Shielding Effectiveness S = A + R + B (dB)
A = Absorption Loss
R = Reflection Loss
B = Correction Factor (for multiple reflections in the shield)
Electromagnetic Filed Shielding (Far Field)
Assuming an electromagnetic wave that propagates perpendicular to the shield surface:
Absorption Loss A = 131.4 * t * SQRT (f * relative permeability * conductivity) dB
• increase due to the skin effect
• is the primary contributor to the shielding effectiveness at high frequencies
t = thicknes of the shield in meters
f = frequency
Reflection Loss R = 168 - (10 * log (relative permeability * f / conductivity)) dB
• decrease with the frequency
• is the primary contributor to the shielding effectiveness at low frequencies
• For sources with high voltages the dominant near-field is an electrical field.
• For sources with high currents the dominant near-field is a magnetic field.
Electric Field Shielding (Near Field)
Reflection Loss R = 322 - (10 * log (conductivity / relative permeability * f^3 * r^2))
r = distance between the source and the shield
electric near-field reflection loss =< far-field reflection loss
Magnetic Field Shielding (Near Field)
Reflection Loss R = 14.57 - (10 * log (conductivity * f * r^2 / relative permeability))
• reflection loss decreases for decreasing frequencies, and is lower than the reflection loss for the plane wave reflection.
• reflection losses are usually negligible for lower frequencies and absorption losses are small for low frequencies too.
Magnetic Field (MF) shielding methods:
• Deviation of the magnetic flux with high permeability material.
• The shorted tuned method, which consists in the generation of opposing fluxes that cancel the magnetic field in the area of interest.
Using magnetic material as shield:
• The permeability of a magnetic material decreases by increasing the frequency (depends only on the material).
• The permeability of a magnetic material decreases by increasing the MF strength (depends on the material and the section of the magnetic circuit).
The steel is a better magnetic field shield at low frequencies than good conductors like aluminium or copper. However at high frequencies, good conductors provide better magnetic shielding.
- For non-magnetic material increases with the frequency, therefore, it is recommended to calculate the attenuation for the lowest frequency of interest.
- For magnetic materials may reduce due to the decrease of the permeability with the frequency.