Measurements are reported on low-Q open resonators at 85 GHz with a large centered coupling hole in each mirror. This is the first study of open resonators with large diffraction losses and efficient input coupling at millimeter wavelengths. The input resonator for a quasi-optical gyroklystron is described with quality factor Q=2000 and 13 round-trip losses where approximately 65 of the incident power in the waveguide is coupled to the fundamental TEM_(00) mode. Two models have been developed to calculate the coupling from the rectangular waveguide through a circular aperture into the open resonator, Both models properly account for the increased diffraction loss of the resonator when coupling apertures are present. Measured results are in reasonably good agreement with values calculated using both scalar diffraction theory and an equivalent-circuit model.
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