An impedance-transforming four-way in-phase Wilkinson power combiner matches an InGaAs power amplifier across the WR-3.4 waveguide band from 220 to 330 GHz. Optimizing the intermediate impedance between the Wilkinson transformer and the two symmetrical parallel PA signal paths achieves broadband large signal bandwidths without the need for additional lossy matching elements. The optimized intermediate impedance is determined by load-pull simulations for maximum output power at several frequencies in the bandwidth to calculate the optimum load impedances, and then solving numerically for the intermediate impedance in the matching network. The design presented achieves 40% large signal bandwidth, 4.6% PAE and 8.1 dBm OP_1db at 300 GHz.