TRAVELLING WAVE TUBES

An improvement in piloted amplifier or locked-oscillator travelling wave tubes of the "M" type, said improvement consisting in a high frequency device for directly modulating the beam in density at the cathode by a grid, this device, in a preferred embodiment of the invention, being constituted by coaxial waveguide of T - form, one of the branches of the T being terminated on the one hand by the cathode supported by the internal conductor of the coaxial section, and on the other hand by a grid supported by the external conductor, and the high frequency signal for modulating the beam being injected into said coaxial waveguide, and having a maximum amplitude in the neighborhood of the cathode.