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Cable Shield Effectiveness

Calculate coaxial cable or shielded cable shield effectiveness (shielding factor) vs frequency using the transfer impedance model.

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Formula

SE = 20·log₁₀(V_no-shield / V_shield)

How It Works

Cable shield effectiveness quantifies how well a cable's metallic shield prevents electromagnetic coupling between the cable interior and the external environment. The key parameter is transfer impedance Z_t (mΩ/m), which is the ratio of the induced open-circuit voltage on the inner conductor to the shield current causing it. At low frequencies, Z_t ≈ R_dc (DC shield resistance). At higher frequencies, skin effect reduces the effective shield thickness, increasing Z_t. For a braided shield, Z_t eventually rises above a few MHz due to leakage through the braid apertures. The shielding effectiveness SE = 20·log₁₀(Z_ref/Z_t) dB, where Z_ref is a reference impedance. A SE > 40 dB is generally required for CISPR 22 compliance. Double-shielded cables (outer braid over foil) achieve SE > 60 dB.

Worked Example

Problem: A shielded cable has R_dc = 10 mΩ/m, length 2 m. Using the simplified transfer impedance model Z_t = R_dc × l × √(1+(f/10MHz)²), calculate SE at 1 MHz, 10 MHz, and 100 MHz. Solution (Z_ref = 10 mΩ for reference): 1. At 1 MHz: Z_t = 10 × 2 × √(1+(1/10)²) = 20 × 1.005 = 20.1 mΩ; SE = 40 − 20·log₁₀(20.1/10) = 40 − 6 = 34 dB 2. At 10 MHz: Z_t = 20 × √(1+1) = 28.3 mΩ; SE = 40 − 20·log₁₀(28.3/10) = 40 − 9 = 31 dB 3. At 100 MHz: Z_t = 20 × √(1+100) = 201 mΩ; SE = 40 − 20·log₁₀(201/10) = 40 − 26 = 14 dB Result: Shielding effectiveness degrades significantly above 10 MHz with this model. Above 100 MHz a double-shield or solid-foil cable is needed.

Practical Tips

  • ✓Always use 360° circumferential bonding of the cable shield to the connector backshell — pigtail grounds add inductance that bypasses the shield at high frequencies.
  • ✓For frequencies above 100 MHz, specify a double-shielded cable (foil plus braid) or solid copper foil rather than a single braid shield.
  • ✓Ferrite clamps on cables can supplement shield effectiveness when cable routing changes cannot be made — place them at both ends near the connectors.

Common Mistakes

  • ✗Assuming a shield terminated at only one end provides full shielding — a shield grounded at one end only blocks electric fields, not magnetic fields above a few kHz; ground both ends for full EMC effectiveness.
  • ✗Relying on shield effectiveness alone without addressing the connector — poor connector transitions (pigtail ground connections) are often the dominant failure mode, not the cable shield itself.
  • ✗Not accounting for cable resonances — a cable acting as a quarter-wave resonator at a problem frequency can enhance emissions at that specific frequency.

Frequently Asked Questions

A solid aluminium foil shield provides 100% coverage and low transfer impedance at low frequencies. A copper braid has lower DC resistance but apertures in the braid degrade high-frequency shielding. For best performance, use a combination: foil plus braid.
In audio and low-frequency signal applications, grounding both ends can create a ground loop that introduces 50/60 Hz hum. Single-end grounding avoids ground loops at the cost of reduced magnetic shielding above ~10 kHz. For RF/EMC, ground both ends and control the ground loop impedance.
There is no fixed minimum, but cables attached to products tested to CISPR 22 Class B typically require SE > 40 dB in the 30 MHz–1 GHz range to avoid cables acting as dominant radiating antennas. The actual requirement depends on the internal noise level of the product.

Shop Components

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