GB2235092A

GB2235092A - Transmit/receive antenna

Info

Publication number: GB2235092A
Application number: GB8625560A
Authority: GB
Inventors: William David Waddoup
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1986-10-25
Filing date: 1986-10-25
Publication date: 1991-02-20
Anticipated expiration: 2006-10-25
Also published as: GB2235092B; GB8625560D0

Abstract

A transmit/receive antenna for a radar system has a microwave focusing lens 32 in the focal plane of which is an array 34 of receive elements, e.g. crossed dipoles. A small transmitter horn 30 is located in a central aperture 31 in the lens, so that the transmitter aperture is substantially smaller than the receive aperture. The transmitted energy floodlight illuminates the targets scanned by the receive elements of the array. A local oscillator source 33 to the rear of the antenna illuminates the elements of the array. In an alternative embodiment the transmitter horn is in an aperture offset from the centre of the lens. <IMAGE>

Description

TRANSMIT/RECEIVE ANTENNA is invention relates to 2 common aperture transmit/receive antenna for a radar system.

A requirement for one type of radar is to produce a tluralitv of close overlapping narrow receiver seams covering a dedined area which is floodlit illuminated by a single transmit iear.-. The single transmit bear, which is non-steerable in relation to the pattern of receive beams obviously has a beamwidth which, for a giver receive aperture size, becomes increasingly large as te number of receive beams instrumented becomes increasingly large.

Hence the transmit antenna aperture becomes correspondingly smaller. To produce tre plurality of close overlapping receiver beams it is known to provide an array of receive elements situated in the focal plane of a microwave lens.

The array may be linear, to form a fan beam pattern, or planar, to form a beam pattern covering both animuth and elevation.

According to the present invention there is provided a transmit freceive anterna for a radar syster comprising a microwave millimetre-wave focussing lens with an array of receive elements situated in the focal plane of the lens, characterised in that a transmit element having ar aperture substantially smaller .ha-. the receive aperture is provided occupying a portion within the tota receive aperture.

In one embodiment of the invention the transmit aperture is positioned in the centre of the anterna lens.

In an alternative embodiment the transmit aperture is positioned adjacent the periphery of the antenna lens.

Embodiments of the invention will now be described with. reference to the accompanying drawings, wherein: Fig. 1 illustrates the principle of a common aperture transmit/receive antenna, Fig. 2 illustrates a common aperture with a central transmit aperture, Fig. 3 illustrates a modified central transmit aperture antenna, Fia. 4 illustrates another form of central transmit aperture antenna, anQ Fig. 5 illustrates a common aperture antenna wIth a perIpheral transmit aperture.

A common aperture transmit/receive antenna is required to transmit a beam, henceforth referred to as a floodlight beam, of sufficient beamwidth to illuminate the solid angle subtended by a set c close overlapping narrow receiver beams, as shown in Fig. 1. The aperture required to fcr a narrow beam is considerably larger than that required to form a wide beam.The invention is based on the assumption that it is possible to tolerate blockage of a small portion cf the wide receiver aperture, necessary to form the multiplicity of narrow receive beams, to accommocate z soma : transmit aperture within the overall wide receIve aperture, thereby achieving a common aperture transm.it/receive antenna.

In the arrangement shown in Fig. 2 an array of receive elements 20 Is situates in the focal plane of a microwave or millimetre-wave focussing lens 21. The elements may each comprise a pair of crossed dipoles one of which is oriented to receive the incoming radiation focussed by the lens whilst the orthogonally arranged dipcle is illuminated by a local oscillator source. The array 20 is conveniently formed on the planar surface cf a secondary plano-convex lens 22. Lens 22 can be a hemispherical lens in the case of a two-dimensional array of elements or a semicylindrical. lens in the case of a linear array of elements.In either case the centre of curvature of the lens 22 is located at the focal plane of the focussing ens 21. Incident radiation parallel to the axis of the lenses will be focussed on the centre element of the array 20 forming the centre receiving beam of the receiver beam pattern. Off-axis radiation kill be focussed on other elements of the array, forming the other beams of the receiver beam pattern. Placed intermediate the lens 21 and 22 is a small double sided mirror 23, angled at 45 to the axis of the lenses. Situated at one sice of the lens assembly is a source of local oscillator energy 24, e.g. a horn, directed toward the rear reflecting surface of the mirror 23.The reflected local oscillator energy is polarised so as to illuminate the relevant dipoles of the array 20. Directly opposIte the local oscillator source 24 there ,s situated transmitter source 25, e.g. a horr, directed toward the front reflecting surface of the mirror 23. The reflected transmitter energy is directed forwards along the lens axis to form the floodlight beam.

Khilst it 15 apparent that the mirror 23 will cause blockage of a sal part of the receiver aperture, an estimate of the effect of thIs central blockage can be derived by calculation from the ratio cf the lens (blockage diemeter to the effective footpront diementer.

Typically for the type of elements described above, a rat@ of 96:575, i.c. the transmit aperture is approximately 1'6 of the diameter cf the receive aperture, will result n a worsening of the sideloberlevel (SLL) of L. 5dB to - 15B In the case of a planar array thus would correspond to a transmatter ansenna floodlighring a partern of 6 receive beams an azimuth x 6 receive beams in elevation, a plaussible number of beams.

An alternative arrangement to the above disposes the ocal oscIllator source 24a to the rear ci te array 2n, in whIch case tre mirror 23 need be reflective o one surface only.

rig. 3 Illustrates a modifcation ci tne antenna structure of Fig. 2. The double-sided mirror @@ of Fig. 2 is replaced by a polarisation sensitive mirror 23a, for example a reflective grating which will reflect, on both sides, radiation of one polarisation only. Both the transmitter source 25 and the local oscillator source 24 are arranged to provide radiation of this one polarisazion only and so the transmitter radiation will be reflected toward the front of the antenna whilst the local oscillator radiation will illuminate the array 20.Radiation reflected from a target in front of the antenna will suffer polarisation rotation to a certain extent, at least some of the target reflected radiation will now be orthogonally polarized with respect to the transmitted radiation and hence will pass through the grating reflector to the array.

Whilst the amount of target reflected energy reaching the array may be significantly reduced, this will be offset to some extent by the reductIon in blocking of the receiver aperture due to the transparency of the reflective grating to the orthogonally polarlsed radiation.

Yet another central blocking arrangement is depicted In Fig. 4 wherein the transmitter horn 30 15 placed within an opening 31 in the focussing lens 32. In this arrangement the local oscillator source 33 is placed behind the array 34. The horn 30 is fed xTia a waveguide 35 the major dimension of which is in a plane passing through te lens axis to minimise the additiona blocking effect of the waveguide In general, antennas are amplitude tapered to produce a low SLL. .In this case the worst location for blockage would be at the centre of the aperture where the a.,pli.ude weighting is highest and hence the effect of blocking greatest. The arrangement of Fic. 5 with non-central blocking would then likely be more attractive.

As in the case of Fig. 4 the local oscIllator horn 40 is to the rear of the array 41. The transmit horn 42 is now disposed within an opening 43 adjacent the periphery of the focussing lens 44. However, with a crossed dipole element array the amplitude distribution across the aperture is controlled by the element patterns which, in the case of an E-plane pattern, do not peak on boresight (lens axis but consideraby off boresight.

The advantages of non-central blocking when compared with central blocking are no not so clear-cut, and have to be ascertained by analysis to determine which configuration is best for a given antenna construction.

Claims

CLAIMS:-

1. A transmit/receive antenna for a radar system comprising a microwave/millimetre-wave focussing lens wIth an array of receive elements situated in the focal plane of the lens, characterised in that a transmit element having an aperture substantially smaller than the receive aperture is provided occupying a portion within the total receive aperture.

2. An antenna according to claim 1 wherein the transmit element aperture is centrally positioned with respect to the receive aperture.

3. An antenna according to claim 2 including a mirror on the lens axis between the focussing lens and the receive element array, the mirror being inclined at 45c to the lens axis, and a transmitter source disposed to one side of the antenna directed toward the mirror so that the transmitter energy is reflected forward of the antenna along the lens axis.

4. An antenna according to claim 3 wherein the mirror is double sides, the antenna further including a local oscillator source disposed to the side of the antenna opposite the transmitter source so that local oscillator energy is reflected backward along the lens axis to llurrinate the array in corjunctior. with radiation received via the focussing lens.

5. An antenna according te claIm 5 wherein the mirror comprises z polarisation sensitive structure which is reflective to radiation of one polarisation only and transparent te radiation of an orthogonal polarisatior., said transr,i.ter source and local oscillator source being arranged both te produce radiation of said one polarisation only.

6. An antenna according to claim 2 wherein the transmit element comprises a horn disposed within an opening in the centre of the microwave focussing lens, the horn being connected via a waveguide feed to a transmitter source remote from the lens.

7. An antenna according to claim 6 including a local oscillator source disposed behind the array and arranged to illuminate the array from the rear.

8. An antenna according to claim 3 including a local oscillator source disposed behind the array and arranged to illuminate the array from the rear.

9. An antenna according to claim 1 wherein the transmit element aperture is positioned adjacent the periphery of the antenna lens.

10. An antenna according to claim 9 wherein the transmit element comprises a horn disposed within an opening in the mIcrowave focussing lens, the horn being connected via a waveguide feed to a transmitter source remote from the lens.

11. An antenne according to claig 10 including a local oscillator source dIsposed behind the array ano arranged to illuminate the arrav fro the rear.

12. A transmit/receive antenna fcr a racar system substantially as described wit reference to tne accompanying drawings.