Simple relations between shielding effectiveness and transfer impedance/admittance for cables

Abstract

J ntroduct ionThe attenuation capability of cable shields are generally described in terms of shielding effectiveness (SE) or in terms of transfer impedance and admittance. For modern weapon system design, a broad range of electromagnetic disciplines levy a combination of both shielding effectiveness and transfer impedance specifications on the same cable hardware. Comparison of these dissimilar specifications is essential to efficient system design and test. This paper presents an overview of a technical approach to perform this comparison and presents some simple shielding relations that predict general common-mode shielding performance over a wide frequency range. Earlier reports of this multi-year effort are summarized and clarifications and extensions added (Ref. 1, 2, 3). ReciDrocityWhether collecting or radiating energy, cables are antennas which must satisfy the antenna reciprocity relation. This relation specifies that the physics of emitting or collecting energy are reciprocal and the shielding relations derived for one case applies to the other. In the following material, much of the development is performed in terms of radiated susceptibility. The resulting equations and conclusions are equally applicable to the corresponding radiated emission cases. Historical ADDroachHistorically, attempts to develop a relationship between the transfer impedance parameter and shielding effectiveness have met with limited success. Attempts to develop such a relation are often based on an incorrect definition of shielding effectiveness that involves the ratio of current or power on the outside of a shield to that on the inside. The correct EMC definition of shielding effectiveness is based on the insertion loss concept which ratios current or power at an unshielded location to the shielded current or power at the same location (Ref. 4). Any attempt to relate shielding parameters by using current or power on the outside of a shield to the current or power on the inside is based on an improper definition of shielding effectiveness and will yield results that are generally incorrect. ADDroachTo develop proper shielding relations between shielding effectiveness and the transfer impedance and admittance parameters, a study was conducted to characterize the various common-mode shielding effects for cables. This study consisted of both analysis and test activities from low kilohertz frequencies to the gigahertz regime. A major output of this study was a promising shielding analysis approach based on the concept of shielding modes. This shielding mode approach formed the basis for the development of improved equational relationships between SE and the transfer impedance and admittance parameters.A shielding mode is defined as an individual, isolated leakage path through the cable shield. For example, when a vertical electric field is incident on a cable-over-ground plane geometry current and voltage are induced on the cable shield. The current and voltage on the shield leak energy through the shield by transfer impedance and ...