The Bell System Technical Journal 



Vol. XVII April, 1938 No. 2 



Studies of Telephone Line Wire Spacing Problems 



By J. A. CARR and F. V. HASKELL 



By spacing a wire and its mate closer than has been the usual 

 custom, and thus obtaining a greater separation between pairs on 

 open wire telephone lines, inductive interference can be diminished 

 and higher carrier frequencies transmitted, thus increasing the 

 message carrying capacit3^ The hazard of the wires swinging 

 into contact in the wind is controlling in the selection of a minimum 

 spacing. This article gives the results of studies made to determine 

 tlie effect of natural winds on variously spaced and arranged wires 

 suspended in accordance with telephone practice. 



UNTIL recent years, it has been the usual practice in the Bell 

 System to use a spacing of 12 inches between adjacent wires on 

 a crossarm with the exception of the wires of the pole pair which are 

 more widely separated to provide climbing space. By reducing this 

 12-inch spacing between the wires of non-pole pairs and obtaining a 

 correspondingly greater separation of pairs, a decrease is obtained in 

 the crosstalk coupling between pairs and also in the induced voltage 

 from such external sources as radio stations and static. This permits 

 the transmission of higher carrier frequencies and provides a greater 

 number of communication channels without impairing the quality of 

 transmission. The increased number of channels that can be obtained 

 is a function, among other factors, of the magnitude of the change in 

 wire spacing. 



It was with a view to ascertaining the limitations that might be 

 imposed upon such a rearrangement of the wires through their swinging 

 into contact in winds that the study herein described was initiated by 

 Bell Telephone Laboratories. 



It is interesting to note here that, since the early results of this 

 study became available, about 75,000 pair-miles of toll line wire have 

 been installed with an 8-inch spacing between the wires of a pair and 

 recently some 2,600 pair-miles with a 6-inch spacing between the 

 wires of a pair. 



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