78 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



ground cables have the same coaxial circuit as the submarine cables but in 

 place of the mechanical protection of jute and armor they are provided with 

 electrical protection of helical steel tapes, layers of paper and over all a lead 

 sheath. 



The 12 kc. to 108 kc. passband yields 24 channels in each cable, each chan- 

 nel occupying a band of 4 kc. The signal-to-noise ratio for these channels is 

 about the same as for the same length of high grade carrier frequency circuit 

 on land. 



The Cable 



The cable has a copper return, as in the case of the earlier installations, 

 but differs from them in being insulated with polyethylene. It also involves 

 some new principles of design that render the cable circuit less subject to 

 change of electrical characteristics due to laying stresses. This is a matter of 

 considerable importance in the case of a system with submerged repeaters, 

 since after the cable has reached the bottom it is impossible to adjust the 

 repeater to compensate for changes in cable attenuation during laying, a mat- 

 ter that in ordinary cables is taken care of by adjusting the equipment on 

 shore. 



In order to avoid undesirable irregularities in transmission characteristics 

 special precautions were taken during manufacture to obtain a higher than 

 usual degree of uniformity of the cable impedance as seen by a repeater. 

 Because of the wide transmission band, schemes heretofore employed for re- 

 ducing the effect of the variation of impedance among the core lengths con- 

 stituting the cable would have called for core lengths so short as to seriously 

 increase the number of joints. The irregularities were therefore minimized by 

 careful control of conductor and insulation diameters and by continuously 

 insulating lengths of the order of 12 n.m., cutting them only as was necessary 

 for handling, and reassembling the shorter lengths as far as possible in insulat- 

 ing order to assure random addition of reflections due to impedance irregu- 

 larities. The success of this technique is evidenced by the impedance devi- 

 ation curves shown in Fig. 10. 



The structure of the cable is shown in Figs. 11 and 12. The central conduc- 

 tor consists of a solid wire .131 inch in diameter on which are laid three copper 

 tape surrounds each .0145 inch thick and .148 inch wide, closely conforming 

 to the solid wire. The interstices of the conductor are filled with polyethylene. 

 The stranded conductor, .160 inch in diameter, is insulated with polyethylene 

 to a diameter of .460 inch. Directly on the polyethylene insulation is laid the 

 return conductor com[)rising six copper tapes, each approximately .016 inch 

 thick by .241 inch wide, preshaped so that when in place they conform to the 

 surface of the insulation. Both the return tapes and the tape surrounds of the 

 central conductor have left hand lay. Over the return conductor is wound a 

 teredo tape approximately .003 inch thick with overlap. Over all is the 



