PROPORTIONING OF CIRCUITS FOR ATTENUATION 



281 



zo 



5 i/i 



D a. 



5^ 



I 2 3 4 5 6 7 8 9 10 II 12 13 14 



RATIO OF CONDUCTIVITIES (INNER TO OUTER CONDUCTOR) 



Pig ig — Variation of optimum diameter ratio of shielded quad with conductivity 



ratio. 



for the same diameter of shield, about 10 per cent higher than that of 

 a shielded pair for its optimum design. 



Conclusion 



There have been discussed a number of different types of individually 

 shielded circuits, both balanced and unbalanced, and the proportioning 

 of these circuits for minimum high-frequency attenuation has been 

 determined. The following table summarizes the optimum pro- 

 portions for the more important circuits treated above. The values 

 given are for the case where all the conductors are of the same material. 



Diameter 

 Circuit Ratio (p) 



Simple coaxial 3.59 



Double coaxial 3-59 



Shielded pair, round conductors and circular shields.. . 5.4 



Shielded pair, round conductors and oval shield 3.7 



Shielded quad 6.8 



spacing 

 Ratio (<t) 



0.46 

 0.47 

 0.49 



Of the transmission characteristics of these circuits, a property of 

 particular interest is the attenuation, since, assuming adequate 

 shielding, it is this which determines either the required repeater 

 spacing for a given transmitted frequency band or the width of fre- 

 quency band obtainable with a given repeater spacing. For each type 

 of circuit considered there has been determined the ideal proportioning 

 whereby the high-frequency attenuation of the circuit may be mini- 

 mized. In addition a variety of methods for the solution of problems 

 in optimum proportioning have been outlined. 



It is, of course, feasible by adjustment of size to obtain the same 

 high-frequency attenuation for all these different types of circuits. 

 However, the size of a structure is usually reflected in its cost. An 

 interesting picture can therefore be drawn by comparing the attenua- 



