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Author Topic: Rooftop mounted antennas and lightning strikes, your experience ?  (Read 6690 times)
W8JJI
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Posts: 291




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« on: July 18, 2013, 04:51:10 PM »


Anyone ever have a roof mounted antenna on a dwelling, on a tripod get struck by lightning ?

What happened ?

(assuming the antenna and mounting system was grounded ...)

Did the lightning follow the ground wire all the way to the ground rod ? Or did it arc off to other objects nearby ?

What about the 3 tripod legs that have screws or bolts going through the roof ? Did the mounting points for each leg discharge lightning into the space under the roof ? (even though a ground system was in place ) ?

Was there damage around the tripod legs where they contact the roof ?

Was your ground wire laying directly on the shingles of the roof ? Did the shingles underneath the ground wire get scorched  or catch fire ?

Info of your "roof mounted" lightning strike experience would be greatly appreciated !

thanks




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K2UE
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Posts: 45




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« Reply #1 on: July 18, 2013, 05:23:49 PM »

I can't give you the exact case you are looking for, but I can give you a real one:

As a kid helping my Dad do TV antenna installs and repairs, we went to a bar at the base of a steep hill that had put up a TV antenna on an old wooden flagpole, using solid dielectric RG59 (this was the 50's).  There was no explicit grounding.  Lightning shredded the moist old flagpole, leaving a ragged stump and a shower of toothpicks.  The coax jacket was reduced to black streaks on a solid copper tube with a basket weave pattern that once was the shield. Droplets of clear polyethylene could be seen on the outside of the copper tube that had oozed thru the voids where the shield strands crossed over.  The balun at the TV was no impediment -- it entered the TV, melted most of the wiring, arced thru the power transformer, fused the zipcord line cord into rigid copper wires showing thru a melted jacket, and blew out most of the fuses in the building.  The TV was "off".

My conclusion is that lighning will follow the ground wire -- and anything else it damn well pleases.  Open switches might as well be closed -- megavolts don't care.
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KE3WD
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Posts: 5694




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« Reply #2 on: July 18, 2013, 05:30:40 PM »

Our entire large repeater system is on the roof of a building at the top of a mountain. 

Those antennas get hit all the time. 

Proper grounding is all the antennas need, although one time we lost a Celwave Stationmaster antenna.  After the changeout, Dave disassembled that poor stationmaster in his backyard - and poured out nothing but gray powder. 

Ground that roof tower properly, its own ground rod and also tie that ground rod through to the service ground rod.  Welding the copper connections is the way to go nowadays. 

Protect your radios with Polyphasor.  Good stuff. 


73
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KE4JOY
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« Reply #3 on: July 18, 2013, 05:32:27 PM »

Code says any metallic items within 3'-0" of the "system" shall be bonded to the system. FWIW  Cool
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W8JJI
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Posts: 291




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« Reply #4 on: July 19, 2013, 03:22:38 AM »

 Huh

thanks for the responses but the question was about tripod mounted antennas on shingle covered roofs and the affects of a lightning strike on that kind of installation in-particular .

Anyone Huh
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K1CJS
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« Reply #5 on: July 19, 2013, 05:15:47 AM »

I can't point to a specific instance either, but as the lightning and its path go, it's very hard to tell where it will strike.  If that tripod and antenna are the highest metallic objects around, they may get hit--but I wouldn't bet the bank on it either.  It's about electrical potential, and how the potential changes in objects around the object being protected.  

There have been cases of antennas up on a house that haven't been touched--but the old, tall tree in the backyard of that house was hit.  Electrical substations with their masses of steel structures not touched, but a nearby structure was struck.  You can never tell where or when lightning will strike.  Outside of being a saying--it's the truth!  If your house/antenna gets hit, unless you've got massive professional lightning protection--and even then, you're going to get some damage.  It's simply unavoidable.

What most of us protect against is the transients from nearby strikes--the induced electrical surges that a good lightning protection scheme WILL stop before they get to your equipment.  THAT is where a good ground will help you avoid getting damage to your radio system.
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K2UE
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« Reply #6 on: July 19, 2013, 05:53:41 AM »

I can't point to a specific instance either, but as the lightning and its path go, it's very hard to tell where it will strike.  If that tripod and antenna are the highest metallic objects around, they may get hit--but I wouldn't bet the bank on it either.  It's about electrical potential, and how the potential changes in objects around the object being protected.  

There have been cases of antennas up on a house that haven't been touched--but the old, tall tree in the backyard of that house was hit.  Electrical substations with their masses of steel structures not touched, but a nearby structure was struck.  You can never tell where or when lightning will strike.  Outside of being a saying--it's the truth!  If your house/antenna gets hit, unless you've got massive professional lightning protection--and even then, you're going to get some damage.  It's simply unavoidable.

What most of us protect against is the transients from nearby strikes--the induced electrical surges that a good lightning protection scheme WILL stop before they get to your equipment.  THAT is where a good ground will help you avoid getting damage to your radio system.

True.  Sitting in my Dad's farmhouse kitchen, the lightning ignored all the antennas, tall trees and the metal-roofed barn, and came down in a blue pillar to a old cedar grapevine post 15 feet outside the kitchen, shredding it.  The stroke spread sidways along the wet cellulose-core clothesline between the posts, turning it into what looked for all the world like cotton candy.
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K5LXP
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« Reply #7 on: July 19, 2013, 06:03:19 AM »

the question was about tripod mounted antennas on shingle covered roofs and the affects of a lightning strike on that kind of installation in-particular.

It doesn't matter, because even if you found someone with that example, the chances of a different lightning event behaving the same way, even at the same location, are pretty small.  I think the examples you've been given so far are indicative of the unpredictable nature of lightning.


Mark K5LXP
Albuquerque, NM
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KE3WD
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Posts: 5694




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« Reply #8 on: July 19, 2013, 06:48:29 AM »

Huh

thanks for the responses but the question was about tripod mounted antennas on shingle covered roofs and the affects of a lightning strike on that kind of installation in-particular .

Anyone Huh


reread my post above. 

I used the word, "tower" in reference to your roof-mounted tripod, substitute the word, "tripod" there. 


73
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KD0REQ
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« Reply #9 on: July 19, 2013, 07:49:43 AM »

I don't want to find out.  just increased my tripod and mast to 30 feet, for a total of 45 feet HAAT.  the trees are still taller.  but I grounded down with #4 aluminum, clamped to the mast, the tower leg, and thence grounding system.  guy wires are airgapped with insulators, and one goes directly to a cyclone fence rail.

lightning, like water, is going to find its own path.  if you provide it with a very nice path that can carry thousands of amps and is bulldozer-strong, it's going to pile on there.  any interruption is going to allow an opportunity to jump to the next best path.

you don't want a next best path, is the bottom line.  guys with towers have a natural best path, assuming they are nicely grounded off the tower to a good radial system, and especially if they break out their feedlines in a metal cabinet as a breakpoint for flashovers.

I presently have an old Blitz Bug at the bottom of the feedline, as well as a protected balun at the top, but I'm going to upgrade to an Alpha Delta this weekend.  yes, the #4 also carries over to the cable entrance panel.

most of what we need to know is on the Polyphaser web page, and much of the rest at w8ji.com
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K8AXW
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« Reply #10 on: July 19, 2013, 09:05:04 AM »

Call me candyass.....whatever.  Before I would put a tripod mount and antenna on my roof, I'd find another hobby! 

While there are thousands of antenna installations that get hit with lightning almost every day, there is one thing that everyone can agree on.  Lightning does what it damn well pleases!  While it might well follow your nice #3 ground wire to an elaborate ground system, there's always enough left over to jump to something in the attic.  MY OPINION, ONLY.

I ALWAYS pull my lead in cables away from my house when not being used.  So far so good!  I want to keep it that way.  Like I said, "Call me a candyass."
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K7JQ
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Posts: 327




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« Reply #11 on: July 22, 2013, 07:56:26 AM »

Talk about lightning going where it wants. I had a  ground-mounted screwdriver antenna (about 10 feet tall) on a hill 350 feet behind my house (highest metal object in the area, and grounded at the base). Last July, it took a direct hit. The antenna was blown to smithereens, and the lightning travelled down the LMR-400 coax, shredding it along the way, and where the coax bent 90 degrees down the back face of the hill, it leaped out at that point to a fence 25 feet from the rear of the house. At least that's what I assumed it did, as I saw a scorch mark where it hit the fence. This resulted in damage that took out my TV's (on a couple of them only the sensitive HDMI inputs), DirecTV boxes, DVD player, a GFCI outlet, tripped a few circuit breakers, blew up a landscape lighting transformer in the back yard and it's attached light fixtures, and destroyed a string of decorative lights attached to the fence.

Fortunately, the coax was disconnected outside the house, radios were unplugged, and nothing in the shack was damaged. Needless to say, during this Arizona monsoon/lightning season, I took the new replacement antenna off the hill until October, and it will come down again next May/June. Don't want to go through that again :>(

73,   Bob K7JQ   
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N8YQX
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« Reply #12 on: July 24, 2013, 12:10:49 PM »

Those antennas get hit all the time. 

I'm curious, how can you tell that the antenna gets hit by lightening?

I ask because I have lightening protection on my antenna as well.  Since my rig hasn't blown up, its working very well, or my antenna hasn't been hit by lightening.
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73,
N8YQX
WB6BYU
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« Reply #13 on: July 24, 2013, 12:44:05 PM »

If you are in the house at the time, you'll know it when it takes a hit!!
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K0ZN
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« Reply #14 on: July 24, 2013, 10:31:09 PM »

N8YQX, to answer your question.  I know three hams who have taken direct hits as well as one neighbor non-ham.  Here is what HAS happened:

Hy-gain vertical on the roof. Large strike. Melted the antenna into a "puddle looking" piece of aluminum on the roof. Some roof damage (burns). Fortunately,
most of the current followed the ground wire to ground.

Long wire antenna in one of my closest ham friends yard:  The antenna was copperweld wire. It was literally vaporized; disappeared. The two end insulators were just hanging there with small stubs of wire left. Literally, the antenna disappeared.

Tribander on a 50' tower. WELL grounded. The guy is an electrician. Several ground rods on each of the three legs and coax ran inside the tower to ground. The lightning
still followed the coax into the house, damaged most of his ham gear and some home appliances. Coax was partially melted and destroyed.

The neighbor who is a non-ham had about a 1 1/2 ft. diameter hole blown in the roof and fortunately a small fire in the roof. Among other things, the lightning followed some 300 ohm TV twin lead from an antenna in the attic and got into house wiring and killed virtually every appliance in the house. The TV was very obviously physically destroyed. Serious damage.

The above said, there are numerous comments on here by guys with seriously large antennas and towers which have had multiple hits and no equipment damage. If you go to the trouble of grounding EXTENSIVELY and correctly, you *usually* will be OK.

Trust me.... if you take a direct hit you WILL know it !!   A small, "minor" hit may not be so obvious....we had a bird feeder suspended from a steel cable that ran between the
house and a tree. It took a hit that melted the wire and dropped the bird feeder.....that is what clued me into the possible hit. During a storm with lightning and a lot of close
thunder it is hard to tell if a very small hit happens. Big strikes will literally rock your house and the flash will light up your entire house like a flashbulb. Again....you will
have no doubt about what happened.

If one lives in a lightning prone area and you do nothing for lightning protection, you are simply going to the Casino and placing a bet......against the house.

There is no absolute 100% "air tight" guarantee you can control a hit, but proper effort in the ground system will place the odds very highly in your favor. Ultimately, lightning is verysimple stuff. All it wants is a very low resistance and low impedance path to ground. You simply give it what it wants via properly placed and grounded LARGE conductors connected to a low impedance/resistance earth ground. Otherwise, it WILL find ground.....through your house wiring, Cable TV wires, phone lines, water pipes, furnace ducts, etc. etc.

Lastly, something like nearly 50% of all lightning damage comes in on the AC powerline and/or possibly the telephone or cable.  It is extremely important to have you
AC power panel well grounded. A whole house surge suppressor at the entrance panel is not a bad idea either. You really MUST protect ALL utility wires that enter your house.

For whatever it is worth:  I live in a very high/bad lightning area and over the years developed a very healthy respect for it. I also have a large, extensive grounding
system including lightning rods on the house!

73,  K0ZN
« Last Edit: July 24, 2013, 10:58:19 PM by K0ZN » Logged
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