Predicted shielding effectiveness of apertures in large enclosures as measured by MIL-STD-285 and other methods

Polarizability theory was applied to the problem of predicting the shielding effectiveness of a conductive shield with a well-defined aperture. Shielding effectiveness was defined by both MIL-STD-285 (ratio of the fields at a prescribed point with and without the shield in place) and the ratio of the magnetic field outside and inside of a shield. Relatively simple relationships were derived when the aperture was electrically small and the shield behaved like a plane conducting sheet. These relationships were consistent with measured values, and allow calibration apertures to be used to establish the credibility of shielding effectiveness measurements. It is noted that shielding effectiveness and surface magnetic-field attenuation are both strong functions of the distance between the measurement location and the shield. Thus, both are not intrinsic electromagnetic parameters