Shielding Effectiveness of Typical Cables from 1 MHz to 1000 MHz

Abstract

Cable shields frequently have two specifica­ tions: one for EMP, and one for EMC. From 0 to 100 MHz, the total transfer impedance of a cable can be measured in a straightforward manner since the cable is electrically small over most of this range. Consequently, transfer impedance is frequently used as the measure of shielding quality of a cable for EMP shielding analysis. Beyond 100 MHz the cable length is not electrically small and it becomes dif­ ficult to reduce measured data. Consequently, some form of shielding effectiveness is often used to specify the shielding quality of cables forEMC analysis. Two alternative definitions of shielding effectiveness (the ratio of power flow out of the interior of the cable to the power flowing on the shield (SEP) and the ratio of current flowing out of the interior of the cable to that flowing on the shield (SEI)), transmission line theory, Ohm's Taw and the transfer impedance parameters (resistance and mutual inductance) were used to calculate shielding effectiveness Up to 10 GHz, The results show that, below a few MHz, the shielding effectiveness was independent of frequency and dependent on cable length. Above a few tens of MHz, when the cable becomes electrically long, the shielding effective­ ness reaches a minimum value which is directly pro­ portional to the shield's mutual inductance and becomes a maximum at the cable electrical length resonances. The high frequency limit of the shield­ ing effectiveness is independent of cable length. The two definitions of shield effectiveness differ by a factor related to the external impedance. Measured data for the shielding effectiveness (the ratio of center conductor to shield current) of several shielded twisted pairs and cable assemblies, from 1 MHz to 1000 MHz using a quadraxial test fixture, will be presented. The measured results agree reasonably well with the theory. The effect of a single aperture was also studied, and measurements show the shielding effectiveness of cables with this type of leakage is particularly poor at the UHF frequencies.