This paper presents the designs of two Ka band spatial power amplifiers using MMIC circuits and multilayer technology. The design approach is based on quasioptical or spatial power combining techniques in a rectangular waveguide and according to the tray topology. An improved matching waveguide to coplanar transition is presented first with integrated bridges on the coplanar section for even mode cancellation. The transition exhibits very low insertion loss, better than 1 dB over the entire Ka band. The designed spatial amplifiers use two trays with either one MMIC amplifier on each tray (2×1) or two MMICs (2×2). The performances of the 2×1 spatial amplifier are presented showing a gain of 20 dB and a combining efficiency higher than 85% between 31 and 36 GHz. Simulation results of the 2×2 spatial amplifier are also presented showing a good combining efficiency level.