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A quasi-optical power combiner for a 4×4 IMPATT oscillator array has been designed and experimentally investigated. The combiner consists of a bi-periodic dielectric phase grating which transforms the near field of a rectangular horn array into a pseudoplane wave. The horn array is excited by oscillators which operate uniformly in both amplitude and phase. A parabolic mirror with a superimposed surface relief couples the pseudoplane wave into a rectangular output horn antenna. In principle, the combiner has no restriction in inter-element spacing and is hence scalable up to submillimeter wavelengths without degradation of power combining efficiency. The quasi-optical design has been verified by scalar field measurements in several planes. The oscillator matrix is injection-locked by a master oscillator from the output port. A continuous wave output power of 1.3 W with an overall power combining efficiency of 70% has been measured at 65 GHz.