EMI gasket Shielding Effectiveness evaluation method using transmission theory

The electromagnetic shielding effectiveness (SE) of EMI gaskets are typically measured using either an aperture attenuation method or a current injection method. The aperture attenuation method measures the radiated field that couple through the seam of an enclosure wall. The ratio of the field intensity levels measured with and without a gasket installed on a seam determines the SE. Since the impedance of the impinging field could not be accurately determined, this method suffers from poor repeatability. The current injection method has better repeatability but limited measurement frequency range. This paper applies transmission theory in predicting the SE of gaskets and an estimation of the measurement errors. The method described in this paper is more accurate and repeatable and the results can be interpolated to lower frequencies below the lowest frequency of measurement. A GTEM was used for the test field generation and a semi-anechoic chamber was used for the radiated field measurements. The SE of Copper-Nickel foam gaskets and Beryllium Copper finger-stock gaskets as used between seams of card-cage circuit-pack faceplates are computed with resulting data presented and discussed.