Interaxial Spacing and Dielectric Constant of Pairs in Multipaired Cables

A cable pair consists of two individually-insulated conductors, of nominally equal circular cross-section, which have been twisted together in a long helix and stranded into a cable core with similar pairs. It has not been possible to analyze rigorously the electrical characteristics of such a circuit in terms of its rather complex physical configuration. For this reason, methods largely of an empirical nature have been used in the past to correlate physical and electrical characteristics of multipaired cables. The capacitance of any system of conductors immersed in a homogeneous medium is directly proportional to the dielectric constant of the medium. The dielectric of a cable pair is not homogeneous, but it can be described in terms of a homogeneous dielectric which would produce the same capacitance. In addition to the dielectric properties of the insulating medium, the capacitance of a cable pair is determined by the disposition of the paired conductors with respect to each other and with respect to the surrounding pairs or sheath. In particular, the interaxial separation between the wires of a pair has a critical effect on capacitance. The interaxial separation is determined by the ability of the insulation to resist deformation due to compressive forces encountered in cabling operations. Thus, the capacitance of a cable pair is largely dependent on the mechanical and dielectric properties of the conductor insulation, and a criterion of the relative efficiency of an insulation is the capacitance level, for a particular conductor gauge, resulting from a given cable space allowance or space-per-pair.