How To Reduce Emi By Shielding - Delta C2000 Series User Manual

4.2 How to reduce EMI by Shielding?

Iron and other metals are high conductivity materials that provide effective shielding at extremely low
frequencies. But conductivity will decrease as:
1. High frequency signals are applied to the conductor.
2. Equipment is located in a strong magnetic field
3. The shielding frame is forced into a specific form by machines.
It is difficult to select a suitable high-conductivity material for shielding without the help from a shielding
material supplier or a related EMI institution.
Metallic Shielding Effectiveness
Shielding Effectiveness (SE) is used to assess the applicability of the shielding shell. The formula
is:
SEdB=A+R+B (Measures in dB)
The absorption loss refers to the amount of energy loss as the electromagnetic wave travels
through the shield. The formula is:
AdB=1.314(f )1/2t
The reflection loss depends on the source of the electromagnetic wave and the distance from that
source. For a rod or straight wire antenna, the wave impedance increases as it moves closer to the
source and decreases as it moves away from the source until it reaches the plane wave impedance (377)
and shows no change. If the wave source is a small wire loop, the magnetic field is dominant and the
wave impedance decreases as it moves closer to the source and increases as it moves away from the
source; but it levels out at 377 when the distance exceeds one-sixth of the wavelength.
where A= Absorption loss (dB)
R= Reflection loss (dB)
B= Correction factor (dB) (for multiple reflections in thin
shields)
where f= frequency (MHz)
= permeability relative to copper
= conductivity relative to copper
t= thickness of the shield in centimetres
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