CABLE DESIGN AND MANUFACTURE 193 



central conductor and core used in the Air Force cable resulted in low 

 unit attenuation and low dc resistance. These advantages resulted in the 

 adoption of this size of cable. However, the outside diameter of the core 

 and the diameter of the central conductor of the coaxial do not fulfill the 

 requirements generally described as optimum for minimum attenuation. 

 Mathematical analysis shows that there is a preferred diameter ratio 

 which results in minimum transmission loss. For the 0.620'' core diameter 

 employed in the Air Force cable, the central conductor diameter chosen 

 was smaller than the ideal central conductor required to satisfy the pre- 

 ferred diameter ratio. The diameter chosen retained the central conductor 

 size which the Key West-Havana and Air Force cables proved to be 

 satisfactory from a manufacturing standpoint. The choice was also com- 

 patible with the dc resistance requirement for transmission of power 

 over the cable to each of the repeaters. 



While production of the cable was proceeding, cable manufactured to 

 the transatlantic specification was tested near Gibraltar in March, 1955. 

 These tests provided a final evaluation of the mechanical and electrical 

 characteristics of this cable before the actual laying of the transatlantic 

 link. 



DETAILS OF STRUCTURAL DESIGN OF CABLE 



The structural features of the coaxial and of types A, B and D armor 

 are summarized in Fig. 2. 



A composite central conductor was chosen to provide a conductive 

 bridge across a possible break in any one of its elements, due to a hidden 

 defect, such as an inclusion of foreign material in the copper. The dimen- 

 sions of the components of the central conductor were preciselj^ con- 

 trolled, and a light draw through a precision die was used to compact 

 and size the assembly. 



Use of high molecular weight polyethylene (grade 0.3) for core in- 

 sulation is a major departure from the materials used in early submarine 

 telephone and telegraph cables. The development of synthetic polymers 

 such as polyethylene had led to the replacement of gutta percha as cable 

 insulation, since polyethylene possesses better dielectric properties and 

 mechanical characteristics and is lighter in weight. 



Ordinary low molecular weight polyethylene is subject to environ- 

 mental cracking, especially in the presence of soaps, detergents and cer- 

 tain oils. High molecular weight material is much less subject to crack- 

 ing, and by adding 5 per cent butyl rubber, further improvement in 

 crack resistance is obtained. 



Six copper tapes applied helically over the core comprised the return 



