A zoom reflector antenna

When a satellite is in an elliptical orbit, the view of the coverage area from the satellite changes as the satellite travels. The size of the coverage area viewed from the satellite is inversely proportional to the distance between the satellite and the earth. Under such circumstance, a zoomable antenna is desirable. In the antenna described by Contu et al. (see "Reconfigurable, zoomable, turnable, elliptical-beam antenna", US Patent no.5,977,923) The radiation pattern is broadened by a defocusing effect, so it is not, technically, a zoom antenna. Furthermore, it cannot provide multiple zoom beams. In an offset Gregorian reflector antenna system, there are three components, feed horn, subreflector and main reflector. In order to zoom the far field pattern, at least two components must be repositioned. In a practical application, the replacements of the main reflector and the subreflector are preferred instead of the feed. This approach has been applied to design a multiple beam system. The results appear to be good enough for a practical application, although the contours of the pattern on the ground are distorted due to the poor optical characteristics of the offset Gregorian geometry. This zoom reflector antenna will not work well if the reflector system is operated in the diffraction limited optics condition.