GB1495298A - Antenna - Google Patents

Abstract

1495298 Aerials KOKUSAI DENSHIN DENWA KK 19 Dec 1974 [21 Dec 1973] 54918/74 Heading H4A A large aperture antenna for use with geostationary satellites having a fixed main reflector is capable of beam scanning over a small angle. The beam from the primary radiator is fed to the main reflector via a plurality of secondary elements, e.g., plane or curved mirrors, or lenses, at least two of which are each separately controlled by one of two control means to displace their position, the displacement of a first altering the position of the phase centre of the beam, to change the radiated beam direction from the antenna, the displacement of the second correcting the distortion in the radiated wave produced by the displacement by the first of the position of the beam phase centre. In Fig. 3 a Cassegrain-type configuration of parabolic main reflector 1 and hyperbolic sub reflector 2 is fed from primary radiator 3, e.g. a corrugated horn, via parabolic reflectors 11 and 13. When reflector 13 is moved by a control means 17, see Fig. 7, to a position 13' thus displacing its focusing point O 2 (which corresponds to the feed point in a true Cassegrain system) to a focal region O 2 <SP>1</SP>, distortion of the main radiated beam from reflector 1 may be corrected by movement of reflector 11 a predetermined distance #z<SP>1</SP>. It is stated that the polarization characteristics and the spill-over from the reflector I are also alleviated by this method. In Fig. 7 a practical arrangement comprises the main reflector 1 fixed to the ground 21 by support structure 20. When an angular error is detected between the main antenna beam and the direction of the satellite, reflector 13 is driven by control motor 17 in the X, Y and Z directions to correct the beam direction, and reflector 11 is similarly displaced to correct the wavefront. In the Fig. 4, 5 and 6 embodiments (not shown) different feed arrangements to the feed point are shown involving plane mirrors or lenses. In Fig. 8 (not shown) the main reflector is an offset parabolic sector reflector. In Fig. 9 (not shown) the main reflector is torus shaped.