From: "Joe Giacobello" <k2xx@swva.net>
> You guys have raised an issue that I have wondered about for some time
> now. I also live out in the country and have noticed that the number of
> houses and other structures with lightning rods seems to have diminished
> over the years. Why has the use of lightning rods fallen out of favor?
I don't know that it has.. I see lots of lightning rods around in "lightning
country" (FL, NM, CO, etc.).. not many in CA, it's true, but we don't see
much lightning here.. Strapping to hold things in place when earthquakes
hit, though...
>
> It was always my belief that the purpose of the rods was to slowly and
> continuously bleed off electrical charge in the air so that the
> likelihood of generating a voltage in excess of the the breakdown
> voltage for air in the vicinity of the structure was minimized.
The field before a lightning strike is many orders of magnitude less than
that for air breakdown (20 kV/m vs 3000 kV/m)... No amount of charge you're
going to feed through some sort of apparatus is going to change the field
all that much. The folks who deliberately trigger lightning use bolder
approaches like rockets trailing wires going many hundreds of feet into the
air, and even then, it's not a sure thing. (Check out the Univ of Fl at
Gainsville web site..)
The ultimate objective was to prevent a sudden, instantaneous, potentially
> destructive, high current discharge through the structure itself.
The shape of your electrode or field isn't likely to change the discharge
characteristics of the lightning much. Once the leader channel is
established (a lowish current sort of thing in the tens of amps range), the
main stroke is going to be the usual 10-20 kA, regardless of what the
endpoint looks like. Lightning doesn't change all that much whether it's
hitting a tower, a tree, a rocket trailed wire, the top of the Empire State
Building (where they did a lot of lightning current research) or the flat
ground.
When
> I proposed on the Antennas mailing list that this mechanism most likely
> also applied to a properly grounded tower near one's house, my
> description was corrected by one of the readers. Apparently, the
> effectiveness of the rod or tower depends on the formation of a cloud of
> charge in the air surrounding the tip. In windy conditions, the cloud
> becomes unstable and must continually reform. (Although I'm not sure
> that it makes any practical difference, I was also informed that the
> flow of charge was from the ground through the rod into the cloud.) Is
> it because windy conditions reduce the effectiveness of lightning rods
> that they are less used today? I'd be interested in hearing some
> comments on this question.
>
> I should also add that I have a pointed multiwire attachment strapped to
> the top of my tower which (allegedly) provides multiple paths for
> discharge and reduces the voltage necessary for (slow) discharge to
> occur. (It can be purchased from the Wireman, and Pres swears that it
> works.)
This sort of thing probably provides a function similar to the "dissipation
wires" on the ends of airplane wings to get rid of "P-static".. The idea
being that rather than charge up a metallic body, and dissipate it in
discrete zaps (think of a Van deGraaff generator), you provide a mechnism to
essentially continuously discharge the stored energy in many small sparks.
This would reduce the magnitude of the zaps. Since even those little
discharges can have significant di/dt, leading to significant induced
voltages in neighboring conductors, reducing the size is a "good thing". It
will also make the sound of the "pop"s from the RF coming from discharges
less noticeable
However, as far as changing the "gross" electric field distribution around
your tower (which is what determines where the lightning will or will not
go) it probably doesn't make much difference. The earth and the cloud form
a capacitor which has a "plate spacing" that is quite large (1000 meters or
more) and where the plates are pretty big (many 1000s of meters). Your
30-40 meter tower isn't all that big in the context of the overall
atmospheric electric circuit.
Compare it to a variable capacitor in an antenna tuner where the plates are
spaced, say, a quarter inch (0.250") apart. Your tower looks like a piece
of dust a few thousands of an inch high on the plate.
I also have a pretty good ground system with about eight ground
> rods connected in a large circle that includes the house ground. I have
> observed major lightning strikes on a ridge about the same height as
> mine and about 3/4 of a mile away, but I have never been hit. Have I
> been lucky or is the system really working?
>
>
> 73, Joe
>
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