Measurements on screened twin cable with an earthed measuring instrument

Various methods of connecting a screened twin cable and an earthed measuring instrument are examined. It is shown that the cable characteristics can be most easily and accurately measured when the connections are such as to produce a nearly symmetrical potential distribution in the cable. In this condition, the impedance measured depends almost entirely upon the constants of the twin system and is only slightly influenced by the earthed system formed by the inner conductors and the screen. Capacitance, attenuation and phase unbalance are discussed. It is shown that a method in which connections are made to both ends of the cable is particularly useful with unbalanced cables, for it automatically takes the average of the two readings which would otherwise be required.