Shielding Theory of Enclosures with Apertures

Abstract

Present methods for computing the shielding efficiency of metallic plates with apertures are based on the analysis of a plane wave incident on an infinite conducting sheet. When applied to actual enclosures with internal radiation sources, these methods lose all validity, and obviously fail to predict the measured results. Semi-empirical formulas are available for special cases, but no serious analytic investigation has ever been conducted. This dissertation develops the theory of electromaqnetic radiation from metallic enclosures with apertures, excited by an internal source at frequencies below the fundamental resonance of the enclosure. The enclosure with an aperture is analyzed from two different points of view: as a cavity with a small aperture in a wall; and as a waveguide section short-circuited at one end and open at the other end. Rectangular geometries are used throughout, since these are by far the most commonly encountered in practical enclosures and cabinets. Using the corresponding dyadic Green's functions, the fields generated inside the enclosure by some simple sources are determined. In addition to the case of a Hertzian dipole - the building block for more complicated sources - a center-fed dipole and a square loop antenna are analyzed. The fields radiated through small apertures in a cavity are determined using Bethe's theory of diffraction by small holes. The radiation from an open waveguide is calculated with the help of field equivalence theorems, with assumptions applicable to the case of evanescent waves. The final step is to derive expressions for the "Insertion Loss" of the shield, defined as the ratio of the field strength at a point external to the shield, before and after the insertion of the enclosure. To accomplish this, the effect of the shield upon the input impedance of the antenna is analyzed, and expressions obtained for the applicable cases. The resulting insertion loss expressions are numerically evaluated for some representative cases, and graphically compared with a series of measurements performed to obtain experimental confirmation. Very good agreement is obtained in all cases, establishing the validity of the analysis. Thus, this work provides accurate prediction capabilities for the design of shielded enclosures with apertures, in the presence of internal or external noise sources (the latter is a consequence of applying the reciprocity theorem). Hence, it constitutes a useful tool in the solution of electromagnetic interference and susceptibility problems.