Usually the side walls of a rectangular cavity are less than one skin depth thick at the field‐modulation frequency. Eddy current creates an opposing magnetic field to the incoming modulation field, which tends to reduce the field at the center of the cavity. Moreover if the walls of the cavity are kept thin, interaction of eddy currents and the static magnetic field will create mechanical vibrations that will modulate the microwave frequency, thus generating spurious signals at the output of the amplifier. In this note a cavity forXband (in 9–10 GHz) is described in which the above problems have been overcome by corrugating the cavity walls in a precise manner. In comparison with a thin sidewall, the EMF that drives the eddy currents remains unchanged, but because the resistance increases, the eddy currents decrease.