140 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



helix to one of the two conductors of a balanced transmission line 

 ("Lecher"-line) and the other hehx to the other. 



Fig. 2.1(c) shows the electric field configuration when fast and slow 

 modes are both present and equally strongly excited. We can imagine 

 the two helices being excited by a voltage source connected to the outer 



(a) FAST WAVE (longitudinal) 



(b) SLOW WAVE (transverse) 



(C) fast and slow waves combined SHOWING SPATIAL "BEAT" PHENOMENON 



Fig. 2.1 — Typical electric field distributions in coupled coaxial helices when 

 thej^ are excited in: (a) the in-phase or lonfritudinal mode, (b) the out-of-phase or 

 transverse mode, and (c) both modes equally. 



