GB2453778A

GB2453778A - An ultra wideband antenna with a high impedance surface reflector

Info

Publication number: GB2453778A
Application number: GB0720459A
Authority: GB
Inventors: Lye Whatt Chua
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2007-10-18
Filing date: 2007-10-18
Publication date: 2009-04-22
Anticipated expiration: 2027-10-18
Also published as: GB2453778B; GB0720459D0

Abstract

An ultra-wideband antenna 10, or a method of making it, comprises a first laminar dielectric substrate 14 with a transmission element formed on a first planar surface of the substrate. The transmission element comprises a radiating element 12 which is tapered towards a narrow end which is connected to a first end of a transmission line 19. The distal, wider end, of the radiating element 12 has a v-shaped notch defining two diverging lobes where the outer edges 21 of the lobes have serrations 17. The second end of the transmission line 19 is connected to a signal connection point 15 and the transmission line is surrounded and spaced from a coplanar ground element 27. The second planar surface of the said substrate 14 is coupled to a first surface of a second laminar dielectric substrate 52 which has a plurality of conductive elements 54 on its first surface which are electrically connected through the dielectric material to a ground plane covering the second surface of the second substrate 52. The conductive elements 54 on the first surface of the second substrate may be of a regular or irregular size, shape or pattern. The second dielectric substrate and its conductive elements provide a photonic circuit element or an artificial magnetic conductor AMC or high impedance surface which is an effective radio frequency reflector such that the compact antenna provides substantially unidirectional radiation.

Description

An Ultra Wideband Antenna

Field of the Invention

The present invention relates to antennas, and more particularly to antennas for radiating ultra wide bandwidth (UWB) pulses.

Background of the Invention

Pulsed electromagnetic (elm) energy transmission and reception systems typically possess wide-band or UWB transmission spectral bandwidths. This UWB characteristic stems from the pulsed nature of the e/m energy transmitted and received by systems.

The shape of such energy pulses in the time domain is typically one of any number of approximations to a delta function, and generally has the property that the width of the frequency spectrum of such impulse increases as the time domain "length" or duration of the pulse decreases. Thus, the shorter the pulse of radiation is, the broader is its spectral bandwidth.

IJWB was previously defined as an impulse radio technology, but those skilled in the art now view it as an available bandwidth set with an emissions limit that enables coexistence without harmful interference. One of the challenges of the implementation of TJWB systems is the development of a suitable antenna that would enhance the advantages promised by a pulsed communication system. UWB systems require antennas that cover up to an octave bandwidth in order to transmit pulses on the order of a sub-nanosecond in duration with minimal distortion.

The UWB performances of antennas result from excitation by impulse or non-sinusoidal signals with rapidly time-varying performances. Thus, when an antenna is used employing such pulses in UWB applications, it is often found that the time-domain behaviour of the antenna is critical to the operation of the antenna. In particular, if an impedance mismatch or discontinuity occurs in such an antenna (such as at the open circuit end of the antenna), the consequence is often the unwanted generation of a standing wave of elm energy within the antenna's radiating element(s) caused by reflections within the antenna of the e/m energy to be transmitted.

This trapped energy not only reduces the efficiency of the transducer of which the antenna forms a part, but also masks, obscures or interferes with signals received by the transceiver while the trapped energy is still present within the antenna.

Thus, in any resonant structure, such as a dipole antenna, an impulse signal injected at the antenna input will typically be partially reflected from the open-circuited end of the dipole causing a residual reflected return signal to appear at the antenna input. This return reflection is often referred to as "ringing" or may be referred to as "aperture clutter" since it clutters/obscures the aperture of the antenna.

Pulsed UWB transceivers aie often employed in applications such as short-distance positioning, or length measurement and so on, where a pulse e/m signal is transmitted from the transceiver and its reflection is subsequently received after a very brief time period. Such an application requires that the entire e/m signal pulse has exited the antenna of the transceiver before any reflection of that signal is expected to be received.

This aims to ensure that the transmitted signal does not interfere with its received reflections and thereby obscure the positioning/measurement process.

However, ringing/aperture clutter results in just such obscurement and is highly undesirable.

Prior art pulsed UWB transceiver systems have attempted to overcome this problem by adding e/m signal absorbing material to the ends of the dipole antennas thereof or by loading the antennas with a distributed series of resistors along their length in an attempt to dampen or attenuate the standing waves therein which cause aperture clutter.

However, such solutions are generally of little effect or most likely result in undesirably excessive attenuation of receivedltransm itted signal energy.

Furthermore, short-range positioning antennas are most desirably small in physical size so as to be not only portable but also useable at close quarters and in confined spaces.

This requires the antenna to be as small as possible. However, reducing the size of an antenna has, in prior art, typically resulted in a corresponding reduction of bandwidth.

US5812081 relates to a time domain communications system in which a pulse-responsive antenna is employed to translate an applied DC impulse into a monocycle signal. Essentially, US5812081 discloses a dipole antenna which is completely the reverse of the conventional "bat wing" antenna and wherein two triangular elements of the dipole are positioned with their bases closely adjacent but DC isolated.

W00161784 relates to a bowtie antenna for use as ground penetrating radar in geometrical applications searching for metal or determining groundwater level, or determining the presence of pipes or electrical wiring. The bowtie antenna comprises a feedpoint for radiating energy, and flat mutually opposing arms that in use radiate the energy supplied at the feedpoint, wherein the feedpoint is positioned in between the opposing arms. Each of the arms comprises a carrier and an electrically conductive element attached to the carrier. Usually the carrier is a substrate to which the conductive element is attached.

Essentially, the above prior art examples disclose antennas that are suitable for ultra wideband applications. However, neither of these prior art examples discloses antennas which produce a substantially shallow radiation null along the direction of the geometrical symmetry axis of the antenna.

A physically small broadband IJWB antenna with low ringing time was published in UX patent application GB2406220. That UWB antenna demonstrates good impedance match from 3.5 GHz to 18 GHz which ensures very low ringing from harmonics of the impulse frequency. It also produces a wide elevation beam width of radiated signals and a shallow radiation null along the direction of the geometrical symmetry axis of the radiating element, which one would not expect from a conventional monopole antenna (as one would expect a complete, zero-signal null along the axis of symmetry.

Although the ground plane of the UWB antenna in GB2406220 provides excellent screening of the associated active circuit from the radiating aperture and may be formed by metallisation of the inter-compartment partition of a hand-held transceiver, the antenna, as a stand-alone component, is a 3-D structure.

Therefore, a small planar antenna structure is desirable for applications which require easy integration of the antenna on a user's clothing or on a PCMCIA PC card for WiFi, Bluetooth and UWB simultaneous applications. Such a small planar antenna structure is described in our earlier UK patent application no. 0611673.5 filed on 13 June 2006.

Different configurations of the small planar antenna are also described in the same document. Generally, the small planar antennas provide similar performance as the UWB antenna described in GB2406220. The antennas also provide omnidirectional characteristics in their azimuthal direction of radiation and a shallow radiation null along its geometrical symmetry axis of the radiating element.

In some applications, it is desirable to provide an antenna with a unidirectional characteristic. This allows the antenna to radiate greater power in a wanted direction of communication to increase performance, and to reduce interference from unwanted sources.

As shown in Figure 9, reflectors 60 are commonly used to modify the radiation pattern of an antenna 62. This is usually achieved by placing a flat sheet reflector 60 of sufficiently large dimension in the plane in the direction of unwanted radiation.

Therefore, the "backward" radiation 64 from the antenna can be significantly reduced (or eliminated) and substantial gain in the "forward" radiation 66 can be increased.

The person skilled in the art would appreciate that the radiation pattern (or the gain in the forward radiation) is determined by the distance, d, between the antenna 62 and the reflector 60. Therefore, the overall size/thickness of the antenna structure 70 could be dependent on the distance, d. In some applications, especially for antennas that are integrated on a user's clothing, it is essential to reduce the overall size/thickness of the antenna.

The present invention provides a small planar antenna structure which has an unidirectional radiation characteristic, by reducing (or eliminating) the distance between the antenna and the reflector.

Summary of the Invention

In general terms, the invention provides a laminar antenna for use in ultra-wideband communications, the antenna comprising: an antenna element including a first electrically conductive layer forming radiation means operable to form a substantially omnidirectional profile, and a dielectric layer defining a first and second opposing planar surfaces, said first electrically conductive layer being formed on said first planar surface; and a photonic Circuit element including a first electrically conductive layer forming an array of conductive elements, a second electrically conductive layer, parallel with the first layer, and providing a ground plane with respect to said array of conductive elements, and a dielectric layer separating the first layer from the second layer, each of said conductive elements is electrically coupled to said second electricailly conductive layer through at least one conductive path; such that when the first electrically conductive layer of said photonic circuit element is coupled with the second planar surface of said dielectric layer of said antenna element, said radiation means is operable to form a substantially unidirectional radiation profile.

In a first aspect of the present invention, there is provided an antenna for use in ultra wideband communications, the antenna comprising a first laminar dielectric substrate defining first and second opposing planar surfaces and a connection point for establishing electrical connection with the antenna, a transmission element formed on said first planar surface of said first laminar dielectric substrate, the transmission element comprising a radiating element and a transmission line providing electrical connection between the radiating element and the connection point, the radiating element being substantially tapered towards a narrow end thereof connected with the transmission line, the distal, wider end thereof having formed therein a substantially v-shaped notch thereby defining two lobes which diverge with increasing distance from said transmission line, wherein outer edges of said lobes have formed therein a plurality of serrations to inhibit propagation of signal waves at said outer edges, a ground element formed on said first planar surface of said first laminar dielectric substrate, substantially corresponding to the extent of said transmission line and said connection point, said transmission line and said connection point having a perimeter thereby separating said ground element and said transmission line and said connection point to provide a coplanar waveguide structure in respect of said radiating element, and a second laminar dielectric substrate, defining first and second opposing planar surfaces, a plurality of conductive elements formed on said first planar surface of said second dielectric substrate and a ground element formed on said second planar surface of said second laminar dielectric substrate, said second laminar dielectric substrate having at least one conductive path for providing electrical connection between said plurality of conductive elements and said ground element of said second laminar dielectric substrate, wherein said first planar surface of said second laminar dielectric substrate is substantially coupled with said second planar surface of said first laminar dielectric substrate.

Preferably, said v-shaped notch extends into said radiating element with an apex angle less than 90 degrees thereby substantially suppressing transverse signal modes of said radiating element.

Preferably, said serrations are log-periodically distributed such that said radiating element is operable over a wide bandwidth of signal frequencies without increasing size of said radiating element.

Preferably, said serrations are formed to enable an enhanced rate of radiative energy loss along said edge thereby reducing reflection signal travelling back along said edge.

The serrations may be formed such that each serration tip is formed by the convergence of two serration edges.

The convergence of said two sen-ation edges may be formed at an angle of between approximately 75° and 105°.

The serrations may be distributed such that corresponding dimensions of successive serrations increase log-periodically whereby the ratio of said corresponding dimensions in respect of successive serrations has constant predetermined ratio value.

The serrations of said opposing edges may be arranged axially symmetrically about an axis extending through the radiating element from said transmission line and between said two edges.

Preferably, said transmission line has a second end connected to said connection point supplying input signal therefrom.

Preferably, said ground element of said first laminar dielectric substrate has a plurality of slots spaced apart from each other at irregular intervals along its two longitudinal edges thereby suppressing resonance of said ground plane, said two longitudinal edges of said ground plane being parallel to said transmission line.

Preferably, said slots have different lengths.

The conductive elements may be arranged in an array on said first surface of said second laminar dielectric substrate.

Preferably, the conductive elements of said array are spaced at regular intervals.

Alternatively, the conductive elements of said array may be spaced at irregular intervals.

Said conductive elements may be substantially geometrically non-identical.

Alternatively, said conductive elements may be substantially geometrically similar.

Each of said conductive elements may be formed of regular or irregular structures.

The person skilled in the art will appreciate that a regular structure includes any structure that is symmetric, for example (but not limited to): circular, triangular, square, or star-shaped structures.

The person skilled in the art would also appreciate that an irregular structure includes any structure that is not regular.

The conductive path is provided between each of said conductive elements and said ground element of said second laminar dielectric substrate.

In a further independent aspect there is provided a method of manufacturing an antenna structure comprising: providing a first laminar dielectric substrate with first and second opposing planar surfaces, forming a connection point for establishing electrical connection with the antenna, forming a transmission element on said first planar surface of said first laminar dielectric substrate, the transmission element comprising a radiating element and a transmission line providing electrical connection between the radiating element and the connection point, the radiating element being substantially tapered towards a narrow end thereof connected with the transmission line, the distal, wider end thereof having formed therein a substantially v-shaped notch thereby defining two lobes which diverge with increasing distance from said transmission line, wherein outer edges of said lobes have formed therein a plurality of serrations to inhibit propagation of signal waves at said outer edges, forming a ground element on said first planar surface of said first laminar dielectric substrate, substantially corresponding to the extent of said transmission line and said connection point, said transmission line and said connection point having a perimeter thereby separating said ground element and said transmission line and said connection point to provide a coplanar waveguide structure in respect of said radiating element, providing a second laminar dielectric substrate with first and second opposing planar surfaces, fonning a plurality of conductive elements on said first planar surface of said second dielectric substrate and forming a ground element on said second planar surface of said second laminar dielectric substrate, providing at least one conductive path for providing electrical connections between said plurality of conductive elements and said ground element of said second laminar dielectric substrate, and coupling said first planar surface of said second dielectric substrate with said second planar surface of said first laminar dielectric substrate.

The sen-ations are formed such that each serration tip may be formed by the convergence of two serration edges.

The convergence of said two serration edges may be formed at an angle of between approximately 75° and 1050.

Preferably, said ground element of said first laminar dielectric substrate has a plurality of slots spaced apart from each other at irregular intervals along its two longitudinal edges thereby suppressing resonance of said radiating element, said two longitudinal edges of said ground plane being parallel to said transmission line.

Preferably, said slots have different lengths.

Preferably, said conductive elements are substantially geometrically non-identical.

Brief description of the Drawings

Embodiments of the present invention will now be described with reference to the accompanying drawings, wherein: Figure 1 shows a plan view of a front surface of an antenna element of an antenna in accordance with a specific embodiment of the present invention; Figure 2 shows a plan view of a back surface of the antenna element illustrated in Figure 1; Figure 3 shows a side view of the antenna element illustrated in Figure 1; Figure 4 shows a plan view of a front surface of a photonic circuit element of the antenna of the specific embodiment of the invention; Figure 5 shows a plan view of a back surface of the photonic circuit element illustrated in Figure 4; Figure 6 shows a side view of the photonic circuit element illustrated in Figure 4; Figure 7 shows a side view of the antenna of the specific embodiment of the present invention assembled from the antenna element illustrated in Figure 1 and the photonic element illustrated in Figure 4; Figure 8 shows a plan view of an array of antennas in accordance with a further embodiment of the present invention; and Figure 9 shows a side view of an antenna structure having an antenna element and a reflector.

Detailed Description

Specific embodiments of the present invention will be described in further detail on the basis of the attached diagrams.

In the following description, a number of specific details are presented in order to provide a thorough understanding of embodiments of the present invention. It will be apparent, however, to a person skilled in the art that these specific details need not be employed to practice the present invention.

Figures 1 to 3 show various views of a planar antenna element 10 produced on a dielectric substrate 14. The planar antenna is capable of being utilised in transmission and reception.

Figure 1 shows a front surface of the planar antenna element 10 comprising a radiating element 12, a transmission line 19 and a ground plane element 27 printed on the dielectric substrate 14. The transmission line 19 has a signal feed point 15 to provide (and to receive) signal to and from the radiating element.

The opposing end of the signal feed point of the transmission line 19 is connected to the radiating element 12. The radiating element 12 is shaped as a segment having two opposed slant edges 21, which diverge outwardly from an apex 16 of the segment.

The two opposed slant edges 21 diverge with increasing distance from the microstrip feed line 19 such that the radiating element 12 tapers outwardly from the transmission line 19. The radiating element 12 possesses two distal peripheral edges (11 and 13) which are arcuate and which respectively bridge the terminal outermost ends of the two opposed slant edges 21 and form curved outermost peripheries of the radiating element 12.

The radiating element 12 has two corresponding series of serrations 17 each formed within a respective one of the two opposed slant edges 21. Each serration of a given series of serrations is formed by a pair of successive angular (tapering) notches 18 which extend into the radiating element 12 from the respective slant edge 21. Each tapering notch has notch edges which converge to terminate within the radiating element 12 at a right-angled apex 18.

Each such serration, and the series of serrations 17 collectively, present a slow-wave structure to a signal propagating along the slant edge 21. Essentially, the slow-wave structure formed along the slant edge 21 of the radiating element 12 is provided with a meander which slows down the progress of a signal wave travelling along the slant edge 21. This is achieved by constraining the signal wave to progress along the longer meandering slant edge rather than to progress directly along a shorter linear slant edge.

As a result, the radiating element is operable over a wide bandwidth of signal frequencies without increasing the physical size of the radiating element 12.

The meanders of the slant edge 21 are shaped such that the Q-factor of the antenna is minimised thereby reducing aperture clutter by reducing the relative magnitude of a signal reaching the terminal (open circuit) end of the slant edge 21 where signal reflection tends to occur, this being the source aperture clutter. The Q-factor of the radiating element 12 is given as: stored energy Q factor c rate of energy loss Thus, the relative magnitude of a signal reaching the terminal outer edge of the slant edge (i.e. relative to the magnitude of that signal at the beginning of the slant edge) is sensitively dependent upon the rate of loss of energy from the signal during propagation along the slant edge. By suitably shaping the meanders of the slow-wave structure, the described specific embodiment of the present invention may enhance the rate of radiative energy loss of the propagating signal as it progresses along the slant edge thereby reducing aperture clutter.

Successive serrations of each series of serrations are shaped to increase in size relative to the preceding serrations in a log-period manner. Thus, the serrations in a given series have a common shape. In this example the common shape is a straight-edged serration with two tapering edges extending from the body of the radiating element 12 at predetermined angles and converging at increasing distance from the body of the radiating element 12 to a terminal right-angular serration tip or apex 18.

Each serration in a given series of serrations 17 possess two tapering edges which each extend from the body of the radiating element 12 at the same predetermined angles as occurs in respect of the edges of an adjacent serration of the series, and also converge at a right angular set-ration apex 18. The ratio of the lengths of the two tapering edges of any given serration is shared by all serrations in the same series since all serrations in a given series share the same general shape. However, due to log-periodic scaling, the lengths themselves increase by a predetermined scaling value such that the ratio of a set-ration edge length of a given serration and the corresponding edge length of the succeeding serration has a constant predetermining ratio value shared by all such neighbouring serrations.

Furthermore, each series of serrations 17 is arranged such that the distance between the location of the apex 18 of the segment of the radiating element 12 and the location of the serration increase log-periodically as one encounters successive serrations of a given series. The result is that the ratio of the aforesaid distance, as between two neighbouring (successive) serrations, is equal to a constant predetermined ratio value shared by all such neighbouring serrations. The location of the serration may be considered to be the location of the apex 18 of the tip of the serration in question, for

example.

The planar antenna element 10 also includes a ground plane 27 formed on the same surface as the radiating element 12. The ground plane 27 is separated from the transmission line 19 and the feed point 15 by the substrate 14 around the perimeter 25 of said transmission line 19 and feed point 15 thereby forming a coplanar waveguide structure.

In order to design an antenna which is capable of operating over a wide bandwidth, biasing and impedance effects of the associated DC networks must be considered from an RF or microwave perspective. DC biasing achieved from the use of RF chokes and resistors is effective only if the chokes are effectively an open circuit with no resonances, and if the combinations of inductance, resistance and capacitance do not limit the ability of the circuit to respond broad band.

The ground plane 27 comprises a plurality of slots 26 along its two longitudinal edges 28. The slots 26 along the longitudinal edges 28 have different lengths 29 and are spaced from each other at irregular intervals. In this configuration, the slots are essentially series inductance that functions as RF choke to attenuate any unwanted signals.

This configuration has an advantage in that all of the metallisation is formed on one surface of the dielectric substrate. This allows surface mount components for the associated circuitry to be mounted on the opposing surface, which is particularly useful for PCMCIA PC card applications.

Figure 4 shows a front surface of a photonic circuit element 50 comprising a plurality of conductive elements 54 printed on one surface of the dielectric substrate 52, and a ground plane 56 printed on an opposing surface of the dielectric substrate. As illustrated in Figure 4, the conductive elements 54 are geometrically similar and have octagonal structures. The conductive elements 54 are arranged in an array and are spaced from each other at regular intervals. Conductive vias 58 are provided through the dielectric substrate 52 to electrically connect each of the conductive elements 54 to the ground plane 56.

Alternatively, the conductive elements can be of different geometries and structures, and can be arranged in a different array pattern with elements spaced at irregular intervals. In fact, this arrangement would substantially increase the operating bandwidth of the photonic circuit element.

Figure 7 illustrates a side view of the antenna of the present invention including the photonic element being positioned under the antenna element. This configuration provides an antenna that produces a unidirectional radiation characteristic. It will be appreciated by the person skilled in the art that the photonic element may not be physically attached to the antenna element in order to produce a unidirectional radiation characteristics. The person skilled in the art will appreciate the antenna would provide the same performance when the photonic element is positioned substantially close the antenna element.

The present invention, for example, as shown in the above embodiments, may provide an ultra wide-band (UWB) electromagnetic impulse transceiver for applications in short range communications and/or positioning systems. The invention may be implemented in the form of a monopole antenna thereby obviating the need for a balun with the antenna circuitry. The antenna according to the present invention in any of its embodiment has the important benefit of being sufficiently small for use as a portable impulse transceiver.

Furthermore, monopole antennas structured according to the present invention in its first aspect display up to an octave bandwidth, have reduced aperture clutter with moderate signal loss and have relatively small physical size.

The antenna described above is unidirectional, having a substantially linearly polarised radiation.

The planar structure of the antenna of the specific embodiment is also easy to manufacture in large volumes. Furthermore, the associated electronics components of a PCMCIA PC card can be incorporated on the same substrate as the planar structure of the antenna. This is also useful in other applications, especially for antennas that are integrated on a user's clothing.

It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalents thereof are intended to be embraced therein.

Claims

CLAIMS: An antenna for use in ultra wideband communications, the antenna comprising: a first laminar dielectric substrate defining first and second opposing planar surfaces and a connection point for establishing electrical connection with the antenna; a transmission element formed on said first planar surface of said first laminar dielectric substrate, the transmission element comprising a radiating element and a transmission line providing electrical connection between the radiating element and the connection point, the radiating element being substantially tapered towards a narrow end thereof connected with the transmission line, the distal, wider end thereof having formed therein a substantially v-shaped notch thereby defining two lobes which diverge with increasing distance from said transmission line, wherein outer edges of said lobes have formed therein a plurality of serrations to inhibit propagation of signal waves at said outer edges; a ground element formed on said first planar surface of said first laminar dielectric substrate, substantially corresponding to the extent of said transmission line and said connection point, said transmission line and said connection point having a perimeter thereby separating said ground element and said transmission line and said connection point to provide a coplanar waveguide structure in respect of said radiating eJement; and a second laminar dielectric substrate, defining first and second opposing planar surfaces, a plurality of conductive elements formed on said first planar surface of said second dielectric substrate and a ground element formed on said second planar surface of said second laminar dielectric substrate, said second laminar dielectric substrate having at least one conductive path for providing electrical connection between said plurality of conductive elements and said ground element of said second laminar dielectric substrate; wherein said first planar surface of said second laminar dielectric substrate is substantially coupled with said second planar surface of said first laminar dielectric substrate such that said radiation element is operable to form a substantially unidirectional radiation profile.
2. An antenna in accordance with claim 1, wherein said v-shaped notch extends into said radiating element with an apex angle less than 90 degrees thereby substantially suppressing transverse signal modes of said radiating element.
3. An antenna in accordance with any one of the preceding claims, wherein said serrations are log-periodically distributed such that said radiating element is operable over a wide bandwidth of signal frequencies without increasing size of said radiating element.
4. An antenna in accordance with any one of the preceding claims, wherein said serrations are formed to enable an enhanced rate of radiative energy loss along said edge thereby reducing reflection signal travelling back along said edge.
5. An antenna in accordance with any one of the preceding claims, wherein said serrations are formed such that each serration tip is formed by the convergence of two serration edges.
6. An antenna in accordance with claim 5, wherein said convergence of said two serration edges is formed at an angle of between approximately 75° and 105°.
7. An antenna in accordance with any one of the preceding claims, wherein said serrations are distributed such that corresponding dimensions of successive serrations increase log-periodically whereby the ratio of said corresponding dimensions in respect of successive serrations has constant predetermined ratio value.
8. An antenna in accordance with any one of the preceding claims, wherein said serrations of said opposing edges are arranged axially symmetrically about an axis extending through the radiating element from said transmission line and between said two edges.
9. An antenna in accordance with any one of the preceding claims, wherein said transmission line has a second end connected to said connection point supplying input signal therefrom.
10. An antenna in accordance with any one of the preceding claims, wherein said ground element of said first laminar dielectric substrate has a plurality of slots spaced apart from each other at irregular intervals along its two longitudinal edges thereby suppressing resonance of said ground element, said two longitudinal edges of said ground plane being parallel to said transmission line.
11. An antenna in accordance with claim 10, wherein said slots have different lengths.
12. An antenna in accordance with any one of the preceding claims, wherein said conductive elements are arranged in an array on said first surface of said second laminar dielectric substrate.
13. An antenna in accordance with any one of the preceding claims, wherein said conductive elements are substantially geometrically similar.
14. An antenna in accordance with any one of the preceding claims, wherein said conductive elements of said arrays are spaced at regular intervals.
15. An antenna in accordance with any one of claims 1 to 13, wherein said conductive elements of said array are spaced at irregular intervals.
16. An antenna in accordance with any one of the preceding claims, wherein said conductive elements are substantially geometrically non-identical.
17. An antenna in accordance with any one of claims I to 15, wherein said conductive elements are substantially geometrically similar.
18. An antenna in accordance with any one of preceding claims, wherein each of said conductive elements are formed of regular or irregular structures.
19. An antenna in accordance with any one of preceding claims, wherein said conductive path is provided between each of said conductive elements and said ground element of said second laminar dielectric substrate.
20. A method of making an antenna structure comprising: providing a first laminar dielectric substrate with first and second opposing planar surfaces, and forming a connection point for establishing electrical connection with the antenna; forming a transmission element on said first planar surface of said first laminar dielectric substrate, the transmission element comprising a radiating element and a transmission line providing electrical connection between the radiating element and the connection point, the radiating element being substantially tapered towards a narrow end thereof connected with the transmission line, the distal, wider end thereof having formed therein a substantially v-shaped notch thereby defining two lobes which diverge with increasing distance from said transmission line, wherein outer edges of said lobes have formed therein a plurality of serrations to inhibit propagation of signal waves at said outer edges; forming a ground element on said first planar surface of said first laminar dielectric substrate, substantially corresponding to the extent of said transmission line and said connection point, said transmission line and said connection point having a perimeter thereby separating said ground element and said transmission line and said connection point to provide a coplanar waveguide structure in respect of said radiating element; providing a second laminar dielectric substrate with first and second opposing planar surfaces, forming a plurality of conductive elements on said first planar surface of said second dielectric substrate and forming a ground element on said second planar surface of said second laminar dielectric substrate; providing at least one conductive path for providing electrical connection between said plurality of conductive elements and said ground element of said second laminar dielectric substrate; and coupling said first planar surface of said second laminar dielectric substrate with said second planar surface of said first laminar dielectric substrate.
21. A method in accordance with claim 20, wherein said ground element has a plurality of slots spaced apart from each other at irregular intervals along its two longitudinal edges thereby suppressing resonance of said ground element, said two longitudinal edges of said ground plane being parallel to said transmission line.
22. A method in accordance with claim 21, wherein said slots have different lengths.
23. A method in accordance with any one of claims 20 to 22, wherein said conductive elements are arranged in an array on said first surface of said second laminar dielectric substrate.
24. A method in accordance with any one of claims 20 to 23, wherein said conductive elements of said arrays are spaced at regular intervals.
25. A method in accordance with any one of claims 20 to 23, wherein said conductive elements of said array are spaced at irregular intervals.
26. A method in accordance with any one of claims 20 to 25, wherein said conductive elements are substantially geometrically non-identical.
27. A method in accordance with any one of claims 20 to 25, wherein said conductive elements are substantially geometrically similar.
28. A method in accordance with any one of claims 20 to 27, wherein each of said conductive elements are formed of regular or irregular structures.
29. A method in accordance with any one of claims 20 to 28 wherein said conductive path is provided between each of said conductive elements and said ground element of said second laminar dielectric substrate.
30. Apparatus substantially as herein described with reference to any of Figures 1 to 8 of the accompanying drawings.
31. A method substantially as herein described with reference to any of Figures 1 to 8 of the accompanying drawings.