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| 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 values. Because metallic shields
have less than infinite conductivity, part of the field is transmitted
across the boundary and supports a current in the metal. The
amount of current flow at any depth in the shield, and the rate
of decay is governed by the conductivity of the metal, its permeability,
and the frequency and amplitude of the field source. The residual
current appearing on the opposite face is the one responsible
for generating the field which exists on the other side.
The current density in a metal shield is not affected by the
shieldĄ¯s thickness. A secondary reflection occurs at the far
side of the shield for all thicknesses. The only difference
with thin shields is that a large part of the re-reflected wave
may appear on the front surface. This wave can add to or subtract
from the primary reflected wave depending upon the phase relationship
between them. For this reason, a correction factor appears in
shielding equations to account for reflections form the far
surface of a thin shield.
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