A TWELVE-CHANNEL CARRIER TELEPHONE SYSTEM 131 



problem, but they also give a cable circuit of low attenuation, — ap- 

 proximately 1.2 db per mile at 140 kilocycles. 



Nevertheless, where spare pairs exist in cables, it has often been 

 found economical to use them for type J transmission. It is possible 

 to use them only non-loaded, in which case the attenuation is very 

 high — 4 to 6 db per mile, depending on the gauge, at 140 kilocycles, 

 and impedance matching transformers are, of course, required at the 

 junction of the open wire and cable. There are cases where this higher 

 attenuation may be permitted and these pairs are used by separating 

 the type J range from the lower frequency range, which is transmitted 

 through pairs equipped with the older type C carrier loading. The 

 separation is accomplished by filters which are usually housed in 

 small filter huts at the junction of the open-wire line and cable. 



In other cases it has been found economical to use the frequency 

 separation method with filters and to install new non-loaded cables of 

 lower attenuation to lead in the type J frequency band alone. Paper 

 insulated 10-gauge pairs or the disc-insulated spiral-four cable of the 

 type described above may be used for this purpose. In either case 

 transformers are used to match the cable impedance to that of the 

 open-wire line over the type J frequency range. 



The reflection requirements are so severe and the effects of even 

 short lengths of cable at the high frequencies so serious, that even short 

 lead-in cables, where the open-wire line actually extends to the re- 

 peater or terminal building, — cables which are only 100 or 200 feet 

 long, must receive special treatment. This has also been accomplished 

 by the use of the disc-insulated spiral-four cables, loaded. 



Interaction Crosstalk 



Because of the higher attenuation there will be many repeater 

 points on a long line at which the type J system will be amplified but at 

 which the other systems and wires on the line will pass through the 

 station without amplification. In this case, evep though the type J 

 pairs are properly transposed to keep down crosstalk between them- 

 selves, there still remains the crosstalk between them and the other 

 pairs on the line, not only pair-to-pair crosstalk but crosstalk from 

 the type J pair to various circuit paths consisting of irregular wire 

 combinations. 



Two difficulties arise in this case: The first is that the crosstalk from 

 the output of one J system into an irregular path may be retransferred 

 into the input of a repeater on another type J system. The second 

 is that the crosstalk from the irregular path may be returned to the 

 input of the same repeater and either influence the overall transmis- 



