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Author Topic: Opinions on proximity to lightning rod ground wire  (Read 11518 times)

Posts: 79

« on: March 11, 2012, 08:28:15 AM »

We have the standard no outside antennas in our HOA clause.  Being busy with work and other activities, I haven't been on the air in a long time.  Now retired and new equipment on the way, I have to think about antennas.  The house is L shaped and has lightning arrestors along the ridge.  (I need them, the house has taken a direct hit before).  What I'm wondering is the possibility of running a dipole along side the wire that connects the lightning arrestors, but I'm wondering what effect, if any, the ground wire would have being so close to the antenna.  I was thinking about having it just a couple of inches from the ground wire, and since these don't face the street, they would be away from prying eyes.  Anyone have any ideas?  Also, what would be a safe distance were I to fire up the Drake L4B. 

The other possibility would be to use a vertical antenna.  When they put in the swimming pool, they electrically tied all the rebar together and then attached it to the common house ground for the the A-C and lightning arrestors.  Would this act as a giant radial?

Oh, if you ask me any questions, it will be a couple of days before i get back to you.  I'm currently in India, but on my way home.

Posts: 120

« Reply #1 on: March 11, 2012, 10:22:46 AM »

Wouldn't putting a wire next to the lightning arrestors on the ridge put your equipment at risk of being fried if you had another strike?

Posts: 7718

« Reply #2 on: March 11, 2012, 11:14:07 AM »

Mounting the vertical close to the grounded wire will cause an almost equal current to travel in a direction opposite to the vertical current. This will cancel much of the antenna magnetic field and a lower radiation resistance will be the result. So, the vertical should not be placed close to the grounded wire.

Additionally a lightning strike to the lightning rod(s) (think 100,000 amps) will induce significant current in the antenna.

So what do you do? For lightning protection the top of the antenna must be lower than the ground rod(s). A lightning rod is said to protect out to a distance that is 20X its height. If the rod is 5' taller than the antenna the antenna can be 100' away and be protected, according to this theory. I would not want to push it this far.

I wonder if a low antenna such as the MFJ-1782 mounted close to the ground and oriented vertically might be suitable? A ground screen under the antenna can reduce ground losses.

For maximum lighting protection the coaxial cable should be routed to the AC service ground where it enters the house and then to the radio. This method eliminates the ground loop that is formed when the radio equipment is connected to two separated grounds.

For example, say the antenna coax is routed directly to the radio equipment and the radio equipment is powered off the AC service. When the antenna is hit by lightning, or lighting current is induced into the antenna by a nearby lightning strike, current travels to the radio via the coax shield and through the house AC wiring to the AC service ground.

Now take the case of the coax routed to the AC service ground first and then to the radio. If hit by lightning the current will travel along the coax shield to the AC service ground. With the radio having no other ground lightning current will not travel through the house AC wiring.

Posts: 79

« Reply #3 on: March 17, 2012, 05:43:12 AM »

Dave, thanks for all you pointed out.  I never thought about what would happen if the lightening rod took a hit. 

It sticks in my mind that when they did the installation that they tied the ground for the lightening rods to the A/C ground as well.  We have sandstone about 2' down, which I got to see first hand when the track-hoe dug the pool.  So what I had the lightening arrestor installers do was to bury copper plates at the corners of the house.  I need to check and see for sure.

After 5 weeks in India without any rain I was getting ready to go to the airport, when we got a quite spectacular display in an electric storm.  We have an apartment on the 5th floor of the Leela in Gurgeon so I got quite a view.  It reminded me how serious lightening strikes can be.  (Also how much shaking there is in a 4.9 earthquake - glad I don't live in 6-land).

I had a Hi-Gain 18HT when I lived in Poughkeepsie that I always kept an external ground on when I wasn't using it.  One time when I forgot to ground it, there was an electrical storm and the lightening strikes, which were not even really close were inducing so much into the tower that it was arcing to ground.  Some serious stuff to just be static electricity.
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