1308 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1956 



4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 



FREQUENCY IN KILOMEGACYCLES PER SECOND 



6.6 



6.8 



7.0 



PERIOD OF PITCH DEVIATIONS 

 0.450" »+« 0.445" 



■^- 



0.455" 



■-^ 





NOMINAL TPI= 40 



EACH POINT REPRESENTS ONE TURN 



DISTANCE ALONG HELIX > 



Fig. 17 — Pitch deviations and internal reflections in an early M1789 TWT. ■ 

 The ordinate of the pitch deviation curve is the difference between the measured 

 spacing between heli.v turns and the nominal value, which for this particular 

 helix was 25 mils. (The tube operated at 1,600 volts.) Each point represents a helix 

 turn. It is seen that the pitch deviations are periodic in nature, repeating about 

 every 0.450 inch. 



The internal reflections were measured by matching the TWT with beam off ' 

 at each individual frequency with a tuner to a VSWR of less than 1.01 (return 

 loss greater than 40 db). The beam was then turned on and the resulting reflection 

 taken as an approximate measure of the internal reflection. There appeared to be 

 no appreciable change in the helix-to-waveguide transducer reflection as a result 

 of turning th(! beam on. Evidence for this is the fact that when the beam was 

 turned on with the lielix voltage adjusted so that the TWT did not amplify, there 

 was little change in the reflection. 



The peaks of the internal reflection curve occur at five, six and seven half wave- 

 lengths i)er i)eriod of the helix pitch deviations, indicating that the reflections 

 from each period arc adding in phase at these frequencies. At the 5,800-mc peak 

 the return loss is positive. This indicates a reflected signal larger than the incident 

 signal. Shorting the TWT output caused the tube to oscillate at this frequency. 



