A Proposed Multiple-Beam Microwave Antenna For Earth Stations and Satellites

Satellite communication systems with large capacities can be achieved if the satellites and earth stations are provided with multiplenarrow-beam antennas. 1 The capacity is proportional to the number of satellites, and thus it is important to use as many as practical in the limited orbital space. A moderate number of the resulting closely spaced satellites can be served by a single antenna at each earthstation site if the antenna is patterned after the offset Cassegrainian antenna shown in Fig. 1. This design allows an orderly expansion in communication capacity by the addition of feed horns. Since only one antenna is needed at each site, the design also permits a large saving in earth-station costs. Good multiple-beam performance can be achieved across all up/down pairs of satellite frequency bands, including those well below 10 GHz. At 20 and 30 GHz, a large earth-station antenna with acceptable thermal and wind distortion is hard to achieve. However, with the design outlined here, these problems can be largely overcome because the main reflector and subreflector can be fixed in position, thus allowing a stiffer structure. The steering of each beam is achieved by moving one of the feed horns, resulting in a steerable angle sufficient for tracking near-synchronous satellites. 1657