264 



BELL SYSTEM TECHNICAL JOURNAL 



ductor, X2 that of the shield, and n the ratio of X,/X2- Expressions for 

 the high-frequency attenuation of this circuit have been given in 

 unpublished formulas developed by S. A. Schelkunoff and by Mrs. 

 S. P. Mead. The approximate formula given below is due to the latter. 



loge 



where a = hjc and v = hlh. 



1 VfT 



X -; — 7= nepers per cm. 

 4c ^f\^ 



(32) 



Fig. 7 — Shielded pair. 



The values of the diameter ratio (p) and what may be termed the 

 spacing ratio {a), which make this expression a minimum for different 

 values of the conductivity ratio n, can be determined in different ways. 

 One possible method is to find the values of h and b which satisfy the 

 equations dajdh = and dajdb = 0. The partial derivatives are, 

 however, very complicated. Accordingly a preferable alternative is to 

 substitute various pairs of values of p and a in (32) and determine, 

 graphically or otherwise, the particular pair which makes it a minimum. 

 In this wav it is found that when the conductors and shield are of the 



