154 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



F(ra) 



7.0 

 6.5 

 6.0 

 5.5 

 5.0 

 4.5 

 4.0 



3.5 



• 

 3.0 



2.5 



2.0 



1.5 



1.0 



0.5 



0.5 (.0 (.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 



/SoaCOTii', 



Fig. 2.9 — ■ Impedance parameter, F(ya), associated with both transverse and 

 longitudinal modes shown for several values of b/a. Also shown is F{ya) for a 

 single helix. 



It is also of interest to consider the impedance of the longitudinal 

 mode in terms of the longitudinal field between the two helices. The 

 factor, ^/(f)/£'/(0), relating this to the axial impedance is plotted in 

 Fig. 2.11. We see that rather high impedances can also be obtained with 

 the longitudinal field midway between helices. This, in conjunction with 

 a hollow electron beam, should provide efficient amplification. 



3. APPLICATION OF COUPLED HELICES 



When we come to describe devices which make use of coupled helices 

 we find that they fall, quite naturally, into two separate classes. One 



