Rethinking Shielding Theory

The traditional "shielding theory" for the shielding effectiveness of enclosures is examined. It is shown that there can be significant effects which are not included within a simple barrier based theory at high frequencies. It is shown that a 1- dimensional box (using the full Maxwell's equations solution) has 1-D resonances that can significantly affect the accuracy of a barrier based shielding calculation. Treating the enclosure as a resonant cavity with perturbations on the walls is a better approach to the analysis of radiated emissions and radiated susceptibility problems at high frequencies (at and above the first resonance). This has the added advantage of having been worked out quite thoroughly for wave guides and resonant cavities.