Constructional and design difficulties of wide-band high-power pulsed travelling-wave tubes in the 10 Gc/s region

Abstract

The paper describes some of the difficulties met in the development of a pulsed travelling-wave amplifier having a maximum power in the region of 1kW and gain greater than 30dB over the frequency band 7.5-11.5Gc/s. This is a considerably higher power level than has been previously attempted in this country with a helix-type tube, and the parts of the development which are described are, in general, only those in which difficulty was met due to the particular combination of high power, high frequency and broad bandwidth.Broad bandwidth in travelling-wave tubes necessitates the use of a helical circuit. This imposes design limitations which restrict the operating voltage in order to avoid backward-wave oscillation. The maximum power that can be achieved is thus limited, even under pulsed conditions, and becomes less as the frequency increases.In a practical design of a waveguide-coupled high-power pulsed travelling-wave tube for the X-band, it was found necessary to maintain a small external diameter. This led to a large dielectric loading factor, which was reduced by a suitable choice of wire diameter and by control of the envelope wall thickness.A simple coupling was developed which gives a voltage standing-wave ratio of less than 1.5 over the range 7.5-11.0Gc/s. An attenuator which is well matched and has substantially constant loss over the same band has also been provided.In operation, the onset of backward-wave oscillation is decreased by the use of an operating voltage considerably higher than the synchronous voltage.From the final valve, pulsed output powers of 1kW are obtained at a gain of 30dB. Small-signal gains of up to 50dB have also been observed.