The oscillation frequency of IMPATT cap oscillators is close to the frequency of the resonator formed by the cap and the diode capsule. In the design of the oscillator or interpretation of its behaviour, knowledge of the cap resonance frequencies and of their dependance upon the geometry and characteristics of the diode is desirable. Assuming radial wave propagation beneath the cap with either shunt or ‘cross’ connection of the diode, good agreement between the measured and estimated resonance frequencies was obtained. Including the cap representative circuit in that for the oscillator, the tuning characteristics were derived assuming a capacitive reactance for the diode. The effect of post inductance was to reduce the potential octave tuning range of Q-band oscillators to a few GHz for the practical range of cap diameters. The study of cap, post, and waveguide coupling effects leading to a more exact representation and the introduction of an active model for the diode, would extend understanding of the frequency range and power capability of the oscillator.