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In the last few years considerable interest has arisen in application of the Cassegrainian principle to paraboloidal antenna systems. In the case of large ground-based tracking antennas, it appears that this type of feed system can offer significant performance and operational advantages over conventional systems. For this application, however, special sidelobe requirements are imposed on the Cassegrainian system. The forward sidelobe distribution must be controlled to reduce the effect of solar noise interference, and the backlobe level must be controlled to reject blackbody radiation from the antenna environment. It is shown that these considerations are the major factor in choosing the feed system configuration. An experimental system utilizing an 85-ft antenna operating at 960 Mc is described. This system has an aperture efficiency of approximately 50 per cent and a measured zenith noise temperature of 9.5Â° K.