206 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



of the coal tar throughout the fibers without having an excess that would 

 make the jute difficult to handle during the armoring process. 



The size, composition, and processing of the armor wires were also 

 placed under close control. Purity, tensile strength, and twist recjuire- 

 ments were designed to ensure that the wire could be applied to the 

 cable, and welded, and that it could withstand the expected tensions 

 during laying and pickup. Strength considerations made it mandatory 

 that inclusions of slag or piping of the wire be eliminated. Piping is an 

 unusual condition encountered during rolling or drawing which results 

 in a hollow shell of steel which may be filled with slag. 



CONTROL OF TRANSMISSION CHARACTERISTICS 



From a broad point of view, the attainment of a final product capable 

 of meeting the stringent transmission requirements was achieved by the 

 following basic steps. 



1. Precision control of the dimensions of the copper conductors, in- 

 cluding the diameter of the fabricated central conductor. 



2. Automatic control of the insulating process to maintain a constant 

 capacitance, thus compensating for deviations in central conductor diam- 

 eter and dielectric constant of the insulation. 



3. Factory process control, by means of a running average of the 

 measured attenuation characteristic of current production, to guide the 

 adjustment of suitable parameters when necessary. 



As indicated in the sections above on the method of manufacture and 

 the control of raw materials, precautions were taken to obtain a central 

 conductor that had predictable electrical performance, and a controlled 

 taper in overall diameter along its length. The need for such effort is ex- 

 plained by consideration of the factors that determine the attenuation of 

 a coaxial structure. 



The attenuation, a, of the cable is directly proportional to the ac re- 

 sistance, /?, and inversely proportional to the characteristic impedance, 

 Zo , as a satisfactory approximation. That is, 



a = -=- db/nm 



where "a" is a coefficient depending on the units. It is thus clear that 

 control of a may be attained by control of R and Zo . Since the resistance 

 is a function largely of the diameter of the central conductor, and since 

 it is held to close tolerances, the constancy of impedance completes the 

 requirement for attenuation control. 



