148 THE BELL vSYSTEM TECHNICAL JOURNAL, JANUARY 1956 



Fig. 2.4.3 — Beat phase-constant plotted as a function of ^^a cot -^x . These 

 curves result from the solution of the field equations given in the appendix. For 

 hi a = 1.75. 



12.6a computed from Fig. 2.6, thus confirming the theoretical prediction 

 rather well. The slight increase in coupling length is attributable to the 

 dielectric loading of the helices which were supported in quartz tubing. 

 The dielectric tends to decrease the dispersion and hence reduce /3,. . This 

 is discussed further in the next section. 



2.10 Effect of Dielectric Material hetween Helices 



In many cases which are of interest in practice there is dielectric ma- 

 terial between the helices. In particular when coupled helices are used 

 with traveling-wave tubes, the tube envelope, which may be of glass, 

 quartz, or ceramic, all but fills the space between the two helices. 



It is therefore of interest to know whether such dielectric makes any 

 difference to the estimates at which we arrived earlier. We should not be 

 surprised to find the coupling strengthened by the presence of the di- 

 electric, because it is known that dielectrics tend to rob RF fields from 

 the surrounding space, leading to an increase in the energy flow through 

 the dielectric. On the other hand, tlio dielectric tends to bind the fields 

 closer to the conducting medium. To find a qualitative answer to this 

 question we have calculated the relative coupling phase constants for 

 two sheath helices of infinite radius separated by a distance "d" for 1) 



