COUPLED HELICES 



143 



3.00 



2.75 



2.50 



2.25 



2.00 



/3ca 



1.75 



1.50 



1.25 



i.OO 



0.75 



0.50 



0.25 



0.5 



1.5 



2.0 



2.5 



/3a 



3.0 



3.5 4.0 



4.5 



5.0 



Fig. 2.2 — Coupling pliase-constant plotted as a function of the single helix 

 phase-constant for synchronous helices for several values of b/a. These curves 

 are based on simple estimates made in Section 2.7. 



There are two opposing tendencies determining the actual physical 

 length of a coupling beat-wavelength: 



1) It tends to grow with the RF wavelength, being proportional to it 

 in the first instance; 



2) Because of the tighter coupling possible as the RF wavelength 

 increases in relation to the heli.x-to-helix distance, the coupling beat- 

 wavelength tends to shrink. 



Therefore, there is a region where these tendencies cancel each other, 

 and where one would expect to find little change of the coupling beat- 

 wavelength for a considerable change of RF freciuency. In other words, 

 the "bandwidth" over which the beat-wavelength stays nearly constant 

 can be large. 



This is a situation naturally very desirable and favorable for any 

 device in which we rely on power transfer from one helix to the other by 



