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The frequency stability of coaxial-cavity oscillators in the 300-1200-mc range can be greatly improved by the addition of a small capacitor in series with the frequency-controlling device. The series reactance thus introduced magnifies the effective capacitance external to the vacuum tube by a factor which is dependent upon the electrical length of the cavity. In theory the stabilization factor can be very high, but practical limitations due to tank-circuit losses, restrictions on reasonable values of cavity characteristic impedance, and practical minimum values of output power restrict the improvement of frequency stability over the nonstabilized oscillator to an order of ten to twenty. In the 600-mc region a preferred form of oscillator employs a tube type 6AF4 operating with an anode voltage of 50-60 volts. In the associated cavity the ratio of diameters of the outer and inner conductors is approximately 2:1 and the characteristic impedance is about 40 ohms. This oscillator, in its better range, exhibits a mean frequency stability of 0.3 cycles/mc/volt, thereby comparing favorably with overtone crystal oscillators in their upper frequency range. The oscillator produces about 80 mw of output power with a plate efficiency of slightly less than 10 per cent. In the frequency range of 700-1000 mc a preferred form is composed of two cavities placed end to end and employs a pencil triode, type 5876. In this oscillator the series capacitor takes the form of an iris which is interposed between the two cavities.