New figures of merit for the characterization of the performance of shielding enclosures

Evaluation and measurement of shielding performance of enclosures and protection structures in general are based on the comparison between the local field values with and without the shield. Apart from some practical problems arising in specific, but relevant, situations like enclosures of small dimensions, such an approach appears rather incomplete and sometime deceitful. Despite this, shielding effectiveness (SE) is a well-established parameter, and it seems that for decades, the attention has been focussed more on how to evaluate and measure the so-called SE, rather on what the goal of any shielding structure is and why each evaluation and measurement should be performed. The main drawback is that SE is a local quantity and its knowledge does not help in the prediction of the real mitigation of undesired effects achieved by means of any shielding structure; undesired effects are mainly due to an integral of an electric or magnetic field, and/or to spatial variations of electric and magnetic field components. In the past, proposals were advanced toward an improved definition and measurement of electromagnetic SE; the proposed new figures of merit were based on the energy (power) penetrating the enclosure, perceived as the key factor for the shielding problem. However, it seems more adequate and correct the direct reference to the mechanism of birth of induced effects, as stemming from Maxwell equations. For these reasons, two new figures of merit are proposed for the comparison of enclosures and shields performance.