The development of mobile satellite communications systems using LEO and MEO satellites has been watched with considerable interest. At the Advanced Space Communications Research Laboratory, a future S-band mobile satellite communications and broadcasting system using GEO satellites is being developed to facilitate use of hand-held communications devices. One of the key technologies in realizing these communications system is a high-performance satellite antenna system consisting of a large deployable reflector and a feed system. This system is capable of generating multiple beams, enabling frequency re-use and achieving high gain. The system can also provide flexible power control for traffic variations. In this paper, we describe the design of an S-band multibeam antenna feed system assumed to use a 10 m diameter reflector, and also describe numerical results.