The interaction of a dense (n∼1012cm−3), mildly relativistic (1.8<&ggr;<3) electron beam and a cylindrical, dielectric lined waveguide has produced tunable microwave radiation in the 150–310 GHz frequency range with an estimated power output of 500 kW at 150 GHz and 10 kW at 310 GHz. The measured output frequency agrees well with the frequency for which the phase velocity of the TM01mode of the dielectric lined guide is synchronous with the electron beam velocity.