TRAVELING WAVE TUBE FOR 6,000-MC RADIO RELAY 1307 



ij The dimensions of this transducer were determined empirically. It 



j was found that the antenna length affects mainly the conductive com- 



! ponent of the admittance referred to the plane of the helix. The length of 



the matching taper affects mainly the susceptive component, and the 



distance from helix to a shorting plunger, which closes off one end of the 



waveguide, affects both components. If for each tube, the position of the 



I waveguides along the axis of the TWT and the position of the shorting 



plunger are optimized, the VSWR of the transducers will be less than 1.1 



I (~26 db return loss) over the entire 500-mc frequency band. With these 



j positions fixed at their best average value, the VSWR will be less than 



I about 1.3 (--^IS db return loss). 



Internal Reiiections 



A problem that has required considerable effort has been that of 

 "internal reflections." By this we mean reflections of the rf signal from 

 various points along the helix as contrasted with reflections from helix- 

 to-waveguide transducers. The principal sources of internal reflections 

 are the edge of the helix attenuator and small variations in pitch along 

 the helix. In the MI789 the pitch variations are the main source of 

 difficulty. 



The type of performance degradation caused by small internal reflec- 

 tions can be illustrated by the following. Consider a signal incident on 

 the TWT output as a result of a reflection from a radio relay antenna. 

 Except for a small reflection at the transducer, energy incident on the 

 TWT output will be transferred to the helix, propagated back toward the 

 input, and for the most part be absorbed in the helix attenuator. How- 

 ever, if there are reflection points along the helix, reflected signals will be 

 returned to the output having been amplified in the process by the TWT 

 interaction. Because of this amplification, even a small reflection of the 

 backward traveling wave can result in a large reflected signal at the TWT 

 output. In the MI789, these amplified internal reflections are con- 

 siderably larger than the reflection from the output transducer. They 

 limit the overall output VSWR to about 1.4, whereas the transducer 

 alone has a VSWR of about 1.1. 



If there is a long length of waveguide between the TWT and the an- 

 tenna, the echo signal resulting from a reflection at the antenna and 

 a second reflection at the TWT will vary in phase with respect to the 

 primary signal as frequency is changed. This will cause ripples in both 

 the gain and in the phase delay of the system as functions of frequency. 

 Suppose the VSWR of the antenna is 1.2 and that of the TWT is 1.4 

 and the two are separated by 100 feet of w^aveguide. The amplitude of 



