The Influence of Slots and Holes on the Shielding Efficiency of a Housing

Slots and holes, which are always present in a conductive shield surrounding a source of electromagnetic radiation, can negatively influence the shielding efficiency (SE). There are several theories describing this influence. In this paper it is shown that the formulas given in the often-cited works by Ott [1] and White et al. [2] do not correlate with the formulas that can be derived using the theoretical approach based on the radiation properties of a small aperture [3,4,5]. It is also proved that the theory based on the radiation properties of a small aperture correlates qualitatively with the measured SE. There is no correlation between the results obtained using the formulas given by Ott and White and the measured SE. It is therefore recommended not to use these formulas when a reasonable prediction of the SE of a conductive shield with holes is needed. In addition, design rules are given concerning the length of the slot and the distance from antenna to slot in relation to the shielding efficiency of the housing.