SPECIFICATIONAntenna systemsThis invention relates to antenna systems for thetransmission and/or reception of electromagneticwaves, including microwave radio waves and infrared waves.
More particularly the invention is concerned I with systems of the kind comprising a parabolicreflector, a transmitting or receiving element (e.g. aprimary aerial for the transmission or reception ofradio waves) located at the focus of the parabolicreflector, and a reflector plate which is mounted tolie opposite the parabolic reflector and which ismovable to enable the direction of a beam of waves(subsequently referred to herein as the "primarybeam") transmitted or received by the system to bechanged, the arrangement being such that, in thetransmitting case, electromagnetic waves from theelement are reflected first by the parabolic reflectorand then by the reflector plate and, in the receivingcase, the received waves reach the element afterreflection first by the reflector plate and then by theparabolic reflector.Examples of antenna systems ofthis kind for transmitting and/or receiving radiowaves are described in the complete specification ofco-pending U.K. Patent Application No. 38126/77.
In addition to transmitting or receiving a primarybeam of waves, a system for the transmission and/orreception of electromagnetic waves is sometimesrequired to have provision for a secondary beamwhich is coaxial with the primary beam and followsthe primary beam as the direction of that beam ischanged. One object of the present invention is toprovide a system in which this requirement is satisfied.
According to the present invention, an antennasystem for ground-based radar equipment, comprises a parabolic reflector, a reflector plate, meansfor varying the angle of tilt of said reflector plate andits angular position about the geometric axis of theparabolic reflector, a first element located at thefocus of said parabolic reflector for the transmissionand/or reception of a primary beam of electromagnetic waves, the parabolic reflector and thereflector plate being arranged to provide a doublereflection of said primary beam, and a secondele- ment positioned on the geometric axis of said parabolic reflector remote from said reflector plate forthe transmission and/or reception of a secondarybeam of electromagnetic waves, by way of reflectionfrom said reflector plate only, the system beingarranged so that in operation, said primary and secondary beams external to the system are coaxial forall positions of the reflector plate.
The second element may be mounted in an aperture in said parabolic reflector.
The first element may be a microwave transmitter/receiver element. The second element may be aninfra-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 ApplicationNo. 38126/77 with particular reference to Figure 2thereof 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.