We propose a method to produce radiation in the centimeter‐to‐millimeter wavelength range, using a short electron bunch traveling along the exit of a waveguide which is at the same time excited by a TM propagating electromagnetic wave. We found that an interesting mode of operation is to drive the waveguide with an excitation frequency very close to the cut off. For such excitation, the corresponding e.m. wave travels with a very large phase velocity which in turn has the effect to increae the wiggling action on the electron bunch. Our method relies also on the coherence of the radiation; that is the bunch length is taken to be considerably shorter than the radiated wavelength. (AIP)