How do I ground a shielded cable for maximum shielding effectiveness at RF frequencies?
Cable Shield Grounding
Proper cable shield grounding is arguably the most important EMI control practice for any electronic system. More EMI problems are caused by improper shield grounding than by any other single factor.
Frequently Asked Questions
When should I ground at one end vs both ends?
Ground at both ends for: all RF cables (> 1 MHz). Any cable where EMI shielding is the primary requirement. Long cable runs (the shield length approaches lambda at the highest frequency). Ground at one end for: audio cables (< 100 kHz) where ground loop hum is the primary concern. Very low-frequency instrumentation (< 10 kHz) where thermoelectric EMF at the ground connections is a concern. DC and low-frequency analog signals in environments with large ground potential differences (industrial plants). The one-end-grounded cable is NOT shielded at RF (the ungrounded end has infinite impedance at RF, and the shield is floating). This is acceptable only for audio/LF applications. For mixed environments (both LF ground loop concerns and RF EMI): ground at both ends and address the ground loop separately (use a common-mode choke or isolate the ground loop with a differential receiver).
What about drain wires?
A drain wire is a bare conductor that runs alongside the foil shield in a shielded cable (common in foil-shielded multi-conductor cables). The drain wire provides a path for soldering or crimping the shield connection (the foil itself is too thin and fragile to crimp). Limitation: the drain wire connects to the foil at a single point (not 360°). This is essentially a pigtail. The SE at RF is limited by the drain wire inductance. For RF: drain-wire-only cables provide poor shielding (20-40 dB) above a few MHz. If forced to use a drain-wire cable for RF: connect the drain wire to ground with the shortest possible length, and wrap the foil itself around the connector backshell for additional 360° contact.
How do I retrofit a pigtail with a 360° bond?
Options: (1) Replace the connector: use a proper coaxial connector or a backshell with 360° shield clamping. This is the best option. (2) Add a hose clamp: for multi-conductor cables, a small stainless steel hose clamp or a conductive band can clamp the braid/foil to a metal grommet or panel fitting, providing near-360° contact. (3) Conductive tape: wrap copper tape tightly around the braid-to-ground junction, covering the pigtail. This provides partial 360° contact. Not as good as a proper clamp but significantly better than the bare pigtail. (4) Cable-mount ferrite: place a ferrite clamp over the cable at the grounding point. The ferrite attenuates common-mode current on the cable shield, reducing the voltage at the pigtail. This does not fix the pigtail but mitigates its effect by 10-20 dB.