Copyright © 2002, 2006 by Harold Melton, KV5R. All Rights Reserved.
I constructed the shunts as follows: Using a piece of 1/8th-by-1 inch aluminum flat or angle stock, cut to about four inches long, drill three holes. Enlarge the two outer holes until you can force-thread two spark plugs into them. Use new, non-resistor-type plugs. Run a 1/4 x 4-inch bolt down through the center hole and place a nut underneath. Now grind a couple of flats on the last inch of the bolt, so it will clamp easily in your ground rod clamp.
Attach the ladder-line as follows: Measure and strip the incoming line and jumper lead. Crimp and solder uninsulated ring lugs. Apply a little grease and then place them under the spark plugs’ caps. Tighten securely with pliers, but be careful not to twist off the small stud on the plugs! Seal the tops off with silicone caulk or Coax-Seal. Be sure to insulate all connections, particularly if you have curious children around, to prevent RF burns.
Note: A gap of .025 will not arc RF at 100 watts on any band. Higher power may require a wider gap, so watch your SWR meter carefully the first time you QRO on each band.
If you want the lightening arrestors to arc at a lower voltage, you’ll need to obtain a couple of gas-filled arc shunts (commonly used in coaxial arrestors), or obtain commercial MOVs rated for RF service.
NOTE: No arrestor will protect your radio! All are meant to arc at a much higher voltage than your radio can stand. Always disconnect during storms! The arrestors shown here are meant to keep the disconnected jumper from arcing inside the house - not to protect the radio!
Figure 1 (Ooooo lookie! RF can change directions 180 degrees!)
How Does it Work?
I have not had a direct lightning strike. The spark plugs should arc across the .025 gaps and shunt inductive impulses (nearby strikes) to ground, and they probably have done so on many occasions. In a direct strike, the arc current will probably ramp up until it vaporizes the ladder-line, which being swagged from the tower, should fall open and might interrupt that leg of the pulse. The tower, about 50 feet away and also grounded, should then ramp up current until it is destroyed or the charge is dissipated. In other words, three successive failure modes are designed to interrupt part of the strike and channel the vast majority of it to ground, away from the operating position. Well, in theory! I do not hope to ever test that scenario. Still, it’s probably better that no planning at all. Remember that, once a lightning arc starts, it converts the air into plasma, which is highly conductive, and nothing will open the arc until the energy dissipates to the point that the power flow will no longer support the plasma state, at which point it converts back to regular gas. (Plasma is simply gas that is so hot that the electrons are stripped from the nuclei. With most of the electrons being free, it becomes a highly conductive gas).
Note that an inductive impulse that would arc the plugs will still damage the radio if the feedline remains connected. Spark “arrestors” are not a replacement for disconnected feedlines! All spark-gap shunts work because the feedline is disconnected, providing a much wider gap (several inches) than the small gap of the device, thus forcing an arc at that path of lower resistance, before the voltage can climb higher. I assume that the plugs should arc at about 10-15 kV, since they do so in engines. That will still ruin your radio if connected.
If I did it over, I’d plant a 4x4 post several feet (maybe 8-10) from the house, bring the ladder-line down it, and place the arrestor assembly there, with its own ground rod. This would carry a large, direct-strike pulse to ground at a greater distance from the house. The jumper would then cross to the operating position, suspended near the ground. This would be a tripping and mowing hazard, but would probably (in theory) shunt the arc to ground at a distance sufficient to keep it from vaporizing the side of the house.
Another idea I have not yet tried is to run the ladder-line underground, by twisting it, wrapping it in bubble wrap, and stuffing it into six-inch PVC sewer pipe. It should work, if the line is well balanced and kept centered in the pipe. This would be done to provide an easy full-stroke arc path to ground starting at some distance from the house, say, 20 feet. The pipe would have to be totally waterproof on both ends. Why not just use coax? Too lossy at high SWR’s! We non-resonant antenna users benefit greatly from ladder-line, in spite of its drawbacks. The most important consideration is to try to keep it from becoming the path for a fat, billion-amp arc right into your operating position!
If you want to get fancy, mount the spark plugs in a piece of brass or copper bussbar and then braze it directly to the top of the ground rod. If dampness or snow becomes a problem, just push 3-inch lengths of old garden hose up onto the threads of the plugs. Keep weeds and grass trimmed below. Making a plastic housing from a food container would be easy, but I have not found it to be necessary.
Are these ideas totally safe? No. Are they better than nothing? Probably, but that is just an untested assumption. The best lightning protection measure would probably be to design an entry point which opens with a window, so that you can just open the window and toss the whole feedline about 25 feet away from the house. My designs are probably a reasonable compromise between safety and convenience.
I now use an outdoor balun, with coax to a grounded bulkhead. Please see 80-Meter Doublet in the 2018 section. Lots of pictures!
I am using dipole wire 12 AWG. Polyethylene coated 259 strands (I bought it from wireman.com) is there better wire I can buy tinned copper would be good better than plain copper wire let me know on what to buy or should I stick with wireman antenna wire thanks. Paul
Better, in what way?
Any wire will work fine if it’s large enough to handle the current, and strong enough to not break.
High-strand count wire is very flexible, easy to handle, typically used for portable operations (QRP backpackers, etc).
I can see no reason to use tinned wire for an antenna.
For most home QTH, permanent antennas, regular 19-strand #14 THHN is all you need. It would be even better if it still cost $25 a roll instead of $90…
Copper-clad steel is better for very long antennas with no center support and a heavy balun and feed line in the middle.
Okay it sounds good for wire for a dipole. I have a muti-band dipole I use it on 160, 75, 40 meters if I feed it with ladder line about 500 or 600 ohms how much ladder line should I use for 160,75,40 meters I have a manual tuner Palstar AT5K and a Palstar auto tuner what length of ladder line would be the best for this set up on transmit and receive when I tune up the dipole with ladder line thanks. Paul PS.for more information I can tell you more I will have to write them and send them to you thanks again
Ladder line length is well-covered, from 3 sources, on page 3 of this article. https://kv5r.com/ham-radio/ladder-line/ladder-line-page-3/
In the event of a direct lightning strike, the best you can hope for is that the feedline vaporises faster than the connected equipment is destroyed (the feedline will work like a fuse). Disconnecting everything, if possible, is well advised.
Trouble is, by the time the line is vaporized, lightning already has a plasma conductor going…
Here’s how I did the spark plug method: 2 lengths of copper pipe driven into the ground, spaced apart to match feedline, with spark plugs screwed into the open end of the pipe, tips down (inside the pipe). The pipe and the plug need to be matched for a very snug fit such that the plug will cut its own threads on the way in. 1/2, 5/8, or 3/4 inch will accomodate most any spark plug.
Alternately you could flare the end of the pipe a bit or cut a couple expansion slots and add a stainless steel hose clamp to tighten the joint. This keeps the tip clean, dry, safe from people, pets, dry grass, etc. Topside I replaced the plug’s screw-on caps with brass hardware and just made a wire loop around the screw without cutting (no joints), between 2 washers, tightened with a rounded cap nut. Finish with a little sealant at the nut//plug joint. Alternately one could crimp or solder a spade lug to the wire and clamp that under the nut but it’s one more connection (to fail!) Top it all off with a scrap of tubing (old garden hose works) for safety. Neat, clean, fairly foolproof maintenance-free, and you can get it apart easily if needed. 73 – K4HSB
Putting spark-plugs in top of driven pipes, I’d be concerned with moisture condensate in the pipe saturating the the ceramic insulator around the center electrode of the plug. I know if plugs ever get wet, they short out and no amount of cleaning and drying will fix them. I had the head on tractor leak coolant a few years ago, and after repairs, I had to put new plugs, the old ones simply would not fire anymore, and I tried everything to dry them out.
For that reason I’d prefer plugs in a bracket, with short open-bottom tubes on them, and silicone caulk or similar on the upper parts, to keep them dry.
But your installation may be fine, just watch your SWR & see if the plugs are getting damp.
Yes, don’t want water in there! Haven’t had any problems but I forgot to mention to seal the pipe — close the bottom off with a pipe cap or by crimping in a vise + solder. You can get an airtight seal which if done on a dry day *might* stay that way. A little silica gel in the pipe could help too. Mine gets full sun which doesn’t hurt either. .025″ works fine for my 100w. 73 – Jim
I see Wireman sells a device similar to your description.
Great article. Thanks!
I want to run a 1100′ feed line to the top of a hill behind my house. You mention being able to purchase spacers to make your own. Can you point me in the right direction to buy those?
Well, if you poke around in usplastics.com they have some uv-resistant pe tubing that might be good for ladderline spacers.
Update (2019): See my 80-Meter Doublet in the 2018 Projects section; I used Zareba (or Fi-Shock) Ribbed-Tube insulators; they are made for electric fence wire, 4″ long, UV resistant. ~$15 for 200, and you’ll also need a bag of 11″ x 1/8th” Ty-Raps to go through the middle of them, twice.
There is also a product called Ladder-Snap that is made for #14 insulated wire.
Have you measured the resistance of a spark plug? I have heard that they are low current devices and have resistors inside to protect the spark coil. In case of an interaction with lightning I would be concerned that a high resistance would convince the current to ground itself elsewhere.
I used non-resistor spark plugs, like a common lawn mower plug Champion J8J. They put resistance in automotive plugs to reduce radio interference – it slows down the rise-time a little and thus makes less RFI. The coil is current-limited by its own internal impedance (and in some cases like my old ford tractor, an external resistor in series with the coil).
But back to spark-gaps. It’s gonna have 2 modes of operation: (1) dissipate a low-amp induction pulse from a nearby strike, or (2) steer a direct strike to a (hopefully) safer path, in which if the amperage ramps up that much, it’s gonna blast your feedline and sparkplugs to smitherines anyway, ionize a conductive plasma path, and finish its drain (hopefully) along the same path. The nearest thing to that path should (again, hopefully) be the top of your ground rod.