Fig. 1 shows a hand-built arc gap, sometimes referred to as a ball gap, for use in AM broadcast transmission systems. Gaps like this are proven technology to “arc over” from RF to ground and help protect AM broadcast equipment against high voltages from static electricity and lightning strikes.
My preference is to put one at the 50 ohm input and another at the antenna of all antenna coupling networks. They also should be installed in AM phasors where transmission lines enter from each tower. Another good place for one is at the input side of the phasor where transmitter feed lines come in. The idea is to limit the maximum voltage to a safe non-destructive value by diverting excess energy to ground. The project I am describing is “on the cheap.”
EASY TO DO
Two arc gaps of different sizes are shown in Fig. 2, but the theme is the same. The larger “L” piece is a standard plated corner brace, sometimes called corner iron or angle bracket, available from local hardware stores. Brass acorn nuts are used on each end of the gap, which is good practice for this kind of device. They have a smooth rounded face where the arc should occur.
I used a 1/4-20 tap to put threads in the bracket. A 1/4-20 machine screw, with lock washer and nut, allows for adjusting the gap dimension without laying hands on RF at the other side of the gap.
The smaller L bracket is made of aluminum and is also threaded to accommodate plated brass hardware. I use non-ferrous metals in RF circuits because they will not vibrate at the radio frequency, heat up and sometimes melt.
Don’t laugh — it has happened in high-power systems. You gotta think about these things!
If in doubt, use an ordinary bar magnet as a test tool. You should not use any hardware for RF if it is attracted to a magnet. Yes, the larger angle bracket is steel, but it is not on the RF side; it is on ground and not conducting RF.
A 2×3/4-inch square porcelain insulator is bolted to the two L brackets. The large arc gap has a 3×1-inch diameter insulator. The bottom bolt for each insulator has a flat head and is recessed to be flush with the bottom surface of the bracket. I do this by using a much larger drill bit to provide a bevel where the mounting hole is. Yes, there are fiber or nylon washers at each end of the insulator to help prevent breakage, especially during temperature changes. Porcelain is brittle and it cracks instead of giving. Also, I like to round off corners, if for no other reason than to prevent injury as my hands work on the device.
Construction of these devices assumes you are handy with tools and like to build things. Parts for this project came out of my junk box but would cost less than $10 new. Machining and assembly time for each one was about 30 minutes. (Maybe I should have been a machinist instead of a broadcast engineer!)
Nice, new arc gaps are available from several sources, including Kintronic Labs (kintronic.com). One of their most popular models is the AG-3-1.5B, which sells for $185.
INSTALLATION & PLACEMENT
Two steel bolts with lock washers and nuts will hold this arc gap assembly down, usually to a metal surface. That surface needs to be at ground potential or a wire must be connected to that end of the arc gap to put it at ground potential.
It is especially critical to install an arc gap wherever the system has a Delta brand antenna ammeter or similar toroid sample transformer. See Fig. 3.
The best location is on the antenna side of the sample coil where lightning is most likely to come in. Run the RF conductor as close to the center of the transformer as possible. Remember, lightning will take the shortest path.
That conductor to transformer bushing distance should be larger than the arc gap spacing. If the transformer has a black mark across its white bushing, chances are it has been hit by lightning. The usual symptom is the sample output voltage will double, causing major metering problems.
Arc gaps of this type should be used inside, not out in the elements. In addition, you want to mount them in such a way that the acorn nuts are horizontal from each other. That will allow an arc to clear quickly. Mounting an arc gap vertically can lead to a continuous arc that does not quench easily.
One important attribute of this design is the arc distance is held constant over time because of its rigid mountings. You don’t want it to be flimsy and cause problems.
DON’T TRY THIS AT HOME
A Jacob’s Ladder, not to be confused with the movie of the same name, is shown in Fig. 4.
Jacob’s Ladders are sometimes used as arc gaps in high-voltage situations to naturally and dramatically quench arcs after they start. This one has #6 solid copper wire where the arc occurs.
Not having any lightning available at the moment, I connected a neon sign lighting transformer and carefully turned on the power. The arc you see started at the lower narrow part, then progressed up the wider space until it popped off the end and went out.
SETTING THE ARC GAP
How much spacing between points is enough or too much?
Nautel’s transmitter people researched the subject and published a paper; you can access the PDF at tinyurl.com/rw-nautel-gap. The company is big on lightning protection to enhance the reliability of their transmitters.
Here is more from another Nautel paper:
All of that assumes 50 ohms impedance with no reactance at sea level. Wider spacing is required for higher altitudes. Diameter of the ball (acorn nut) plays into this as well. High positive or negative reactance can be a real wildcard in the equation. AM modulation, as you know, adds 50 percent or more RF power on peaks and of course more voltage.
For me, it is easier to set the gap wide, then slowly reduce the spacing while the station is fully modulated. It can be an exciting moment when the arc occurs! I then back off the gap to twice the distance and tighten it down.
Storms with lightning can even happen during winter months in Minnesota. It is surprising but true that we get thundersnow. That’s right, snowstorm conditions in dry air can bring high static charges and lightning at times.
Make it part of your routine maintenance to check all arc gaps for cleanliness. You might be surprised to find gaps badly pitted from arcing. Filing the edges smooth might be required. Be glad they helped save the equipment while sacrificing themselves in the process. It is cheap insurance and makes perfect sense.
Mark Persons, WØMH, is a Certified Professional Broadcast Engineer and has more than 40 years’ experience. His website is www.mwpersons.com.
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