COUPLED HELICES 



165 



28 



26 



24 



22 



20 



18 



)6 



in 



_i 



LU 



m (4 

 u 



12 



10 



1.5 



2.5 3 4 



FREQUENCY IN KILOMEGACYCLES 



Fig. 3.3 — • The return loss and directivity of an experimental 100 per cent 

 coupled-helix transducer. 



wave components which have variations with angle around the helix- 

 axis, and coupling between such components on two helices wound in 

 opposite directions must be small. Finally, there are the inevitable me- 

 chanical inaccuracies and misalignments. 



Fig. 3.3 shows the results of measurements on a coupled-helix trans- 

 ducer with no termination at the far end. 



3.4.2 Coupled-Helix Attenuator 



In most TWT's the need arises for a region of heavy attenuation 

 somewhere between input and output; this serves to isolate input and 

 output, and prevents oscillations due to feedback along the circuit. Be- 

 cause of the large bandwidth over which most TWT's are inherently 

 capable of amplifying, substantial attenuation, say at least 60 db, is 



