The Surface Transfer Impedance of Coaxial Cables

The surface transfer impedance of a coaxial cable is generally accepted as a useful measure of the screening properties of the cable. A convenient experimental method of determining surface transfer impedance is to surround a known length of the cable by a third coaxial conductor which is connected to the outer conductor of the cable through a shorting plate at one end, and through a voltage measuring instrument at the other end. The voltage measured by this instrument is then divided by the current flowing along the cable and by the length of cable under test to give the surface transfer impedance of the cable. This paper firstly gives a theoretical analysis of this experimental arrangement, showing that from a few cycles/second this technique does give the surface transfer impedance of the cable with a negligible error other than that in the measuring instruments. Secondly, a theoretical analysis for the comparison of the variations with frequency of the surface transfer impedances of copper sheathed cables and similar steel sheathed cables is given, and results obtained experimentally are shown. Finally, it is shown how this technique can be used to give the variation of the permeability of steels with both frequency and field strength, thus giving information that can be useful in calculations in electromagnetic shielding.