For the repeater system of the satellite digital multimedia broadcasting (DMB) service (2630~2655 MHz) requiring a high-gain antenna and enough isolation between antennas, a cavity-backed slot antenna array (CBSAA) fed by a single waveguide is proposed to suppress side lobe level (SLL) and to increase front-to-back ratio (FBR). The SLL and FBR are enhanced with an optimized magnitude ratio of the electric field at each slot and an additional vertical reflector. The measured SLLs in the Hand E-planes are under -33.24 and -35.78 dB, respectively. An FBR over 37.84 dB and a peak gain over 17 dBi are measured. The proposed CBSAA consists of 2 X 4 slot elements backed by a single cavity and has the characteristics of high power handling capability, high radiation efficiency, low feed line loss, and simple feed network.