WHAT ARE EMI SHIELDING PRINCIPLES?
The importance of wave impedance is shown by an electromagnetic wave encountering an obstacle such as a metal shield. If the impedance of the wave differs greatly form the natural impedance of the shield, much of the energy is reflected and the rest is transmitted across the surface boundary, where absorption in the shield further attenuates it . Because most metals have an intrinsic impedance of only milliohms, less low impedance H-field energy is reflected and more is absorbed. This is because the metal is more closely matched to the impedance of the field. This is also why it is difficult to shield against magnetic fields. On the other hand the wave impedance of electric fields is high, so most of the energy is reflected for this case. At higher frequencies, typically over 10 MHZ, EMI shielding is governed mostly by absorption. Shielding effectiveness of metallic enclosures is not infinite, because the conductivity of all metals is finite. They can, however, approach very large valu
