Investigation on shielding effectiveness of reverberant enclosures with contents

Shielding is a method to protect electronic devices from electromagnetic interference and the protection is usually provided by using an enclosure. This ability of protection is quantified by shielding effectiveness (SE). Most of methods to predict SE are about empty enclosures. It has been shown that the presence of contents affects the SE and hence they should be considered. The power balance (PWB) method is widely used for analysing shielding problems of populated enclosures. By using this method, the contents are represented by their absorption cross sections (ACS). Previous ACS measurements were performed when the contents were unpowered. This might be a problem if the contents contain active devices. In order to investigate the influence of power states on ACS, we measured the ACS of some computer components when they were powered on and off. Comparison suggests that their ACS barely change in different power states. Therefore, it can be concluded that power state has little effect on ACS measurements. The PWB method assumes that in the steady state, the internal field of an enclosure is uniform, which is not true when the internal loss is high. In order to overcome this limitation, a 2D diffusion model has been proposed. It stems from acoustics and is a generalization of the PWB method. In this work, the 2D model has been expanded to a 3D one. The 3D diffusion model was verified by using it to predict the power density in a populated enclosure and compare the predictions with those obtained by full wave simulation and measurement. The result indicates that when the loss in an enclosure is high, the internal field is not uniform and the 3D diffusion model is able to describe the variation of field.