Abstract: The present invention relates to a backward wave mode oscillator tube, the delay line of which is in two sections of dissimilar length both operating in the backward wave mode, aligned in the direction of propagation of the electrons of the beam, the second being substantially shorter than the first and exhibiting a structure homothetic with the first in the ratio 1/n, n being an integer, with a load arranged at the end of this second section which is adjacent the electron-gun, in which load is picked-up electromagnetic energy on the n.sup.th harmonic of the predominant component of the wave in the first section.

Abstract: The present invention relates to a backward wave mode travelling wave oscillator tube.The delay line is in two sections of dissimilar length, 40 and 42, aligned in the direction of propagation of the electron beam (the cross-hatched area); the second 42 being the shorter, receives the beam already modulated by the first, 40, operating in the backward wave mode. The load 8 is arranged at that end which is adjacent the electron-gun 1, 2, of the second delay line section, also operating in the backward wave mode.Application to the design of high-output backward wave tubes for operation in the millimetric sub-millimetric wavebands.