GB2393855A

GB2393855A - An antenna

Info

Publication number: GB2393855A
Application number: GB0222738A
Authority: GB
Inventors: Lye Whatt Chua
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2002-10-01
Filing date: 2002-10-01
Publication date: 2004-04-07
Anticipated expiration: 2022-10-01
Also published as: GB2393855B; GB0222738D0

Abstract

An omnidirectional antenna 1 for transmitting and/or receiving circularly polarised electromagnetic radiation. Antenna 1 comprises a first non-planar antenna element 2 formed from a twin start spiral mounted on a dielectric substrate 4a, and a second non-planar antenna element 3 formed from a twin start spiral mounted on a dielectric substrate 4b. The substrates 4a and 4b are curved and arranged such that the antenna elements 2 and 3 are on opposite outward facing sides, thus forming a partial wrap around structure. A ground plane 5 is arranged between the antenna elements 2 and 3 to form an electromagnetic transmission line therebetween.

Description

l An Antenna The present invention relates to antennas for transmitting

and/or receiving electromagnetic radiation.

In situations where communication with a moving object is 10 to be performed using radio or microwave telecommunications techniques, it is often desirable that the object in question has the ability to receive and/or transmit communications signals from/in as many directions as possible. The desire is to enable 15 reception/transmission of communications signals irrespective of the relative position or orientation of the object relative to those who wish to communicate with it. 20 In attempts to provide telecommunication receivers/ transmitters having the above ability, some existing telecommunications systems employ an array of many separate flat (i.e. "planar") antennas. Each such planar antenna is typically able to transmit/receive radiation 25 prir-cipally confined to one "forward" hemisphere, the variation of signal amplitude within which is determined

by the antenna gain. This is particularly the case with antennas arranged to transmit/receive circularly polarised radiation and which comprise, for example, a flat spiral antenna conductor arrangement.

S Transmission/reception from or to flat spiral antennas occurs principally along the "spiral axis" of the antenna (i.e. perpendicular to the plane containing the spiral) about which the antenna conductor "spirals".

10 Planar anLcnrlas such as these are sometimes arranged in a generally circular array such that the array is able to transmit and receive radiation propagating in any generally radia] direction towards/from that circle array. Provided that sufficient antennae are used, such 1S arrangements are able to "see" in all such radial directions from/towards the array of antennae with no substantial "blind spots".

However, such prior art antenna arrays often require a

20 large number of separate planar antennae. Typically at least four separate antennae are required (Lhe first being forward-looking, the second rearward-looking, third and fourth looking side-to-side) and ofCcr1 employ many more in order to ensure a subsLanLial]y uniform 360 25 transmiL/r-eceive capability. 'l'he result is a complex and typically bulky composite antenna which rn,-y be expensive

to produce and difficult to accommodate in the often limited spaces in which such antenna are often desired to be used.

5 The present invention aims to provide a substantially omnidirectional antenna which overcomes at least some of the deficiencies in the prior art.

In the following, the term "omnidirectional", in the 10 context of antenna transmission or reception, is intended to indicate the ability of an antenna to simultaneously transmit or receive radiation to or from all locations on a continuous locus of points which surround/enclose the antenna. For example, the locus could comprise all 15 locations surrounding the antenna which reside in one mutual plane intercepting the antenna. This example gives 360 visibility within that plane without any "blind-spots". 20 At its most general, the present invention proposes a composite antenna employing separate non-planar antenna clcmcots formed so as to be equivalent to planar antenna elements that have been deformed out of their planes and towards each other, so as to provide at least a partial AS "wrap-around" effect thereby. This arrangement is particularly, though not exclusively, applicable to

antennae arranged to transmit and/or receive circularly polarised radiation.

As few as two separate antennae may be employed in this 5 way, the "wraparound" configuration of these antennae assists in providing an omnidirectional transmit/receive capability to the composite antenna structure. The present invention also proposes employing antennae dimensioned to transmit and receive a circularly 10 polarized radiation so as to provide a compact omnidirectional antenna responsive to circularly polarised radiation and employing as few as two separate antennae. 15 The present invention also proposes arranging ground plane means between the non-planar antenna elements of the composite antenna in such a way as to form a waveguide or electromagnetic transmission line structure in conjunction with the antenna elements. This is found 20 to permit efficient use of energy emanating from the antenna elements. For example, the ground plane means may be arranged to lie substantially on an equipotential surface on a virtual ciisplacement current path between the non-planar antenna elermcnUs. The advantages of such 25 an arrangement shall be discussed below.

The antenna elements may comprise a conductive strip or strips arranged over the ground plane means such that the strip or strips and ground plane together may behave as a wave guide/transmission line when the antenna is used.

5 The conductive strip or strips may be arranged in any suitable pattern (e.g. in a spiral pattern) over the ground plane means.

It is to be understood that the term "ground plane" used 1() herein is intended to be inclusive of structures which are not planar or flat, but which possess surface curvature. In a first of its aspects, the present invention may 15 provide an antenna for transmitting and/or receiving electromagnetic radiation, the antenna including: a first non-planar antenna element dimensioned to transmit and receive circularly polarized radiation; a separate second non-planar antenna element opposing 20 the first non-planar antenna element and dimensioned to transmit and receive circularly polarised radiation; ground plane means arranged between the first non-

planar antenna element and the second non-pJanar antenna element and spaced therefrom so as to form an 25 electromagnetic transmission line therewith; the ftr,L r-or--planar antenna element being deformed

such that parts of the first non-planar antenna element are displaced from other parts thereof towards the second non-planar antenna element; and, the second non-planar antenna element being deformed 5 such that parts of the second non-planar antenna element.

are displaced from other parts thereof towards the first non-planar antenna element.

Thus by taking two antenna elements each arranged to 10 transmit/recei.ve circularly polarized radiation, and forming those antenna elements so as to have portions which are displaced or "bent" towards each other, it is possible to achieve composite antenna which is both substantially omnidirectional and capable of operating 15 with circularly pol.arised radiation. Those portions of a given antenna element which are displaced towards the other antenna element are preferably the non-central parts of the given antenna clement. Preferably, the central area of any given one of the two antenna elements 20 is most distant from the other of the two antenna elements. For example, the central portions of each of the first and second antenna elements may be deformed into a convex structure with the central portions being arranged at the apex of the respective convex structure 25 formed thereby.

The term "transmission line" used above and hereafter is intended to be inclusive of structures also known generally as "waveguides" which employ electromagnetic waveguide techniques as will be apparent to the skilled 5 person.

The provision of ground plane means for each of the first and second nonplanar antenna elements enables a waveguide or electromagnetic transmission-line structure 10 to be formed and may provide an electromagnetic coupling between the first and second antenna elements. As shall be discussed below, this coupling may enhance the ahi.l.ity of the antenna to transmit/receive radiation omnidirectionally. Preferably, the ground plane means includes a first ground plane portion arranged in spaced opposition to the first non-planar antenna element, and a second ground plane portion arranged in spaced opposition to the second 20 nonplanar antenna element, wherein the first and second ground plane portions are in electrical contact with each other SUC}1 as to permit electrical conduction therebetween. The first and second ground plane portions may he:integrally formed as one continuous ground plane 25 means, or may be separately formed ground plane portions which are brought into electrical contact with each other

either by virtue of being in direct physical contact or being connected together by any other means which permit electrical conduction therebetween.

5 In a conventional spiral antenna, a back to back arrangement of spirals is not beneficial. This is because the mutual interaction of the inwardly directed electromagnetic fields associated with the bi-directional

radiation of each spiral will have a detrimental effect 10 on the performance of each spiral. The transmission line geometry of the proposed invention, not only eliminates the above problem, but also provides a relatively low loss transmission line path from the spiral input terminal to the active radiating area identified for each 15 input microwave frequency.

Careful choice of the separation between the antenna elements and the ground plane ensures that efficient wave-guiding is achieved by the transmission line 20 structure formed thereby. Accordingly, it is preferable that the first ground plane portion has a first ground plane surface arranged in substantially uniformly spaced opposition to the first non-planar antenna element, and the second ground plane portion has a second ground plane 25 surface arranged in substantially uniformly spaced opposition to the second non-planar- antenna clement,

wherein the first and second non-planar antenna elements are deformed to conform to, and to correspond with, the opposing first and second ground plane surfaces respectively thereby to provide substantially uniform 5 spacing therefrom.

It will be readily apparent that the value of the aforementioned uniform spacing should be appropriately chosen such that it is not so small as to suppress 10 radiation from an antenna element, but is not too large that radiation loss from the transmission line/waveguide structure formed thereby is not excessive.

Preferably, each of the first and the second antenna IS elements comprise one or more strips of electrically conductive material which, in conjunction with the ground plane means, form a microstrip transmission line.

Preferably, the antenna includes signal feed means 20 operatively connected to the first non-planar antenna element and to the second nonplanar antenna element and arranged to direct electrical currents through the first non-planar antenna element and through the second non-

planar antenna element such that electrical currents 25 directed through opposing parts of the first and second non-planar antenna elements are oppositely directed.

By ensuring that signal currents passing through the opposing parts of the first and second antenna elements are at all times oppositely directed, an electromagnetic 5 coupling may be provided between the opposing antenna parts which, in conjunction with the ground plane means between the opposing antenna elements, supports a virtual displacement current loop pathway which couples the opposing antenna elements.

Consequently, the virtual displacement current loop may enable the transmission and reception of circularly polarised radiation to occur therefrom as if the loop itself were a continuous physical antenna loop. This may 15 enable the antenna to transmit/receive circularly polarised radiation at portions of the composite antenna where no physical antenna loop is present.

Preferably the structure of the second non-planar antenna 20 element substantially matches the structure of the first non-planar antenna element and the second non-planar antenna element is arranged in opposition to the first non-planar antenna clement such that those parts of the second non-planar antenna element which oppose the first 25 nonplanar antenna element oppose equivalent parts of the first non-planar antenna elements. As a result of the

above manner of electromagnetic excitation of the conductors of the two antenna elements the electric field

vectors of the two sets of associated transmission lines appear codirectional, thus creating a virtual 5 displacement current between the two antenna elements.

This will be elaborated further below.

Preferably, the first and the second non-planar antenna elements are arranged in close proximity in a back-to 10 back arrangement with the convex portions thereof facing in opposite outward directions.

For example, when in a back-to-back arrangement, the oppositely facing first and second antenna elements may 15 be aligned such that the peripheral portions of the first antenna element which have been deformed towards the second antenna element arc closest to corresponding peripheral parts of the second antenna element which have been deformed towards the first antenna element. In this 20 way a relatively symmetrical back-to-back alignment may be achieved which may assist in providing ornnidirectionality within the antenna.

The form and structure of each of the first and 25 second non-planar antenna elements may be any structure which is capable Of transmitting or receiving circularly

polarized radiation as would be readily apparent to the skilled person. Each antenna element may be in the form of a continuous conducting structure and may itself comprise a number of unconnected sub-elements.

5 1 Preferably, the first non-planar antenna element is a spiral antenna element, andi the second non-planar antenna element is a spiral antenna element) 10 the first spiral antenna element being deformed such that inner spiral parts of the first antenna element are displaced from outer spiral parts thereof towards the second spiral antenna element) and, the second spiral antenna element being deformed such 15 that inner spiral parts of the second antenna element are displaced from outer spiral parts thereof towards the first spiral antenna element.

As each non-planar antenna element may be a non-planar 2() spiral configuration with, for example, the peripheral spiral parts of the first antenna element being aligned to oppose (back-to-back) the peripRc-ral spiral portions of the second non-planar antenna element. Any generally spiral geometry or formation may be employed for each of 25 the antenna elements.

The term "spiral" is intended to include any structure in the form of an elongate arm extending outwardly of a point so as to form a continuous succession of open loops which turn in a common direction/sense (when viewed from 5 a fixed relative position) about a mutual spiral axis such that any succeeding loop extends from the terminal end of a preceding loop whereby any given loop at least partia]l.y circumscribes a preceding loop when viewed along the spiral axis.

The term "spiral" is intended to include not only planar spirals but also non-planar spirals in which the arm of the spiral extends not only in a direction perpendicular to the spiral axis but also in a direction parallel to 15 that axis. It is also intended to include arcuate arm structures as well as non-arcuate structures such as "square" spirals comprising loops each of which comprises four successive linear loop portions of increasing length which each extend at right angles from the end of a 20 respective preceding linear loop portion.

Wherc the antenna elements are spiral in form, it is preferable that each of the first and second spiral antenna elements are a twin-start spiral comprising: 25 a first sub-spiral element; and, a complementary second sub-spiral element being a

spiral of shape and dimensions corresponding to those of the first subspiral element; the second sub-spiral and the first sub-spiral arranged about a mutual spiral axis in mutually 5 interleaving orientation (without touching each other) such that corresponding parts of the first sub-spiral and the second sub-spiral are located at substantially equal distances from the mutual spiral axis at opposite sides thereof. An example of a planar equivalent of such a spiral structure is often referred to a "twin-start" spiral.

The term "twin-start" spiral herein shall be intended to relate to nonplanar equivalents.

Thus, each sub-spiral element provides a conductor which, for example, has a starting point near the centre of the spiral antenna element with which it is associated, and an end point at the periphery of the antenna element.

20 Each spiral antenna element may be energised at its centre by means of, for example, a coaxial cable with one of the conductors of the coaxial cable being used to eneryise one of the sub-spiral elements of a given antenna clement with clectrical signals while the other 25 conductor of the coaxial cable is connected to the ground plane means. The two subspiral antenna elements of a

given antenna elements are connected in parallel in this way, as would be readily apparent to the skilled person.

Where oscillating currents are driven through the antenna element in this way, via the coaxial cable, circularly 5 polarized electromagnetic radiation may be caused to propagate outwardly from the antenna element.

Furthermore, the spiral nature of such twin-start spiral antenna elements typically causes the transmitted 10 radiation to be substantially broadband. This can be appreciated by considering that each of the twinstart spiral antenna elements behaves as if it were a two-wire transmission line with the first sub-spiral element comprising one wire of the line and the complementary 15 second sub-spiral element representing the other wire thereof. Typically, a two-wire linear transmission line in which the spacing between the wires is small relative to the wavelength of radiation to be transmitted thereby, with produce substantially no (or at least very little) 20 radiation when energised at its terminals with oscillating electrical signals of opposite instantaneous signal polarities. In such a case, since the currents being driven through the two wires are in antiphase (oppositely directed), the radiation emanating from one 25 dine substantially cancels that emanating from the other dine. The result is subst-ant:i-Jlly no radiation.

However, the two-wire spiral transmission line represented by a twinstart spiral antenna may, as one progresses along the spiral gradually and continuously S transform from a non-radiating structure to a radiating structure for a given wavelength of radiation. For example, oscillating signal currents input to the centre of each of the sub-spiral elements substantially in antiphase (opposite instantaneous polarity) cause 10 currents in nei.ghbouring spiral loop segments of a given antenna element to be initially oppositely directed at the spiral centre. However, since such neighbouring loop segments are reached by traversing different electrical path distances along the two sub-spirals of a twin- start IS spiral, whenever that path difference is equivalent to a half- integer multiple of the signal wavelength the neighbouring oscillating signals are brought into phase.

Consequently, radiation transmission reaches a maximum value at that point of the twin-start spiral where such a 20 phase condition exists and for radiation having a wavelength equal to that of the alternating currents in question. In genera], a spiral loop of a giver circumference will 25 preferentially transmit or receive radiation having a wavelength substantially equal. to Che magnitude of that

circumference. Since successive spiral loop circumferences increase continuously from a minimum value to a maximum value, the transmission and reception bandwidth of the spiral antenna element similarly extends 5 across a corresponding range of wavelength values. In this way, the twinstart spiral antenna elements of the antenna may provide substantially omnidirectional broadband circularly polarized electromagnetic radiation transmission or reception.

The form of the spiral or spirals employed in each antenna element may be of any generally spiral type and need not be of a uniformly increasing spiral arm radius.

15 For example, each of the first and second spiral antenna elements may comprise a square spiral (when viewed along the spiral axis). The spirals may be Archernedian spirals, logarithmic spirals or the like.

20 The spiral or spirals of each spiral antenna element may be deformed or displaced in a direction perpendicular the spiral axis such that then viewed along the spiral axis the resulting spiral appears squashed in a given direction or stretched in a given direction or otherwise 25 irregularly shaped. The spiral may comprise a succession of linear spiral portions forming a spiralling polygon

structure with linear spiral sections extending between successive vertices of the polygon. For example, the spirals may be Archemedian-like, but squashed/stretched into a generally square spiral shape. This structure 5 makes efficient use of space when spirals are mounted on a straight-edged substrate for example.

Preferably, portions of the first spiral antenna element extending between successive vertices thereof are 10 arranged in parallel orientation relative to opposing portions of the second spiral antenna element extending between successive vertices thereof. This enables better coupling between the opposing portions when creating the aforementioned virtual displacement current loops 15 therewith. Indeed with this aim in mind the antenna may be arranged such that each portion of the first spiral antenna element extending between successive vertices thereof, is arranged in parallel orientation to an opposing portion of the second spiral antenna element 20 extending between successive vertices thereof.

The antenna may include a dielectric spacer means located in the space between the ground plane means arid the first non-planar antenna element and in the space between the 25 grour-d plane means and the second nonplanar antenna elcrnerlt. 1hc dielectric spacer means may comprise any

suitable dielectric material as will be apparent to the skilled person. The dielectric may simply be an air gap.

The anterloa may preferably further comprise: 5 substrate means upon which the first and second non planar antenna elements are arranged.

Where a substrate means is provided, it may comprise oppositely facing curved surfaces upon which the first 10 and second non-planar antenna elements are respectively arranged. For example, the oppositely facing curved surfaces may correspond with oppositely-facing surfaces of a 15 cylindrical surface as a right-circular or a right-

elliptical cylindrical surface.

Alternatively, the substrate surfaces may correspond to oppositely facing hemispherical surfaces. The antenna 20 elements and the ground plane means are preferably each formed from an electrically conductive material.

Preferably, the substrate means is formed from a dielectric material and, the first and the second non 25 planar antenna elements comprise lines of conductive material printed upon the substrate means. The lines of

conductive material may be printed on the outwardly facing opposite substrate surfaces.

The signal feed means of the antenna may comprise one 5 more balanced-tounbalanced (Balun) signal transformer to at least one of which Balun(s) each of the first and the second non-planar antenna elements are operatively connected. 10 Each of the first and the second non-planar antenna elements may be operatively connected to a mutual Balun but may be connected to separate Baluns.

The or each Balun signal transformer is preferably 15 arranged to transform an oscillating electrical input signal received from a coaxial cable into two separate oscillating electrical drive signals of equal instantaneous magnitude (being half the magnitude of the oscillating signal received by the Balun), and each 20 having an instantaneous polarity which is opposite to the other relative to a common Earth. The Balun signal transformer or transformers are preferably arranged to feed a drive signal of one instantaneous polarity to one of the fir-et and second sub-spiral elements of a given 25 antenna element and to feed a drive signal of the opposite instantaneous polarity to the other of those

sub-spiral elements. The Balun(s) and the ground plane means are preferably connected to a mutual Earth such that the magnitudes of the opposite-polarity electrical drive signals are magnitudes (and polarities) as measured 5 relative to the voltage of the ground plane means.

rl'he present invention may provide a microwave transmitter, receiver or transceiver comprising an antenna according to the first aspect of the invention.

10 Thus it will be appreciated that the above described signal transformation of the Balun(s) (so as to produce drive signals from input signals) may operate in reverse whereby received electromagnetic radiation induces "drive" signals in receiving antennas and the Balun(s) IS transform such induced signals into a single-polarity receive signal for output to a signal receiver.

It will be appreciated that the invention in its first aspect, according to any number of the preferable 20 features and variants described above represent the physical implementation of a corresponding method for transmitting and/or receiving electromagnetic radiation.

It is intended that the present invention may provide such a corresponding method or methods.

In a second of its aspects, the present invention may provide a method for transmitting and/or receiving electromagnetic radiation, the method including: providing a first non-planar antenna element S dimensioned to transmit and receive circularly polarized radiation; providing a separate second non-planar antenna clement dimensioned to transmit and receive circularly polari.sed radiation; and, 10 providing ground plane means arranging the first non-planar antenna element in opposition to the second non-planar antenna element such that parts of the first non-planar antenna element are displaced from other parts thereof towards the second 15 non-planar antenna element; arranging the second non-planar antenna element such that parts of the second non-planar antenna element are displaced from other parts thereof towards the first non-

planar antenna element; and, 20 arranging the ground plane means between the first non-

planar antenna element and the second non-planar antenna element being spaced therefrom so as to form an electromagnetic transmission line therewith.

25 PreLe.rably, the method includes:

providing a ground plane means in the form of a first ground plane portion and a second ground plane portion; arranging the first ground plane portion in spaced 5 opposition to the first non-planar antenna element; and, arranging the second ground plane portion in spaced opposition to the second non-planar antenna element; wherein the first and second ground plane portions are arranged in electrical contact with each other such 10 as to permit electrical conduction ti-erebetween.

Accordingly, the method may provide that the first ground plane portion is provided with a first ground plane surface and the second ground plane portion has a second 15 ground plane surface, and the method may preferably include; arranging the first ground plane surface in substantially uniformly spaced opposition to the first non-planar antenna element; and, 20 arranging the second ground plane surface in substantially uniformly spaced opposition to the second non-planar antenna element; wherein the first and second non-planar antenna elements are deformed to conform to, and to correspond 25 with, the opposing first and second ground plane surfaces

respectively thereby to provide substantially uniform spacing therefrom.

Preferably, each of the first and the second antenna 5 elements may be provided with one or more strips of electrically conductive material which in conjunction with the ground plane means form a microstrip transmission line.

10 The method preferably further includes: directing electrical currentsthrough the first non-

planar antenna element and through the second non-planar antenna element such that electrical currents directed through opposing parts of the first and second non-planar 15 antenna elements are oppositely directed.

In accordance with this method it is preferable that the structure of the second non-planar antenna element substantially matches the structure of the first non 20 plar-ar antenna element and the second non-planar antenna element is arranged in opposition to the first non-planar antenna element such that those parts of the second non-

planar antenna element which oppose the first non-planar antenna elemerlloppose equivalent parts of the first non 25 planar antenna element.

Preferably, the method includes employing antenna elements in which the first non-planar antenna element is a spiral antenna element, and; the second non-planar antenna element is a spiral 5 antenna element; the first spiral antenna element being deformed such that inner spiral parts of the first antenna element are displaced from outer spiral parts thereof towards the second spiral antenna element; and, 10 the second spiral antenna element being deformed such that inner spiral parts of the second antenna element are displaced from outer spiral parts thereof towards the first spiral antenna element.

15 Preferably, each of the first and second spiral antenna elements are twin-start spirals comprising: a first sub-spiral element; and, a complementary second sub-spiral element being a spiral of shape and dimensions corresponding to those of 20 the first sub-spiral element; the second sub-spiral and the first sub-spiral being arranged about a mutual spiral axis in mutually int:>rlcavirl orientation such that corresponding parts of the first sub-spiral and the second sub-spiral are 25 located at substantially equal distances from the mutua] spiral axis at opposite sides thereof.

For example, each of the first and second spiral antenna elements may comprise a square spiral.

5 The method preferably includes the step of arranging the antenna elements such that portions of the first spiral antenna element extending between successive square vertices thereof, are arranged in parallel orientation relative to opposing portions of the second spiral 10 antenna element extending between successive vertices thereof. The method may further comprise: providing a dielectric spacer means; and, 15 locating the dielectric spacer means in the space between ground plane means and the first non-planar antenna element and in the space between ground plane means and the second non-planar antenna element.

20 The method may also comprise: providing a substrate means; and, arranging the first and second non-planar antenna elements thereupon.

25 The substrate means, preferably comprises oppositely facing curved surfaces upon which the first and second

non-planar antenna elements are respectively arranged.

It is preferable the substrate means is a dielectric substrate and, the first and the second non-planar antenna elements comprise lines of conductive material 5 printed upon the substrate means.

Exemplary and non-limiting embodiments of the present invention shall now be described with reference to the attached drawings in which: 10 Figure 1 illustrates a top view of an antenna comprising two opposing spiral antenna elements; Figure 2a illustrates a side view of the antenna illustrated in Figure 1, and illustrates one of the spiral antenna elements in the form of a twin-start 15 spiral being a square spiral when viewed along the spiral axis as shown in the figure; Figure 2b illustrates a side view of the antenna element of Figure 1, the side view being of the side of the antenna opposite to that shown in Figure 2a.

20 Figure 3 illustrates schematically opposing peripheral portions of the two antenna elements of the antenna of Figure 1, and indicates electrical current directions therethrough.

25 In the figures, like parts and elements have been assigned like rofererlce numbers.

2g Referring to Figure 1 there is illustrated an antenna 1 comprising a first non-planar antenna element 2, comprising a twin-start spiral formed upon a first 5 dielectric substrate structure 4a a second non-planar antenna element 3 comprising a twin-start spiral formed upon a second dielectric substrate structure 4b arranged in back-to-back alignment with the first non-planar antenna element 2. The dielectric substrate structures 4a 10 and 4b each possess an outward surface portion having a curvature corresponding to that of opposing surfaces of a hollow right cylinder. Figure 1 illustrates the antenna as viewed along the cylindrical axis of the (imaginary) cylinder to which the outward surfaces of substrates 4a 15 and 4b conform. That is to say, the view illustrated in Figure 1 represents a cross-sectional view across the cylindrical axis (the /,-axis in this case), whereby the outer surface of the substrate structures 4a and 4b define an ellipse in the Y-X plane.

Each of the first and the second non-planar antenna elements (items 2 and 3 of Figure 1 respectively) are deformed so as to conform to the aforementioned convex outer elliptical surface on opposite outward-facing sides 25 thereof. To this extent each of the first and second

non-planar antenna elements, 2 and 3, partially "wrap-

around" the surface of the substrate structure 4.

In the space between the antenna elements 2 and 3 is 5 housed a ground plane structure 5 and a broadband Balun 6. The ground plane structure 5 of the antenna 1 takes the form of a right cylinder formed of conductive material electrically connected to "Earth" and being of slightly smaller dimensions than those of the right lO cylinder associated with the curvature of the substrate structures 4a and 4b. The dimensions of the ground plane structure 5 are chosen to be such that the separation X between the outer cylindrical surface of the ground plane and the opposing inner cylindrical surface of the IS substrate structure 4 its substantially constant wherever those two surfaces oppose. A dielectric spacer material ]00 is arranged in between the ground plane structure 5 and the first and second antenna elements 2 and 3. The dielectric spacer 100 has a uniform thickness X and 20 comprises expanded polystyrene foam having a dielectric constant of 1.0. The dielectric spacer 100 takes the form of a hollow eliptical right cylinder upon the outer curved surface of which the first and second antenna elements are arranged (together with their respective 25 dielectric substrates 4a and 4b).

Figure 2 illustrates a side-view of the antenna l illustrated in Figure l as viewed along the -Y axis thereby bringing the second antenna element 3 into view.

5 'lhe second antenna element 3 takes the form of a twin-

start spiral printed upon a dielectric substrate 4b, the spiral comprising a first sub-spiral element 3A and a complementary second sub- spiral element 3B. Each of the first and second sub-spiral elements 3A and 3B take the 10 form of square spirals arranged about a mutual spiral axis in mutually interleaving orientation (without touching each other) about that mutual spiral axis such that corresponding parts of the first and the second sub-

spirals are located at equal distances from, but at 15 opposite sides of, their mutual spiral axis. In the example shown in Figure 2 the mutual spiral axis is the -Y axis extending into the plane of Figure 2 toward the origin of co-ordinates.

20 The first sub-spiral element 3A begins from point A in the central portion of the spiral antenna element 3 and spirals outwardly therefrom terminating at a peripheral part A' of the spiral. antenna element 3. Similarly, the second sub-spiral element 3B begins from a central region 25 B of the spiral antenna element 3 and spirals outwardly therefrom terminating at a peripheral region B' of the

spiral antenna element. It is to be noted that the origin A of the first sub-spiral element 3A and the origin B of the second sub-spiral element 3B, are each located at equal distances from but on opposite sides of 5 the mutual spiral axis of the two sub-spiral elements.

This is also the case for the terminal points A' and B' of the two subspiral elements.

Each of the first and second sub-spiral elements 3A and 10 3B comprise "square" spirals comprising loops each of which comprises four successive linear loop portions of increasing length which each extend at right angles from the end of a respective preceding linear loop portion.

In this example each of the sub-spiral elements takes the 15 form of a square spiral when viewed along the spiral axis thereof as illustrated in Figure 2. The spirals are effectively the square-shaped equivalents of Archemedian spirals (which are usually circular in form).

20 Figure 2B illustrates a side view of the antenna l as viewed looking down the Y axis so as to bring the first antenna element 2 into full view. The shape and dimensions of the first antenna element 2 are substarrLially identical lo the shape and dimensions of 25 the second antenna element 3. Indeed, the relative orientations of the first and second antenna elerner-rLs, 2

and 3, are such that the second antenna element 3 as viewed down the -Y axis toward the origin of co-ordinates provides a view which is substantially identical to that obtained by viewing the first antenna element 2 down the 5 Y axis towards the same origin of co-ordinates.

I'hus, tic first antenna element 2 also comprises a first suh-spiral element 2A and a second sub-spiral element 2B being of shape and dimensions corresponding to those of 10 the first sub-spiral element 2A and arranged about a mutual spiral axis in mutually interleaving orientation such as has already been discussed in respect of the sub-

spiral elements 3A and 3B of the second antenna element 3 illustrated in Figure 2.

Each of the first and second sub-spiral elements (2A, 2B and 3A, 3) of both the first and second anter-na elements, 2 and 3, comprise strip lines of conductive material printed upon the outer cylindrical surface of 20 the respective dielectric substrate structures 4a and 4b.

Such strip lines, and means and methods of printing those lines upon a substrate surface may be any such methods as wo.l.c.} be readily available to tt--'se of skill in the art.

The substrate structures 4a and 4b each comprises a 25 material having a low dielectric constant, and the conductive material of the strip lines may be a metal or

conductive ink as is used in thick film printed circuit fabrication. The Balun 6 has a first input/output means arranged to 5 receive or transmit voltage signals from or to an unbalanced coaxial transmission cable (not shown) one conductor of which is grounded (i.c. zero Volts) and the other of which is driven at an oscillating voltage representing a signal (either a received signal or a 10 signal to be transmitted). Other input/output means of the Balun signal transformer 6 comprise a first signal line and a second signal line for conveying signals of an instantaneous magnitude, equal to, but a polarity opposite to, those conveyed by the first signal line.

15 The first Balun signal line is connected to the central starting points (i.e. points A) of the sub-spiral element of each of the first and the second antenna elements 2 and 3 and the second Balun signal line is connected to the central starting point (i.e. point B) of each of the 20 first and second antenna elements 2 and 3.

The Balun 6 is arranged to couple balanced oscillating voltage signals from either (or both) of the first and Lhc second antenna elements 2 and 3 and to transform 25 those balanced signals into an unbalanced oscillating signal (e.g. oscillating between 0 volts and V volts) for

transmission along the unbalanced coaxial cable connected thereto (not shown). It wild be readily appreciated that such an arrangement is able to convert an unbalanced oscillating input signal, emanating from the unbalanced 5 coaxial transmission line, into balanced oscillating signals with which the first and second antenna elements (2 and 3) may be driven.

In respect of each of the first and second antenna 10 elements, 2 and 3, the Balun 6 is arranged to couple an oscillating signal conveyed by its first signal line to the spiral start point A of the first sub-spiral element 2A of the first antenna element 2, and to couple an oscillating signal conveyed by its second signal line to 15 the start point B of the second sub-spiral antenna element 2B thereof. Conversely, the Balun 6 inputs oscillating signals from the second signal line into the start point A of the first sub-spiral element 3A of the second antenna element 3, and feeds the oscillating 20 signals from the first signal line to the start point B of the second sub-spiral element 3B thereof. The signals conveyed by the first signal line have the same instantaneous magnitude as those covered by the second signal line but, since the two signal lines are balanced, 25 have opposite sign (polarity).

In this way, the sub-spiral element of the first antenna element 2, and the corresponding sub-spiral elements of the second antenna element 3 are driven with oscillating signals of equal instantaneous magnitude but opposite 5 polarity.

Figure 3 illustrates an alternative view of the antenna I in which peripheral portions of both the first and the second antenna elements, 2 and 3, are shown in back-to 10 back opposition across the low-curvature region of the outer surface of the dielectric spacer 100 (not shown for clarity). Only the opposing peripheral portions, 9 and lo, of the first and second antenna elements, 2 and 3, are illustrated in Figure 3 and internal spirals of each 15 antenna element are not shown. Spiral portions of either antenna elements in the proximity of the high- curvature region 8 of the elliptical dielectric spacer are the second sub- spiral element 2B of the first antenna element 2 and the first sub-spiral element 3A of the second 20 antenna element 3. Opposing linear portions 9 and lo of the first and second spiral elements are aligned in parallel opposition and face one another across the ground plane structure 5. Each of the first and second non-planar antenna elements, ? and 3, are uniformly 25 spaced from the opposing surface of the ground plane means 5 the dielectric spacer 100 by a constant

3G separation distance X. This separation is carefully chosen to ensure that a waveguiding structure is produced, in the form of a microstrip transmission line by each portion of a given antenna spiral and the 5 opposing conductive ground plane surface 5 adjacent thereto. The separation X between the ground plane structure 5 and the opposing antenna elements, 2 and 3, is carefully 10 selected such as to ensure that electromagnetic radiation may be effectively guided across preferably the entire transmission line structure so formed should the separation X be too large then guided electromagnetic energy may be lost from the transmission line structure IS before being radiated outwardly from an antenna element.

Conversely, should the separation X be too small, then the coupling between the ground plane structure 5 and a given antenna element may be so great as to inhibit outward radiation from the antenna element. As will be 20 readily appreciated by the skilled person, a balance between these two extremes should be struck when selecting the value of the separation X between the grouTld plane structure 5 and the opposing antenna elements 2 and 3. The choice of microstrip line height 95 is a fuTlctiorl of radiation efficiency, absolute frequency bandwidth and the loss of elec-tromac3netic energy throuc3h

the excitation of surface wave modes, as would be readily appreciated by the skilled person.

By appropriately directing the electrical currents 5 passing through opposing portions of the first and second antenna elements, one may generate a virtual current loop which enables transmission and/or reception of electromagnetic radiation of circular po]arisation in/from directions substantially facing the virtual 10 current loop so formed.

Referring to figure 3, a virtual current loop is generated by causing the currents I2 and Is, passing along the opposing spiral portions 9 and lO, to be oppositely 15 directed at any point in time. This condition is achieved by applying to the first and second sub-spiral elements (2A and 2B) of the first antenna element 2 electrical drive signals of an instantaneous polarity which is opposite (instantaneously) to the electrical 20 drive signals simultaneously applied to the and second subantenna elements (3A and 3B) of the second antenna element 3 respectively.

The current I: and the current IS flowing througi-r the 25 opposing 1inear portions, 9 and lO respectively each comprise a conduction current portion and a displacement

current portion. The opposing portions 9 and 10 of the first and second antenna elements 2 and 3 are driven so as to have oscillating electrical potentials, relative to Earth, which are equal in magnitude (instantaneously) but 5 opposite in polarity.

By connecting the underlying ground plane structure 5 to "Earth" and arranging it such that it has substantially uniformly zero electrical potential ensures that the 10 "equal and opposite" oscillating electrical potentials of the overlying antenna element portions 9 and 10 are "equal and opposite" relative to the same potential of the ground plane means 5. Consequently, displacement current passing between the antenna element portion 9 of 15 the first antenna element 2 and the underlying ground plane 5 has the same magnitude as (instantaneously), but opposite direction to, displacement current passing between the ground plane 5 and the overlying antenna clement portion 10 of the opposing second antenna element 20 3. Thus, for example, referring to Figure 3, at a given instant in time, the displacement current portions lla of the current I2 passes from the ground plane 5 to the 25 overlying portion 9 of tie first antenna elements 2.

Simultaneously with this, the displacement c:.rrrer-L

portion llb of the current IS passes from the portion lO of the second antenna element 3 to the underlying ground plane 5.

5 Displacement currents l]a and llb have equal magnitude (instantaneously) , but opposite senses of direction, and the arrangement can be regarded as a flow of displacement current from the second antenna element 3 to the first antenna element 2 via different parts of the mutual 10 ground plane means 5. However, since the ground plane means has substantially uniformly zero electriGal potential (Earth) there can be substantially no identifiable current path along the surface of the ground plane. However, one may regard the ground plane means as providing a "virtual" displacement current path between the points at which displacement current paths of equal magnitude and opposite direction (e.g. lla and lib) meet 20 the surface of the ground plane from opposing antenna elements. This "virtual" current path is indicated by path segment "Vl" extending across the surface of the ground plane 5 between oppositely directed displacement current path sections lla and lib. (Dielectric space 25 lOO omitted for the sake of clarity.)

The result is a composite "virtual" path of current from antenna element portion 10 to opposing antenna element portion 9. A similar "virtual" path from antenna element portion 9 to opposing antenna element portion 10 S simultaneously arises at other parts of the opposing antenna elements where displacement current paths 11c and define flow from the first antenna element 2 to the second antenna element 3 via the ground plane means 5 and the "virtual" path "V2".

Thus, opposing portions, 9 and 10, of the first and second antenna elements, the displacement current paths lla, lib, tic and lid and "virtual" paths V1 and V2 together define a virtual instantaneous current loop.

IS this way a virtual current loop is generated.

A consequence of such a virtual loop is the ability of the composite antenna 1 to transmit/receive circularly polarised radiation to/from locations facing the high 20 curvature edge portion of the antenna where no physical antenna loop exists, thereby enI-rarlcing the omnidirectionality of the antenna 1.. The same condition arises in respect of substantially all opposing antenna portions of the first and second antenna elements. This 25 provides a multiplicity of virtual current loops coupling substantially al] opposing portions of the first and

second antenna elements and is not limited to only the peripheral loop portions illustrated in Figure 3. This enhances the ability of the antenna 1 to "see" to either side of the first and second antenna elements 2 and 3.

5 The spirals of the first and second spiral antenna elements 1 and 2 are formed such that sufficient space exists between neighbouring conductive antenna loop elements to enable electromagnetic radiation to pass therebetween towards (or from) the underlying ground 10 plane structure 5 as viewed along the X axis. This enables underlying virtual current loops to radiate electromagnetic radiation outwardly from the antenna 1 or to receive electromagnetic radiation from beyond the overlying antenna elements 2 and 3 without complete 15 absorption of such radiation thereby.

Furthermore, as has been discussed above, radiation of electromagnetic energy of a given wavelength occurs preferentially from spire] antenna loops having a 20 circumference of an appropriate size. However, because of the finite Q-factor of each turn of a spiral antenna loop (whereby each spiral loop has a non-constant loop radius, making it a spiral rather than a closed loop) each loop of such spirals preferentially radiate across a 25 narrow range of different frequencies. By appropriately choosing the loop micro-strip width and/or the separation

between successive loops adjacent spiral turns effectively may radiate over overlapping (partially) frequency ranges to couple so as to reinforce the radiating efficiency of each other. In the present 5 embodiment, the value of the spatial separation between neichbouring portions of adjacent spiral turns is carefully chosen so as to ensure such reinforcement of adjacent turns, while also enabling electromagnetic radiation to pass therebetween.

It is to be appreciated that the orientation of the first and second antenna elements and 3, may be such that they are folded across the highcurvature regions, 7 and 8, of the elliptically curved dielectric spacer lOO 15 rather than across the low-curvature regions as illustrated in Figure l. In this geometry, the spirals are revolved by 90 about the principle axis of the right elliptic cylinder relative to the spacer lOO and the ground plane such that the spiral Gentles lie at the 20 apices of the high curvature regions 7 and 8 of Fig. 3.

Revolution by exactly 90 is not essential and 90 4- X up to 40 say) is also possible. The exact choice of angle would depend on mechanical assembly constraints.

25 To alternative arrangements the cylindrical cod (top and bottom) portions of the antenna structure l, whether- it

be generally formed as an elliptical cylinder or otherwise, may be lined with radar absorbing materials so as to suppress radiation from, for example, induced circulating currents occurring along the edges of the S cylinder ends. This aim may alternatively be achieved by inserting photonic band gap (PBG) filters at the two cylinder end locations of the ground plane such as by etching holes in the ground plane structure 5 at the terminal cylindrical end surfaces thereof. Photonic band 10 gap filler structures such as would be readily apparent to the skilled person may be employed.

In alLcrnative arrangements the first stages of the receiver may be integrated with the printed circuit board 15 of the Balun. This may improve the overall sensitivity of the receiver by reducing signal loss through the shorter length of signal interconnecting cable required between receiver and Balun.

20 It is to be appreciated that the present invention is not limited to substrate surfaces having right elliptical cylindrical form, or any cylindrical form in general.

Any suitable substrate shape or structure may be employed to enable a "wrap-around" of the first and second antenna 25 elements thereupon. For example, the substrate surface may comprise two hemispherical surfaces (either separate

or forming opposite hemispheres of a mutual sphere) upon which the first and second antenna elements may be printed or wound in a generally hemispherically spiral form. s It is to be appreciated that the present invention, in any of its aspects, is intended to include any and all variants and alternatives to the embodiments described herein as would be readily apparent to the skilled 10 person.

Claims

Claims:

1. An antenna for transmitting and/or receiving electromagnetic radiation, the antenna including: 5 a first non-planar antenna element dimensioned to transmit and receive circul.arl.y polarized radiation; a separate second non-planar antenna element opposing the first no-planar antenna element and dimensioned to transmit and receive circularly polarised 10 radiation; ground plane means arranged between the first non-

planar antenna element and the second non-planar antenna element and spaced therefrom so as to form an electromagnetic transmission line therewith; 15 the first non-planar antenna element being deformed such that parts of the first non-planar antenna element are displaced from other parts thereof towards the second non-planar antenna element; and, the second non-planar antenna element being deformed 20 such that parts of the second non-planar antenna element are displaced from other parts thereof towards the first non-planar antenna element.

2. An antenna according to Claim 1 wherein the ground 25 plane means includes: a first ground plane portion arranged in spaced

opposition to the first non-planar antenna element; and, a second ground plane portion arranged in spaced opposition to the second non-planar antenna element; wherein the first and second ground plane portions 5 are in electrical contact wi th each other such as to permit electrical conduction therebetween.

3. An antenna according to Claim 2 wherein: the first ground plane portion has a first ground 10 plane surface arranged in substantial! y uniformly spaced opposition to the first non-planar antenna element) and, the second ground plane portion has a second ground plane surface arranged in substantially uniformly spaced opposition to the second non- planar antenna element; 15 wherein the first and second non-planar antenna elements are deformed to conform to, and to correspond with, the opposing first and second ground plane surfaces respect) vely thereby to provide substantially uniform spacing therefrom.

4. An antenna element accord) ng to any preceding claim wherein each of the first and the second antenna elements corner ise one or more strips of electrically conductive material which in con junction with the ground plane means 25 form a microstrip tran.smi ssion line.

5. An antenna according to any preceding claim including: signal feed means operatively connected to the first non-planar antenna element and to the second non-planar 5 antenna element and arranged to direct electrical currents through the first non-planar antenna element and through the second non-planar antenna element such that electrical currents directed through opposing parts of the first and second non- planar antenna elements are 10 oppositely directed.

6. An antenna according to any preceding claim wherein the structure of the second non-planar antenna element substantially matches the structure of the first non 15 planar antenna element and the second non-planar antenna element is arranged in opposition to the first non-planar antenna element such that those parts of the second non-

planar antenna element which oppose the first non-planar antenna element oppose equivalent parts of the first non 20 planar antenna element.

7. An antenna according to any preceding claim wherein the first nonplanar antenna element is cJ spiral antenna element, and; 25 the second non-planar antenna element is a spiral antenna element;

the first spiral antenna element being deformed such that inner spiral parts of the first antenna element are displaced from outer spiral parts thereof towards tile second spiral antenna element; and, 5 the second spiral antenna element being deformed such that inner spiral parts of the second antenna element are displaced from outer spire] parts thereof towards the first spiral antenna element.

10 8. An antenna according to Claim 7 wherein each of the first and second spiral antenna elements are twin-start spirals comprising: a first sub-spiral element; and, a complementary second sub-spiral element being a 15 spiral of a structure corresponding to those of the first sub-spiral element; the second sub-spiral and the first sub-spiral being arranged about a mutual spiral axis in mutually interleaving orientation such that corresponding parts of 20 the first sub-spiral and the second sub-spiral are located at substantially equal distances from the mutual spiral. axis at opposite sides thereof.

9. An antenna according to any of preceding claims 7 or 25 8 wherein each of the first and second spire] antenna elements comprises a square spiral.

10. An antenna according to Claim 9 wherein portions of the first spiral antenna element extending between successive square vertices thereof are arranged in 5 parallel orientation relative to opposing portions of the second spiral antenna element extending between successive square vertices thereof.

11. An antenna according to any preceding claim further lO comprising a dielectric spacer means located in the space between ground plane means and the first non-planar antenna element and in the space between ground plane means and the second non-planar antenna element.

15 12. An antenna according to any preceding claim further comprising: substrate means upon which the first and second non-

planar antenna elements are arranged.

20 13. An antenna according to Claim 12 wherein the substrate means comprises oppositely facing curved surfaces upon which the first and second non-planar anter-rTla elements are respectively arranged.

25 l4. An antenna according to any of preceding claims 12 to 13 wherein the substrate means is a dielectric

substrate and the first and the second non-planar antenna elements comprise lines of conductive material printed upon the substrate means.

5 15. An antenna according to any of preceding claims 5 14 wherein the signal feed means comprises one or more balanced-to-unbalanced (Balun) signal transformers to at least one of which each of the first and the second non-

planar antenna elements are operatively connected.

16. An antenna according to Claim 15 wherein each of the first and the second non-planar antenna elements are operatively connected to a mutual Balun.

15 17. A microwave transmitter comprising an antenna according to any preceding claim.

18. A microwave receiver comprising an antenna according to any of preceding claims l to 17.

19. A microwave transceiver comprising an antenna according to any of preceding claims 1 to 18.

20. A method for transmitting and/or receiving 25 electromagnetic radiation including: providing a first non-planar antenna element

dimensioned to transmit and receive circularly polarized radiation; providing a separate second non-planar antenna element dimensioned to transmit and receive circularly 5 polarized radiation; and, providing a ground plane means; arranging the first non-planar antenna element in opposition to the second non-planar antenna element such that parts of the first non-planar antenna element are 10 displaced from other parts thereof towards the second non-planar antenna element; arranging the second non-planar antenna element such that parts of the second non- planar antenna element are displaced from other parts thereof towards the first non 15 planar antenna element; and, arranging the ground plane means between the first non-planar antenna element and the second non- planar antenna element being spaced therefrom so as to form an electromagnetic transmission line therewith.

21. A method according to Claim 20 including: providing a ground plane means in the form of a first ground plane portion and a second ground plane portion; 25 arranging the first ground plane portion in spaced opposition to the first non-planar antenna clement; and,

arranging the second ground plane portion in spaced opposition to the second non-planar antenna element; wherein the first and second ground plane portions are arranged in electrical contact with each other such 5 as to permit electrical conduction therebetween.

22. A method according to Claim 2I wherein the first ground plane portion is provided with a first ground plane surface and the second ground plane portion has a 10 second ground plane surface, and the method includes; arranging the first ground plane surface in substantially uniformly spaced opposition to the first non-planar antenna element; and, arranging the second ground plane surface in 15 substantially uniformly spaced opposition to the second non-planar antenna element; wherein the first and second non-planar antenna elements are deformed to conform to, and to correspond with, the opposing first and second 20 ground plane surfaces respectively thereby to provide substantially uniform spacing therefrom.

23. A meshed according to any of preceding claims 20 to 22 wherein each of the first and the second antenna 25 elements are provided with one or more strips of electrically Conductive material which in conjunction

with the ground plane means form a microstrip transmission line.

24. A method according to any of Claims 20 to Claim 23 S including: directing electrical currents through the first non-

planar antenna element and through the second non-planar antenna element such that electrical currents directed through opposing parts of the first and second non-planar 10 antenna elements are oppositely directed.

25. A method according to any of preceding claims 20 to 24 wherein the structure of the second non-planar antenna element substantially matches the structure of the first IS non-planar antenna element and the second non-planar antenna element is arranged in opposition to the first nonplanar antenna element such that those parts of the second non-planar antenna element which oppose the first non-planar antenna element oppose equivalent parts of the 20 first non-planar antenna element.

26. A method according to any of preceding claims 20 to 2.5 wherein the first non-planar antenna element is a spiral antenna clement, and; 25 the second non-plarlar antenna element is a spiral antenna element;

the first spiral antenna element being deformed such that inner spiral parts of the first antenna element are displaced from outer spiral parts thereof towards the second spiral antenna element; and, 5 the second spiral antenna element being deformed such that inner spiral parts of the second antenna element are displaced from outer spiral parts thereof towards the first spiral antenna element.

10 27. A method according to Claim 26 wherein each of the first and second spiral antenna elements are twin-start spirals comprising: a first sub-spiral element; and, a complementary second sub-spiral element being a 15 spiral of shape and dimensions corresponding to those of the first sub-spiral element; the second sub-spiral and the first sub-spiral being arranged about a mutual spiral axis in mutually interleaving orientation such that corresponding parts of 20 the first sub-spiral and the second sub-spiral are located at substantially equal distances from the mutual spiral axis at opposite sides thereof.

28. A method according to any of preceding claims 26 or 25 27 wherein each of the first and second spiral antenna elements comprises a square spiral.

29. A method according to Claim 28 wherein portions of the first spiral antenna element extending between successive square vertices thereof, its arranged in 5 parallel Orientation relative to opposing portions of second spiral antenna element extending between successive vertices thereof.

30. A method according to any of preceding claims 20 to 10 29 further comprising: providing a dielectric spacer means; and, locating the dielectric spacer means in the space between ground plane means and the first non-planar antenna element and in the space between ground plane 15 means and the second non-planar antenna element.

31.A method according to any preceding claim further comprising: providing a substrate means; and, 20 arranging the first and second non-p] .anar antenna elements thereupon.

82. A method according to Claim 31 wherein the substrate means comprises oppositely facing curved 25 surLac:es upon which the first and second nonplanar antenna elements are respectively arranged.

33. A method according to any of preceding claims 3] to 34 wherein the substrate means is a dielectric substrate and, the first and the second non-planar S antenna elements comprise lines of conductive material printed upon the substrate means.

34. An anter-na element substantially as described in any one embodiment hereinbefore with reference to the 10 accompanying drawings.

35. A method substantially as described in any one embodiment hereinbefore with reference to the accompanying drawings.