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Absfract-A microwave circuit suitable for use with solid-state microwave sources in L-band system applications has been developed. The use of this circuit with high-efficiency silicon avalanche diodes is discussed. This circuit incorporates coupled TEM bars with variable high-Q capacitors. The circuit is about A/8 long and an L band can be fit into a box 1 1/2 by 2 by 5/8 inches high. A computer analysis of the coupled bar structure has been performed. Thk has been used to calculate the fundamental and harmonic impedances at the device terminals for various values of circuit parameters. Measurements made with a Hewlett-Packard network analyzer are in good agreement with the calculated results. The fundamental impedance of the circuit can be varied to match a 50-ohm load to a wide range of device impedances. The harmonic impedances can be tuned independently by the addition of idler bars to obtain optimum device performance.With these circuits 30-mil silicon avalanche diodes have yielded 220 watts of peak output power with 29 percent efficiency. The devices were operated with 0.5-,M pukes and up to 1 percent duty cycle. The addition of a capacitor shunting the device, which tunes higher order harmonics, has made it possible to achieve stable jitter-free operation. Devices in this circuit have been run over the temperature range from -50°to + 125 "C with less th~a 5-MHZ variation in frequency and a 0.7-dB shift in power.