What are the different types of cable shields?

Cable shields fall into four main categories: braided wire shields (woven mesh of tinned copper or aluminum wires providing 65-97% optical coverage and 30-80 dB SE), metallic foil shields (thin aluminum or copper foil laminated to a polyester carrier providing 100% coverage but lower SE at high frequencies due to skin effect in thin foil), spiral/serve shields (helically wound wires providing good flex life but poor SE above 10 MHz), and solid shields (seamless copper or aluminum tubes providing 100+ dB SE but no flexibility). Combination shields such as foil+braid provide both 100% coverage and good high-frequency performance, achieving 80-100 dB SE. Semi-rigid coaxial cables use solid copper or silver-plated copper tubes for the highest possible shielding in RF applications.

How is cable shielding effectiveness measured?

Shielding effectiveness (SE) is measured using two primary methods: transfer impedance testing per IEC 62153-4-3, which measures the voltage coupled through the shield per ampere of current per meter of cable length (lower Zt = better shielding), and surface transfer impedance testing which is related to SE by SE = 20*log10(Z0/(2*pi*f*Zt*L)) where Z0 is the characteristic impedance and L is the cable length. Direct SE measurement per IEEE 299 injects a field outside the cable and measures the attenuated field inside (or vice versa). Transfer impedance is preferred below 1 GHz because it separates shield quality from termination quality, while triaxial test methods extend measurement capability to 10+ GHz.

When should I use double-shielded cable?

Use double-shielded cable when single-braid shielding (typically 40-60 dB SE) is insufficient for your EMC or signal integrity requirements. Common scenarios: military systems requiring MIL-STD-461 compliance (RE102 limits often require 60+ dB SE), medical devices near MRI equipment, sensitive receiver cables where coupled noise must be below -120 dBm, and cables routing through high-interference environments (near switching power supplies, motor drives, or radar transmitters). Foil+braid provides 80-90 dB SE at moderate cost. Double braid provides 70-80 dB SE with better flex life than foil+braid. For maximum shielding (100+ dB), use semi-rigid or corrugated solid-wall cable, accepting the loss of flexibility.

What is Cable Shielding? | RF EMI Shield Types

Definition & Purpose

Cable shielding is a conductive layer surrounding the signal conductors within a cable assembly, designed to attenuate electromagnetic coupling between the cable's internal signals and the external environment. The shield serves dual purposes: preventing externally generated electromagnetic fields from inducing noise on the signal conductors (immunity), and preventing the cable's own signals from radiating and interfering with nearby equipment (emissions). Shielding effectiveness (SE) quantifies this attenuation in decibels.

The shield works through three physical mechanisms: reflection of incident electromagnetic waves at the conductor surface (dominant at low frequencies where shield impedance is much lower than free-space impedance), absorption of wave energy as it passes through the conductive material (proportional to shield thickness relative to skin depth), and internal re-reflection at the second surface. For braided shields, a fourth mechanism matters: the apertures between braid wires allow field leakage that limits SE above 100 MHz, making braid optical coverage percentage and termination quality the dominant factors at RF frequencies.

Key Formulas

Shielding Effectiveness (solid shield):

SE(dB) = R(dB) + A(dB) + M(dB)

R = reflection, A = absorption (8.7 t/δ), M = re-reflection correction

Skin Depth:

δ = √(2ρ / (2πfμ))

Copper at 1 GHz: δ = 2.1 µm; at 100 MHz: δ = 6.6 µm

Transfer Impedance to SE:

SE ≈ 20 log₁₀(Z₀ / (2πf × Z_t × L))

Shield Type Comparison

Shield Type	Coverage	SE (100 MHz)	Flex Life	Weight	Cost
Single braid (85%)	85%	40-50 dB	Excellent	Low	Low
Single braid (95%)	95%	55-65 dB	Good	Medium	Medium
Double braid	98%+	70-80 dB	Good	Medium-High	Medium-High
Foil only	100%	50-60 dB	Fair	Low	Low
Foil + braid	100%	80-90 dB	Good	Medium	Medium
Spiral/serve	90-95%	20-40 dB	Excellent	Low	Low
Solid tube (semi-rigid)	100%	100+ dB	None	High	High

Practical Application

A medical device manufacturer selects cable shielding for a patient monitoring cable that must operate within 3 meters of a 1.5T MRI system generating fields at 63.9 MHz (Larmor frequency). The MRI's gradient coils produce 40 dBµV/m field strength at the cable location. To keep induced noise below the monitor's 1 µV sensitivity threshold, the cable needs SE > 60 dB at 64 MHz. A single braid (85% coverage) provides only 45 dB, insufficient. A foil+braid combination achieves 82 dB SE, providing 22 dB margin. The cable uses an aluminum/polyester foil with drain wire plus a 90% tinned copper braid, terminated with 360-degree backshells at both connectors. Transfer impedance testing confirms Z_t < 5 mΩ/m from 1-200 MHz, meeting IEC 60601-1-2 immunity requirements.

Frequently Asked Questions

What are the shield types?

Braid (woven wire, 30-80 dB SE, flexible), foil (100% coverage but thin, 50-60 dB), foil+braid (80-90 dB, best general-purpose), spiral (good flex, poor HF SE), solid tube (100+ dB, no flex). Semi-rigid cables use solid copper for maximum shielding.

How is shielding measured?

Transfer impedance (IEC 62153-4-3): voltage coupled per amp per meter (lower Z_t = better). SE = 20log(Z₀/(2πfZ_tL)). Direct SE measurement per IEEE 299. Transfer impedance preferred below 1 GHz; triaxial methods for higher frequencies.

When use double-shielded cable?

When single braid (40-60 dB) is insufficient: military MIL-STD-461 compliance, medical devices near MRI, sensitive receivers below -120 dBm, or high-interference routing. Foil+braid: 80-90 dB. Double braid: 70-80 dB with better flex.

Cable Shielding