Experimental and theoretical analysis of the magnetic field attenuation of enclosures

The surface magnetic-field attenuation (MFA) of a wide range of enclosures and panels has been measured by injecting a uniform current across the surface and measuring the ratio of the surface magnetic field to the magnetic field inside the enclosure. A simple theory has been developed to explain and extend the results. The measurements show that relatively simple structures provide on the order of 50 dB of attenuation for frequencies above 1 MHz. In well-constructed enclosures, the electromagnetic performance was limited by joint resistance. Current diffusion was not evident for frequencies above a few tens of kilohertz, except in panels with minimal metal content (metal screens and flame-sprayed copper). Aperture coupling can be predicted using polarizability theory