High quality shielding with predictable and verifiable effectiveness

Shielding is an important technique often needed to control EMI. The shielding effectiveness is poorly defined, however, in EMC literature. Measurement procedures, such as MIL-STD-285, are useless and also conceptually false. The theories about the absorption of electromagnetic waves inside of a conducting medium and their reflection from the boundary surface are scientifically correct. They predict for 1 mm thick sheetmetal enclosures hundreds of dB attenuations, but only for perfectly closed/bonded structures, which are of little practical use. The shielding effectiveness of useful structures with gasketed removable panels, doors, cooling air and/or water ducts and wire feed-throughs, is a small fraction of what is calculated for the “ideal case”. Thus shielding efficiency is essentially determined by the quality of gasketing and the design of the various feed-throughs, not by the sheet metal walls. This paper offers an outline for a comprehensive shielding theory, which focuses on these components of primary significance. It cover the following topics are: enclosures as cavity resonators and as antennas; gasketing design and gasket evaluation; contact resistance and valid transfer impedance measurements; waveguides below their cut-off frequency; feed-through capacitors and filters; and novel, theoretically correct and unambiguous, definition of shielding effectiveness