Geometrical Effects on Shielding Effectiveness at Low Frequencies

A frequent approach to computing the magnetic shielding effectiveness of enclosures is to consider the effect of a plane wave impinging on a sheet of infinite extent. This permits an analysis based on a transmissionline characterization. However, when the wavelength is large compared to the dimensions of the enclosure, other analytical approaches provide better results. It has been shown that the current distribution on a box-like object scattering in the Rayleigh region tends to concentrate at the edges and corners of the box. This leads to concentrations of the magnetic field in the vicinity of edges and corners both inside and outside the enclosure. Since the effects of the current concentrations are localized, the magnetic shielding problem can be simplified by assuming a uniform current distribution on the exterior of the enclosure. Under this assumption the socalled "circuit approach" can be applied. The box-like enclosure is characterized as a series of shorted turns which shield a sensor within the enclosure. Based on the geometry, the mutual and leakage impedances between the source and sensor are used to compute the magnetic shielding effectiveness. This approach yields valid results for shields constructed of either wire mesh or sheet metal. It can also be extended to account for degradation due to bad bonds. A comparison of results, both transient and steady state, of the circuit approach and scattering theory show close agreement for spherical enclosures.