A microwave tube including: a) an electron gun capable of producing a beam of electrons in the form of a hollow axisymmetrical cylinder in repetitive operation; and b) a body in which the beam is intended to propagate, the body ending with a collector and being provided with means for applying an axial magnetic field for guiding the beam and, upstream from the collector, with a resonant structure crossed by the beam, intended to group the electrons into a succession of packets (P). The resonant structure is formed with two coupled cavities which follow along the axis (Z) of the beam, the coupling being achieved via an inter-cavity region crossed by the beam, this structure having axial symmetry the axis of which is that of the beam and being dimensioned so that the transformation of the electrons in packets is accomplished at a frequency which is that of the π resonant mode of the resonant structure.