1310 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1956 



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 FREQUENCY 



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6.0 6.5 



KILOMEGACYCLES PER SECOND 



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DISTANCE ALONG HELIX (EACH POINT REPRESENTS ONE TURN) 



Fig. 18 — Pitch deviations and internal reflections in a recent M1789 TWT. 

 By precise helix winding techniques the pitch deviations have been reduced by a i 

 factor of about 10 over those occurring in early tubes. The resulting internal re- 

 flections have been improved by about 25 db although there is still a residual 

 periodicity remaining. 



For return losses greater than about 25 db, we begin to see internal reflections 

 originating from the edge of the heli.\ attenuator. At these values of return loss, 

 the measurements also begin to be in appreciable error as a result of the residual 

 transducer reflections. 



Helix Attenuator 



Attenuation is applied to the helix by spraying aquadag directly on 

 the heUx assembly and then baking it. The result is a deposit of carbon on 

 the ceramic rods and on the glaze fillets. The attenuation is held between 

 65 and 80 db and is distributed as shown in Fig. 19. Evidently most of 

 the loss is caused by a conducting bridge which is built up between 

 helix turns. This was indicated by one experiment in which we cleaned 

 the deposit off the rods of a helix by rubbing them with emery paper. 

 Only the carbon directly between helix turns then remained. This de- 

 creased the total attenuation by less than 20 per cent. Having the helix 

 glazed to the support rods is apparently necessary in order to get good 

 contact between the winding and the carbon "bridge." We have been 

 able to obtain about four times as much loss per unit length with glazed 

 hehces as with non-glazed ones. Using our method of applying attenua- 

 tion we can add in excess of 80 db/inch to a glazed helix. The ability to 

 obtain such high rates of attenuation allows us to concentrate the loss 

 along the helix thereby minimizing the TWT length. 



The machine used for spraying aquadag on the helix is shown in Fig. 



