On the operation of the travelling wave tube at low level

The small-signal theory of the travelling wave tube is developed on the assumption of finite distributed attenuation in the helix and beam voltages other than the “synchronous” voltage. On restricting the gain parameter z to values less than 2, it appears that a simplified analysis gives results in error, by not more than about 6 per cent. The voltage gain is then derived for the particular case of the first “zero-gain” point, when the travelling wave tube presents an infinite attenuation to a signal at the input. It is shown that, under these conditions, a measurement of beam current enables an estimate of the effective impedance of the helix to be made, a parameter that is of considerable importance in designing travelling wave tubes. Theoretical and experimental results are presented.