Interaxial Spacing and Dielectric Constant of Pairs in 

 Multipaired Cables 



By J. T. MAUPIN 



(Manuscript Received April 24, 1951) 



A major handicap in the evaluation of different designs and manufacturing 

 processes, in respect to efficiency of space utilization inside the sheath of multi- 

 pair telephone cable, has been the lack of a simple and accurate method of meas- 

 uring the dielectric constant of such cable pairs. This paper describes a simple 

 non-destructive method of determining both the interaxial spacing between 

 conductors of a cable pair and the dielectric constant. An important by-product 

 of the work is the demonstration of the fact that e = L X C is not a valid means 

 of determining the dielectric constant of cable pairs. 



Introduction 



A cable pair consists of two individually-insulated conductors, of nomi- 

 nally 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 pos- 

 sible 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 capaci- 

 tance. 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 

 abiUty 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 con- 

 ductor 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. 



Experience with paper ribbon and paper pulp insulated cables has shown 

 that occasional wide deviations in capacitance can occur even though the 

 space-per-pair allowance is substantially constant. The aforementioned em- 



652 



