440 



BELL SYSTEM TECHNICAL JOURNAL 



Another complicating factor is, however, involved, as is the case 

 with all long wire circuits, namely, the variation encountered with 

 changes in temperature. The loss in a 10-mile repeater section varies 

 materially from summer to winter. If the cable is hung overhead, 

 this variation is about as shown in Fig. 4 and amounts to a change of 

 about ± 7 per cent in the attenuation. If the line is buried under- 

 ground at the normal depth used for telephone cables, the actual 

 variation is about one-third as much. Automatic transmission regu- 

 lators were developed to compensate for these changes. These 



z 

 o 



D 

 Z 



H 2 

 < 



200 



400 600 800 1000 1200 1400 



FREQUENCY IN KILOCYCLES PER SECOND 



1600 1800 2000 



Fig. 2 — Attenuation per mile of the New York-Philadelphia coaxial cable, 

 proportion of that attenuation due to conductance losses. 



Also the 



regulators depend, for their operation, on the transmission of a pilot 

 channel. At each point where it is desired to regulate the transmission, 

 the pilot channel is selected by a very narrow band filter and its 

 amplitude used to control an automatic device which changes the gain 

 of the repeater until the amplitude of the pilot at the output of the 

 repeater reaches a certain predetermined value. The regulators on 

 the New York-Philadelphia circuit have operated with such accuracy 

 that it has been unnecessary to make manual adjustments to take 

 care of temperature changes. 



