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BELL SYSTEM TECHNICAL JOURNAL 



ground the annual variation is only about one-third of the above 

 value and the changes occur much more slowly. On a transcontinental 

 line the annual variation might total about 1500 db. Inasmuch as it 

 is desirable to hold the transmission on a long circuit constant within 

 about ± 2 db, it is obvious that the regulation problem is a serious one. 

 In a single repeater section of aerial line the variation might amount 

 to ± 2.5 db per day or ± 5 db per year. Such variations, if allowed 



Fig. 12 — Photograph of 1000-kilocycle repeater. 



to accumulate over several repeater sections, will drop the signal down 

 into the noise or raise it so as to overload the tubes. It is, therefore, 

 advisable to provide some regulation at every repeater in an aerial 

 line so as to maintain the transmission levels at approximately their 

 correct position. For underground installations the regulating mech- 

 anism may be omitted on two out of every three repeaters. 



In choosing a type of regulator system the necessity for avoiding 

 cumulative errors in the large number of repeater sections has been 

 borne in mind. In view of the wide band available, a pilot channel 

 regulator system was naturally suggested. Such a scheme employing 

 two pilot frequencies has been used experimentally to adjust the gain 

 characteristic in such a way as to maintain the desired levels through- 

 out the band. The accuracy with which this has been accomplished 

 for a single repeater section is illustrated in Fig. 13. Over the entire 

 band of frequencies and the extreme ranges in temperature which may 

 be encountered, the desired regulation is obtained within a few tenths 

 of a db. 



