An analytical model based on the Bethe's theory of diffraction by small holes is presented to predict the shielding effectiveness (SE) of metallic rectangular enclosure with electrically large aperture under plane wave illumination over a wide frequency range (0~3 GHz). In this model, the aperture is represented as electric and magnetic dipoles located at the center of the aperture, and the coupling relation between external plane wave and electromagnetic field inside the enclosure is established. The approximate solution of electromagnetic field distribution inside the enclosure is obtained in terms of the integrals of the electric and magnetic dynamic Green function. Finally, the influence of enclosure thickness on SE is calculated by introducing thickness attenuation coefficient. The model considers the effect of the thickness on the calculation results and is simple with low computation complex and high estimation accuracy. Besides, the effects of parameters like enclosure and aperture dimensions, aperture and observation point positions, incident and polarization direction of the plane wave on SE can be analyzed comprehensively based on the model. Simulation results of the proposed model are in accord with that of the TLM method, which verifies the accuracy and reliability of the model.
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