Skip to content
RFrftools.io
EMC

Cable Shield Effectiveness

Calculate cable shielding effectiveness and transfer impedance vs frequency. Evaluate coaxial and shielded cable EMI performance for EMC compliance.

Loading calculator...

Formula

SE=20⋅log10(Vno−shield/Vshield)SE = 20·log₁₀(V_no-shield / V_shield)

How It Works

The Cable Shield Effectiveness Calculator computes transfer impedance and shielding effectiveness for shielded cables — essential for EMC compliance, system-level radiated emissions control, and immunity to external interference. EMC engineers use this to achieve 40-80 dB cable shielding required for CISPR 32 Class B compliance and MIL-STD-461G RE102.

Per Henry Ott's 'EMC Engineering' and MIL-HDBK-1857, the key parameter is transfer impedance Z_t (mohm/m), which relates shield current to induced voltage on the inner conductor: V_inner = Z_t x I_shield x L. At low frequencies (below approximately 1 MHz), Z_t equals shield DC resistance. At higher frequencies, skin effect reduces Z_t initially, but braid apertures cause Z_t to rise above approximately 10 MHz.

Shielding effectiveness SE = 20 x log10(Z_ref / (Z_t x L)), where Z_ref is typically 10 mohm reference. A cable with Z_t = 10 mohm/m at 100 MHz and L = 2m has SE = 20 x log10(10/(10x2)) = -6 dB at 100 MHz — negative SE means the cable is actually coupling noise IN. Per CISPR 32, cables should achieve SE > 40 dB to avoid being dominant emission sources.

Double-shielded cables (foil plus braid) achieve SE > 60 dB by combining foil's 100% coverage with braid's low DC resistance. Per MIL-C-17, triaxial cables achieve SE > 100 dB. For most industrial applications, a single braid with 85%+ coverage provides adequate 30-50 dB SE below 100 MHz.

Worked Example

Problem: Evaluate cable shielding for 2m USB cable with single braid shield (Z_t = 20 mohm/m at DC, rising as sqrt(1 + (f/10 MHz)^2)). Is it adequate for CISPR 32 Class B?

Solution per Ott:

  1. At 30 MHz (CISPR 32 radiated starts): Z_t = 20 x sqrt(1 + 9) = 63 mohm/m
  2. At 100 MHz: Z_t = 20 x sqrt(1 + 100) = 201 mohm/m
  3. At 300 MHz: Z_t = 20 x sqrt(1 + 900) = 600 mohm/m
  4. SE at 100 MHz: SE = 20 x log10(10/(201 x 2)) = 20 x log10(0.025) = -32 dB
  5. With 10 mA internal shield current at 100 MHz: V_coupled = 201e-3 x 0.01 x 2 = 4 mV
  6. This voltage across 50-ohm LISN = 4mV/50 = 80 uA, radiating field approximately 66 dBuV/m at 3m
Analysis: This cable provides NEGATIVE shielding above 10 MHz — it acts as an antenna. For CISPR 32 compliance, need double-shielded cable (Z_t < 10 mohm/m at 100 MHz) or add ferrite clamps for 20 dB additional suppression.

Practical Tips

  • ✓Use 360-degree shield termination to connectors — per MIL-STD-461G, pigtail grounds add 20-50 nH inductance that degrades SE by 10-20 dB above 30 MHz. Backshell clamp or crimp terminations provide <1 nH.
  • ✓Add ferrite clamps at both cable ends — per Murata, snap-on ferrites provide 10-20 dB additional CM attenuation from 30-500 MHz, supplementing cable shield when termination quality is uncertain.
  • ✓Specify double-shielded cables for frequencies above 100 MHz — per CISPR 32 design guide, single braid SE degrades significantly above 100 MHz; double shields (foil + braid) maintain 60+ dB to 1 GHz.

Common Mistakes

  • ✗Grounding shield at one end only — per Ott, single-point grounding only shields electric fields; magnetic field coupling (dominant above approximately 1 MHz) requires shield current flow, which needs both-ends grounding. Exception: audio frequencies below 20 kHz where ground loops cause 50/60 Hz hum.
  • ✗Relying on shield effectiveness rating without checking termination — per MIL-HDBK-1857, pigtail ground terminations add 10-30 nH inductance that bypasses the shield above 10 MHz. Use 360-degree circumferential bonding to connector backshells.
  • ✗Assuming foil shields are better than braid — foil provides 100% optical coverage but has higher Z_t than braid at DC due to thin aluminum (typically 10 um). Per Ott, foil-braid combination provides best performance: foil for high frequencies, braid for low frequencies.

Frequently Asked Questions

Depends on frequency per Ott: foil provides 100% coverage (no apertures) but thin aluminum has higher DC resistance; braid has lower DC resistance but 80-95% coverage with apertures. Below 10 MHz, braid wins; above 100 MHz, foil wins. Best performance: foil + braid combination achieves SE > 60 dB across full spectrum per MIL-C-17 specifications.
Per Ott, single-point grounding prevents ground loop currents at 50/60 Hz that cause audible hum in audio systems. However, single-point grounding provides NO magnetic field shielding above approximately 1 kHz. For EMC applications (30 MHz to 1 GHz), always ground both ends. For audio, use balanced (differential) signaling to reject common-mode hum instead of single-point grounding.
No fixed minimum — depends on internal noise level. Per Ott, typical rule: internal emissions should be 20 dB below limit before cable becomes dominant source. If PCB emissions are 50 dBuV/m and limit is 40 dBuV/m, cable SE must be >30 dB (to achieve 50-30=20 dBuV/m cable contribution). In practice, 40+ dB SE is recommended for reliable compliance margin.
SE decreases 6 dB per doubling of length — Z_t is per-meter, so longer cables couple more noise. Additionally, cables longer than lambda/4 at problem frequency become resonant antennas with enhanced radiation. Per MIL-HDBK-1857, a 1m cable resonates at 75 MHz (quarter-wave); a 2m cable at 37.5 MHz. Keep cables as short as possible for EMC.
Yes — per IEC 62153-4-3 (triaxial method) or MIL-STD-1377. Inject known current on shield outer surface; measure induced voltage on inner conductor per unit length. Equipment: signal generator, RF amplifier, injection fixture, spectrum analyzer. Transfer impedance test fixtures available from Fischer Custom Communications and others. Results are manufacturer datasheet curves.

Shop Components

As an Amazon Associate we earn from qualifying purchases.

Copper Foil Tape

Copper foil tape for EMI shielding and grounding

Amazon →

Ferrite Bead Kit

SMD ferrite bead assortment for suppressing high-frequency noise

Amazon →

Related Calculators

EMC

RF Shielding

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

EMC

Radiated Emissions

Estimate radiated emissions from PCB current loops using the small-loop model. Compare E-field against CISPR 22/FCC Class B limits instantly.

EMC

EMI Margin

Calculate EMI compliance margin with measurement uncertainty and safety margin. Predict CISPR/FCC pre-compliance test pass or fail instantly.

EMC

Ferrite Bead

Calculate ferrite bead impedance, insertion loss, and DC voltage drop at any frequency. Select EMI filter beads for power supply decoupling.