2 BELL SYSTEM TECHNICAL JOURNAL 



munication circuits, the increase in the strength of interfering fields and 

 the development of highly efftcient amplifiers has led to constantly more 

 exacting requirements in electrical balance of telephone circuits. 



The development of multichannel systems by carrier methods em- 

 ploying constantly increasing frequency ranges has placed particularly 

 exacting requirements on such electrical balance. In a recent paper, 

 "Carrier in Cable," ^ are described the balancing methods which have 

 been developed to permit the use of an increased frequency range in 

 such cables. The recently published paper, "Systems for Wide Band 

 Transmission Over Coaxial Lines''^ points out the possibilities and 

 possible requirements for very much wider frequency ranges. In that 

 paper, coaxial lines are proposed which are particularly interesting in 

 that they abandon electrical balance altogether and depend entirely on 

 metallic shielding. 



For such wide frequency range transmission, very interesting and 

 important questions are raised, first as to the extent to which such wide 

 bands can be placed on existing types of structure which are based on 

 balance and, second, as to whether new construction designed par- 

 ticularly for such wide bands should depend on balance or shielding 

 alone or a combination of the two. It is the purpose of this present 

 paper to discuses these questions. 



As noted in the Espenschied-Strieby paper, the apparatus described 

 for broad-band transmission on concentric structures would also serve 

 for other types of line structures. There would, of course, be problems 

 in either balancing the apparatus or isolating its unbalances from the 

 line structure. For the purposes of the present paper it is assumed 

 that there will be no important reaction from the apparatus stand- 

 point on this consideration of line balance and shielding. 



Existing Cables 



The attenuation of pairs in existing cables has a characteristic with 

 respect to frequency generally similar to that of coaxial conductors but 

 is, naturally, considerably higher because of the smaller physical di- 

 mensions and higher dielectric losses of the cable pair. For example, 

 at a million cycles an ordinary 19-guage cable pair has an attenuation of 

 about 18 db per mile, an ordinary 16-gauge pair about 14 db per mile, 

 while a small-sized coaxial structure has an attenuation of about 6 db 

 per mile. This means more repeaters for the cable pairs, three times as 

 many for 19-gauge and a little more than twice as many for 16-gauge. 

 Also, it means more difficulty in maintaining stability of trimsmission, 



••'A. W. Clark and H. W. Kendall, Elcc. Engg., July, 1933; Bell Sys. Tech. Jour., 

 July, 1033. ss.J .. 



