A fast circuit model description of the shielding effectiveness of a box with imperfect gaskets or apertures covered by thin resistive sheet coatings.

This paper presents an intermediate level circuit model (ILCM) for the prediction of the shielding effectiveness (SE) of a rectangular box containing one or more rectangular gaskets of known transfer impedance or constitutive parameters. The box may also possess a rectangular aperture covered by a thin resistive sheet. The ILCM takes into account multiple waveguide modes and is thus suitable for use at high frequencies and with relatively large boxes. The gaskets may be positioned anywhere in the irradiated front face of the box, and the SE at any point within the box may be found when irradiated by a plane wave. Solution times using the ILCM technique are three orders of magnitude less than those required by traditional numerical methods such as finite difference time domain (FDTD), transmission line matrix (TLM), or method of moments (MoM), even when using a relatively slow interpreted language such as MATLAB. Accuracy, however, is not significantly compromised. Comparing the circuit model with TLM over eight data sets from 4 MHz to 3 GHz resulted in an rms difference of 3.90 dB and a mean absolute difference of 2.35 dB in the predicted SE values. The ILCM accurately reproduces the detailed structure of the SE curves as a function of frequency and observation point.