he two primary shielding mechanisms are reflection and absorption. The two combined result in the familiar expression SE(dB) = R(dB) + A(dB), where SE(dB) is shielding effectiveness in decibels, R represents Reflection Loss, and A represents Absorption Loss (both also expressed in decibels). Often, absorption loss (also called penetration loss) is the energy that is not first reflected by the shield that is absorbed by it – the topic of this article.
Note: A third term involved in the SE equation called the multiple reflection factor B is often ignored if A is greater than 9 dB or if the field is electric or plane wave.
Skin depth (or skin effect as it is often called) is the distance required for the impinging wave to be attenuated to 1⁄e or 37% of its original value, where e, Euler’s number (pronounced “Oiler”), equals approximately 2.71828.
The equation for skin depth in meters (m) is:
δ = Skin depth
w = 2πf (f is frequency)
μ = permeability of the shielding material
σ = conductivity of the shielding material
Also, notice the square root of frequency term in the above equation. This means that as frequency increases, absorption becomes the dominant term exponentially.
- Williams, T., EMC for Product Designers, 5th Edition, Newnes, 2017.
- Kimmel Gerke Associates, Ltd., EDN Designer’s Guide to Electromagnetic Compatibility, 2005.
- Ott, H.W., Electromagnetic Compatibility Engineering, Wiley, 2009.