EMC Shielding Effectiveness Calculator

Q: What is Schelkunoff's Theory of Shielding?

Formulated by Sergei Schelkunoff in 1943, shielding theory models an electromagnetic shield using transmission line analogies. Total shielding effectiveness (in decibels) is the sum of three mechanisms: reflection loss ($R$) at the boundary interfaces due to impedance mismatch, absorption loss ($A$) as the wave decays exponentially inside the conductive bulk, and a multiple internal reflection correction factor ($B$).

Q: Why is low-frequency magnetic field shielding so difficult?

In the near field of a low-impedance magnetic source (e.g., $50/60\text{ Hz}$ transformers or switching inductors), the wave impedance is extremely low ($Z_w \ll 377\ \Omega$). Because the shield's intrinsic impedance is also low, the impedance mismatch is minimal, yielding almost zero reflection loss ($R \approx 0$). Consequently, shielding relies almost entirely on absorption or magnetic flux shunting, requiring thick high-permeability ferromagnetic materials like Mu-Metal.

Q: When does the multiple reflection correction term B become significant?

The multiple reflection correction factor $B$ is a negative penalty (reducing total SE) that occurs when a shield is electrically thin ($t < 1\text{ to }2$ skin depths $\delta$), causing internal reflections to re-radiate out the back face. When absorption loss $A \ge 15\text{ dB}$, internal reflections are completely attenuated before reaching the opposite interface, making $B \approx 0\text{ dB}$.

Electromagnetic Compatibility: Compute total shielding attenuation ($SE = R + A + B$), skin depth ($\delta$), and near- vs far-field reflection losses per Schelkunoff.

Shield Material & Geometry

Shield Material Preset:

Shield Thickness t (mm): ≈ 39.4 mils

Frequency f:

EM Wave Field Type:

Source Distance r (m): Used for near-field reflection

Shielding Effectiveness Breakdown

Total Shielding (SE)

-- dB

Absorption Loss (A)

-- dB

Reflection Loss (R)

-- dB

Multiple Refl. (B)

-- dB

Skin Depth (δ)

-- μm

Field Boundary r_b

-- m

Effective Shielding Class: Excellent Military Shielding (> 100 dB)

Wavelength in Air (λ): -- m

Thickness in Skin Depths (t / δ): --

Recommended Tools & Equipment

Tested hardware and components for high reliability

100% Free Tool Zero Sign-Up

Engineering Fundamentals of Electromagnetic Shielding

Electromagnetic shielding attenuates unwanted radiated emissions and protects sensitive analog and digital circuitry from high-energy radio-frequency interference (RFI) and electromagnetic pulses (EMP).

1. Schelkunoff Transmission Line Analogy

The total Shielding Effectiveness ($SE$) in decibels is expressed as:

SE (dB) = R + A + B

Absorption Loss ($A$): Exponential attenuation through skin depth $\delta$:
A (dB) = 8.686 · (t / δ) = 131.4 · t [mm] · √(f_MHz · μ_r · σ_r)
Reflection Loss ($R$): Caused by wave impedance mismatch between the incident medium ($Z_w$) and the metal shield ($Z_s$):
Far-field Plane Wave: R_p = 168 - 10 · log₁₀((μ_r · f) / σ_r)
Multiple Reflection Correction ($B$): Modifies thin-sheet performance when $A < 15\text{ dB}$:
B (dB) = 20 · log₁₀|1 - exp(-2t / δ)|

2. Skin Depth (δ)

The skin depth is the penetration distance at which the wave amplitude drops to $1/e$ ($36.8\%$):

δ = √(1 / [π · f · μ · σ])  ≈  0.066 / √(f_MHz · μ_r · σ_r)  [mm]

Frequently Asked Questions

What is Schelkunoff's Theory of Shielding?

Formulated by Sergei Schelkunoff in 1943, shielding theory models an electromagnetic shield using transmission line analogies. Total shielding effectiveness (in decibels) is the sum of three mechanisms: reflection loss ($R$) at the boundary interfaces due to impedance mismatch, absorption loss ($A$) as the wave decays exponentially inside the conductive bulk, and a multiple internal reflection correction factor ($B$).

Why is low-frequency magnetic field shielding so difficult?

In the near field of a low-impedance magnetic source (e.g., $50/60\text{ Hz}$ transformers or switching inductors), the wave impedance is extremely low ($Z_w \ll 377\ \Omega$). Because the shield's intrinsic impedance is also low, the impedance mismatch is minimal, yielding almost zero reflection loss ($R \approx 0$). Consequently, shielding relies almost entirely on absorption or magnetic flux shunting, requiring thick high-permeability ferromagnetic materials like Mu-Metal.

When does the multiple reflection correction term B become significant?

The multiple reflection correction factor $B$ is a negative penalty (reducing total SE) that occurs when a shield is electrically thin ($t < 1\text{ to }2$ skin depths $\delta$), causing internal reflections to re-radiate out the back face. When absorption loss $A \ge 15\text{ dB}$, internal reflections are completely attenuated before reaching the opposite interface, making $B \approx 0\text{ dB}$.