open- Wire Crosstalk * 



By A. G. CHAPMAN 



Introduction 



THE tendency of communication circuits to crosstalk from one to 

 another was greatly increased by the advent of telephone 

 repeaters and carrier current methods. Telephone repeaters multi- 

 plied circuit lengths many times, increased the power applied to the 

 wires, and at the same time made the circuits much more efficient in 

 transmitting crosstalk currents as well as the wanted currents. Carrier 

 current methods added higher ranges of frequency with consequently 

 increased crosstalk coupling. Program transmission service added to 

 the difficulties since circuits for transmitting programs to broadcasting 

 stations must accommodate frequency and volume ranges greater than 

 those required for message telephone circuits. 



As these new types of circuits were developed, their application to 

 existing open-wire lines was attended with considerable difficulty from 

 the crosstalk standpoint. Severe restrictions had to be placed on the 

 allocation of pairs of wires for different services in order to keep the 

 crosstalk within tolerable bounds. In many cases the existing lines 

 were retransposed but, nevertheless, there were still important re- 

 strictions. While great reduction in crosstalk was obtained by the 

 transposition arrangements the crosstalk reduction was finally limited 

 by unavoidable irregularities in the spacing of the transposition poles 

 and in the spacing of the wires, including differences in wire sag. 

 To further improve matters it was, therefore, necessary to alter the 

 wire configurations so as to reduce the coupling per unit length between 

 the various circuits. 



Recently this study of wire configurations has resulted in extensive 

 use of new configurations of open-wire lines in which the two wires of 

 a pair are placed eight inches apart instead of 12 inches, the horizontal 

 separation between wires of different pairs being correspondingly 

 increased. With these eight-inch pairs it has usually been found 

 desirable to discard the time-honored phantoming method of obtaining 



* This paper gives a comprehensive discussion of the fundamental principles of 

 crosstalk between open-wire circuits and their application to the transposition design 

 theory and technique which have been developed over a period of years. In this 

 issue of the Technical Journal the first half of the paper is published. In the April 

 1934 issue will be the concluding part, together with an appendix entitled "Calcula- 

 tion of Crosstalk Coefficients." 



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