Antenna

Abstract

An antenna for a communication device is disclosed. The antenna has a structure including a ground plane and a lid component. The lid component is conductive, substantially planar and has a planform shape which is lesser in a first lid component dimension (L.sub.1) than it is in a second lid component dimension (L.sub.2) perpendicular to the first lid component dimension (L.sub.1). The ground plane is conductive and substantially planar, and the size of the ground plane is greater than the size of the lid component. The lid component is conductively connected to the ground plane but also spaced apart from the ground plane, such that there is a space between the lid component and the ground plane, and the antenna is center fed.

Claims

1. An antenna for a communication device, the antenna having a structure including a ground plane and a lid component, wherein: the lid component is conductive, substantially planar and has a planform shape which is lesser in a first lid component dimension (L1) than in a second lid component dimension (L2) perpendicular to the first lid component dimension (L1), the ground plane is conductive, substantially planar and has a planform shape which has a first ground plane dimension (G1) and a second ground plane dimension (G2), where the first and second ground plane dimensions (G1 and G2) are parallel to the first and second lid component dimensions (L1 and L2) respectively, the size of the ground plane in the first ground plane dimension (G1) is greater than the size of the lid component in the first lid component dimension (L1) and the size of the ground plane in the second ground plane dimension (G2) is greater than the size of lid component in the second lid component dimension (L2), and the lid component is conductively connected to the ground plane but also spaced apart from the ground plane such that there is a space between the lid component and the ground plane, and the antenna is center fed, wherein energy/radiation radiated/emitted by the antenna emanates from between the ground plane and edge(s) of the lid component that extend in the direction of the second lid component dimension (L2) and wherein no energy/radiation is radiated/emitted from between the ground plane and edge(s) of lid component that extend in the direction of the first lid component dimension (L1).

2. An antenna for a communication device, the antenna having a structure including a ground plane and a lid component, wherein: the lid component is conductive, substantially planar and has a planform shape which is lesser in a first lid component dimension (L1) than in a second lid component dimension (L2) perpendicular to the first lid component dimension (L1), the ground plane is conductive and substantially planar, the size of the ground plane is greater than the size of the lid component; the lid component is conductively connected to the ground plane but also spaced apart from the ground plane, such that there is a space between the lid component and the ground plane, and the antenna is center fed, wherein energy/radiation radiated/emitted by the antenna emanates from between the ground plane and edge(s) of the lid component that extend in the direction of the second lid component dimension (L2) and wherein no energy/radiation is radiated/emitted from between the ground plane and edge(s) of lid component that extend in the direction of the first lid component dimension (L1).

3. The antenna as claimed in claim 1, wherein the lid component is spaced apart from but also parallel to the ground plane.

4. The antenna according to claim 1, wherein energy/radiation radiated/emitted by the antenna emanates from between the lid component and the ground plane.

5. The antenna according to claim 1, wherein the communication device is an RFID reader operable to be used in an application involving road vehicle detection and/or identification, and wherein, of the parts and components of the RFID reader, at least the antenna's ground plane is operable to be installed on the surface of the road.

6. The antenna according to claim 1, wherein the lid component is substantially rectangular with dimensions L1×L2, energy/radiation radiated/emitted by the antenna emanates from between the ground plane and the long edges of the substantially rectangular lid component that extend in the direction of the second lid component dimension (L2), and no energy/radiation is radiated/emitted from between the ground plane and the short edges of the substantially rectangular lid component that extend in the direction of the first lid component dimension (L1).

7. The antenna according to claim 1, wherein the planform shape of the lid component is lesser in the first lid component dimension (L1) than in the second lid component dimension (L2) by a factor f, where 0.3≤f≤0.75.

8. The antenna according to claim 1, wherein the second lid component dimension (L2) is approximately half the antenna's operating signal wavelength (A) plus or minus a matching factor (x) of up to 20%, the antenna's operating signal is about 800 MHz to 1 GHz in frequency and in the direction of the second lid component dimension (L2) the lid component extends for between approximately 90 mm and approximately 260 mm.

9. The antenna according to claim 1, wherein the lid component is supported at a location spaced apart from the ground plane by one or more conductive support members, the distance that the lid component is spaced apart from the ground plane is defined by the length of the support members, the distance with which the support member(s) support the lid component apart from the ground plane is approximately the antenna's operating signal wavelength (λ) divided by a factor h, where 10≤h≤35.

10. The antenna according to claim 1, wherein the ground plane includes a base plate, and the lid component is spaced apart from but also parallel to the base plate, such that the space between the lid component and the ground plane is the space between the lid component and the base plate, both of the lid component and the base plate are formed from a substantially rigid and conductive material, wherein the base plate is substantially planar and with a plan form shape that is larger than that of the lid component but smaller than that of the ground plane.

11. The antenna according to claim 1, wherein a filler or supporting material is provided in the space between the ground plane and the lid component.

12. The antenna according to claim 1 further including a protective cover.

13. The antenna as claimed in claim 12, wherein the protective cover is in contact with the ground plane and extends over the lid component in order to protect the lid component.

14. The antenna as claimed in claim 13, wherein the protective cover is in contact with the ground plane all the way around the lid component, and the lid component and the space between the ground plane and the lid component are enclosed within the ground plane and the protective cover.

15. The antenna according to claim 12, wherein the protective cover functions as a radome.

16. The antenna according to claim 12, wherein the protective cover is operable to assist the ground plane to lower a radiation pattern of the antenna.

17. The antenna according to claim 12, wherein the protective cover has one or more edges, which extend from the ground plane to the level of the lid component, and the one or more edges thereof have at least a portion which is sloping to assist in reducing impact or shock to a vehicle tire or the like that contacts or rolls over the protective cover or a portion of the protective cover.

18. The antenna as claimed in claim 17, wherein the one or more edges of the protective cover are straight along their length.

19. An RFID reader incorporating or operable to be used with an antenna as claimed in claim 1.

20. The antenna according to claim 1, wherein the lid component is spaced apart from the ground plane thereby forming a cavity between the lid component and the ground plane.

21. The antenna according to claim 2, wherein the lid component is spaced apart from the ground plane thereby forming a cavity between the lid component and the ground plane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1—schematic representation of the required read-zone for an on road RFID reader antenna.

(3) FIG. 2—“dropped doughnut” (or “squashed toroid”) shaped antenna radiation pattern, which is omnidirectional in the azimuth plane, and which has previously been considered desirable for an on road RFID reader antenna.

(4) FIG. 3—schematic illustration for the way that “crosstalk” may arise for a vehicle's RFID tag where multiple RFID reader antennas each of which provides an omnidirectional radiation pattern are used.

(5) FIG. 4—elevation/height and directional/horizontal offsets of the radiation communication path between a vehicle license plate's RFID tag and an on road RFID reader antenna, relative to the plate's “face-on” direction.

(6) FIG. 5—plan (or planform) view of a three lane road with an RFID reader antenna placed on 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, there will be an RFID reader antenna placed in the middle of each lane—see FIG. 1. Note also: reference numeral 3 in this Figure represents the radiation pattern of the RFID reader antenna, where that radiation pattern is omnidirectional (i.e. equally in all radial directions) in the azimuth plane, as has previously been considered desirable.

(7) FIG. 6—plan view (i.e. when viewed in planform) of a single road lane with an RFID reader antenna placed on the road in the middle of the lane. Note: reference numeral 3 in this Figure again represents the radiation pattern of the RFID reader antenna, where that radiation pattern is omnidirectional (i.e. equally in all radial directions) in the azimuth plane, as has previously been considered desirable.

(8) FIG. 7—(i) schematic representation of the potential reduction in the width of the effective read zone 9 as a consequence of increased directionality of on-plate RFID tag antenna radiation (e.g. due to vehicles which have large, bluff fronts); and (ii) a possibly preferred RFID reader antenna radiation pattern shape (or at least a preferred shape when viewed in plan form) 3′ which may help to accommodate for this.

(9) FIG. 8—(i) schematic representation of a possible alternative approach for addressing the potential reduction in the width of the effective read zone, as depicted in FIG. 7(i), where the radiation pattern shape is made to switch between pointing diagonally left and diagonally right using time division multiplexing; and (ii) schematic representation of the need for the multiplexing to be synchronised as between nearby antennas

(10) FIG. 9—perspective view of a typical conventional retroreflective (“cat eye”) road marker.

(11) FIG. 10—perspective view of a typical conventional retroreflective (“cat eye”) road marker installed on the road (between double lines separating adjacent road lanes).

(12) FIG. 11—side-on view of an RFID reader structure (or the portion thereof including the reader antenna structure) in accordance with one possible embodiment of the invention. Note: in this Figure, the base plate (which is part of the ground plane) is shown, but other parts of the ground plane that surround the base plate are not shown. The ground plane, which includes/incorporates the base plate visible in this Figure, sits directly on the road (not shown.

(13) FIG. 12—perspective view of the RFID reader structure (or the portion thereof including the reader antenna structure) in accordance the same embodiment. In FIGS. 12 and 13, the base plate (which is part of the ground plane) is shown, but other parts of the ground plane that surround the base plate are not shown. The ground plane, which includes/incorporates the base plate visible in these Figures, sits directly on the road (not shown).

(14) FIG. 13—exploded perspective view of the RFID reader structure (or the portion thereof including the reader antenna structure) in accordance the same embodiment

(15) FIG. 14—side-on view of the RFID reader (antenna) structure, which sits on and above the road surface, in accordance with the same embodiment, but also showing (by way of non-limiting example) other electronics that could possibly be associated with the RFID reader and which may be (at least in this particular installation, although they need not always be) located in the road (i.e. buried beneath the road surface and beneath the antenna etc).

(16) FIG. 15—schematic illustration of the dimensions of the ground plane and of the antenna's lid component relative to a single road lane. Note that this Figure shows the whole ground plane and also the lid component, but other components such as the protective cover, base plate, etc, are not illustrated

(17) FIG. 16—graphical representations of the shape and strength/power of the radiation pattern produced by an antenna in accordance with one possible embodiment of the invention.

(18) FIG. 17—graphical representations of the shape and strength/power of the radiation pattern produced by an antenna in accordance with another possible embodiment of the invention, different to the embodiment whose radiation pattern is represented in FIG. 16, and which has (in particular) a lid of different length relative to width dimensions compared to the embodiment whose radiation pattern is represented in FIG. 16.

(19) FIG. 18—(i)a and (i)b are graphical representations of the shape of the radiation pattern produced by an antenna (a wafer antenna) in accordance with another possible embodiment of the invention, and (ii) and (iii) are graphical representations of the shape of the radiation pattern produced by the same (wafer) antenna compared to the shape of the radiation pattern produced by an alternative type of (mushroom) antenna, being an antenna of the type described in patent application '994

DETAILED DESCRIPTION

(20) FIG. 11, FIG. 12, FIG. 13 and FIG. 14 all illustrate an RFID reader structure, or at least they all illustrate the portion of it that includes the RFID reader antenna, in accordance with one possible embodiment of the invention. As shown in these Figures, the RFD reader structure (or the portion of it that contains the antenna) includes a base plate 61 (which is itself part of the antenna's ground plane—see below), a protective cover 62 (which in this case takes the form of a transparent, generally flat, rectangular “dome” made of a strong/structural (and preferably transparent or translucent) material such as polycarbonate, an engineering plastic like acetal (also known variously by such names as Delrin, Celcon, Ramtal, and others) or the like), four corner support members or “pillars” 63, a lid component (hereafter simply the “lid”) 64, a block 66 of support or filler material (the “support block” 66), and a feeding conductor/pin 67. These various parts and components of the RFID reader antenna structure will be discussed in greater detail below.

(21) This particular embodiment of the invention will be described with reference to, and in the context of, its use in road applications where the RFID reader antenna communicates with RFID tags which are located on (or integrated as part of) vehicle license plates. This embodiment of the invention will also be explained below with reference to a situation in which the RFID reader antenna is installed on the road (and commissioned and used) in a manner that causes the reader antenna's radiation pattern to extend more across the road (i.e. more in a direction perpendicular to the direction of vehicle travel on the road) than it does along the road, as shown in FIG. 7(ii). However, it is to be clearly understood that this and other embodiments or variants of the invention may also be capable of installation on the road (and commissioning and use) in a manner that causes (or enables) the long dimension of the reader antenna's radiation pattern to extend at least somewhat more along the road than simply directly across, and possibly with the additional ability to rapidly switch (i.e. between diagonally-left and diagonally-right) using multiplexing, as discussed above with reference to FIG. 8. This last, however, will not be described in detail.

(22) Referring to the base plate 61, as mentioned above, this is (or it becomes, when the antenna is fully assembled and installed) an integral part of the antenna's overall ground plane. The ground plane is conductive overall (at least at the antenna's operating frequency), and so the base plate 61, which is part of the ground plane, is also made from a conductive material. Typically, the base plate 61 will be made from a substantially rigid, conductive material, e.g. such as aluminium (or some other substantially rigid, conductive metal), although other materials (e.g. such a carbon) might also be used. Because the base plate 61 is made from a material which is substantially rigid in addition to being conductive, the base plate 61 therefore provides a structural base upon which other components of the antenna structure can be mounted, including the pillars 63, the lid 64, the support block 66 which is between the base plate 61 and the lid 64, and the protective cover 62.

(23) The way in which the base plate 61 is integrated (or made to be an integral part of the overall larger ground plane) is not narrowly critical and any means for achieving this may be used. Typically, the fact that the base plate 61 is made from a conductive material, and that other surrounding portions of the overall ground plane, which are in contact with at least the edges of the base plate 61, are also conductive (at least at the antenna's operating frequency) may suffice to ensure that the overall ground plane, including the base plate 61 and the other portions of the ground plane that surround it, is conductive. In any case, it is to be emphasised again (and clearly understood) that the base plate 61 depicted in FIG. 11, FIG. 12, FIG. 13 and FIG. 14 is not itself the ground plane (or not the whole of the ground plane—the whole of the ground plane is illustrated in FIG. 15). Rather, the base plate 61 is a conductive component that becomes an integral part of the larger, overall ground plane when the antenna is assembled and installed, and the base plate 61 forms a rigid structural component upon which other components of the antenna structure may be mounted. Further explanations relating to particular features and functions of the base plate 61 will be given below.

(24) The antenna's overall ground plane, including the base plate 61 and the portions of the ground plane that surround it, should be applied to (or installed directly onto) the surface of the road. The actual size of the ground plane (in terms of its length and width on the road, and also its overall shape) will be discussed below, but it should be noted again that in FIG. 11, FIG. 12, FIG. 13 and FIG. 14 it is only the base plate 61 that is shown, not the whole ground plane. The whole ground plane is shown in FIG. 15.

(25) In general terms, the ground plane overall (and in particular the portions of it that surround the base plate 61) forms a fairly thin layer which is typically applied immediately onto or on top of the road surface (the thickness of the ground plane is not necessarily critical to the invention, and it may vary from embodiment to embodiment or depending on how the ground plane is made, but by way of indication (albeit without limitation) the thickness of the ground plane may vary from several millimetres up to a few centimetres). Typically, the portions of the ground plane that surround the base plate 61 will be formed as discussed below, and the base plate 61 will then be installed somewhere within the boundaries of this. Typically the base plate 61 will be installed at the geometric centre of the ground plane; however this is not necessarily critical, and it may often be sufficient for the base plate 61 to be located somewhere towards the centre or middle of the ground plane, if not in the exact geometric centre. But the base plate 61 generally should not be right near the perimeter edge of the overall ground plane, otherwise other parts of the antenna may not be adequately shielded by the ground plane—see below.

(26) In this embodiment, rest of the antenna structure sits (or is mounted) directly onto the upper side/surface of the base plate 61 once the base plate is installed on the road, or possibly even before the base plate is installed on the road or relative to the other portions of the ground plane. In this particular embodiment (see FIG. 13 in particular) a somewhat thinner or recessed portion 65 is provided in the middle on the upper surface of the base plate 61. The short vertical wall that extends around and defines the recess 65 in the base plate 61 is actually the same shape as the outer perimeter of the base of the protective cover 62. Therefore, when the protective cover 62 is installed onto the base plate 61 (with the other components contained beneath the cover 62 and between the cover 62 and the base plate 61) the outer perimeter edge of the recess 61 provides outer support for the perimeter base portion of the cover 62. This may help to reinforce the base portion of the cover 62 and prevent it from deforming or flexing outward, e.g. in the event that a car or vehicle drives over the antenna thereby imposing a downward force that might otherwise tend to squash the cover 62 and make it deform outwards. Reinforcing the base of the cover 62 and helping to prevent it from deforming/flexing outwards in this way also helps to reinforce the overall cover 62 (including the upper portions thereof) in the vertical direction. This is because preventing the base of the cover 62 from deforming/flexing outwards also thereby helps to prevent the upper portions of the cover 62 from being forced to move downwards, toward the surface of the road. In other words, it helps to prevent the overall cover 62 from “flattening out”, and this in turn may help to provide additional protection for the components house between the cover 62 and the base plate, such as the lid 64 and the pillars 63.

(27) As has been mentioned, the overall ground plane should be conductive. For the avoidance of doubt, unless the context clearly dictates otherwise, reference herein to the ground plane being “conductive”, or to the word “conductive” generally, should be understood as meaning (or including) fully conductive but also partially conductive but effectively fully conductive at the antenna's operating frequency (typically around 1 Ghz, although other operating frequencies are also possible) even if not necessarily so other frequencies.

(28) The ground plane overall must generally be of a certain size, or at least a certain minimum size. One important reason why the ground plane should generally be of a certain size is to help ensure that it (i.e. the ground plane) operates to adequately shield other parts (particularly conductive and radiating parts) of the antenna structure from the potentially widely and dynamically variable radio frequency influences of the underlying road, other “near ground” effects, etc. Another reason why the ground plane should generally be of a certain size is to help ensure that it operates to adequately shield any electrical cables, electronics, etc, that may be located beneath the ground plane from the potentially very strong magnetic fields that are created by electric vehicles which are becoming increasingly common on public roads.

(29) The overall ground plane can actually have any shape, provided its size (in all directions along the ground) is sufficient to provide adequate shielding for other portions of the antenna. And as mentioned above, the other conductive and radiating components of the antenna should be located sufficiently towards the middle of the ground plane, and away from the perimeter edge of the ground plane, to be adequately shielded.

(30) In the particular embodiment described herein, and e.g. shown in FIG. 15, the overall ground plane has a planform shape (i.e. a shape which when viewed in orthographic projection) which is greater in a first ground plane dimension (G.sub.1) than it is in a second ground plane dimension (G.sub.2) perpendicular to the first ground plane dimension (G.sub.1) (i.e. G.sub.1⊥G.sub.2 and G.sub.1>G.sub.2). However, as has been mentioned, the ground plane could potentially be shaped in other ways.

(31) The ground plane should preferably be installed on the road surface (as discussed above) and with, in this particular example, the second ground plane dimension (G.sub.2) oriented parallel to the direction of vehicle travel on the road (i.e. G.sub.2=G.sub.along).

(32) In the particular embodiment presently described, the ground plane is essentially planar (i.e. a thin layer on the road) and rectangular in plan form shape with dimensions G.sub.1 (or G.sub.across)×G.sub.2 (or G.sub.along), where G.sub.1 (or G.sub.across)>G.sub.2 (or G.sub.along) as mentioned above. More specifically, in a particularly preferred version of the present embodiment, and where the other parts of the reader and antenna structure have the particular dimensions discussed below, the ground plane should be a generally thin, planar rectangle with dimensions of G.sub.1=4 m (or thereabouts) and G.sub.2=3 m (or thereabouts). Note that, in relation to the first ground plane dimension G.sub.1 (or G.sub.across)=4 m (approx), this corresponds to the full width of the single lane on most roads. For roads that have lanes even wider than this, it may be that the size of the first ground plane dimension G.sub.1 (or G.sub.across) is even greater than 4 m, so as to extend all the way across the road lane (although this also may not always be necessary). It is to be clearly understood, however, that in other embodiments, and particularly if the other parts of the reader and/or antenna structure have sizes or dimensions different to those of this particular embodiment (which may occur e.g. if the antenna is to operate with a different signal frequency), or perhaps in other operational examples, the absolute and relative dimensions of the ground plane may also change compared with that just described.

(33) Without limitation to what has been said about this elsewhere, in order for the ground plane to adequately shield other parts of the antenna structure from the potentially variable radio frequency influences of the underlying road (and from other “near ground” influences), the ground plane (and hence the material or substance from which it is formed) may (at least when “finished” and ready for use) need to have a minimum conductivity. Or in other words, the ground plane may (when finished/installed and ready for use) need to have resistivity which is below a certain maximum. For the particular antenna structure(s) proposed herein, and given the antenna power, desired radiation pattern shape, antenna gain, antenna return loss, etc, the ground plane (and hence the material/substance from which it is formed) should, it is thought, preferably (when installed, finished and ready for use) have a conductivity of approximately 10.sup.3 S/m or more (i.e. the conductivity should preferably be approx. equal to or more than 1000 Siemens per meter). To put this another way, the conductive ground plane (and hence the material/substance from which it is formed) should, it is thought, preferably (when finished) have a resistivity below approximately 10.sup.−3 Ωm (i.e. the resistivity should preferably be equal to or less than 0.001 ohm meters).

(34) In relation to the creation/formation/installation/deployment of the conductive ground plane, and in particular those portions of it other than the base plate 61, this should preferably be as economical and non-disruptive as possible, both in terms of the time, cost, complexity, etc, involved in the creation/formation/installation of the ground plane itself, and also given that it will usually be necessary to close the road (or at least a section of the road or the lane(s) involved) while this is taking place.

(35) It was mentioned above that the ground plane may need to 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.sup.3 S/m or more. If the conductivity of the ground plane is greater than approximately 10.sup.6 S/m, this 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 very effectively with ground planes where the conductivity is considerably less than “fully” conductive.

(36) A conductive ground plane for which the conductivity is greater than approximately 10.sup.6 S/m could be created if it (or the portions of it other than the base plate 61) were to be made from a mesh made solely or mainly of, for example, stainless steel, copper, aluminium or certain other suitably conductive metal alloys, or perhaps from steel wool or metal cloth. However, the practicalities and difficulties associated with applying such a metal mesh to the road surface (at least or especially if the mesh is a separate, stand-alone object and not embedded in or as part of some other object or substance that can be more easily applied to the road) mean that creating portions of the ground plane that surround the base plate 61 from nothing (or little) more than such a metal alloy mesh may perhaps be less attractive than other possible alternatives (some of which are discussed below). Also, a ground plane which (around the base plate) is made from nothing (or little) more than a metal mesh may also have certain associated risks/hazards, particularly e.g. if the mesh were to lift off the road surface due to improper or imperfect installation, or as a result of wear and tear, etc. Therefore, whilst the use of a ground plane made (apart from the base plate) from nothing (or little) more than a metal alloy mesh could be highly effective in terms of its ability to shield the antenna structure from the potentially variable radio frequency influences of the underlying road (and from other “near ground” influences), and whilst embodiments of the invention could well operate with a ground plane (apart from the base plate) made from such a simple metal alloy mesh, nevertheless for practical reasons it is thought that this is less likely to be used (or perhaps it will be used less often) than other possible alternative means for forming the ground plane (apart from the base plate).

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

(38) Another consideration that may affect the means chosen for forming the ground plane (apart from the base plate) is that the surfaces of roads generally expand and contract and change shape somewhat with time. For instance, when a road is loaded as a vehicle wheel presses down thereon as it passes, the road surface will momentarily compress/change shape slightly beneath and due to the pressure imposed by the vehicle wheel. Also, expansion and contraction of the road surface can occur due to temperature fluctuations (e.g. between day and night, or with the change of season, etc). This expanding and contracting and changing of shape, often repeatedly/cyclically, can consequently create cyclic loading/stress and hence fatigue in any structure which is connected or bonded thereto. This may in turn to lead to fatigue-related failure, for example, of any ground plane (or ground plane layer) which is provided thereon, especially if the ground plane (or ground plane layer, apart from the base plate) is in the form of a rigid or brittle structure. On the other hand, the ground plane (or ground plane layer, apart from the base plate) 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.

(39) With the foregoing in mind, one means for providing the ground plane (apart from the base plate) 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 (or perhaps particulate aluminium or other metal, or the like) may be incorporated or blended into the paint, epoxy or polymer. Other conductive components/substances (i.e. other than graphite powder) may of course also be used. Nevertheless, referring for instance to a ground plane (or ground plane layer, apart from the base plate) which is formed from an epoxy/graphite blend, as a comparative example of the hardiness of a ground plane/layer formed in this way, epoxy/graphite blends are often also used in yacht building for load-bearing structures and surfaces. Also, epoxy/graphite blends can have a conductivity of up to approximately 10.sup.4 S/m (which it will be noted is easily sufficient for the purposes of the present invention).

(40) Another means which is thought to be possibly suitable for forming the ground plane (apart from the base plate) is to use carbon cloth (which can have a conductivity in excess of 10.sup.5 S/m) which is painted or epoxied onto the road surface. Such a carbon cloth may alternatively be embedded in polymer sheets which can themselves be melted onto the road surface. In other applications and industries, such as boat and yacht building and repairs etc, it has been shown that maintenance and repair of carbon cloth layers/surfaces/structures, and similarly maintenance and repair of carbon cloth infused epoxy/polymer layers/surfaces/structures, can be relatively easy, cost and time efficient, and effective, using well-understood processes and techniques (none of which require detailed explanation here).

(41) The component, substance or element within the ground plane (apart from the base plate), which provides the conductivity, should preferably be close (ideally as near as possible) to the upper surface of the ground plane when the ground plane (or layer) is applied/formed/installed on the road. In other words, once the ground plane (apart from the base plate) has been applied/formed/installed on the road, within the vertical thickness of the structure/layer of the ground plane, 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 other parts of 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.

(42) Yet another means which is thought to be possibly suitable for forming the ground plane (apart from the base plate) is to use a form of prefabricated “patch” type product which can be applied to the road. These could be similar to in many ways to, for example, the road repair/modification product produced by South African company A J Broom Road Products (Pty) Ltd and referred to by them as the BRP Road Patch. Hence, the ground plane (apart from the base plate) could possibly be created using something similar to the BRP Road Patch; that is to say, the ground plane (apart from the base plate) 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 thin sheets (i.e. prefabricated sheets) which are dimensioned to suit the intended application (see above in relation to the size of the ground plane). The base plate 61 could potentially be installed before, after, or at the same time as, the patch is installed on the road to form other portions of the ground plane.

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

(44) A prefabricated patch like that described above may be adhered to the road surface to form the ground plane (apart from the base plate) in any suitable way or using any suitable technique. By way of example, such patches may be adhered using cationic emulsion or anionic emulsions.

(45) In order for a prefabricated patch like that described above to have sufficient conductivity, a conductor or some form of conductive component or substance could be included in the mixture (along with the aggregate, etc) bound within the bitumen rubber binder. Alternatively, an aluminium alloy or other metal conducting mesh could be incorporated into (or as part of the patch) such that the said conductive metal mesh (rather than simply being applied to the road as a standalone mesh) is applied to the road as part of (or within) the patch product. As a further alternative, particulate or granular aluminium (or other metal) could actually be included in (i.e. as part of) the aggregate which is 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.

(46) As well as providing shielding, the conductive ground plane may also assist with one or more of the following: concentrating the radiation emitted by the antenna into the desired azimuth zone (which is preferably in the shape of an ellipse or other shape is discussed below); reducing the angle of elevation of the path of maximum gain in the colon and concentrating the radiation pattern below the path of maximum gain.

(47) The overall ground plane of the RFID reader antenna structure (which is part of an RFID reader structure) has been explained above. It has also been explained that parts of the reader antenna (and of the reader) other than the ground plane sit or are mounted on top of the ground plane, and in particular on top of the base plate 61. It has further been explained that the conductive ground plane may need to have a certain minimum size, for instance in order to adequately shield the antenna structure. In situations where only a single antenna (corresponding to a single RFID reader) is used (e.g. installed in the road) at a given location, the antenna structure will have its own associated ground plane. However, there may be situations where multiple RFID reader antennas are used at a given location. To help visualise this, consider FIG. 5. FIG. 5 actually shows a situation where only a single RFID reader antenna is used at the depicted location—on the surface of the road in the middle of the centre lane. However, in other situations, it could be that multiple antennas are used, e.g. in a line across the road. For instance, there could be situations in which there is an antenna mounted in the centre of each lane of the road, such that the antennas together define a line across the road. In such situations, the multiple antenna structures need not necessarily each have their own unique ground plane separate from the ground plane of any of the other antennas. Instead, a single conductive area could potentially (possibly) be provided and shared by some or all of the antennas, such that the single area operates as the ground plane for two or more separate antennas. As one possibility, a single partially conductive area shared by all of the antenna structures (where the multiple antenna structures form a line across the road) could be provided as a wide strip (3 m or more wide) extending across all lanes (i.e. across the total width) of the road. This is depicted in FIG. 1.

(48) It should be noted though that, in situations where multiple antennas are used at a given location (e.g. as just discussed), each one (or one or more of them) could still have its own associated (i.e. unique and un-shared) ground plane separate from the ground plane of any of the other antennas. This could possibly occur, say, if the reader antenna in one lane were to be located somewhat further down the road than a reader antenna in an adjacent lane, such that a simple partially conductive strip extending perpendicularly across the road (i.e. like shown in FIG. 1) would not provide adequate coverage around each antenna. However, from a practical point of view, the time, cost, effort, etc, associated with installing or creating a separate ground plane for each antenna structure may be greater than for installing or creating a single larger partially conductive area (e.g. like the wide strip extending across the road mentioned above) which is shared by some or all of the antennas and operates as the ground plane for those antennas, so providing a common/shared ground plane for multiple reader antennas may be desirable where possible. Another possible benefit is that such a strip could be coloured, or it could have markings (e.g. edge markings extending across the road before and after the antenna structures in the vehicles' direction of travel), or it could have a different surface texture or stone/particle size or the like, etc, in order to make the strip clearly visible (or perhaps audible when driven over), which (like above) may be of use where vehicle operators need to be able to see when they are about to pass over an area/location where their vehicle will be detected and/or identified (or at least know or be alerted when this happens). Also, like above, the strip may incorporate lighter coloured or reflective particles to assist in minimising heating and heat retention, etc.

(49) Returning again to consider the RFID reader antenna structure generally, as has been explained, this also includes a lid component (lid) 64. The lid has a planform shape (i.e. a shape which when viewed from above in orthographic projection) which is lesser in a first dimension (L.sub.1) than it is in a second dimension (L.sub.2) perpendicular to the first dimension (L.sub.1) (i.e. L.sub.1⊥L.sub.2 and L.sub.1<L.sub.2). The lid 64, in this embodiment at least, is essentially thin, generally planar and rectangular in plan form shape with dimensions L.sub.1 (or L.sub.across)×L.sub.2 (or L.sub.along), where L.sub.1 (or L.sub.across)<L.sub.2 (or L.sub.along), as mentioned above. More specifically, the planform shape of the lid 64 is preferably lesser in the first dimension (L.sub.1) than it is in the second dimension (L.sub.2) by a factor f, where 0.3≤f≤0.75. (i.e. L.sub.1=f L.sub.2 (or L.sub.across=f L.sub.along, where 0.3≤f≤0.75). L.sub.2 (or L.sub.along) should be approximately half the antenna's operating signal wavelength (λ) plus or minus a matching factor (x) of up to 20%. (i.e. L.sub.along=λ/2±x, x≤20%). In the particular embodiment presently described and shown in FIG. 11, FIG. 12, FIG. 13, FIG. 14 and FIG. 15, in the direction of the second dimension (L.sub.2) the lid extends for approximately 90 mm to 260 mm (i.e. L.sub.2=90 mm to 260 mm). Actually, it is envisaged that the embodiment of the antenna depicted may be implemented in practice using an operating frequency of 920 MHz, which means a wavelength of approximately λ=0.326 m. This means that if L.sub.along=137 mm, which is what is presently considered most desirable for an operating frequency of 920 MHz (and this is considered to be the desirable operating frequency), then x=−0.026 or about 19%. Where L.sub.along=137 mm, L.sub.across may be anywhere in the range from about 40 mm to about 110 mm. In another example though, for an operating frequency of 1 GHz, which means λ=0.3 m, this means that if L.sub.along=180 mm, then x=0.03 or about 16%. Where L.sub.along=180 mm, L.sub.across may be anywhere in the range from about 54 mm to about 135 mm. For a given length of lid (i.e. L.sub.along, which is determined with reference to operating frequency) the width of the lid (i.e. L.sub.across) may be varied or adjusted in order to tune the antenna or adjust the shape of the radiation pattern, as discussed below.

(50) The lid 64 is made from a thin plate of conductive, and preferably fairly stiff and resilient material, typically metal (although other non-mental conductive materials are potentially possible). A range of conductive metals are thought to be potentially suitable, including silver, aluminium, copper and other like metals known for their conductivity. However, whilst it is quite possible that metals such as this that are known for their conductivity (and alloys thereof) may be used, it is thought that it may actually be desirable for the lid 64 to be made from a metal more commonly known more for its strength, but which also has high (or adequately high) conductivity, like e.g. steel or titanium. The reason steel or titanium (or possibly other metals or alloys having generally similar properties to these) are considered to be potentially highly suitable is because, not only are they adequately conductive, but they are also strong and highly resilient (i.e. they “spring back” if deformed, provided of course the deforming force does not cause the material to reach or exceed its elastic deformation or yield stress limit). These metals (i.e. steel, titanium and the like) also have a high fatigue resistance, meaning that repeated elastic deformation should not cause the metal to fatigue (i.e. weaken) quickly. The reason these properties (i.e. strength, resiliency and fatigue resistance) are considered potentially important is because, in the road applications in which the antenna is to be used, the antenna will be frequently run over by vehicles (including large heavy vehicle such as trucks), and this will consequently cause some (even if relatively small) deformation of the various parts of the antenna, including the lid 64, even though the lid 64 is encased and protected within the cover 62.

(51) The size of the lid 64 in the L.sub.1 (or L.sub.across) and L.sub.2 (or L.sub.along) dimensions was discussed above. In terms of thickness, as also mentioned above, the lid 64 is (or it will usually be) a generally thin plate. However, the actual thickness of the lid 64 is not critical. In fact, as has been mentioned elsewhere, the lid 64 is not a radiating component of the antenna. Accordingly, it is quite possible for the thickness of the lid 64 to be changed or varied (e.g. depending on the material used), without affecting the radio/signalling properties/performance/operation of the antenna. Nevertheless, depending on the material from which it is made (and in particular the strength, resiliency, etc, properties) the lid 64 will typically have a thickness ranging from less than a millimetre up to several millimetres. However, as has been said, no limitation as to the actual thickness of the lid 64 is to be implied. Because the lid 64 will generally be fairly thin, it might be thought that it might be quite easily bent/deformed beyond the material's yield stress limit. However, as will be explained below, the lid 64 (as well as being protected beneath the cover 62) is supported underneath by the support block 66, which prevents the lid 64 from being (plastically) deformed beyond the material's yield stress.

(52) As shown most clearly in FIG. 13, there is a conductive feeder pin 67 associated with (and connected to) the lid 64. As those skilled in the art will appreciate, the feeder pin 67 carries an electric current to the lid 64. However, it is very important to appreciate that the antenna in this embodiment (and in the present invention generally) is NOT a patch antenna (or anything like it). Therefore, whilst the feeder pin 67 carries an electric current to the lid 64, it is not the lid 64 that radiates the energy emitted by the antenna. Rather, as has been explained elsewhere, it is thought that the open side face(s) of the cavity, namely between the ground plane (base plate 61) and the edge(s) of the lid 64 that extend along the long sides of the lid (L.sub.2), on either side of the lid, that resonate. It is therefore thought to be these long side gaps between the lid 64 and the base plate 61 that form virtual cavity resonators, and which therefore radiate the energy emitted by the antenna.

(53) In the particular embodiment shown in the Figures, feeder pin 67 connects to the lid 64 (from the underside) at a location that is exactly halfway between the short ends of the rectangular lid (i.e. halfway along the lid 64 in the L.sub.2 dimension) and which is also exactly halfway between the long sides of the rectangular lid (i.e. halfway across the lid 64 in the L.sub.1 dimension). The lid 64, and the antenna generally, it is therefore “centrally fed” or “centre fed” in the particular embodiment shown.

(54) As shown in FIG. 11 and FIG. 12 in particular, when the RFID reader antenna structure is assembled, the lid 64 is mounted relatively above, but parallel to, the base plate 61, and it is supported in this position by four pillars 63. The pillars 63 are conductive, and they therefore serve to conductively connected the conductive base plate 61 (and therefore the ground plane) to the conductive lid 64. In terms of the materials from which the pillars 63 may be made, the same general considerations apply as discussed above in relation to the lid 64, and the same materials may potentially be used (although it is to be clearly understood that the material used for the pillars 63 need not necessarily be the same material used for the lid 64). There is one pillar 63 provided for (and beneath) each corner of the rectangular lid 64. Each pillar 63 is actually made up of three sub-pillars, as can be most clearly appreciated from FIG. 13. In the case of each of the pillars 63, the three sub-pillars that make up the pillar are arranged with: one of the sub-pillars right in the corner, i.e. forming a corner sub-pillar a second sub-pillar immediately adjacent (i.e. very close to, if not in direct contact with) the corner sub-pillar on the inward side thereof in the L.sub.1 direction, and a third sub-pillar immediately adjacent (i.e. very close to, if not in direct contact with) the corner sub-pillar on the inward side of the corner pillar in the L.sub.2 direction.
Thus, on each of the pillars 63, the three sub-pillars together define a corner (specifically a right angled corner), and these corners help to correctly and securely locate the support block 66, which is in the shape of the rectangular prism and has dimensions in the L.sub.1 and L.sub.2 directions sized to just fit (i.e. it fits snugly) between the pillars 63, so that the corners of the rectangular support block 66 slot into the corners defined by the pillars 63. The support block 66 will be discussed further below.

(55) As will be appreciated, in simple terms, it is the height of the pillars 63 that defines the size of the vertical separation between the ground plane (base plate 61) and the lid 64. The height of the pillars 63 therefore plays a significant role in defining (and adjusting their height can be used to tune the antenna by altering) the size in the vertical dimension of the gaps, both along the long and the short sides of the lid component, between the lid component 64 and the ground plane (base plate 61). However, it should also be borne in mind that, in this particular embodiment at least, the base plate 61 has a recessed portion 65, and the pillars 63 are located within this recessed portion 65. Actually, the pillars are located on a very slightly raised platform that is itself formed in the base of the recessed portion 65. Thus, the pillars 63 extend between the upper surface of the base plate 61 where they connect to the base plate 61, which is within the recessed portion 65, on the slightly raised platform portion, and the underside of the lid 64. It is therefore perhaps correct to say that, in this embodiment, it is the vertical height of the pillars 63, together with the depth of the recess 65 (and the height of the raised platform) in the base plate 61, that defines the “effective” vertical dimension/size of the long side (and short side) gaps, namely the gaps on the long and short sides between the lid 64 in the upper surface of the base plate 61 on the portions of the base plate that surround the recessed portion 65.

(56) In fact, it is actually thought that the recess 65 in the base plate 61, as well as providing structural outer support for the cover 62, also has some influence on the antenna's radiative properties. In particular, it is thought that the depth of the recess 65, and more specifically the consequent height of the short, vertical perimeter wall of the recess 65, may influence how much the antenna's radiation is concentrated below the angle of elevation of the path of maximum gain (all around the antenna in the azimuth plane). Concentrating the antenna's radiation low down, including below the angle of elevation of the path of maximum gain, is advantageous for reasons that have been explained previously. It is thought that if the depth of the recess is made greater (deeper), such that the height of the perimeter wall of the recess is made greater (higher), this may have the effect of concentrating more of the antenna's radiation below the angle of elevation of the path of maximum gain. Conversely, if the depth of the recess is made less (shallower), such that the height of the perimeter wall of the recess is made less (lower), this may, it is thought, have the effect of causing less of the antenna's radiation to be concentrated below the angle of elevation of the path of maximum gain. As a further possible alternative, instead of (or possibly in addition to) making the depth of the recess 65 in the base plate 61 deeper in order to increase the height of the recess' perimeter wall and thereby concentrate more of the antenna's radiation pattern down lower below the path of maximum gain, it may instead (or also) be possible to incorporate into the antenna structure one or more additional components or conductive elements that serves as a “wall extension” (i.e. a height extension for the perimeter wall of the recess 65). A single such component or element could be, for example, a narrow strip of metal (or conducting material) formed into a “loop” that is placed onto the base plate 61 immediately above the perimeter wall of the recess 65 and which extends around in the shape of and immediately above the perimeter wall of the recess 65, such that the inner surface of this loop effectively forms an extension of (i.e. it increases the effective height of) the perimeter wall of the recess 65 itself. Alternatively, as it may not be necessary or important to provide a height extension for those parts of the recess perimeter wall which are at (or below) the short end gaps (i.e. below the short end edges of the lid), because the short end gaps are non-radiating, it may therefore be possible to provide, say, a pair of narrow strips of metal (or conducting material) which are placed onto the base plate 61 immediately above those parts of the recess perimeter wall which are at (or below) the long side gaps (i.e. below the long side edges of the lid) and which extends along and immediately above the long edge lengths of the perimeter wall of the recess 65, such that the inner surfaces of these strips effectively form extensions of (i.e. they increase the effective height of) the long edge lengths of the perimeter wall of the recess 65. Such component(s) or element(s) could be provided as separate, additional component(s) of the antenna structure, or alternatively it/they could be incorporated into one of the other components, such as by being incorporated into the cover 62, such that the component(s) become correctly positioned relative to the perimeter wall of the recess 65 when the cover 62 is installed. In any case, providing such component(s)/element(s) (or something similar) may serve to effectively increase the height of the (relevant parts of the) perimeter wall of the recess 65, without necessarily increasing the actual depth of the recess 65 itself (or not by as much as the height of the (parts of the) wall is effectively increased), and thereby helping to cause more of the antenna's radiation to be concentrated at an angle of elevation below the path of maximum gain.

(57) It should also be recognised, however, that the extent to which the depth of the recess 65 can be increased (i.e. made deeper), or effectively increased by the introduction of additional component(s)/element(s) (say), etc, may be limited due to the very limited overall height of the antenna structure and its components, which may actually allow for only limited variability/adjustment in this regard. Also, it should also be borne in mind that, because it is thought to be the long side gaps that resonate, and because the resonant properties of these are thought to be determined not only by the length in the L.sub.2 dimension of the lid (or the distance between the pillars 63 in the L.sub.2 dimension) but also at least in part by the vertical separation between the ground plane (base plate 61) and the lid 64 (which is essentially what defines the effective height of the long side gaps as discussed above). Therefore, because the height of the long side gaps is also thought to be important in determining (and providing) the antenna's resonant properties, the extent to which alterations may be made which affect this height (i.e. the height or effective height of the long side gaps) may be further limited by the need or desire not to overly impede or compromise these resonant properties for the antenna's tuning.

(58) On each of the four pillars 63, there are small round detents or lugs on the top of each of the three sub-pillars. Also, in each corner of the lid 64, there are three holes all of a diameter corresponding to the diameter of the lugs on top of the sub-pillars, and the three holes in each corner of the lid 64 are formed in a corresponding arrangement to the arrangement of the lugs on top of the sub-pillars on the respective corresponding posts 63. Therefore, when the lid is placed on top of the pillars 63, the lugs on top of each pillar insert into the holes in the respective corners of the lid thereby correctly locating the lid 64 relative to the pillars 63 (and relative to the recessed portion 65 in the base plate 61, etc). Note that the corners of the lid, where the pillars connect thereto, are locations of ground potential (or nulls) in the lid, and it is significant that the pillars connect at locations of ground potential or nulls.

(59) The antenna pillars 63 (or one or more of them, or one or more sub-pillars of one or more of the pillars 63) may be hollow along the length thereof. For example, there could be a through-bore extending axially through the (or each) relevant sub-pillar. This hollow interior extending through the one or more sub-pillars may provide one or more conduits for cables, wires or the like to extend from below the base plate 61 (or otherwise below the ground plane) and connect to any electronic parts and/or equipment that may be located, say, in a space that may be provided above the lid 64 but below the underside of the protective cover 62. A space for other electronic parts and/or equipment might also or instead be provided, say, adjacent but just outside/beyond the short side gap on one or both ends of the lid 64, but still within the confines of the cover 62 when the coverage installed. Or indeed, electronic parts and/or equipment could also be located at a range of other locations provided this does not substantially interfere with the radiative properties of the main antenna. These electronic parts and/or equipment could include any electronics associated with the RFID reader, e.g. like a modem or filters or amplifiers or the like, or communication equipment such as a supplemental Wi-Fi or Bluetooth antenna, etc, or illuminating component as discussed elsewhere herein.

(60) It was mentioned above that the RFID reader antenna structure includes a support block 66. It was also explained that this support block is sized so as to fit snugly between the corners defined by the respective pillars 63. The support clock 66 resides beneath the lid when the antenna structure is assembled, and together with the posts 63, the support block 66 helps to provide structural support for the lid 64. Because the support block 66 is located beneath the lid 64, the support block 66 must of course be installed on the base plate 61 between the pillars 63 before the lid 64 is mounted on top of the pillars 63. Actually, when the antenna structure is assembled, after the base plate 61 has been initially installed on the road and the pillars 63 have been installed on the base plate 61, the support block 66 can then be inserted between the pillars 63, as discussed above. The thickness of the support block 66 in the vertical dimension is such that the support block 66 fills (in the vertical direction) the space between the underside of the lid 64 and the upper surface of the (slightly raised platform within the recess 65 in the) base plate 61.

(61) Therefore, as mentioned above, the pillars 63 and the support block 66 together help to provide structural support for the lid 64 in its position mounted above and parallel to the ground plane. As mentioned above, the posts 63 will typically be made from metal, and they therefore provide a quite ridged support beneath each of the four corners of the lid 64. The support block 66, which fills the entire space inside the corners defined by the pillars 63 and between the base plate 61 and the underside of the lid 64, and which is therefore in contact with both the base plate 61 and the underside of the lid 64, may be made from a wide range of different materials. The support block 66 is not a conductive or radiating component of the antenna, and it should therefore be substantially non-conductive (or at least substantially non-conductive at the frequencies at which the antenna operates). Preferably, the support block should be made from a material that has appropriate dielectric properties, preferably a low dielectric constant with uniform dielectric properties throughout the material. Also, in order to help support the lid 64 above, and specifically in order to help support the inner portions of the lid 64, inwards from the four (rigid/stiff) corner pillars, against downward deformation (as may occur when a large load is applied from above, as when a vehicle runs over the antenna, etc) the support block should be made from a solid material of some kind. However, the support block 66 need not necessarily be a highly rigid material (i.e. not necessarily like the strong material from which the protective cover 62 is formed, or anything like that). Instead, the support 66 may be made, and indeed it may be desirable for it to be made, from a material which, whilst solid, also has a reasonable degree of resiliency or “give”. Possible examples of such materials include closed cell foams like styrofoam or the like, or paper or cardboard formed with a cellular (or honeycomb) like structure, or indeed possibly a range of other materials of the kind commonly used as padding in packaging around objects, consumer appliances and the like when they are shipped. The reason materials such as this, which are solid but which also have a reasonable degree of give or deformability, may be suitable (or even desirable) can be understood by first remembering that the lid 64 is a fairly stiff (typically metal) plate. The lid 64 also lies directly on top of the support 66 and the underside of the lid 64 is in contact with the entire upper surface (or most of it) of the support 66. Therefore, when a vertically downward load is applied to the antenna structure, and if this is sufficiently large to cause deformation of the protective cover 62 and also of the lid 64 underneath (and anything located in between the underside of the cover 62 and the upper surface of the lid 64), then if this load causes downward deformation or flexure of the lid 64, even if the load (after passing through or being transmitted by the cover 62 etc) becomes applied only to a small/localised region somewhere in the middle of the lid 64 (between the corners which are supported by the stiff pillars 63), the fact that the lid 64 is itself fairly rigid will help to cause that localised load to be distributed and borne by a much greater area of the support 66 underneath. This will, in turn, cause that greater area of the support 66 to become compressed, and the compression may also spread out through the material of the support 66 in such a way that an even larger proportion (if not the whole of) the support 66 underneath the lid 64 helps to bear the load (even if the load is applied as a fairly localised load where it is transmitted onto the lid 64).

(62) It was mentioned above that it may in fact be preferable for the support 66 to be made from a material which, whilst solid, also has a reasonable degree of resiliency or “give”. The reason this may be preferable over, say, a highly rigid material is because highly rigid materials are (generally by their nature) less resilient (i.e. less flexible or able to deform). Many are even brittle or susceptible to fracture. As a result, if a highly rigid material were to be used for the support 66, this could potentially be susceptible to cracking, or possibly to fatigue failure over time. Therefore, whilst no limitation whatsoever is to be implied as to what material may be used for the support 66, it is considered that it may often be preferable for the material to have a degree of resiliency or give, rather than being very highly rigid, as this may in fact perform better in providing support beneath the lid 64.

(63) The protective cover 62, which as mentioned above, takes the form of a transparent, generally flat and rectangular “dome” made of a strong/structural (and transparent or translucent) material such as polycarbonate or the like, is installed over the top of the lid 64, and hence over the support 66, pillars 63, etc, located beneath the lid 64. The protective cover or “dome” 62, as well as serving a structural protective function, may actually also function as a radome. (According to Wikipedia: “a “radome” (which is a portmanteau of radar and dome) is a structural, weatherproof enclosure that protects a (e.g. radar) antenna. [A] radome is constructed of material that minimally attenuates the electromagnetic signal transmitted or received by the antenna”. However, further in addition to this, the protective cover 62 may also serve (along with the ground plane) to lower the antenna's radiation pattern (i.e. reducing the elevation angle of the path of maximum gain and directing the bulk of the radiation (i.e. concentrating the radiation) to the area below the path of maximum gain, between the path of max gain and the ground plane).

(64) Incidentally, the path of maximum gain, in elevation, of the antenna's radiation pattern, and the radiation distribution above and below the path of maximum gain, are significantly influenced by the height of the long side gaps (this has been explained previously) and also significantly by the ground plane which is proportionally much more massive than the lid component. However, in further addition to this, the material thickness and the angle of attack (i.e. the angle of slope) of the long side edges of the cover 62, and the dielectric value of the material from which the cover 62 is made, may (it is thought) further effect the elevation angle of the path of max gain and the radiation distribution above and below the path of maximum gain. Therefore, these properties, namely the angle of slope of the long side edges of the cover 62, and the material thickness of the cover along these long sides, and the dielectric value of the material from which the cover is made, are further properties that can potentially be altered or modified in order to tune the antenna or alter its radiation pattern. However, again, the extent of possible alteration or variation that may be possible may often be limited by other considerations. For example, the ability to alter or make adjustments to the angle of attack (i.e. the angle of slope) of the long side edges of the cover 62 may be restricted significantly by the need to maintain an angle of slope which provides adequate safety for vehicle wheels which might contact and roll over the cover 62, and this may also be affected by the provisions of applicable road safety regulations and the like.

(65) In order to fit over the other components, the protective dome 62 has a generally rectangular-prism-shaped opening formed in its underside. This opening in the underside of the dome 62 is most clearly visible on its own in FIG. 13. The way in which the other components of the RFID reader antenna are received within this opening in the underside of the dome 62, when the dome is installed thereon, is clearly shown in FIG. 11, FIG. 12 and FIG. 14. Thus, when the dome 62 is installed over the other components, the outer perimeter portions of the dome 62 (i.e. those perimeter portions which surround and between them define the opening in the underside of the dome 62) extend down and cover the top and sides of the other components. In fact, the dome is mounted in contact with the base plate 61 in a manner that forms a seal preventing the ingress of moisture, dirt or other contaminants into the inside thereof where the other components are housed. Appropriate sealants or adhesives may be used to form this seal between the peripheral undersides of the dome 62 and the base plate.

(66) The way in which the outer perimeter base portions of the dome 62 are supported by the vertical sides of the recess 65 in the base plate 61 has been explained above.

(67) It is an important aspect of the design of the RFID reader antenna in this particular embodiment that when the RFID reader antenna structure is fully assembled (i.e. when the dome 62 has been finally installed to form a protective cover over the other assembled components), the total “real” height of the resulting structure is less than 25 mm, more preferably around 20 mm. In this regard, the “real” height means the vertical distance between the upper surface on the base plate 61 in the areas immediately surrounding (i.e. on the outside of) the dome 62 and the top surface of the dome 62. By way of example, if the “real” height of the assembled antenna structure is 20 mm, the actual height of the dome 62 could be a few millimetres greater than this, however it will be noted that, like other parts of the antenna structure, the dome 62 is received into the recessed portion 65 in the centre of the base plate, so even if the vertical height of the dome 62 is slightly greater than 20 mm (perhaps 21-23 mm), nevertheless the “real” height of the overall antenna structure (which is the height that it will appear to have from the point of view of a vehicle approaching it) will still only be 20 mm.

(68) Limiting the height of the overall RFID reader antenna structure to less than 25 mm, and preferably around 20 mm, is important because, as discussed above, government and regulatory authorities responsible for authorising the installation and/or use of any form of equipment (or objects of any kind) on or near public roads are often highly conservative and therefore highly wary of allowing the installation and/or use of new types or forms of equipment which have not been used previously on public roads, particularly if the form (i.e. the size and/or shape and/or general configuration or appearance, etc) of the new equipment is unfamiliar, unconventional or different to types or forms of equipment that have previously been authorised for use. However, in this regard, in most countries/jurisdictions, the regulatory authorities responsible for authorising the installation and use of equipment on roads have granted permission for the installation and use of conventional retroreflective (“cat eye”) road markers, like those depicted in FIG. 9 and FIG. 10, and these are indeed used extremely widely. Importantly, the height of these conventional retroreflective road markers is typically about 25 mm. Thus, the RFID reader antenna structure presently described will have a height no greater (and possibly less than) that of conventional retroreflective road markers which are widely authorised for use, commonly accepted and used.

(69) It should be noted that, in a direction parallel to the direction in which vehicles travel along a road, the total length of the protective cover/dome 62 will often be considerably longer (typically several times longer) than the typical length in this direction of a conventional retroreflective (“cat eye”) road marker like the ones shown in FIG. 9 and FIG. 10. However, in a direction perpendicular to the direction which vehicles travel along the road (i.e. in a direction across the road), the total width of the protective cover/dome 62 will be roughly the same as (or possibly smaller than) the width of a conventional retroreflective road marker. And importantly, from the point of view of an oncoming vehicle (or the vehicle's drivers), it is the width (i.e. the size in a direction across the road), as well as the height, of an object on the road that determines the apparent size of that object (i.e. it is the width and height of the object on the road that largely determines how big that object appears to be from the point of view of the driver of the oncoming vehicle). The length of the object in a direction parallel to the direction of vehicle travel is generally much less significant in providing the driver of an oncoming vehicle with an appreciation for the size of an object they are approaching on the road, and in fact given the viewing angles involved when the object is viewed by the driver from a distance away from the object, the driver may not even be able to fully appreciate how long the object is in the direction parallel to the direction of vehicle travel. Therefore, even though the protective cover/dome 62 of the antenna structure in the present embodiment, which is what determines its apparent size from the point of view of a driver of an oncoming vehicle, is longer than a conventional retroreflective road marker, nevertheless this is much less significant (and it may not even be noticed) by the driver, who will comprehend the size of the object (the cover 62) based on its width and height, and from this it (the cover 62) will appear to be essentially little or no different in size and shape than a conventional retroreflective road marker (which they are perfectly accustomed to seeing and driving over).

(70) To put this another way, in the example presently described, the RFID reader antenna structure is installed such that it is one of the short edges of the rectangular RFID reader antenna structure (i.e. one of the edges parallel to the lid's L.sub.1 dimension) that points along/up/down the road. Therefore, from the point of view of a vehicle (and its driver) approaching the RFID reader structure, it is this short edge (and in particular the short edge of the cover 62) that the vehicle (and its driver) will “see”. For reasons discussed above, even for a given length of the lid 64 (L.sub.2, which is determined according to antenna operating frequency), the antenna with (L.sub.1) can still vary. However, it is anticipated that the width (L.sub.1) of the lid 64 will often be less than 100 mm, and often less than 90 mm (widths of around 75 mm to 80 mm are expected to be typical). As can be seen from FIG. 12 and FIG. 13, the width of the cover/dome 62 parallel to the lid's L.sub.1 dimension will be somewhat larger than the lid's L.sub.1 dimension. This is because the dome 62 extends beyond, and overhangs, the lid 64 on both sides in the L.sub.1 direction (in fact the dome 62 overhangs the lid on all sides). Nevertheless, if it is assumed that the width of the lid 64 is 80 mm and that the dome 62 extends beyond this by 20 mm on either side in the L.sub.1 dimension, this means that the total width of the RFID reader antenna structure “as seen” (i.e. from the point of view of) an approaching vehicle will be approximately 120 mm. This, again, is approximately the same as the width of conventional retroreflective road markers which are widely authorised for use, commonly accepted and used.

(71) It is also important that the edge of the structure which a vehicle “sees” as it approaches, namely the forward facing edge of the cover 62, is a straight edge (i.e. this edge extends in a straight line across the road from the point of view of an oncoming vehicle). This is important because this is actually quite different to, e.g., the alternative RFID reader antenna structure previously proposed in patent application '994 above, which was an RFID reader antenna structure having an overall circular plan form shape. As a consequence, in the case of the RFID reader antenna structure previously proposed in patent application '994, the edge of the structure which a vehicle would “see” as it approached along the road is a curved, rather than straight, edge. And in fact, in the case of the RFID reader antenna structure previously proposed in patent application '994, the edge of the structure which the vehicle's wheel/tire would initially struck/contact upon driving over the antenna structure would also (naturally) be a curved, rather than straight, edge. For vehicle such as cars, trucks and the like, this is not perceived to be a significant problem. However, there is at least a perception that this might be problematic for vehicle such as, for example, motorcycles, bicycles and the like, for which there is perceived to be a danger that if the vehicle's front wheel were to strike the curved edge at a slight angle (i.e. at an angle other than perfectly “direct on” to the edge), this could cause the vehicle's front wheel to be knocked off course, potentially leading to accident and injury. However, in the antenna structure in the embodiment presently described, this problem is resolved or moot, because the edge of the structure which a vehicle “sees” as it approaches (i.e. the forward facing edge of cover 62) is a perfectly straight edge extending directly across the road, and so again, the antenna structure in the present embodiment should be perceived to create no more danger on the road than a conventional retroreflective road marker of the kind commonly accepted and used (and which are deemed not to pose an unacceptable risk).

(72) Furthermore, it can be seen from FIG. 11, FIG. 12, FIG. 13 and FIG. 14 that the sides of the dome 62, whilst straight along their length, are not simply straight, vertical sides. Rather, there is at least an upper portion on each of the sides of the dome 62 (and this typically extends for more than half the height of the dome) which slopes inwardly and upwardly. It should generally be the case that the amount by which the dome 62 extends out and overhangs the other components of the antenna is sufficient to allow these sloped portions to have a slope of about 45° or less relative to the plane of the base plate/ground plane/road. This (along with the height which is limited to 25 mm or less) may help to allow the wheels of cars and other road going vehicles to roll over the said devices without an undue jolt or impact. And again, this angle of slope of the upper portions on the sides of the dome 62 is similar to that which is widely used and accepted (and deemed not to pose an unacceptable risk) on conventional retroreflective road markers. Also, as mentioned above, in addition to helping the wheels of cars and other vehicles roll over the device without an undue jolt or impact, the angle of attack (i.e. the angle of slope) of the sloped portions of the long side edges in particular of the cover 62, and the material thickness along the long sides and the dielectric value of the material from which the cover 62 is made, may effect the elevation angle of the path of max gain in the antenna's radiation pattern and the radiation distribution above and below the path of maximum gain.

(73) Note that, whilst it has been suggested that polycarbonate, or acetal, or the like, may be a particularly suitable material to use in making the protective cover/dome 62, no absolute limitation is to be implied in this regard. Indeed, there are potentially a range of other structurally strong and dielectrically suitable materials that could also be used, and any of these may indeed be used.

(74) Without limitation to the foregoing, it has been mentioned that the reason why polycarbonate has been selected as one possible material from which the protective cover (dome) 62 may be made is due to the strength of this material (and also its durability, toughness, resistance to UV on other elemental degradation) and consequently the protection it can therefore provide for the lid 64 and other components of the antenna covered thereby. However, the use of polycarbonate may have the additional benefit that this material can be made 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 that may be provided in or as part of the RFID reader, 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 and even from a distance away from the RFID reader (especially at night or in low light conditions). These lights or LEDs (or indeed other electronic components) could be housed in a small space that may (sometimes) remain between the upper surface of the lid 64 and the underside of the dome 62, or possibly they could be mounted inside hollows or openings formed in one or more of the peripheral portions on the dome, i.e. horizontally out from the other antenna components that have been described. In any case, such lights or LEDs could be used, for example, to provide indications as to the current operational status of the RFID reader or individual parts or functions of it. For instance, as a simple example, a red light/LED could be provided which “turns on” in situations where there is a fault or malfunction or warning associated with the operation of the RFID reader (e.g. where there is a component malfunction, or a power supply failure or disruption, or an “almost empty” battery or backup battery, etc). However, such lights, LEDs or the like which may be contained within (but visible from without) the RFID reader might also be used for a range of other purposes. For example, because the RFID reader in these applications is positioned on the surface of the road (i.e. on the surface on which vehicles are travelling and to which the vehicle's drivers are paying close attention), LEDs or lights in the RFID reader may also be used to provide various forms of signalling to vehicles. For example red and green lights could be used for indicating lanes that are open or closed for vehicle travel, or for indicating the permitted direction of travel in a lane (this last might be useful e.g. in places which implement “tidal flow” traffic management which facilitates vehicular travel, within a given lane, in different directions at different times of day, to help accommodate large volumes of traffic flow in one direction or other at different times of day). There could also be other possible uses, for example a flashing light could be used to provide a warning to road users of an upcoming incident or danger further down the road. Or, red, yellow and green signals could be provided in an RFID reader located just before an intersection with traffic lights, and the red, yellow or green lights in the RFID reader could be changed instantaneously/simultaneously and correspondingly with the change in signal at the traffic lights. The illumination of, or light signals emitted from, any lights or LEDs inside the RFID reader could also be visible and detectable to cameras or other imaging devices, for example those located at the side of the road and used for law enforcement or traffic management purposes. It will be appreciated that the possible uses mentioned above for lights, LEDs or the like which may be provided in or as part of the RFID reader are merely examples, and there may be many other uses or applications for this.

(75) Where the cover 62 is instead made from a material, like e.g. acetal, which is not necessarily transparent or translucent, light guides may be provided within the cover 62 to still allow LEDs or the like to be used in a similar manner to that described above.

(76) It is to be noted now that FIG. 14 is a view of an RFID reader which incorporates the proposed antenna as well as other RFID reader equipment that is not shown in FIG. 11, FIG. 12 and FIG. 13. It should also be noted from the outset that FIG. 14 depicts a situation where at least some parts of the RFID reader, and other associated equipment, are located at or below the level of the road surface, whereas other parts (particularly parts associated with the antenna that have been described in detail above) are located on or above the level of the road surface. And as will be readily appreciated, FIG. 14 is a side-on cross-sectional view, and hence parts of the RFID reader as well as other associated equipment which are located both above and below the level of the road surface can be seen. The particular parts and electronics of the RFID reader shown in FIG. 14 will not be discussed in detail herein; however these are essentially the same as (or at least similar to) the parts and electronics associated with the RFID reader described in earlier patent application '994.

(77) Whilst FIG. 14 depicts a scenario where at least some (and in that case most) of the parts and electronics associated with the RFID reader are buried beneath the level of the road, below the antenna, it is to be clearly understood that no limitation is to be implied as to what the various parts and electronics are, and how and where they may be mounted. Hence, parts and electronics associated with the RFID reader need not necessarily be buried beneath the reader antenna. Indeed, in other embodiments, electronics associated with the RFID reader could, instead, be located (say) to the side of the road and connected to the antenna located in the middle of the road (or the road lane) by wires or cables installed into small slots or channels which are initially cut into the road and then covered over after the cables have been installed.

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

(79) In terms of powering the antenna (and the other electronic components incorporated in or associated with the RFID reader), this may be done in any manner of ways. For example, by using an induction loop, or by connecting one or more current (power) carrying cables directly to the RFID reader structure. Such current (power) carrying cables could be installed in shallow slots or trenches formed in the road (e.g. cut/dug in the road and then covered over after the cable has been laid).

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

(81) Turning now to FIG. 16 and FIG. 17, these provide graphical representations of the “shape” of the radiation pattern produced by antennas in accordance with embodiments of the present invention. Note that the radiation patterns represented in FIG. 16 and FIG. 17 were produced using a mathematical model; however actual measurements taken from actual prototype antennas in accordance with embodiments similar to the one depicted in FIG. 11 to FIG. 15 appear to confirm the accuracy with which the mathematical model represents actual (real-world) antennas in accordance with embodiments of the invention.

(82) Referring first to FIG. 16(i), this is an illustration (i.e. a “wireframe” visualisation) of the geometry of the nodes used in mathematically modelling one particular antenna, and the radiation pattern representations in FIG. 16(ii)-(vii) were produced from this particular mathematical simulation. Note that there is nothing actually shown in FIG. 16(i) which graphically represents the antenna's ground plane; however this is not to suggest that the ground plane is not represented in the mathematical model. In any case, it will be easily appreciated from FIG. 16(i) how the geometry of the nodes in the mathematical model (as represented in the “wireframe” visualisation) correspond to the geometry of the rectangular (L.sub.1×L.sub.2) lid component 64 supported on the pillars 63 at the four respective corners in the particular antenna being simulated.

(83) In the remainder of FIG. 16: FIG. 16(ii) and FIG. 16(iii) are plan form views (i.e. “top-down” views from directly above) of graphical representations of the simulated antenna's radiation pattern, and if the antenna simulated in these views is considered to be located on the surface of a road in the centre of a road lane, the direction of vehicle travel on the road lane would be horizontally from right to left (or left to right); FIG. 16(iv) and FIG. 16(v) are end-on views of graphical representations of the simulated antenna's radiation pattern, i.e. as if looking at the antenna's radiation pattern in a direction along/down the road in the direction of vehicle travel; and FIG. 16(vi) and FIG. 16(vii) are side on views of graphical representations of the simulated antenna's radiation pattern, i.e. as if looking at the antenna's radiation pattern in a direction across the road, perpendicular to the direction of vehicle travel.

(84) As the various views in FIG. 16 illustrate, the simulated antenna's radiation pattern has a shape that extends further across the road (or more in a direction perpendicular to the direction of vehicle travel on the road) than down/along the road. In other words, the antenna emits more energy, or a greater energy density, transversely across the road than it does along the road. And as explained in the Background section above, the effect this may have is that, as a consequence of the geometries of a vehicle's RFID tag antenna radiation pattern and of the RFID reader antenna radiation pattern (whose radiation pattern is depicted in these views), and as a result of the interaction between the two, the effective read zone should, for example, cover the full width of the road lane, as shown in FIG. 7(ii), despite any increased directionality of a vehicle's tag antennas' radiation (again, discussed above).

(85) Turning now to FIG. 17(i), like FIG. 16(i), this is an illustration (i.e. a “wireframe” visualisation) of the geometry of the nodes used in mathematically modelling one particular antenna, and the radiation pattern representations in FIG. 17(ii)-(iii) were produced from this particular mathematical simulation. However, a very important thing to note about FIG. 17(i) is that the actual geometry of the nodes represented is different to the geometry represented in FIG. 16(i). More specifically, in FIG. 17(i), the shape/geometry with which the lid component 64 is simulated, as defined by the length:width (i.e. L.sub.1:L.sub.2) ratio of its rectangular shape, is different to the shape/geometry with which the lid component 64 is simulated in FIG. 16(i). Thus, FIG. 17(i) shows that the particular antenna simulated therein and whose radiation is represented in the other view is FIG. 17 has a different geometry to the antenna simulated and represented in FIG. 16, and this is why the shape of the radiation pattern depicted in FIG. 17(ii)-(iii) differs from the shape of the radiation pattern depicted in FIG. 16(ii)-(vii). And indeed, a comparison of FIG. 16 with FIG. 17 provides an example of the way in which the geometry of the present antenna (and in particular the relative length:width ratio of the antenna's rectangular lid component) can be altered in order to alter the shape of the radiation pattern produced by the antenna. In the particular example given by FIG. 17, the particular antenna simulated therein has a lid component geometry that is thinner (i.e. narrower in the L.sub.1 dimension) than the particular antenna simulated in FIG. 16, and the result of this geometry change is (at least in simple terms) to cause the antenna's radiation pattern to extend even more across the road (or even more in a direction perpendicular to the direction of vehicle travel on the road) and even less down/along the road in comparison.

(86) An important point to note, in the simulations in both FIG. 16 and FIG. 17, is that the radiation pattern has a “null” (or at least a virtual/effective null) located above the geometric centre of the lid component—this can be seen most clearly in FIG. 16(ii) and FIG. 17(ii). The reason this is important is because it means that, moving inward towards the centre of the antenna in any radial direction, the overall shape of the antenna's radiation pattern effectively “curves over” (or the density of the energy in the radiation pattern effectively drops off) approaching this geometric centre/null location. And the effect of this is that the amount of energy that the antenna radiates in a vertical upward direction is limited, which is important in order to prevent e.g. blinding reflections from the undersides of vehicles (as has been discussed elsewhere).

(87) Another point to be made is that, whilst the antenna's radiation pattern may be described as extending further in one direction than another (i.e. more across the road (or more in a direction perpendicular to the direction of vehicle travel on the road) than down/along the road), and whilst the various views in FIG. 16 and FIG. 17 may appear to show that the radiation pattern consequently has a generally elliptical shape, in fact (i.e. in reality) the radiation pattern does not actually have any definite edge or boundary. Therefore, it is not correct to say that something is either inside, or outside, the antenna's radiation pattern. The antenna's radiation pattern (at least in a theoretical sense) actually extends in all directions and into all regions of space around the antenna (theoretically to infinity—i.e. the radiation pattern theoretically does not ever stop or end). However, the strength (or the energy density) of the antenna's emitted radiation drops or becomes lower (quite quickly) as distance from the antenna increases, and also energy is not radiated out by the antenna with the same/equal strength or intensity in all directions. On the contrary, energy is radiated by the antenna much more strongly in some directions and much less strongly in other directions. Thus, the seemingly elliptical shape of the antenna's radiation pattern is related to (or it comes about partly as a consequence of) the directions extending outward into the regions of three-dimensional space around the antenna where the density of the energy radiated by the antenna is greatest (i.e. the long axis of the ellipse generally corresponds to the direction in which the antenna emits energy with the greatest intensity—but see below for further discussion on the edge/boundary of the elliptical shape).

(88) Following on from the above, whilst in theory the antenna's radiation pattern may be considered to extend to infinity, nevertheless due to the nature of digital electronics, there is (or there may be said to be) an edge or boundary within the antenna's radiation pattern, which may (in this instance) be thought of as defining the outer edge or boundary of the radiation pattern's elliptical shape. This edge or boundary is not, however, a feature of the radiation pattern itself, for the reasons discussed above. Rather this edge or boundary becomes defined as consequence of the relationship between the energy radiated by the antenna (as an RFID reader antenna) and the operation of an RFID tag that exchanges information with the (RFID reader) antenna. More specifically, the said edge or boundary within the (RFID reader) antenna's radiation pattern takes its shape (i.e. the surface shape of the ellipse e.g. as depicted in the Figures in this case) and it is defined by the locus of points in three-dimensional space where the density of the energy radiated the (RFID reader) antenna becomes great enough to communicate with an RFID tag that is within the (RFID reader) antenna's radiation pattern. This may be conveniently explained with reference to so-called passive RFID tags, although it is to be clearly understood that the present invention is by no means limited to use with only passive RFID tags (i.e. the invention could also be used with so-called active RFID tags and indeed any other forms of RFID tags). A passive RFID tag is an RFID tag that does not contain its own battery or other power source. Instead, a passive RFID tag is itself (i.e. the tag's antenna and also all of the tag's operating electronics are) powered by the energy radiated by the RFID reader antenna. Now, due to the nature of digital electronics, there will always be a certain minimum amount of power that is required in order to operate a given passive RFID tag (e.g. to enable it to power on and transmit a signal using its own antenna back to the RFID reader antenna, etc). Naturally, however, the amount of power that is required to operate different passive RFID tags may differ (note that the amount of power that a passive RFID tag requires to power on and operate is often described as the tag's sensitive). Accordingly, some passive RFID tags with lower sensitivity may need more power before they can power up and operate etc, and so these may need to get closer to the RFID reader antenna (where the density of the energy radiated by the antenna is greater) in order to operate and communicate with the RFID reader antenna. On the other hand, other passive RFID tags with higher sensitivity may require less power to turn on and operate, and therefore they may be able to turn on and operate at a greater distance from the RFID reader antenna. The point is that, as a result of this, the above-mentioned edge or boundary within the radiation pattern (i.e. the surface shape of the ellipse of the radiation pattern in this case, in three-dimensional space), which is defined by the locus of points where the density of the energy radiated by the antenna becomes great enough to enable an RFID tag to communicate with the RFID reader antenna, is not actually fixed. Rather, its location (i.e. how far out from the antenna this edge or boundary is) is dependent, assuming the amount of energy radiated by the antenna remains fixed/set, on the sensitivity of the RFID tag. Therefore, in the context of the present invention, the “size” of the ellipse of the antenna's radiation pattern (i.e. how “big” the ellipse is relative to the size of the antenna), assuming a set power output from the RFID reader antenna, will be larger for more sensitive tags and smaller for less sensitive tags.

(89) However, a further point that must then be made is that, when the present invention is put into practice, the RFID tags used on vehicle license plates (regardless of whether they are passive RFID tags or some other form of tag) should have a sensitivity such that the “required read zone” (inside which the RFID reader must be able to communicate with a vehicle's plate-mounted RFID tag if the vehicle's tag is within the said region), the size and shape of which is described above with reference to FIG. 1 and FIG. 5 etc, falls within the ellipse of the antenna's radiation pattern. In other words, the power output from the RFID reader antenna should be such that, and in combination the sensitivity of the RFID tags on vehicle license plates should also be such that, there is no part of the required read zone described above that is outside the edge or boundary of the ellipse of the antenna's radiation pattern.

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

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

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