Spark Plug to arrest static

Thread Starter


Joined Jun 30, 2022
Hello my friends. I saw this pic below which is claiming that this set up with a car sparkplug will stop static electricity going into the bottom cable.
1) Is this setup true or fake? Will this work?
2) If it is a true setup, then where does the static electricity flow to? Where is the 'lower potential'? Is the lower potential, the air at the end of the spark plug? In order for a spark to occur, we do need a second terminal with lover potential so that the static electricity can jump to that other end. Isn't that correct?


Thank you my friends.


Joined Feb 24, 2006
1) The picture shows that the setup is not fake. It exists.
2) What happens depends on the construction of the spark plug

Static discharges come from a high potential with very little current. When the potential on the center conductor of the cable system goes to extreme levels, that allow a spark to form across the gap, the potential is discharged to the cable shield, the aluminum box, and a driven ground in the lower right-hand corner of the aluminum box.

As long as the potential on the center conductor does not exceed the threshold set by the spark plug gap (0.020" typical) signal energy will bypass the spark gap and proceed down the cable system. What may be less obvious is that every impedance discontinuity provides an opportunity to produce attenuation and reflections, I'm assuming the presence of a transmitter in the system and these reflections, if not controlled, could be a problem.


Joined Mar 14, 2008
It only drains static electricity that has a voltage greater than the breakdown voltage of the spark-plug electrode gap, so it's mainly for protecting against large static voltages, such as could build up on an antenna during an electrical storm.


Joined Mar 10, 2019
It will technically work, as did earlier lightning arrestors that used spark gaps. However, for modern radios, spark gaps don't fire until there is thousands of volts on the center lead. That will blow most modern solid state radios. Most modern lightning arrestors use gas discharge tubes, and a 100W radio will benefit with an arrestor with a 230V or 350V gas tube. For regular receivers (receive only, no transmit), most modern arrestors will have a 75V gas tube in them.
Spark gap technology was ok when radios used tubes, those could withstand a small surge in voltage until the gap arced over. Modern solid state equipment should never rely on a spark gap protection device. One could use the same setup as in the picture, but solder in a gas discharge tube instead for better protection.
Remember that spark plugs used to fire at 10,000V to 20,000V. Modern ones may work at less firing voltages, but you still need thousands of volts to arc across the spark plug gap.
One could use the spark plug design as a front end as long as you used a modern arrestor between the spark plug and the radio. That is, have two protection devices between the antenna and the radio. One for a large voltage, the other to handle the residual voltage to a lower, safer level.


Joined Feb 19, 2018
Spark plugs often have a much larger gap (or resistor) inside the ceramic portion too, so theres that to consider. A better approach would be to replace the spark plug with a sharp pointed ground wire ALMOST touching the wire the plug is currently soldered to. That way, RF doesn't see the spark plug path and you still retain the same (IMO useless) spark gap. With how easily new radios are damaged by over voltage, a lighning strike at your neighbors place is enough. It don't take kilovolts or a direct strike, it only takes a dozen volts or so depending on the radio. I'm still waiting for someone to come on the forum here, on qrz, or anywhere radio people are, and excidedly report how their spark arrestor saved their modern gear. I don't think such stories exist outside of tube equipment.

As for static, just use a matching network that provides DC ground.


Joined Feb 19, 2018
There are many kinds of plugs out there. I figured pictures were easier than following links.

Resistor plugs are common (my generator uses them), but I only saw a gas-filled plug once. I forget what it came out of, but when it broke, the contact inside wasn't snapped. It had a squarely machined end on both broken pieces that would obviously not reach eachother so I figure that had been one of those gas-coupled capacitive plugs.


Joined Feb 19, 2018
The damage from a lightning strike is something that no amount of preparation can prevent. But you can go a long way toward preventing the strike in the first place. You can make lightning not want to strike there by installing lightning rods (i used to think they were for lightning to hit.... I was wrong, its cool how they work).


Joined Sep 30, 2009
I forget what it came out of, but when it broke, the contact inside wasn't snapped. It had a squarely machined end on both broken pieces that would obviously not reach eachother so I figure that had been one of those gas-coupled capacitive plugs.
I think different. Back in the dark ages of my youth they sold plugs that were like that, a small internal spark gap, for cars that burned oil. The extra spark gap was supposed to make the actual spark stronger, to burn off the fouling.

Dirt bikers did the same type of thing to clear a fouled plug, shut off the motor, pull the plug wire and let the plug boot still on the plug, restart the engine and let it run till the sound showed that the plug was no longer fouled. All of this was done with the bike not being ridden while doing it. Some will try to say it doesn't work but I did it and it did work.


Joined Jan 15, 2015
It looks like an antenna selector switch using Amphenol PL259 connectors. The line to the receiver / transmitter has a spark plug across the line, a spark gap. The idea being the spark plug acts as a lightening arrester. It serves the same purpose as a commercial production one of these. May help for a lightening strike in the general vicinity but if lightening direct strikes an antenna in that network you can figure everything is toast. Anyway I see a home brew lightening arrester. :) Will it work? To a point but not for a direct hit on any antenna out there selected or not.



Joined Aug 21, 2008
Agreed it looks like a sparkplug being used as a lightning arrester. The telephone company here attached a more professional spark arrester to my phone line where the line crossed the property line.


Joined Jan 23, 2018
IF the spark plug used has just an ohm or two of resistance between the connection and the gap electrode, THEN there is some chance of it providing some protection. BUT most spark plugs do have an internal resistor to reduce radiated noise. And a 10K ohm resistor in series with the spark gap prevents it from protecting very well. So it should be very easy to check the resistance and discover that it is a resistor plug and offers no protection at all. OR it is SOILD CONDUCTOR PLUG THAT CAN PROVIDE A BIT OF PROTECTION.