GB2031655A

GB2031655A - Dual beam antenna system

Info

Publication number: GB2031655A
Application number: GB7931389A
Authority: GB
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1978-09-08
Filing date: 1979-09-10
Publication date: 1980-04-23
Also published as: DE2936372A1; GB2031655B

Abstract

An antenna system having provision for simultaneously scanning both a primary beam and a secondary beam so that the two beams are always coaxial. The primary beam, of radio waves, is formed by means of a parabolic reflector 2, a primary element 1 located at the focus of the parabolic reflector and a reflector plate 4 which lies opposite the parabolic reflector and through which the primary element projects. An assembly of an infra-red receiver 46 and an associated focussing arrangement 48 project through an aperture 47 in the parabolic reflector so that infra-red waves of the secondary beam are passed to the receiver after reflection by the reflector plate. Both beams are scanned by angular movement of the reflector plate. <IMAGE>

Description

SPECIFICATION Antenna systems This invention relates to antenna systems for the transmission and/or reception of electromagnetic waves, including microwave radio waves and infra red waves.

More particularly the invention is concerned I with systems of the kind comprising a parabolic reflector, a transmitting or receiving element (e.g. a primary aerial for the transmission or reception of radio waves) located at the focus of the parabolic reflector, and a reflector plate which is mounted to lie opposite the parabolic reflector and which is movable to enable the direction of a beam of waves (subsequently referred to herein as the "primary beam") transmitted or received by the system to be changed, the arrangement being such that, in the transmitting case, electromagnetic waves from the element are reflected first by the parabolic reflector and then by the reflector plate and, in the receiving case, the received waves reach the element after reflection first by the reflector plate and then by the parabolic reflector.Examples of antenna systems of this kind for transmitting and/or receiving radio waves are described in the complete specification of co-pending U.K. Patent Application No. 38126/77.

In addition to transmitting or receiving a primary beam of waves, a system for the transmission and/or reception of electromagnetic waves is sometimes required to have provision for a secondary beam which is coaxial with the primary beam and follows the primary beam as the direction of that beam is changed. One object of the present invention is to provide a system in which this requirement is satis fied.

According to the present invention, an antenna system for ground-based radar equipment, com prises a parabolic reflector, a reflector plate, means for varying the angle of tilt of said reflector plate and its angular position about the geometric axis of the parabolic reflector, a first element located at the focus of said parabolic reflector for the transmission and/or reception of a primary beam of elec tromagnetic waves, the parabolic reflector and the reflector plate being arranged to provide a double reflection of said primary beam, and a secondele- ment positioned on the geometric axis of said para bolic reflector remote from said reflector plate for the transmission and/or reception of a secondary beam of electromagnetic waves, by way of reflection from said reflector plate only, the system being arranged so that in operation, said primary and sec ondary beams external to the system are coaxial for all positions of the reflector plate.

The second element may be mounted in an aper ture in said parabolic reflector.

The first element may be a microwave transmit ter/receiver element. The second element may be an infra-red receiving element.

The means for varying the angular position of the reflector plate about said geometric axis may be effective to vary said angular position through 360".

An antenna system in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawing which shows diagrammatically a partially-sectioned perspective view ofthe system.

Referring to the accompanying drawing, a primary radio aerial in the form of a waveguide horn 1 is arranged to radiate vertically upwards and immediately above the horn 1 there is a downwards facing parabolic reflector 2, the horn 1 lying in the focus of reflector 2. Reflector 2 is mounted at the top of a conical radome 3, so as to be coaxial therewith. The collimated beam reflected from the reflector 2 illuminates a flat reflector plate 4 having a centre hole 5 through which the horn 1 projects. Radio waves reflected by the reflector plate 4 pass through the radome 3 and the emergent beam is defined by a pair of broken lines 6. For the purpose of varying the elevation of the beam, the angle of tilt of the reflector plate 4 is arranged to be varied between the position shown in the drawing and the position shown by the broken outline 4'.

By rotating the plate 4 about the geometric axis of the parabolic reflector 2 (on which axis lies horn 1), the azimuth direction of the emergent beam can be swept through 360".

The reflector plate 4 is in fact mounted on the end of a tube 7 by means of a hinge or pivot 8 (which is shown somewhat diagrammatically in the drawing).

The tube 7 is mounted via bearings (not shown) in another tu be 9 which in turn is mounted via bearings (also not shown) on a supporting platform 10. Platform 10 also locates and supports the conical radome 3 which carries the parabolic reflector 2. The waveguide feeder 13 passes through the tubes 7 and 9 and is terminated at its upper end by the horn aerial 1.

At its upper end, the tube 9 is secured to a cam 11 and a cam follower 12 is secured to the back of the reflector plate 4, the cam follower 12 being urged into contact with the cam 11 by means of a spring (not shown). The angle of tilt of the reflector plate 4 and thus the elevation of the beam of radio waves transmitted by the system may be changed by rotation of the tube 9. By rotating the tubes 7 and 9 together the azimuth of the beam may be changed.

Although the system under consideration has so far been described only as operating in the transmitting mode, it will of course be understood that it is equally suitable for operating in the receiving mode.

In fact when used in conjunction with ground-based radar equipment, this system would normally be required both to transmit and to receive.

As so far described, the present system is the same as the antanna system described in the complete specification of co-pending U.K. Patent Application No. 38126/77 with particular reference to Figure 2 thereof and reference is made to that specification for further details of the mechanism by which the tubes 7 and 9 are driven by electric motors 20 and 22.

The drawing originally filed was informal and the print here reproduced is taken from a later filed formal copy.

In addition to the primary beam of radio waves so far considered, the present system also has provision for receiving a secondary beam of infra-red waves. For that purpose, the parabolic reflector 2 has a central aperture 47 through which passes infra-red waves between the reflector plate 4 and an infra-red receiver 46. The receiver 46 lies on the geometric axis of the parabolic reflector 2 and points downwards towards the reflector plate 4 so that the emergent primary and secondary beams external to the system are necessarily coaxial for all positions of the reflector plate 4. The receiver 46 may have its own focusing arrangement 48, for example a lens system.

Clearly the infra-red receiver could be mounted completely inside the radome if its size were such as not to obstruct the primary beam excessively.

Equally it could be entirely external to the radome if the parabolic reflector were of such construction as to betransparentto infra-red.

When used in association with other equipment (including radar equipment) for the detection and location of aircraft, the infra-red arrangement described in the last paragraph enables the equipment to be used in a purely passive role to detect the presence of an aircraft and, after such a detection has been made, for the radar to be made operative for measurement of the range (and a more accurate measurement of bearing) of the aircraft to be made.

It is to be understood that within the scope of the present invention, the single aerial element constituted by the horn 1 may be replaced by a plurality of aerial elements. One well-known form of system to which the invention is applicable is for use with a mono-pulse radar equipment, the aerial in that case consisting typically of four separate waveguide horns which all radiate together when the equipment is operating in its transmitting mode while signals picked up by the individual horns in the receiving mode are subjected to differential interpretation to give the position of a body detected by the radar equipment.

Furthermore, the secondary beam is not necessarily of infra-red waves and may be of eithertransmitted or received radio waves, the unit 46 being changed accordingly.

Claims

1. An antenna system for ground-based radar equipment, comprising a parabolic reflector, a reflector plate, means for varying the angle of tilt of said reflector plate and its angular position about the geometric axis of the parabolic reflector, a first element located at the focus of said parabolic reflector for the transmission and/or reception of a primary beam of electromagnetic waves, the parabolic reflector and the reflector plate being arranged to provide a double reflection of said primary beam, and a second element positioned on the geometric axis of said parabolic reflector remote from said reflector plate for the transmission and/or reception of a secondary beam of electromagnetic waves, by way of reflection from said reflector plate only, the system being arranged so that in operation, said primary and secondary beams external to the system are coaxial for all positions of the reflector plate.

2. An antenna system according to Claim 1, wherein said second element is mounted in an aperture in said parabolic reflector.

3. An antenna system according to Claim 1 or Claim 2, wherein said first element is a microwave transmitter/receiver element and said second element is an infra-red receiving element.

4. An antenna system according to any preceding claim, wherein said means for varying the angular position of the reflector plate about said geometric axis is effective to vary said angular position through 360".

5. A combined microwave/infra-red antenna system for ground-based radar equipment substantially as hereinbefore described with reference to the accompanying drawing.