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WO2016008004A1 - Antenne - Google Patents

Antenne Download PDF

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Publication number
WO2016008004A1
WO2016008004A1 PCT/AU2015/050384 AU2015050384W WO2016008004A1 WO 2016008004 A1 WO2016008004 A1 WO 2016008004A1 AU 2015050384 W AU2015050384 W AU 2015050384W WO 2016008004 A1 WO2016008004 A1 WO 2016008004A1
Authority
WO
WIPO (PCT)
Prior art keywords
antenna
ground plane
slots
rfid
radiation pattern
Prior art date
Application number
PCT/AU2015/050384
Other languages
English (en)
Inventor
Yifan Wang
Albertus Jacobus Pretorius
Seppo Aukusti SAARIO
Amin Mahmoud ABBOSH
Original Assignee
Licensys Australasia Pty Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from AU2014902707A external-priority patent/AU2014902707A0/en
Application filed by Licensys Australasia Pty Ltd filed Critical Licensys Australasia Pty Ltd
Priority to AU2015291782A priority Critical patent/AU2015291782B2/en
Publication of WO2016008004A1 publication Critical patent/WO2016008004A1/fr

Links

Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q9/00—Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements
    • H01Q9/04—Resonant antennas
    • H01Q9/30—Resonant antennas with feed to end of elongated active element, e.g. unipole
    • H01Q9/40—Element having extended radiating surface
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06K—GRAPHICAL DATA READING; PRESENTATION OF DATA; RECORD CARRIERS; HANDLING RECORD CARRIERS
    • G06K7/00—Methods or arrangements for sensing record carriers, e.g. for reading patterns
    • G06K7/10—Methods or arrangements for sensing record carriers, e.g. for reading patterns by electromagnetic radiation, e.g. optical sensing; by corpuscular radiation
    • G06K7/10009—Methods or arrangements for sensing record carriers, e.g. for reading patterns by electromagnetic radiation, e.g. optical sensing; by corpuscular radiation sensing by radiation using wavelengths larger than 0.1 mm, e.g. radio-waves or microwaves
    • G06K7/10316—Methods or arrangements for sensing record carriers, e.g. for reading patterns by electromagnetic radiation, e.g. optical sensing; by corpuscular radiation sensing by radiation using wavelengths larger than 0.1 mm, e.g. radio-waves or microwaves using at least one antenna particularly designed for interrogating the wireless record carriers
    • G06K7/10346—Methods or arrangements for sensing record carriers, e.g. for reading patterns by electromagnetic radiation, e.g. optical sensing; by corpuscular radiation sensing by radiation using wavelengths larger than 0.1 mm, e.g. radio-waves or microwaves using at least one antenna particularly designed for interrogating the wireless record carriers the antenna being of the far field type, e.g. HF types or dipoles
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q1/00—Details of, or arrangements associated with, antennas
    • H01Q1/27—Adaptation for use in or on movable bodies
    • H01Q1/32—Adaptation for use in or on road or rail vehicles
    • H01Q1/3208—Adaptation for use in or on road or rail vehicles characterised by the application wherein the antenna is used
    • H01Q1/3233—Adaptation for use in or on road or rail vehicles characterised by the application wherein the antenna is used particular used as part of a sensor or in a security system, e.g. for automotive radar, navigation systems
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q9/00—Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements
    • H01Q9/04—Resonant antennas
    • H01Q9/30—Resonant antennas with feed to end of elongated active element, e.g. unipole
    • H01Q9/32—Vertical arrangement of element
    • H01Q9/36—Vertical arrangement of element with top loading

Definitions

  • the present invention relates primarily to an antenna with a low physical profile and a particular radiation pattern.
  • the antenna can be placed in or on the surface of a road, a driveway, or the like, and can be used to perform radio- frequency identification (RFID) with RFID capable tags (RFID tags) which are located on the front and/or the back of passing vehicles.
  • RFID tags RFID capable tags
  • the antenna would be a part of a RFID reader which is operable to communicate with RFID tags.
  • the RFID tags will be located on the vehicles' license plates. (Or more specifically, for vehicles which have a license plate on the front and the rear, a RFID tag will preferably be placed on one or both of a said vehicle's license plates, or for vehicles which have only one license plate, a RFID tag will preferably be placed on the single license plate).
  • Vehicle licence plates (and hence the RFID tags) may be mounted to the vehicle centrally, or at an off-centre location, relative to a vehicle's widthwise direction. Also, there will often be variation in the position/trajectory of travelling vehicles; for example within a given lane one vehicle may be travelling quite close to the left- hand side of the lane whereas another vehicle may be travelling closer to the right-hand side (or a vehicle may even intentionally travel between or across multiple lanes in an attempt to avoid detection). Consequently, although the tags on vehicle licence plates may sometimes pass directly over the RFID reader (and its antenna), often the tag on a vehicle may pass to one side or other of the RFID reader. This can have a significant impact on RFID read performance. However, the present invention may be well suited to accommodate such issues.
  • embodiments of the antenna may be particularly suited for use with vertically polarised RFID tags.
  • the use of the antenna for communicating with vertically polarised RFID tags may help to facilitate, for example, a RFID tag read environment which is directionally independent (at least in a plane parallel to the surface of the ground on/in which the antenna is located) and which may thus help to reduce the number of required antennas (the antenna count) and the associated system complexity in applications involving detection of RFID tags on vehicles.
  • the invention is not necessarily limited to use with vertically polarised RFID tags.
  • the antenna could potentially be used in a wide range of other areas and/or applications as well.
  • the antenna could instead potentially find use in side and/or overhead placements to read/communicate with RFID tags on vehicles, or on goods or products which are moving past the antenna (e.g. goods or products being carried past the antenna by a machine, or on a conveyor, etc).
  • the antenna will hereafter be described with reference to, and in the context of, the above application where the antenna is used with vertically polarised RFID tags which are located on vehicle license plates.
  • the invention relates to an antenna for a communication device, wherein:
  • the physical construction of the antenna includes a number of parts, one of the parts is a substantially planar ground plane, and the size of the antenna in a direction perpendicular to the ground plane (i.e. perpendicular to the plane of the ground plane) is less than the largest dimension of the ground plane (e.g. less than the largest of the diameter or length or width, etc, of the ground plane, as applicable); and
  • the antenna's radiation pattern extends further in a direction parallel to (the plane of) the ground plane than it does in a direction perpendicular to (the plane of) the ground plane.
  • the dimension of the antenna (that is to say, its size) perpendicular to the ground plane is less (preferably much less) than the largest dimension (diameter, length, width, etc) of the ground plane.
  • the antenna when it is in use, it will be positioned such that the ground plane is oriented horizontally (e.g. on or parallel to the actual ground, which is part of planet earth) and such that the other parts of the antenna extend vertically upward relative to the ground plane. Accordingly, for convenience (albeit without limitation to the scope of the invention or the orientations in which the antenna may be used), the dimension (i.e.
  • the height of the antenna should be less (preferably much less) than the largest dimension of the ground plane.
  • the antenna should have what may be termed a "height-restricted" configuration, in that the antenna's height should be less (preferably much less) than the diameter or length or width, etc, of the ground plane. In some embodiments, the height of the antenna (i.e.
  • the antenna's dimension perpendicular to the ground plane may be less than half the largest dimension of the ground plane, and preferably less than a quarter of the largest dimension of the ground plane.
  • the height of the antenna may be approximately 10%-20% of the size of the largest dimension of the ground plane.
  • the antenna's radiation pattern should extend further in a direction parallel to the ground plane than it does in a direction perpendicular to the ground plane.
  • the antenna's radiation pattern should extend further in a direction parallel to the plane of the ground plane than it does in the antenna's "height" direction. The reason for this will be explained below.
  • the antenna's configuration i.e. the nature of its physical construction
  • makes the antenna a type or form or species of dipole or monopole antenna i.e. the nature of its physical construction
  • a part of the antenna's physical construction i.e. another part in addition to the ground plane
  • the monopole element may have two ends, namely a first end and a second end.
  • the monopole element may extend between its first end and its second end.
  • the first end of the monopole element may be positioned near, but not in connection with, the ground plane.
  • the second end may be positioned further from the from the ground plane than the first end such that the monopole element is oriented perpendicular to the ground plane.
  • yet another part of the antenna's physical construction may be a substantially planar top load element.
  • This top load element may be connected to the monopole element on the second end of the monopole element, such that the plane of the top load element is substantially parallel to the plane of the ground plane.
  • the antenna might be referred to as a type or species of "top loaded monopole" antenna. It is thought that the inclusion of a top load element connected on the second end of the monopole element (i.e. the end of the monopole element that is further away from the ground plane) may help to restrict the amount of radiation emitted by the antenna in a direction perpendicular to the ground plane.
  • the inclusion of such a top load element may help to ensure that the antenna's radiation pattern extends further in a direction parallel to the ground plane than it does perpendicular to the ground plane (i.e. in the antenna's "height" direction). Also, the inclusion of a top load element may help to maintain radiation efficiency without increasing the "height" of the antenna. This may, in turn, assist with allowing the antenna to have a "height-restricted" configuration.
  • a dielectric material may also be provided on the opposite side of the ground plane from the monopole element.
  • a layer of (typically unbroken) conductive material (which may perform a shielding function) may also be provided on the opposite side of the dielectric material from the ground plane.
  • this optional dielectric material, and likewise the optional (shielding) layer of conductive material may be used in a range of different embodiments. Therefore, for example, one or both of these may form part of an antenna in embodiments like the one described with reference to Figure 24 below, or like the ones described with reference to Figure 25 or Figure 27 below, etc.
  • the ground plane may be thin and circular. That is, the ground plane may be shaped, for example, like a thin disc, or a thin circular plate, or the like.
  • the monopole element may have a short, wide (i.e. squat) cylindrical shape, and the location of the monopole element relative to the ground plane may be such that a cylindrical axis of the monopole element (or a hypothetical extension of such an axis) would extend through the centre of the circular ground plane.
  • the top load element may also be thin and circular.
  • the top load element may also be shaped, for example, like a thin disc, or a thin circular plate, similar to the ground plane, although the top load element may not be (and often will not be) the same size as the ground plane.
  • the top load element is circular, as just described, the location of its connection to the monopole element may be such that the cylindrical axis of the monopole element would extend through the centre of the circular top load element.
  • the antenna may be (configured so as to be) operable for use with a radio signal having a predetermined wavelength ( ⁇ ).
  • ⁇ a radio signal having a predetermined wavelength
  • the diameter of the ground plane may be approximately 2/5 the signal wavelength (2 ⁇ /5).
  • the distance between the ground plane and the top load element in a direction perpendicular to the ground plane
  • the diameter of the monopole element i.e. the dimension of the monopole element parallel to the ground plane
  • the diameter of the top load element may be approximately 1 /3 the signal wavelength (A/3).
  • an antenna is still considered to conform to the relative size proportions above even if the actual relative size of different parts of the antenna (or some of them) is not exactly in proportion with (or varies somewhat from) the relative ratios expressed as exact fractions of signal wavelength ( ⁇ ) above. It should also be noted that some dimensions and/or other design attributes of the antenna may be independent of the signal wavelength ( ⁇ ).
  • antennas in accordance with some particular embodiments of the invention may be operable with signals of wavelength ( ⁇ ) around 350 mm or lower, or in other words with signals of frequency around 860 MHz or higher (although no limitation whatsoever is to be implied from this as to the possible ranges of frequencies/wavelengths with which the antenna of the invention might potentially be used) .
  • Some antennas in accordance with embodiments of the invention may operate with signals around 1 GHz (approximately).
  • the antenna should consequently be sized to fit within a housing having an overall outer diameter of 180 mm or less and an overall height/thickness (i.e. perpendicular to the overall diameter) of 40 mm or less.
  • the antenna's physical construction may further include a plurality of pole elements.
  • Each pole element may have one end connected to the ground plane and an opposite end connected to the top load element.
  • each pole element may extend perpendicularly between the ground plane and the top load element.
  • Each pole element may also have a thin elongate shape (e.g. an elongate cylindrical shape, or an elongate rectangular prism shape, or the like), and the location of each pole element may be outwards (radially outwards if the ground plane is circular) from the centre of the ground plane.
  • pole elements there may be four pole elements located an equal distance from, and at equally spaced locations around, the monopole element, and in some particularly preferred embodiments the spacing between the axial centres of the pole elements may be approximately 1/7 the signal wavelength (A/7) (although again this should be treated as approximate only - see above).
  • one or more (or possibly all) of the pole elements may be hollow. This may help to enable, for example, electronics or other equipment, which may be positioned on the opposite side of the top load element from the ground plane, to be connected with electronics or a power source, etc, located on the opposite side of the ground plane from the top load element. Hence, things like electrical connectors, cables, fibre-optic lines, etc, may extend through the hollow inside of one or more of the pole elements in order to connect such electronics, etc.
  • the ground plane of the antenna may incorporate one or more slots which extend part way through, or all the way through, the thickness of the ground plane.
  • the slots may also extend into, or all the way through, the dielectric layer adjacent the ground plane (if present). It is thought that these slots may function to help compensate/account for the ground effect, including the "near ground” effect".
  • the "near ground effect” is the ground effect caused by the ground (which is part of planet Earth), or by the surface on which the antenna is mounted, in the immediate vicinity of the antenna (e.g.
  • ground effect i.e. the ground effect from the “near ground” in particular may be highly variable and even dynamically variable (i.e. subject to change with time and/or due to changes in conditions, etc). This is discussed further below.
  • a first point is that, when an antenna in accordance with the present invention is used in, for example, a RFID vehicle detection and/or identification application, the antenna is effectively being used in a way that may be considered generally similar or analogous to an antenna in a RADAR transmitter/sensor.
  • RADAR essentially involves a radio signal that is first transmitted by a sensor; that radio signal is then reflected by the object to be observed, and the reflected signal is received and interpreted by the sensor (e.g.
  • a signal may be emitted by an RFID reader (which includes an antenna in accordance with the present invention), and a "reflected" signal may then be sent back from e.g. an RFID tag on a vehicle, back to the RFID reader.
  • RFID both of these signals (i.e. both the signal emitted by the RFID reader and also the "reflected" signal sent back from the RFID tag to the RFID reader) can be modulated to carry information/data (this modulation of data onto the signals is at least part of what distinguishes RFID from traditional RADAR wherein the signals are unmodulated).
  • information can be modulated onto the signal emitted by the RFID reader such that information is sent from the reader to the tag, and similarly information can be modulated onto the signal sent (reflected) by the RFID tag such that information is sent back from the tag to the reader.
  • the exchange of information may be used to perform (and in fact this may be what makes it possible to perform) the identification (i.e. ID detection/recognition) of a specific vehicle.
  • this signal (even if it is an unmodulated signal) may immediately signify the presence of a RFID tag (and hence a vehicle) within the read range of the reader (although which specific vehicle it is - i.e. the specific vehicle identity/ID - may not in this case be determinable, at least not from the signal sent by he RFID tag alone).
  • the way the said signal changes with time i.e. the way the signal which is sent from the RFID tag and received by the reader changes with time, even if it is an unmodulated signal
  • antennas in accordance with the present invention when used in e.g. RFID vehicle detection and/or identification applications, may be used in a similar or analogous way to traditional RADAR antennas (see above), nevertheless at the same time the region within which antennas in accordance with the present invention may need to operate, and the required transmission ranges, radiation pattern shapes, and even the physical position of the antenna (and hence the physical location in which, and from which, the antenna's signal is transmitted) may all be vastly different to antennas used in conventional RADAR. Indeed, for reasons explained in detail below, antennas in accordance with the present invention will often need to be located at ground level, typically on or in the surface of the ground (i.e.
  • the antenna will generally need to be configured to be positioned at (and such that its signal radiation is emitted from) ground level on planet Earth.
  • ground level typically at least 2 wavelengths above the ground (i.e. the height from which a conventional RADAR antenna operates is generally at least twice the wavelength of the RADAR signal it transmits).
  • traditional RADAR antennas are generally not required to accommodate for much (if any) change in signal transmission propagation conditions due to the "near ground effect”.
  • the effect on signal transmission propagation caused by planet Earth may often be assumed negligible or at least constant, e.g. regardless of time and/or position variant changes in weather or ambient conditions or ground conditions.
  • This is very different to the antenna in the present invention which must operate on/in the ground and where the effect on signal transmission propagation caused by the ground on/in which the antenna is located (especially the near ground) can change drastically both between different locations and also dynamically at the same location (i.e. signal transmission propagation conditions can change drastically with time even at a single location, e.g.
  • RADAR antennas generally have a very focussed/directional radiation pattern intended to transmit over large or very large transmission distances (typically in a broadcast manner). So, not only are conventional RADAR antennas normally positioned well above ground level, but they have narrow focussed/directional radiation patterns and transmit over large distances (i.e. they operate in what is often termed the far field - a.k.a. the Fraunhofer region). In contrast, antennas in accordance with embodiments of the present invention may (and typically will) need to transmit over and within a range that is very much closer to the antenna, possibly even within the antenna's radiating near field a.k.a. Fresnel region.
  • antennas in accordance with embodiments of the present invention may (and typically will) need to provide a radiation pattern that is non-focussed, and which extends further in a direction parallel to the plane of the ground plane than it does in a direction perpendicular to the plane of the ground plane.
  • the shape of the antenna's radiation pattern is discussed in detail elsewhere herein.
  • the antenna which is part of an RFID reader located on/in the road surface, may be used to (so to speak) "radar" detect and identify one or more vehicles within a radius of about 5 or 6 m around the antenna, where the RFID tag(s) on the vehicle(s) is/are at or below a height of about 2 m.
  • this example is given to provide context only and should not be considered limiting in any way.
  • the ground plane may in fact incorporate one or more sets of slots.
  • Each said set may comprise a plurality of slots with each slot in a set being approximately parallel to other slots in that set.
  • the slots in each set may also be spaced apart from one another in a direction extending away from the monopole element.
  • two or more sets of slots may be provided.
  • the slots in each set may be substantially arc-shaped, and the arcuate length of the respective slots within a set may increase with increasing distance from the monopole element.
  • a specific embodiment of an antenna incorporating sets of slots, as just described, is discussed further below. It is to be noted that there is no limitation in terms of the way the slots in the ground plane may be formed. For example, the slots may be formed by cutting or otherwise removing material from the ground plane.
  • the slots might be formed by adding additional material to the ground plane in some places such that slots are formed in between thickened areas of the ground plane.
  • the ground plane may be provided with a "corrugated” or “rippled” configuration (the corrugations or ripples might extend in one or more shapes or directions) and areas of the ground plane which are recessed (i.e. troughs, as distinct from raised/elevated areas/peaks) may serve as the slots.
  • a range of other possibilities may also be evident to those skilled in the art. Everything that has just been said about the way in which the slots in the ground plane may be formed also applies equally to the optional formation of slots in a dielectric layer (if present) and/or in a shielding layer (if present).
  • the antenna's radiation pattern should extend further in a direction parallel to the plane of the ground plane than it does in a direction perpendicular to the plane of the ground plane.
  • the antenna's radiation pattern extends further in all directions parallel to the plane of the ground plane than it does in a direction perpendicular to the plane of the ground plane.
  • the arrangement of the one or more slots in the ground plane may be operable to (i.e.
  • the slots may, because of their particular arrangement) help provide at least approximate symmetry of the antenna's radiation pattern about an axis or a plane that is perpendicular to the plane of the ground plane.
  • the arrangement of the one or more slots in the ground plane may also be operable (i.e. the slots, because of their particular arrangement, may function) to help accommodate/compensate for the potentially dynamically variable ground effect, as discussed above.
  • the antenna's radiation pattern may have a generally toroid-like shape. Even more preferably, the antenna's radiation pattern may have a "dropped doughnut" or “toroid on the ground” shape which, compared to the shape of a regular toroid, is lower/flatter in the toroid's axial direction and extends more broadly radially (i.e. similar to the shape of a doughnut that has been dropped flat onto the ground and thus flattened somewhat).
  • a "dropped doughnut” or “toroid on the ground” shape which, compared to the shape of a regular toroid, is lower/flatter in the toroid's axial direction and extends more broadly radially (i.e. similar to the shape of a doughnut that has been dropped flat onto the ground and thus flattened somewhat).
  • One such radiation pattern shape which is considered to be especially desirable/advantageous is discussed further below.
  • Embodiments of the invention have been described above in which the antenna's configuration (i.e. the nature of its physical construction) makes the antenna a type or form or species of dipole or monopole antenna. However, it is also envisaged that there may well be embodiments of the invention that do not fall within this general category. By way of example, embodiments may be provided in which
  • the antenna (again) has a substantially planar ground plane, and the size ("height") of the antenna in a direction perpendicular to the ground plane is less than the largest dimension of the ground plane;
  • the antenna's radiation pattern (again) extends further in a direction parallel to the ground plane than it does in a direction perpendicular to the ground plane; and furthermore another part of the antenna's physical construction is an inverted F antenna element (I FA element) which is shaped like a sideways capital letter "F".
  • I FA element inverted F antenna element
  • a single long side of the F-shaped IFA element which would be the vertical part of the "F” if the "F” were in a normal upright orientation, may be oriented parallel to the ground plane.
  • the F-shaped IFA element may have two “prongs” depending from the single long side, namely a second prong which would be the lower of the two if the "F” were in a normal upright orientation and a first prong which would be the upper of the two if the "F” were in a normal orientation.
  • the second prong may extend vertically downwards from partway along the single long side to near (or through), but not in contact with, the centre of the antenna's ground plane.
  • the first prong may extend vertically down from one end of the single long side and connect to the antenna's ground plane slightly to one side of the second prong.
  • the ground plane may be thin and rectangular.
  • the ground plane may incorporate one or more slots which extend part way through, or all the way through, the thickness of the ground plane.
  • the ground plane may actually incorporate one or more sets of slots, each said set comprising a plurality of slots with each slot in a set being approximately parallel to other slots in that set, and the slots in each set being spaced apart from one another in a direction extending away from the location of the connection between the ground plane and the second prong of the IFA element.
  • two or more sets of slots may be provided, the slots in each set may be substantially arc-shaped, and the arcuate length of the respective slots within a set may increase with increasing distance from the second prong of the IFA element.
  • a specific embodiment of an antenna incorporating sets of slots, as just described, is discussed further below. (It should also be noted that the slots in the ground plane described in this paragraph could also extend into, or through, the thickness of a dielectric layer adjacent the ground plane (if present).)
  • the antenna's radiation pattern should preferably extend further in at least two opposed collinear directions (i.e. forward and backward along a single linear axis) parallel to (the plane of) the ground plane than it does in a direction perpendicular to (the plane of) the ground plane.
  • the arrangement of the one or more slots in the ground plane may be operable to cause the antenna's radiation pattern to extend in the said at least two opposed collinear directions.
  • the arrangement of the one or more slots in the ground plane may also (again) be operable (i.e. the slots, because of their particular arrangement, may function) to accommodate/compensate for the potentially dynamically variable ground effect.
  • another part of the antenna's physical construction may be a substantially planar top load element (or cap).
  • the top load element (cap) in these embodiments may be located on top of the single long side of the F-shaped IFA element, such that the (plane of the) top load element is substantially parallel to the (plane of the) ground plane.
  • the ground plane may be thin and circular.
  • the top load element may also be thin and circular, and the top load element may be positioned with an edge of its circumference above the second prong of the IFA element and with its diameter aligned with the linear axis of the single long side of the IFA element.
  • the ground plane may (again) incorporate one or more slots which extend part way through, or all the way through, the thickness of the ground plane (and/or into or through the thickness of a dielectric layer (if present)). And the arrangement of the one or more slots in the ground plane may (again) be operable (i.e.
  • one or more sets of slots may be provided in the ground plane, each said set comprising a plurality of slots with each slot in a set being approximately parallel to other slots in that set, and the slots in each set being spaced apart from one another in a direction extending away from the location of the connection between the ground plane and the second prong of the IFA element. Two or more sets of such slots may be provided.
  • the slots in each set may be substantially arc-shaped, and the arcuate length of the respective slots within a set may increase with increasing distance from the second prong of the IFA element.
  • the antenna may comprise a first antenna operable for radio communication, and a second antenna may also be provided for data communication in addition to the radio communication performed by the first antenna.
  • a top load element operable for radio communication
  • the top load element be provided with an elongate slot therein.
  • the elongate slot may function as the second antenna. (That is, the second antenna may be a slot antenna.)
  • controlling electronic equipment associated with the antenna may be located on the opposite side of the ground plane (and also on the opposite side of the dielectric layer and/or conductive shielding layer) from the other resonant (radiating) parts/elements of the antenna. This may help to shield the electronics, and the resonant parts/elements of the antenna, from one another. Therefore, for example, in the embodiments of the invention discussed above which incorporate a monopole element, controlling electronic equipment associated with the antenna may be located on the opposite side of the ground plane (etc) from the monopole element. Similarly, in the embodiments of the invention discussed above which incorporate an IFA element, controlling electronic equipment associated with the antenna may be located on the opposite side of the ground plane (etc) from the IFA element.
  • Antennas which conform to the present invention may be operable for use as an antenna of a radio frequency identification (RFID) reader, and the RFID reader may be operable to communicate with RFID capable tags mounted on vehicles (e.g. cars, buses, trucks, motorcycles, etc).
  • RFID tags may be mounted on vehicles' licence plates.
  • the RFID reader may be configured to be positioned on, or partly in, or in, the surface of a road, driveway, carpark, or the like, on which the vehicles travel.
  • the RFID reader may be configured to be positioned to the side of, or above, where the vehicles travel (e.g.
  • the RFID tags may preferably be linearly polarised, and where this is the case the antenna (and the RFID reader with which the antenna is associated) may be correspondingly linearly polarised. Even more preferably, the RFID tags and the antenna may both be vertically polarised. It should be noted that all references herein to polarisation refer to polarisation of the electric field (E-filed) and not polarisation of the magnetic field (H-field).
  • the invention in another possible broad form, may reside in (or provide) a RFID reader including an antenna of the kind described above and wherein the RFID reader is operable to communicate with RFID capable tags mounted on vehicles.
  • the RFID tags may be mounted on vehicles' licence plates, and the RFID reader may be configured to be positioned on, or partly in, or in, the surface of a road, driveway, carpark, or the like, on which the vehicles travel.
  • the power of the antenna and the RFID tags, respectively may be such that the beam shape associated with the antenna's radiation pattern extends approx. (or at least) 5 or 6 m in all directions around the antenna in a plane parallel to the antenna's ground plane. Where this is the case, the majority of the beam shape associated with the antenna's radiation pattern may also be contained within the region 0-2 m perpendicularly to one side of the antenna's ground plane (the same side as the monopole, or I FA element).
  • Figure 1 , Figure 2 and Figure 3 together help to illustrate the importance of beam width and direction in successfully reading (communicating with) a RFID tag.
  • Figure 4 is a plot of the radiation pattern (including the 3 dB beam width) for a directional narrow beam antenna.
  • Figure 5 is a schematic representation of a typical construction of a patch antenna.
  • Figure 6 illustrates, from the side, the use of a RFID reader/antenna located on an overhead gantry to read a RFID tag on a vehicle windscreen and/or license plate.
  • Figure 7 illustrates a RFID overhead reader and a RFID side reader scenario, when viewed in a direction opposite to the direction of travel of the vehicles in the depicted lanes of the road.
  • Figure 8 illustrates the travel path of a windscreen-mounted RFID tag on a vehicle within an overhead RFID reader beam.
  • Figure 9 illustrates factors that contribute to create non-linear variation of the signal between an overhead RFID antenna and a windscreen-mounted RFID tag.
  • Figure 10 shows a vehicle license plate mounted within a cavity to protect it from damage.
  • Figure 1 1 illustrates the travel path of a vehicle's front and rear license plate within an overhead RFID reader beam.
  • Figure 12 helps to explain the required (or at least a desirable) beam shape for a RFID reader antenna placed in/on the road.
  • Figure 13 illustrates, inter alia, the impact of a short following distance between, on the one hand, a classic patch antenna beam shape, and on the other hand, a flat antenna beam shape, as emitted from an in/on-road reader.
  • Figure 14 illustrates one possible example of a vertically polarised horizontal slotted upright antenna.
  • Figure 15 illustrates the radiation pattern of the vertically polarised horizontal slotted upright antenna in Figure 14.
  • Figure 16 illustrates how the ground effect can effect the direction of maximum gain in a radiation pattern.
  • Figure 17 illustrates antenna beams which are pushed upwards due to a conductive ground effect.
  • Figure 18 illustrates the beam shape, in free space, of a hypothetical/idealised upright half-wave dipole antenna.
  • Figure 19 illustrates the read-zone for a RFID enabled vehicle license plate.
  • Figure 20 schematically illustrates the orientation of a RFID tag (which is mounted on a vehicle license plate) within the read-zone of an in/on-road RFID reader.
  • Figure 21 illustrates the effective read-zone for a RFID tag which is on a vehicle license plate when read using a RFID antenna of the kind provided by the invention.
  • Figure 22 illustrates example uses of the kind of antenna provided by the invention, and the resulting effective read-zone, in different scenarios.
  • Figure 23 illustrates the desired radiation pattern for an antenna of the kind provided by the invention.
  • Figure 24 illustrates an antenna in accordance with one possible embodiment of the invention which is suitable for use as an on/in road RFID reader antenna.
  • the antenna has a top loaded monopole configuration with a round and periodically slotted ground plane. This configuration has been found to achieve a radiation pattern as illustrated in Figure 23.
  • Figure 25 illustrates an inverted F antenna with a square periodically slotted ground plane, which has been found to achieve the radiation pattern shown in Figure 26, and where the direction of maximum gain is up and down the road, somewhat similar to the antenna in Figure 14.
  • Figure 27 illustrates an antenna in accordance with another possible embodiment of the invention having an inverted F antenna (IFA) configuration with a round, periodically slotted ground plane, and which has been found to achieve a radiation pattern that is somewhat similar to the one depicted in Figure 23.
  • This antenna also has a slot in the top load cap which may provide an additional antenna which may help to allow or facilitate data communications using WiFi.
  • the radiation pattern for this additional/cap antenna is shown in Figure 28.
  • Figure 29 illustrates possible desirable placements of a RFID reader antenna (or the placement of the device which houses the RFID reader and its antenna) on or in a road surface.
  • Figure 30 illustrates the approximate general dimensions of the antenna in the particular possible embodiment in Figure 24.
  • RFID technology particularly passive backscatter UHF RFID technology, as described by ISO/IEC 18000 part 6, is thought to be suitable for use in vehicle identification.
  • RFID and like terms herein includes passive backscatter UHF RFID technology, as described by ISO/IEC 18000 part 6.
  • the present invention may be useful for all types of RFID, and may also be useful for RF communications between vehicles and a roadside unit.
  • the benefits may be much particularly significant (even disruptive in nature) in that they may (at least help to) overcome several limitations associated with current commercially available electronic vehicle identification systems that use passive backscatter UHF RFID.
  • RFID herein may be thought of as referring primarily (although not necessarily exclusively) to passive backscatter UHF RFID.
  • ISO/IEC 18000 part 6 type C might be considered to be the current de facto standard for passive backscatter UHF RFID technology.
  • Passive backscatter RFID is, in fact, similar in some ways to RADAR (the term "RADAR" actually originated as an acronym of RAdio Detection And Ranging).
  • RADAR essentially involves a radio signal transmitted by a sensor that is then reflected by the object to be observed and the reflected signal is interpreted by the sensor.
  • the signal emitted by the RFID reader, and the "reflected" signal i.e. the signal sent from the RFID tag back to the RFID reader
  • the tag i.e. the signal sent from the RFID tag back to the RFID reader
  • the beam shape of a RFID reader may be defined as the locus of points at which a RFID tag receives enough energy from the reader to switch on and communicate intelligently with the reader. This is generally a sharp, but moving edge due to the nature of digital electronics and electric field propagation. Normally, the beam shape closely follows (or is closely related to) the radiation pattern of the reader antenna. It is for this reason that RFID systems are often designed with only the reader antenna radiation pattern in mid.
  • Tag sensitivity refers to the minimum signal power at a specific locus (in the air) at which a given tag switches on. This sensitivity can be influenced by a number of factors including chip power levels, tag antenna radiation pattern, tag construction and tag orientation.
  • the reader and tag should preferably each be positioned and oriented in the direction of maximum power/sensitivity of the other.
  • Figure 1 illustrates the radiation pattern (in free space) of a high gain, directional antenna of a kind typically found in (or used in) current/conventional RFID systems.
  • Figure 2 and Figure 3 both illustrate a RFID tag positioned in front of the RFID reader of Figure 1.
  • Figure 2 and Figure 3 both show the RFID tag's radiation pattern, represented in both cases by solid black lines, superimposed on the radiation pattern of the reader antenna of Figure 1 .
  • the RFID tag is located further away from the reader than in Figure 3.
  • the RFID tag is oriented at an angle (approximately 45°) relative to the reader, whereas in Figure 2 the tag is oriented directly "face on" to the reader.
  • the RFID tag's radiation pattern (and hence its beam) points directly towards the reader
  • Figure 3 the RFID tag's radiation pattern (and hence its beam) points somewhat upwards toward a point above the reader.
  • the tag in Figure 2 is therefore more likely to be read than the tag in Figure 3, even though the tag in Figure 3 is closer to the reader.
  • These Figures also help to illustrate the importance, in terms of read performance, of not just the reader radiation pattern and tag radiation pattern, but also of "angles of read", and furthermore the significance of the antenna's aperture. The meaning and importance of the antenna aperture will be discussed further below.
  • Focussed (i.e. high gain, directive, narrow beam) antennas have become a de facto standard for RFID use, largely because they are thought to reduce radio noise by focusing the radiation pattern to the area of intended read.
  • Figure 4 plots a typical radiation pattern for such an antenna. In other words, Figure 4 is a plot of the radiation pattern for a focussed antenna.
  • the radiation pattern plot in Figure 4 is, in effect, a representation of the antenna's "directivity" ("directivity" is the way the antenna's gain varies with direction).
  • dBi is shorthand for dB(isotropic) and signifies the directional gain of an antenna compared with a hypothetical isotropic (point) antenna which distributes energy uniformly in all directions.
  • the indicated 3 dB beam width in Figure 4 may be considered to be the "aperture" of the antenna. From an RFID perspective, a 3 dB reduction at a specific locus means a 50% reduction in energy (i.e. a 50% reduction in the "energy in the air") at that locus.
  • the amount of energy that is available to switch on a tag located from the reader in a direction on the edge of the reader's 3 dB beam width is only half of the amount which is available to switch on a tag located in the reader's direction of maximum gain (which is at 90° in Figure 4). Furthermore, the amount of energy that is available to switch on a tag located outside the reader antenna aperture (i.e. outside the 3 dB beam width) will be less than half of that which is available in the reader's direction of maximum gain, and with further movement outside (away from) the reader antenna aperture the amount of energy reduces rapidly.
  • a tag which is located in the direction of (or on the edge of) the reader's 3 dB beam width i.e. a tag which is on the periphery of the reader antenna aperture
  • a tag which is outside the reader antenna aperture needs to be closer (possibly much closer) to the reader in order to be switched on as compared to a similar tag located in the direction of the reader antenna's maximum gain.
  • the antenna in Figure 4 is most effective (i.e. its RFID read range is greatest) for a tag located in its aperture (i.e. within the 3 dB beam width).
  • FIG. 4 actually relates to an antenna design that is a conventional patch or parabolic design, which is one form of focused antenna currently used in e.g. point to point communications and RFID.
  • Figure 4 and the associated discussion above help to illustrate generally how focused antennas reduce radio noise by focusing the radiation pattern to the area of intended read, and thus why focused antennas have become a de facto standard for use in conventional RFID systems.
  • focussed antennas i.e. high gain, directive, narrow beam
  • such focussed (i.e. high gain, directive, narrow beam) antennas may not be well suited for use in the applications considered herein, specifically given the drastic RFID read range reduction away from the direction of maximum gain (and also when considering issues such as the predictability of the orientations of both the reader and the tag on the vehicle/license plate).
  • FIG. 5 is a schematic representation of a typical construction of a patch antenna.
  • a patch antenna is one conventional form of focused/directional antenna. It is also important to note how the construction of such a conventional patch antenna impacts on the antenna's radiation pattern.
  • the antenna beam (and its overall radiation pattern) points perpendicularly away from the ground plane (i.e. it points vertically upward relative to the ground plane in Figure 5).
  • the ground plane typically has an area of more than 300 mm x 300 mm.
  • RFID has become (and is continuing to become) increasingly popular, and its use is becoming more common/widespread, including in the identification of vehicles on the road.
  • Gantry based RFID reader infrastructure is, however, complex and typically very time consuming and extremely expensive to deploy and maintain. The result is that RFID vehicle identification is predominantly used only in revenue earning and cost saving applications; for example in freight logistics, toll and congestion charging, and the like. These applications are, however, generally loyalty based, meaning that they rely on compliance by users.
  • Such current RFID vehicle identification systems are generally not well suited to cope with, or adapt to, situations where a person takes steps to prevent detection of their vehicle, or to cause misdetection (incorrect identification) of their vehicle.
  • Current RFID vehicle identification systems are also typically of a closed loop nature (which means that all elements in the systems are specified and regulated by a single entity).
  • the high cost of current RFID infrastructure is one of the major factors currently inhibiting wider deployment of RFID, especially for compulsory and/or regulatory identification of vehicles (e.g. for law enforcement purposes) in an open loop manner (as posed to closed loop).
  • authorities in one European country should preferably be able to read and verify all vehicles (the RFID tags thereon) travelling on that country's roads, including vehicles visiting from other countries and where the RFID tags on other countries' vehicles may have been issued by separate authorities.
  • the information needed for these purposes is currently not readily available (if at all), or where it is available (to some extent) it is often incomplete and obtained from various (often incompatible) sources using expensive and convoluted technologies and methods.
  • the proposals discussed herein may help (or may be used) to provide RFID reader infrastructure which is more cost efficient and easier to deploy and maintain.
  • Figure 8 illustrates the travel path 8-3 of windscreen-mounted RFID tags (such as windscreen-mounted RFID tag 8-2) within an overhead RFID reader beam 8-4.
  • the vertical width of the tag travel path 8-3 which extends from approximately 1 m above the ground to approximately 2 m above the ground, exists due to the fact that RFID tags will be positioned at different heights in different vehicle types. For example, a RFID tag installed in the windscreen of a large truck will typically be higher (closer to 2 m) above the ground than a RFID tag installed in the windscreen of a low-slung sports car (which may be closer to 1 m above the ground). It must also be noted that, for different vehicle types, the vehicle windscreen orientation varies from approximately vertical (as found on e.g.
  • the orientation of the RFID tag antenna when installed on the inside of different vehicles' windscreens can vary from approximately vertical to almost horizontal (and this is in addition to the possible variation in the height of the RFID tag placement for different vehicle types discussed above). The reason this is important is because of the significant influence relative antenna position and orientation can have on read performance, as discussed above.
  • the reader antenna 8-1 is placed 6 m above the road, which is a typical road clearance height.
  • a minimum read range of approximately 6.5-7 m is required to read a windscreen- mounted RFID tag reliably.
  • This minimum 6.5-7 meter (approx.) read range is depicted in Figure 8 by the shape of the RFID reader antenna's effective beam 8-4, which (in this two- dimensional, cross-sectional representation) has a "70° sector" shape with a radius of 6.5-7 m (in Figure 8 the radius shown slightly less than 7 m).
  • this reduces the effective read range requirement (possibly to below 6 m) for them.
  • the scenario in Figure 8 is well within the limits of what can be achieved with RFID, given RFID technology read performance limitations and the geometry imposed by the locations of the RFID reader 8-1 and RFID tags 8-2 in Figure 8.
  • the reason why the scenario in Figure 8 is well within the limits of what is possible can be appreciated from the fact that, in Figure 8, the minimum required tag travel path/distance 8-5 (which is 4 m long for reasons discussed above) easily fits within the effective beam shape 8-4 of the reader antenna 8-1 .
  • the vehicle's windscreen-mounted RFID tag 8-2 will be within the effective beam shape 8-4 of the RFID reader 8-1 for (more than) enough time to be reliably read.
  • metal body parts of a vehicle can deform/distort/complicate the radiation pattern of the RFID tag's antenna.
  • the vehicle's metal body generally surrounds the RFID tag antenna and tends to generate a mutual-coupling effect that distorts the antenna properties both in radiation features and signal fidelity.
  • the windscreen/headlamp glass/plastic due to both its composition and thickness, often displays an uncertain dielectric variance and may even act as a radio shield as a result of tinting and/or hardening.
  • Figure 9 illustrates certain factors that contribute to create non-linear variation of the signal between an overhead RFID reader antenna and a windscreen-mounted RFID tag, including as a result of movement of the vehicle. More specifically, Figure 9 illustrates the direct communication path 9-4 between the overhead RFID reader antenna 9-1 and the windscreen- mounted RFID tag 9-2, together with a number of multi-path factors which contribute to create signal nonlinearity associated with the direct communication path.
  • the multiple reflected communication paths 9-3 (which are inherently unpredictable due to varying vehicle windscreen and body shapes/configurations, and also bearing in mind that each of these reflected paths 9-3 is also subject to communication path length decrease and the issues associated therewith) combine to result in an overall/net communication signal to the RFID reader 9-1 that incorporates the multiple variable signals, each having an exponential tangent (i.e. a highly nonlinear) Doppler shift.
  • a RFID tag on or in a metal plate on a vehicle may help to largely avoid the radio influences of the vehicle (like those discussed above).
  • a metal plate on a vehicle (such as the vehicle's license plate) can also have a highly consistent shape/construction (i.e. the shape/configuration of the metal plate will typically vary very little, if at all, from vehicle to vehicle).
  • Such a metal plate (license plate) can also function as a ground plane which largely shields the antenna beam from the reflective effects of the rest of the vehicle structure.
  • the metal plate (preferably a license plate) on/in which the RFID tag is mounted is itself mounted in such a way that a clear line of sight is maintained to the plate (so that there are no (or few) intervening reflectors/reflections between the RFID tag and the RFID reader).
  • the applicable governing legislation requires vehicle license plates to be installed in such a way that they can be clearly seen (i.e. such that there is a clear line of sight to the license plate). This thus makes the vehicle license plate a particularly suitable placement location on a vehicle for a RFID tag if the tag is to be reliably read by a RFID reader.
  • a RFID tag may preferably be placed on one or both of a said vehicle's license plates, and for vehicles which have only one license plate, a RFID tag may preferably be placed on the single license plate.
  • Figure 10 shows a licence plate mounted within a cavity (i.e. within the channel section of a metal girder) to protect it from damage.
  • This mounting does not obstruct the reading of the plate by a human, but an overhead RFID reader would likely have problems reading a RFID tag installed on such a plate (due to the shielding effect of the section of the metal girder that extends out above the plate).
  • Figure 1 1 illustrates the travel path 1 1 -3 of a vehicle's license plate, where the license plate has a RFID tag thereon (making it a "RFID plate” 11 -2), within an overhead RFID reader beam 1 1 -4.
  • Figure 1 1 actually illustrates that a vehicle may have a RFID plate 1 1 -2 mounted on the front and/or the back thereof.
  • the vertical width of the tag travel path 1 1 -3 in Figure 1 1 which extends from approximately (just above) ground level to approximately 1 m above the ground, exists due to the fact that RFID plates 1 1 -2 may be positioned at different heights (i.e. different distances off the ground) on different vehicle types.
  • a RFID plate 1 1 -2 installed on a large truck will typically be higher (closer to 1 m) above the ground than a RFID plat 1 1 -2 installed on a low-slung sports car (which might be as little as 20 cm or less above the surface of the ground).
  • vehicle license plates generally display little (if any) variation from vehicle to vehicle in terms of the orientation (angle) at which they are installed.
  • Vehicle license plates are typically required to be installed vertically, such that the plane of the license plate is perpendicular to the direction of travel of the vehicle. Consequently, RFID tags on/in vehicle license plates (hence “RFID plates” like the plates 1 1 -2 in Figure 1 1 , and the antennas thereof) generally have highly consistent orientation from vehicle to vehicle, even across different vehicle types.
  • the orientations of the antennas of RFID tags when the tags are installed on/in vehicle license plates, will generally vary very little, even across different vehicle types. The importance of this should not be underestimated given the significant influence that relative antenna orientation can have on read performance (see above).
  • the influence of the vehicle body on a license plate tag may be related to the size of the metal (conducting) background to the plate, which may be small for a sedan and larger for a bus, as an example.
  • a larger conducting background may have a general effect of making the antenna aperture more narrow and perpendicular to the plane of the background. This can have an overall negative effect for a gantry placed reader, but a positive effect for an in/on road reader; see Figure 12 and the relevant discussions below.
  • This minimum 7.5 m (approx.) read range is depicted in Figure 1 1 by the shape of the RFID reader antenna's effective beam 1 1 -4, which (in this two-dimensional, cross-sectional representation) has a "30° sector" shape with a radius of around 7.5 m (the radius of the beam 1 1 -4 extends to midway between the 7 m and 8 m arcs from the location of reader 1 1 -1 ).
  • the scenario in Figure 1 1 is on the edge (i.e. it is approaching the limit) of what can be achieved with RFID, given RFID technology read performance limitations (within spectrum regulations) and the geometry imposed by the locations of the RFID reader and RFID plates (tags) in Figure 1 1 .
  • the reason why the scenario in Figure 1 1 is approaching the limits of what is possible can be appreciated from the fact that, in Figure 1 1 , the minimum required tag travel path/distance 1 1 -5 (which must again be at least 4 m long for reasons discussed above) only just fits within the effective beam 1 1 -4 of the reader antenna 1 1 -1.
  • the vehicle's license plate mounted RFID tag (RFID plate) 1 1 -2 will be within the effective beam 1 1 -4 of the RFID reader 1 1 -1 for only just enough time to be reliably read (or possibly, due to the possible influence of communication distorting/inhibiting factors, the vehicle's RFID plate 11-2 may not be within the effective beam 1 1 -4 for quite long enough, in which case a reliable read may not be, or may not always be, possible).
  • an in/on-road location is thought to be a preferable placement location for a RFID reader, especially if vehicles' RFID tags are on or part of the vehicle license plate (which is also thought to be highly preferable).
  • the multi-path problem discussed above with reference to Figure 9 may be largely alleviated since the only real reflectors which might reflect a signal between the in/on-road RFID reader and an on-plate RFID tag are the road itself and other vehicles in an adjacent lane.
  • the road is a weak reflector which tends to scatter the signal (rather than produce the much more problematic near-perfect, but slightly out of phase, reflections typically associated with the vehicle bonnet etc for windscreen mounted tags). And adjacent vehicle multi-path reflections typically display a close to linear Doppler shift which can be filtered relatively easily.
  • Figure 12 illustrates a desirable radiation pattern, and hence a desirable beam shape 12-4, for a RFID reader antenna 12-1 which is placed in/on the road. More specifically, Figure 12 illustrates a cross-section of the said desirable beam shape 12-4, in a vertical plane which extends parallel to the vehicle's direction of travel and through the centre of the antenna radiation pattern, with the said plane viewed from one side. It will be noted that the beam shape 12-4 is quite low (relative to vehicle height) and long/wide (relative to travel direction). Contrast this with the radiation pattern 13-2 on the right hand side in Figure 13 which is a radiation pattern for a conventional directional (and upward-pointing) patch antenna.
  • the RFID tag 12-2 is placed in or on the vehicle's front and/or rear license plate resulting in a potential tag travel path 12-3 which is typically the space between about 200 mm and about 1200 mm above the road surface.
  • a potential tag travel path 12-3 which is typically the space between about 200 mm and about 1200 mm above the road surface.
  • its license plate, with the RFID tag thereon will typically pass through this region 12-3 which is approximately 200 mm-1200 mm above the ground as the vehicle passes the reader.
  • the bulk of the space inside the effective beam 12-4 of the antenna 12-1 in Figure 12 i.e. the bulk of the space within which the RFID tag on the vehicle license plate will receive sufficient energy to "switch on" and communicate with the RFID reader
  • this beam shape 12-4 is thought to be highly suitable.
  • Figure 13 illustrates, on the right-hand side thereof, the radiation pattern (and hence beam shape) 13-2 associated with a reader having an upward-pointing conventional patch antenna.
  • Figure 13 also illustrates, on the left hand side, a low antenna radiation pattern (beam shape) 13-3 as emitted from a form of in/on-road reader 13-1 having an antenna like those discussed below. (Note that the radiation pattern/beam shape 13-3 on the left-hand side in Figure 13 is the same as the radiation pattern/beam shape 12-4 illustrated in Figure 12.)
  • the metal surface under a vehicle can act as a reflector, and it is close to the reader antenna 13-1 . This may result in a blinding energy reflection which, in the case of an upward- pointing conventional patch antenna (or any other kind of upward-pointing focused, narrow beam antenna), will be very high (as indicated by the amount of depicted energy within the region 13-6 in Figure 13).
  • An antenna with a radiation pattern which possesses low radiation in the vertical direction, especially directly above the antenna may help to reduce this reflected blinding energy substantially (this is illustrated by the substantially lesser amount of energy within the region 13-5 in Figure 13, as compared with the amount of energy in the region 13-6 above the patch antenna).
  • Figure 13 illustrates that even though an in/on-road location is a preferable placement location for a RFID reader, it is also the case that focused, narrow beam antennas (as conventionally used in other RFID applications) may be inappropriate for use in this application, due to the possibility for a blinding reflection from the underside of a vehicle. Accordingly, it would appear that an antenna with a radiation pattern having an overall low, flat shape would be preferable.
  • a low, flat shaped antenna radiation pattern could possibly be achieved by turning a directional antenna (like the one illustrated in Figure 5) on its side.
  • a directional antenna like the one illustrated in Figure 5
  • Such a physical structure is obviously not feasible for use on the road as it would obstruct traffic and would likely be destroyed by the first vehicle to collide with it (not to mention the damage caused to the vehicle, potential accident injuries, etc).
  • simply turning a directional antenna on its side in order to achieve the desired radiation pattern may not be an option.
  • a "height-restricted" antenna should be interpreted as a reference to an antenna which meets, at least, this last-mentioned configuration criterion (i.e. a "height- restricted” antenna is a reference to an antenna that has a low profile physical structure, or in other words, a low physical height).
  • a height-restricted antenna which also has an overall low, flat shaped radiation pattern may be particularly desirable.
  • FIG 14 depicts an example of an upright slotted antenna and Figure 15 illustrates its radiation pattern.
  • the antenna in Figure 14 is merely one example of an upright slotted antenna, and the term slotted antenna really defines an entire category of antennas.
  • the particular example upright slotted antenna depicted in Figure 14 uses an upright slotted radiator 14-1 which has a generally forward and backward pointing radiation pattern, and the forward and backward pointing portions of the radiation pattern are each (individually) somewhat similar to the radiation pattern of a patch antenna (see Figure 15).
  • a reflector 14-2 is used to push the radiation to one side, and at the same time functions (to some extent) to neutralise the ground effect.
  • this type of antenna although simple to construct, may not be suitable for use in the particular applications discussed herein due to its inability to cover the desired read zone. Further explanations relating to the read zone are given below.
  • the RFID reader will preferably be located on/in the surface of the road. Accordingly, within the range of communication distances currently under consideration (typically 8 m or less and usually around 5 or 6 m) the ground will always be the surface of a road. In other words, if the RFID reader is placed on/in the road surface, the ground within a communication distance radius from the RFID reader (i.e. within at least 8 m or less and usually around 5 or 6 m) will usually be constituted by the road surface (and normally/often nothing else).
  • This "close ground” or “near ground” has determinable, but highly changeable radio properties. Antenna design taking into consideration such "near ground” effect (i.e. the ground effect caused by this "near ground”) has not been widely investigated in the past. It is thought to be preferable that the variation in road dielectric properties in different weather conditions should also be taken into account, as discussed in the Summary of the Invention section above.
  • the ground effect arises due to a change in the material(s) which the antenna's radiation propagates through or reflects from.
  • the ground (or more specifically in the present context, the road and its base) demonstrates various dielectric properties (permittivity and conductivity). This is due to the materials used in the road's construction, and also due to moisture, the latter of which is highly variable/changeable and uncontrollable.
  • the typical impact of a conducting ground effect is to push the direction of maximum gain upwards.
  • Figure 17 illustrates, for the situation where a patch antenna is on its side so that its beam extends horizontally, the way the radiation pattern of the patch antenna is pushed upwards because of the ground effect.
  • This effect may be present where, for example, metal reinforcing is present in the road and/or conductive fluids (e.g. water due to recent rainfall) are on or in the road surface.
  • the reader antenna 17-1 (a patch antenna pointing horizontally) is placed on the road.
  • the direction of maximum gain 17-3 is pushed up, in this case to -30 degrees.
  • a narrow aperture beam shape like the one identified as 17-4 in Figure 17, does not provide enough energy in the vehicle's license plate/RFID tag potential travel path 17-2. In other words, for the narrow aperture beam 17-4, there is not enough of the plate tag travel path 17-2 within the beam (and hence the vehicle's RFID tag will not be within the beam for long enough) for a reliable read of the vehicle's RFID tag to be achieved.
  • the beam aperture could be widened to increase the amount of the plate tag travel path 17-2 that is within the beam.
  • This possibility is shown by the wider beam aperture 17-5 in Figure 17.
  • this illustrated possibility i.e. 17-5 in Figure 17
  • the ground effect would likely actually push such a directional radiation pattern further away from the road (meaning that the amount of the plate tag travel path 17-2 which is within the beam may not actually increase very much).
  • the way in which the direction of maximum gain of an antenna is pushed upwards by the ground effect can also be of benefit, and embodiments of the present invention may take advantage of this, in relation to issues associated with radio noise and interference.
  • the use of multiple radio transmitters (and the antennas thereof) which are operating on the same or similar frequency and located nearby one another can (in general terms) give rise to interference. That is, the transmitters/antennas can interfere with one another (often referred to as "cross talk").
  • One way to overcome or reduce this problem is to ensure that radio transmitters/antennas that are located near one another operate on different frequencies (or ideally frequencies that are mathematically orthogonal to one another so as to prevent additive or subtractive interference).
  • the number of frequencies that are actually available for use can be very limited. Also, the frequencies themselves that are actually available may not meet the desirable condition of orthogonality.
  • the amount of energy from the reader that is available to switch on a tag located from the reader in a direction outside the reader antenna aperture is (as explained above) much less than the amount that is available for a tag at the same distance from the reader but located in a direction inside the reader's aperture, and the further outside the aperture the less energy is available.
  • the amount of energy (as depicted by the plotted locus) in the direction approximately along (or just above) the ground is between -20 dB and -30 dB, meaning that the amount of energy from the antenna in this direction is drastically less than in directions that are within the antenna's aperture. What this indicates in practice is that there may be little or no signal (i.e.
  • antenna having a low profile physical structure specifically an example upright slotted antenna
  • Another type of antenna is a dipole antenna.
  • a conventional dipole antenna consists of two conductive elements such as metal wires or rods, which are usually bilaterally symmetrical. The driving current from the transmitter is applied, and for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the feedline to the transmitter/receiver is connected to one of the conductors.
  • dipole antenna The most common form of dipole antenna, referred to as a half-wave dipole antenna (or simply a half-wave dipole), has two straight rods or wires oriented end to end on the same axis, with the feedline connected to the two adjacent ends.
  • Dipole antennas in general are resonant antennas, meaning that the elements serve as resonators, with standing waves of radio- frequency current flowing back and forth between their ends.
  • the most common form of dipole antenna is the half-wave dipole, an in a half-wave dipole each of the two rod elements is approximately 1/4 of a wavelength long, so that the whole antenna is a half- wavelength long (hence the name "half-wave" dipole).
  • Figure 18 illustrates the theoretical radiation pattern of an upright half-wave dipole antenna in free space, with the antenna located at the centre of the depicted radiation pattern.
  • This perfect/around doughnut (or "toroid") shaped radiation pattern provided by a theoretical half-wave dipole antenna seems intuitively suitable for use in the road vehicle identification application presently under consideration, especially if the centre point of the dipole antenna were to be at the ground/road surface (assuming any effects caused by the road and the surface of the road can be ignored).
  • Such an upright dipole antenna with its "doughnut" beam shape would be directionally independent in the plane of the surface of the road.
  • a significant difficulty is that vehicles often may not travel precisely in line with or along the direction of intended read. For instance, a vehicle may travel to one side or other of the reader, or it may pass the reader at an angle relative to the reader antenna's direction of maximum gain, such that the vehicle (and the associated antenna/tag) does not move directly into or along the reader antenna's direction of maximum gain.
  • the antenna/tag on the vehicle must be read from the side and/or at an angle, and as has been explained, the amount of energy required to read the tag in such situations may be higher (compared to an ideal "face on” read), even if the tag is actually quite close to the reader.
  • one of the main benefits of a toroid shaped radiation pattern is that it is inherently non-directional (in a horizontal plane).
  • the antenna's maximum power extends in all horizontal directions around the antenna, and this can help to significantly reduce the problems discussed above.
  • a dipole antenna emits linearly polarised energy/radiation. Consequently, if a dipole antenna is used as the antenna for a RFID reader, this in turn requires any RFID tags which are to be read by the RFID reader (such as RFID tags on vehicle license plates) to "reflect" a signal (or produce a modulated reply/response signal) with the same polarisation.
  • RFID tags such as RFID tags on vehicle license plates
  • polarisation is not predictive or fixed. Reflections also change the direction of polarisation. Therefore, it has previously been considered preferable, in the field of RFID, to use circularly polarised antennas (due to the ability to better cope with unpredictable polarisation) rather than linearly polarised antennas.
  • a vehicle license plate including a RFID tag (and it's antenna) thereon
  • linear polarisation (as emitted by dipole antennas) is thought to be potentially suitable.
  • linear polarisation of the reader and tag antennas which may preferably be vertical polarisation (see below), may provide additional benefits.
  • any component of a noise signal having different polarisation e.g.
  • any component of the noise signal having horizontal polarisation, where the reader and tag antennas are vertically polarised may be more easily (or even naturally) filtered out.
  • Another potential benefit of linear polarisation is that the efficiency of energy utilisation may be improved (as there may be zero polarisation mismatch, e.g. between the reader and tag).
  • a RFID vehicle license plate i.e. a vehicle license plate having mounted thereon (or incorporating) a RFID tag
  • the RFID tag incorporates a slotted antenna which is linearly (preferably vertically) polarised.
  • a tag antenna may emit linearly polarised energy/radiation, and preferably the energy/radiation emitted by the tag antenna may be vertically polarised.
  • the reason vertical polarisation may be preferable is because, if the RFID reader antenna is a kind of dipole antenna oriented upright, the reader antenna will emit vertically polarised energy/radiation.
  • a RFID vehicle license plate wherein the antenna thereon is also vertically polarised may be an appropriate match for a RFID reader incorporating an upright dipole antenna (located at an "on-road” or "inter-road” level).
  • dipole antennas may well be suitable for use in RFID readers in the present application
  • a simple half-wave dipole antenna (being the most common form of dipole antenna) may not be the most suitable or ideal.
  • the form of antenna used may be a type or species of dipole antenna (i.e. the antenna proposed for use in the present context might be said to fall into the general category of dipole antennas or dipole-like antennas), however it may not be merely a simple half-wave dipole antenna.
  • the antenna may be an adapted form of dipole antenna, or a variation or modification of a traditional dipole (or dipole-like) antenna configuration.
  • the antenna should also be a "height-restricted" antenna.
  • the term “height-restricted” herein refers to an antenna that has a low profile physical structure, or in other words, a low physical height.
  • the antenna should be configured to provide a low, flat radiation pattern.
  • Figure 19 illustrates, for one possible scenario, the read-zone for a vehicle equipped with a RFID enabled license plate.
  • the RFID plate travel path in Figure 19 is 4 m wide with the read-zone starting at 5 m before the reader antenna and ending at 5 m beyond the reader antenna (the reader in this instance is located in the centre of the road lane at the marked 0 m point).
  • the space from 1 m before to 1 m beyond the reader antenna is excluded from the read- zone in an attempt to reduce the blinding effect of radiation reflection (discussed above with reference to Figure 13), and also because of angled-read problems that may arise in this region, especially for vehicles (and the plates thereof) which are moving near the side of the lane (rather than down the centre of the lane directly in line with the reader).
  • Figure 20 is a schematic representation of what is depicted pictorially in Figure 19.
  • Figure 20 shows the license plate/RFID tag orientation within the read-zone (detect area) of an in/on-road RFID reader.
  • Figure 21 illustrates the effective read-zone 21 -5 for a RFID tag 21 -4 located on a vehicle license plate, as read using an in-road RFID reader 21 -1 with an adapted/modified and height-restricted form of upright dipole antenna.
  • the required read-zone 21 -7 based on the travel path 21 -3 of the vehicle, covers the typical lane width of (2Ly) 4 m and the required 4 m in-beam travel path (Lx).
  • the required read zone 21 -7 in Figure 21 corresponds to the read zone/detect area depicted in Figure 19 and Figure 20.
  • the RFID reader's (wide and flat) "dropped doughnut" shaped radiation pattern (this being a highly preferable shape for the radiation pattern) is represented in Figure 21 by the circle labelled 21 -2, however it will be understood that this beam shape 21 -2 (which is represented as large a circle in Figure 21 ) is actually a dropped-doughnut-like or squashed-toroid-like radiation pattern preferably having a shape approximating the one shown in Figure 23.
  • the RFID reader's radiation pattern 21 -2 with a face-on read range of approximately 6 m, combined with the effect of the angle of read 21 -6 on the plate's RFID tag 21 -4, results in the illustrated effective beam shape 21 -5 (or effective read zone 21 -5).
  • the effective beam shape (read zone) 21 -5 is the area in which a RFID tag which is on/in a vehicle license plate will receive enough power from the RFID reader 21 -1 to be switched on and effectively reflect a modulated signal.
  • the effective read zone 21 -5 is roughly "figure 8"-shaped, with the centre of the figure 8 located at the position of the RFID reader 21 -1 and the two lobes of the "figure 8" on either side thereof in the direction of vehicle travel.
  • the RFID reader's antenna being an adapted/modified and height-restricted form or variation of dipole antenna 21 -1 , is non- directional and therefore the orientation of the "figure 8" shaped effective read zone 21 -5 - i.e. in line with the vehicle's direction of travel - arises due to the geometry of the required read zones 21 -7, and the convergence of the "figure 8" lobes near the reader arises due to angle of read issues.
  • These factors concerning the orientation of the "figure 8" shaped effective read zone 21 - 5 are therefore not a result of the design/configuration of the antenna 21 -1 itself).
  • Figure 22 illustrates example uses of a RFID reader equipped with an adapted and height-restricted dipole antenna 22-1 (and which provides a doughnut shaped radiation pattern, as in Figure 21 ), or multiple such RFID readers, with the resulting effective read-zone 22-2, in different read scenarios.
  • the potential travel path of a license plate RFID tag 22-3 is indicated (indicated as 22-3 and also coloured blue in Figure 22), based on where a vehicle may physically drive, on each different type of road. All road lanes in these examples are 3 m wide, which is average for many road lanes.
  • a bi-directional (single carriageway) narrow road 22-4 that is approximately 6 m wide can be covered with a single reader which will read vehicles in both directions (this is the example in the top left of Figure 22). This is because the plates will typically be placed in the centre on the front and back of a vehicle, meaning that the 4 m wide read-zone will actually cater for a 6 m wide vehicle travel zone (this is ignoring motorcycles and the like).
  • a road with a shoulder, or a wide shoulder, 22-5 (the presence of the shoulder increases the width of the area in which a vehicle can travel) may however often require two readers (as illustrated in the top-middle example in Figure 22).
  • a four lane single direction road with shoulders 22-6 may require three readers (as illustrated in the lower left example in Figure 22).
  • a road crossing of two narrow roads 22-7 could potentially require only one reader (which is why this is illustrated in the example on the right-hand side in Figure 22); although a crossing of a narrow road with a road having wider shoulders may require two readers.
  • the example scenarios in Figure 22 help to illustrate that, for example in applications such as law enforcement applications, one or more RFID readers should be deployed on or in the road such that it is not possible (or at least it is very difficult) for a vehicle to avoid detection (i.e. avoid having its license plate RFID tag read by one of the readers). So, for example where the RFID readers are used in law enforcement applications, it should not be possible for a vehicle to easily avoid detection by "driving around” the reader by skirting around (and not entering) the reader's detection zone. On the other hand, there may be situations where a RFID reader is intentionally placed so as to only cover a portion of the road.
  • the RFID reader might be placed so as to only detect vehicles travelling in the lane(s) in question, but so as not to detect vehicles travelling in other lanes.
  • an antenna which provides a radiation pattern as illustrated in Figure 23 is desirable for use in RFID readers which are to be used in "on-road” or “in-road” placement locations in vehicle identification applications.
  • a radiation pattern concentrates the maximum power in the zone where a RFID tag on a vehicle license plate is most likely to travel, which is typically 8 m or less from the antenna. (This was also explained above with reference to Figure 12.)
  • This radiation pattern is also directionally independent in relation to the travel of the vehicle, with a low level of power directly above the antenna (this last is important for reasons discussed above).
  • Figure 23 actually shows the calculated radiation pattern of a particular adapted/modified and height-restricted form/variation of dipole antenna (i.e. an antenna which is adapted/reconfigured compared to a conventional half-wave dipole antenna to have a low- profile or low-height physical structure but so as still to provide an overall radiation pattern shaped like a dropped-doughnut as shown), and which is placed in or on the road.
  • dipole antenna i.e. an antenna which is adapted/reconfigured compared to a conventional half-wave dipole antenna to have a low- profile or low-height physical structure but so as still to provide an overall radiation pattern shaped like a dropped-doughnut as shown
  • this radiation pattern is quite wide and flat (approximately toroidal and similar to the shape of a doughnut that has been dropped flat onto the ground and flattened somewhat).
  • the antenna should preferably be small enough - more preferably less than 50 mm tall and less than 300 mm in diameter - so as to be easily installed in or on the surface of a road.
  • This antenna configuration should preferably also be such as to neutralise ground and surface effects which may occur.
  • a RFID reader incorporating an adapted form or variant of dipole antenna (being vertically polarised), which is operable to be placed in or directly on the surface of the road, which has a low physical profile, and which meets the radiation pattern shape requirements outlined above, may be highly desirable in the context of the presently discussed application involving reading RFID enabled vehicle license plates.
  • a monopole antenna may be able to provide the presently desired radiation pattern.
  • a monopole antenna might be said to be an adapted type of (or a variant of or a special case of) dipole antenna.
  • a monopole antenna is a type of antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over a conductive ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the lower end of the monopole and the ground plane. One side of the antenna feedline is attached to the lower end of the monopole, and the other side is attached to the ground plane (which is often the Earth).
  • a monopole antenna is a resonant antenna in that the rod functions as a resonator with standing waves of radio-frequency current flowing back and forth between its ends.
  • the most common form of monopole antenna is the quarter-wave monopole, in which the rod length is approximately 1/4 of a wavelength of the radio waves (hence the name quarter-wave monopole).
  • monopole antennas such as a quarter-wave monopole may be able to provide a radiation pattern that is desirable for the vehicle identification applications discussed herein
  • standard monopole antennas are generally too tall (i.e. they don't meet the "height-restricted" requirement).
  • current knowledge on monopole antennas relates mostly to long distance communications where the bottom half of a conventional vertical dipole is replaced by a conducting ground plane (of sufficient size to approximate an infinite ground plane). This is especially the case for lower frequency, long distances transmission applications, and where earth may be approximated as an infinite conducting ground plane.
  • the ground plane in the kinds of applications discussed herein will, however (as discussed above), generally be close to the antenna and imperfect and changing in nature.
  • Figure 24 illustrates the configuration of an antenna in accordance one particular embodiment of the invention that is thought to be particularly suitable for use in the vehicle identification applications discussed herein.
  • the antenna in Figure 24 is actually a form of adapted/modified monopole antenna.
  • the parts of this antenna, as labelled in Figure 24, include:
  • dielectric layer 24-4 of the same shape and located immediately beneath the ground plane (note that the inclusion of this dielectric layer 24-4 is optional, albeit preferable, and there may also be a (again optional) layer of conductive material (not shown in Figure 24) on the opposite side of the dielectric layer 24-2 from the ground plane - this layer (if present) will often be non-slotted such that it forms a non-slotted ground shield),
  • an upright cylindrical monopole 24-2 (which in this case is in the shape of a short, squat cylinder) oriented vertically relative to the horizontal ground plane and perpendicular to the centre of the ground plane (note that the lowermost first end of the monopole 24-2 does not actually connect with or contact the ground plane 24-3; rather the monopole 24-2 hangs above the ground plane (suspended from the top load 24-1 ) such that the lowermost first end of the monopole 24-2 is separated from the upper surface of the ground plane 24-3 by a small gap (this gap is visible in Figure 30),
  • shortening poles 24-5 each extending vertically upwards from the ground plane, and the shortening poles 24-5 are all located at an equal radial distance outward from the monopole (i.e. they are all at an equal distance from the central vertical axis of the antenna) and with equal spacing around the monopole (note that the shortening poles form what might be termed a "bird cage” configuration), and a circular top load plate (a.k.a.
  • top load 24-1 which in this embodiment is of slightly lesser diameter than the ground plane 24-3, and which is mounted on top of the shortening poles 24-5 such that the top load 24-1 is parallel to, but spaced vertically above, the ground plane (and as mentioned above the monopole 24-2 is connected to, and effectively “hangs” from, the underside of the top load 24-1 ).
  • the dielectric layer 24-4 (if present), the ground plane 24-3, the monopole 24-2 and the top load 24-1 are all mounted such that their circular centres coincide with the antenna's central vertical axis.
  • the top load 24-1 is shown transparently. However this is merely so that other components (e.g. the monopole and the shortening poles etc) can be more easily made out. In practice, it is expected that the top load 24-1 will be made from a conductive material such as high-grade conductive copper or the like (and as such the top load 24-1 will likely be opaque). Similarly, other parts of the antenna including the monopole 24-2, the shortening poles 24-5, the ground plane 24-3 and the non-slotted ground shield (if present) will also likely be made from conductive materials, although there is no necessary requirement for them all to be made from the same material. Materials that may be suitable for these conductive parts of the antenna, and the issues associated with the selection of appropriate materials, will be familiar to those skilled in the art and therefore need not be discussed in detail. In any case, the invention is by no means limited to by any particular materials.
  • the solid dielectric layer 24-4 which (if present) is located beneath the ground plane, should be made from a suitable solid material having appropriate dielectric properties. Possible candidate materials might include plastics, polymers, ceramics, some metal alloys or oxides, etc. In any case, any solid dielectric material known by those skilled in the art to be suitable (and preferably with ordinary permittivity and zero (or low) conductivity) may be used for the dielectric layer 24-4 (if present).
  • the solid dielectric layer (and the material chosen for the formation of the dielectric layer) may also help to improve the mechanical strength of the antenna overall, for example if the ground plane is formed from a thin (and consequently flexible or insufficiently rigid) layer of conductive copper.
  • Electronics associated with the antenna should preferably be mounted (or otherwise located) vertically underneath the ground plane 24-3 (or beneath the dielectric layer 24-4 and/or ground shield (if present)). This is so that these electronics are shielded from the antenna by the ground plane 24-3 (or by the ground plane plus the dielectric layer/ground shield), and so that the antenna is shielded from the electronics.
  • further electronics and/or a further antenna may also be provided.
  • an additional antenna might be used for non-RFID purposes, for example Wi-Fi communication or the like which is in addition to the main antenna's RFID function.
  • further electronics and/or additional antenna may be positioned on top of (or otherwise vertically above) the top load 24-1 . If positioned above the top load 24-1 , such electronics and/or further antenna may be shielded from the main antenna by the top load 24-1 .
  • Figure 24 does not depict any further electronics or additional antenna (it merely depicts the main antenna).
  • the circular ground plane 24-3 is actually slotted. More specifically, the ground plane 24-3 contains periodic slots. In other words, the ground plane 24-3 is a periodically slotted ground plane.
  • each set there are four distinct sets of slots.
  • Each slot is shaped like a short arc oriented concentrically with the ground plane's circumference.
  • the respective slots are radially spaced equally from one another, and each set extends radially outwards from the position of one of the respective shortening poles 24-5.
  • the arcuate length of the individual slots becomes greater as the radial distance from the centre of the antenna increases.
  • Each set of arcuate slots is separated from the adjacent set of slots by a solid, un-slotted portion of the ground plane 24-3.
  • the slots are cut (or otherwise formed to extend) through the thickness of the ground plane, although in other embodiments the slots might be merely indented into the surface of the ground plane without extending through the full thickness of the ground plane. In any case, in the depicted antenna, the slots do not extend into (or at least they do not extend all the way through) the thickness of the dielectric layer 24-4 (although the slots may do in other embodiments).
  • the number, the relative shape, the relative size, the relative depth (into the ground plane and/or the dielectric layer), the relative position, etc, of the slots may be varied in order to alter the performance of the antenna (i.e. these things may be varied in order to "tune" the antenna).
  • the ability to alter the configuration of the slots is one of the important ways in which an antenna of this kind may be tuned.
  • the function of the periodic slots in the ground plane is (it is thought) to help ensure uniformity of the antenna's radiation pattern for the desired read zone, and also to help to negate (or minimise) the variable ground effect.
  • the slotted ground plane is also thought to help reduce the antenna return-loss but without requiring undesirably large increases in the ground plane size/dimension. In other words, it is thought that the use of a slotted ground plane may help to reduce return-loss whilst also allowing the ground plane size/dimension to remain sufficiently small/compact.
  • the actual way in which the configuration of the slots is varied, and the affect different changes in slot configuration may have in terms of antenna performance, is outside the scope of the present disclosure.
  • the radiation pattern of the antenna in Figure 24 is a highly desirable (possibly near perfect or near ideal) "dropped doughnut" or a "toroid on the ground” shape, as depicted by Figure 23. Accordingly, the particular antenna in Figure 24 provides a radiation pattern of a shape (shown in Figure 23) which is thought to be highly desirable/beneficial/functionally suited for RFID readers which are to be used in "on-road” or “inroad” placement locations in vehicle identification applications. By way of further explanation, the shape of the antenna radiation pattern depicted in Figure 23 is still generally “toroid” like.
  • the shape of the radiation pattern in Figure 23 (which is the radiation pattern for the antenna depicted in Figure 24) is generally lower and flatter. That is, it is slightly “squat” or “squashed” in the vertical direction, and this is actually thought to be desirable/advantageous because it means that the antenna's energy extends generally more in the horizontal plane (in all directions) and less in the vertical direction (which means the antenna's beam may extend further outwards horizontally but there may also be less “blinding" from the underside of vehicles etc due to the comparatively lesser amount of energy directed in the vertical direction).
  • the shape of the radiation pattern is similar to that labelled 12-4 in Figure 12 and 13-3 in Figure 13, which is thought to be highly desirable/beneficial/functionally suited for RFID readers for reasons discussed above.
  • the positioning of the shortening poles 24-5 around the monopole 24-2 forms what might be termed a "bird cage" configuration. Accordingly, the configuration of the antenna in Figure 24 might be termed a birdcage configuration (or the antenna therein might be termed a birdcage antenna).
  • the birdcage antenna in Figure 24 is quite substantially modified/reconfigured, as compared to say a conventional half-wave dipole or quarter-wave monopole for example, nevertheless the birdcage antenna is still a species or kind of dipole (or monopole) antenna.
  • the birdcage antenna in Figure 24 like other conventional dipole (or monopole) antennas, is a resonant antenna. Accordingly, the sizes of the various parts of the birdcage antenna are inherently and necessarily dependent on the frequency of the radio signal with which the antenna is to operate. Or equivalently, it might be said that the sizes of the various parts of the birdcage antenna are inherently and necessarily dependent on the wavelength of the radio signal at the specified operating frequency.
  • the thickness of the antenna ground plane 24-3 (in Figure 30 this is actually the combined thickness of the antenna ground plane 24-3 and the dielectric layer 24-4) which in this example is -3 mm,
  • the underside of the antenna is either the underside of the ground plane, or the underside of the dielectric layer if the dielectric layer is present) and the bottom of the cavity into which the RFID reader (of which the antenna forms part) is inserted, and
  • FIG. 30 there is a small horizontal gap between the upper surface of the ground plane and the lower end of the monopole. Also, there is a solid black line extending along the bottom/underside of the dielectric material. This is intended to represent the (typically non-slotted) conductive ground shield (which, like the dielectric material, is optional).
  • box 30-1 illustrated in Figure 30. The vertical side edges of the box 30-1 extend along the side edges of the cavity in which the antenna is located, the lower horizontal side of the box extends along the base of the cavity, and the top of the box extends slightly above the level of the ground.
  • the box 30-1 is intended to represent (the outline of) the housing or casing of a RFID reader. That is, the housing or casing within which the antenna (plus other reader electronics, power source or power connections, etc) are located.
  • the overall antenna should still fit within a structure, such as the housing of a RFID reader, having a diameter of, say, 180 mm or less and a height of, say, 40 mm or less. In fact, the antenna should fit within a housing of this size and still leave room for the frame and supporting parts of the housing, etc.
  • the housing itself (even if fully recessed/buried in the surface of the road, with no part left above the road surface) should not penetrate the road by much (if any) more than 40 mm. This should therefore allow such an RFID reader housing to be placed in the road without compromising the integrity of the road.
  • the gaps between the antenna (or the RFID reader housing) and the road structure generally should not cause the radiation properties of the antenna to change, however, this may influence the antenna return loss. In any case, this minor effect might be compensated for by adjustment to the input power level to help guarantee sufficient power emitted from antenna body.
  • an "inverted F antenna” as depicted in Figure 25 may be a simpler alternative construction to achieve an antenna that has a low profile physical structure and which is able to provide a low, flat radiation pattern.
  • the IFA therein has an upstanding antenna element (the resonant part of the antenna) which is shaped like a sideways capital letter "F". The single long edge of the "F” is oriented parallel to the antenna's ground plane.
  • the "F"-shaped antenna element also has two "prongs”. The prong which would be the lower of the two if the "F" were in a normal upright orientation will be referred to here as the second prong.
  • the other prong namely the one which would be the upper of the two if the "F" were in a normal orientation, will be referred to here is the first prong.
  • the second prong extends vertically downwards from partway along the long horizontal portion of the "F” and inserts through a small hole in the centre of the antenna's ground plane (note that the second prong inserts through the hole in the ground plane but does not contact with the ground plane).
  • the first prong extends vertically down from one end of the long horizontal portion and connects to the antenna's ground plane slightly to one side of the second prong.
  • the ground plane of the IFA in Figure 25 is rectangular. In Figure 25, even though no dielectric layer or underlying conductive shield layer is depicted, these could optionally be provided. Also, the configuration of the periodic slots in the ground plane differs. In the particular IFA depicted in Figure 25, there are two sets of slots. In each set, there is a number (1 1 ) of arcuate slots. Each slot is shaped like an arc, although it will be noted that the slots in this IFA ground plane are shaped like longer arcs than the slots in the ground plane in Figure 24.
  • the slots in the ground plane of the IFA in Figure 25 are similar to the slots in the ground plane in Figure 24 in that they are oriented to form concentric arcs centred on the centre of the antenna. And in each set of slots in Figure 25, the respective slots are radially spaced equally from one another, and each set extends radially outwards in a direction perpendicular to the axis of the horizontal portion of the "F".
  • the arcuate length of the individual slots becomes greater as the radial distance from the centre of the antenna increases, and one set of arcuate slots is separated from the other set of slots, on both sides, by a solid, un-slotted portion of the ground plane.
  • the slots in the IFA ground plane are formed through the thickness of the ground plane.
  • the number, the relative shape, the relative size, the relative depth (into/through the ground plane and/or into any underlying dielectric layer), the relative position, etc, of the slots may be varied in order to alter the performance of the IFA (i.e. these things may be varied in order to "tune" the antenna).
  • One difficulty associated with an IFA is that the asymmetrical configuration of an IFA can result in a non-perfect (in particular non-symmetrical) toroid radiation pattern.
  • the particular configuration of the periodic slotted ground plane is used to for several reasons including: to help correct the said asymmetry of the radiation pattern, to reduce the size of the ground plane, to manipulate the surface impedance to help ensure a uniform radiation pattern, and to limit static and changing ground effects. It is thought that, in this way, the periodic slotted ground plane may be matched (at least to a suitable extent) to direct the beam of the IFA up and down the road.
  • the IFA is matched with a rectangular periodic slotted ground plane, etc, as discussed above, and this particular configuration results in a radiation pattern up and down the road as illustrated in Figure 26.
  • the periodic slotted ground plane of the IFA could possibly also be used/changed to correct/adapt the IFA's radiation pattern to be nearer to the preferred "dropped doughnut" or “toroid on the ground” shape shown in Figure 23, but with such an alternative configuration the IFA would likely still generate substantially more vertically upward energy than the particular top loaded monopole (i.e. "birdcage") antenna depicted in Figure 24.
  • a cap or top load may be used to reduce upwards radiation (as is indeed the case in the birdcage antenna in Figure 24). Such a cap or top load might therefore also be used with an IFA.
  • an IFA configuration may be used to provide a height-restricted antenna which also has an overall low, flat shaped radiation pattern, as desired.
  • an additional (typically higher frequency) antenna may be integrated into the top load or cap of an IFA.
  • Such an additional antenna may be used to provide data communications from the device (where the device incorporates the main antenna with the "dropped doughnut" radiation pattern used for RFID) to another device such as a controlling device.
  • the additional antenna may be used for Wi-Fi or WAVE, as described in the IEEE 802.1 1 standard set.
  • Figure 27 illustrates a possible example of a capped IFA with an elongated slot formed in the cap which functions as the additional antenna. In this particular example the slot forms a 2.4 GHz Wi-Fi antenna.
  • the periodic slotted circular ground plane in this example is used to correct for or accommodate the resulting imbalanced radiation pattern.
  • Figure 28 illustrates the Wi-Fi radiation pattern, which may be ideal for an in/on road device to communicate with another device located on the roadside, or on a vehicle or on a pole, etc.
  • the combined use of the RFID antenna and the additional data communications capability that may be provided by the additional antenna may help to reduce deployment and maintenance costs.
  • the antenna proposed herein will often be (and should be suitable to be) placed on the ground, or in the ground just below the surface, or at any position in between, as conditions or the application requirements dictate. This is illustrated in Figure 29.
  • a conducting reflector (preferably one having a diameter ⁇ ) may be mounted on the underside of the overhead structure, such that the reflector becomes mounted between the structure and the antenna on the underside of the structure.
  • a birdcage antenna like the one in Figure 24 for example may thus be mounted beneath and in the centre of this reflector with a standoff between the reflector and the antenna ground plane of ⁇ ⁇ /16.
  • Such a configuration may thus use the same birdcage antenna as described above but the toroid radiation pattern may be pushed downwards, preferably with the angle of maximum gain at ⁇ 45°. This change in the radiation pattern (i.e.
  • This radiation pattern (i.e. with the angle of maximum gain pushed down preferably by -45°), which may be suitable for use with windscreen tags that are mounted so as to be effectively vertically polarised, may create a RFID beam which is effective for reading windscreen tags, possibly nearly as well as plate tags, in 2 lanes in any direction. It is believed that the vertical polarisation may (even in this "upside down" configuration) alleviate some of the multi-path problems described above.
  • the toroid radiation pattern may also alleviate blinding reflections of the roof of, for example, buses.
  • the small size of the antenna may also be very useful where space is restricted, as under bridges and in tunnels.

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Abstract

La présente invention concerne une antenne pour dispositif de communication. La construction physique de l'antenne comprend un certain nombre de pièces. L'une des pièces est un plan de sol sensiblement plat. La taille de l'antenne dans une direction perpendiculaire au plan de sol est inférieure à la dimension la plus grande du plan de sol. En outre, le motif de rayonnement de l'antenne s'étend plus dans une direction parallèle au plan de sol que dans une direction perpendiculaire au plan de sol.
PCT/AU2015/050384 2014-07-14 2015-07-08 Antenne WO2016008004A1 (fr)

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CN108232418B (zh) * 2016-12-20 2021-02-02 莱森西斯澳大利亚私人有限公司 用于通信装置的天线及rfid读取器
CN108232418A (zh) * 2016-12-20 2018-06-29 莱森西斯澳大利亚私人有限公司 天线
US10402601B2 (en) 2016-12-20 2019-09-03 Licensys Australasia Pty. Antenna
TWI758485B (zh) * 2017-05-30 2022-03-21 澳大利亞商萊森西澳洲私人有限公司 天線
RU2754305C2 (ru) * 2017-05-30 2021-08-31 Лайсенсиз Аустралазиа Пти Лтд Антенна
WO2018218279A1 (fr) * 2017-05-30 2018-12-06 Licensys Australasia Pty Ltd Antenne
US11309630B2 (en) 2017-05-30 2022-04-19 Licensys Australasia Pty Ltd Antenna
AU2018276303B2 (en) * 2017-05-30 2022-11-03 Licensys Australasia Pty Ltd An antenna
KR102347047B1 (ko) * 2020-11-04 2022-01-04 주식회사 담스테크 안티 드론용 광대역 전방향 모노폴 안테나
DE102022110002A1 (de) 2022-04-26 2023-10-26 Tönnjes Isi Patent Holding Gmbh Vorrichtung und Verfahren zum Auslesen von Kennzeichen sich bewegender Objekte
DE102022110042A1 (de) 2022-04-26 2023-10-26 KATHREIN Sachsen GmbH Antennenanordnung zum Auslesen von UHF RFID Signalen
EP4270637A1 (fr) * 2022-04-26 2023-11-01 KATHREIN Sachsen GmbH Dispositif d'antenne pour la lecture de signaux rfid uhf
WO2023208423A1 (fr) * 2022-04-26 2023-11-02 Tönnjes Isi Patent Holding Gmbh Système et procédé de lecture d'identifiants d'objets en mouvement
TWI856527B (zh) * 2022-04-26 2024-09-21 德商滕尼耶施Ｉｓｉ專利控股有限公司 用於讀取移動物體之標識的裝置及方法
CN119510964A (zh) * 2025-01-22 2025-02-25 中汽研新能源汽车检验中心(天津)有限公司 参考接地平面辐射场强评估方法、系统、装置及介质

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