Shielding Effectiveness
Understanding Shielding Effectiveness
SE quantifies how much an enclosure or barrier reduces the electromagnetic field strength at a given point. It is the ratio of incident field to transmitted field, expressed in dB. A 60 dB SE means the field is attenuated by a factor of 1,000.
Loss Mechanisms
- Reflection loss: Impedance mismatch at material surfaces reflects energy back. Dominant at low frequencies for good conductors (copper, aluminum)
- Absorption loss: Energy dissipated as heat within the material. Increases with frequency, conductivity, permeability, and thickness. 1 skin depth = 8.7 dB
- Re-reflection correction: Internal reflections within thin shields. Significant only when absorption loss is below 10 dB
Factors Affecting SE
- Material: Copper, aluminum, steel, mu-metal, conductive composites. Higher conductivity = better reflection; higher permeability = better absorption
- Apertures: Seams, ventilation holes, connector cutouts are the dominant leak paths. A slot radiates when its length exceeds lambda/2
- Gaskets: Conductive elastomer, beryllium copper finger stock, or knitted wire mesh gaskets maintain SE at enclosure seams
- Frequency: SE typically increases with frequency for solid barriers but aperture leakage worsens
Measurement Standards
- IEEE 299: Standard for enclosure SE measurement using plane wave, E-field, and H-field sources
- MIL-STD-285: Military standard, largely superseded by IEEE 299 but still referenced
- ASTM D4935: Material-level SE testing using coaxial transmission line fixtures
Key Equations
SE = R + A + B dB
R = reflection loss, A = absorption loss
B = re-reflection correction
Absorption:
A = 8.686t/δ = 131.4t√(fμrσr) dB
t = thickness
Reflection (E-field):
R = 20log(Z0/(4Zs))
Comparison
| Material @1G | R (dB) | A (dB) | SE total | Thickness |
|---|---|---|---|---|
| Copper 0.1mm | 88 | 35 | 123 | 0.1 mm |
| Aluminum 0.5mm | 80 | 62 | 142 | 0.5 mm |
| Steel 0.5mm | 45 | 155 | 200 | 0.5 mm |
| Conductive paint | 15–30 | 5–15 | 20–45 | 25 μm |
| Metalized fabric | 20–40 | 2–10 | 22–50 | 0.1 mm |
Frequently Asked Questions
What is shielding effectiveness?
SE measures how well a barrier attenuates EM fields, in dB. SE = reflection loss + absorption loss + re-reflection. 60 dB means 1,000x field attenuation. Measured per IEEE 299.
How is SE measured?
Per IEEE 299: transmit antenna outside enclosure, receive inside, compare to free-space reference. Material-level uses coaxial fixtures per ASTM D4935 or waveguide methods.
What affects shielding effectiveness?
Material conductivity/permeability, thickness, frequency, and apertures. Seams and slots are the dominant leak path. A single untreated slot can compromise an otherwise perfect enclosure at high frequencies.