The authors present a quasioptical power combining transmission amplifier for increasing the power level available from solid-state circuits. Receiving and transmitting arrays of patch antennas, input/output isolation, MESFETs, bias and matching circuitry are contained on a single substrate, making monolithic millimetre-wave integration possible. The flexibility of selecting input polarisation with respect to the output while maintaining amplifier stability is demonstrated. A 24-MESFET patch antenna amplifier array is presented.