A dual band low PIM feed system for Cassegrain applications

Multi-frequency band, multi-function satellite communications antenna systems, and in some instances, limited ground facilities, have in recent times required complex single aperture feed system designs. The normal approach involves the inherent frequency selection characteristics of the periodic structure of the corrugated horn. Interesting design problems occur in the coupling junction at which frequency selection/separation takes place, to ensure complete lossless decoupling between operational frequency bands. More importantly though, maintaining stringent low sidelobe envelope and polarization discrimination performance consistent with modern satellite communication requirements, have a dramatic effect on the nature of the design of the horn and the frequency selective coupling junctions. A number of computational design routines have been developed for these junction designs, and refined to offer compliant antenna performance. The article describes the more unusual aspects of the 1.75-1.85/2.2-2.3 GHz and 7.25-7.75/7.9-8.4 GHz feed for a 9 m Cassegrain having also to comply with a very low level passive intermodulation products (PIMPs) specification in the X-band.