GB2267603A - Electronically scannable array of antenna elements. - Google Patents

Abstract

An apparatus, for example, a radar apparatus, having a plurality of antenna elements 14a in an electronically scannable array 14, has a calibration antenna 15 located forward of the array in a determinable position, the calibration antenna being coupled by optical means 19 to a transmit and/or receive calibration module 20 located outside the solid angle of transmission and/or reception of the array. The calibration antenna, which consists of a dipole 4, patch loop printed on a dielectric substrate, may be mounted on a radome and used to adjust the gain or phase of the antenna elements in the array 14. The location of the calibration antenna may be predetermined or determined by means of an optical source located with it and 2 or 3 photodetectors. The calibration antenna may operate in either transmit or receive modes. <IMAGE>

Description

Improvements in or Relating to Phased Array Antenna This invention concerns apparatus particularly, but not exclusively, radar apparatus, having an electronically scanned antenna formed by an array of antenna elements. The invention is concerned with the in-situ calibration of such an apparatus.

Electronically scanned antennae comprise a plurality of separate radiating ( & receiving) elements in a one or two dimensional array which constitutes the antenna aperture of, for example,-a radar transceiver.

A radiated beam (or a plurality of beams) is formed by accurately controlling the amplitude and phase of the r.f.

transmit signal at each antenna element such that, in accordance with diffraction theory, the radiated signals combine coherently to give a desired polar distribution.

Conversely, receive "beams" (i.e. high sensitivity "directions") are created by accurately controlling the gain and transmission phase of the various antenna elements acting as receivers such that they give the necessary polar distribution of sensitivity in a particular direction when combined coherently in the receiver. With electronic steering, it is possible to create multiple "beams" simultaneously.

By their very nature, high-directivity electronically steered antennae must consist of many elements (eg > 1000) and the quality (i.e. directivity, sidelobe levels, null positions and null depth) of the formed beams is critically dependant on the precision of control of amplitude/gain and, in particular, the phase of each individual transmit/receive channel (associated with a respective antenna element) of the transceiver.

-In general terms, it is not practicable to make all channels ideal (i.e. to have, to all intents and purposes, identical characteristics in terms of amplitude/gain and phase as a function of frequency, power level, operating temperature and lifetime). It is therefore necessary to find some way to: 1. calibrate the antenna in order to assess the actual performance of individual transmit/receive (Tx/Rx) channels; 2. calculate the necessary compensation in signal combining networks in order to correct for deviations from the ideal for each element; and 3. adjust individual transmit/receiver (Tx/Rx) channels.

Ideally, the correction data is held within a respective module associated with each channel -- this then, makes each individual channel look "ideal" to an overall control system and allows the array to be built using a common module for each of the Tx/Rx channels.

The manner of adjustment of gain and phase is an intrinsic part of the basic system for an electronically steered antenna. Also, given that one knows the gain/phase/frequency relationship for each Tx/Rx channel, there are known techniques for providing digital look-up tables and/or interpolating algorithms in order to effect the necessary respective correction. Calibration, on the other hand, presents a far more difficult problem for an operational system.

For practical purposes, it is assumed that a reference plane (in microwave terminology) can be established in a relatively straightforward manner at the start of an r.f.

manifold (i.e. at the point where the individual channels are combined into an overall receiver channel; or alternatively, where an overall transmitter r.f. signal is split into the various channel feeds). Given this, the main calibration task is primarily one of measuring the S-parameters between the manifold reference plane and a calibration Tx/Rx reference plane via each active array channel in turn. It is then necessary to calculate the deviation from the ideal response; modify the channel gain/phase inputs and repeat the measurement. This cycle is repeated until the modified channel comes within desired limits. The "correction inputs" are stored for that element/frequency/phase/gain combination. This storage may take the form of a look-up table or interpolation formulae, either centralised or distributed within each channel.

At its simplest, calibration can take place using a calibrated single element receiver/transmitter on a near-field or far-field test range. This has the disadvantage, however, that, for an operational system, it is an infrequently performed activity and thus cannot cope with environmental and aging effects likely to be encountered in an operational system.

Preferably, it is desirable to have an in-situ calibration facility that can periodically re-assess and up-date the antenna calibration. This implies that there should be calibration facilities either (a) within or (b) permanently attached to the antenna.

In the former case, it has been proved to be impractical to incorporate an internal distribution network for a calibration test-tone/receive-channel, as this implies a distribution network almost as complex as that for the active antenna itself, and thus almost as difficult to calibrate, as one cannot be certain whether an error is in the main or the calibration matrix.

In the latter case it is necessary to provide a test transmit/receive source.

(a) ruggedly and permanently attached to the antenna structure: (b) whose position can be accurately measured with respect to an antenna face and centre-line (C/L); and (c) which does not perturb the far-field radiation pattern in any significant way (e.g. it must not introduce large amounts of metal in front of the antenna).

According to the present invention, an apparatus comprises an array of transmit and/or receive antenna elements having an associated solid angle of transmission and/or reception, a calibration antenna, located forward of said array within the solid angle, for receiving signals transmitted by the antenna elements and/or for transmitting signals to the antenna elements, and optical means coupling the calibration antenna to receive and/or transmit calibration modules located outside of the solid angle.

As only the calibration antenna is located within the solid angle and operates at low power levels and hence may be physically small, the far field radiation pattern of the apparatus is substantially unperturbed.

Particularly, the invention is concerned with radar apparatus and will be so described hereinafter. It will be appreciated, however, that the invention is also applicable to communication apparatus having a phased array antenna.

The calibration antenna is notionally located at a predetermined position within the solid angle but preferably includes an optical source whereby any position variation, from the predetermined position, may be determined.

An X-Y photodetector is conveniently arranged for projection of an image of the optical source thereon whereby to determine X-Y position variations of the optical source and the calibration antenna.

Preferably, at least two photodetectors are provided to enable X-Y-Z position variations of the optical source and the calibration antenna.

The or each photodetector may comprise a CCD array having a field lens for imaging the optical source thereonto.

In a preferred embodiment, the radar apparatus is arranged to energise each antenna element and its associated transmit and/or receive channel separately whereby a "test tone" signal may be transmitted to or received from the calibration antenna and the variation of each element and its channel from an "ideal" determined.

As is conventional in electronically scanned antenna, it is possible to vary the gain and/or phase of a signal in each channel. In accordance with this invention, the variation from ideal of each element and channel is monitored and a correction or corrections applied substantially to eliminate the variation. The correction or corrections are advantageously digital in form.

Conveniently, storage means are provided for storing the correction needed in respect of each element and channel.

The calibration antenna is preferably positioned at a predetermined location on a boresight of the radar apparatus whereby to minimise the processing needed to determine a correction to be applied.

The calibration antenna may be a dipole thin film printed on a dielectric substrate. The dipole, and its associated circuits, advantageously has dimensions less than 20 mm x 15 mm.

If Wavelength Division Multiplexing/Demultiplexing is employed, the optical means coupling the calibration antenna to the transmit and/or receive calibration modules may comprise a single optical fibre. The calibration antenna includes an opto-electric and/or an electro-optical transducer for converting a modulated optical test tone, for transmission, from the transmit calibration module or for converting a received test tone radar signal, on reception, into a modulated optical signal for sending to the receive calibration module.

Separate optical fibres may be provided for transmission, for reception and/or for provision of the optical source. For example, two optical fibres may connect the calibration receive module to the transducer located at the calibration antenna. A first fibre may conduct an unmodulated optical signal to the calibration antenna whereat the optical signal is modulated by the electro optical transducer, upon receipt of a test tone radar signal by the antenna, and the modulated optical signal returned via the second optical fibre to the receive calibration module.

Similarly, two optical fibres from the transmit calibration module may feed respectively a DC bias optical signal and an X band modulated optical signal to the opto-electric transducer to feed the calibration antenna with a transmit test tone.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a radar apparatus incorporating a calibration antenna, in accordance with the present invention; Figure 2 is a diagrammatic perspective view indicative of calibration antenna position determination in the apparatus of Figure 1; Figure 3 is a diagrammatic perspective view of a radar apparatus in accordance with the present invention; Figure 4, is a diagrammatic perspective view of a Tx calibration antenna for calibrating the reception characteristics of a radar apparatus, in accordance with the present invention; and Figure 5 is a diagrammatic perspective view, similar to Figure 4, of an Rx calibration antenna for calibrating the transmission characteristics of a radar apparatus, in accordance with the present invention.

The present invention makes use of the fact that, in receive mode, the radar receiver is extremely sensitive and thus needs only very low levels of irradiation for operation. Thus, we can use novel techniques for feeding a low-power test-tone to a physically-small and inefficient broad-beam antenna in a way which is not intrusive in the electromagnetic sense. Furthermore, the feed method can be used to provide means for accurately measuring the calibration antenna position relative to the array face (to allow phase corrections to be calculated for non-deal positioning of the calibration antenna). Finally, the feed may be configured to operate in a reciprocal way, in order to calibrate Tx channels, by connecting the calibration antenna to a calibration receiver. The essential ingredients of the invention are as follows: Referring firstly to Figures 1 and 2, a test transmit signal is generated in a calibration Tx module 1 and is transmitted to a calibration antenna 4 by means of broad-band modulation in an opto-modulator 2 of an optical carrier emitted at a wavelength A, by a semiconductor laser 2a, transmitted to and detected by a fast photodiode 3 at the calibration antenna 4 by means of a single-mode optical fibre 5 Ei.e. a dielectric feed]. It is possible to modulate an optical beam at rates spanning dc to 20GHz and to transmit 10-100mW of optical power with little loss.

The calibration antenna 4 is positioned at a predetermined location which may be within 1-5 active antenna diameters of the face of a circular array 14 preferably on, or close to, the centre-line of the array 14 under test.

For example, the calibration antenna 4 may be housed within, or be an integral part of, a radome surrounding the active antenna. The calibration antenna 4 then lies, preferably centrally, of the solid angle over which the array may transmit a radar signal or may receive radar echoes.

At the calibration antenna 4, in transmit mode (i.e.

calibrating Rx channels of the array 14), the optical signal is detected by a high-speed photodiode 3 and a microwave-frequency test signal extracted as an electrical signal which is coupled directly to the test antenna 4. In receive mode (i.e. calibrating Tx channels of the array 14) the signal from the calibrating antenna 6 directly modulates a guided wave optical beam in an optical fibre 7) using an external (eg. Mach-Zehnder) modulator 8. This modulated beam is then transmitted via a single-mode optical fibre 9 to a calibration photo-receiver 10 behind or away from the active antenna 14 and certainly outside of the aforesaid solid angle. Given that the calibration antennae 4 can be relatively small (eg A /10), a non-intrusive feed is provided without the need for any power supplies at the head. Modulators and photodiodes capable of operating in the dc - 20 GHz band are known.

A radiating monomode fibre output 11 driven by a semiconductor laser of wavelength A 2 is also located at the test antenna thereby to provide an optical source and to act as a position indicator (typically the radiating diameter will be a few ym) which is imaged by a lens 12 onto a focal plane detector 13.

The focal plane detector 13 is preferably a CCD array.

Modern CCD camera chips have element spacings in the lOOgm region which means that a l00mm focal length imaging system can resolve the angular position of a monomode fibre-end to within 1 mr (sufficient accuracy for most radars).

Two miniature camera arrangements (12a, 13a; 12b, 13b), referenced to the array-face 14, could thus position the calibration source relative to the array face to an accuracy of lmr. (Figure 2 for example shows a more detailed geometry illustrating how deviations in calibration antenna position can be measured using lenses (L1 & L2) and CCD camera chips (FP1 & FP2) and giving a digital readout).

Referring to Figure 2, the calculation of the absolute position of the calibration antenna 4 is effected as follows: The antenna array 14 has its centre position denoted X=O, Y=O. A normal to the X Y plane (the boresight of the antenna) extends in a direction Z from the centre position and the calibration antenna 4 is positioned at a location D so as to have X,Y and Z coordinates 0,O,Z . Any element of the array 14 transmitting or receiving radar signals along the boresight, for a particular frequency, has a predetermined phase and amplitude relationship to that of an element situated at X=O, Y=O.

An optical source positioned at D (the end of the fibre 11), is imaged by a pair of lenses L1 and L2 onto respective XY photodetectors set up so that the images I1 and I2 are at positions x1=0, y1=O and x2=O, y2=0. Any variation of the optical source from the position D provides images at positions x1, y1 and x2 and y2 due to an actual location of the source at a position X=0+#x, Y=0+#y and Z=Z+#z for < < f1 9 < < CL2 and , 6 and az < < z Z, #y and #z can be determined: x1#f1.#x CL1 y1 # f1. (#z sin &alpha;1 + #y cos &alpha;1 CL X2 z ! #z sin #2-#x cos #x COS i2) CL2 y2 # f2.

CL2 Relative Phase of an antenna element at position X,Y is given by:

where Arf is the free space wavelength of the r.f.

signal.

If the position of the calibration antenna 4 is accurately known, calculation of free-space propagation delays, between the calibration antenna and each of the channel antennae is enabled and prediction of the relative amplitude and phase distribution expected across the active array face for each Tx/Rx channel in the ideal case is possible. Given ideal identical performance for all channels in the active array 14, it is possible to calculate the expected relative phase of each channel output in receive mode when irradiated by the "point" calibration antenna in its determined position. Conversely, it is possible to calculate the expected received phase at the "point" calibration antenna 4 for each "ideal" channel (when in transmit mode).

It is then possible to inject test tones either from the calibration transmitter via the calibration antenna into the RX channels of the array, or into the TX channels of the array for reception by the calibration antenna and for feeding to the calibration receiver 10.

The deviation from the computed "ideal" (allowing for the determined calibration antenna position) can then be measured. This, in turn, can be used in a closed-loop system to generate a digital correction command to the channel module.

Finally, when actual response for a demanded frequency, gain and phase of transmission or reception by the particular element and channel of the array 14 comes within the desired limits, the correction "word" can be stored locally within the particular channel module memory.

This then provides permanent correction (until the next calibration) for that set of demand parameters, and so makes all channels look ideal to a central controller.

Furthermore, all channels will have a common hardware configuration.

The feed to the calibration transmitter/receiver is common to all measurements. In cases therefore where it is only the relative gain/amplitude distribution across the active face of the array 14 that is important, there is no requirement to calibrate the feed itself. In cases where there is need to have tight control of gain/phase flatness over a range of operating frequency, it will be necessary to calibrate the feed in terms of amplitude/phase/frequency performance. With the feed to the calibration Tx and Rx modules being a single, permanent channel however, calibration of this feed should be relatively straightforward.

There will be some need to calibrate, on a one-off basis, the polar distribution of the calibration antenna but with this being an "electrically small" structure, the polar distribution should be broad, with little fine structure and should thus pose little problem.

Referring now the Figures 3, 4 and 5, an electronically scanned antenna 14 is shown. The antenna 14 is circular in shape and substantially planar. As is conventional, it is comprised of a plurality of Tx/Rx elements 14a each fed via a respective channel 16 from a Common microwave source/demodulator and receive signal processing circuit 17. Each element channel 16 includes gain control and phase shifting means 18 so that the amplitude and phase of the signal transmitted thereby can be controlled. Similarly, the amplitude and phase of a received signal can be controlled. The gain control and phase shifting means 18 of each channel may be "corrected" by a master control module 17a in dependence upon frequency and desired beam angle and generally includes a look-up table for applying appropriate factors to the gain control and phase shifting means.

The appropriate factors need to be established initially and also subsequently by calibration and recalibration.

In accordance with the present invention, a calibration feed 15 is positioned (for example, mounted on a nose cone radome (shown dashed) enclosing the array 14) on a boresight of the array 14 and includes a Tx/Rx antenna 4,6 and an optical source 11. Optical means 19 (e.g. the monomode fibres 5,7 and 9) couple the calibration head 15 to a remote optical transmitter receiver module 20. Whereas the calibration head 15 is located within the solid angle of transmission and/or reception of radar signals of the array 14, the module 20 is located outside such solid angle and the calibration head 15 and the optical coupling means provide minimum perturbation of the far field radiation pattern produced or seen by the array 14.

As far as the radar antenna array 14 is concerned, Rx characteristics are determined by Tx operation of the calibration means and Tx characteristics are determined by Rx operation of the calibration means.

The calibration head 15 includes a photodetector 21 associated with the coupling 19 and the Tx calibration antenna 4 and an optical modulator 22 associated with the coupling 19 and the Rx calibration antenna 6.

Referring particularly to Figure 4, one form of the Tx calibration antenna 4, photodector 21 and optical coupling 19 is shown. The module 20 (here shown as a Tx module 20a) is coupled to the head 15 by two optical fibres 23,24. An optical signal is fed from a laser source via an X-band modulator (see Figure 1) of the module 20a along the fibre 23 to an r.f. photodiode 25 coupled directly to the Tx antenna 4. The fibre 24 carries an unmodulated optical signal which is split by a multiple splitter 26 in the head 15 and the signal feeds a plurality of photodiode chips 27 connected in series to provide a DC bias as indicated by the thumbnail circuit diagram of Figure 4. One of the outputs of the splitter 26 may serve as the optical source 11 shown in Figures 1 and 2. The bias signal provides sufficient power to maintain the reverse bias of the photodiode 25 and thus causes the antenna 4 to radiate the r.f. signal detected by the photodiode 25. If the signal carried by the fibre 24 is switched off. The diode chips 27 do not provide the necessary D.C. bias to the photodiode 25. In this way, the photodiode 25 is not active during transmission of radar signals by the array 14 and is protected.

The DC bias signal is conditioned by a chip capacitor C and a distributed thin film inductor L. An appropriate printed resistor may be included in the electrical circuit.

The output power radiated by the antenna 4 is very small, for example, of the order of 1.4 .W to produce a signal level of -80 dBm at the antenna element 14a under calibration. Thus, if the receiver has a a -101 dB input noise level, this is equivalent to a 21 dB signal to noise ratio in a 10 NHz noise bandwidth. In the module 20a, a lmW r.f. input to the optical modulator 2 (see Figure 1) suffices and an optical coupling 19,23 signal to noise ratio of 51dB in a 10 NHz noise bandwidth is assumed.

Instead of two fibres 23,24, a single fibre may be used employing wavelength division multiplexing (WDM) or a similar technology. A PIN diode limiter, optically switched, may additionally be provided to protect the r.f.

photodetector 25 during radar operation if the photodetector 25 is not itself switched off (see above).

The illustrated dipole antenna 4 may be replaced by a patch or loop antenna.

The described components of the photodiode 21 can be encapsulated in a solid dielectric or sealed in a ceramic package.

Referring now to Figure 5, the other half 15b of the calibration head 15 is shown. A receive antenna 6, again shown as a dipole printed upon a dielectric substrate 28b (which is preferably integral with the substrate 28a of Figure 4) is connected to an integrated modulator chip 8 also mounted on the substrate. An optical fibre 29 of the optical coupling means 19 carries an unmodulated optical signal from an Rx calibration module 20b to the modulator 8.

An r.f. signal received by the antenna 6 is used to drive the modulator 8 to modulate the signal fed thereto by the fibre 29 and the so-modulated optical signal is returned by an optical fibre 30 of the coupling 19 to the module 20b.

As can be seen from Figure 1, the module 20b includes a semiconductor laser of wavelength A3 and a photoreceiver 10 and associated electronic circuitry.

Instead of two fibres 29,30, a modulator chip and a single fibre having a reflective termination may be used.

The dipole 6 could be integrated with the modulator chip 8 or the chip 8 could be mounted on the back of a patch antenna.

Again, the assembly could be encapsulated in a solid dielectric or hermetically sealed in a ceramic package.

Assuming an antenna element 14a radiates a 5W signal and the calibration head 15 is situated at a distance of 2 metres, a 36.W r.f. signal would be available at the halfwave dipole antenna 6. The application of such a signal to a known form of integrated optical modulator would produce an output level of -42 dBm (36 dB signal to noise ratio in a 10 MHz noise bandwidth).

The dynamic range of the Rx module of the calibration head 15 is sufficient to enable it to be used without overload to monitor the status of the whole array 14 when 1200 T/R elements 14a are operating. It would be possible therefore to monitor the transmission characteristics of the array 14 during normal radar operation. The accurate positioning of the antenna is of secondary importance here and would be difficult and impractical to determine in real time but an immediate warning could be given of malfunction of any or all of the antenna elements 14a merely by monitoring the radar beam as received by the calibration antenna 6.

If the integrated modulator chip 8 were to be in the form of an optimised bandpass modulator device with 2 GHz total bandwidth, such a device would be 6 dB more sensitive than a wideband modulator having a d.c. to 20 GHz bandwidth.

In practice, all three functions at the calibration antenna (position indication; Tx calibrate; Rx calibrate) could be effected via a single fibre link together with optical wavelength division multiplexing and T/R switching.

However, the intrusion penalty of two additional optical fibres may be outweighed by that of the wavelength Mux/Demux and its cost.

A single vertically polarised antenna could be used on the calibration head for both transmit and receive calibration antennae. Switching of the common antenna between the two calibration circuits could be implemented using an optically controlled PIN diode as a T/R switch.

The invention is not confined to the precise details of the foregoing example and variations may be made thereto within the scope of the appended claims.

Claims (24)

1. An apparatus comprising an array of transmit and/or receive antenna elements having an associated solid angle of transmission and/or reception, a calibration antenna located forward of the array within the solid angle, for receiving signals transmitted by the antenna elements and/or for transmitting signals to the antenna elements, and optical means coupling the calibration antenna to receive and/or transmit calibration modules located outside of the solid angle.

2. An apparatus as claimed in claim 1 wherein the calibration antenna is located at a predetermined position.

3. An apparatus as claimed in claim 1 or 2 wherein the calibration antenna includes an optical source whereby its position may be determined.

4. An apparatus as claimed in claim 3 further including an X-Y photodetector whereonto an image of the optical source is projected whereby X-Y co-ordinates of the calibration antenna relative to the array may be determined.

5. An apparatus as claimed in claim 4 including a plurality of the photodetectors whereby X-Y-Z co-ordinates of the calibration antenna may be determined.

6. An apparatus as claimed in claim 4 or 5 wherein the or each photodetector comprises a CCD array and a field lens for imaging the optical source thereonto.

7. An apparatus as claimed in any preceding claim wherein the calibration antenna is located on the boresight of the apparatus.

8. An apparatus as claimed in any preceding claim including a radome for protecting the array and wherein the calibration antenna is mounted on the radome.

9. An apparatus as claimed in any preceding claim wherein each antenna element is part of an associated channel, and wherein means are provided for activating each channel separately in conjunction with the calibration antenna to determine the variation of the channel from an ideal response.

10. An apparatus as claimed in claim 9 wherein each channel includes means for altering the phase and/or gain thereof and wherein the altering means is controlled so as to alter the phase and/or gain of the channel in accordance with the determined variation of the channel from the ideal in order to minimise the variation.

11. An apparatus as claimed in claim 10 including storage means for storing the alternation necessary to each channel to minimise the variation of that channel from the ideal.

12. An apparatus as claimed in claim 11 wherein a respective one of the storage means forms part of each channel.

13. An apparatus as claimed in any preceding claim wherein each antenna element is arranged to transmit radar signals and the calibration antenna includes an antenna and an electro-optical transducer for producing an optical signal in accordance with the received radar signal.

14. An apparatus as claimed in claim 13 wherein the optical coupling means includes means for transmitting an unmodulated optical signal to the calibration antenna and the electro-optical transducer comprises an integrated electro-optical modulator.

15. An apparatus as claimed in claim 13 or 14 wherein a receive calibration module comprises an optical source for generating an unmodulated optical signal, and a photodetector means for detecting modulation of the modulated optical signal received via the optical coupling means from the calibration antenna.

16. An apparatus as claimed in any preceding claim wherein each antenna element is arranged to receive radar echoes and wherein the calibration antenna includes an antenna and a photo-electrical transducer for feeding an electrical signal to the antenna for transmission.

17. An apparatus as claimed in claim 15 wherein the photoelectrical transducer comprises a photodiode and the optical coupling means includes means for feeding an r.f.

modulated optical signal to the calibration antenna.

18. An apparatus as claimed in claim 16 or 17 including means for providing a DC bias to the antenna of the calibration antenna.

19. An apparatus as claimed in claim 16, 17 or 18 wherein a transmit calibration module comprises an optical source, a modulator for modulating an optical signal from the source with an r.f. modulation for transmission of the modulated optical signal via the optical coupling means to the calibration antenna.

20. An apparatus as claimed in any preceding claim wherein the optical coupling means includes at least one optical fibre.

21. A radar apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

22. A calibration antenna for use in a radar apparatus as claimed in any preceding claim comprising an antenna and a photoelectric and/or an electro-optical transducer arranged for mounting within a solid angle of the radar apparatus, optical coupling means, and a transmit and/or receive calibration module locatable outside the solid angle of the apparatus.

23. A calibration antenna substantially as hereinbefore described with reference to and as illustrated in Figure 3 or Figure 4 or Figure 5 of the accompanying drawings.

24. An apparatus as claimed in any of claims 1 to 21 further including means for monitoring signals transmitted by the array of antenna elements during normal operation thereof and for indicating the malfunction of any or all of the antenna elements in real time.