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
(http://home.earthlink.net/~jimux/hv/sphgap.htm for the design of the gap
and http://home.earthlink.net/~jimlux/hv/spherev.htm 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. (http://home.earthlink.net/~jimlux/hv/paschen.htm)
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
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