282 BELL SYSTEM TECHNICAL JOURNAL 



tions, at the same high frequency, of different types of circuits having 

 the same cross-sectional area and of the same material. For structures 

 with solid wall conductors and air insulation the comparison works 

 out as shown in the table below, the attenuation of the coaxial circuit 

 being used as a standard of reference. 



Coaxial circuit 1.00 



Shielded pair, round conductors and circular shield 1.50 



Double coaxial circuit 2.00 



Shielded pair, round conductors and oval shield, approximately 1.3 



Shielded pair, circular shield with quasi-elliptical conductors, ap- 

 proximately 1 .47 



In each case the cross-sectional area is taken as that enclosed within 

 the shield. This neglects any differences in the thickness of shield 

 that may be required. 



A specific comparison of considerable interest is that between an 

 unbalanced coaxial circuit and a shielded pair, the latter being taken 

 as representative of shielded balanced circuits. The table shows that, 

 for the same attenuation, the cross-sectional area included within the 

 shield is larger for the shielded pair than for the coaxial circuit. On 

 the other hand, the use of balance in addition to shielding is advan- 

 tageous in that it reduces the amount of shielding needed. The shielded 

 pair makes possible the utilization of the entire frequency range, if 

 desired, whereas with a coa.xial circuit it is necessary to discard the lower 

 frequencies where it is uneconomical to provide adequate shielding. 



A thorough-going comparison of the relative advantages and fields 

 of application of the various types of circuits which have been dis- 

 cussed w^ould extend to great length. Clearly a large number of 

 factors enter into the choice of the configuration of shielded high- 

 frequency circuit to be used in any given instance. These factors 

 include the width of frequency band to be transmitted, the degree of 

 shielding required, the relative economy of manufacture of different 

 structures, etc. While a complete exposition of these factors has not 

 been attempted, the principles of optimum proportioning which have 

 been discussed should be helpful in selecting the best configuration to 

 meet given requirements, and the particular configuration chosen 

 should be made to conform reasonably closely to the optimum. 



Refkrkn'ces 



1. L. Espenschied and M. E. Strieby, "S\ stems for Wide-Band Transmission Over 



Coxial Lines," Elec. Engg., Vol. 53, October, 1934, p. 1371; also Bell System 

 Technical Journal, \'ol. 13, October, 1934, p. 654. 



2. A. B. Clark, "Wide Band Transmission Over Balanced Circuits," Elec. Engg., 



Vol. 54, January, 1935, pp. 27-30; also Bell System Technical Journal, \'o\. 14, 

 Januar\-, 1935, p. 1. 



3. British Patent No. 284,005, C. S. Franklin, January 17, 1928. 



