74 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



pected effects would be encountered due to the immersion of the repeater in 

 water. 



A large scale test was needed, however, to establish the practicability of 

 the repeatered cable. This is because of the fact that during the laying opera- 

 tion the suspended length of cable trailing the ship may be as great as ten 

 nautical miles or more, and in this length there occur complex mechanical 

 phenomena which cannot be simulated in the laboratory with a great degree 

 of assurance. After preliminary trials from a barge in Long Island Sound, a 

 deep water test of the repeatered cable was made in 1948 in the Bahamas. 

 The cable ship LORD KELVIN of the Western Union Telegraph Company 

 was chartered for the purpose. Lengths of cable up to 15 n.m. were paid out 

 along with repeaters in depths of water up to 2 n.m. Several repeaters were 

 laid, measured while on the ocean bottom and then hauled back to the ship, 

 a procedure that involves much more severe treatment than a mere laying 

 operation. The repeater shown in Fig. 1 experienced this treatment. Tests 

 were also made with repeater housings containing specially designed acceler- 

 ometers to determine the shocks resulting from possible abuse during laying. 

 The results indicated that the repeaters as well as the cable could take the 

 punishment with considerable margin of safety. 



The Transmission System 



Designing the electrical circuit of the repeater was largely a matter of 

 getting the most out of the long life electron tube in the way of stability of 

 repeater gain and low modulation while obtaining as much gain as the system 

 permits. For most efficient use of tubes and to simplify the structure a uni- 

 directional repeater design was decided upon. 



The repeater employed in the Key West-Havana cables has three stages 

 with negative feed back, the circuit being as shown in Fig. 6. The gain fre- 

 quency characteristic is shown in Fig. 7. The transmission band is from 12 

 kc. to 120 kc. The insertion gain at 108 kc, the top frequency employed in 

 traffic, is 65 db. The repeater gain equalizes the loss of about 36 n.m. of cable, 

 the attenuation frequency characteristic of which is shown in Fig. 8. For com- 

 parison the characteristics of the earlier cables are also shown. 



The layout of the new repeatered cable installation is shown in Fig. 9. 

 There are two cables, one for each direction of transmission. The East, or 

 No. 5 cable, transmitting South, is 114.55 n.m. in length. The West, or No. 6 

 cable, transmits north and is 124.97 n.m. in length. Each cable has three 

 repeaters spaced approximately 36 n.m. apart. Two of the six repeaters are 

 in a depth of .9 n.m. and two in about .35 n.m. The last repeater in each 

 cable is located as close as* possible to deep water so as to strengthen the 

 signal before it enters shallow water and land sections of cable where static 



