> Date: Thu, 04 Jun 1998 15:51:17 -0400
> From: Bill Coleman AA4LR <firstname.lastname@example.org>
> Well, I'm not a lightning expert,
There might very well be no such expert, because lightning
is so esoteric. Roy Lewallen would call this a "high-Q" thread,
because it could ring on and on. Wes Stewert calls it a tar-baby. I
think this is a high-Q tar baby. ;-) There are some things that are
not debatable, however, because they can be confirmed by
> A grounded conductor aloft assumes the same potential as the ground it is
> connected to. It therefore becomes difficult for it to build up a charge
> and make it a potential target.
Actually not according to physics. Charges DO move up in the
conductor. The problem is we can't really measure those charges by
normal means we are accustomed to in conventional circuits, and they
move up in the conductor even if the conductor is an insulator!!!
Earlier I gave the example of a charged paper cone. That's a common
physics experiment. That charges all collect on the outside of the
cone. If you use a string to turn the paper cone inside out, the
charges all move to the outside again even though the cone is an
> An ungrounded conductor aloft assumes whatever potential it wants. If it
> is bombarded with rain, it picks up the charge of the rain. Wind friction
> can also generate a sizable static charge. It may pick up enough of a
> charge to become a target.
Here's how I view the problem, I hope it makes sense. Imagine the
great potential difference between the earth and cloud required to
start an arc. It certainly is much more potential than a few dry
cells, hi hi.
Now how on earth (pun intended) can that conductor "collect" enough
charge to arc over to a cloud thousands of feet away without arcing
over to earth LONG before that point? The charges physicists speak of
"collecting" do so because of the force on charges due to the
electric field, not because of something like water into a bucket.
We can't "pour" those charges anywhere because more charges will
quickly fill the void EVEN when we talk about charge in an insulator!
> Of course, lightning is dumb, and it doesn't understand theory - it
> strikes where and when it wants....
Things can sometimes be too complex for a simple "one-fits-all"
explanation. You can bet there are a lot of things going on that
control getting a hit or not, but changing the potential of a
conductor tower with reference to earth would make no difference at
all. Draw it on paper. Lets say the cloud is charged to +5000 volts
with reference to earth. A conductor connected to earth would
have a potential difference of 5000 volts. If we let that tower float
to 2500 volts, the potential difference to the cloud and to earth
would EACH be 2500 volts, for a total path potential difference of
The only way to lessen the charge is to transport charges BACK to
the cloud that were given up, and that requires some direct path for
ions or charges.
1.) I don't disagee that adding a static brush MIGHT reduce the
potential for a hit, because the corona from the brush would make the
object appear fatter and blunter. But the very same thing could be
accomplished by making the object fatter and blunter WITHOUT the
noise generated by corona.
2.) I don't disagree a wider copper flashing has less inductance than
a narrower "something else", but I disagree with the contention that
the flat shape is always better because it is flat. A one inch
wide flat conductor has about the same inductance as one inch
diameter conductor of the same length. I don't rush out to buy
copper flashing UNLESS I need a whole lot of area. Three inch wide
copper flashing is better than a 3/4 inch copper pipe, but I simply
elected NOT to mess with three inch wide copper flashing running all
around my house because I can disconnect the antennas when storms
I am curious about determining some "rule of thumb" for the performance
of various lengths and materials for broadband grounds.
This morning there was a loud boom. Marsha was standing in the
carport watching the storm, and tells me lightning hit directly on my
200 foot tower. This PC was also on and running. The only damage was
to a digital wall clock sitting on the refrigerator and plugged into
a separate outlet. SK clock.
Does that prove anything? Not a thing except nothing is greatly
wrong with my system. One or two data points mean nothing in a random
event. Certainly everything survived without expensive gizmo's with
what I consider proper grounding, but who knows about the next time.
With another 200-300 foot tower going up on the highest point for
several miles, I'll probably enjoy that next time soon enough.
73, Tom W8JI
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Administrative requests: towertalk-REQUEST@contesting.com