Why Equipotential Bonding Can Backfire on HF Antennas
PE grounding an antenna is not always necessary when it is close to the home. The braid of the coax already provides a continuous protective earth back into the house ground system. For balcony, attic, or roof-edge installations within about 10–15 m of the electrical entry, this is generally sufficient — adding separate rods or mid-cable clamps often complicates RF behavior without tangible safety benefits.
When Ground Rods Become Necessary
Beyond roughly 30 m from the house, the coax shield alone is no longer an adequate PE path. Soil resistance, lightning surge currents, and local potential gradients make a local rod system near the antenna essential. Without it, surge and fault currents will try to return on the coax shield, stressing equipment and increasing risk.
Close to Home: When Equipotential Plays Against You
Near the house, everything metal tied to PE — balcony railings, mast brackets, window frames — sits on the same equipotential. At HF, those bonded objects can behave like parasitic elements, detuning the antenna, skewing patterns, and increasing common-mode pickup. In other words, the “perfect” equipotential that’s great for safety can be too perfect for RF.
Fix: Keep Safety DC, Block RF with a PE Choke
The practical cure is to preserve DC/low-frequency safety bonding while introducing high impedance at HF in the PE bond path. You can do this with a ferrite choke on the PE conductor so nearby bonded metal remains safe, but is much less “visible” to RF.
Recommended Hardware & Turns
Use a Type 31 split-core clamp (25 mm ID) on the PE lead. Wind 6–9 turns through the core (looping the PE conductor through the opening repeatedly) to raise HF impedance while leaving DC continuity intact.
Values below are indicative ranges for well-packed turns and good lead dress. Actual results vary with wire routing and installation.
| Turns | ~3.5 MHz | ~7 MHz | ~14 MHz | ~21 MHz | ~28 MHz |
|---|---|---|---|---|---|
| 6 turns | ~1–2 kΩ | ~2–3 kΩ | ~3–4 kΩ | ~2–3 kΩ | ~1–2 kΩ |
| 9 turns | ~2–3 kΩ | ~3–5 kΩ | ~4–6 kΩ | ~3–4 kΩ | ~2–3 kΩ |
Rule of thumb: Use the fewest turns that give you ≥3 kΩ around the band(s) of interest. Keep the loop area compact and avoid long pigtails to prevent adding stray inductance.
Practical Notes
- Safety first: The PE choke should be on a short, direct PE lead. You are adding HF impedance, not removing the safety bond.
- Don’t cut the coax to “tap PE”: Use bulkhead plates and proper bonding; mid-cable clamps on braid create leakage paths and corrosion risks.
- Combine with a 1:1 CMC on the coax: A proper common-mode choke on the feedline plus a PE choke on local bonds reduces both antenna detuning and shack noise.
Mini-FAQ
- Do I need PE at the balcony? — The coax braid typically suffices. Extra PE runs can couple HF unless you choke them.
- When do I add rods? — When the antenna is ~30 m or more from the house, install local rods near the antenna.
- How many turns on the clamp? — 6–9 turns on the Type 31 split-core (25 mm ID) covers most HF cases.
- Will the choke affect lightning grounding? — It adds HF impedance, not DC resistance. You still need appropriately sized bonding conductors and surge protection.
Questions or experiences to share? Feel free to contact RF.Guru.
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