Low-frequency shielding effectiveness of inhomogeneous enclosures

This paper presents a two dimensional analytical solution for electromagnetic shielding problems at frequencies below resonances. The application of the method to three dimensional shields yields good approximations, except for errors nearby edges which are not parallel to the magnetic field. The solution is valid for enclosures of arbitrary shape with any given combination of materials and wall thicknesses. Considering skin effects, the shielding effectiveness is derived by solving the Helmholtz equation for individual parts of the wall with different thicknesses and materials. The solution for the total shield is then found by applying Faraday´s law in integral form. Three cases are discussed: heterogeneous enclosures consisting of several wall sections with different materials and wall thicknesses, coated, laminated and nested shields and multi-cavity enclosures. Different examples of combinations of non-magnetic and magnetic materials are discussed in detail. Finally, the potential of saving material and weight by using multiple shields is investigated