Measurement of EMI shielding of plastic composites using a dual chamber facility

Abstract

The use of plastic materials as housings for electronic equipment depends on the ability to change the plastic composition from an insulator into an electrical conductor. Because there is a wide latitude in the level of conductivity that is achievable in such a composition, it is important to be able to relate the level of electromagnetic interference (EMI) shielding attained to the bulk resistivity of the conductive plastic composition. This report describes the use of a dual chamber method for measuring the EMI shielding effectiveness of polymer-based compositions. The system described is capable of measuring the shielding effectiveness of filled composites, coated samples, flat and molded structures and gaskets, and the effect of void spaces and joints. The dynamic range of this system is 80 dB, which is sufficient for most plastic compositions. Finally, a correlation between shielding effectiveness and bulk resistivity for filled compositions is presented, based on plane wave analysis. Theoretical predictions and experimental results are shown to be in excellent agreement.