COUPLED HELICES 



173 



5.0 



0.5 



2.0 2.5 3.0 



/3o a COT y/ 



5.0 



Fig. 4.1.2 — The radial propagation constants associated with the transverse 

 and longitudinal modes on coupled coaxial sheath helices given as a function of 

 ^ofl cot \}/i when h/a — 1.50. 



yt obtained from (29) and similarly the power in the longittidinal mode is 

 found by using values of yi . 



II. FINDING r 



When coaxial helices are used in a transverse field amplifier, only the 

 transverse field mode is of interest and it is important that the helix 

 parameters be adjusted such that there is no longitudinal field at some 

 radius, f, where the cylindrical electron beam will be located. This condi- 

 tion can be expressed by equating Ez to zero at r = f and from (8) 



BMyr) + B^,{yf) = 



(33) 



