In the mobile satellite communications system, an adaption of small and economical earth terminals, by realizing the large satellite EIRP is particular important. In order to satisfy this requirement, a large antenna and high power output transponders are necessary, simultaneously, reducing the mass of communications equipments in the satellite is required. In this paper, the relations between the total mass of communications equipments and each characteristics of transponder and antenna in the satellite are derived. As a result, the optimum transponder output power, antenna diameter and beam number which give the large satellite EIRP with minimum mass of communications equipments are shown.
