Improved aperture model for shielding prediction

In this paper, we combine the standard expression for the impedance of a capacitive diaphragm inside a rectangular waveguide with a recent model for the shielding effectiveness of a rectangular box with a rectangular aperture in its front face. The result is an improved, very rapid and simple intermediate level circuit model which allows the shielding effectiveness of a rectangular box with a rectangular aperture or slot in its face to be evaluated. Irradiation is by a vertically polarized plane wave falling on the front face of the box. The apertures in the improved model must occupy the entire width of the box, though may be set at any vertical height. Existing circuit models for shielding effectiveness require the slot to be placed centrally (in height and width) in the front face of the box, though they allow the slot to be of any length. However, such models are found to fail when the length of the slot corresponds to an integral number of wavelengths of free space radiation. In such cases the shielding effectiveness (SE) is erroneously predicted to be infinite. In contrast, the model presented here gives good predictions for SE even when the horizontal width of the slot is an integral number of wavelengths long.