Electromagnetic Shielding

Abstract

The most common method of protecting a system from undesirable electromagnetic interferences is by placing it inside a shielded enclosure of highly conducting walls. In practice, an enclosure is never made perfect in the sense that it has cracks and seams to allow for energy penetration through apertures, and the conductivity of its walls, albeit high, is not infinite, thus permitting low-frequency magnetic field penetration through diffusion. The diffusive penetration mechanism is particularly effective at low frequencies where the skin depth of the enclosure’s wall is greater than the wall’s thickness. At these frequencies the familiar attenuation and scattering losses of the wall are negligible and the only means to shield against the magnetic field penetration is by way of the induced currents in the shield. Hence the geometrical shape of the enclosure becomes most critical, since it determines the induced current distribution.