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Transmission Line Model for Shielding Effectiveness Estimation of a Rectangular Enclosure with Apertures

In this paper, an approach to the shielding effectiveness evaluation of a metallic enclosure with apertures is discussed. It has been previously shown that a simple analytical transmission line formulation gives good predictions of the shielding effectiveness of a metallic enclosure with apertures. In this work the model is extended to include the fundamental formulas to deal with the cases of a rectangular enclosure with numerous small apertures, and arbitrary angle of polarization. Simulation results indicate that: when the frequency is under resonant frequency, the closer to the enclosure, the more coupling energy is; resonance appears between the aperture and the enclosure, which results in low even negative shielding effectiveness in the metal shell; shielding effectiveness increases when polarization angle increases, and shielding of low frequency is better than that of high frequency; for the same areas, shielding effectiveness of a single hole is worse than that of multi-one. Introduction With the development and application of electronic technology, the electromagnetic interference problem becomes more and more serious. Shielding is one of the effective facilities used to reduce the interference of electronic equipments. Shielding effectiveness (SE) is the ratio of the field strength in the presence and absence of the enclosure for both electric and magnetic fields. SE is primarily affected by wave penetration through apertures and slots used to accommodate visibility, ventilation, or access to interior components. Several analytical and numerical techniques are suggested to estimate SE of an enclosure with apertures. Analytical methods are accurate but can just be applied to very simple geometries with some approximations. A simple analytical method based on transmission-line parameters has been introduced by Robinson et al. [1]. In this method, the rectangular enclosure and the aperture is modeled by a short-circuited rectangular waveguide and a coplanar strip transmission line, respectively. Using the voltage and the current at a point in the equivalent circuit, the electric and magnetic SE is obtained. This straightforward approach is limited to center aperture, is limited in that the incident plane wave can only have one polarization direction of travel and ignores the mutual admittance between apertures. There are numerous numerical techniques such as finite-difference time-domain [2], finite element [3], transmission-line matrix [4], and hybrid [5] methods that offer good accuracy over a broad frequency band but at the cost of large memory and CPU time. Hence, numerical techniques are severely limited in analyzing realistic enclosures that have large numbers (hundreds) of small holes. In this paper, for an enclosure with many openings, we present more accurate aperture array admittance for use in the waveguide equivalent circuit model of Robinson et al. [1]. While the enclosure is modeled as a short-circuited waveguide, the aperture array is characterized by admittance [6], [7]. Additionally, the incident wave is adapted it to any polarizing angle [8]. Theory A rectangular aperture in an empty rectangular enclosure is represented by the equivalent circuit of Robinson et al. [1], which is shown in Fig. 2. The longer side of the slot is shown normal to the E-field, which is the worst case for shielding.