Investigation of intermodulation and crossmodulation distortion in traveling-wave tubes

Intermodulation and crossmodulation distortion are items of special interest in connection with the two-signal operation of traveling-wave tubes. The traveling-wave tube is a non-linear device which exhibits power saturation, limiting effects, and also exhibits a change in relative phase shift as a function of signal input drive. Theoretical expressions are developed for two-signal third and fifth-order intermodulation products from a power series expansion of the single-signal amplitude transfer function. Experimental data have been obtained On many existing high-efficiency medium-power and low-noise tubes. To study the effects that traveling-wave tube design parameters have on distortion phenomena, several specially-designed tubes have been constructed and tested. The experimental tubes, each a distinct version of the 2- watt PPM focused L-band high-efficiency traveling-wave tube, were constructed with one or more of the following features: high perveance, low perveance, tapered helix (both compressed and expanded), uniform severed helix, and severed helix with pitch change. Single-reversal and PPM focusing methods are compared directly on the same tube.