DUAL REFLECTOR ANTENNA CAPABLE OF STEERING RADIATED BEAMS

1367331 Aerials NIPPON TELEGRAPH & TELEPHONE PUBLIC CORP 10 Sept 1971 [28 Sept 1970] 42242/71 Heading H4A The surfaces of the main reflector and of the sub-reflector of a Cassegrain type aerial are specially shaped, so that when the sub-reflector is inclined in either one of two directions from the axis of the main reflector a substantially plane equi-phased wavefront is radiated from or received by the aerial. A graphical construction is described for arriving at the shapes of the reflector surfaces, and equivalent formulµ are derived. Such an aerial for selectively tracking a plurality of geo-stationary communication satellites comprises a main reflector 40, Fig. 8, with feed horn 52 and sub-reflector 43, mounted on a support 47 with foundation 46, and arranged for rotation in the equatorial plane about an axis 51. A motor 50 drives the reflector &c. through gearing, to enable it to be pointed generally towards a particular satellite. Thereafter, tracking is maintained by movement of the sub-reflector 43 by means of a device 42, which is carried on supports 41 fixed to the reflector 40. A device similar to that shown in Fig. 8, for a sub-reflector 60, Fig. 9, comprises three motors 67, 76, 79 which respectively rotate the sub-reflector about a transverse axis through a point 61 on the axis 78 of the main reflector, move the sub-reflector along the axis 78, and rotate the sub-reflector about the axis 78. The motors are controlled by a unit (Fig. 10, not shown) in which a detector responsive to any difference between the directions of the satellite and of the beam radiated from the aerial is coupled to two generators, which produce signals respectively representative of errors about, and perpendicular to, the axis of the aerial. The first error signal operates a control circuit for the motor 79. The second error signal is resolved by an analogue computer into two components, which operate respective control circuits for the motors 67, 76.