Safety / Grounding Question for Mag-Mount Rooftop Antenna

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Gilligan

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I am in a peculiar situation in that I have access to a commercial building roof that is fairly high up (4 stories) and overlooks a decent area, but I can only put a mag-mount antenna on the roof (nothing more permanent for now). The roof hatch from the 4th floor is about 30 inches square and has seemed to provide a sufficient ground plane for the mag-mount antenna to give me good reception, sometimes up to 60 miles away. I simply run the 14-foot cable down through the roof hatch to my scanner which is running 24/7 in a storage room on the 4th floor.

Here is my question: If there is a lightning storm in my area, am I running the risk of starting a fire by having the antenna up on the roof without any kind of lightning rod, etc...? I am willing to unplug my scanner if it's going to storm (since it's just a matter of going in the storage room) but I don't want to be constantly unmounting and remounting the mag-mount up on the rooftop every time it's going to storm. More importantly, I don't want to cause any damage to the building I'm working in. If lightning were to strike the antenna (which I highly doubt, since it's also not the highest metal structure on the roof), and there was no radio connected to the antenna, but the BNC tip was just dangling from a shelf, what would happen? Am I taking a big risk by having this mag-mount on the roof access door like that? Thanks for any help on this.
 

prcguy

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Its impossible to say what would happen from a direct hit but you might want to ground the antenna per code, see NEC article 810. For TV, satellite and similar antennas on commercial buildings you can usually meet code with a short run (less than about 30ft) of #10 copper wire to the closest metallic conduit on the roof that has a home run to a main breaker box for the building.

That will probably not save anything from a direct hit but at least you can tell the boss your antenna is grounded per code. And I would disconnect the antenna during a lightning storm anyway.
prcguy
 

Gilligan

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For TV, satellite and similar antennas on commercial buildings you can usually meet code with a short run (less than about 30ft) of #10 copper wire to the closest metallic conduit on the roof that has a home run to a main breaker box for the building.
How would I actually connect the wire to the antenna? Where would the connection be made?

And I would disconnect the antenna during a lightning storm anyway.
I will probably just do that since you are recommending it. :)
 

prcguy

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Without having a lightning arrestor with ground lug you could get a big hose clamp to go around the magnet and sandwich the ground wire between the magnet and clamp. The typical ground blocks for TV and satellite antenna feedlines only ground the coax shield anyway.

What you would be doing is placing the feedline at the same potential as the building ground to meet NEC safety requirements. A slightly better approach would be to cut the coax close to the magnet, install a new connector and put a good lightning arrestor there. You could also upgrade the coax into the closet with an inline arrestor outside.
prcguy


How would I actually connect the wire to the antenna? Where would the connection be made?

I will probably just do that since you are recommending it. :)
 

prcguy

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I was fortunate to have some ham and scanner antennas on the roof of my office for the last 17yrs of working and here is a pic of the above ceiling drop into my office. There was a conduit with weather head on the roof that penetrated the roof and the conduit was also bonded to the roof and metal frame of the building.

As the three coax cables exited the conduit inside the building it hit three lightning arrestors (long black boxes in the pic) that were bonded to the conduit and building frame with 3" wide copper strap and #2 green wire. Then it was a short 20ft run into my office.

The building originally had about 75 ground rods and a 500MCM ground ring wire buried around the building and that was bonded to all the I beam supports and frame of the building. The indoor lightning arrestors are not the best solution but they were the second line of defense since two of my three antennas on the roof had separate lighting arrestors outside at the antenna with 500MCM wire straight down to the ground ring.
 
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