Abstract: A variable pitch delay line for a travelling wave tube, is constituted by cells constructed in waveguide sections in which takes place the propagation of the electromagnetic waves which are used in the operation of the tube. All the cells are constituted by the same components, namely tops or covers, a ring, supporting rods for the ring and a short-circuit. The variable pitch is obtained by expansion or contraction of the dimensions within the guide cross-section. A significant improvement of the tube efficiency is obtained by the use of lines formed from three sections with different pitches, which succeed one another along the path of the beam, whereof the second section has a smaller pitch and the third section a larger pitch than the first section.