Multi-conductor transmission line modelling of transfer impedance measurement methods

Abstract

Electromagnetic shielding is of high importance to avoid coupling between cables or pick-up of external fields. The use of metal braids around cables improves its protection. Shielding effectiveness of these braids is characterized by the transfer impedance of infinitesimally short braids. In practice, the transfer impedance is measured along a braid of finite length by either a line injection or a triaxial method. Analytical models for the calculation of transfer impedance do not include the finite length and terminations of the measurement setup. Mismatches in the terminations cause resonances starting from frequencies where the length of the transmission line becomes significant in terms of wavelengths. This paper presents a multi-conductor transmission line model for line injection and triaxial methods, which takes into account the finite length and the terminations of the measurement setup. By the application of this model a clear difference is observed between resonance effects caused by mismatches of terminations and by differences in propagation speeds inside and outside of the coax under test.