PROPORTIONING OF CIRCUITS FOR ATTENUATION 275 



it has already been seen, should have a value of approximately .486 

 for niininuini capacitance. It is evident that the value of the ratio 

 //ii/(^o + /'n) for minimum capacitance in Fip;. 14 should be very close 

 to .486, but in view of the concentricity of the conductors with the 

 semi-circular parts of the shield it should be slightly less than this 

 value. It has been found that to obtain minimum capacitance for 

 an oval shielded circuit the spacing ratio should be approximately 



^" # .47. (57) 



Ca + h 



It has been seen for Fig. 7 that to obtain minimum high frequency 

 attenuation the spacing ratio is shifted from the value of .486, which 

 gives minimum capacitance, to a value of about .46. For F"ig. 14, 

 however, the current density in the shield is more uniform, so that the 

 proximity effect between conductors is less completely compensated 

 by the shield currents. Hence the spacing ratio for minimum high- 

 frequency attenuation for the oval shielded circuit should be approxi- 

 mately the same as that for minimum capacitance, as given in (57) 

 above. 



There remains to be determined the second condition for minimum 

 high-frequency attenuation for an oval shielded circuit of given cross- 

 sectional area, namely, the optimum value of the diameter ratio Co/^o. 

 Comparison with Fig. 13 indicates that the optimum value of this 

 ratio should be fairly close to the optimum value of 3.6 for the coaxial 

 circuit. Comparison with Fig. 7, Co being equal to about .69c for equal 

 areas in the two cases, shows that the optimum value of the ratio 

 Co/6o should be slightly greater than 3.6. For practical purposes the 

 optimum may be taken as 



|-"=3.7. (58) 



With this ratio the size of the conductors with oval shield is, for the 

 same cross-sectional area, approximately the same as that of the 

 optimum size of conductors with circular shield. 



The capacitance of the pair with oval shield is smaller than the 

 capacitance of the pair with circular shield, because the inner conduc- 

 tors of the former are more widely separated and are farther from the 

 shield. It is very slightly larger than the capacitance of the double 

 coaxial circuit. 



The part of the resistance of the oval shielded circuit which is due 

 to the shield will be less than that for a circular shield because of the 

 more uniform current density in the shield. However, as has been 



