How does transfer impedance relate to shielding?

Transfer impedance directly determines how much interference voltage appears on the signal conductor when current flows on the cable shield: V_noise = Zt × I_shield × L_cable. Lower Zt = less coupled voltage = better shielding. For a solid tube shield: Zt is determined entirely by the skin effect. At DC: Zt = R_dc = rho/(2*pi*r*t). As frequency increases, the skin depth becomes less than the wall thickness, and the external current is confined to the outer surface, reducing the coupling to the interior. Zt_tube = R_dc × (t/delta) × exp(-t/delta) / sinh(t/delta). Above the skin-depth frequency: Zt drops at 20 dB/decade. For braided shields: Zt initially drops with frequency (skin effect), then INCREASES above a few MHz due to porosity coupling (magnetic field penetrating through the braid apertures). This is why braided shields have a minimum Zt at an intermediate frequency and then increasing Zt at higher frequencies.

How is transfer impedance measured?

Standard measurement methods: Triaxial method (IEC 62153-4-3): the cable under test is placed inside an outer tube (triaxial fixture). Current is injected between the cable shield and the outer tube. The voltage induced on the inner conductor is measured. Zt = V_measured / (I_injected × L). Frequency range: DC to ~1 GHz. Line injection method (IEC 62153-4-6): uses a clamp-on injection device to couple current to the cable shield. Simpler setup but limited frequency range. Suitable for installed cables. Reverberation chamber: for frequencies above 1 GHz where transfer impedance transitions to shielding effectiveness (SE). Irradiate the cable in a mode-stirred chamber and measure the induced signal. Results: Zt is plotted vs frequency. Key features: DC value, skin-effect roll-off, braid resonance (minimum point), and porosity rise. Standards: MIL-DTL-17, IEC 62153, SAE AS23053.

How do different shield types compare?

Solid copper tube (semi-rigid): Zt < 1 mΩ/m at DC, drops continuously with frequency (pure skin effect). Best possible shielding. No porosity. Used for high-performance RF cables (SMA semi-rigid, corrugated hardline). Double braid: Zt = 1-5 mΩ/m at DC. Two layers of braid reduce porosity coupling. Minimum Zt around 1-10 MHz, then slow rise. Used for RG-214, double-shielded cables. Single braid: Zt = 5-20 mΩ/m at DC. Porosity coupling causes Zt to rise above 10-50 MHz. Typical coverage: 85-95%. Used for RG-58, RG-316. Foil + braid: foil eliminates porosity, braid provides mechanical strength and low-frequency shielding. Zt: 2-10 mΩ/m, relatively flat vs frequency. Used for RG-6, LMR-series. Spiral shield: highest Zt above 1 MHz (spiral acts as an inductor). Only useful for audio/low-frequency applications.

EMC / Shielding

Cable Shield Transfer Impedance

Z_t = V_int / (I_ext × L)

V_noise = Z_t × I_shield × L. Units: mΩ/m. Solid tube: <1 mΩ/m, drops with f (skin effect). Double braid: 1-5 mΩ/m. Single braid: 5-20 mΩ/m, rises above 10 MHz (porosity). Foil+braid: 2-10 mΩ/m flat. Measured: triaxial method (IEC 62153). Standards: MIL-DTL-17.

Solid tube: <1 mΩ/m

Single braid: 5-20 mΩ/m

Std: IEC 62153

Understanding Transfer Impedance

Transfer impedance is the most meaningful metric for cable shield performance in EMC applications. Unlike shielding effectiveness (which is a far-field concept), Zt directly tells you how much voltage appears on your signal conductor for a given current on the shield. It captures both the resistive coupling (DC through cable resistance) and the inductive coupling through braid apertures, giving a complete picture of how well the shield protects the internal signal.

Transfer Impedance Physics

Cable Shield Transfer Impedance:
V noise = Z t × I shield × L. Units: mΩ/m. Solid tube: <1 mΩ/m, drops with f (skin effect). Double braid: 1-5 mΩ/m....

Key specifications:
1 m | -5 m | -20 m | 10 MHz | -10 m

Power: P(dBm) = 10log(P_mW), 0dBm = 1mW

Shield Type Zt Comparison

Shield Type	Z_t @DC	Z_t @100 MHz	Trend	Cable Example
Solid tube	<1 mΩ/m	<0.01 mΩ/m	Falls	Semi-rigid, hardline
Double braid	1-5	0.5-5	Min then rises	RG-214
Foil + braid	2-10	2-10	Flat	LMR-400, RG-6
Single braid	5-20	10-100	Rises	RG-58, RG-316
Spiral	10-50	100-1000	Rises fast	Audio cables

Key Equations

Maxwell’s equations (time-harmonic):
∇×E = −jωμH
∇×H = jωεE + J

Wave equation:
∇²E + k²E = 0, k = ω√(με)

Skin depth:
δ = 1/√(πfμσ)

Comparison

Connector	Freq Max	Impedance	Power	Interface
SMA	18 GHz	50 Ω	0.5 W	Threaded
N-Type	11 GHz	50 Ω	5 W	Threaded
2.92mm (K)	40 GHz	50 Ω	0.3 W	Threaded
1.85mm (V)	67 GHz	50 Ω	0.2 W	Threaded
1.0mm (W)	110 GHz	50 Ω	0.1 W	Threaded

Common Questions

Frequently Asked Questions

Shielding relationship?

V_noise = Zt × I_shield × L. Lower Zt = less coupled noise. Solid tube: pure skin effect, Zt drops with frequency. Braid: skin effect + porosity coupling. Porosity rises above 10 MHz. This is why braids have a Zt minimum.

Measurement?

Triaxial method (IEC 62153-4-3): cable inside outer tube, inject current, measure induced voltage. DC to ~1 GHz. Line injection (clamp-on) for installed cables. Plot Zt vs frequency. Standards: MIL-DTL-17.

Shield types?

Solid tube (<1 mΩ/m, best). Double braid (1-5, good). Foil+braid (2-10, flat). Single braid (5-20, rises at HF). Spiral (10-50, audio only). Choose based on frequency range and required isolation.

Related Terms

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