WO2000059071A1

WO2000059071A1 - Antenna system

Info

Publication number: WO2000059071A1
Application number: PCT/GB2000/001166
Authority: WO
Inventors: James Christopher George Lesurf
Original assignee: The University Court Of The University Of St. Andrews
Priority date: 1999-03-31
Filing date: 2000-03-27
Publication date: 2000-10-05
Also published as: AU3447400A; EP1166391A1; JP2002540705A; GB9907317D0

Abstract

An antenna system comprises an antenna element (42; 104; 204) and at least one feed unit (44, 46, 50, 52; 92, 94; 192, 194; 142, 144) coupled to the antenna element so as to allow passage of a signal therebetween, wherein the system includes delay means for introducing a phase delay into a component of said signal to remove quadrant ambiguity in signals received by the antenna element or to allow phase/amplitude control of a signal transmitted by the antenna unit.

Description

Title: Antenna System

Field of the invention

This invention relates to an antenna system to receive or transmit electromagnetic radiation, and in particular to an antenna system for use with microwave radiation.

Background to the invention

Antenna systems, such as those used in the mobile telecommunications field, are conventionally based on dipole-based arrays which transmit and receive electromagnetic radiation. However such systems tend to be limited in the area they can cover effectively.

Antenna systems using a ring of feed horns have been developed. Such systems can provide coverage over effectively 360° in azimuth, as the system produces a large number of lobes of radiation distributed over 360° about a central origin. In general each lobe has slightly different energy characteristics to the other lobes. There is a wide energy distribution associated with the ring of feed horns arrangement and this results in degradation of the antenna pattern when, for example, irradiating point sources, and can also produce ambiguities in detecting the direction from which power is received.

It is an aim of the present invention to provide an antenna system which at least in part overcomes these difficulties.

Summary of the invention In accordance with one aspect of the invention, there is provided an antenna system comprising an antenna element, and at least one feed unit coupled to the antenna element so as to allow passage of a signal therebetween, wherein the signal passed between the antenna element and the feed unit is composed of two components, and the feed unit includes means for introducing a phase delay between the respective signal components.

The introduction of a phase delay between the signal components ensures that quadrant ambiguity in signals received by the antenna element is removed, or that in transmission, angular phase/amplitude control of a signal radiation pattern from the feed unit incident on the antenna element is available.

A system embodying the invention is capable of transmission or reception of signals, and is generally applicable for indoor and outdoor communication, such as telecommunications networks and local area networks.

The means for introducing the phase delay is preferably an element adapted to introduce a delay into one signal component which varies by an amount that depends on azimuthal angle, to introduce direction dependent phase changes into one signal component. Typically the optical element is a spiral phase plate where the phase delay varies azimuthally, and the phase plate introduces the direction dependent phase changes into the signal.

The signal preferably has a wavelength in the microwave or mm- wave region, although by using an appropriate feed unit the signal may have a wavelength up to and including the visible region.

For certain applications, it is desirable for the signal to be circularly polarised.

The feed unit desirably comprises a first feed with a first radiation axis and a second feed with a second radiation axis, with the first and second radiation axes arranged to intersect each other, a polariser positioned at the intersection of the first and second radiation axes, and means for introducing a phase delay disposed between the second feed and the polariser, such that the polariser also acts as a beam splitter to ensure that the first and second components follow a common path between the feed unit and the antenna element.

Preferably the first radiation axis and the second radiation axis are disposed orthogonally with respect to each other, with the polariser positioned at an angle of 45° with respect to each radiation axis.

Preferably a beam convertor is placed between the feed unit and the antenna element, the beam converter acting to alter the polaristion of a beam passing therethrough. Typically the beam convertor will be a transmission quarter-wave plate, which converts, for example, a circularly polarised beam into a largely plane polarised beam for input into the feed unit, or correspondingly to convert a plane polarised beam into a circularly polarised beam for input to the antenna element. Where a polariser is included within the feed unit, the principal axis of the quarter- wave plate is desirably aligned at an angle of 45° to the polariser axis.

Where a single feed unit is used in the system, it is desirable to use the feed unit with a beam convertor, and for radiation input to/received by the antenna element to be circularly polarised, so that direction ambiguity is removed.

The antenna system may include two or more feed units, and thus in accordance with another aspect of the invention, the antenna system may comprise an antenna element coupled by means of a common coupling to first and second feed units as aforesaid. Typically the coupling is a beam splitter or polariser disposed external to the feed units, such that the signals passed between the antenna element and each feed unit follow a common path between the antenna element and the beam splitter or polariser.

The invention also lies in retrofitting of means for introducing a phase delay to existing antenna systems, so as to create an antenna system in accordance with the present invention. As employed herein, the term feed unit includes within its meaning an emitter for conveying energy from a transmitter output to be transmitted by an antenna element and also a receiver for receiving energy gathered by an antenna element, for conveying to the input circuit of a receiver.

The invention will now be described by way of example, and with reference to, the accompanying drawings in which:

Figure 1 shows a schematic view of prior art antenna system using a feed horn;

Figure 2 shows electric field patterns observed in the prior art antenna system for two differently placed received signals;

Figure 3 shows a schematic view of modified prior art antenna;

Figure 4 shows a schematic view of one embodiment of an antenna system in accordance with the present invention;

Figure 5 shows a schematic view of a second embodiment of the invention;

Figure 6 shows a schematic view of a third embodiment of the invention;

Figure 7 shows a schematic view of a first modification for use with any of the embodiments so as to control the antenna pattern;

Figure 8 shows a schematic view of a second modification for use with any of the embodiments so as to control the antenna pattern;

Figure 9 shows the antenna pattern produced by the third embodiment of the invention; and

Figure 10 diagrammatically shows an antenna system in accordance with the present invention when in use; and

Figure 11 shows a modified version of the embodiment shown in Figure 6.

Description

A prior art antenna system is shown in Figure 1. This antenna system comprises a feed horn 10 which radiates a beam along a nominally vertical optical axis 12 onto a metallic cone 14 whose vertex 16 is on-axis and points towards the feed 10. By setting a cone half-angle of approximately 45°, the result is that the antenna radiates power uniformly into the horizontal plane in all (azimuthal) directions. Thus for observers in, or near to, the horizontal plane the antenna appears to radiate in all directions, i.e. is an omni radiator.

With this prior art system, the beam radiated onto the cone 14 from the feed 10 is generally circularly polarised, as otherwise the polarisation state radiated in the horizontal plane is direction, or azimuth, dependent. Also the beam should be passed through a mode converter to produce a beam which has a null on-axis, to minimise the component of radiation impinging on the cone axis. This reduction of the axial component suppresses the diffraction effects caused by irradiating a sharp point which otherwise degrades the antenna pattern in elevation.

The electric field pattern observed for this system, when looking up the optical axis during illumination of the cone 14 with a plane polarised field with its E-vector vertical, is illustrated schematically in Figure 2.

Figure 2(a) shows the electric field pattern observed when the signal received comes from the North, and Figure 2(b) shows the electric field pattern for a signal received from the North-west. The amplitude distribution and the apparent polarisation are both direction dependent. Thus for the simplest form of directional information an arrangement is used as shown in Figure 3, with two feeds 20, 22 positioned at 45° relative to a plane polariser 24, with one feed 20 having a common axis 26 with a cone 30, and one feed 22 having an axis 32 orthogonal to the cone axis 26. As a received beam passes down from the cone 30 along cone optical axis 26, the polariser 24 detects the beam and separates the beam into two separate polarisation states, resulting in differing signals reaching the respective feeds 20, 22. The electric field orientations at the respective feeds are shown by vectors H and V.

The power levels seen by the feeds or detectors 20, 22 will be in the proportion cos² θ to sin² θ , where θ is the bearing angle of arrival of the incoming signal with respect to one of the principle planes of polarisation defined by the polariser orientation. Although this simple system provides directional information, it does not offer unambiguous information about the power received around an entire 360° of azimuth. The power ambiguity is because the polariser only analyses the detected power ratio and loses information about the sign of θ , and as such has quadrant ambiguity with four possible direction angle values that will give an observed power ratio.

The present invention seeks to provide a way of resolving the bipolar directional ambiguity of the above system. One way to do this is to switch from simple power detectors to heterodyne down converters and compare the detected components coherently.

One embodiment of the present invention is shown in Figure 4. An antenna system 34 comprises two feed units 36, 38 which are coupled by means of polariser 40 to antenna element, or cone 42. Each feed unit 36, 38 comprises a pair of feed horns 44, 46; 50, 52, a spiral phase plate 54; 56 and a polariser/beam splitter 60; 62. In feed unit 36, one feed 44 is arranged along a common axis 64 to the cone 42. The second feed 46 is placed so that its axis 66 is orthogonal to the axis of the cone 42. The polariser/beam splitter 60 is placed between each feed 44, 46 and the cone 42, and at 45° to each axis, to ensure signals transmitted by the second feed 46 travel along a common path with signals from the first feed 44 to reach cone 42. The spiral phase plate 54 is disposed between the second feed 46 and the beam splitter 60, with the spiral phase plate 54 having a common optical axis with the second feed 46.

The second feed unit 38 is of the same internal layout as the first feed unit, but the feed equivalent to feed 46 is placed parallel to the axis of cone 42 and feed 50 at 90° to the cone axis. A signal from the second feed unit is deflected by polariser 40 to travel along a common path with signals passed between the first feed unit and antenna, for the journey between polariser 40 and antenna 42.

In Figure 4 and the remaining figures, each polariser is shown as an isometric view of a "square sheet of parallel lines" which represent the wires of a grid polariser. In such a situation, the polariser reflects the E-field component parallel to the wires, and transmits the field component perpendicular to the wires. In each case, it is the projection of the beam field which is relevant. The orientation of the polariser tends to be arbitrary, achieving the function of resolving or overlying orthogonal plane components. The polarisers are shown as a grid of lines by way of example only, and many other forms of polariser would be readily apparent to the person skilled in the art.

When the system is set to transmit, the signals 70, 72 from the first and second feed units 36, 38 are combined along the cone axis 64 to create the antenna input signal 74. Each signal has two components, one component phase delayed with respect to the other, and the four components are coherently added together. Due to the phase delay in two of the components, part of the signal is reinforced with respect to the rest of the signal, so producing a controlled pattern of radiation incident on the cone 42.

The antenna system 34 can act as a receiver, or by reciprocity as a transmitter. In use when receiving signals, the cone 42 receives radiation reflected/transmitted from sources external to the antenna system, and this received input signal passes to beam splitter/polariser 40 where it is split into two components. Each component is passed to one of the feed units, and split once more so that there are a total of four components, two components for each feed unit. Within each feed unit 36, 38, one feed 44; 50 sees the original input signal to the cone 42. The second feed 46; 52 of each feed pair sees the signal input to the unit 36; 38 via the spiral phase plate 54; 56 which introduces a phase delay into the original input signal by an amount that depends on angle, i.e. the phase delay varies azimuthally. When the output signals from the feed units 36, 38 are added together, coherent adding means that part of the original signal is reinforced, to produce a controlled reception pattern which is without direction ambiguity.

In this arrangement, simple power detectors are replaced by heterodyne down converters, with two feed units 36, 38. Each of the pair 44, 46; 50, 52 of feeds per unit will (assuming ideal splitters/polarisers) see the same power level. When the antenna is without the spiral phase plate, 'NSl NS 'EW = E EW (1, 2)

where E_NS is the electric field in the direction North to South, and E_EW is the electric field in the direction East to West.

However because the spiral phase plates introduce azimuth dependent phase changes, by suitable alignment of the plates, the cross products can be arranged such that,

E_NSE NS = pNSl if signal from 'North' (3)

E_NSE NS = - _NS| if signal from 'South' (4)

and similarly,

> I |2

E_EWE E = I EEW I if signal from 'West' (5)

E_EWE EW = -pEWl if signal from 'East' (6)

Hence the spiral plates allow the quadrant ambiguity to be removed by observing the time- averaged signs of the cross products.

The set up of receivers in Figure 4 is for a vertical input polarisation, with a suitably modified system being used for other polarisations. With such a system, the output 'decoding' required to determine the direction/azimuth of the input signal is quite simple. The system provides a simple sine/cosine variation so ensuring the system is easy to build, calibrate, and use.

The system is also easily time-reversible to make an electronically 'steered' antenna. By choosing the appropriate signal levels and phases injected via the four feeds 44, 46, 50, 52, a pattern can be placed on the top most cone 42 which preferentially radiates in a specific direction. The resulting pattern is smoother, with less complex side lobes than a conventional 'ring of horns' arrangement. Hence the system has the advantage of good performance for the level of simplicity. As a quasi-optical arrangement, the system can operate over a reasonably wide frequency range, typically low microwave to visible radiation, covering energies from below mW to beyond kW, and is capable of receiving femto watts of energy.

The system shown in Figure 4 assumes a nominally plane polarised input. Generally a circular polarisation state is more convenient, and detection or transmission of signals of arbitrary polarisation, and recovering information regarding the polarisation of these signals, may also be required. To achieve this, a modified system as shown in Figure 5 is used. This is similar to the arrangement shown in Figure 4, and corresponding reference numerals are used for common components. However a transmission quarter-wave plate (QWP) 80 is positioned between the cone 42 and the polariser 40, with the principle axes of the QWP 80 aligned at 45° to those of the polariser 40 when viewed along the beam axis 64.

When the system acts as a receiver, and the input beam is circularly polarised, the QWP 80 converts the beam received by the cone 42 into a beam which is plane polarised predominantly in one of the principle orientations of the polariser 40. Hence the bulk of the signal will be sent to one pair of the feeds acting as receiving detectors. Here the polarisation directions directed to a detector are identified by the subscript C or A for 'clockwise' or 'anticlockwise' to refer to the corresponding input states. As before, the spiral phase plates cause the portion of the signal directed to the second detectors 46, 52 of each pair to be phase delayed by an amount which depends upon the azimuthal angle. Hence the phases seen by each detector of a pair can be compared to determine the azimuthal angle. The relative levels (and phases) between the pairs enable the input signal's polarisation state to be identified .

This system is simpler to use than the one shown in Figure 4 and provides more information about the input beam signal. Thus generally this embodiment is of particular use for signal reception.

For signal transmission, either of the embodiments shown in Figures 4 and 5 is suitable, although each has different advantages. The system of Figure 5 is simple to use and should provide electronic control over both the nominal direction of transmission and the transmitted polarisation state. However, the system shown in Figure 4 provides better control of the modal illumination of the cone, and as such provides a more directional transmission beam with better side lobe control.

The systems shown in Figures 4 and 5 are versatile in providing general polarisation and direction coverage. They also allow using one pair of feeds for transmission and the other for reception in suitable polarisation states, either co-polar or cross-polar depending upon the arrangement.

When transmission or reception of only a single, circular, polarisation state is required, the system can be simplified further as now only a single feed unit, with a pair of feed horns, is required. An arrangement of this simpler form is shown in Figure 6. This system comprises a single feed unit 90 comprising two feed horns 92, 94, spiral phase plate 96, polariser/beam splitter 100, quarter wave plate 102 and cone 104. As with Figure 4, one feed 92 is arranged along a common axis to the cone 104. The second feed 94 is placed so that its axis is orthogonal to the axis of the cone 104. The polariser/beam splitter 100 is placed between each feed and the cone 104, and at 45° to each axis, to ensure signals transmitted by the second and first feeds travel along a common path between polariser 100 and cone 104. The spiral phase plate 96 is disposed between the second feed 94 and the beam splitter 100, with the spiral phase plate 96 having a common optical axis with the second feed.

When the signal components from the first and second feed are combined along the cone axis to create feed unit output signal 106, coherent adding takes place. Due to the phase delay in one component, part of the signal is reinforced with respect to the rest of the signal. This signal passes through QWP 102 and produces a controlled pattern of radiation incident on the cone. As before, the electric field orientations at the feeds are shown by vectors H and V, although these are only by way of example as generally the E-field orientations depend on the composite signal created by the circumstances of use. In general, the polarisations in some locations can be defined by the orientation of the polariser.

In use, a signal of a certain power is split to form two equal input signals 110, 112 to be fed to feed horns 92, 94. A controlled phase delay is introduced into one of the input signals 110, and this modified signal fed to the second feed 94. The first feed 92 receives the input signal 112 with no phase delay. If desired, it is possible to arrange for the antenna system to be electronically steered by phase modulating the electrical input signal 110 to the second feed horn 94. These are various methods of achieving the phase modulation. One method is to use two or more input signals of differing frequencies, and subtract one input signal from another so as to produce a transmitter where the output beam scans around at the difference frequency. In this case, phase modulation occurs simply because the differing frequencies beat in and out of phase.

It is worth noting that higher transmission directionality may be obtainable by 'cascading' higher/expand order spiral phase plates to control the antenna pattern.

Figures 7 and 8 show two different methods of introducing phase delays into the feed signals. The arrangements shown can be used with any of the embodiments of the invention described with appropriate modification, and may be combined together if required. However for ease of explanation, the two principal ways of introducing phase differences are shown separately.

In Figure 7, a serial way of introducing phase delays or differences is illustrated for when the antenna is in a transmission state. A succession of spiral phase plates 120, 122, 124 is disposed in the basic feed unit 36 between feed 46 and polariser 60. Each spiral phase plate is chosen to introduce a different phase delay into the signal passing through it. Thus, as shown in Figure 7, for a signal passing from the feed 46 to the polariser 60, a signal α has a phase delay of φi introduced at the first spiral phase plate 120, a second phase delay of φ₂ is introduced by spiral phase plate 122, with a third phase delay introduced by spiral phase plate 124, such that the signal reaching polariser 40 is the original signal from the feed with phase delays φi, φ and φ₃ added in.

Instead of this serial approach to introducing phase delays, alternatively or in combination therewith, a branching or parallel approach can be taken as shown in Figure 8. Again for simplicity of explanation, the phase delays are shown assuming that the feed unit is transmitting a signal to the antenna. Assuming that the signal from feed 46 is represented as α, a phase delay of φi is introduced by spiral phase plate 54 so that a combined signal of α + φ] reaches polariser 60. Feed 46 produces a signal represented as γ, and at polariser 60, these signals are combined along a common path, such that the output signal from feed unit 36 is combined of two signals ot + φ_{1 }} and also γ. When fed into another feed unit 36, further phase delays are introduced, such that the output from the second feed unit consists of a component being α + φi + φ₂ as introduced by the second phase plate within feed unit 130, and also γ + φ₅. Whichever type of cascade arrangement, or combination of arrangements is chosen, the aim is to compose, or decompose the signal depending on direction of travel, into a chosen set of modes or beams which at the cone have specific azimuthal mode numbers as well as defined relative amplitudes and phases. However many arrangements for achieving this are possible, for example one or more input signals from the feeds can be split, passed through more than one feed unit, then recombined before further processing. Whilst the modifications shown in Figure 7 and 8 have been described in the transmission state, they are equally applicable for the reception of signals.

The embodiments described here combine omnidirectional coverage with electronic control of directionality for transmission use, and azimuth measurement for reception use. In addition, they can also provide polarisation control for transmission, or measurement for reception and direction finding.

The embodiments shown here are simple examples of a general 'class' of systems which can be assembled and have their performance tailored. For example, the form of the antenna pattern can be chosen to be almost independent of the chosen transmission axis bearing when these systems are used to transmit power. This contrasts with spaced horns which can generate 'grating' sidelobe effects which vary with the relative phase/amplitude contributions. Similarly, determinations of signal direction for reception benefit from the simple sine-like behaviour of the directional dependence presented to the feeds.

Figure 9 shows the approximate horizontal (azimuthal) pattern of the simplest form of directional omni antenna shown in Figure 6. The pattern is in the form of 'beam' whose direction can be steered electronically to radiate in any direction in a horizontal plane. It does not exhibit the 'petals of a flower' behaviour of conventional arrays, but rather has one lobe which simply rotates and keeps the same shape no matter which direction it is aimed in. In Figure 9, the antenna is nominally aimed in the '90 degree' direction. More complex arrangements, using more spiral phase plates, would be more directional or could, if required, simultaneously aim in two or more directions.

As shown, the directional pattern of a transmit system covers a reasonably wide azimuthal angle. Further modifications possible to improve the transmission directionality and sidelobe performance include the addition of higher-order spiral modes to obtain more detailed angular phase/amplitude control of the pattern illuminating the cone, and the use of a suitable anisotropic/expand 'sheath' around the cone to alter the exiting field pattern.

Also if employed with a suitable quasi-optical circulation system, the antenna systems discussed can continuously illuminate a 360° horizontal view, and determine the bearing of a reflecting object. The system would require no mechanical or electronic scanning. The system requires a single antenna, and the internal circulation would reduce the antenna's 'glint' detectability. The absence of 'swept' power would reduce the resulting radar's vulnerability to detection.

Figure 10 demonstrates how an antenna system 140 can be used to simultaneously receive and transmit. Feeds 142, 144 which simultaneously transmit and receive are connected into a closed loop 146 via circulators 150, 152. The two circulators 150, 152 are connected in loop 146 such that in a first arm 154 of the loop, a transmit source 156 and phase delay unit 160 are provided. In a second arm 162 of the loop, a phase measurement unit 164 is provided which enables the direction (azimuthal angle) of arrival of an incoming signal to be determined by measuring the relative phases of the signal components. The direction of the incoming signal through the feeds 142, 144 is shown by 166, 170, each incoming signal passing through a circulator. The transmit source 156 provides an output signal for feed 144 via circulator 152, and to the other feed 142, a phase delay is introduced into the signal from the transmit source 156 by unit 160. The phase delay is set so as to control the direction of the transmitted signal to a chosen azimuthal angle as a result of setting the transmit phase delay between the components sent to the respective feeds. It should be noted that a time delay or time offset is equivalent to a phase delay in these representations and throughout the antenna usage.

Thus with the present invention, a "directional omni" antenna, i.e. an antenna which can receive from anywhere in a horizontal plane and provide information about the direction of arrival of an incoming signal, is provided. The same type of antenna can, by symmetry, be used as an electronically steerable antenna to radiate as required in any direction in the horizontal plane without any mechanical adjustment. The design is particularly suitable for the microwave/mm-wave region where conventional dipole-based arrays tend to be impractical. The Figure 11 shows how the aπangement of Figure 6 can be modified for transmission purposes. The components corresponding to those of Figure 6 are indicated by the reference numerals of Figure 6 raised by 100.

The system of Figure 11 has an extra polariser 220 and a beam load (absorber) 222. This alters the power/polarisation characteristics. The system is the same (as the Figure 6 system) in the manner in which it exploits the spiral phase plate 196 to induce directional behaviour which depends upon the two contributions, Ec and Ec' but the effect is manifested in a different way. In the Figure 6 configuration, the transmitted polarisation is direction dependent and may be selected at reception by suitable polarisation filtering or processing. This is valuable in applications where the system is intended to transmit to a receiver that can then use the detected polarisation to determine their bearing with respect to the TX angular co-ordinates. This may be useful for locating purposes, etc. The system of Figure 11 can use angled polarisers to control the total power transmitted as a function of direction hence does not assume polarisation filtering at the receiver when used as a transmitter.

Similar alterations may be applied to the other described embodiments of the invention. There are also a range of other alterations depending upon what is required which include surrounding the cone with a cylinder-wall of polariser or ferrite to obtain similar effects to the above.

Claims

1. An antenna system comprising an antenna element, and at least one feed unit coupled to the antenna element so as to allow passage of a signal therebetween, wherein the signal passed between the antenna element and the feed unit is composed of two components, and the feed unit includes means for introducing a phase delay between the respective signal components.

2. A system according to claim 1, in which the means for introducing the phase delay is an element adapted to introduce a delay into one signal component which delay varies by an amount that depends on azimuthal angle, to introduce direction dependent phase changes into one signal component.

3. A system according to claim 2, in which the element is a spiral phase plate for causing the phase delay to vary azimuthally, and thereby introduce direction dependent phase changes into the signal.

4. A system according to any of the preceding claims, in which the system is configured to transmit and/or receive a signal having a wavelength in the microwave or mm- wave region.

5. A system according to any of the preceding claims, in which the feed unit comprises a first feed with a first radiation axis and a second feed with a second radiation axis, with the first and second radiation axes arranged to intersect each other, a polariser positioned at the intersection of the first and second radiation axes, and means for introducing a phase delay disposed between the second feed and the polariser, the arrangement being such that the polariser acts as a beam splitter to ensure that the first and second components follow a common path between the feed unit and the antenna element.

6. A system according to claim 5, in which the first radiation axis and the second radiation axis are disposed orthogonally with respect to each other, with the polariser positioned at an angle of 45° with respect to each radiation axis.

7. A system according to any of the preceding claims, in which a beam convertor is situated between the feed unit and the antenna element, the beam convertor acting to alter the polaristion of a beam passing therethrough.

8. A system according to claim 7, in which the beam convertor comprises a transmission quarter-wave plate.

9. A system according to claim 8, in which the quarter-wave plate is arranged to convert a circularly polarised beam into a largely plane polarised beam for input into the feed unit, and/or correspondingly to convert a plane polarised beam into a circularly polarised beam for input to the antenna element.

10. A system according to claim 9, when appended to claim 5 the principal axis of the quarter-wave plate is desirably aligned at an angle of 45° to the polariser axis.

11. A system according to any of the preceding claims, in which the system includes two or more feed units, the system comprising an antenna element coupled by means of a common coupling to first and second feed units as aforesaid.

12. A system according to claim 11, in which the coupling comprises a beam splitter or polariser disposed external to the feed units, such that the signals passed between the antenna element and each feed unit follow a common path between the antenna element and the beam splitter or polariser.

13. A system according to any of the preceding claims, in which the term feed unit comprises an emitter for conveying energy from a transmitter output to be transmitted by an antenna element and a receiver for receiving energy gathered by an antenna element, for conveying to the input circuit of a receiver.