838 



hELI. SYSTR\r TFXnxrCAL JOURNAL 



with tlic lowest mode,'' tluit which is the coiuUerpart of the dominant wave* 

 in a metal pipe. If a dielectric-lihed metal guide is excited at the dominant 

 mode, and if the metal shield is abruptly terminated, the wave energy will 

 continue on in the unsheathed dielectric rod and will be confined almost 

 exclusively to the lowest hybrid mode. This is, indeed, the most common 

 way of exciting the dielectric wire. 



The extent to which the power is concentrated within the dielectric 

 is a function of the rod diameter and dielectric constant. This is shown^ 

 in Fig. 1. If the curves for the two different dielectric constants are re- 



DVe 



plotted against tlie effective diameter, 



, they become more nearly 



0.004 0.01 002 004 0.1 0.2 0.4 



1.0 2.0 4.0 



10 20 40 



Fig. 1 — Ratio of power inside Wi to jiowcr outside Wo for a cylindrical 

 dielectric wire. 



coincident. A universal curve cannot be given, however, because the field- 

 retaining effect of a dielectric-air interface increases with increasing dielec- 

 tric constant. 



The phase velocity within the rod is also a function of the diameter and 



dielectric constant, as shown^ by Fig. 2. Wlien — is very small comi)ared 



A 



with unity, the rod exerts negligible guiding action, and the transmission 

 is close to that in free s[)ace. For rods of large diameter, the power is con- 



' Unliki' all olJuT modes in a dielectric wire and all modes in a conduclinij; pil)e, the 

 lowest dielectric wire mode llieoreticall>- has its cutoff at zero fre(|uency. ("f. SchelkunofI, 

 loc. cil., p. 428. 



■•That is, the TEn mode in circular i)ipc or the TKio mode in rectangular \n\ic. 



^ Figs. 1 and 2 are based on calculations by Dr. Marion C. Gray. 



