The median frequency in the emission from a microwave generator (14), which operates in a pulsed manner, is shifted when the generator (14) operates between passive reflectors (23) whose axes are parallel thereto. In the axial direction, the reflectors (23) are approximately the same length (L) as the generator (14), with length matching through telescopic rods being preferable. However, the frequency is governed primarily by the distance (D) between the reflectors (23-23) between which the generator (14) is operated. Once desired resonance conditions have been found, for example for injection of microwave radiation into the mechanical structure of an external arrangement, this frequency relationship can be fixed by the magnitude of the length of the preferably telescopic coupling piece (16) between the resonator (13) and the antenna (18).