A modified subreflector design and diffraction analysis of Voyager´s high gain antenna for Cassini spacecraft

The Cassini mission requires that the reflector antenna be able to operate at four different frequencies. This means that in addition to the direct-fed S-band and Cassegrain-fed X-band operations, direct-fed Ku-band and Cassegrain-fed Ka-band operations are added on the Cassini antenna. A design scenario involving relatively simple and minor modifications to the existing Voyager dual-shaped symmetric antenna configuration is discussed. It is concluded that the subreflector displacement and the concomitant required surface reshaping, based on the phase-only geometrical-optics synthesis, is a good design alternative with an overall loss of less than 0.4 dB compared to that achievable with the original-shaped reflector, but still about 0.4 dB more than a nonshaped standard paraboloid/hyperboloid Cassegrain configuration, 5 dB more than a simple hyperboloid used with the shaped reflector, and several dB more than that achieved by simply moving the original shaped subreflector to a 1-in offset position.<>