PROPORTIONING OF CIRCUITS FOR ATTENUATION 



273 



In such a double coaxial circuit, used at hi^h frefjuencies, eciual and 

 opposite currents will How on the inner and outer conductors of each 

 coaxial unit. The resistance and inductance of the balanced circuit 

 will, therefore, be twice, and the capacitance and leakance one-half, 

 the corresponding values for one coaxial unit. The attenuation of the 

 balanced circuit is equal to the attenuation of one coaxial unit and may 

 be expressed by the formulas previously given, where the various 

 symbols are understood to refer to one unit of the circuit. Accordingly 

 the optimum high-frequency proportions are the same as those pre- 

 viously derived for ordinary coaxial circuits of different types. '^ 



As the frequency is reduced, the optimum proportions become dif- 

 ferent from those for coaxial units, since the circuit inductance ap- 



Fig. 13 — Double coaxial circuit. 



proximates more closely that for a simple pair of wires occupying the 

 positions of the inner conductors, while the capacitance remains equal 

 to one-half of that of one coaxial unit. As a result the optimum 

 diameter ratio is larger than at high frequencies. 



Shielded Pair — Round Conductors and Oval Shield 



The shield around a pair does not have to be cylindrical. Upon 

 consideration of a pair of round conductors with a cylindrical shield, 

 as shown in Fig. 7, it is evident that the shield approaches quite close 

 to the conductors at the sides, while it is well removed from them at 

 the top and bottom of the figure. This means that for a given area 

 enclosed by the shield the capacitance of the circuit is greater than 

 would be the case if the shield were kept at a more nearly uniform 



