Measured Transfer Impedance of Braid and Convoluted Shields

Abstract

The surface transfer impedance of 36 samples of cable shields were measured from 1 kHz to 100 MHz. Measurements were made in 1 and 6 meter long quad­ raxial test fixtures using computer controlled net­ work analyzers. The apparatus was calibrated using shields made from copper pipes, having a known resis­ tance and leakage mutual inductance. These calibra­ tion shields also allowed the noise floor of the system to be measured. The cable shield samples included tubular and machine braids, magnetic and non-magnetic convolutes, tape wound aluminum and stainless steel conduits, metal tapes and combina­ tions. All samples used a 3/4" diameter core made up of 19 shielded twisted pairs. The convolute and conduit samples were 1-1/4" in diameter.