Braided Shield
Woven metallic mesh providing EMI shielding for RF cables and interconnects
Definition & Operating Principle
A braided shield is a tubular mesh of interwoven metallic wires that surrounds the dielectric and center conductor of a coaxial or multiconductor cable. The woven structure forms a Faraday cage that attenuates electromagnetic interference by reflecting incident fields off the metallic surface and absorbing residual energy through ohmic losses in the wire strands. Unlike solid tube shields, braided construction allows cable flexibility while maintaining shielding integrity through thousands of overlapping wire crossover points.
The braid is characterized by its carrier count (number of wire groups), picks per inch (crossovers per unit length), wire diameter, and braid angle. These parameters determine the optical coverage, which is the fraction of the cable surface physically covered by metal. Higher optical coverage directly correlates with lower transfer impedance and greater shielding effectiveness. Typical RF cable braids use tinned copper, silver-plated copper, or aluminum wire depending on the frequency range, weight constraints, and environmental requirements of the application.
Key Formulas
Optical Coverage (single braid):
C = 2F − F²
where F = (N · d · PPI) / (cosα · π · D), N = carriers, d = wire diameter, PPI = picks per inch, α = braid angle, D = braid diameter
Transfer Impedance:
Zt = V / (I · L) [mΩ/m]
where V = voltage induced on inner conductor, I = shield current, L = cable length
Dual-Braid Coverage:
Cdual = 1 − (1 − C1)(1 − C2)
For two 90% braids: Cdual = 1 − (0.1)(0.1) = 99%
Shield Type Comparison
| Parameter | Single Braid | Dual Braid | Foil | Foil + Braid | Spiral |
|---|---|---|---|---|---|
| Optical Coverage | 85-95% | 97-99% | 100% | 100% | 95-98% |
| Transfer Impedance (DC) | 1-5 mΩ/m | 0.5-2 mΩ/m | 3-10 mΩ/m | 0.5-3 mΩ/m | 5-20 mΩ/m |
| Shielding Effectiveness (1 GHz) | 60-80 dB | 80-100 dB | 70-90 dB | 90-110 dB | 40-60 dB |
| Flex Life (cycles) | 10M+ | 5M+ | 10-50K | 500K-2M | 50M+ |
| Weight (relative) | Medium | Heavy | Light | Medium | Light |
| Cost (relative) | Medium | High | Low | Medium-High | Low |
| Best For | General RF | MIL-spec | Static install | Precision RF | Audio/low freq |
Practical Application
In a typical SATCOM ground terminal, the coaxial cable run from the outdoor BUC/LNB assembly to the indoor modem can exceed 30 meters. Using a single-braid RG-6 cable with 85% coverage might pass residential installations, but a defense-grade terminal requires MIL-DTL-17 cables with dual-braid construction achieving 98%+ coverage and transfer impedance below 2 mΩ/m. The cable shielding must maintain performance through the full 950 MHz to 2,150 MHz L-band IF range while surviving continuous outdoor flexing from wind loading on the antenna pedestal. Engineers specify silver-plated copper braid for these applications because silver oxide remains conductive, unlike the resistive copper oxide that forms on bare copper braid exposed to salt air environments.
Frequently Asked Questions
What optical coverage does a braided shield need for effective EMI protection?
For general-purpose EMC compliance, 85% optical coverage is the minimum acceptable threshold. Most MIL-spec and precision RF cable assemblies target 90-95% coverage using a single braid layer. Dual-braid constructions achieve 97-98% coverage by offsetting the diamond-shaped apertures between layers, reducing leakage by 20-30 dB compared to a single braid at the same carrier count.
How does transfer impedance relate to braided shield performance?
Transfer impedance (Zt) quantifies how much external electromagnetic energy couples through the shield to the inner conductor. A high-quality single braid achieves Zt below 5 mΩ/m at DC and below 20 mΩ/m through 1 GHz. Above 1 GHz, transfer impedance rises due to skin depth effects and aperture leakage. Dual-braid cables maintain Zt below 2 mΩ/m through 1 GHz.
When should you use a braided shield versus a foil shield?
Braided shields provide superior low-frequency magnetic field shielding and mechanical durability through repeated flexing. Foil shields offer better high-frequency electric field shielding above 1 GHz due to their continuous construction but crack under bending. Most high-performance RF cables combine both: foil for complete high-frequency coverage plus braid for mechanical strength and low-frequency performance, achieving 100+ dB shielding effectiveness from DC through 10 GHz.