An equivalent circuit for designing a coherent power combiner using a
quasi-optical resonator has been developed. In the resonator, large nu
mbers of devices (HEMT, HBT, etc.) are arrayed two dimensionally and m
ounted on a surface of a metal grooved-mirror. A newly developed equiv
alent circuit for the resonator has been constructed using a transmiss
ion-line model. Experiments performed at Ku-band have shown that oscil
lation frequencies in a 3x3 HEMT array oscillator can be predicted wit
h errors of less than 1% by using this equivalent circuit.