GB2280787A - Radar antenna - Google Patents

Abstract

A scanning antenna includes an assembly rotatable in a horizontal plane about a first axis whereby to achieve 360 DEG coverage in azimuth. The assembly comprises a plane mirror 7 pivotable about a point 6 on the first axis, a non-planar reflector 5, multiple feed means 1 3 and associated lens means 4. The multiple feed means and lens means receive multiple stacked beams which have been scanned in elevation by pivoting of the plane mirror. The lens axis can be tilted at an angle to the first axis whereby to improve the efficiency of scanning. A high degree of transmitter/receiver isolation can be achieved (Fig. 6 not shown) by locating the transmitter between the receiver 1 - 3 and the mirror 7 and directing it at the mirror so that the transmitter and receiver are "looking" in opposite directions and by transmitting polarised radiation blocked in the receive path by a screen (24). The antenna may be used for air defence target tracking or locating jammers. <IMAGE>

Description

RADAR ANTENNA This invention relates to a radar antenna and in particular to an antenna for tracking targets over a wide range of angles to very high precision.

According to the present invention there is provided a scanning antenna including an assembly rotatable in a horizontal plane about a first axis whereby to achieve coverage in azimuth, the assembly comprising a plane mirror pivotable about a pivot axis transverse to the first axis, a non-planar reflector, multiple feed means and associated lens means, the multiple feed means and the associated lens means being disposed between the non-planar reflector and the plane mirror; and the reflector, the multiple feed means and the associated lens means being such as to produce/receive multiple stacked beams, which multiple stacked beams can be scanned in elevation by pivotting of the plane mirror.

Embodiments of the invention will no be described with reference to the accompanying drawings, in which: rig. 1 illustrates an antenna comprising a basic hybrid lens/reflector and plane mirror arrangement for achieving external coverage in elevation; Fig. 2 illustrates the arrangement of Fig. 1 with the plane mirror tilted relative to the plane of the reflector for an elevation of 8 ; Figs. 3a and 3b illustrate pivoting of the plane mirror for elevations of 700 and 00, respectively; Figs. 4a and 4b illustrate the positions of the plane mirror for elevations of 700 and 0 but with the whole antenna arrangement tipped backwards by 35 ; Fig. 5 illustrates an alternative Cassegrainian version of the antenna, and Fig. 6 illustrates an embodiment of tipped-back antenna incorporating a transmitter aperture.

For accurate low-elevation-angle tracking, it is consIdered that a VSRAD (Very Short Range Air Defence) radar antenna should produce 10 x 10 basic beams. By comparison a proposed FADS (Future Air Defence Systems) antenna has been postulated with beams which are 10 (in azimuth) x 80 (in elevation). In addition the 10 x 1" beams must be provided through 3600 in azimuth (as in FADS) and 0 to 700 in elevation (as opposed to 0 to 400 for FADS).

In order to achieve the 360 coverage in azimuth it is proposed to mechanically spin the antenna assembly in the horizontal plane. This is the case for both FADS and VSRAD.

To achieve the extended coverage in elevation with narrower beams than FADS, without vastly increasing the number of beams/feed elements, for VSRAD we now propose to scan multiple stacked beams, produced by a hybrid lens/reflector antenna with multiple feeds, using a pivoting plane mirror. This is illustrated in Fig. 1.

For simplicity only three feeds 1, 2, 3, and thus three receive beams, are shown although in practice the number of feeds would lie between 5 and 25, for example, typically being 10. The feeds 1, 2 , 3 are in a linear array, and may be comprised by a linear array of feed horns or dipoles. A plane mirror 7, non-planar reflector 5, and lens 4 combination direct energy from the basic received beams from different directions to their appropriate feeds. The basic received beams cover a narrow elevation sector, but tilting the plane mirror 7 about a pivot point 6, lying on the axis of symmetry of the non-planaor reflector (mirror 5) and lens 4, scans the basic multiple stacked beams as a whole over the wider elevation sector 0 to 700. The distance "d" indicates the footprint for a feed on the lens axis i.e.

feed 1. The reflector needs to be larger than "d" to allow for off-axis beams corresponding to the feeds 2 and 3, which are off the lens axis. The feeds are operated simultaneously, te ensure there are no blind spots, and receive corresponding beams which converge to one another from the elevational position under consideration. As the plane mirror is tilted (pivoted) the beams are scanned in the corresponding direction. The mirror pivots in coarser angular increments than a single beamwidth. In addition, the precise angular increment is not critical provided it is accurately known (for example by means of an optical shaft encoder). Hence, the mechanical specifications of the mirror servo drive are greatly relaxed.In fact, mirror angular increments of the order of 2 BPBW are required, where n is the number of basic beams in the stacked beam cluster, e HPBW is the basic half-power beamwidth, and the factor of 2 accounts for the angular multiplication factor of the mirror.

rig. 2 illustrates the arrangement of Fig. 1 with the mirror tilted for an elevation angle-of-arrival of e0 with respect to the plane of the reflector.

Analysis of Fig. 2 with a feed on the lens axis and the diameter of the footprint on the reflector given by d, shows that the required length x of the plane mirror is given by: d x = d sinp (Tr/2-0)/2 If e = 700 then x/d = 5.76 whereas if e = +350 then x/d = 2.17.

In terms of the plane mirror aperture, therefore, it is more efficient to scan 6 = +350 and to cover elevations of 0 to 700 by tilting the antenna backwards, than to scan 6 = 0 to 700 without antenna tilt-back. This is illustrated in Figs. 3a and 3b and Figs. za and 4b.

Whilst it might be considered possible similarly to use a pivoting plane mirror in a Cassegrainian type of hybrid antenna, as illustrated in Fig. 5, in order to miniaturise the mirror still further, such an arrangement has disadvantages. In such an arrangement a pivoting plane mirror 1G is placed near the focus of the primary reflector 11 and serves to reflect to a lens 12 a received beam reflected to the mirror 10 by the reflector 11. The non-planar primary reflector 11 must view the entire field of view (FOV), that is +35 rather than + ((nxl) /2)0 as in the Fig. 1 arrangement. With state of the art equipment, scanning aberrations in such arrangements become intolerable at + 80 FOV and hence the solutions of Figs. 1 to 4 are currently preferred, particularly the tilted-back arrangement.Primary scanning performed solely by the plane mirror has the unique property of being aberation-free. Another disadvantage of the Fig. 5 arrangement is that the non-planar primary reflector 11 would be used very inefficiently.

The proposed arrangement facilitates the incorporation of a transmitter aperture as is illustrated in Fig. 6. The transmitter aperture 20 is disposed on the axis of lens 21, the antenna also including a reflector 22 and a pivotable plane mirror 23, which is pivotable through a total angle of 350 for covering elevations of 0 to 700. The whole arrangement is mechanically rotatable about an axis through the pivot point of the mirror, which is also the focus of the reflector, in order to achieve the required 3600 rotation in azimuth. The transmitter aperture floodlights the nxn 10 received beams as they are both (transmit Tx and receive Rx) scanned in elevation by the pivoting of the plane mirror.It should be noted that the floodlight requirements implies that the transmitter aperture 1s only 1/n (linearly) that of the receiver. The transmit aperture, therefore, produces only marginal worsening of the blockage already presented by the receiver lens.

The primary advantage of this arrangement, apart from the common usage of the plane mirror 23 for primary elevation scanning, is the very high degree of transmitter/receiver isolation, since the transmitter and the receiver are "looking" in opposite directions.

If even this degree of isolation proves insufficient, a further amount (possibly 20 to 30dE) could be obtained by transmitting one linear polarisation (for example Tx (HP)-horizontally polarised), receiving on the orthogonal linear polarisation (Rx(VP)-vertically polarised), and providing an isolatIng linear polarisation sensitive screen 24 between the transmitter and receiver as shown in Fig. 6. This relies on the fact that scattering by a target will transform a measure of the incident energy upon reflection from one polarisation to another. The planarity of the plane mirror greatly helps in preserving the purity of polarisation of the transmit signal reflected from it, be it in the desired direction or low-level back scattering towards the receiver. If a curved scanning reflector had been employed instead of the planar mirror, possible low-level back-scattering towards the receiver would have been of impure polarisation, allowing an increased measure of it to penetrate an isolating polarisation-sensitive screen equivalent to screen 24.

In use of the antenna arrangements of the present invention the whole arrangement is spun constantly in azimuth by suitable drive means, and at the same time the plane mirror is gradually tilted, that is gradually driven between its positions corresponding to minimum and maximum elevation and back again. There is thus spiral acquisition of a beam reflected by a target.

Since it is necessary to know accurately the position (elevation) of the plane mirror in order to accurately identify or track a target, optical encoder means may be associated with a mechanical shaft driving the plane mirror.

Elevation information is obtained by determining which feed receives the highest amplitude signal, for example, and the elevation of the mirror.

The tilted-back antenna arrangements permits tracking of targets over a wide range of elevation angles to very high precision using fine optical determination of the coarse mechanical beam steering provided by the pivoting of the plane mirror, and fine electronic tracking by comparison of the multiple stacked beams.

The coarse mechanics permits much relaxed servo drive requirements in comparison with prior art arrangements.

By incorporating a transmit aperture as described there is achieved very high transmit/receive- isolation. This isolation is crucial to the successful operation of a CW radar to prevent saturation of the receiver chain by transmit leakage. Whereas VSRAD applications have been referred to above, the antenna arrangement may alternatively be employed for example for locating jammers.

Claims (10)

CLAIMS:-

1. A scanning antenna including an assembly rotatable in a horizontal plane about a first axis whereby to achieve coverage in azimuth, the assembly comprising a plane mirror pivotable about a pivot axis transverse the first axis, a non-planar reflector, multiple feed means and associated lens means, the multiple feed means and the associated lens means being disposed between the non-planar reflector and the plane mirror; and the reflector, the multiple feed means and the associated lens means being such as to produce/receive multiple stacked beams, which multiple stacked beams can be scanned in elevation by pivotting of the plane mirror.

2. A scanning antenna as claimed In claim 1 wherein the non-planar reflector and the lens means have an axis of symmetry, which is parallel to or comprises said first axis.

3. A scanning antenna as claimed in claim 1, wherein the non-planar reflector and the lens means has an axis of symmetry, which extends at an angle to said first axis whereby the antenna assembly is tilted with respect to said first axis.

4. A scanning antenna as claimed in claim 3 wherein the multiple feed means receives said stacked multiple beams, and including a transmitter aperture whose axis is on said second axis, which transmitter aperture is disposed between the multiple feed means and the plane mirror, the transmitter aperture and the multiple feed means looking in opposite directions.

5. A scanning antenna as claimed in claim 4 and including a polarisation sensitive screen disposed between the transmitter aperture and the multiple feed means.

6. A scanning antenna as claimed in any one of the preceding claims including servo drive means for driving the pivotable plane mirror and associated optical encoder means for providing information as to the angular position of the plane mirror for use with information available from the multiple feeds to derive target elevation information to high precision.

7. A scanning antenna as claimed in any one of the preceding claims wherein the multiple feeds comprise a linear array and are operated simultaneously.

8. A scanning antenna as claimed in claim 7 wherein the multiple feeds comprise a linear array of feed horns or dipoles.

9. A scanning antenna substantially as herein described with reference to and as illustrated in Figs. 1 and 2 with reference to either Figs. 3a and 3b or Figs.

4a and 4b; Fig. 5 or Fig. 6 of the accompanying drawings.

10. A radar system including a scanning antenna as claimed in any one of the preceding claims.