156 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



helix will inevitably excite both modes equally. When it is desired to 

 excite one mode exclusively a more difficult problem has to be solved. 

 Therefore, in section 3.1 we shall first discuss methods of exciting one 

 mode only before going on to discuss in sections 3.2 and 3.3 devices 

 using one mode only. 



In section 3.4 we shall discuss devices depending on the simultaneous 

 presence of both modes. 



3.1 Excitation of Pure Modes 



3.1.1 Direct Excitation 



In order to set up one or the other normal mode on coupled helices, 

 voltages with specific phase and amplitudes (or corresponding currents) 



E|(f) 

 E|(o) 



10^ 

 5 



10^ 



10^ 



10' 



10 



10" 



3 A 



/ho a COT 1fi^ 



Fig. 2.11 — -The relation Ijetween the impedance in terms of the longitudinal 

 field between couj)led helices excited in the in-phase mode, and the impedance in 

 terms of the longitudinal field on the axis shown as a function of /3offl cot \pi . 



