Abstract: A microwave oscillator having very low phase noise characteristics has a reflection oscillator circuit coupled to a right-circular cylindrical resonant cavity designed to operate in the TE.sub.011 mode. The resonator length L and inner diameter D are restricted to a range of values such that (D/L).sup.2 is less than 2.5 but greater than 1.7. To suppress the TM.sub.111 resonant modes in the resonator, two techniques are employed. A single tuning post is disposed on either resonator end plate at a point where the TM.sub.111 electric field Z-component is a maximum. The post length is adjustably fixed to maximize the output microwave power at the cavity output port. Each end plate also has a continuous gap separating each plate from the housing to suppress electric field currents of the TM.sub.111 mode flowing on the inner surface of the resonator.