The design and performance of a circularly polarized feed system for an 11 m diameter earth-station antenna, which operates in the frequency band 7.25 to 8.4 GHz, is described. A dominant factor in the design of the feed system is a requirement of 40 dB isolation at the centre of the band. To achieve this high isolation the feed system has several distinctive features, including low-reflection waveguide components and a matched radome on the feed horn. As well as meeting the isolation requirement, the feed system that is now installed in the earth station has an insertion loss 0.15 dB, an axial ratio 0.5 dB and a VSWR 1.3.
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