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Re: [TowerTalk] grounding compromises

Subject: Re: [TowerTalk] grounding compromises
From: K4SAV <>
Date: Sun, 06 Aug 2006 21:14:52 -0500
List-post: <>
Most people on this reflector are used to thinking in terms of grounding 
towers, not dipoles. The same rules apply to dipoles, but there is 
another problem with dipoles that puts it into the hard pile.

Most any kind of grounding arrangement will protect you against close 
strikes or static discharge, but if you are unfortunate enough to 
receive a direct hit, here is the most likely outcome:

The difficulty in protecting a dipole is in controlling where the 
current is going to go, if hit by a strike. Coax and ladder line will 
not carry the current of a lightning strike.  When those vaporize, you 
will be left with an ionized trail where the feedline was. Of course, if 
that feedline goes directly into your house, guess where that ionized 
trail will be.  If it goes directly to your entrance panel, even a well 
grounded one, the outcome will probably not be good.  A direct strike on 
the side of your house is not good. When the feedline vaporizes, and you 
have only the ionized trail left, other fingers are likely to split off 
and hit anything else that looks like a good ground, such as your 
chimney, outside faucets, electrical outlets, or rebar in your concrete 
blocks. These small secondary strikes can do a lot of damage.

If you really want to provide protection for a dipole you have to do a 
lot of work. First you need to move the place, where the main strike is 
going to go, away from your house.  That means your dipole should not be 
close to your house.  You have to provide a conductive center support 
that can handle the lightning current, and run your feedline down it.  
At the base of that center support you need a grounding system that is 
the same as you would install for a tower. Then you route the feedline 
to the house after grounding to the radial system. You also need the 
single point grounding system you have heard discussed on this reflector 
many times.

Now you know why many people don't provide lightning protection for a 
dipole. If you happen to have an inverted vee hanging off your tower, 
this protection is easy. Also with an inverted vee off a tower, the 
tower is much more likely to be hit than the inverted vee.

Whether you should install such a system depends a lot on where you live 
and the amount of lightning you have. I think if I lived on the west 
coast, Wa, Or, or Ca I wouldn't bother. If lived at my previous location 
(exact center of highest density lightning strikes in the US) there is 
no way I would put up a dipole without it.

Jerry, K4SAV

David Talkington wrote:

>Hash: SHA1
>Salutations --
>I'm trying to arrive at the most appropriate grounding and lightning 
>protection solution for an 80m dipole at my small residence.
>The house electrical system ground is at the opposite end of the house 
>from the station.  To reach it from the bulkhead would require about a 
>50 foot run of either wire or strap, and would require three bends 
>around corners.  There is concrete around the perimeter for the entire 
>distance, and no crawl space or basement for that portion of the trip, 
>so a straight line isn't possible, and I can't put the entry point there 
>To avoid relying on this relatively high-impedance path to ground, I 
>have driven a copper-clad steel bar into the ground near the bulkhead, 
>and have a 10 foot, nearly straight line to it from the bulkhead.  The 
>coax will have a lightning suppressor inline at that point.  The entry 
>is into the basement, and runs up a couple of feet to the first floor 
>where the station is.  I will bond the water pipe (which is yet another 
>path to ground), heating ducts, and house electrical ground in that area 
>to the station ground.
>So ... there are now at least three paths to ground in the structure: 
>the electrical service ground, the copper water pipe, and my 
>supplemental electrode near the coax bulkhead.  This creates the 
>possibility of a potential or rise time difference between grounding 
>points.  Is this worth concern, or simply a necessary compromise in a 
>residential structure pressed into service as a radio station?  How have 
>others approached similar challenges?
>Cheers -d
>- --
>David Talkington
>PGP key:
>Version: GnuPG v1.4.5 (GNU/Linux)
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