What is the transfer impedance of a shielded cable and how does it relate to shielding quality?
Cable Transfer Impedance
Transfer impedance is the gold standard for quantifying cable shielding quality. It is measured per MIL-STD-1377, IEC 62153-4-3, or similar standards.
- 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
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Frequently Asked Questions
Which cable should I use for a sensitive receiver?
For maximum shielding: (1) Semi-rigid coax (best): Z_T < 0.1 mΩ/m at all frequencies. SE > 100 dB. Rigid, not suitable for frequent flexing. Use for fixed installations between the antenna and the receiver. (2) Double-shielded flex cable: Z_T ≈ 1-10 mΩ/m. SE > 70 dB. Flexible. Use for test cables and connections that need occasional flexing. (3) Foil + braid (RG-6 type): Z_T ≈ 5-50 mΩ/m. SE > 60 dB. Good balance of shielding and flexibility. Use for general instrumentation and CATV. (4) Single braid (RG-58): Z_T ≈ 10-500 mΩ/m. SE = 50-75 dB at low frequencies, degrading at high frequencies. Adequate for low-frequency applications (< 100 MHz) but not recommended for sensitive microwave receivers.
How does connector quality affect Z_T?
The cable Z_T is only part of the shielding system. The connector and the cable-to-connector transition can dominate the overall shielding: a poor connection between the braid and the connector body adds transfer impedance locally. A pigtail ground connection (braid gathered into a wire attached to a ground lug) adds 10-100× more Z_T than a 360° clamp connection. Best connector connection: the braid is clamped 360° to the connector body (standard for SMA, N-type, BNC coaxial connectors). The effective Z_T of the connector joint: < 1 mΩ for a properly installed coaxial connector. Worst: the braid is hand-soldered to a drain wire with a pigtail to a ground terminal. Z_T can be 10-50 mΩ or worse. For high-shielding applications: always use coaxial connectors with 360° shield bonding. Never use pigtails.
Does cable length affect shielding?
Yes. A longer cable has more surface area for external field coupling. The coupled voltage is proportional to cable length: V_coupled = Z_T × I_external × L. A 10 m cable picks up 10× the voltage of a 1 m cable (for the same external field). Conversely: the SE (in dB) decreases by 20×log10(L_longer/L_shorter) for a longer cable. Doubling the cable length reduces the SE by 6 dB. For sensitive applications: use the shortest cable possible. If a long cable run is unavoidable: use a higher-quality cable (double-shielded or semi-rigid) to compensate for the length penalty.