GB2324911A - Antenna array - Google Patents

Abstract

An antenna array (130) comprises for example a plurarity of antenna elements (136) distributed in a 3-dimensional space. Signals are conveyed to and from the elements (136) through optical fibres (140) so that only a small proportion of the space contains reflective metallic parts. The elements (136) are operative to convert optical signals conveyed thereto to microwave radiation either by optical non-linear mixing processes within the elements or by photodiode detection of microwave modulation conveyed in the optical signals. Incident received microwave radiation at the array (130) is similarly converted at the elements (136) to optical signals for coupling into optical fibres (140). This conversion involves either mixing the microwave radiation with an optical local oscillator signal conveyed to each element along optical fibres (140), as a floodlight source 142 or by arranging for the incident microwave radiation to apply modulation to a light source and then conveying the resulting modulated optical signal from each element along optical fibres (140) to signal processing units remote therefrom. The use of microwave transparent elements close to the antenna element array allows a wider variation in the direction of the beam formed by the array and lower distortion of the beam pattern.

Description

ANTENNA ARRAY This invention relates to antenna arrays, in particular arrays for use in microwave systems.

Antenna arrays incorporate a plurality of individual antenna elements. Such arrays are becoming increasingly employed in communication and radar systems because their gain direction may be altered electronically without applying mechanical scanning, namely by phase and amplitude control of signals applied to and received by elements. Conventional pianar arrays have elements arranged in a planar array surface with transmitter and receiver (TX/RX) units in close proximity to each element; such units are often located immediately behind each element.

A problem with an electronically steered planar array is that the gain direction may only be steered within a limited angle of x/2 radians relative to a direction (normal direction) which is normal to the planar array surface. Moreover, directional gain characteristics become difficult to control for gain direction angles of greater than approximately s13 radians relative to the normal direction. Such difficulties include unsatisfactory side lobe suppression and diminishing gain compared to array characteristics in approximately the normal direction. Planar arrays have other disadvantages, for example they are not aerodynamic; this is important for a forward facing radar in an aircraft where a radome is usually added for improving aerodynamics but represents under-utilised space.

These difficulties may be ameliorated by arranging elements in a convex conformal array surface. A conformal surface is defined as a surface that is not all within one plane and which may have curvature along one or more axes. It may be incorporated as part of a structure, for example a section of a fuselage of an airborne vehicle. A problem which arises from this conformal configuration is that there is less space behind the elements for accommodating TX/RX units compared to a planar array.

Current design philosophy envisages using minimal TXIRX units in close proximity to the elements, mounting other parts of TXIRX units remotely from the array and employing optical fibres for linking remote TX/RX units to minimal TXIRX units local to each element; the use of optical fibres provides an advantage of conveying signals with negligible crosstalk from neighbouring fibres. The optical fibres may convey optical radiation modulated at the frequency of signals to be radiated from or received at the array. A further problem with convex conformal arrays occurs when the gain direction is steered away from an axis normal to the convex conformal surface at the centre of it; some antenna elements may be unable to contribute to array performance for some gain directions, particularly directions having angles approaching ld2 radians relative to the normal axis. Furthermore, even when optical fibres are used for linking remote TX/RX units to minimal TXJRX units at the convex conformal array, there is still a requirement for electrical bias supplies to be brought to the elements for powering the minimal T)(/RX units.

A further development of multielement arrays is a volume array comprising a number of magnetic loop antenna elements supported on metallic coaxial shafts parallel to one another within a 3-dimensional space. The loop antenna elements are separated by an inter-element distance in the range 0.5S to A, where k is an array wavelength.

A practical example of a volume array is a crow's nest array. The coaxial shafts convey microwave signals to and from the loop elements. The polarisation of microwave radiation emitted from the loop elements is arranged to be orthogonal to the metallic support shafts so that the shafts provide minimal microwave obscuration.

This enables gain direction steering angles from the crow's nest array in a range ux/3 to 47to3 radians relative to an axis parallel to the support shafts to be achieved, although steering angles of less than 7r/4 radians to the axis becomes problematic because directional gain and side lobe performance become unsatisfactory. A planar array may be integrated with a crow's nest array to form a composite antenna array providing a wide range of steering angles in a range in excess of s/2 radians; however, a disadvantage of this composite array is that it is a complex configuration with complex ground plane arrangement and that polarisation direction diversity from the crow's nest array is restricted.

It is an object of the invention to provide an alternative form of antenna array.

The present invention provides an antenna array incorporating antenna elements, element support means and signal conveying means which are at least partially transmissive to radiation irrespective of radiation polarisation direction.

The invention provides the advantage that it enables all array elements to contribute to array directional gain characteristics for a large range of gain direction angles and polarisation orientations because it reduces the effects of element obscuration by other array regions.

The elements may each incorporate an antenna of non-linear dielectric optical material such as ammonium dihydrogen phosphate (ADP), potassium dihydrogen phosphate (KDP), caesium dideuterium arsenate (CDA) or lithium niobate (LiNbO3); these materials exhibit birefringence which is proportional to the square of radiation electric field strength present in the materials.

The antenna may be covered by a dielectric coating which is reflective, ideally greater than 95% reflective, at optical radiation frequencies for creating multiple propagation paths within the antenna thereby enhancing optical mixing effects occurring therein; a region through which optical radiation is coupled into the antenna from an optical fibre may remain uncoated. Optical radiation frequencies in this case may be in the range from far infrared radiation to ultraviolet radiation. The dielectric coating may be transmissive, ideally greater than 95% transmissive, to radiation at microwave frequencies.

The optical fibre may carry two coherent optical signals differing in frequency by an amount corresponding to the frequency of radiation to be emitted from the antenna.

A dielectric matching transformer may be added to the antenna for ensuring efficient coupling of radiation from the antenna into free space and vice versa. The matching transformer may contain a dielectric polymer such as polymethylmethacrylate.

The antenna may also be arranged to mix incident microwave radiation with a locally generated optical signal conveyed to the antenna through an optical fibre for generating a mixed heterodyne signal comprising optical sidebands at a frequency separation from the locally generated optical radiation similar to the frequency of incident microwave radiation. This mixed heterodyne signal may be coupled out of the antenna along an optical fibre for undergoing signal processing such as optical filtering, amplification and detection remotely from the antenna.

In an alternative embodiment, each array element may comprise a microwave dipole incorporating a detection photodiode activatable by modulated optical radiation conveyed along an optical fibre thereto. The detection photodiode may be activated with photovoltaic biasing means, namely bias photodiodes, responsive either to flood optical illumination or to optical radiation conveyed thereto along optical fibres.

Optical attenuators and phase shifters may be employed for controlling optical signals conveyed to the detection photodiodes along the optical fibres.

In order that the invention might be more fully understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which Figure 1 illustrates three types of prior art antenna arrays, namely a planar array, a convex conformal array and a (crow's nest) volume array; Figure 2 illustrates an antenna element incorporating an optically non-linear dielectric antenna; Figure 3 illustrates an antenna element incorporating a dipole antenna and a detection diode; Figure 4 is a schematic diagram of a multielement system including an antenna array of the invention incorporating the antenna element of Figure 3; and Figure 5 illustrates an alternative form of antenna array of the invention.

Referring to Figure 1, a planar array, a convex conformal array and a (crow's nest) volume array are indicated by 2, 4 and 6 respectively. The planar array 2 has a planar ground plane surface 8 incorporating a number of antenna elements such as element 10 arranged in a common plane and each orientated in the direction of an axis 12 normal to the surface 8. The conformal array 4 has a convex ground plane surface 14 incorporating a number of antenna elements arranged over the surface, such as elements 16,18 orientated in different directions relative to one another and an axis 20. The volume array 6 is contained within a three dimensional radome 22.

It has a number of loop antenna elements such as 24 which are distributed in three dimensions with an inter-element spacing in the range 0.5S to X, where X is an array wavelength. The elements 24 are supported on metallic coaxial shafts such as 26 attached to a ground plane 28. The antenna elements 24 are orientated to emit microwave radiation polarised orthogonally to the shafts 26 and to an axis 30 normal to the plane 28. The radome 22 has a point P located at a central region of it.

In the case of the planar array 2, each element has a signal control unit associated with it. The control unit attenuates and phase shifts or time delays signals from a source and supplies them to the respective element. The attenuation and phase shift or delay applied determines a direction in which radiated microwave energy propagates from the array 2; this direction is indicated by a ray 150 which subtends an angle o relative to the normal axis 12. The ray 150 is steerable in any direction provided that o is between 0 and err/2 radians. It is not possible for o to exceed s/2 radians because the ray 150 would be obscured by the ground plane 8 and behind this by the presence of TXIRX units (not shown).

This angular restriction on o is ameliorated in the conformal array 4. Radiation may be emitted from this array 4 along a path which subtends an angle o which exceeds s/2 radians relative to the axis 20. A disadvantage of this type of array is that all elements may not always be able to contribute to radiation reception or emission even for path angles of 6 less than id2 radians where, for example, curvature of the array face 14 screens elements 18 in one region of the array from elements 16 in another.

Convex conformal arrays may assume many different forms, for example elements may be located on a hemispherical or conical surface, or on the fuselage and wings of an aircraft.

The crow's nest array 6 is an array configuration capable of providing a gain direction, represented by a ray 152, subtending an angle 0 greater than s/2 radians relative to the axis 30. The loop antenna elements 24 radiate microwave radiation which is polarised orthogonally to the shafts 26 so that they do not provide excessive microwave obscuration. However, this polarisation constraint results in the crow's nest array 6 providing inferior performance for a ray angle o of less than gut/3 radians relative to the axis 30 and a limited range of possible radiation polarisations.

The ground plane obscuration described above for planar and conformal arrays, and the polarisation and gain direction angle limitations for crow's nest arrays may be circumvented if the requirement for a ground plane is avoided, and if mechanical supports associated with the array elements are transmissive, for all polarisation directions, to radiation emitted from and received at antenna elements.

Referring now to Figure 2, an optically stimulated microwave antenna element 40 is shown together with associated peripherals to be described later. The element 40 is for use in an array of the invention. It incorporates a dielectric antenna 42 in the form of an elongate rectangular block fabricated from ammonium dihydrogen phosphate (ADP), potassium dihydrogen phosphate (KDP), caesium dideuterium arsenate (CDA) or lithium niobate (LiNbO3). The antenna 42 has a reflective coating which is reflective at optical radiation frequencies and transmissive at microwave radiation frequencies. The reflective coating is absent in a circular region 46 at one longitudinal end of the antenna 42. The antenna 42 is surrounded by a dielectric matching transformer 44 containing polymethylmethacrylate. The antenna 42 has a length corresponding to a half-wavelength for radiation emitted from or received at it through the transformer 44. An optical fibre 48 is connected to the antenna 42 at the region 46. The optical fibre 48 is divided into an optical fibre 50 and an optical fibre 52 for connecting to a laser 54 and a laser 56 respectively. The optical fibre 48 is also connected to an optical processor unit 58. The laser 54 has an optical output to the fibre 50 at a radiation frequency f1, whereas the laser 56 has an optical output to the fibre 52 at a radiation frequency f2. There is a frequency difference of Af between the frequencies f1 and f2.

The antenna 42, the matching transformer 44 and optical fibre 48 incorporate dielectric materials and are each transmissive to microwave radiation. As compared to the prior art, these items incorporate relatively few metallic parts which would reflect incident microwave radiation and thereby cause obscuration. In an array of elements 40, there will be a degree of obscuration but only to an extent which is acceptable for normal purposes.

The operation of the element 40 in transmission mode will now be described. The lasers 54,56 provide optical radiation at frequencies of fi and f2 respectively at an output power level of several Watts. The frequency difference Af between frequencies fl and f2 is selected to be approximately 10 GHz, namely within the Xband microwave frequency band of S12 GHz. Laser output radiation is conveyed along the optical fibres 50, 52 to the fibre 48 which illuminates the region 46 on the antenna 42. As has been said, the region 46 is not reflectively coated, and the incident radiation passes through it. The incident radiation is thereby coupled into the antenna 42 which functions as a waveguide and internally reflects the radiation.

The reflective coating ensures that optical radiation is largely constrained within the antenna 42. Optical mixing occurs in the antenna 42 by virtue of its optical birefringence which is proportional to the square of radiation electric field strength present within it. This results in generation of TE-mode microwave energy within the antenna 42. The microwave energy passes through the antenna coating and couples as a TE-polarised wave into free space through the matching transformer 44.

The antenna 42 behaves as a point dipole having omnidirectional emission characteristics.

The operation of the element 40 in receive mode will now be described. The laser 54 is temporarily disabled so that only the laser 56 outputs optical radiation which is conveyed along the optical fibres 52, 48 to the antenna 42. Microwave radiation which is incident on the matching transformer 44 is coupled through the optically reflective coating into the antenna 42. Non-linear mixing of the coupled microwave radiation within the antenna 42 at a frequency fm with the optical radiation at frequency f2 from the laser 56 generates optical sideband signals at frequencies of f2+frn and f24rn. These sideband signals are coupled out of the antenna 42 along the optical fibre 48 to the processor unit 58 for further signal processing such as amplification and detection.

Referring to Figure 3, another type of microwave antenna element for use in an array of the invention is indicated generally by 60. The element 60 incorporates two elongate conductive metal strips 62, 63. Each strip 62, 63 has a square crosssection, has a length of a quarter wavelength for radiation emitted from or received at it, and has two end faces such as 64 and four side faces such as 66, 67. The strips 62, 63 are bonded at one of their end faces to opposite faces of a cuboid quartz insulating spacer 68. Each face of the spacer 68 has the same area as the end face 64. A bias generating photodiode 70 is bonded along the side faces 66, 67 of the strips 62, 63 respectively and also to an exposed face 72 of the spacer 68. The exposed face 72 and the side faces 66, 67 are co-planar. The bias photodiode 70 is a rectangular 0.4 mm-thick slice of silicon into which a shallow optically illuminable p-n junction has been formed. The photodiode 70 is approximately twice as long as each of the strips 62, 63 and approximately half as wide as an edge of the spacer 68. A high frequency detection photodiode 76 is bonded to an exposed face 74 of the insulating spacer 68. The face 74 is orthogonal to the face 72. The detection photodiode 76 is electrically connected to the strips 62, 63 near the spacer 68 by small wires. A bias connection wire 78 is connected from the bias photodiode 70 to the detection photodiode 76. An optical fibre 80 is connected to the detection photodiode 76 for conveying a modulated optical signal to it. A power connection wire 82 is connected from the bias photodiode 70 to a microwave receiver circuit 84.

The circuit 84 is bonded to the face 72 next to the bias photodiode 70 and furthest from the detection photodiode 76. It is electrically connected to the strips 62, 63 near the spacer 68 by small wires. An optical fibre 86 is connected to the circuit 84 for relaying an optical receiver signal therefrom to remote processing units (not shown).

The bias photodiode 70 is transmissive to radiation at microwave frequencies on account of the slice of silicon having a high bulk resistance.

The antenna element 60 differs from the element 40 in that former does not use nonlinear optical mixing processes. During operation, the element 60 is exposed to optical illumination and the bias photodiode 70 acts in response as a photoelectric source providing power and bias potential for the detection photodiode 76 and the receiver circuit 84.

The spacer 68 and strips 62,63 form a dipole. The strips 62, 63 cause a degree of microwave obscuration. However, the element 60 is spaced sufficiently far apart from similar neighbouring elements in an array so that obscuration is at an acceptably low level for practical purposes.

The operation of the element 60 in transmission mode will now be described. The optical fibre 80 conveys a modulated optical signal to the detection diode 76. The modulated signal comprises an optical carrier signal which is amplitude modulated by a microwave signal. The detection photodiode 76 converts the modulated optical signal to a microwave signal which is applied to the strips 62, 63 from whence it is radiated into free space. The bias photodiode 70 applies a bias potential via the connection 78 to the detection photodiode 76 for obtaining adequate high frequency operation by ensuring that minority carriers in the photodiode 76 are rapidly absorbed.

The operation of the element 60 in reception mode will now be described. Microwave radiation which is incident on the element 60 generates an altemating voltage in the strips 62, 63. This alternating voltage is conveyed by wire connections to the receiver circuit 84 where it modulates optical radiation generated either by a solid state laser contained within the circuit 84 or radiation conveyed through the optical fibre 86 from a remote laser source (not shown) for producing a modulated output signal. The modulated output signal couples into the optical fibre 86 for further signal processing in remote units (not shown). Power for the laser within the circuit 84 is provided by the bias photodiode 70.

Referring to Figure 4, a multielement antenna array system is indicated generally by 90. The system 90 incorporates an array 92 incorporating a number of antenna elements such as 94. Each element 94, represented by a bar, is of the same form as the element 60 described with reference to Figure 3. The system 90 includes an element bias supply 96 consisting of a source of optical radiation conveyed as general flood illumination 98 to the elements 94. The elements 94 are connected to respective optical phase or true time delay shifters such as 100. The shifters 100 are connected via a bundle of optical fibres 102 to an optical coupler 104. The coupler 104 is connected through an optical fibre 106 to an optical transmitter unit 110. The transmitter unit 110 incorporates a laser power supply unit 112, a laser 114 and a modulator 116. The coupler 104 is connected to the modulator 116 via the optical fibre 106. The power supply unit 112 is connected to the laser 114. An optical output from the laser 114 is connected to the modulator 116. The modulator 116 is also connected to a X band (8-12 GHz) microwave oscillator 118.

The bias supply 96 provides optical illumination 98 for bias generating diodes incorporated within each antenna element such as 94. The power supply unit 112 provides power for operating the laser 114 which produces an optical output beam of several Watts power. The output beam is conveyed to the modulator 116 which applies amplitude modulation to it for generating a modulated output signal for the coupler 104. A X band microwave modulating signal is provided for the modulator 116 by the oscillator 118. The optical fibre 106 conveys the modulated output signal to the coupler 104 which couples the modulated output signal into the bundle of optical fibres 102. Each fibre in the bundle 102 conveys the modulated signal to a phase shifter such as the phase shifter 100; the phase shifter operates by imposing a phase change or by imposing a true time delay. The shifters 100 are required for correcting for phase delays associated with each element location in the array 92 as well as imposing phase and amplitude changes as required for steering a beam of microwave radiation from the array 92. Output signals from the shifters 100 are conveyed along optical fibres to the antenna elements 94 which convert the optical signals into microwave signals at detection diodes contained therein for emission as microwave radiation into free space at the strips 62, 63. On account of the antenna elements 94 and associated optical fibres 80, 86 being transmissive to microwave radiation as a result of a relatively high proportion of dielectric components incorporated therein, the microwave radiation emitted at each element 94 combines to form a microwave beam which may be steered through a wide angle from 0 radians to in excess of 4s/3 radians relative to an axis 120.

Referring now to Figure 5, a multielement antenna array of the invention is indicated generally by 130. The array 130 incorporates a hemispherical radome 132 and a planar base region 134. The radome 132 is transmissive to microwave radiation and internally reflective to optical radiation. The array 130 incorporates a number of antenna elements such as 136, a number of element support shafts such as 138, element optical fibre signal links such as 140 and a number of bias supply units such as 142 for providing general flood illumination such as 144 within an interior region 146 of the radome 132. The elements are distributed in three dimensions within the radome 132 with an inter-element spacing in the range 0.5k to X, where X is an array wavelength.

The antenna array 92 in Figure 4 may be implemented in a similar manner to the array 130 illustrated in Figure 5. The radiation transmissive elements 136 are suspended by the element support shafts 138 at a number of positions within the radome 132. The support shafts 138 are manufactured from a dielectric material so that they are also transmissive to microwave radiation and therefore provide low obscuration. Optical fibres such as 140 convey optical signals to the suspended antenna elements 136. Optical flood illumination from the bias supply units 142 is reflected within the radome 132 for producing multiple rays 144 so that bias generating photodiodes 70 incorporated within the elements 136 are adequately illuminated despite the support shafts 138 and elements 136 causing some optical obscuration.

The antenna array 130 may also incorporate antenna elements as illustrated in Figure 2. Optical flood illumination is not required for this type of element.

Although the conversion efficiency of optical signals to microwave signals within the elements illustrated in Figures 2 and 3 may be relatively low, for example -130 dB in the case of the element 40 in Figure 2, an antenna array may incorporate several thousand elements which collectively emit useful amounts of microwave radiation.

Solid state lasers now commercially available provide in excess of 10 Watts continuous optical output power and these enable construction of an antenna array incorporating elements 60 and producing several Watts of microwave output power.

Several lasers may be configured in parallel in the system 90 in Figure 4 in place of the laser 114 when greater output power is required. Moreover, a dual mode laser providing an output at two optical frequencies simultaneously may be employed for exciting the antenna 42 in Figure 2 rather than employing two separate lasers 54, 56 as illustrated. Optical radiation conveyed through the optical fibres may have wavelengths between and including far infra red light and ultraviolet light.

The antenna array 130 may incorporate elements which are orientated in a variety of different directions relative to one another in order to provide the array with a capability of emitting radiation with controlled continuously varying polarisation.

Crossed dipole elements, incorporating individual antenna elements as illustrated in Figures 2 and 3 which are collocated and orientated orthogonally to one another1 may be incorporated into the antenna array 130 for example for achieving this characteristic. The bias supply units 142 may, in an alternative form of the antenna array 130, convey optical radiation to bias photodiodes on the elements through optical fibres rather than as flood illumination within the radome 132.

It is possible to configure the elements illustrated in Figures 2 and 3 to provide an array which has nearly constant radio cross section (RCS), and hence aperture size, for a large range of incident microwave radiation angles. The elements may, for example, be regularly distributed at half-wavelength intervals within a spherical radome in order to achieve this characteristic.

The array 130 may initially find an application as part of a communications link. The optical fibres employed to convey signals to and from the array 130 allow the array to be located several kilometres from a control position. This enables the array 130 to be employed with submarines, surface craft, dirigibles and a variety of land-based or airborne vehicles.

Claims (25)

1. An antenna array incorporating antenna elements, element support means and signal conveying means which are at least partially transmissive to radiation irrespective of radiation polarisation direction.

2. An array according to Claim 1 wherein the antenna elements incorporate optical mixing means for generating microwave signals from mixing of optical signals therein.

3. An array according to Claim 1 or 2 wherein the antenna elements incorporate optical mixing means for generating optical heterodyne signals from mixing of microwave and optical signals therein.

4. An array according to Claim 2 or 3 wherein each antenna incorporates optical mixing means comprising optically non-linear dielectric material.

5. An array according to Claim 4 wherein the dielectric material is ammonium dihydrogen phosphate, potassium dihydrogen phosphate, caesium dideuterium arsenate or lithium niobate.

6. An array according to Claim 4 or 5 wherein the antenna has a surface coating which is transmissive at microwave frequencies for transmission and reception purposes and reflective at optical radiation frequencies for confining optical radiation within the antenna.

7. An array according to Claim 6 wherein the coating extends over all of the antenna surface other than a region through which optical radiation is coupled.

8. An array according to Claim 4, 5, 6 or 7 wherein the antenna incorporates dielectric matching means for coupling microwave signals from the antenna into free space.

9. An array according to Claim 8 wherein the matching means is of polymethylmethacrylate.

10. An array according to any preceding claim wherein the signal conveying means is arranged to convey optical signals and comprises optical fibre means.

11. An array according to Claim 10 wherein the signal conveying means for conveying the optical heterodyne signals from the element is an optical fibre.

12. An array according to Claim 10 wherein the signal conveying means for conveying the optical signals to the element is an optical fibre.

13. An array according to Claim 1 wherein each element incorporates a microwave dipole for radiating and receiving microwave radiation, a detection photodiode for converting an optical signal modulated by a microwave signal into a microwave signal, and a bias photodiode for biasing the detection photodiode.

14. An array according to Claim 13 wherein the detection photodiode is connected to the microwave dipole for radiating the microwave signal.

15. An array according to Claim 13 or 14 wherein the bias photodiode is attached onto the dipole.

16. An array according to Claim 13, 14 or 15 wherein the bias photodiode is a rectangular silicon slice disposed longitudinally of the dipole.

17. An array according to Claim 13, 14, 15 or 16 wherein the dipole comprises two elongate metallic strips which are each attached at one of their ends to opposite sides of an insulating spacer.

18. An array according to Claim 17 wherein the spacer is of quartz.

19. An array according to Claim 17 or 18 wherein the detection photodiode is attached to the spacer.

20. An array according to Claim 13,14,15, 16, 17,18 or 19 wherein the detection diode is connected to generating means for generating the modulated optical signal.

21. An array according to Claim 13, 14, 15, 16, 17, 18, 19 or 20 incorporating illuminating means for illuminating the bias photodiode to provide bias for the detection diode.

22. An array according to Claim 20 or 21 including optical attenuating and phase shifting or signal delaying means operative to control phase or amplitude of optical signals conveyed to the detection photodiodes from the generating means.

23. An array according to any preceding claim wherein the antenna elements are orientated in different directions relative to one another for emitting and receiving radiation having a number of different relative polarisation directions.

24. An array according to any preceding claim wherein the antenna elements are arranged to provide a relatively similar radio cross section for most incident radiation angles.

25. An array according to Claim 24 wherein the elements are distributed within a spherical radome.