Scaling Laws for Large Shields in Quasi-Stationary Magnetic Fields

P r o t e c t i o n of electronic e q u i p m e n t against electromagnetic interference is o f t e n achieved by providing a metallic enclosure. L a r g e electronic complexes, such as r a d a r installations a n d d a t a processing centers m a y be protected by covering the entire building with a m e t a l lic shield. (Some penetrations into this enclosure are usually required for t h e purpose of air-vents, cable-inlets, access t u n n e l s etc.) T h e p e r f o r m a n c e of t h e enclosure is measured by t h e shielding effectiveness, which is the ratio of the field strength at an exterior location where t h e field is u n d i s t u r b e d by t h e shield to the field strength at a point inside t h e enclosure. A first a p p r o x i m a t i o n of the shielding effectiveness can be obtained a n a l y t i c a l l y . 1 In this case, (i) c o n s t a n t p e r m e a b i l i t y is assumed, and (ii) t h e a c t u a l shape of t h e enclosure is replaced by a geometrically simpler shape, such as an infinite cylindrical shell or a spherical shell. 2323