MEASUREilENTS IN MULTIPAIRED CABLES 665 



fields are completely terminated on the same shielding surface. The shield 

 around a cable pair resembles a Faraday "cage" of wires which is inherently 

 transparent to the magnetic field at low frequencies. Furthermore, at the 

 highest frequency investigated (about 5 mc), the magnetic shielding of a 

 cable pair by the adjacent surrounding conductors is still much less effec- 

 tive than the essentially perfect static shielding. Thus, it is concluded that 

 the L X C method should not be used for cable pairs when good accuracy 

 is desired. 



Acknowledgments 



The writer wishes to acknowledge the valued assistance of his associates 

 in this work, which was directed by Mr. O. S. Markuson. 



APPENDIX I 



The following formulas were developed by Mrs. S. P. Mead: 

 I. Formula for the mutual capacitance of a balanced shielded pair: 



0.01944 € 



^mut — /l 1 ^\ 



Cmut is in fii/mi. 



u = d/2S 



V = S/D 



d = conductor diameter 



5 = interaxial separation 



D = inside diameter of shield 



€ = dielectric constant (unity for air) 



8i2 is a complicated function of u and v. It increases for larger values 

 of u and/or smaller values of v. The term 0.1086 5i2 amounts to 

 from J% to about 7% over the ranges of u and v values plotted in 

 Fig. 3. 

 n. Formula for the capacitance to ground of a balanced shielded pair: 



_ 0.03889 € 



^' ~ , /[3 - Vl + 4^2] [1 _ y\i -I- ^u^)]\ -I- 0.4343A1 



log 



Suv" 



') 



Cg is in /zf/mi. 



e, u and v are defined in section I. 



Ai is also a compUcated function of u and v. The term 0.4343 Ai amounts 

 to from less than J% to about 3% over the ranges of u and v plotted in 

 Fig. 2. It increases for larger values of u and/or larger values of v. 



