static discharge on radio tower

[blueangel-eric]

a few yrs ago during a thunderstorm i heard on the scanner a fire dispatch for a radio tower sparking. i hurried down there and beat the FD there. sure enough it was sparking. this is a FM and AM broadcast tower with several insulators inline on the guy wires. it seems one guy wire was getting static electricity and the sparking was from the electricity jumping the insulators. now i didn't see any lightning strikes on the tower but it was sparking over and over for at least 10 mins while i was there and before i was there and after i left. is it normal for a tower to get repeated charges like that? it was at least 2 times a minute. the fire department called the radio station and then left the scene after making sure nothing was on fire. i think they ruled it as being static electricity.

 

[Don_Burke]

We get some interesting static buildup on our towers.

It sounds like that station might have the spark gap set wrong. The insulators should not be the first thing to arc.

 

[zz0468]

I've once heard a tower humming like a hive of bees from a continuous static discharge. This was during a thunderstorm, but the tower never did get hit. The buzzing faded away after a few minutes.

 

[kb0nly]

If the tower was properly grounded, or in the case of it being isolated and loaded and a proper spark gap as Don mentioned, then it shouldn't happen that way. That's a good sign actually that something is wrong if the guy wire insulators are arcing over before something else does.

 

[blueangel-eric]

We get some interesting static buildup on our towers.

It sounds like that station might have the spark gap set wrong. The insulators should not be the first thing to arc.

what is the spark gap and what do you mean?

 

[kb2vxa]

AM towers are series fed, that means they are the radiating element(s) and are not grounded whether or not there are any other antennas on them. Those by the way are fed through isolators so they and the transmission lines going up the tower become electrically part of the tower. At the base there is a horn gap (shaped like a V) so that a lightning strike may jump and discharge to ground, then any RF arc created may extinguish by traveling upward to the point it can no longer jump the gap.

That's the way it goes if all is well, if there is a problem like the grounding strap became disconnected or broken the juice finds another path like over the guy insulators. If they were arcing at the rate of two per minute that tower was accumulating one heck of a charge! I'm surprised it didn't or maybe it did damage the tuning network in the dog house, phasor (if it was a directional array) or even the transmitter. All things considered those guy insulators were probably damaged or carbon tracked and need replacement. Their purpose is to break up the steel cables into non resonant lengths so they become invisible to RF.

 

[Scott_PHX_APP]

Many years ago, I had a small antenna on the top of a vent pipe about 2 feet from the roof of our trailer. The coax was run down the side of the unit and in under the floor into the room where I plugged it in to my scanner, (not connected at the time). We had a thunder storm moving in, but not overhead yet, still about 20 miles off. As I was walking by the room I heard what sounded like water dripping from the roof to the floor and in a puddle? I looked all over the celing and couldn't find a wet spot and besides, it wasn't raining! After listing real close, I ended up at the end of the cable, (old Motorola type connector) yep, it was arcing over from the center to the shield... Nothing a 1 Meg resistor didn't fix...
Later...

 

[richardc63]

I've once heard a tower humming like a hive of bees from a continuous static discharge. This was during a thunderstorm, but the tower never did get hit. The buzzing faded away after a few minutes.

Yes, I've witnessed the same but with two riggers on it at the time. I couldn't understand why they were all but "running" down the ladder! They got the hint when a spanner through a spark... It never got hit that day but there was a severe storm nearby.

Richard

 

[blueangel-eric]

AM towers are series fed, that means they are the radiating element(s) and are not grounded whether or not there are any other antennas on them. Those by the way are fed through isolators so they and the transmission lines going up the tower become electrically part of the tower. At the base there is a horn gap (shaped like a V) so that a lightning strike may jump and discharge to ground, then any RF arc created may extinguish by traveling upward to the point it can no longer jump the gap.

That's the way it goes if all is well, if there is a problem like the grounding strap became disconnected or broken the juice finds another path like over the guy insulators. If they were arcing at the rate of two per minute that tower was accumulating one heck of a charge! I'm surprised it didn't or maybe it did damage the tuning network in the dog house, phasor (if it was a directional array) or even the transmitter. All things considered those guy insulators were probably damaged or carbon tracked and need replacement. Their purpose is to break up the steel cables into non resonant lengths so they become invisible to RF.

so what kind of antenna is the AM antenna then? So are you saying the tower itself is the antenna if it's the radiating element?

 

[mancow]

Yes, it's one enormous antenna.

Think of the size of the wavelength at those low frequencies and then it makes sense.

 

[blueangel-eric]

Yes, it's one enormous antenna.

Think of the size of the wavelength at those low frequencies and then it makes sense.

i always though it was a big wire antenna or the guy wires that acted as the antenna.

 

[kb2vxa]

Yes, an AM broadcast tower is actually a ground plane in the truest sense. The tower is the radiating element working against the Earth itself with radials either laid on the surface or slightly buried only to improve ground conductivity, unlike a raised ground plane they are not active elements. Nope, the guy wires only support the tower being insulated from it and broken by insulators into non resonant lengths so they don't interfere with the antenna itself.

Certain ELF antennas are the exception, one design is the "umbrella" where the guys are electrically connected to the tower, continuous and insulated from ground just above the anchors. It is as the name implies an umbrella, you need a really huge antenna when you get down around 16KHz where the early wireless stations operated. I used to live near Tuckerton NJ where one operated, just Google "tuckerton wireless" for some rather interesting web sites on the subject.

 

[kb0nly]

I've seen two types of AM setups, one where the tower is loaded and one where there is a LONG wire antenna element on standoffs on the tower that's loaded. Both really cool to check out, especially the matching in the doghouse.

 

[ipfd320]

its amazing that u all turned me on to this...believe it my inlaws live approx 1/2 mile from the tuckerton transmitter site off of radio rd...and i know the bldg well...