The Coax-to-Earth Clamp Myth: When Good Intentions Kill Your Choke
It looks so reassuring: a shiny clamp around the coax, tied to a ground rod. "That’ll keep RF away from my shack," people say. Sadly, this little trick is more snake oil than science. In reality, it’s one of the fastest ways to re-introduce common-mode currents right back into your station.
Why People Do It
On paper it sounds great: ground the coax shield, dump noise into the earth, keep the station quiet. The problem? RF doesn’t behave like DC plumbing, and coax clamps don’t work like magical drains.
Why It Fails in Practice
- Most antennas already provide a DC path to ground. Verticals with tapped coils, EFHWs with UNUNs, loops, or dipoles fitted with a bleeder network are all DC grounded. A plain center-fed dipole, however, is not.
- Placed behind a common-mode choke, the clamp kills it. We see this often: operators ground their radial plate, then add a clamp after the choke. The choke becomes defunct, return currents happily float back to the shack, and RX noise has the time of its life.
- Ground loops galore. By tying coax to PE earth again, you create multiple return paths. Congratulations: you’ve just built an RF highway directly into your radio room.
Advanced: When a High-Z Earth Reference Actually Helps
Use this only on the antenna side of your choke. Putting any earth bond after the choke bypasses it.
- RX lines: Add a bleeder 220 kΩ–1 MΩ from shield to a small RVS ground rod, then place your CM choke, then run the feedline to the shack. Optionally add a GDT (90–230 V) to the rod for surge protection. No capacitors—at HF they become low-Z.
- EFHW counterpoise (far end): A bleeder 10–47 kΩ (2–5 W) from the tail to a long RVS ground rod gives a lossy, high-Z reference that tames float and static without handing RF a low-impedance return. For high power, favor the higher end of that range.
- Hardware hygiene: Avoid Cu↔RVS direct contact; use tin-plated lugs/bimetal interfaces. For aluminum interfaces, apply Würth AL-1100.
The Sarcasm Section
If coax clamps really worked, every RFI problem could be solved by hammering more rods into the lawn. Imagine it: an RF-proof moat of copper stakes! Sadly, RF ignores our plumbing fantasies and follows current distribution, not garden décor.
What To Do Instead
- Ground the mast for lightning and safety. That’s non-negotiable.
- Use proper chokes to stop common-mode currents at the source.
- Keep your grounding strategy simple—avoid redundant clamps that undo the work of your choke.
Mini-FAQ
- Does clamping coax to earth reduce RFI? — No. It usually re-introduces common-mode noise, especially if done after a choke.
- Isn’t more grounding always better? — Not in RF. Too many “extra” grounds just create loops that invite noise.
- So I only ground the mast? — Yes. Ground the mast for safety, choke the coax for RF. Simple and effective.
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