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Long ago, it was fairly standard to provide lightning protection for outdoor radio antennas by installing a large double-throw knife switch -- antenna leads to the center, radio on one side, ground on the other. Double pole for a two-wire feed, single for a plain long wire for a receiver.

This, installed outdoors with suitable thermal protection between the switch/wires and house structure, seems like it ought to still be a good method, and one that's in reach of the cash-strapped starting amateur or SWL. Just step outside and throw the switch before operating and after shutting down, and your antenna becomes a grounded wire (I've read fairly recent research that suggests lightning rods actually repel lightning by grounding off charge build-up on high points of the protected structure; this would work the same way).

Now, heavy knife switches are less visible than they used to be, but they're still around -- look inside any industrial high-voltage or -current shutoff box, and you'll find a knife switch with copper bus bars as thick as a heavy chef's knife and as wide as your thumbnail; on three-phase, it'll be three pole.

Further, knife switches are well within the capability of a hobbyist to build, though heavy gauge copper sheet and bar isn't as easy to find as it once was (and costs plenty) -- one could source all the needed materials in the plumbing section of the local Big Box home improvement store (copper pipe can be flattened to make bus bars and contact clips).

Clearly, there's something missing from my picture of this kind of lightning protection, for it to have been replaced with (potentially sketchy and impossible to genuinely test at the consumer level) gas discharge systems and similar high voltage switching.

Are knife switches to alternately connect my antenna to the radio or to ground obsolete for good electrical or safety reasons, or have they just fallen victim to profit motive on the part of equipment makers and sellers?

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The dimensions of a typical "knife switch" make it well suited to the high impedance of the open-wire or "ladder" transmission lines which were ubiquitous "Long ago." While the wide spacing between the switch's "protected" and "operating" positions provides a significant level of lightning protection, it would also present a substantial impedance discontinuity to a low-impedance coaxial transmission line, which could create unwanted reflections.

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  • $\begingroup$ Not at 160 thru about 20m, Brian. :-) Think about it. $\endgroup$ – Mike Waters Jul 28 at 22:03
  • $\begingroup$ "Think about" what, exactly, Mike? $\endgroup$ – Brian K1LI Jul 28 at 22:14
  • $\begingroup$ Aren't they an insignificantly small portion of a wavelength, especially on 80 and 40m? $\endgroup$ – Mike Waters Jul 28 at 22:24
  • $\begingroup$ I guess it depends on the spacing. Mine was 1". $\endgroup$ – Mike Waters Jul 28 at 22:39
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    $\begingroup$ I cobbled together some transmission lines in SPICE. The effects are much more pronounced at 30MHz than at 3MHz, which may highlight another reason they were popular long ago: when did hams get privileges above 20 meters? $\endgroup$ – Brian K1LI Jul 28 at 22:43
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Well ... IMHO, they have not totally vanished.

Just because you don't see them for sale on the Web or in QST, doesn't necessarily mean that they are not in common use.

This ceramic knife switch and lightning arrestor were at my previous QTH, for my ladder-line-fed multiband 125' inverted-V dipole. I bought the knife switch at a local hardware store. Lots of these are for sale on eBay.

Ceramic knife switch


Lightning arrestor.

The red wire in the center is ground.

If I use this here again, I would put static drain resistors across the gaps.

OWL lightning arrestor

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  • $\begingroup$ That lightning arrestor seems similar to the "spark plug" arresters I've seen in drawing form from long ago. $\endgroup$ – Zeiss Ikon Jul 29 at 11:07
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A regular household light switch is actually an enclosed knife switch. Those are spring-loaded so they close completely every time. If you want to ground your antenna, put a SPDT light switch in a weather-resistant NEMA enclosure and wire it up.

Classic open-air knife switches are hard to find and not that cheap to build. As you point out, copper is more expensive than it used to be. Home workshops are harder to find, too.

Exposed to weather, copper will corrode quickly. Corroded or half-closed knife switch connections can have a lot of resistance.

Exposed knife switches don't meet electrical safety standards from the past few decades, so good-quality ones just aren't made. Those "mad scientist" switches in the 1940s electrical labs just aren't available now. Electrical service shutoff enclosed knife switches are good, but don't ground and cost about $100.

Knife switches are a pain to use with coax, because you need to break out the coax to the knife switch, then go back to coax to get to the rig. The knife switch will add a big impedance discontinuity with associated reflections.

This all assumes a good ground, which isn't cheap. I expect only a few hams actually have a quality safety ground where the RF enters the shack. That would be two 8-foot ground rods 4 feet apart, with 4 gauge wire run between them. For details, see the (great) ARRL book "Grounding and Bonding for the Radio Amateur".

http://www.arrl.org/shop/Grounding-and-Bonding-for-the-Radio-Amateur/

With a coax feedline, it makes sense to spend your money on a coax switch that grounds unconnected feedlines. You can also use a lightning arrester with a gas tube and a DC blocking capacitor. Use the right voltage rating gas tube for the power level.

If you are going to walk outside, just disconnect the coax. This is much easier if you use BNC or even Type N connectors. Those connect and disconnect more reliably than UHF connectors. It is too easy to get a bad connection with a UHF connector.

Push-on UHF connectors are also popular. At $6, this one is a lot cheaper than any knife switch.

https://www.americanradiosupply.com/uhf-male-female-push-on-quick-coaxial-adapter-connector-pl-259/

Equipment doesn't disappear because of the "profit motive". That actually keeps popular stuff available. Equipment disappears because not enough people want it.

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  • $\begingroup$ "You can also use a lightning arrester with a gas tube and a DC blocking capacitor. Use the right voltage rating gas tube for the power level." If these aren't the $30 items I see on GigaParts and Amazon, I don't have any idea how to identify what I actually need. But for that, I need to ask another question, don't I? $\endgroup$ – Zeiss Ikon Jul 28 at 2:48
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    $\begingroup$ @ZeissIkon I buy my GDTs from Mouser. The large ones are less than three bucks each. $\endgroup$ – Mike Waters Jul 28 at 21:50

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