Analysis of finite aperture effect in the modified MIL-STD-285 shielding material´s test method

The theoretical basis for SE (shielding effectiveness) values obtained from the Modified MIL-STD-285 Shielding Material´s Test Method is investigated. The magnetic loop measurement is considered with a circular aperture on an infinitely-thin perfect conductor wall. When the aperture size is large enough to the wall-to-antenna distance, a rigorous integral formula for dipole source shielding by an infinitely-large plane shield gives the asymptotic value. When the aperture dimension is small enough, the loaded aperture´s magnetic polarizability formula quasi-static approximation) can be used, provided the shield is quite thinner than skin depth. For the intermediate region between these two limited cases, an approximate approach is evaluated which is based on the aperture integral. For more general cases such as for non-circular apertures filled with weak shielding material, the possibility of the use of the IBC (Impedance Boundary Condition) with a full-wave analysis code is discussed