130 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



1 . The helix itself, determining the beam voltage for optimum beam- 

 circuit interaction ; 



2. The helix ends and matching stubs, etc., all of which have to be 

 positioned very precisely with relation to the waveguide circuits in 

 order to obtain a reproducible match ; 



3. The loss, in the form of "aquadag" on the support rods, which 

 greatly influences the tube performance by its position and distril)ution. 



In spite of the enormous bandwidth over which the traveling-wave 

 tube is potentially capable of operating — a feature new in the field of 

 microwave amplifier tubes — it turns out that the positioning of the tube 

 in the external circuits and the necessary matching adjustments are 

 rather critical; moreover the overall bandwidths achieved are far short 

 of the obtainable maximum. 



Another fact, experimentally observed and well-founded in theory, 

 rounds off the situation: The electro-magnetic field surrounding a helix, 

 i.e., the slow wave, under normal conditions, does not radiate, and is 

 confined to the close vicinity of the helix, falling off in intensity nearly 

 exponentially with distance from the helix. A typical traveling-wave 

 tube, in which the helix is supported by ceramic rods, and the whole 

 enclosed by the glass envelope, is thus practically inaccessible as far as 

 RF fields are concerned, with the exception of the ends of the helix, 

 where provision is made for matching to the outside circuits. Placing 

 objects such as conductors, dielectrics or distributed loss close to the 

 tube is, in general, observed to have no effect whatsoever. 



In the course of an experimental investigation into the propagation of 

 space charge waves in electron beams it was desired to couple into a long 

 helix at any point chosen along its length. Because of the feebleness of 

 the RF fields outside the helix surrounded by the conventional sup- 

 ports and the envelope, this seemed a rather difficult task. Nevertheless, 

 if accomplished, such a coupling would have other and even more im- 

 portant applications; and a good deal of thought was given to the 

 problem. 



Coupled concentric helices were found to provide the solution to the 

 problem of coupling into and out of a helix at any particular point, and to 

 a number of other problems too. 



Concentric coupled helices have been considered by J. R. Pierce, 

 who has ti'cated the problem mainly with transverse fields in mind. 

 Such fields were thought to be useful in low-noise traveling-wave tube 

 devices. Pierce's analysis treats the helices as transmission lines coupled 

 uniformly over their length by means of nuitual distributed capacitance 

 and inductance. Pierce also recognized that it is necessary to wind the 



