COUPLED HELICES 



155 



class contains those devices which depend on the presence of only one of 

 the two normal modes of propagation. The other class of devices depends 

 on the simultaneous presence, in roughly equal amounts, of both normal 

 modes of propagation, and is, in general, characterized by the words 

 "spatial beating." Since spatial beating implies energy surging to and 

 fro between inner and outer helix, there is no special problem in exciting 

 both modes simultaneously. Power fed exclusively to one or the other 



/bo a COT jfi, 



Fiji;. 2.10 — The relation l)et\veen the impedance in terms of the transverse 

 field between conpled helices excited in the out-of -phase mode, and the impedance 

 in terms of the longitudinal field on the axis shown as a function of /3oa cot tpi . 



