High-efficiency three-way Ka-band waveguide unequal power combiner

This article presents the design, simulation and characterisation of a novel high-efficiency Ka-band (32.05 +/- 0.25 GHz) rectangular waveguide three-way serial combiner for monolithic microwave integrated circuit power amplifiers (PAs) with unequal output power. The combiner presented here is suited for low data rate communications from deep space using a single carrier with constant envelope (amplitude) type modulation. The three-way combiner consists internally of two branch-line hybrids connected in series by a short length of waveguide. The two branch-line hybrids were each designed to combine two input signals that are equal in phase and with an amplitude ratio of two, although in general the amplitude ratio could be arbitrary. The combiner was fabricated in an E-plane split-block arrangement and precision machined from blocks of aluminium with standard WR-28 waveguide ports. S-parameter measurements demonstrated a close achievement of the design goals. The measured return loss at the output port was greater than 16 dB and the isolation between the three input ports of the three-way combiner were greater than 22 dB. The combining of three PAs was successfully demonstrated in which the measured combining efficiency was greater than 90% at the centre frequency of 32.05 GHz.