PROPORTIONING OF CIRCUITS FOR ATTENUATION 267 



diameter of conductor, the ratio of the interaxial separation of the 

 conductors to the inner diameter of the shield which gives maximum 

 characteristic impedance lies between the limits 0.486 and 0.500. 

 For practical purposes a value of about 0.49 may generally be used. 



Effect of Dielectric 



The effect of dielectric for a shielded pair is similar to that for a 

 coaxial circuit. When the insulation is so disposed between conduc- 

 tors and shield that a line of dielectric flux passes through only one 

 kind of dielectric material, the second term of the attenuation formula 

 is independent of the proportioning of conductors and shield, so that 

 the optimum proportions as given in Figs. 8 and 9 are unchanged. 

 These values will also serve for most practical cases where a line of 

 dielectric fiux may pass through more than one kind of material. 



Effect of Frequency 



At frequencies where the approximate formulas no longer hold, 

 the conditions for minimum attenuation as given by Figs. 8 and 9 

 undergo some change, especially the former. As the frequency is 

 decreased the attenuation is minimized by increasing the size of con- 

 ductor for a given size of shield. In other words, the optimum 

 diameter ratio grows less. The optimum spacing ratio increases from 

 0.46 toward the value which gives minimum capacitance, i.e., ap- 

 proximately 0.49. 



Pair in Space 



It is interesting to digress for a moment to consider briefly the case 

 shown in Fig. 10 of a pair of round conductors in space. This may 



I'i.H. 10 — I*air in space. 



be regarded as a pair surrounded by a shield of infinite diameter. If 

 the conductors are of solid material, the attenuation of the circuit at 

 high frequencies is 



