Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q1/00—Details of, or arrangements associated with, antennas
  • H01Q1/04—Adaptation for subterranean or subaqueous use
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q1/00—Details of, or arrangements associated with, antennas
  • H01Q1/12—Supports; Mounting means
  • H01Q1/22—Supports; Mounting means by structural association with other equipment or articles
  • H01Q1/2208—Supports; Mounting means by structural association with other equipment or articles associated with components used in interrogation type services, i.e. in systems for information exchange between an interrogator/reader and a tag/transponder, e.g. in Radio Frequency Identification [RFID] systems
  • H01Q1/2216—Supports; Mounting means by structural association with other equipment or articles associated with components used in interrogation type services, i.e. in systems for information exchange between an interrogator/reader and a tag/transponder, e.g. in Radio Frequency Identification [RFID] systems used in interrogator/reader equipment
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q1/00—Details of, or arrangements associated with, antennas
  • H01Q1/48—Earthing means; Earth screens; Counterpoises
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q19/00—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic
  • H01Q19/06—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic using refracting or diffracting devices, e.g. lens
  • H01Q19/09—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic using refracting or diffracting devices, e.g. lens wherein the primary active element is coated with or embedded in a dielectric or magnetic material
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q9/00—Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements
  • H01Q9/04—Resonant antennas
  • H01Q9/30—Resonant antennas with feed to end of elongated active element, e.g. unipole
  • H01Q9/32—Vertical arrangement of element
  • H01Q9/36—Vertical arrangement of element with top loading
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q9/00—Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements
  • H01Q9/04—Resonant antennas
  • H01Q9/30—Resonant antennas with feed to end of elongated active element, e.g. unipole
  • H01Q9/40—Element having extended radiating surface
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q1/00—Details of, or arrangements associated with, antennas
  • H01Q1/27—Adaptation for use in or on movable bodies
  • H01Q1/32—Adaptation for use in or on road or rail vehicles
  • H01Q1/325—Adaptation for use in or on road or rail vehicles characterised by the location of the antenna on the vehicle
  • H01Q1/3275—Adaptation for use in or on road or rail vehicles characterised by the location of the antenna on the vehicle mounted on a horizontal surface of the vehicle, e.g. on roof, hood, trunk
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q1/00—Details of, or arrangements associated with, antennas
  • H01Q1/27—Adaptation for use in or on movable bodies
  • H01Q1/32—Adaptation for use in or on road or rail vehicles
  • H01Q1/325—Adaptation for use in or on road or rail vehicles characterised by the location of the antenna on the vehicle
  • H01Q1/3283—Adaptation for use in or on road or rail vehicles characterised by the location of the antenna on the vehicle side-mounted antennas, e.g. bumper-mounted, door-mounted
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q13/00—Waveguide horns or mouths; Slot antennas; Leaky-waveguide antennas; Equivalent structures causing radiation along the transmission path of a guided wave
  • H01Q13/02—Waveguide horns
  • H01Q13/04—Biconical horns

Definitions

• the present invention involves an antenna with a low physical profile and a particular radiation pattern.
• 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.
• RFID tags RFID capable tags
• the antenna would be a part of (or associated with) a RFID reader which is operable to communicate with RFID tags.
• the RFID tags will be located on (or integrated as part of) the vehicles' license plates.
• 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).
• the antenna could potentially be used in a wide range of other areas and/or applications as well.
• 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).
• ⁇ 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
• ⁇ 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.
• Patent applications ⁇ 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 .
• 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).
• Patent applications ⁇ 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.
• 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.
• Patent applications ⁇ 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.
• 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 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.)
• 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 .
• Patent applications ⁇ 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.
• Patent application ⁇ 61 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 ⁇ 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.
• 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.
• 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:
• 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) ...
• 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).
• 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.
• 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).
• 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.
• 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.
• 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).
• 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
• 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.
• 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).
• 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).
• 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.
• 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 ⁇ 61 and '384].
• 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 ⁇ 61 and '384].
• antennas for an] antenna ... configured to operate with signals of frequency around
• 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
• 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.
• 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.
• 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).
• 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.
• this low gain of antennas having a small-footprint periodic slotted ground design can consequently create a need to increase power output from the RFID reader (which powers the antenna) to thereby compensate for low antenna gain.
• 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.
• 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.
• 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.
• the invention relates broadly to an antenna for a communication device, the antenna having a structure comprising:
• 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.
• an encompassing side i.e. a side that goes all the way around the circumference of the 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.
• 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.
• 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).
• 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.
• 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.
• 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.
• 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.
• 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.
• 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:
• 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;
• ⁇ the diameter of the base plate is less than 190mm
• 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°.
• the invention in a second form, 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").
• 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.
• 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.
• 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 ( ⁇ ) of the signals to be transmitted and/or received by the antenna. Also, the partially conductive area may have a conductivity of approximately 10 3 S/m or more.
• 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.
• 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.
• the path of max gain (relative to the azimuth plane) may be at approximately 30° elevation.
• 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).
• 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).
• 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.
• 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.
• 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.
• 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 3 ⁇ 4 ⁇ , and preferably not more than 1 ⁇ 4 ⁇ .
• 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.
• Figure 1 required read-zone for an in/on road antenna.
• Figure 2 “dropped doughnut” (or “squashed toroid") shaped antenna radiation pattern.
• 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.
• FIG 4 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).
• Figure 5 traditional disk cone antenna with the disk and cone each formed from discrete elongate rod elements.
• Figure 6 traditional disk cone antenna having a solid disk and solid cone.
• Figure 7 RFID reader (including antenna structure) with a surrounding partially conductive area of radius R at least twice the signal wavelength ⁇ .
• Figure 8 schematic illustration of a shape corresponding to that of the proposed antenna structure.
• FIG. 9 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.
• Figure 10 annotated cross-sectional view of the proposed antenna structure etc in Figure 9.
• FIG. 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.
• Figure 12 annotated cross-sectional view of the proposed antenna structure etc in Figure 1 1 .
• 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.
• Figure 14 schematic representation of the possible shape and configuration of the main frusto-cone body of the antenna structure
• FIG. 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
• FIG. 17 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.
• Figure 18 side-on pictorial view of the slight alternative or variant on the proposed antenna structure etc in Figure 17.
• 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)
• 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.
• 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”.
• 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.
• an antenna structure 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.
• 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).
• partially conductive substructure means a substructure which is at least partially conductive
• 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.
• 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.
• 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.
• the partially conductive area of course need not be circular. Indeed it may take any number of other shapes (or indeed any shape).
• the size of the partially conductive area should still be sufficient to provide adequate shielding to the antenna structure.
• 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 ( ⁇ ) of the signals to be transmitted and/or received by the antenna (i.e. the radius of a circular partially conductive area should be >2 ⁇ ).
• 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.
• the partially conductive area happens to be circular in shape with radius R of >2 ⁇ , as discussed immediately above.
• R radius
• 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.
• the partially conductive area 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.
• 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 10 3 S/m or more (i.e. the conductivity should preferably be approx. equal to or more than 1000 Siemens per meter).
• the partially conductive area 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).
• 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.
• 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.
• 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.
• 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).
• 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 10 3 S/m or more. If the conductivity of the partially conductive area is greater than approximately 10 6 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.
• a partially conductive area for which the conductivity is greater than approximately 10 6 S/m 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.
• 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.
• 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.
• 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.
• the partially conductive area 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.
• 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.
• 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.
• epoxy/graphite blends are often also used in yacht building for load-bearing structures and surfaces.
• epoxy/graphite blends can have a conductivity of up to approximately 10 4 S/m (which it will be noted is easily sufficient for the purposes of the present invention).
• 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 ofl0 5 S/m) which is painted or epoxied onto the road surface.
• carbon cloth which can have a conductivity in excess ofl0 5 S/m
• Such a carbon cloth may alternatively be embedded in polymer sheets which can themselves be melted onto the road surface.
• 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).
• 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.
• 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.
• 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.
• 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.
• 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 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.
• 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.
• markings e.g. border or edge markings
• 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.
• 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).
• 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.
• patches may be adhered using cationic emulsion or anionic emulsions.
• 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.
• 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.
• 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.
• multiple of the antenna structures are used, e.g. in a line across the road.
• the multiple antenna structures need not necessarily each have their own associated partially conductive areas.
• a single partially conductive area could be provided and shared by some or all of the antenna structures.
• a single partially conductive area shared by all of the antenna structures could be provided as a wide strip extending across all lanes (i.e. across the width) of the road.
• the width of this partially conductive strip which extends across the road may need to be approximately 4 ⁇ 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.
• 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).
• the strip may incorporate lighter coloured or reflective particles to assist in minimising heating and heat retention, etc.
• a number of the important differences between a conventional disk-cone antenna structure and the presently proposed antenna structure relate to relative sizes and proportions of different parts of the 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
• the maximum diameter of the antenna which is generally on the cone.
• antenna height » antenna diameter in a conventional disk-cone antenna.
• the height of the antenna i.e.
• the diameter of the antenna's disc is generally smaller than the maximum diameter of the antenna's cone.
• cone diameter > disk diameter.
• the diameter of the antenna's disc is larger than the maximum diameter of the cone (i.e. cone diameter ⁇ disk diameter).
• 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 ( ⁇ ) 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.
• ⁇ wavelength
• Figure 8 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.
• Figure 8 shows that the antenna structure has an overall circular-based and upwardly-tapering frusto-conical shape 10.
• 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.
• a comparatively inverted circular cone shaped opening 12 tapers inwards from its widest point at the top down to a convergence point 14.
• the convergence point 14 is the lowermost point on the inverted cone shaped opening 12.
• 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).
• 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.
• 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”.
• the 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.
• 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 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°);
• the inner diameter of the rim 16 (again, if present) is approximately 80 mm
• 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.
• 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.
• 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.
• 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.
• the partially conductive area that surrounds the antenna structure 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.
• 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.
• 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).
• 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
• 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.
• the glass or other strong/structural and dielectric material should have a relative permittivity (or dielectric constant) of between approximately 3 and approximate 6.
• a relative permittivity or dielectric constant
• 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).
• 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.
• 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).
• components such as the cone 12 are made from a conductive material (typically metal).
• 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.
• the antenna structure 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).
• thin layers or films of metal such as this, especially if elevated/upstanding and unsupported in "free space", can be very flexibly and flimsy.
• the proposed antenna structure may often be directly run over by vehicles travelling along the road in/on which the antenna is installed.
• the main frusto-cone body 10 of the antenna structure helps to provide.
• 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.
• 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.
• the RFID reader 100 in Figure 9 is installed in an "in-road" installation.
• an appropriately-shaped recess/hole/cavity hereafter the “cavity” 1 10
• the cavity 1 10 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.
• 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.
• 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).
• This optional bore/shaft portion 1 13 should be shaped to receive a heat sink 105.
• 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.
• 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.
• 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.)
• 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).
• 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.
• 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).
• 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.
• 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.
• 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 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.
• certain commercial silicon (or silicon-based) and bitumen (or bitumen-based) adhesives may be suitable for use as the adhesive.
• an adhesive which is comparatively weak but also highly effective at heat transfer i.e. an adhesive which conducts heat well
• 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.
• 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.
• 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.
• the partially conductive area is indicated by reference numeral 90.
• the two sides of the Figure are cut off and therefore the full width of the partially conductive area 90 is not necessarily illustrated.
• 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.
• 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.
• 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.
• the glass used is a form of soda lime glass.
• the antenna structure's cone is labelled with reference number 120 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.
• 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.
• 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.
• 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).
• 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.
• 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.
• 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.
• 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.
• the way in which the antenna structure is attached to the container 160 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.
• 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
• Figure 9 generally correspond to those dimensions described above with reference to Figure 8.
• the diameter of the cone 120 is approximately 104 mm
• the height of the cone 120 is 25 mm or less.
• the base plate (disc) has an outer diameter of approximately 180 mm.
• these dimensions are all given by way of illustrative example only.
• 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.
• 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.
• 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).
• RFID readers 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.
• 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.)
• 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.
• 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.
• 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)
• 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.
• 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.
• 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.
• 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.
• 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.
• a flashing light could be used to provide a warning to road users of an upcoming incident or danger further down the road.
• 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.
• 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.
• FIG. 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.
• 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.
• the antenna screws 190 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.
• Figure 15 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the antenna structure e.g.
• 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.
• the RFID reader 200 in Figure 15 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.
• 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.
• 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.
• 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.
• 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).
• Figure 17 and Figure 18 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.
• 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.
• FIG. 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.
• FIG. 17 and Figure 18 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.
• the vibration absorber 260 screws into an internally threaded portion on the top of the heat sink 205
• 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.
• 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.
• 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 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.
• 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 .
• 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.
• 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.
• 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.
• 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.
• 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).
• 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.
• Figure 17 and Figure 18 actually also differ slightly from one another in one other way.
• this end plate 390 located on the bottom end of the main heatsink 305.
• 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.
• 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.
• the end plate 390 therefore forms a connecting plate between the upper (main) section of heatsink 305, and the lower heatsink extension.
• 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.
• 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.
• 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).
• Figure 2 and also Figure A23
• Figure 2 merely provides a visually appreciable illustration of what shape is meant by "dropped doughnut” or “squashed toroid”.
• Figure 19 and Figure 20 illustrate the antenna's radiation pattern (and parameters thereof) for a signal frequency of 860-940 MHz.
• 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.
• the amount of energy the antenna emits, and the way the energy intensity varies with elevation 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.
• 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.
• Figure 20 illustrates that, in the desired radiation pattern
• ⁇ the elevation range of the critical read zone is from 3° to 30° elevation
• ⁇ the 3dB beam width is 40°, extending from 10° to 50° elevation;
• an RFID reader which, again, incorporates the presently-proposed antenna structure
• an RFID reader 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.
• 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.
• 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.
• the partially conductive substructure takes the form of a partially conductive on-road-locatable cradle 300 (hereafter the "on-road cradle" 300).
• 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.
• 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.
• 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.
• 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 screw directly into these threads in the on-road cradle.
• 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.
• 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.
• Figure 1 1 One difference in Figure 1 1 , compared with Figure 9, relates to the size of the heat sink 105.
• 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.
• 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.
• 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).
• 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.
• 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.
• 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 3 ⁇ 4 ⁇ , and preferably not more than 1 ⁇ 4 ⁇ (the closer to 1 ⁇ 4 ⁇ the better the shielding).
• 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.
• 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.
• batteries typically rechargeable batteries although replaceable batteries might also be used
• associated power supply electronics are provided inside the on-road cradle 300 itself and these are used to power the RFID reader.
• 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.
• the RFID reader is located above the road surface.
• 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.
• the height to which the antenna's ground plane is elevated when the reader is mounted to the on- road cradle 300 is approximately ⁇ /4.
• 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.
• 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.
• FIG. 5 is a schematic representation of a typical construction of a patch antenna.
• FIG. 6 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.
• FIG. 7 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.
• FIG. 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.
• FIG. 1 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.
• FIG. 30 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.
• RFID and like terms herein includes passive backscatter UHF RFID technology, as described by ISO/IEC 18000 part 6.
• 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.
• 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.
• 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.
• 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
• the tag i.e. the signal sent from the RFID tag back to the RFID reader
• the beam shape of a RFID reader 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.
• 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.
• 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.
• 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 .
• the RFID tag is located further away from the reader than in Figure A3.
• the RFID tag is oriented at an angle
• Figure A4 plots a typical radiation pattern for such an antenna.
• 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).
• 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.
• 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).
• 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
• 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
• 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.
• 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).
• 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.
• 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.
• focussed antennas i.e. high gain, directive, narrow beam
• 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).
• FIG A5 is a schematic representation of a typical construction of a patch antenna.
• 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).
• the ground plane typically has an area of more than 300 mm x 300 mm.
• 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.
• 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 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).
• 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 urgent 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.
• 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.
• 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.
• the reader antenna 8-1 is placed 6 m above the road, which is a typical road clearance height.
• 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).
• 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.
• 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.
• 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.
• metal body parts of a vehicle can deform/distort/complicate the radiation pattern of the RFID tag's antenna.
• 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.
• 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.
• the metal surfaces and edges of the vehicle body 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.
• a RFID tag on or in a metal plate on a 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.
• 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).
• 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.
• 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).
• 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.
• 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).
• 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.
• RFID plates like the plates 11-2 in Figure Al l, and the antennas thereof
• 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.
• 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.
• 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).
• Sensors which are positioned on or in the road have previously been proposed and trialled.
• 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.
• 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).
• 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.
• a potential tag travel path 12-3 which is typically the space between about 200 mm and about 1200 mm above the road surface.
• 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
• this beam shape 12-4 is thought to be highly suitable.
• FIG. 12 how an in/on-road placement may also alleviate or at least reduce read issues associated with short following distances, tailgating, etc.
• 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.
• in/on-road location 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 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).
• 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.
• a directional antenna like the one illustrated in Figure A5
• 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).
• simply turning a directional antenna on its side in order to achieve the desired radiation pattern may not be an option.
• 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).
• 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.
• 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.
• 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.
• the ground will always be the surface of a road.
• 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
• 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
• 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
• 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.
• 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.
• 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 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.
• this illustrated possibility i.e. 17-5 in Figure A17
• 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 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.
• 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.
• 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.
• 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).
• 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.
• dipole antenna 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.
• 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.
• 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.
• 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.
• 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.
• RFID tags such as RFID tags on vehicle license plates
• 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.
• a vehicle license plate including a RFID tag (and it's antenna) thereon
• linear polarisation (as emitted by dipole antennas) is thought to be potentially suitable.
• linear polarisation of the reader and tag antennas which may preferably be vertical polarisation (see below), may provide additional benefits.
• 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).
• a RFID vehicle license plate i.e. a vehicle license plate having mounted thereon (or incorporating) a RFID tag
• the RFID tag incorporates a slotted antenna which is linearly (preferably vertically) polarised.
• 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.
• 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).
• 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.
• the antenna should also be a "height-restricted" antenna.
• the term “height-restricted” herein refers to an antenna that has a low profile physical structure, or in other words, a low physical height.
• 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.
• Figure A20 shows the license plate/RFID tag orientation within the read-zone (detect area) of an in/on-road RFID reader.
• 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 covers the typical lane width of (2Ly) 4 m and the required 4 m in-beam travel path (Lx).
• 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.
• 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
• 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.
• 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.
• 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
• 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).
• 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.
• 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.
• an antenna which provides a radiation pattern as illustrated in Figure A23 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.
• 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
• 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).
• 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.
• 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.
• a monopole antenna may be able to provide the presently desired radiation pattern.
• a monopole antenna might be said to be an adapted type of (or a variant of or a special case of) dipole antenna.
• 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).
• 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).
• monopole antennas such as a quarter-wave monopole may be able to provide a radiation pattern that is desirable for the vehicle identification applications discussed herein
• standard monopole antennas are generally too tall (i.e. they don't meet the "height- restricted" requirement).
• 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 perpendic
• 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).
• 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.
• 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-
• the antenna 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).
• 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.
• 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.
• Electronics associated with the antenna 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.
• further electronics and/or a further antenna may also 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.
• 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.
• Figure A24 does not depict any further electronics or additional antenna (it merely depicts the main antenna).
• 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.
• 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.
• 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).
• 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).
• 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.
• 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.
• 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.
• 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.
• the shape of the antenna radiation pattern depicted in Figure A23 is still generally "toroid" like.
• 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).
• 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.
• 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).
• 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.
• 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.
• operating frequencies will generally be in the ultrahigh frequency (UHF) range.
• UHF ultrahigh frequency
• typical operating frequencies for an antenna such as this might be in the range of, say, 860-960 MHz.
• 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.
• FIG. 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.
• FIG. 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).
• 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.
• 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.
• 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.
• an "inverted F antenna” 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.
• 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.
• 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.
• 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.
• 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".
• 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.
• 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).
• the asymmetrical configuration of an IFA can result in a non -perfect (in particular non-symmetrical) toroid radiation pattern.
• 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.
• 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.
• an IFA configuration may be used to provide a height -restricted antenna which also has an overall low, flat shaped radiation pattern, as desired.
• 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.
• 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.
• FIG. 1 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.
• 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.
• a conducting reflector (preferably one having a diameter ⁇ ) 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 ⁇ ⁇ /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.
• 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.

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• 2016
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