PROPORTIONING OF CIRCUITS FOR ATTENUATION 279 



determining the attenuation for a given shape of conductor, there may 

 be great difficulty in finding the arrangement and number of filaments 

 which will produce an equipotential surface to coincide with the given 

 shape. 



By applying this method to a series of approximately elliptical 

 conductors previously shown to be of the shape that would be expected 

 to have lower attenuation than circular conductors, what is considered 

 a close approximation to the optimum shape of conductor for a pair 

 with circular shield has been arrived at. This is approximately an 

 ellipse whose major axis is about 5 per cent longer than its minor axis, 

 the latter being in line with the center of the shield. The high- 

 frequency attenuation of a circuit with circular shield and conductors 

 of this shape is approximately 2 per cent lower than that for the same 

 shield with round conductors. This reduction does not appear enough 

 to offset the practical difficulties involved with conductors of such 

 shape. 



Shielded Quad 



The number of conductors enclosed within a shield, instead of being 

 one, as in the coaxial, or two, as in the shielded pair, may be more. 

 By placing four conductors within a common shield, two separate 

 balanced-to-ground circuits may be obtained. If sufficiently good 

 balance can be obtained between these circuits, the total frequency 

 band which can be transmitted within a given cross-sectional area may 

 be increased. To obtain balance, the plane of the conductors of one 

 circuit needs to be at right angles to that of the other circuit and all 

 conductors should be equidistant from the axis of the shield. The 

 pairs may be twisted or spiralled about the axis of the shield. 



An arrangement of this kind is show^n in Fig. 16, where four round 

 conductors are placed within a circular shield to form a shielded quad, 

 or, as it is frequently described when the conductors are twisted, a 

 "shielded spiral four." Diagonally opposite conductors are used as 

 the sides of a circuit. 



Approximate formulas for the high-frequency attenuation of either 

 circuit of Fig. 16, when the enclosed conductors are solid, have been 

 derived in unpublished work of Mrs. S. P. Mead and S. A. Schelkunoff. 

 The optimum high-frequency proportioning of the system, assuming 

 the same conductivity for both enclosed conductors and assuming 

 gaseous dielectric, has been determined by Mrs. Mead. The results 

 are shown in Figs. 17 and 18, where the optimum diameter ratio and 

 spacing ratio are plotted as functions of the ratio of the conductivity 

 of the enclosed conductors to that of the shield. For the case of 



