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David Robbins K1TTT (ex KY1H)
k1ttt@berkshire.net or robbins@berkshire.net
http://www.berkshire.net/~robbins/k1ttt.html
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Message-ID: <35492148.BBA190F8@berkshire.net>
To: <towertalk@contesting.com>
Date: Fri, 01 May 1998 01:11:36 +0000
From: David Robbins <k1ttt@berkshire.net>
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To: Pete Smith <n4zr@contesting.com>
Subject: Re: [TowerTalk] precip static (was402-CD vs. EF-240X)
References: <00b201bd73be$54c32ea0$6b7a95ce@seay.alaska.net>
<3.0.1.16.19980430110914.423ff616@popd.ix.netcom.com>
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in my work recently i have been learning quite a bit about charge
distribution in and under clouds and along towers. i will have to
give it some more detailed thought but here are a few initial
things that probably contribute to the observed differences between
antennas and storms.
1. the static electric field under a thunderstorm typically reaches
10kv/m, now of course you would expect less than this under a non
thunderstorm with the exact value depending on the magnitude of the
convection currents in the storm, and all the other factors that
cause charge to separate in a cloud.
2. a tower is much more conductive than air so the static voltage
distribution along a tower is essentially a constant and is equal
to the ground voltage since that connection is a much lower
impedance than the connection to the air.
3. one factor i don't quite understand yet that is considered a
factor in how those 'porcupine' lightning protectors work is that
a corona source (sharp points are best) create an ionized cloud
that locally neutralizes or shields lower structures from the
electrostatic field under the cloud. the exact nature of this
space charge, its persistance and magnitude probably depends on lots of
factors including wind, precipitation, humidity, etc.
now, if you combine all this one possible scenario that explains
what we have seen here many times would go like this:
1. a charged cloud approaches the tower. say the field is 1kv/m
which is well below a typical thunder storm, but very possible to
find in 'normal' conditions.
2. at the top of a 50m tower the voltage difference between the
tower which is essentially at ground potential and the air is
about 50kv.
3. now at 50kv any sharp point (bolts, hose clamps, pop rivets,
cable clamps, etc) is going to start to generate corona. this
corona will of course be more intense near the top of the tower
because that is where the potential difference is the highest.
Also the corona will generate that space charge that will tend
to reduce the field strength below the top of the tower therefor
preventing corona on the lower antennas.
the critical factor would be the electric field strength in
the air near the tower. if it is below the corona inception
point the storm would be 'quiet', but go just above that point
and you would quite suddenly get noise.
now what i need to do is instrument a tower with some voltage
probes and borrow the corona camera from work to see if i can
prove any of that.
(note, this is all off the top of my head so i may have slipped
a few digits here and there.)
Pete Smith wrote:
>
> >the noise generated by atmospheric phenomena, such as "precipitation"
> >noise, rather than the man made stuff that might be avoided by an
> >antenna being higher in the air.
> >
> Amen ... in earlier discussions, many people have also observed anecdotally
> that high antennas are more affected than low ones, and that some
> rainstorms are much noisier than others. Anyone run across a systematic
> study in the literature?
>
> Time and again, I've found that one rainstorm will be relatively quiet,
> while another (with similar rainfall rate and presence/absence of
> lightning) will generate S9 +20 dB noise.
>
--
David Robbins K1TTT (ex KY1H)
k1ttt@berkshire.net or robbins@berkshire.net
http://www.berkshire.net/~robbins/k1ttt.html
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