Experiments have been carried out to investigate the processes leading to the generation of intense microwave signals, resulting from the nonadiabatic motion of an electron beam along a rippled magnetic field. The observations made are consistent with an instability occurring when phase‐matching conditions for a cyclotron wave on the beam and a waveguide mode are satisfied. The injection of the beam through the rippled field then serves the primary purpose of converting some of the electron motion from the parallel to the perpendicular sense. The microwave signal is maximized when this conversion is most efficient. Measurements are reported of diamagnetism, beam drift currents, and the microwave power and frequency spectrum. The results presented are interpreted as far as possible using existing theories.