Design of a dual beam spacecraft antenna for simultaneous elliptical and circular coverages

In order to provide flexible communications service in the K-band (10.95-12.5 GHz for transmit and 14.0-14.5 GHz for receive) INTELSAT has implemented two fully steerable spot beams on the INTELSAT V and VI spacecraft. On INTELSAT V the west spot is circular and the east spot is elliptical, while on INTELSAT VI both beams are elliptical. For future applications the two elliptical coverages defined as S1 and S2 are of interest. However, in addition to these two beams a third, circular K-band coverage must be provided for particular orbital locations of the spacecraft in the Pacific Ocean region. It can be assumed that the S2 beam will then be pointed towards Japan, and the circular S2A beam will cover Southern Australia. Because the S2 beam pointing remains fixed whenever the S2A beam is active, it is possible to integrate the two beams into one antenna system. The author presents one approach to designing the S2/S2A antenna, with the design objective being to optimize the transmit performance and to avoid any blockage introduced by the additional feed needed for the S2A beam. It is shown that, while the S2 reflector is designed to produce an elliptical beam, the scanned beam generated from the S2A feed actually turns out to be almost circular and better focused than the S2 beam.<>