Clarification of basic concepts for electromagnetic interference shielding effectiveness

TLDR

Misunderstandings about electromagnetic interference shielding effectiveness (EMI SE) persist in the literature, with confusion over its definition and decomposition into reflection, absorption, and multiple‑reflection losses. The paper aims to clarify common misconceptions about EMI SE through theoretical derivation and case analysis. The authors analyze the Calculation theory, recommend using power coefficients to compare reflection and absorption contributions, and explain how multiple‑reflection effects are incorporated into the loss terms. The study shows that relying solely on absorption loss is inadequate, demonstrates the proper role of multiple‑reflection losses, and provides a correct framework for assessing reflection and absorption contributions to overall shielding.

Abstract

There exists serious miscomprehension in the open literature about the electromagnetic interference shielding effectiveness (EMI SE) as a critical index to evaluate the shielding performance, which is misleading to the graduates and newcomers embarking on the field of electromagnetic shielding materials. EMI SE is defined as the sum of three terms including reflection loss, absorption loss and multiple reflection loss in the classical Schelkunoff theory, while it is decomposed into two terms named reflection loss and absorption loss in practice, which is called Calculation theory here. In this paper, we elucidate the widely-seen misconceptions connected with EMI SE via theoretical derivation and instance analysis. Firstly, the terms in Calculation theory are often mistakenly regarded as the approximation of the terms with the same names in Schelkunoff theory when multiple reflection loss is negligible. Secondly, it is insufficient and unreasonable to determine the absorption-dominant shielding performance in the case that absorption loss is higher than reflection loss since reflection loss and absorption loss cannot represent the actual levels of reflected and absorbed power. Power coefficients are recommended to compare the contribution of reflection and absorption to shielding performance. Thirdly, multiple reflection effect is included in the definitions of reflection loss and absorption loss in Calculation theory, and the effect of multiple reflections on shielding property is clarified as against the commonly wrong understandings. These clarifications offer correct comprehension about the shielding mechanism and assessment of reflection and absorption contribution to the total shielding.