Quarter-Wave Stub: Best Lightning Arrestor for Monoband
For single-band systems, a quarter-wave shorted stub is the most robust, low-cost lightning arrestor you can deploy on coax. Properly cut and bonded, it is electrically invisible at the operating frequency yet provides a near-zero-ohm path to ground for lightning impulses. No consumables, no moving parts, and—unlike GDT cartridges—no “one-and-done.”
A stub is not a substitute for bonded masts, entry panels, equipotential grounding, and AC/mains surge protection. It complements them.
How a Quarter-Wave Stub Works
- At RF: A shorted ¼λ coax section transforms to a very high impedance, so the radio sees almost nothing.
- At DC and lightning surges: The same stub looks like a short, instantly dumping surge energy to ground.
Limitations to Keep in Mind
- Single band only: A 20 m stub won’t protect you on 40 m or 10 m.
- Length critical: Must be cut precisely to ¼ λ (including coax velocity factor).
- Grounding is everything: The stub is useless unless tied into a proper grounding system.
- Antennas with built-in DC ground: Some commercial models (e.g., Procom, Kathrein) already have a DC-grounded radiator. This handles static buildup and slow surges, but the stub still adds extra protection against the nanosecond rise-time of lightning impulses.
- Regulated sites: Commercial and broadcast stations may require UL/IEC-certified surge devices. A DIY stub may not be legally acceptable, even though technically effective.
How to Terminate a Quarter-Wave Stub
The stub is a length of coax with the far end shorted. That short (inner conductor bonded to shield) must then be tied directly into the ground system:
- Bond to a copper rod, ground bar, or entry plate with the lowest possible inductance path (wide strap, minimal bends).
- Install it at the coax entry point so energy is shunted before entering the shack — placement at the antenna feedpoint is not required.
- Keep leads short: Lightning rise-times are nanoseconds, so every centimeter of wire adds inductance that reduces effectiveness.
Frequency Range of Usability
Quarter-wave stubs work reliably from LF through UHF. Above a few GHz, the required lengths become very short and practical construction tolerances, losses, and coupling make them less effective. In HF and VHF repeater or contest-band scenarios, they remain the most durable solution.
When to Use a Quarter-Wave Stub
- Repeater sites: Fixed-frequency VHF/UHF installations where reliability is critical.
- Monoband contest stations: Dedicated arrays for 20 m, 40 m, etc.
- Remote stations: Where maintenance-free protection is a must.
Mini-FAQ
- Does a quarter-wave stub work on multiple bands? — No. It’s strictly single-band, tuned to one frequency range.
- Is it really better than a PolyPhaser? — For monoband use, yes. It never wears out and costs almost nothing.
- Can I make one myself? — Yes. Use coax of the same type as your feedline, short the end, and cut to ¼ λ × velocity factor.
- Do I need a stub if my antenna is already DC-grounded? — Yes. DC grounding handles static and slow surges, but the stub handles the fast lightning impulse.
- Where does the shorted end go? — Bond the shorted inner+shield directly to a ground rod or entry plate with a low-inductance strap.
- Can I use this in a commercial setup? — Technically yes, but regulatory bodies often require certified surge arrestors. A DIY stub may not meet UL/IEC compliance.
- Does the stub have to be at the feedpoint? — No. The ideal place is at the shack entry point, bonded into the station ground system.
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