34 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



having many repeaters in tandem, automatic gain control (gain regu- 

 lation) in the repeaters provides an ideal method for minimizing the 

 amount of the total system margin which must be allocated to environ- 

 mental loss variations. However, this would have required adoption of 

 elements of unproved integrity which might have increased the proba- 

 bility of system failure. So the more simple and reliable alternative was 

 adopted — • fixed gain repeaters with built-in system margins. 



System Inaccessibility 



The inaccessibility of the undersea system for periodic or seasonal 

 adjustment and the decision to avoid automatic gain regulation were 

 major factors in the allocation of system margins between undersea 

 and shore locations. To avoid wasting such a valuable commodity as 

 margin, required the most careful consideration of means of trimming 

 the system during laying. Equalization for control of misalignment in 

 the undersea link is a function of the match between cable attenuation 

 and repeater gain. Generally speaking, these are fixed at the factory. 

 Very small unit deviations from gain and loss objectives could well add 

 to an impressive total in a 3,200-db system. Accordingly, it was neces- 

 sary to plan for periodic adjustment of cable length during laying, and 

 where necessary, insertion of simple mop-up undersea equalizers at the 

 adjustment points, 



DESIGN OF HIGH-FREQUENCY LINE 



Terminology 



The high-frequency line, as the term is used here, includes the cable, 

 the undersea repeaters, the undersea equalizers, and the shore-station 

 power separation filters, transmitting and line-frequency receiving 

 amplifiers, and associated equalizers. 



Repeater Spacing and System Bandwidth 



As in most transmission media, the attenuation of the cable increases 

 with increasing frequency. Hence the greater the bandwidth of the sys- 

 tem, the greater the number of repeaters needed. 



In this system, powered only from its ends, the maximum permissible 

 number of repeaters and thus the repeater spacing is determined by a 

 dc voltage limitation, as explained in a later section. 



With the repeater spacing fixed, and the type of cable fixed, the re- 

 quired repeater gain versus frequency is known to the degree of ac- 

 curacy that the cable attenuation is known. The frequency band which 



