A model study is described in which a rotating capacitor is coupled to a large re‐entrant cavity. The size and capacitive loading (∼500 pF) are such as to make modulation by ferrites or broadband amplifiers difficult and expensive. A nominal frequency range of 3 to 1 is achieved with a single‐plate six‐bladed rotor. On a possible fullscale cavity with a range of approximately 9 to 3 Mc, the data are scaled to a voltage of 5 kV peak for rf power requirements of 1 to 4.4 kW.