GB2401994A

GB2401994A - Dual band antenna system with diversity

Info

Publication number: GB2401994A
Application number: GB0411033A
Authority: GB
Inventors: James William Kingsley; Christopher Morgans; Richard Baggaley; Steven Martin; Andrew Wyatt; Simon Philip Kingsley
Original assignee: Antenova Ltd
Current assignee: Antenova Ltd
Priority date: 2003-05-19
Filing date: 2004-05-18
Publication date: 2004-11-24
Anticipated expiration: 2024-05-18
Also published as: GB2401994B; GB0411033D0; WO2004105182A1; GB0311361D0

Abstract

There is disclosed an antenna device comprising a dielectric substrate (1) with first and second opposed surfaces. A conductive ground plane (2) is formed on a first area of the second surface of the substrate (1), leaving a second area of the second surface blank. At least two conductive feedlines (15, 16) are formed on the first surface of the substrate (1) and extend from an area corresponding to the first area of the second surface to an area corresponding to the second area of the second surface. Each feedline (15, 16) is provided with a dielectric element (19, 20) in the area corresponding to the second area of the second surface, and at least one slot or discontinuity (23, 24, 25) is formed in the conductive ground plane (2) at an interface (14) between the first and second areas in a region between the feedlines (15, 16). The antenna device displays good dual band operation with good diversity and isolation between the antenna elements.

Description

2401 994

DUAL BAND ANTENNA SYSTEM WITH DIVERSITY

The present invention relates to a dual band antenna for use in mobile telephone handsets, PDAs (Personal Digital Assistants), PCMCIA cards, WLAN access points and other electrically small radio platforms. Embodiments of the present invention seek to provide a dual band antenna device having wide bandwidth in both bands and including two antenna elements so as to provide diversity.

Diversity is required to combat the multipath problem and is particularly needed when high data transmission rates are required.

Embodiments of the present invention may incorporate various types of antenna devices, including dielectric resonator antennas (DRAs), high dielectric antennas (HDAs), dielectrically loaded antennas (DLAs), dielectrically excited antennas (DEAs) and traditional conductive antennas made out of electrically conductive materials.

DRAs are well known in the prior art, and generally are formed as a pellet of a high permittivity dielectric material, such as a ceramic material, that is excited by a direct microstrip feed, by an aperture or slot feed or by a probe inserted into the dielectric material. A DRA generally requires a conductive groundplane or grounded substrate.

In a DRA, the main radiator is the dielectric pellet, radiation being generated by displacement currents induced in the dielectric material.

HDAs are similar to DRAB, but instead of having a full ground plane located under the dielectric pellet, HDAs have a smaller ground plane or no ground plane at all.

DRAs generally have a deep, well-defined resonant frequency, whereas HDAs tend to have a less well-defined response, but operate over a wider range of frequencies.

Again, the primary radiator in the dielectric pellet.

A DLA generally has the form of an electrically conductive element that is contacted by a dielectric element, for example a ceramic element of suitable shape. The primary radiator in a DLA is the electrically conductive element, but its radiating properties are modified by the dielectric element so as to allow a DLA to have smaller dimensions than a traditional conductive antenna with the same performance.

A further type of antenna recently developed by the present applicant is the dielectrically excited antenna (DEA). A DEA comprises a DRA, HDA or DLA used in conjunction with a conductive antenna, for example a planar inverted-L antenna (PILA) or planar inverted-F antenna (PIFA). In a DEA, the dielectric antenna component (i.e. the DRA, HDA or DLA) is driven, and a conductive antenna located in close proximity to the dielectric antenna is parasitically excited by the dielectric antenna, often radiating at a different frequency so as to provide dual or multi band operation. Alternatively, the conductive antenna may be driven so as parasitically to drive the dielectric antenna.

An important problem facing antenna designers, in particular today where many portable appliances such as computers, mobile telephones, computer peripherals and the like communicate with each other in a wireless manner, is to provide good diversity within a small space. In telecommunications and radar applications it is often desirable to have two or more antennas that give a different or diverse 'view' of an incoming signal. Generally speaking, the different views of the signal can be combined to achieve some optimum or at least improved performance such as maximum or at least improved signal to noise ratio, minimum or at least reduced interference, maximum or at least improved carrier to interference ratio, and so forth.

Signal diversity using several antennas can be achieved by separating the antennas (spatial diversity), by pointing the antennas in different directions (pattern or directional diversity) or by using different polarizations (polarization diversity).

Antenna diversity is also important for overcoming the multi-path problem, where an incoming signal is reflected off buildings and other structures resulting in a plurality of differently phased components of the same signal.

A significant problem arises when diversity is required from a small space or volume such that the antennas have to be closely spaced. An example of this is when a PCMCIA card, inserted into a laptop computer, is used to connect to the external world by radio. Most high data rate radio links require diversity to obtain the necessary level of performance, but the space available on a PCMCIA card is generally of the order of about 1/3 of a wavelength. At such a close spacing, most antennas will couple closely together and will therefore tend to behave like a single antenna. In addition, there is little isolation between the antennas and, consequently, there is little diversity or difference in performance between the antennas. As a rule, about -20dB coupling (isolation) is the target specification between antennas operating on the same band for a PCMCIA card. For access points (in WLAN and the like applications), which are rather like micro-base stations, even greater isolation is required, about -40dB being desirable. Such high isolation is extremely hard to achieve with conventional antennas when the access points are the size of domestic smoke alarms and less than a wavelength across. Similarly with laptop computers, isolation between WLAN and Bluetooth0 antennas of-40dB or more is seen as desirable.

A method of creating good diversity at the Wireless Local Area Network (WLAN) frequency of 2.4 GHz has been published ["Printed diversity monopole antenna for WLAN operation", T-Y Wu, et. al., Electronics Letters, 38, 25, December 2002].

This paper describes how to remove the ground plane on the underside of a printed circuit board (PCB) so that the end section of a microstrip on the top surface becomes a radiating monopole. This is shown in Figure 1 of the present application. Wu et al. also describe how a T-shaped section of ground plane between the two antennas can help to increase port isolation between them. Further details are presented in ["Planar Antennas for WLAN Applications", K-L Wong, National Sun Yat-Sen University, Taiwan, presented at the 2002 Ansoft Workshop and available on the Ansoft website].

The antenna system discussed above is relatively narrow band and no method of extending the bandwidth or other aspects of antenna performance, is offered. As described in the paper by Wu et al., this type of antenna does not have sufficient bandwidth to be used in a mobile communications system.

It is part of accepted antenna theory that 'fat' monopoles can be designed to have wider band performance than 'thin' monopoles, see for example, ["The handbook of antenna design", O. Rudge, et. al., Peter Peregrinus Ltd. 1986] where rectangular and conical shaped monopoles are shown to have very broadband responses. A recent paper ["Annular planar monopole antennas", Z. N. Chen, et. al., FEE Proc.-Microw.

Antennas Propag., 149, 4, 200 - 203, 2002] describes how a monopole shaped as a circular disk or annulus can have broadband impedance and radiation characteristics.

A recent book ["Broadband microstrip antennas", G. Kumar & K. P. Ray, Artech House, 2003] describes how the fat dipole concepts can be extended to printed microstrip antennas (MSAs). Figure 2 shows the general design of an MSA and Kumar & Ray show that rectangular, triangular, hexagonal and circular printed microstrip antennas all have broadband properties.

None of the references above make any mention of diversity or of using more than one monopole at a time, nor do they provide robust techniques for increasing isolation between antenna elements.

All of the references identified above are hereby incorporated into the present i application by way of reference, and are thus to be considered as part of the present

disclosure.

According to the present invention, there is provided an antenna device comprising a dielectric substrate with first and second opposed surfaces and having a width and a length, a conductive ground plane formed on a first area of the second surface of the substrate and leaving a second area of the second surface blank, at least two conductive feedlines formed on the first surface of the substrate or between the first and second surfaces and extending from an area corresponding to the first area of the second surface to an area corresponding to the second area of the second surface, each feedline being provided with a dielectric element in the area corresponding to the second area of the second surface, and at least one slot or discontinuity being formed in the conductive ground plane at an interface between the first and second areas in a region between the feedlines.

In some embodiments, a second dielectric substrate may be provided on the second surface so as to sandwich the groundplane between the dielectric substrate and the second dielectric substrate. It will be appreciated that this is entirely equivalent to a single dielectric substrate having the groundplane formed between the first and second surfaces. In these embodiments, the dielectric substrates are preferably the same size and shape, although this need not always be so.

In preferred embodiments, the interface extends generally across the width of the dielectric substrate, and may be formed as a substantially straight line except at the location of the slot or discontinuity. Where the dielectric substrate has a rectangular shape, the interface may extend generally parallel to an end edge of the dielectric substrate. Alternatively, the interface may be curved or may meander or may be formed at an angle or slope relative to an end edge of the dielectric substrate.

In a first preferred embodiment, the slot or discontinuity is formed as a gap or cut-out in the groundplane that extends generally towards a central part of the dielectric substrate from the interface. The gap or cut-out may have a length of around 5mm to 20mm or more, and may have a width of about 2mm to lOmm or more. The precise dimensions of the gap or cut-out will depend on the desired operating characteristics of the antenna device. The gap or cut-out may be generally rectangular, or may be trapezoidal or re-entrant in configuration, or may have any other desirable shape.

What is important is that the gap or cut-out extends between the two feedlines on the side of the interface facing away from the location of the dielectric elements. A plurality of gaps or cut-outs, for example two, three, four or more, may be provided between the feedlines, for example by providing a castellated or crenellated edge to the interface between the feedlines. It will be appreciated that in these embodiments, the gaps or cut-outs extend generally into the groundplane from a basis line of the interface.

Alternatively, the discontinuity may be formed as one or more, preferably several, projections of the groundplane from the basis line of the interface into the second area, the projections being between the feedlines. The projections may be generally rectangular, or may be trapezoidal or any other suitable shape. Again, a castellated or crenellated profile may be formed in the groundplane at the interface and between the feedlines.

In a yet further embodiment, there may be provided at least one gap or cut-out in the groundplane extending generally away from the second area and between the feedlines, and a groundplane extension provided that extends from a base of the slot or cut-out, between (but not touching) sides of the slot or cut-out, towards an end edge of the dielectric substrate, the extension passing between the dielectric elements. The extension may optionally pass over the edge from the second surface of the dielectric substrate to the first, and then double-back along the first surface, generally following its line on the second surface, between the dielectric elements and the feedlines. The extension may terminate on the first surface at a point corresponding to the location of the gap or cut-out on the second surface, preferably behind the basis line of the interface and in the first area. In some embodiments, a second conductive groundplane may be provided on the first surface of the dielectric substrate (but electrically isolated from the feedlines). The second groundplane may terminate at an interface on the first surface that is somewhat set back from the interface on the second surface, for example at a position on the first surface corresponding to the base of the slot or cut-out on the second surface. The second groundplane can help to isolate the feedlines from external electromagnetic interference.

Instead of projecting from a gap in the interface on the second surface, the groundplane extension may instead project directly from the interface.

A plurality of groundplane extensions may be provided, either extending from gaps in the interface or directly from the interface, or some combination of both.

Indeed, all embodiments of the present invention may include a second groundplane provided on the first surface for isolation purposes.

In a particularly preferred embodiment, the feedlines generally diverge from each other on the first surface of the dielectric substrate in the second area. For the best polarization diversity, a divergence angle of substantially 90 is preferred, although a smaller or larger divergence angle may be used so as still to give beam diversity albeit with some sacrifice of polarization diversity.

Dielectric elements are provided in association with the feedlines on the first surface in the second area. The dielectric elements may be made of dielectric ceramics materials with a dielectric constant (relative permittivity) greater than 5, and more preferably greater than 10 or greater than 100. The dielectric elements may various forms depending on the operational characteristics desired. For example, the elements may be formed as oblongs, wedges, trapezoids, parallelepipeds or other shapes, and may have edges or surfaces thereof ground or filed down so as to provide chamfers and curves.

One or more surfaces of each dielectric element, for example a top surface, a bottom surface andlor one or more side surfaces can be metallised or provided with a conductive coating. In particular, metallising a top surface of a dielectric element can help to reduce the size of the element required for a given operational frequency, or can help to adjust or change an operating frequency of the antenna thus formed.

In currently preferred embodiments, two feedlines are provided, each feedline having one dielectric element associated therewith.

The feedlines may be formed as printed microstrip transmission lines or in any other appropriate manner.

The dielectric elements may be bonded to the feedlines in the second area on the first surface, for example by way of soldering or using an electrically conductive epoxy resin, or may be otherwise associated with the feedlines so as to form a dielectric antenna. Specific types of dielectric antenna configuration that are useful with the present invention include DRAB, HDAs, DLAs and DEAs as defined in the introduction to the present application. Full operational and structural details of these types of dielectric antennas are outside the scope of the present application, but further information may, for example, be found in the present applicant's co-pending application GB 2 388 964 the disclosure of which is hereby incorporated into the present application by reference.

In most embodiments, the dielectric elements will be mounted on top of the feedlines on the first surface.

The feedlines may extend beyond the dielectric elements, for example towards corners of the dielectric substrate in the second area, thus forming "tails" or "overhangs". The tails or overhangs may be substantially in line with the rest of the feedlines in the second area, or may include end portions that are bent or curved, for example towards or away from or generally parallel to a longitudinal centre line of the dielectric substrate. The tails or overhangs, andlor the feedlines generally, especially in the second area, may be curved or may meander or take various other configurations.

In some embodiments of the invention, PIN diode switches or other switches may be provided so as to switch in or out additional sections of feedline, for example at one or both of the tails or overhangs, thereby allowing an operating frequency of one or other of the dielectric antennas to be adjusted. This can be useful when the antenna device is to be used in different countries where one or other or both of an upper and a lower band needs to be raised or lowered to a different frequency.

In yet another embodiment, instead of providing the feedlines with tails or overhangs, a pair of isolated conductive plates or patches may be provided on the second surface of the substrate underneath the dielectric elements which are located on the first surface. The conductive plates or patches each have a slot which is arranged generally orthogonal to the respective feedline, which crosses over the slot.

Advantageously, a groundplane extension is provided on the second surface so as to extend from the interface to an edge of the second area and passes between the conductive plates or patches.

In this embodiment, the dielectric elements may be formed as generally flat rectangular elements, which may be substantially square. The plates or patches may have a shape and area substantially the same as the dielectric elements, or may be slightly smaller or slightly larger. By making the elements flat, a low profile antenna device can be constructed, which is particularly advantageous in PCMCL card applications. Preferably, the first and second feedlines are substantially orthogonal to each other where they pass under their respective dielectric elements, thus providing the best polarization diversity.

One of the Redlines may follow (on the first surface) a path of the groundplane extension (on the second surface) before extending under its dielectric element.

In this embodiment, the slots radiate in one frequency band, and the plates or patches radiate in a second frequency band, thus providing dual band operation. The dielectric elements serve to provide a dielectric loading for both bands of operation and thereby help to make the antenna device more compact.

For a better understanding of the present invention and to show how it may be carried into effect, reference shall now be made by way of example to the accompanying drawings, in which:

FIGURE 1 shows a prior art WLAN antenna device;

FIGURE 2 shows a prior art printed 'fat' monopole antenna device; FIGURE 3 shows a first embodiment of the present invention; FIGURE 4 shows an alternative first embodiment of the present invention; FIGURE 5 shows a second embodiment of the present invention; FIGURE 6 shows measured and simulated return losses of the antenna device of Figure 3 and the isolation between the antenna elements; FIGURE 7 shows a perspective view of a third embodiment of the present invention; FIGURE 8 shows a schematic plan view of the third embodiment; FIGURE 9 shows a plan view of a part of the third embodiment; FIGURE 10 shows an underplan view of a part of the third embodiment; FIGURE 11 shows return loss and isolation for the third embodiment; FIGURE 12 shows a schematic view of a fourth embodiment of the present invention; and FIGURE 13 shows return loss for the antenna device of Figure 12.

Figure 1 shows a prior art printed microstrip dual monopole antenna device, including a dielectric substrate 1 in the form of an FR4 PCB, a main conductive groundplane 2 on the underside of the substrate 1, two printed microstrip lines 3 on the upper side of the substrate 1, the lines 3 terminating in two radiating sections 4, and a small 'T'-shaped section of groundplane 5 on the underside of the substrate 1 in a location between the two radiating points 4.

Figure 1 also shows the device in cross-section, where it can be seen how the two microstrip lines 3 pass from the upper side of the substrate 1 to its lower side through a pair of gaps or holes 6 in the groundplane 2, and terminate in a pair of SMA connectors 7 which are electrically isolated from the groundplane 2 by insulating washers 8.

The two microstrip lines 3 are configured such that the radiating sections 4 point towards corners 9 of the substrate 1 and are disposed at 90 degrees to each other. No groundplane 2 is provided underneath the radiating sections 4.

This prior art antenna device has a narrow bandwidth in operation, and is acknowledged in the prior art to be unsuitable for mobile communications for this reason.

Figure 2 shows another prior art antenna device, also comprising a dielectric substrate 1 with a conductive groundplane 2 on its underside and a printed microstrip line 10 on its upper side. The line 10 terminates in a 'fat' section 11, which is significantly wider then the main section of the line 10, so as to define a radiating section 1 1. No groundplane 2 is provided under the radiating section 11. An edge 12 of the groundplane 2 acts as a groundplane for the radiating section 11. This antenna device has good bandwidth, but does not provide antenna diversity.

Figures 3 and 4 show a first preferred embodiment of the present mention, comprising a dielectric substrate 1 in the form of an FR4 or Duroi PCB. An underside of the substrate 1 is provided with a conductive groundplane 2 by metallization or any other suitable process, the groundplane 2 defining a first area of the substrate 1. An end portion 13 of the underside of the substrate 1 is not provided with a groundplane 2, this defining a second area of the substrate 1. An interface 14 is thus defined between the first and second areas of the substrate 1. this embodiment, the interface 14 follows a generally straight line across the width of the substrate 1. First and second microstrip feedlines 15, 16 extend across the first area from feed points 17, 18 towards the second area, the feedlines 15, 16 being formed on the topside of the substrate 1. The feedlines 15, 16 run generally parallel to each other as they approach the second area, but as they cross the interface 14, the feedlines 15, 16 diverge at substantially 90 degrees to each other. Two dielectric elements 19, 20 in the form of oblong ceramics pellets are soldered or otherwise mounted on the divergent feedlines 15, 16 in the second area. The feedlines 15, 16 extend beyond the dielectric elements 19, 20 towards corners of the second area, thus defining tails or overhangs 21, 22. In the embodiment shown, the tails 21, 22 are bent towards each other, but in alternative embodiments the tails 21, 22 may be bent away from each other or may curve back towards the interface 14. This bending allows the antenna device as a whole to be made smaller, in particular the second area of the dielectric substrate. The tails 21, 22 act as dielectrically-loaded monopole antennas for operation in a lower frequency band (in this case, 2.1 to 3.1 GHz at a -10 dB return loss level), while the dielectric elements 19, 20 act as HDAs for operation in a higher frequency band (in this case, 4.9 to 6.1 GHz at a -10 dB return loss level).

A key feature of the invention in this embodiment is a gap or cut-out 23 formed in the groundplane 2 at the interface 14 and between the feedlines 15, 16. The gap 23 is generally rectangular in shape, and extends into the first area. The gap 23 serves to improve isolation between the two antennas formed by the respective tails 21, 22 and dielectric elements 19, 20. The gap 23 extends a predetermined distance from the interface 14 to a base 100 of the gap 23.

The antenna device of Figures 3 and 4 has been designed to cover the Rem) Bluetoot /802.11g WLAN band at 2.4 GHz and all the high frequency 802.11a WLAN bands between 4.9 and 5.9 GHz. However, the technology is equally applicable to mobile phones or any other device requiring diversity and/or dual band functionality. Diversity is required to combat the multipath problem and is particularly needed when high data transmission rates are required.

Figure 5 shows an alternative embodiment, like parts being labelled as for Figures 3 and 4. The key difference is that, instead of a gap or cut- out being formed in the groundplane 2 at the interface 14 on the underside of the substrate 1, a set of three projections 24 is provided, the projections 24 having a castellated or crenellated configuration. The projections 24 extend into the second area from the first area on the underside of the substrate 1, and are located between the feedlines 15, 16 located on the topside of the substrate 1. The projections 24 serve to improve isolation between the two antennas formed by the respective tails 21, 22 and dielectric elements 19, 20.

It will be appreciated that multiple gaps or cut-outs 23 may be formed at the interface 14 of the embodiment of Figures 3 and 4, this resulting in a similar castellated or crenellated configuration to that of Figure 5, except that the base line of the interface 14 will define the tops of the castellations rather than the bottoms thereof.

The lengths of the tails 21, 22 may be increased so as to Ale operation at lower frequencies in the lower band, for example to cover t1MT or GSM1800 bands. In some embodiments, additional lengths of feedline may be switched in or out of the tails 21, 22, for example by using PIN diode switches (not shown).

Several prototype antenna pairs of the type shown in Figures 3 and 4 have been built and tested. Figure 6 shows the measured Sir return loss (line 25) for the left hand antenna of a prototype and the S22 return loss (line 26) for the right hand antenna.

The measured -10 dB upper frequency limit for this prototype is about 5.9 GHz, but the simulated return losses (using Ansoft HFS _) show that 6.1 GHz should be I achieved - see the lines 27 and 28.

Also shown in Figure 6 are the measured SO isolation between the antennas (line 29) and the upper band isolation from the simulation (line 30). There is very good isolation between the antennas in both the 2.4 GHz Bluetooth/802.11g band and the 5-6 GHz 802.11 a bands. (RS) The antennas have good gain and efficiency. In the lower band the gain is of the order of 2 dBi and in the upper band it is nearer 5 dBi. The efficiency is over 80%.

A further alternative embodiment is shown in Figures 7 to 10, with like parts being labelled as for the previous Figures. In addition to the groundplane 2 on the underside of the dielectric substrate 1, there is provided an additional groundplane 2' on the topside of the substrate. The additional groundplane 2' does not extend towards the end of the substrate 1 as far as the groundplane 2 on the underside, but terminates on the topside at a line corresponding to the location of the base 100 ofthe gap 23 formed in the groundplane 2 on the underside. A groundplane extension 25 is provided, which extends from the base 100 of the gap 23, but not touching the sides of the gap 23, to the edge of the underside of the substrate 1, and then returns back on itself on the topside of the substrate 1 to a point corresponding to a point just past the line of the interface 14. The feedlines 15, 16 on the topside are insulated from the additional groundplane 2', for example by being formed between the topside and the underside of the substrate 1, only moving up to the topside by way of vies 101 near the interface 14, and are provided with dielectric elements 19, 20 which are slightly shorter than those of the embodiments of Figures 4 and 5. As before, tails 21, 22 extend beyond the dielectric elements 19, 20 to act as dielectrically-loaded monopole antennas. The tails 21, 22 in this embodiment have a zig-zag configuration, which allows the tails 21, 22 to have a greater length in the space available than if the tails were straight.

This embodiment is particularly useful in PCMCIA applications, since the feedlines 15, 16 can be isolated from a PCMCIA chassis by way of being sandwiched between the groundplanes 2, 2'. The gap 23 and the extension 25serve to provide improved isolation between the antenna elements 19, 21 and 20, 22 respectively.

The return loss and isolation between the antenna pair 19, 21 & 20, 22 can be seen in Figure 11. The antenna device as a whole has good coverage of both the WEAN 802.1 lb band at 2.4 GHz and the 802.11 a band at 5-6 GHz.

Figure 12 shows a fourth embodiment of the present invention, comprising an antenna device including a dielectric substrate 1 having a first, upper surface and a second, lower surface, a conductive groundplane 2 on the second surface, and first and second conductive feedlines 15, 16 formed on the first surface and extending from feed points 102 in the first area on the first surface (located above the groundplane 2) to the second area of the first surface (located above regions with no groundplane). A groundplane extension 30 extends from the interface 14 on the second surface towards an edge 31 of the substrate 1. Two dielectric elements 19, 20, here in the form of low-profile squares of dielectric ceramics material, are provided on the first surface over the blank parts of the second surface (i.e. where there is no groundplane 2). The feedlines 15, 16 pass under the dielectric elements 19, 20 on the first surface, and are arranged orthogonal to each other at these points for maximum polarization diversity. Two conductive plates or patches 32, 33 are provided on the second surface, located directly beneath the dielectric elements 19, and having a similar size and shape. The conductive plates or patches 32, 33 each have a central slot or aperture 34, 35, the slots also being arranged orthogonal to each other. The feedlines 15, 16 are configured so as to cross over the slots 34, 35 generally orthogonal thereto, the feedlines 15, 16 and the dielectric elements 19, 20 being on the first surface of the substrate 1 and the groundplane 2, groundplane extension 30 and the patches 32, 33 being on the second surface of the substrate 1.

The slots 34, 35 radiate in one frequency band, and the patches 32, 33 radiate in a second frequency band, thus providing dual band operation. The dielectric elements 19, 20 serve as dielectric loads for both the slots 34, 35 and the patches 32, 33, thereby allowing the antenna as a whole to be more compact.

By making the dielectric elements 19, 20 relatively flat, a low profile antenna device can be constructed, which is particularly advantageous in PCMCLk card applications.

Figure 13 shows the return loss performance of the antenna device of Figure 12, demonstrating excellent dual band operation.

The preferred features of the invention are applicable to all aspects of the invention and may be used in any possible combination.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and are not intended to (and do not) exclude other components, integers, moieties, additives or steps.

Claims

CLAIMS: 1. An antenna device comprising a dielectric substrate with first

and second opposed surfaces and having a width and a length, a conductive ground plane formed on a first area of the second surface of the substrate and leaving a second area of the second surface blank, at least two conductive feedlines formed on the first surface of the substrate or between the first and second surfaces and extending from an area corresponding to the first area of the second surface to an area corresponding to the second area of the second surface, each feedline being provided with a dielectric element in the area corresponding to the second area of the second surface, and at least one slot or discontinuity being formed in the conductive ground plane at an interface between the first and second areas in a region between the feedlines.
2. An antenna device as claimed in claim 1, wherein the interface extends across the width of the substrate on the second surface.
3. An antenna device as claimed in claim 2, wherein the interface is a substantially straight line except at a location of the slot or discontinuity.
4. An antenna device as claimed in claim 2, wherein the interface is curved or meanders across the width of the second surface.
5. An antenna device as claimed in any preceding claim, wherein the slot is formed as a gap or cut-out in the groundplane and extends by a predetermined distance from the interface into the first area.
6. An antenna device as claimed in any one of claims 1 to 4, wherein the discontinuity is formed as a projection of the groundplane and extends by a predetermined distance from the interface into the second area.
7. An antenna device as claimed in claim 5 or 6, wherein a plurality of slots or discontinuities is provided at the interface, and wherein the plurality of slots or discontinuities has a castellated or crenellated configuration between the feedlines.
8. An antenna device as claimed in claim 5 or claim 7 depending from claim 5, wherein an additional groundplane extension line extends from a base of the gap or cut-out or at least one of the gaps or cut-outs on the second surface, over an edge of the substrate, and then across the first surface between the dielectric elements and the feedlines.
9. An antenna device as claimed in claim 6 or claim 7 depending from claim 6, wherein an additional groundplane extension line extends from the interface on the second surface, over an edge of the substrate, and then across the first surface between the dielectric elements and the feedlines.
10. An antenna device as claimed in any preceding claim, wherein the feedlines extend beyond the dielectric elements on the first surface so as to form tails or overhangs.
11. An antenna device as claimed in claim 10, wherein the tails or overhangs are bent or curved towards each other.
12. An antenna device as claimed in claim 10, wherein the tails or overhangs are bent or curved away from each other.
13. An antenna device as claimed in any one of claims 10 to 12, wherein the tails or overhangs have a zig-zag or meandering configuration.
14. An antenna device as claimed in claim 6, wherein the groundplane projection extends adjacent parts of the second area having no groundplane, and wherein the adjacent parts of the second area on the second surface are each provided with a conductive plate or patch that is electrically isolated from the groundplane.
15. An antenna device as claimed in claim 14, wherein the conductive plates or patches on the second surface each have a slot, and wherein the feedlines pass under the dielectric elements on the first surface in such a way that each feedline crosses the slot in the respective plate or patch.
16. An antenna device as claimed in any preceding claim, wherein an additional groundplane is provided on the first area of the first surface, the additional groundplane being insulated from the feedlines.
17. An antenna device as claimed in any preceding claim, wherein the feedline diverge from each other on the first surface in the second area.
18. An antenna device as claimed in claim 17, wherein the Redlines diverge from each other at an angle of substantially 90 degrees.
19. An antenna device as claimed in claim 17, wherein the feedlines diverge from each other at an angle of less than 90 degrees.
20. An antenna device as claimed in claim 17, wherein the feedlines diverge from each other at an angle of more than 90 degrees.
21. An antenna device as claimed in any preceding claim, further comprising at least one switch and at least one additional section of feedline which can be switched in and out of one of the feedlines by the switch so as to change an operating frequency of the antenna device.
22. An antenna device as claimed in any preceding claim, wherein at least one surface of each dielectric element is metallised or provided with a conductive coating.
23. An antenna device as claimed in claim 22, wherein the surface is a top surface.
24. An antenna device substantially as hereinbefore described with reference to or as shown in Figures 3 to 13 of the accompanying drawings.