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Re: [TowerTalk] Lightnig Arrestors at tower base or house entrance?

To: K4SAV <>
Subject: Re: [TowerTalk] Lightnig Arrestors at tower base or house entrance?
From: Jim Lux <>
Date: Tue, 26 Feb 2008 11:11:26 -0800
List-post: <">>
K4SAV wrote:
> ...."Is it because of some overvoltage issue on the shield? Maybe 
> letting the coax be the sacrificial fuse (i.e. let it arc
> through the jacket) might be a good approach?".....
> That is exactly what you are trying to avoid by grounding the coax to 
> the tower.  Replacing all the coax on a tower after a lightning strike 
> is a difficult way to implement a fuse.  Pin holes in the coax jacket 
> means death of the coax shortly after due to water entrance.
> Jerry, K4SAV

Indeed.. if the goal is to protect the coax, you're right.  If the goal 
is to protect the equipment inside the shack, and you're willing to 
sacrifice the coax, then...

But an interesting question arises.  Say you have a piece of coax 
running down the tower, and it's connected to the tower at the top 
(where you could put a bulkhead connector where you transition to a 
jumper to the antenna, for instance).   Just how high would the voltage 
get between shield and tower at the bottom of the tower (or at some 
point along the way).

Would it get over, say, 1kV?  (which would be a guess as to the 
breakdown voltage of the jacket)

If there's a direct strike to the tower, I can see the voltage 
differential being pretty high.  But, for a lot of people, the direct 
strike isn't the concern.. they're relatively rare.  The concern is 
often the induced voltage from a nearby strike.  Then that gets into a 
discussion of how near, etc.

But, for discussion, say you have the shield also grounded at the 
entrance to the house, and that the coax runs on the ground, and is 
taped or fastened to the tower.  It's not a very large loop (area wise) 
that's going to pickup the magnetic field from the nearby stroke. If the 
shield resistance is, say, no more than 10 ohms, and the tower 
resistance is the same, then it would take a induced current of 50 Amps 
to get the voltage difference to be 1kV between tower and shield.

Obviously, if you bring your coax from the tower to the house on an 
overhead line 10 feet off the ground over a span of 20 feet, it's a 
different matter.  Then you have a huge single turn coil to pick up the 
magnetic field.

Maybe all this guestimation is totally out of the ballpark.
I know that if I were doing it, I'd probably just put grounded bulkhead 
connectors at the base, if only for convenience.  But, I'd be interested 
if someone can point to an actual analysis or measurement of the system.

Jim, W6RMK


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