Feed-Reflector design for large adaptive reflector antenna (LAR)

A novel Feed-Reflector system for large Cassegrain antennas of radio astron omy and deep-space communication applications is investigated. This Feed-Re flector is used to illuminate a hyperboloid subreflector with a 5-10 m diam eter located 500 m above the ground. Because the subreflector is located in the near field of the Feed-Reflector antenna, a theory based on the near f ield focusing properties of paraboloid reflectors is established. The focus ing at near distance is formed by moving the feed horn away from the focal point of the Feed-Reflector. In this theory, the properties of axial defocu sed paraboloid reflectors at near distance are investigated in more detail. By using equivalence path law, the subreflector shape is obtained. It is f ound that the hyperbola can approximate the subreflector well. A detailed r ay tracing is performed on the entire system which reveals that the feed sy stem uses some part of the subreflector three times. The gain, side lobe le vel, cross polarization, and aperture distribution are calculated for diffe rent feed horn locations and taper at the edge of the Feed-Reflector and al so for different sizes and eccentricities of the subreflector. Peak efficie ncy in excess of 74.8% and side lobe level around -20 dB are obtained for u nshaped system. The performance of the system over the operating band (1-22 GHz) is also studied and shown that the lower-frequency limit is dependent on subreflector and Feed-Reflector sizes.