Abstract: A modular microwave power combiner having a plurality of identical microwave diode amplifier circuits grouped in sets within the body of a modular unit which are electrically coupled in parallel to a central cavity sized to be resonant at a single frequency to minimize mismoding thereby enhancing combiner efficiency. A number of waveguides are provided for coupling of the microwave diode in each amplifier circuit to the resonant cavity, and the waveguides are situated on one side of a PC board at the top of each modular unit. Since each of the modular units is essentially identical in physical size and shape, the overall power output of the power combiner can be increased by merely adding additional modular units. The individual modular units are sized to fit top-to-bottom such that the respective resonant cavities are spaced apart by one-half wavelength (.lambda./2) for the parallel electrical coupling of all of the resonating cavities.