Abstract: The thickness of the fingers of the line is modified, while the pitch of the fingers is kept constant, so that the capacitance between two successive fingers varies substantially proportionally to the product P.sub.HF .times.dI/dx, in which P.sub.HF represents the microwave power at any point x in the line and dI/dx the gradient, as a function of the position x on the line, of the current I delivered by the voltage supply creating a continuous electrical field E.sub.o between the electrodes of the tube. The values of P.sub.HF and of I are measured on the tube having a constant coupling resistance or are calculated by a computer program.