EMI shielding measurements of conductive polymer blends

Shielding efficiencies for a number of blends containing a variety of conductive fillers, including intrinsically conductive polymers, have been measured in the near-field with a dual-chamber box and in the far-field with a transmission line fixture. Although all samples tested satisfied the classical good conductor approximation, most of them exhibited a cross-over from being electrically thin (thickness<skin depth) to being electrically thick (thickness>skin depth) over the frequency range of interest, 1 MHz to 3 GHz. The theoretical relations for both near-field and far-field shielding which are prevalent in the literature do not accurately describe this region of cross-over. Expressions are defined that describe the behavior accurately over the entire range of interest. Far-field shielding efficiencies as high as 70 dB at 1 GHz were measured for purely organic composites consisting of an intrinsically conductive polymer dispersed in a thermoplastic matrix