Measurement of the Magnetic Field Reduction Provided by Generic Equipment Cases and Enclosures

One measure of the shielding provided by enclo­ sures is the ratio of the magnetic field inside the enclosure divided by the surface magnetic field out­ side the enclosure. This ratio is defined here as the magnetic field reduction or attenuation. The magnetic field attenuation of three types of equip­ ment cases and two enclosures was measured for fre­ quencies up to 100 MHz, using several types of Trans­ verse Electromagnetic (TEM) cells. The equipment cases included a common aluminum chassis with a simple cover, a pair of deep drawn aluminum boxes attached to each other by a machined flange and a commercially-available steel box with an "RFI" gasket. The enclosures included two enclosures approximately 8' x 4' x 2' each divided into four equipment bays and a cable bay across the back. The measured magnetic field attenuation of the generic equipment cases and enclosures was always less than that predicted by plane wave shielding calculations. The measured electromagnetic performance was consis­ tent with a simple theory that considered the finite impedance of a joint as the mechanism for coupling current from the exterior to the interior of the enclosure.