GB1488590A - Antenna with a beam of variable aperture for operation in the millimetre waveband - Google Patents

Abstract

1488590 Aerials THOMSON-CSF 7 March 1975 [12 March 1974] 9703/75 Heading H4A A variable beam width aerial comprises a radiation source, a main reflector or refractor which has a real focus, a secondary reflector or refractor for forming an image of the source at the said focus to irradiate the main reflector or refractor, and first and second means for, in a joint action, respectively translationally moving the source and either translationally moving the secondary reflector or refractor or modifying its shape, without displacing the image of the source. The invention has particular utility in the millimetre band, when setting up a link between two fixed stations, as a wide beam is available for initial alignment, and a narrow beam for normal working. As described, an aerial comprises a main paraboloidal reflector 1, Fig. 1, with a focus F, a sub-reflector 2, with an apex S, and a feed horn 3. In normal operation, the sub-reflector 2 is an hyperboloid 21, with one focus at F and the other at A, the apex of the main reflector 1. The horn 3 is also located at A. For a wider beam, a different reflecting surface 22 is used for the sub-reflector 2, and the horn 3 is axially displaced so that it is located at a point B, nearer the apex S. The image of the horn source, as before, is located at the point F. In one arrangement, different sub-reflector surfaces are located on respective sides of a barrel device which is rotated about an axis which intersects and is perpendicular to the aerial axis, so that any one of said surfaces may be used (Fig. 2, not shown). In another arrangement, the subreflector surface is on a diaphragm which is deformable by variation of air pressure (Fig. 3, not shown). In a second embodiment, the subreflector is moved towards the main reflector as the feed horn is moved away from the main reflector, but its shape is not changed (Fig. 4, not shown). In a third embodiment, the feed horn 3 irradiates the main paraboloidal reflector 1 through a lens 4. Normally, the horn is at P and the lens is at L. For a broader beam, the horn is moved to Q, and the lens to M. In both cases, the horn source is imaged at F.