How to protect an antenna from lightning?

I don't think there is any absolute way to protect from lightning strikes, especially direct ones. I had one at home and wiring was vaporized. Someone else can explain the concept of good grounding, but it is important to have the ground rods as close as possible to your tower and make sure that it is electrically sound. A good grounding system allows your tower's electrical charge to vary with that of the earth. By keeping the charge the same a lightning strike is less likely to take place.

It's hard to protect a transmitter which has to be mounted on the antenna, but you might consider running your antenna lead through an in-line protector. It was designed for CATV, Satellite and TV coax. It probably wouldn't be enough to protect your transmitter from a direct strike, but it might make a difference if the lightning hit nearby. From my experience even a nearby hit can be devasting as far as damage goes. If you go back through this forum you'll find some posts about isolating your audio equipment and power leads from your antenna/transmitter. That could prevent a serious injury or even a fire from lightning.

A study of the operation of lightning shows, strikes are intiated from a protrusion attached to the ground with the opposite potential of that in the cloud. The sharper the protruding object the more likely the lightning strike will occur. A lightning strike's made up of nearly unlimited voltage and mega-amperes of current, certainly enough to melt sandy soils and disintegrate large trees.

Ground to cloud strikes, as near as several hundred feet, can totally destroy all kinds of electronic equipment (including audio gear, transmitters, ect.). The only sure fire way to eliminate the possibility is to "unplug" all such equipment from commercial AC service (the power company), unplug all the RF feedlines from transmitters/receivers and unplug any connections to wire telephone equipment. Make sure all cable ends are away from combustibles and other equipment. Lightning is looking for the easiest and shortest path to ground.

My expereince has been, radio stations DO NOT survive direct strikes. Something WILL get destroyed. I have replaced thousands of dollars in commercial broadcast equipment as a result of "non-direct" lightning strikes. Even months after the strike, I have had to replace and repair equipment weakened by an electrical storm. After a direct strike, you put out the fire and clean up the mess.

Disappative ground systems, large low-inductance conductors (copper strap), spark gaps, gas discharge tubes, lightning chokes, Static-Cat charge diffusers, MOV shunts and other methods are used widely to protect equipment that DOES NOT take a direct hit.

Bottom-line: You can take limited measures to prolong equipment life in lightning prone areas of the country. However, if your numbers up, its up. Ka-Boom! Put out the fire, clean up the mess and rebuild. Part 15 AM transmitters and antennas are only "cannon fodder" in the face of a lightning strike.

Marshall Johnson, Sr.
Rhema Radio - The Word In Worship
http://www.rhemaradio.org
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So if I can equalize the charges between the ground and the antenna then I will improve my chances of survival, right?
Would it help to connect a 2 - 3 Megaohm resistor between the antenna and my grounding system? Or would I lose a lot of signal doing that? It seems like that would help equalize charges.

And thanks to everybody who has replied!

Large wire-wound resistors are used for static discharge drains on tall towers. Unless you have winds exceeding 20-30 miles per hour and humidity below 20 percent on a consistent basis, adding a static discharge resistor is a waste.

To use the same component for lightning protection is also a waste of time and money. Lightning is super high frequency energy seeking a path to earth. Anything, and I mean anything, in the way will be destroyed. The resistor becomes a fuse. After it burns apart, what happens to the follow strike?

A high current, high inductance choke (several millihenry) would help the most with static discharge. However, the reality is, the Part 15 AM antenna is a "sky-needle" by its very construction. And as such, the higher the antenna in relationship with surrounding terrain, buildings and trees makes more of a difference than anything else.

Equalizing charges would be impossible to accomplish. One potential is in the cloud, the other is on the ground. The build up of a charge in the thunder cloud actually develops the charge on the ground. When the potential on the ground is great enough, the lightning begins on the ground, tree, building, antenna or whatever, and begins to discharge from ground to cloud. The cloud follows by discharging causing the visible lightning bolt.

Here is the smart thing to do. Drive four or five 8 foot ground rods into the ground approx. 8-12 feet apart, covering as much ground as possible. The tops of the rods should be 4-6 inches below ground level. The ground rods should be the copper coated variety, not the galvanized (zinc coated) variety. Connect the ground rods with 3-4 inch wide copper strap (available from a variety of vendors online) using heavy duty bolts or clamps. Use enough strap to connect all the rods and at least two of the tower legs together electrically. This system helps to dissipate and spread the discharge out (it has to handle a HUGE amount of electrical current). Then, you can connect your transmitter ground to the system. Your AM transmitter will still probably turn into a briquet after a strike, but at least you have a chance of not having the house burn down.

Please understand, when it comes to the destructive potential of lightning, there is nothing you humanly can DO to discourage or stop it. Even follow strikes and finger strikes in your vicinity are potentially very dangerous.

Marshall Johnson, Sr.
Rhema Radio - The Word In Worship
http://www.rhemaradio.org
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Just a bit of reminiscing. I've been playing with radio since my teens 55 years ago, and just completed working 50 years on weekends with the local radio station. When I was a teenager in the mid 50's a lot of people still had their big 30's and 40's consol floor model radios which required outdoor antennas, or aerials as they were then called, 50 to 100 feet long and at least 20 feet high. One of the antenna items sold and used then were lightning arrestors. These contained two carbon blocks with a piece of mica in between, one block connected to the antenna, the other to ground, the theory being that any high static discharge to the antenna would choose the ground route instead of the radio. Bell telephone used a similar device on home phones and may still do. Maybe such a device would work with a part 15 antenna, not for direct lightning strikes, but heavy static discharges that may occur and otherwise do damage. Jim B

I remember replacing those devices. The phone company calls them "line fuses". They work good on devices that are primarily "voltage" driven pieces of equipment (phones, amplifiers, ect.). You know, good ole electron tubes.

Today however, most devices are solid-state and current driven. If the least bit of high voltage were to be present on the junction of a solid state device (transistor, FET, JFET, MOSFET or integrated circuit), it would simply weaken and fail. I have replaced tons of them for that very reason. A metal oxide varistor, shunted across the line out or in, helps to knock down the destructive voltage. However, I have found nothing helps in the face of a direct strike. Poof! Pow!

Which brings me to the "Electronic Theory of Smoke". All electronic devices work on smoke not electrons. The proof is, when you hook something up wrong and you let the smoke out the device, it quits working. Try it out. Its fool proof. On second thought, don't try it out.

Thank you for the walk down memory lane.

Marshall Johnson, Sr.
Rhema Radio - The Word In Worship
http://www.rhemaradio.org

We called it the "Smoke Test". If the equipment doesn't (smoke), it passes!

Frank
www.easthillradio.com

Frank,

So, you have seen the smoke...and it is not your friend. Or is it?

Marshall Johnson, Sr.
Rhema Radio - The Word In Worship
http://www.rhemaradio.org

Marshall,

I've seen smoke more on more projects than I ever want to enumerate!

Am I mistaken, or did we work together at KPIC?

Frank
www.easthillradio.com

Frank,

Is that really you? Another reject from mountain top TV. Have you made the leap into hobby broadcasting yet? Well, it looks like you have. Good show. Send me an email at the station website and let me know how to get a hold of you.

Marshall Johnson, Sr.
Rhema Radio - The Word In Worship
http://www.rhemaradio.org

Boy, what a small world. Frank (Easthill Radio) and I used to work together over 20 years ago at KPIC TV in Roseburg, Oregon. He, another fellow at the station and I used to talk for hours on ham radio back in the old days. Radio and television is a very small fraternity, no matter at what level. We meet again after wives, kids and years of time back at the microphone (this time on the internet).

Marshall Johnson, Sr.
Rhema Radio - The Word In Worship
http://www.rhemaradio.org