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I have an antenna that's well above the treeline, and I've heard I can add some small amount of protection by placing two bolt heads a short distance from each other, one connected to the antenna, the other connected to the ground rod.

What distance should I have the bolt heads apart to induce spark over on lightning strikes, but not interfere with my normal transmissions? I won't be using more than 100W in the HF bands with this antenna.

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    $\begingroup$ Wikipedia says the electrical breakdown of air occurs at around 4-30kV/cm. You might want to find some other sources before trusting that, though. Also, 1000 Volts will easily zap your radio even if most of the discharge is grounded. $\endgroup$ – Paul Dec 7 '13 at 21:09
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    $\begingroup$ Possible duplicate of How can I protect equipment against a lightning strike? $\endgroup$ – a CVn Dec 8 '13 at 12:59
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One enthusiast reports, ".029" spacing for a KW station, and .045" spacing for 2.5 KWs"

Keep in mind that a lightning arrestor doesn't stop an electrical discharge event, it merely shunts most of the energy to ground. There's still a lot of damaging current that ends up in the wire which will damage attached equipment. See How can I protect equipment against a lightning strike? for a better overview of all aspects of lightning protection.

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    $\begingroup$ Also, regardless of gap spacing, wouldn't this depend heavily on the ground used as well? $\endgroup$ – David VK2VXK Dec 10 '13 at 23:08
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    $\begingroup$ @DavidVK2VXK Yes, a spark gap is only as effective as its ground. $\endgroup$ – Adam Davis Dec 10 '13 at 23:09
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    $\begingroup$ I've seen illustration of using a spark plug (from a gasoline engine), inset into the top of a copper pipe driving in the ground, to provide the gap. This is within the gap range that can be set in conventional side-electrode plugs. $\endgroup$ – Zeiss Ikon Jan 11 at 14:35
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Get a two terminal GDT and call it a day, use very short low inductance leads to attach it between the center conductor and earth ground. GDT (Gas Discharge Tube), Bourns make them, Digikey or Mouser may have them. GDTs are high-impedance, very fast-acting, very high-energy devices for surge protection. MOV (Metal Oxide Varistor) devices may have low enough off-state leakage to parallel the GDT - pick a 600VDC device. These two together should really nail down any spikes. You will find lots of application information at the Bourns and Little Fuse web sites.

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    $\begingroup$ I tidied this up as best as I could. Please don't type in ALL CAPS. $\endgroup$ – Scott Earle Jan 11 at 7:42

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