  The usually continuous and incoherent Cerenkov radiation (CR) is converted
into coherent and narrow-band radiation from within a dielectric-line cavity
excited by a dc electron beam. We have studied the self-oscillating behavior of
a device that operates at $\sim22$ GHz by calculating explicitly the electron
self-bunching and radiation growing processes. The nonlinear gain of the device
and the radiation spectrum are also calculated. The agreement to the
experiments indicates that the developed formalism could be used in designing a
THz source based on cavity Cerenkov radiation.
