We describe a microwave source that utilizes a plasma to bunch the electron beam. The electron beam interacts with the plasma via the wakefield produced by the head of the beam. The resulting plasma oscillations bunch the beam at a frequency proportional to the plasma frequency. Thus the device can be tuned by varying the plasma density. The theory of the device is described and numerical simulations of the interaction are presented. The theory and simulations were validated by a proof-of-principle experiment using a simple klystron configuration. ©1997 American Institute of Physics.