Line Problems in the Development of the Twelve-Channel 

 Open-Wire Carrier System * 



By L. M. ILGENFRITZ, R. N. HUNTER, and A. L. WHITMAN 



The development of the type J twelve-channel carrier telephone 

 system for open-wire lines required an increase of nearly 5 to 1 in the 

 transmission frequency range of the lines. In the provision of 

 suitable line facilities a number of new problems were encountered 

 with respect to attenuation, noise and crosstalk. Methods for 

 meeting these problems and the results obtained are described. 



Introduction 



ANEW carrier telephone system for open-wire telephone lines 

 has been described recently.^ This system increases the number 

 of two-way telephone circuits which can be obtained on a single pair of 

 wires from the previous maximum of 4 to a total of 16. This has been 

 achieved by extending the frequency range from a maximum of about 

 30 kilocycles to more than 140 kilocycles. The exploitation of this new 

 range of frequencies on open wire has involved the solution of a number 

 of interesting problems, among which are these : 



(1) Not only does the attenuation of an open-wire line under ordi- 

 nary weather conditions rise substantially with frequency but ex- 

 tremely large increases in attenuation occur at the higher frequencies 

 when ice forms on the wires.^ • ^ In spite of these effects a high degree of 

 stability of transmission has been secured on all channels by the 

 provision of automatic control of repeater gain and equalization. 



(2) New crosstalk problems created by the extension of the fre- 

 quency range have been solved by the development of transposition 

 designs with numbers of transpositions not greatly in excess of those 

 employed for the lower frequency systems. Problems have also 

 arisen in controlling the crosstalk around the repeaters and in reducing 

 the effect of impedance departures between the line circuits and the 

 equipment. 



Frequency Allocations 



The type J system operates on circuits on which type C carrier 

 systems were already operating in the frequency range up to about 30 

 kilocycles. To provide enough frequency separation between the two 



* Presented April 18, 1939 before the A. I. E. E., in Houston, Texas. 

 1 Reference numbers refer to the list of references appearing at the end of the 

 article. 



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