48 BELL SYSTEM TECHNICAL JOURNAL 



tions. If the helices were wound in the same direction, the "transverse" 

 and "longitudinal" modes would cease to be clearly transverse and longitu- 

 dinal should the phase velocities of the two helices by accident differ a little. 

 Further, even if the phase velocities were the same, the transverse and longi- 

 tudinal modes would have almost the same phase velocity, which in itself 

 may be undesirable. 



Field analyses of coupled helices confirm these general conclusions. 



Fig. 3.17 — Currents and voltages of concentric helices. 



3.5 About Loss in Helices 



The loss of helices is not calculated in this book. Some matters concern- 

 ing deliberately added loss will be considered, however. 



Loss is added to heUces so that the backward loss of the tube (loss for a 

 wave traveling from output to input) will be greater than the forward gain. 

 If the forward gain is greater than the backward loss, the tube may oscillate 

 if it is not terminated at each end in a good broad-band match. 



In some early tubes, loss was added by making the helix out of lossy wire, 

 such as nichrome or even iron, which is much lossier at microwave frequen- 

 cies because of its ferromagnetism. Most substances are in many cases not 

 lossy enough. Iron is very lossy, but its presence upsets magnetic focusing. 



When the helix is supported by a surrounding glass tube or by parallel 

 ceramic or glass rods, loss may be added by spraying aquadag on the in- 

 side or outside of the glass tube or on the supporting rods. This is advan- 

 tageous in that the distribution of loss with distance can be controlled. 



It is obvious that for lossy material a finite distance from the helix there 

 is a resistivity which gives maximum attenuation. A perfect conductor would 

 introduce no dissipation and neither would a perfect insulator. 



If lossy material is placed a little away from the helix, loss can be made 

 greater at lower frequencies (at which the field of the helix extends out 

 into the lossy material) than at higher frequencies (at which the fields of 



