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Re: [TowerTalk] Grounding a vertical

To: "Mike Bragassa, K5UO" <>,<>
Subject: Re: [TowerTalk] Grounding a vertical
From: Jim Lux <>
Date: Sun, 26 Mar 2006 07:39:43 -0800
List-post: <>
At 06:43 AM 3/26/2006, Mike Bragassa, K5UO wrote:
>...speaking of lightning:
>What is a simple and effective ground for my base-loaded 80 meter vertical 
>(Rohn 25g)? "Spark gap": How can one be made and what gap is correct?

In a uniform field gap (which you are not likely to get, but more later), 
breakdown, in air, at sea level, is about 70 kV/inch (or 30kV/cm).

Almost any practical gap will breakdown at a lower voltage, depending on 
how nonuniform the field is, how rough the surfaces are, etc.

A sphere gap (two smooth spheres separated by much less than the diameter 
of the spheres) will have a breakdown slightly lower.  There's no simple 
analytical equation for it, so most resort to tables 
( for the design of the gap 
and for the table).

A needle gap (basically, anything with reasonably sharp points) will 
breakdown at about a third the voltage of the uniform gap.  call it around 
25 kV/inch or 10kV/cm.

Below around 350V, gap breakdown won't occur, regardless of how low the 
pressure or spacing. (

If your air density is lower than standard sea level, the breakdown voltage 
is reduced proportionally (at 18,000 ft, the breakdown will be half, at 
5000 ft, about 80%)

The challenge in RF applications is getting a gap that has a close enough 
spacing to breakdown at a low enough voltage, but that doesn't have 
significant L or C, so that your circuit isn't perturbed.

A couple bolts with the right spacing works fairly well, and is reasonably 

>Mike, K5UO
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