Cold tests and modeling of low-Q quasi-optical resonators

Measurement are reported on low-Q open resonators at 85 GHz with a large ce ntered coupling hole in each mirror. This is the rst study of open resonato rs with large diffraction losses and efficient input coupling at millimeter wavelengths, The input resonator for a quasi-optical gyroklystron is descr ibed with quality factor Q = 2000 and 13% round-trip losses where approxima tely 65% of the incident power in the waveguide is coupled to the fundament al TEM,, mode. Two models have been developed to calculate the coupling fro m the rectangular waveguide through a circular aperture into the open reson ator, Both models properly account for the increased diffraction loss of th e resonator when coupling apertures are present. Measured results are in re asonably good agreement with values calculated using both scalar diffractio n theory and an equivalent-circuit model.