Abstract: This invention relates to a microwave tube (3) for generation of an electromagnetic wave with frequency F. The tube comprises mechanical means for varying the frequency F composed of a set of rings (A, B, C, D) defining a periodic structure inside the tube and mechanical means (4, 5, 2, G) for displacing rings with respect to each other while maintaining a periodicity for periodic structure during displacement of the rings. The invention is applicable to any type of microwave tube, such as a PWT, a BWO type tube, a klystron, a magnetron, a carcinotron, a maser, etc.