A compact, easily tunable, silicon avalanche diode oscillator for pulsed operation in the L-band which has an efficiency of about 40 percent, equal to that of vacuum tube oscillators operating at these frequencies, is obtained by utilizing a coaxial line composed of three serially connected sections in which the intermediate section has a characteristic impedance significantly larger than either of the other sections. One end of the coaxial line is short circuited; the avalanche diode is coupled to the coaxial line at a point between the short circuit and the beginning of the intermediate section; a variable capacitance is connected across the intermediate section at a given point thereof; and the other end of the coaxial line is connected to the output of the oscillator. The short circuit is made movable with respect to the position of the avalanche diode and the variable capacitance is made adjustable.