenna.

22. The RFID reader as claimed in claim 20 or 21 wherein the partially conductive area has a conductivity of approximately 10³ S/m or more.

23. The RFID reader as claimed in any one of claims 17 - 22 wherein the antenna is operable to, in use, generate a radiation pattern having a "dropped doughnut" or "squashed toroid" shape.

24. The RFID reader as claimed in any one of claims 17 - 22 wherein the antenna is omnidirectional in the azimuth plane.

25. The RFID reader as claimed claim 24 wherein, where the antenna operates with a signal frequency of 860-940 MHz, the elevation range of the critical read zone in the radiation pattern is from approximately 3° to approximately 30° elevation.

26. The RFID reader as claimed claim 24 or 25 wherein, in the said radiation pattern, the path of max gain is at approximately 30° elevation.

27. The RFID reader as claimed claim 24, 25 or 26 wherein, in the said radiation pattern, the 3dB beam width is approximately 40°, extending from approximately 10° to approximately 50° elevation.

28. The RFID reader as claimed any one of claims 24 - 27 wherein, in the said radiation pattern, there is an effective radiation null at 90° elevation.

29. The RFID reader as claimed any one of claims 24 - 28 wherein the effective read range of the RFID reader is from approximately 1 m to approximately 6.4 m from the antenna in any direction along the road surface.

30. An RFID reader, incorporating an antenna as claimed in any one of claims 1 -16, wherein the RFID reader is operable to be used in an application involving road vehicle detection and/or identification and wherein at least the antenna is operable to be installed or mounted in or on a partially conductive structure.

31 . The RFID reader as claimed in claim 30, wherein the partially conductive structure is operable to be placed on the surface of the road, and when the partially conductive structure is placed on the surface of the road with the antenna installed or mounted therein or thereon, the antenna is located a distance vertically above the road surface.

32. The RFID reader as claimed in claim 31 , wherein when the antenna is installed or mounted in or on the partially conductive structure, the antenna is located at or near the top of the partially conductive structure.

33. The RFID reader as claimed in any one of claims 30 - 32, wherein the partially conductive structure is substantially frusto-conical in shape.

34. The RFID reader as claimed in claim 33, wherein the angle of slope of the side on the frusto-conical partially conductive structure substantially matches the angle of slope on the side of the antenna's main frusto-conical body.

35. The RFID reader as claimed in claim 32, or claim 33 or 34 when dependent on claim 32, wherein, where the antenna operates with a signal frequency (λ) of 860-940 MHz, the configuration of the partially conductive structure should be such that the height of the antenna's base plate, when it is mounted on the partially conductive structure and the partially conductive structure is on the road surface, is not more than ¾λ, and preferably not more than ¼ λ.

36. The RFID reader as claimed in any one of claims 33-35, wherein the construction and/or configuration of the partially conductive structure, including in relation to its height, the angle of slope of its side, internal construction, and positioning of internal components, can be chosen and/or varied to tune the partially conductive structure so that, when the partially conductive structure is used in conjunction with (at least) the antenna, the radiation pattern has a desired "dropped doughnuf-shape.