COUPLED HELICES l,']! 



two helices in opposite directions in order to obtain well defined trans- 

 verse and axial wave modes which are well separated in respect to their 

 velocities of propagation. 



Pierce did not then give an estimate of the velocity separation which 

 might be attainable with practical helices, nor did anybody (as far as we 

 are aware) then know how strong a coupling one might obtain with such 

 heUces. 



It was, therefore, a considerable (and gratifying) surprise^' ^ to find 

 that concentric helices of practically realizable dimensions and separa- 

 tions are, indeed, very strongly coupled when, and these are the im- 

 portant points, 



(a) They have very nearly equal velocities of propagation when un- 

 coupled, and when 



(b) They are wound in opposite senses. 



It was found that virtually complete power transfer from outer to 

 inner helix (or vice versa) could be effected over a distance of the order 

 of one helix wavelength (normally between i^fo and 3^^o of a free-space 

 wavelength. 



It was also found that it was possible to make a transition from a co- 

 axial transmission line to a short (outer) helix and thence through the 

 glass surrounding an inner helix, which was fairly good over quite a con- 

 siderable bandwidth. Such a transition also acted as a directional coupler, 

 RF power coming from the coaxial line being transferred to the inner 

 helix predominantly in one direction. 



Thus, one of the shortcomings of the "conventional" helix traveling- 

 wave tube, namely the necessary built-in accuracy of the matching 

 parameters, was overcome by means of the new type of coupler that 

 might evolve around coupled helix-to-helix systems. 



Other constructional and functional possibilities appeared as the 

 work progressed, such as coupled-helix attenuators, various tj^pes of 

 broadband couplers, and schemes for exciting pure transverse (slow) or 

 longitudinal (fast) waves on coupled helices. 



One central fact emerged from all these considerations: by placing 

 part of the circuit outside the tube envelope with complete independence 

 from the helix terminations inside the tube, coupled helices give back to 

 the circuit designer a freedom comparable only with that obtained at 

 much lower frequencies. For example, it now appears entirely possible 

 to make one type of traveling wave tube to cover a variety of frequency 

 bands, each band requiring merely different couplers or outside helices, 

 the tube itself remaining unchanged. 



Moreover, one tube may now be made to fulfill a number of different 



