What is transfer impedance and why does it matter for cable EMC?

Transfer impedance (Z_t) is the voltage developed on the outside of a cable shield per unit length per ampere of current flowing on the inner conductor, measured in milliohms per meter. It quantifies how effectively the shield prevents internal signals from coupling to the external environment (and vice versa). A solid copper tube has Z_t near zero; a single-braid shield has Z_t of 5-20 milliohms/m below 10 MHz, rising rapidly above 100 MHz due to braid porosity. Double-braid cables achieve 1-5 milliohms/m. Semi-rigid cables with solid outer conductors provide Z_t below 0.5 milliohms/m. Lower transfer impedance means better shielding, which is critical for EMC compliance where cables are often the primary radiation mechanism.

Why are cables often the main source of radiated emissions?

Cables act as efficient antennas when common-mode currents flow on their shields or outer surfaces. Even a few microamperes of common-mode current on a 1-meter cable at 100 MHz can produce electric field strengths exceeding CISPR 32 Class B limits at 3 meters distance. The cable is typically the longest conductor in the system, making it a better antenna than PCB traces. Common-mode currents are generated by ground potential differences between connected devices, asymmetric signal return paths, and imperfect cable shield terminations. Mitigation includes 360-degree shield termination at connectors (not pigtails), ferrite common-mode chokes, and ensuring signal return currents flow through the shield rather than through external ground paths.

How much does cable shield coverage affect EMC performance?

Shield coverage percentage directly correlates with shielding effectiveness, but the relationship is not linear. A single braid with 85% optical coverage provides approximately 40-50 dB shielding effectiveness at 100 MHz. Increasing to 95% coverage improves this to 55-65 dB. A double braid (95%+95%) achieves 70-80 dB. However, shield termination quality often matters more than coverage: a 95% coverage shield terminated with a 25 mm pigtail loses 20-30 dB of shielding effectiveness at GHz frequencies compared to a 360-degree backshell termination. Foil+braid combinations provide 90+ dB shielding but the foil is fragile and requires a drain wire connection.

What is Cable EMC? | Shielding & Emissions Control

Definition & Scope

Cable EMC encompasses the electromagnetic compatibility design, testing, and installation practices that ensure cable assemblies do not act as sources of radiated or conducted emissions and are not susceptible to external electromagnetic interference. In most electronic systems, cables are the weakest link in the EMC chain: they are the longest conductors, most exposed to external fields, and most likely to carry common-mode currents that radiate efficiently as unintentional antennas.

The key metrics for cable EMC performance are shielding effectiveness (SE, in dB), transfer impedance (Z_t, in mΩ/m), and common-mode rejection. Shielding effectiveness measures the attenuation of electromagnetic fields passing through the cable shield. Transfer impedance quantifies the coupling between the cable interior and exterior, combining both resistive leakage (dominant below 1 MHz) and field penetration through braid apertures (dominant above 10 MHz). Proper cable EMC design addresses not just the cable itself but also connector shielding, backshell termination, cable routing, bundling, and grounding practices.

Key Formulas

Radiated Field from Common-Mode Current:

E(V/m) = 1.26 × 10^-6 × f × I_CM × L / d

100 MHz, 10 µA, 1 m cable, 3 m distance: E = 42 dBµV/m (Class B limit: 40)

Transfer Impedance Coupling:

V_coupled = Z_t × I_inner × L_cable

Z_t = 10 mΩ/m, I = 100 mA, L = 1 m: V = 1 mV

Pigtail Inductance Penalty:

Z_pigtail = 2πf × L_pigtail

25 mm pigtail (~25 nH) at 1 GHz: Z = 157 Ω (shield bypassed)

Cable Shield Performance Comparison

Shield Type	Z_t (1 MHz)	SE (100 MHz)	Flex Life	Cost
Single braid (85%)	5-20 mΩ/m	40-50 dB	Good	Low
Single braid (95%)	3-10 mΩ/m	55-65 dB	Moderate	Medium
Double braid	1-5 mΩ/m	70-80 dB	Good	Medium-High
Foil + braid	1-3 mΩ/m	80-90 dB	Fair (foil fragile)	Medium
Semi-rigid (solid)	<0.5 mΩ/m	100+ dB	None (fixed)	High
Corrugated (solid)	<1 mΩ/m	90-100 dB	Limited	High

Practical Application

An industrial controller fails CISPR 32 Class A radiated emissions at 156 MHz (3rd harmonic of a 52 MHz clock) with the emission traced to a 2-meter shielded cable connecting to a remote I/O module. The cable uses single-braid shielding terminated with a 30 mm pigtail at the connector. At 156 MHz, the pigtail presents 29 ohms of inductive impedance, effectively creating a 29-ohm gap in the shield. Replacing the pigtail with a 360-degree EMI backshell termination reduces the emission by 22 dB. Adding a snap-on ferrite clamp (impedance 180 ohms at 156 MHz) near the controller end provides an additional 8 dB of common-mode suppression. The combined 30 dB reduction brings the emission 12 dB below the Class A limit with comfortable margin.

Frequently Asked Questions

What is transfer impedance?

Voltage developed per unit length per ampere through the shield (mΩ/m). Single braid: 5-20 mΩ/m. Double braid: 1-5 mΩ/m. Semi-rigid solid: <0.5 mΩ/m. Lower Z_t = better shielding = better EMC performance.

Why are cables the main emission source?

Cables are the longest conductors, acting as efficient antennas. Just 10 µA of common-mode current on a 1 m cable at 100 MHz approaches CISPR Class B limits. Caused by ground differences, asymmetric returns, and poor shield termination.

How much does shield coverage matter?

85% braid: 40-50 dB SE. 95%: 55-65 dB. Double braid: 70-80 dB. But termination quality matters more: a 25 mm pigtail loses 20-30 dB vs. 360-degree backshell at GHz frequencies.

Cable EMC