An overview of the theory of shielding (as proven by test) is presented. This consists of (1) the generation of an electromagnetic wave; (2) the penetration of the wave into and through a shielding barrier; and (3) the penetration of a wave through an EMI gasketed seam.
KeywordsElectromagnetic wave Barrier penetration EMI gasket EM wave Wave generation Gasket penetration Shielding theory Shielding practice EMI seam Reflection loss Shielded seam
The theory of shielding  begins with the generation of an electromagnetic (EM) wave. This is accomplished with the use of two (2) of Maxwell’s equations and class notes from a 1960 UCLA course on EM theory. The result is a wave consisting of lines of flux (defined as displacement current dD/dt) that radiates away from the source. This displacement current generates the E and H fields, where the value of the H field in A/m is equal to the value of the displacement current. When the wave is impinged onto a shielding barrier, the current (displacement current) in the wave is coupled to the barrier. The current coupled to the barrier (surface current density JS) is attenuated by skin effect, where the values of the E and H field in the barrier are attenuated equally. The value of the E and H fields that exit the barrier is equal to that at the exit surface of the barrier. If there is an EMI gasketed seam in the shield, the surface current density JS that crosses the seam will create a voltage across the seam. The value of the wave that penetrates the seam is directly proportional to that voltage .