文档介绍：Chomerics - SHIELDING METHODS
● EMI Shielding Principles
● EMI Gasketing
● Shielding and EMI Gasket Equations
● EMI Gasket Types
● EMI Gasket Selection
● Gasket Mounting Choices
EMI Shielding Principles [top]
The importance of wave impedance is shown by an ic wave encountering an obstacle
such as a metal shield (see Figure 3). If the impedance of the wave differs greatly from 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 ic 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 values. Because metallic shields
have less than infinite conductivity, part of the field is
transmitted acros