The Transmission Characteristics of Open- Wire 

 Telephone Lines ^ 



By E. I. GREEN 



Values of the primary transmission constants R, L, G, and C for open-wire 

 telephone lines are presented, and the factors which affect these constants 

 in practise are discussed. Consideration is then given to the constants which 

 are of principal interest in telephone work, namely, the attenuation, the 

 characteristic impedance, the phase constant, and the velocity of propaga- 

 tion. Data regarding these characteristics are given for the frequency 

 range from to 50,000 cycles. 



NEARLY 3,000,000 miles of open wire are now furnishing toll 

 service in the Bell System, and this total is increasing at a rate of 

 more than 100,000 miles a year. Hence, the subject of the transmission 

 characteristics of open-wire circuits, in addition to being of considerable 

 natural interest, is of no little importance in many branches of tele- 

 phone work. In the design of apparatus to be associated with the 

 open-wire circuits as w^ell as in the engineering and maintenance of the 

 facilities derived from them, a knowledge of these transmission charac- 

 teristics is indispensable. 



The problem of determining the characteristics of the open-wire 

 circuits has, of course, been coexistent with the circuits them- 

 selves, and hence dates back to the beginnings of telephony. Of 

 late years, however, there has been a very decided change in the 

 nature and scope of the problem. This has resulted from many factors, 

 particularly {a) the extensive application of carrier telephone and 

 telegraph systems- and {b) the constantly increasing length of the long 

 distance circuits. The first of these factors has extended the trans- 

 mission range upward from about 3000 cycles to about 30,000 cycles, 

 and may well extend it higher in the future. The second, in combina- 

 tion with the higher standards which are now applied in long distance 

 transmission, has required greater accuracy in the data, emphasizing 

 especially the importance of time and space variations in the charac- 

 teristics. Also, recent changes in the construction of open-wire lines 

 (to be described later) have necessitated substantial additions to the 

 data. 



1 Presented at the Summer Convention of the A. I. E. E., Toronto, Ont., Canada, 

 June 23-27, 1930. 



2 See "Carrier Systems on Long Distance Telephone Lines," by H. A. Affel, C. S. 

 Demarest, and C. W. Green, A. I.E. E. Trans., Vol. 47, 1928, pp. 1360-1386. Bell 

 System Tech. Jl., July 1928, pp. 564-629. 



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