Total Shielding Effectiveness of a Shield: SE = R + A + B
An electromagnetic wave is attenuated as it passes through a shield because it loses energy. That loss divides into reflection loss and absorption loss, and a correction for repeated reflections inside the shield completes the total shielding effectiveness.
Components of SE
The total shielding effectiveness is SE = R + A + B (dB).
- R: attenuation from a single surface reflection. A wave entering a shield crosses two interfaces and is reflected twice, so the total reflection loss is the sum of the losses at both.
- A: absorption loss, the energy consumed as the wave travels through the shield.
- B: correction for multiple reflections inside the shield. When A exceeds 10 dB, B can be ignored.
Reflection loss and wave impedance
The formula for R depends on the type of wave, so it is calculated separately for a far-field plane wave, a near-field electric field and a near-field magnetic field, each using its own wave impedance. In general, the larger the mismatch between the wave impedance and the impedance of the shield, the higher the reflection loss.
How the parameters affect R and A
| Parameter increased | R | A |
|---|---|---|
| Thickness t | Unchanged | Increases |
| Frequency f | Decreases | Increases |
| Relative permeability μr | Decreases | Increases |
| Relative conductivity σr | Increases | Increases |
Changing a parameter in the opposite direction reverses the trend. A thicker shield raises A and so makes B less important. When selecting a material, consider these parameters together with the frequency range and the type of field that has to be suppressed.