TRAVELING WAVE TUBE FOR 6,000-MC RADIO RELAY 1309 



the gain fluctuations will be about 0.25 db, the amplitude of the phase 

 I fluctuations will be about 0.9 degree and the periodicity of the fluctua- 

 tions will be about six mc. This effect may be eliminated by using an 

 I isolator between the TWT and the antenna to eliminate the echo signal. 

 I In addition to echo signals that occur between the TWT and the 

 1 antenna there are echoes which occur wholly within the TWT as a result 

 of a reflection of the signal from the output transducer and a second 

 reflection from some point along the helix. Thus even if a TWT is operat- 

 ing into a matched load it may have ripples in gain or phase characteris- 

 tics. These ripples may be controlled by minimizing the internal re- 

 flections. In the MI789 they are less than ±0.1 db in gain and one-half 

 degree in phase. Their periodicity is about 100 mc. 



In addition to causing transmission distortions, internal reflections 

 can seriously reduce the margin of a TWT against oscillation. Outside 

 of the frequency band of interest, the helix-to-waveguide transducer 

 may be a poor match or the TWT may even be operating into a short 

 circuit in the form of a reflection type bandpass filter. At such fre- 

 quencies, the internal reflections must not be large enough so that an 

 echo between transducer or filter and an internal reflection point will 

 see any net gain, or else the TWT will oscillate. 



With many types of helix winding equipment, variations in helix 

 pitch are periodic in nature. This causes the helix to exhibit a filter-like 

 behavior with respect to internal reflections. At frequencies at which 

 the period of the pitch variations is an integral number of half-wave 

 lengths, the resultant reflections from each individual period will add in 

 phase, thereby causing the helix to be strongly reflecting at these fre- 

 quencies. This effect can perhaps best be illustrated by considering some 

 results obtained in an early stage of the MI789 development. Fig. 17 

 shows measurements of the spacing between turns of an early helix. 

 Also shown is the return loss as a function of frequency that a signal 

 incident on the output of an operating TWT would see as a result of 

 internal reflections alone. Helix-to-waveguide transducer reflections were 

 eliminated with waveguide tuners during this experiment. The deviations 

 in helix pitch from nominal are rather large and are markedly periodic 

 in nature. The resulting internal reflections show strong peaks at fre- 

 quencies corresponding to five, six and seven half-wavelengths per 

 period of the pitch deviations. 



In the present M1789 this situation has been considerably improved 

 b}^ increased precision in helix winding and by insuring that the re- 

 maining periodicity does not produce a major reflection peak in the band. 

 Fig. 18 shows pitch measurements and internal reflections for a recently 

 constructed tube. 



