Lightning strikes

[AmpedNow]

All of this severe weather got me thinking about this...

How often are broadcast towers hit directly? And are certain towers more prone to strikes than others?

It would seem that a 2,000 ft tower in the middle of Texas or a tower farm on a very high mountain could not not be struck during virtually any storm. But say, a 300-foot AM tower or an FM antenna on top of a moderate sized building could avoid being hit for a long period of time.

Also, does RF itself draw lightning strikes? And how do typical ground systems keep the equipment from being fried?

I've noticed that small market stations seem to be more prone to being knocked off the air during storms. The major markets seem to almost never go off for any length of time.

Just some random questions...

[The-Hammer]

I've noticed that small market stations seem to be more prone to being knocked off the air during storms. The major markets seem to almost never go off for any length of time.

That's mostly a function of large market stations being able to afford complete auxiliary transmission sites. Not to mention more complete engineering staffs to carry out emergency repairs.

[genlock]

Larger towers (1,000ft or so) will have a larger mass to dissipate a strike. Also generally a good lightening ground, which is different from an rf ground (radials). Just the larger foundation of a large tower helps quite a lot. Those towers cost a lot and part of the expense for a TV transmitter is surge suppression on the incoming AC lines. Expensive but worth it. Most of the large towers will be struck several times during each storm that goes over. Sounds like someone hits the tower with a hammer when it does. It is not always the strike that causes the direct damage. the inductive field will kill a lot of stuff too. Don't think that when your engineer is at the transmitter he is goofing off, he is usually working hard to insure that he is not called out in the middle of an important nap or football game. Grounding is mundane but important.

[Tom Taggart]

Not the size of the station, but how well it is engineered. Our tower gets minor strikes all the time, we have several ground radials off the tower and studio ground into the leach bed behind the building.

We've lost three things to lightning strikes over the years:

1. A system 90 (on its last legs anyway) when a burst followed a cable TV feed into the building.

2. Output chips in our Auditronic console. We drove the (at the time) 8100 through about 25 feet of audio cable which is buried between the main building and the transmitter building. Cured by placing a pair of 111C transformers on the output of the console.

3. Most recently, the switch on our internal network got fried from a burst that---ta-da! followed in on the shield of the TV cable. Also fried the cable modem. Found ground wire attached to TV cable at entry point to the building--followed ground wire only to find it ~on the ground~ not attached to anything.

[engineer]

Many times smaller and medium market stations have no auxiliary power generators. You would have to have them at the studio and transmitters. Most of the time it’s a loss of power that causes stations to go off during storms. Most if not all major market stations have generators that automatically switch upon power loss. They also have two, sometimes three transmitters.

RF does not attract lightning. I don't care what anybody says, lightning is an unknown science. AM towers are the worst since they are insulated from ground at AC, but have a small DC path for static. The taller AM towers can have more problems than shorter towers. Not because being taller equals more strikes; but being taller does mean more lighting signal (rf portion) is received by the tower. Some sections of lightning strikes are AC, while others are DC. The RF side of lighting (around 100 kHz) can cause havoc with ATU's that may be resonant with those frequencies. The tower bandwidth and network Q must be designed to be non-resonant with lightning frequencies. (I'm fighting one of those battles now). For years ground radials served as the main ground on many AM towers. Studies over the last twenty years have shown a system of ground rods in addition to the radials is a much better system. Of course, a system of rods around the transmitter building is also a must. One problem often found at transmitter sites is the station ground is not common with the main power panel ground / neutral. This can spell disaster when a lightning or surges enter on AC lines.

[Mr. Jones]

Here are some wicked cool shots of the WVAH tower getting hit time after time the other day.

june4promo3.jpg

With some video goodness to go along with it.

That tower has to be good for something.

[AmpedNow]

Awesome vid! Awesome page! Thanks!



I guess that box thing is their HD antenna. Rock 105 is supposed to be up there somewhere.

Cool beans.

[AmpedNow]

Most of the large towers will be struck several times during each storm that goes over. Sounds like someone hits the tower with a hammer when it does.

I actually witnessed this from about a mile away from a TV tower one time. Sparks flew off the tower, and it sounded more like a pop-boom than a hammer.

What about antenna farms on high mountaintops? There's not a lot of tower, but the height is still there.

[amayo]

Back in the day at the Great 98 in Fairmont, we had a lightning strike on the STL tower at the studios. It managed to fry the STL, processing, and studio equipment before it was through. Went on air from the production studio for about 3 days before we got everything repaired.

The only outward sign of any trauma to the equipment rack, was a small scorch mark at opposit corners of the top and bottom. Inside--devastation....don't f**k with mother nature.

[Clay JD Walker]

Scared the crap out of me while at WQBE in my first month doing 7-mid. All of that spring-time storm crap hit, and I was in the middle of a break, and lightning hit the transmitter tower. Didn't take me off the air, but I did have to replace my headphones after a loud 'pop' blew them up. Of course, it was worse on my ears, and I'm certain the listener had a blast hearing me attempt to recover with a pant load.

That stuff doesn't happen here...we're HD, so we don't monitor off-air, just a faked processed program feed.

But, I can always punch up the off air and hear my breaks as if I were dead...spooky.