TRAVELING WAVE TUBE FOR 6,000-MC RADIO RELAY 1311 



20. A glass cylinder and photocell arrangement is used to monitor the 

 amount of carbon deposited. In this manner the attenuation added is 

 made independent of both the aquadag mixture and the nozzle setting 

 of the spray gun. This machine has been checked alone by using it to 

 spray glass slides which are then made into attenuator vanes. Over a 

 two-year period we have found that a gi\'en light transmission through 

 the monitor slide results in the same vane attenuation within ±2 db out 

 of 40 db. 



After a helix has been sprayed, it is vacuum fired at 800°C for thirty 

 minutes and then the loss is measured. About 60 per cent of the helices 

 fall within the desired range of 65-80 db. The principal cause of the 

 differences in attenuation is believed to be variation in the condition of 

 the glaze fillets. Helices not meeting specifications are sprayed and fired 

 a second time (after cleaning off excess acpadag if necessary) . This second 

 treatment, brings the attenuation of almost all helices to within the 

 desired range. 



3.4 The Collector 



It is desirable to operate the collector at the lowest possible voltage 

 in order to minimize the dc power input to the TWT. This increases the 

 overall efficiency and simplifies the collector cooling problem. On the 



0.5 1.0 



1.5 2.0 2.5 3.0 3.5 

 DISTANCE FROM INPUT 



HELIX INPUT 



4.0 4.5 5.0 



HEL 



5.5 



t 

 X OUTPUT 



Fig. 19 — Distribution of helix attenuation. The attenuation pattern has a 

 gradually slanting edge facing the output to provide a smooth transition into the 

 loss for any signals traveling backwards toward the input. Reflections of these 

 signals must be \evy small since the reflected signals will be amplified in the 

 process of returning toward the output. Cold measurements (i.e., measurements 

 on the heli.x without electron beam) made by moving a sliding termination inside 

 the helix, indicate that the return loss from the attenuator output is better than 

 45 db, the limiting sensitivity of our measurement. The input side of the helix 

 attenuator is also tapered to minimize reflections but this taper is much sharper 

 than that on the output side because there is comparatively little gain lietween 

 input and attenuator. Cold measurements with a sliding termination showed a 

 return loss for this taper of about 40 db. (Surprisingly, even a sharp edge pro- 

 duces a reflection with a return loss of almost 30 db.) 



