Cable Shielding Effectiveness

Formula

Description

Cable shielding attenuates electromagnetic interference through absorption and reflection. This simplified formula shows that shielding effectiveness increases with frequency, shield conductivity, permeability, and the square of the shield thickness. At low frequencies, magnetic shielding is difficult because the absorption loss is small, requiring high-permeability materials like mu-metal. At high frequencies, even thin copper or aluminum shields provide excellent attenuation. Braided shields are the most common type, offering good flexibility and moderate SE (40-90 dB), while solid tube shields provide the best performance (>100 dB).

Variables

• SE — Shielding effectiveness (dB)
• f — Frequency (Hz)
• µ — Permeability of shield material (H/m)
• σ — Conductivity of shield material (S/m)
• t — Shield thickness (m)

Practical Notes

Braided cable shields have coverage percentages of 60-98%, and SE improves with higher coverage. Double-shielded cables (braid + foil) achieve 90-120 dB SE. For copper: σ = 5.8×10⁷ S/m, µ = µ₀ = 4π×10⁻⁷ H/m. Cable shield grounding is critical: ground at one end only for low frequencies (to avoid ground loops) or at both ends for high frequencies (to maintain shield current return path).