FLEXIBLE REPEATER DESIGN 91 



1930's. It is virtually identical to the structure of the repeaters manu- 

 factured by the Bell Telephone Laboratories for the cables laid in 1950 

 between Key West and Havana.' 



The functions of the enclosure are to protect the repeater unit from 

 the effects of water at great pressure at the ocean bottom; to provide 

 means of connecting the repeater to the cable before laying; and to be 

 slender and flexible enough to behave like cable during laying. How these 

 functions are met in the design may be more readily understood by refer- 

 ence to Fig. 17. 



The repeater unit, described earlier, is surrounded by a two-layer 

 carcass of steel rings, end to end. The rings are surrounded in turn by a 

 copper tube If inches in diameter and having a ^-inch wall. 



When a repeater is bent during laying by passing onto the cable-ship 

 drum, the steel rings separate at the outer periphery of the bend and the 

 copper tube stretches beyond its elastic limit. As the repeater leaves the 

 drum under tension the rings separate and the copper stretches on the 

 opposite side, leaving the repeater in a slightly elongated state. At the 

 ocean bottom, hydraulic pressure restores the repeater to its original 

 condition with rings abutted and the copper tube reformed. 



The system of seals in each end of the tube consists of (1) a glass-to- 

 Kovar seal adjacent to the repeater unit, (2) a rubber-to-brass seal sea- 

 ward from the glass seal, and (3) a core tube and core sleeve seal sea- 

 ward from the rubber seal. 



The glass seal, although capable of withstanding sea bottom pressures, 

 is primarily a water vapor barrier and a lead-through for electrical con- 

 nection to the repeater circuit. In service it is normally protected from 

 exposure to sea pressures by the rubber seal. 



The rubber seal, capable of withstanding sea bottom pressures, is in- 

 deed exposed to these pressures for the life of the repeater, but is not 

 exposed to sea water. It is likewise a lead-through for electrical connec- 

 tion from the cable to the glass seal. 



The core sleeve seal is an elastic barrier betw^een sea water on the out- 

 side and a fluid on the inside. This fluid, polyisobutylene, is a viscous 

 honey-like substance, chemically inert, electrically a good insulator, and 

 a moderately good water vapor barrier. It fills the long thin annular 

 space outside the cable core and inside a copper core tube and thus 

 becomes the medium of transmitting to the rubber seal the sea pressure 

 exerted on the core sleeve. It can be seen that the core sleeve seal has 

 nominally no pressure resisting function and no electrical function. 



The same fluid is also used to fill the space between the glass and rub- 

 ber seals. Voids at any point in the s^^stem of seals are potential hazards 

 to long, trouble-free life. Empty pockets, for instance, lying between the 



