[TowerTalk] Vertical antennas and lightning

[K1TTT]

You are not 'bleeding static from the clouds'.  The clouds are giant
generators powered by the huge updrafts and charged by the freezing moisture
droplets colliding, they separate many coulombs of charge for each stroke.
What you see on the ground as corona and streamers is a tiny fraction of
that which has been attracted to the charge at the base of the cloud.  Look
at it from the lightning's standpoint, start off 10000' or more up looking
down at your little vertical, do you really think that a storm that is miles
in diameter and miles tall generating many coulombs of charge is going to
care in the least what your single little point is doing?  Or that your
single ground connection is passing a few microamps of current into the air
a couple of miles below it?  not a chance... the only time your little
vertical has any effect is when a downward leader is within striking
distance, that last 100m or so... by that time if the leader is close
enough, and your vertical has enough charge to start a streamer, and nothing
else beats it to leader, then your vertical gets fried.  Otherwise you are
just.... (pick your favorite teacup/ocean hopelessness image)

Remember, air is an excellent insulator... and in a lightning storm there
are literally tons of charged droplets draining charge from the cloud
continuously, and yet it still charges up enough to create huge discharges
right through thousands of feet of air.  I have tried to measure either the
attractiveness or draining capability of large antenna farms and huge power
line corridors, and couldn't see it on years of lightning data... so don't
expect to see any effect from single towers or antennas, grounded or not.
Focus instead on making sure that you provide good paths for the current to
go around your equipment once it is on a wire or even in the ground.


David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
 

> -----Original Message-----
> From: David Gilbert [mailto:xdavid at cis-broadband.com]
> Sent: Sunday, August 01, 2010 20:02
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Vertical antennas and lightning
> 
> 
> That doesn't pass my perception of the physics involved.
> 
> How does bleeding static from the clouds not initiate an ionized path
> itself?  It seems to me that's like sticking a pin in a balloon to let
> out enough air so that it won't pop.
> 
> Dave   AB7E
> 
> 
> On 8/1/2010 10:36 AM, Bill Aycock wrote:
> > David--
> > This argument has always had holes, as far as I am concerned. It is  not
> the
> > strike energy that "Bleeding" will handle, but the static charge that
> helps
> > create an ionized path, that the strike can follow.  Diminishing that
> path
> > HAS to help.
> > Additionally, I know this is not absolute; I merely want to improve the
> odds
> > a little.  Other protection is also needed.
> > Comment?
> > Bill--W4BSG
> >
> > ----- Original Message -----
> > From: "David Gilbert"<xdavid at cis-broadband.com>
> > To:<towertalk at contesting.com>
> > Sent: Sunday, August 01, 2010 11:35 AM
> > Subject: Re: [TowerTalk] Vertical antennas and lightning
> >
> >
> >
> >> Look at it this way ... the magnitude of arcing you get across the end
> >> of the coax if you don't have the antenna DC-shorted to ground is
> rather
> >> tiny.  My experience over the years (when I had a dipole or vertical
> >> that was not DC-shorted to ground) was that I'd get a spark across the
> >> end of a PL-259 (roughly half inch spacing) every few seconds.  Add up
> >> the energy from all those little arcs over maybe a ten or fifteen
> minute
> >> period and compare it to the energy from a single lightning strike.
> >> Then consider the likelihood that the portion of the cloud system that
> >> generated the lightning strike wasn't even near your QTH ten or fifteen
> >> minutes ago.
> >>
> >> It's like trying to drop the level of a flowing river by removing water
> >> with a teacup.
> >>
> >> DC-shorting an antenna to ground is important to protect both equipment
> >> and people from static buildup.  I once drew a really thick (lots of
> >> current) 2 inch long bright blue arc to my left hand from the shack end
> >> of the coax coming from an unterminated 80m dipole (my right hand was
> on
> >> the floor) ... that calculates out to about 300,000 volts and the
> biceps
> >> of both arms were sore for three days.  Imagine what the energy that is
> >> capable of generating those half inch arcs might do to a receiver front
> >> end or the contacts of a small relay.
> >>
> >> But grounding the antenna isn't going to even come close to bleeding
> off
> >> enough charge from the clouds overhead to prevent a lightning strike.
> >>
> >> 73,
> >> Dave   AB7E
> >>
> _______________________________________________
> 
> 
> 
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