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EMC

Shielding Effectiveness Calculator

Calculate electromagnetic shielding effectiveness, absorption loss, reflection loss, and skin depth. Evaluate enclosure materials per MIL-STD-285.

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Formula

SE=A+R=8.686×(t/δ)+20×log10(∣1+η0/ηs∣/2)SE = A + R = 8.686×(t/δ) + 20×log₁₀(|1+η₀/η_s|/2)

Reference: MIL-STD-285, Schulz et al.

SE— Total shielding effectiveness (dB)
A— Absorption loss (dB)
R— Reflection loss (dB)
δ— Skin depth (m)
t— Shield thickness (m)
σ— Conductivity (S/m)
μ_r— Relative permeability

How It Works

The Shielding Effectiveness Calculator computes electromagnetic attenuation for conductive enclosures — essential for EMC compliance (CISPR 32, FCC Part 15), medical device immunity (IEC 60601-1-2), and military specifications (MIL-STD-461G). EMC engineers use this to achieve 40-80 dB shielding required for sensitive electronics protection.

Per Henry Ott's 'EMC Engineering' and MIL-HDBK-419A, shielding effectiveness SE = A + R + B, where A is absorption loss, R is reflection loss, and B is re-reflection correction (negligible when A > 10 dB). Absorption loss A = 8.686 x t/delta, where t is thickness and delta = sqrt(2/(omega x mu x sigma)) is skin depth. At 1 GHz, copper skin depth is 2.1 um; a 1mm copper sheet provides A > 400 dB.

Reflection loss R = 20 x log10(Z0/4Zs), where Z0 = 377 ohm (free space) and Zs = sqrt(omega x mu/sigma) is shield impedance. Copper at 1 GHz has Zs = 0.026 ohm, giving R = 20 x log10(377/(4 x 0.026)) = 67 dB. Total SE for copper exceeds 100 dB — but real enclosures have apertures.

Per Ott, apertures dominate shielding failure. A single slot of length L reduces SE to approximately 20 x log10(lambda/(2L)) at frequencies where L > lambda/2. A 10cm slot (f_cutoff = 1.5 GHz) provides only 0 dB shielding at 1.5 GHz and negative SE (resonant amplification) above. CISPR 32 Class B requires 40 dBuV/m limit — enclosure apertures must be sized to provide 20+ dB margin.

Worked Example

Problem: Design aluminum enclosure (sigma = 3.77e7 S/m, mu_r = 1) with 2mm wall thickness for 40 dB shielding at 1 GHz. Maximum ventilation slot length?

Solution per Ott:

  1. Skin depth at 1 GHz: delta = sqrt(2/(2 x pi x 1e9 x 4 x pi x 1e-7 x 3.77e7)) = 2.6 um
  2. Absorption loss: A = 8.686 x 0.002/2.6e-6 = 6680 dB (wall is not limiting factor)
  3. Reflection loss: Zs = sqrt(2 x pi x 1e9 x 4 x pi x 1e-7/3.77e7) = 0.032 ohm; R = 20 x log10(377/(4 x 0.032)) = 66 dB
  4. Enclosure SE without apertures: >100 dB
  5. For 40 dB at 1 GHz with apertures: SE_aperture = 20 x log10(lambda/(2L)); lambda = 0.3m at 1 GHz
  6. 40 = 20 x log10(0.3/(2L)); 100 = 0.3/(2L); L = 1.5mm maximum slot length
  7. For 20 ventilation slots: use honeycomb waveguide-beyond-cutoff filter (5mm cells provide >60 dB at 1 GHz)
Result: 2mm aluminum provides >100 dB material SE, but slots must be <1.5mm for 40 dB. Use honeycomb filter for ventilation.

Practical Tips

  • ✓Size apertures to lambda/20 maximum — per Ott, this provides 26 dB margin versus lambda/2 resonance. At 1 GHz (lambda=30cm), max aperture = 15mm; at 3 GHz, max = 5mm.
  • ✓Use conductive gaskets at all seams — EMI gaskets (BeCu fingerstock, conductive foam) maintain <10 mohm contact resistance required for 40+ dB SE per MIL-HDBK-419A.
  • ✓Place EMI filters at cable entry points — feedthrough capacitors provide 40-60 dB; Pi-filters provide 60-80 dB. Filter must be bonded to enclosure for proper ground reference.

Common Mistakes

  • ✗Assuming material SE equals enclosure SE — material provides 60-100+ dB but apertures (seams, ventilation, displays) typically limit real enclosures to 20-60 dB. Per Ott, a single untreated seam can reduce SE to <10 dB.
  • ✗Using DC conductivity for high-frequency calculations — skin effect confines current to surface; surface finish (oxidation, paint) can add 10-20 dB loss. Use measured surface resistance or specify conductive finish.
  • ✗Ignoring cable penetrations — unfiltered cables act as slot antennas inside shielded enclosures. Per MIL-STD-461G, all cables must be filtered at point of entry or use shielded/filtered connectors.

Frequently Asked Questions

Copper (sigma = 5.8e7 S/m) and aluminum (sigma = 3.77e7 S/m) are most common, providing >80 dB material SE above 100 kHz. For magnetic shielding below 100 kHz, use mu-metal (mu_r = 20,000-100,000) which achieves 40-60 dB by redirecting magnetic flux. Per Ott, 0.5mm copper provides equivalent SE to 1mm aluminum due to higher conductivity.
Absorption loss A = 8.686 x t/delta. When t >> delta (typical for metals above 1 MHz), SE increases 8.7 dB per skin depth of thickness. At 1 MHz, copper skin depth = 66 um; 1mm copper provides 130 dB absorption. At 1 GHz, delta = 2.1 um; even 0.1mm copper provides >400 dB absorption — apertures always dominate.
Yes significantly — per MIL-HDBK-419A, non-conductive paint adds 20-40 dB loss at seam interfaces by increasing contact resistance. Solutions: (1) mask mating surfaces from paint; (2) use conductive paint (nickel or copper filled); (3) specify chromate conversion or conductive anodize for aluminum. Surface oxidation alone can add 10 dB loss.
1-10 GHz is most challenging because: (1) apertures of practical size (>5mm) approach resonance; (2) cable/connector transitions have significant leakage; (3) gasket contact impedance creates slots. Per CISPR 32, radiated limits extend to 6 GHz. Above 10 GHz, smaller wavelength makes aperture control easier. Below 100 kHz, reflection loss decreases — magnetic shielding required.
Per MIL-STD-461G: (1) Overlap seams by 1/4 wavelength at highest frequency (7.5mm at 10 GHz); (2) Use continuous conductive gaskets — BeCu fingerstock for removable panels, conductive foam for permanent seams; (3) Space fasteners at lambda/20 intervals; (4) Ensure <2.5 mohm contact resistance across full seam length. EMI gaskets add $2-10/meter but provide 20-40 dB improvement over bare metal contact.

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