Why Faraday Shields Are Easily Compromised by Conductor Penetrations with a Practical Example

Faraday shields are used to confine high frequency electromagnetic fields generated by sources such as switching electronics. Here, a simple cylindrical model of a Faraday shield is used to explain why such shields can be compromised by one or more conductors that penetrate through holes in the shield. It is shown that electromagnetic fields within the shield are reflected many times and that the energy carried by the fields exits via the penetration periodically in small amounts over time and may become electromagnetic interference (EMI). The fields inside the shield are enhanced at resonant frequencies within the shield and these resonances are reflected in the fields that escape and become EMI. In cases for which filtering of these exit ports are not practical, it is shown that placing absorbing material within the shield can reduce the energy that escapes and hence the EMI.