Towards an improved definition and measurement of electromagnetic shielding effectiveness

At present, the only viable measurement of electromagnetic shielding effectiveness relates the field at the centre of an enclosure to the external 'threat' field. Although this allows the relative effects of different enclosure types to be studied this parameter is inadequate because it accounts neither for the disposition of objects within the enclosure, the spatial variation of the internal field or the absorption of energy by the contents. In redefining the shielding effectiveness, it is important that the values used be measurable in a consistent and repeatable manner, ideally using equipment which is readily available in EMC test labs. Also, parameters should be easily obtainable from simulations for theoretical work. The results generated should be of use to designers of equipment, for example, to allow them to compare the performance of different enclosures, and the effect of the disposition of contents within the enclosures. An example pair of alternative definitions involving standardised absorbing contents are: the shielding aperture and the shielding energy ratio, both of which are discussed.