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THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



the same velocity an overlap of one half of a beat wavelength will result 

 in a 100 per cent power transfer from one helix to the other. A signal in- 

 troduced into the outer helix at point A by means of the coaxial line will 

 be all on the inner helix at point B, nothing remaining on the outer helix. 

 At that point the outer helix can be discontinued, or cut off; since there 

 is no power there, the seemingly violent discontinuity represented by the 

 'open" end of the helix will cause no reflection of power. In practice, un- 

 fortunately, there are always imperfections to consider, and there will 

 often be some power left at the end of the coupler helix. Thus, it is de- 

 sirable to terminate the outer helix at this point non-reflectively, as, for 

 instance, by a resistive element of the right value, or by connecting to it 

 another matched coaxial line which in turn is then non-reflectively ter- 

 minated. 



It will be seen, therefore, that the coupled-helix transducer can, in 

 principle, be made into an efficient device for coupling RF energy from 

 a coaxial line to a helix contained in a dielectric envelope such as a glass 

 tube. The inner helix will be energized predominantly in one direction, 

 namely, the one away from the input connection. Conversely, energy 

 traveling initially in the inner helix will be transferred to the outer, and 

 made available as output in the respective coaxial line. Such a coupled- 

 helix transducer can be moved along the tube, if required. As long as the 

 outer helix completely overlaps the inner, operation as described above 

 should be assured. By this means a new flexibility in design, operation 

 and adjustment of traveling-wave tubes is obtained which could not be 

 achieved by any other known form of traveling-wave tube transducer. 

 Naturally, the applications of the coupled-helix transducer are not 

 restricted to TWT's only, nor to 100 per cent power transfer. To obtain 



Fig. 3.2 — A simple coupled helix transducer. 



