Abstract: A Cassegrainian antenna having a feed horn, a subreflector and a main reflector exhibits subreflector reflections back toward the feed horn. Some of this energy is reflected by the feed horn, and some by the main reflector adjacent to the feed horn, back toward the subreflector. This doubly reflected energy constitutes an echo which is reduced in the prior art by using a flat reflector plate mounted near the subreflector to cancel energy radiated by the subreflector back toward the region of the feed horn. Herein is disclosed the addition of a frequency sensitive reflecting wire grid between the flat plate and the feed horn. A combined reflection from the grid, plate, and subreflector provides echo cancellation in two frequency ranges. The plate may be recessed in a hole in the subreflector. A guard ring surrounding the plate prevents leakage through the subreflector hole.

Abstract: A compact offset Cassegrainian antenna having a paraboloidal main reflector and a convex subreflector brings microwave beams to a focus. A small flat reflector placed in one of the beams at the focus deflects it for interception by a concave reflector and horn. Another beam passing by the flat reflector is intercepted by a second concave reflector and horn. The flat reflector is smaller than a focally located horn, allowing more closely-spaced beams to be accommodated. The flat reflector may be replaced by a curved reflector, a prism, or a microwave lens. Another flat reflector is added for each additional beam. Polarized beams are accommodated by adding a polarization screen.