Also known as: surge arrestor, coax surge protector, lightning arrester, GDT protector
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A coax lightning arrestor is a small in-line device that sits in the feedline where it enters a building and shunts a voltage surge to ground before it can reach your receiver. Inside is usually a gas-discharge tube (GDT) between the coax centre conductor and a grounded shell: invisible to normal signals, an instant short to a spike. For any outdoor antenna, it is the cheapest insurance you can buy for your SDR’s front end — and for the gear and house behind it.
How it works
Under normal conditions the arrestor is an open circuit across the line: the gas tube does not conduct, and RF passes through with only a small insertion loss, typically a fraction of a dB across HF through UHF. When a nearby lightning strike or a static build-up drives the line voltage past the tube’s breakdown threshold — often around 90 V — the gas ionises and conducts, dumping the transient into the grounded shell and clamping the voltage the downstream equipment sees. Once the surge passes, the gas de-ionises and the tube resets itself, ready for the next event. On many models the GDT is a replaceable cartridge, since a big hit can wear it out.
Crucially, the arrestor protects the centre conductor of the coax. Grounding the mast and the cable shield — which a grounding kit does — leaves the inner conductor unprotected; the arrestor closes that gap.
How it mounts
Install it where the feedline enters the building, bolted to a grounded bulkhead or ground plate and bonded with a short, heavy, straight wire to the same ground rod the mast uses. The short bond is not a detail — a fast transient sees the inductance of every inch and every bend, so a long or curly ground lead badly undercuts the device. Match the connectors to your run: UHF (PL-259/SO-239) is common on scanner and ham gear, N-type on low-loss outdoor cable, and SMA on the SDR end, with adapters as needed from the cables and connectors kit.
Relevance to GopherTrunk
The arrestor is transparent to GopherTrunk — it passes signal and only acts during a surge — but it is what lets you responsibly leave an outdoor antenna connected to a 24/7 Raspberry Pi scanner. A $35 dongle running unattended on a rooftop discone is exactly the setup that a static discharge or nearby strike can quietly kill; the arrestor plus a grounded mast is the standard, inexpensive defence. It has nothing to do with encryption — that is a separate wall — but it keeps the hardware alive to decode everything that is in the clear.
Where to buy
A gas-discharge coax arrestor like the Proxicast (around $25) is the standard pick: low insertion loss, a replaceable GDT element, and UHF/N/SMA connector options to match your feedline. Mount it at the building entry, bonded short and heavy to the mast’s ground.
Pair it with a grounding kit; see the mast and mounting guide for the full safe stack and the outdoor base build for a parts list.
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Frequently asked questions
Do I need a lightning arrestor for an SDR antenna?
If the antenna is outdoors, yes. A coax lightning arrestor (around $20–30) installs in-line where the feedline enters the building and shunts surge to ground through a gas-discharge tube before it reaches your SDR. It is cheap insurance for a $35 dongle and, more importantly, for the equipment and house behind it. Bond it to the same ground as the mast.
How does a coax lightning arrestor work?
Most use a gas-discharge tube between the coax centre conductor and the grounded shell. Under normal signal levels it is an open circuit and passes RF with minimal loss; when a surge spikes the voltage past its breakdown, the gas ionises and conducts, dumping the transient to ground. It is a fast, self-resetting shunt — the same idea as a lightning rod, applied to your feedline.
Where do I install the arrestor?
At the point where the feedline enters the building, mounted to a grounded bulkhead or ground plate and bonded with a short heavy wire to the same ground rod as the mast. Putting it at the entry keeps surge outside your walls; a short, direct ground bond is what makes it work, since a fast transient sees the inductance of a long or bent wire.
Does an arrestor replace grounding the mast?
No — they work together. The grounding kit bonds the mast and bleeds static; the arrestor protects the inner conductor of the coax, which grounding the shield alone does not. And neither fully stops a direct strike — for real storm risk, unplug the feedline. The surest lightning protection is still an antenna cable that is not connected to anything.