Unfortunately the physics of arcs across large gaps is much different
than that of sparks across small gaps. The small gap between your
finger and the doorknob breaks down in a single avalanche once the gap
is small enough, on large air gaps the progression is slower and has
much less predictability. It is so variable that even high voltage
insulators are rated by a value that is statistically derived from a
whole series of flashover tests. And even after all the tests it only
gives a 50% chance of preventing a flashover at the rated voltage....
given certain waveforms and atmospheric conditions.
And the response of air to an electric field changes radically once the
field gets high enough to cause corona... its not high enough from your
finger, it is at the tip of a leader progressing out of a thunder cloud.
It changes so much that when predicting lightning performance of power
lines we have to take into account the diameter of the corona around the
wires when computing the characteristic impedance of the wires to
properly predict currents and voltages.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Jerry Keller
> Sent: Wednesday, July 07, 2004 20:37
> To: (Reflector) TowerTalk; Bill Aycock
> Subject: Re: [TowerTalk] Lighting
>
> I don't see a significant difference except the obvious one of scale.
> Excess electrons will travel to where there is a home for them, one
way or
> another... a (relative) few at a time, as they accrue... or en masse
if
> they build up. If it were not so, how would a capacitor work? Or
anything
> else, for that matter.
>
> Except for scale and the huge number of earth and sky variables that
> determine where, when, etc... why should a lightning strike be any
> different?
>
> again, just my unschooled, unformula'd, two common cents :-) (I just
> gotta quit this thread)
>
> 73, Jerry K3BZ
>
>
>
>
> ----- Original Message -----
> From: Bill Aycock
> To: Towertalk@contesting.com
> Sent: Wednesday, July 07, 2004 8:01 AM
> Subject: Re: [TowerTalk] Lighting
>
>
> Funny how some simple things are forgotten. I have two "grounding
> straps"
> that came with solid state components. The instructions told me to
wrap
> the
> velcro band around my wrist, with the metal plate in contact with my
> skin,
> and clip the alligator clip to the chassis of the place where the
> component
> was intended to go. The purpose was to bleed off static charge
through
> the
> wire, rather than by an arc to the component.
>
> How is this different from the effect of the ground provided by a
tower
> coupled to the earth by a wire or rod?
>
> Bill
>
>
> at 02:38 AM 7/7/2004 -0400, you wrote:
> > > Many people have described observing this anecdotally, like W0UN
> with his
> >50
> > > to 100-mile view in Colorado, lighting strikes would cease when
they
> >neared
> > > his antenna farm and pick up again a couple miles on the other
side.
> > >
> >The strikes may decrease, but I doubt it's due to the grounded
towers.
> >Strikes ocurr due to a potential difference between the cloud
(source)
> and
> >ground. As a storm moves localized and moving areas of charge form
on
> the
> >ground as well as in the clouds(NWS). A grounded tower only serves
to
> move
> >opposing portions of the charged area closer together. OTOH the
> charged
> >area my be to one side of the tower at a distance greater than the
> height of
> >the tower. In this case the ground feeder will probably initiate
from
> the
> >nearby ground instead of the tower. Strikes are preceeded by
what
> are
> >called leaders, or feeders. These leaders extend both from the
cloud(s)
> >downward and from the earth upward as well as between clouds.
> *Generally*,
> >but not always, the downward strike comes after an initiating
upward
> >current.
> >
> >The theory behind lightning rods and static balls is they help
bleed
> off the
> >charge faster than it can accumulate much like the corona discharge
> from a
> >Vandagraph generator. It's basically a high voltage corona with
little
> >current. It takes time for the charge to build high enough for a
> strike to
> >occurr. This takes a fine point, or a lot of fine/sharp points.
Still,
> in
> >many storms the charge builds far faster than the lightening rod(s)
or
> >static balls can bleed off.
> >
> >It is not the job of the rod or ball to protect the structure
during a
> hit,
> >but hopefully to prevent the hit from happening, or lowering the
charge
> so
> >the strike is not as strong as it could have been.
> >
> > > Many of us have also observed the tic-tic-tic of static
discharge
> coming
> >in
> > > on a coax.
> >
> >Everything is relative. Looking at it from the lightning's
perspective
> that
> >spark is the antenna *charging* to the localized, higher potential
> >difference of the ground.
> >
> > >
> > > I doubt these things "prevent" a lightning strike, but I
personally
> do
> > > believe they help discharge the atmosphere near the antenna
farm,
> which
> > > helps prevent some strikes. This effect may be much more
pronounced
> with
> >a
> > > multi-tower station than with just one tower.
> >
> >I don't think any one knows for sure, but logically all those high
> points
> >could help bleed off charge, ... or they could just pose as a
> generalized
> >ground at a higher altitude. After all, it is generally the taller
> objects
> >that *tend* to get hit. Look at the repeated hits the Empire State
> Building
> >takes.
> >
> > >
> > > On a dry winter day when you're walking on the carpet at the
office
> and
> >then
> > > touch the metal drinking fountain you get a sudden static
discharge,
> >
> >Ever see snow static cause arcs across a PL-259? I've never had a
mag
> that
> >put out that much juice. IF it's cold and really blowing you can
get
> an arc
> >every 5 seconds or even less and that is one potent strike. They
are a
> real
> >good reason for not disconnecting the cable and letting it lay on
the
> floor.
> >The choke from center conductor to ground at the coax connector on
most
> rigs
> >prevent the snow static from building to much of anything.
> >
> >When static starts to build here, I can hear the popping of the
flash
> overs
> >at the center insulators on some antennas. One reason I now have
> normally
> >grounded relays and the antennas are at DC ground.
> >
> >Like Tom I never, or rarely ever, disconnect a rig during a storm.
I
> do
> >quit operating at times though. <:-))
> >
> > > analogous to a lightning strike (though micro). Touch something
> metal
> >every
> >
> >Kinda, sorta, almost, similar...<:-)) The dynamics and as you say,
> scale,
> >are different but there are similiarities.
> >
> > > few steps and the charge cannot build up to such a size. You
are
> > > discharging the buildup before it gets so big.
> >
> >Which is what lightning rods and static balls are supposed to do.
> >
> >Roger Halstead (K8RI, EN73 & ARRL Life Member)
> >N833R, World's Oldest Debonair (S# CD-2)
> >www.rogerhalstead.com
> > >
> > > 73 - Rich, KE3Q
> ><Snip>
> >
> >
> >_______________________________________________
> >
> >See: http://www.mscomputer.com for "Self Supporting Towers",
"Wireless
> >Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041
with
> >any questions and ask for Sherman, W2FLA.
> >
> >_______________________________________________
> >TowerTalk mailing list
> >TowerTalk@contesting.com
> >http://lists.contesting.com/mailman/listinfo/towertalk
>
> Bill Aycock - W4BSG
> Woodville, Alabama
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers",
"Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041
with
> any questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers",
"Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041
with
> any questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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