Measured electromagnetic shielding of small composite enclosures/junction boxes

The electromagnetic shielding performance of three shielded composite enclosures was characterized by measuring the surface transfer impedance, with and without covers and calculating their shielding effectiveness according to a specified definition. Measurements with covers showed that apertures were not apparent. Below 100 MHz, the transfer impedance was relatively frequency independent-175 m/spl Omega/ for polyetherimide with nickel coated graphite fibers, 10 m/spl Omega/ for polyetherimide with glass fibers plated with copper and nickel on the interior and exterior surfaces and 22 m/spl Omega/ for a similar enclosure plated on the interior only. The effect of the inter-surface inductance was apparent in the enclosure with two metallic surfaces. After the covers were removed, the high frequency transfer impedance was proportional to frequency up to 100 MHz. Above 100 MHz inductance of the sense wire caused the loop to be self-integrating.