COUPLED HELICES 129 



181 , 2 May be considered as axial phase constant of inner and outer 

 helices, respectively 



(Sft Beat phase constant 



jSc Coupling phase constant, (identical with ^b when /3i = JS2) 



I3ce Coupling phase constant when there is dielectric material be- 



tween the helices 



/3d Difference phase constant, [ /3i — /32 [ 



(8f Axial phase constant of single helix in presence of dielectric 



^t, ( Axial phase constant of transverse and longitudinal modes, re- 

 spectively 



7 Radial phase constant 



jt, ( Radial phase constant of transverse and longitudinal modes, 

 respectively 



r Axial propagation constant 



Tt. ( Axial propagation constant for transverse and longitudinal 

 coupled-helix modes, respectively 



e Dielectric constant 



e' Relative dielectric constant, e/eq 



En Dielectric constant of free space 



X General circuit wavelength; or mean circuit wavelength, \/XiX2 



Xo Free space wavelength 



Xi, 2 Axial wavelength on inner and outer helix, respectively 



X6 Beat wavelength 



Xc Coupling wavelength (identical with Xb when (5i = /So) 



yj/ Helix pitch angle 



i/'i, 2 Pitch angle of inner and outer helix, respectively 



CO Angular frequency 



1. INTRODUCTION 



Since their first appearance, traveling-wave tubes have changed only 

 very little. In particular, if we divide the tube, somewhat arbitrarily, 

 into circuit and beam, the most widely used circuit is still the helix, and 

 the most widely used transition from the circuits outside the tube to the 

 circuit inside is from waveguide to a short stub or antenna which, in 

 turn, is attached to the helix, either directly or through a few turns of 

 increased pitch. Feedback of signal energy along the helix is prevented 

 by means of loss, either distributed along the whole helix or localized 

 somewhere near the middle. The helix is most often supported along its 

 whole length by glass or ceramic rods, which also serve to carry a con- 

 ducting coating ("aquadag"), acting as the localized loss. 



We therefore find the following circuit elements within the tube en- 

 velope, fixed and inaccessible once and for all after it has been sealed off: 



