Measurement techniques establish shielding values

Features of tests defined by MIL-STD-285 for evaluation of gasket and shielding materials for effectiveness in EM applications are discussed, along with additional tests to assay the transfer impedance (TI). The military standard produces measurements of EM properties of shielded enclosures, with effectiveness expressed as the decibel ratio of the field intensity at fixed points on both sides of the barrier. The TI method is a measurement of voltage induced on one side of a gasket by a current injected on the other side. The military method is limited to a frequency range of 10 kHz-10 GHz, and an example is provided of a typical military test set-up. TI trials are confined to frequencies below 1 GHz in order to avoid cavity resonances caused by the gasket. Documentation of test results of a material's shielding effectiveness is outlined.