How does the shield construction of a coaxial cable affect its shielding effectiveness?
Cable Shielding Comparison
Shielding effectiveness is the ratio of electromagnetic field strength without the shield to the field strength with the shield, expressed in dB. Higher SE means less signal leakage and less external interference pick-up. The shield construction determines SE through two mechanisms: reflection (the shield surface reflects incident electromagnetic waves) and absorption (induced currents in the shield dissipate energy).
| Parameter | Semi-Rigid | Conformable | Flexible |
|---|---|---|---|
| Loss (dB/m at 10 GHz) | 0.8-2.5 | 1.0-3.0 | 1.5-5.0 |
| Phase Stability | Excellent | Good | Fair |
| Bend Radius | Fixed after forming | Hand-formable | Continuous flex OK |
| Shielding (dB) | >120 | >90 | >60-90 |
| Cost (relative) | 2-5x | 1.5-3x | 1x |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
Does braid coverage percentage directly predict SE?
Not directly. Going from 90% to 95% coverage improves SE by about 10 dB (not 5 dB) because the gap area decreases non-linearly. Going from 95% to 98% adds another 5-8 dB. Above 98% coverage, the braid is extremely dense and approaches semi-rigid performance.
Is foil shielding better than braid?
Foil provides 100% coverage with no gaps, giving excellent high-frequency SE. However, foil is thin and fragile, providing poor mechanical durability and limited low-frequency SE (thin conductor = high resistance). Foil + braid combines the high-frequency coverage of foil with the mechanical strength and low-frequency conductivity of braid.
What about corrugated outer conductors?
Corrugated outer conductors (used in conformable and some flexible waveguide-style cables) provide SE between braid and solid tube: typically 80-90 dB. The corrugations allow bending while maintaining a continuous (though not smooth) outer conductor. They are a good compromise between flexibility and shielding.