? on grounding TV antenna

Posted by zzznot on October 15, 2009, 12:02 pm

Y'know, that is interesting.

The logic seems to be that a conductive path disperses the buildup of
static, that would increase likelihood of a strike.

But also, it would seem that a tall conductive path would increase
the likelihood of a strike to a large-area charge buildup that
causes lightning.

So, do ground systems have a high resistance, to attempt to achieve
the one and not the other?

J.

> Was talking with someone the other day and mentioned that I had just
> grounded the TV antenna (the mast, actually) to protect against lightning
> strikes, and they said that was not such a good idea because lightning is
> more likely to strike a path that goes straight to ground. Now I am not
> sure what to do. Anyone have any input on this topic? Ideas are
> gratefully
> received...
> Thanks,
> Dave
>

Posted by Heathcliff on October 15, 2009, 12:26 pm

> Y'know, that is interesting.
> The logic seems to be that a conductive path disperses the buildup of
> static, that would increase likelihood of a strike.
> But also, it would seem that a tall conductive path would increase
> the likelihood of a strike to a large-area charge buildup that
> causes lightning.
> So, do ground systems have a high resistance, to attempt to achieve
> the one and not the other?
> J.

> > Was talking with someone the other day and mentioned that I had just
> > grounded the TV antenna (the mast, actually) to protect against lightni=

> > strikes, and they said that was not such a good idea because lightning =

> > more likely to strike a path that goes straight to ground. =A0Now I am =

not

> > sure what to do. =A0Anyone have any input on this topic? =A0Ideas are
> > gratefully
> > received...
> > Thanks,
> > Dave

From what I have read, for cloud-to-ground lightning, the sequence
generally goes like this. A "stepped leader" comes down from the
cloud base in a series of steps or jumps. When it gets within a
couple hundred feet of the ground, now there is kind of a target area
on the ground, maybe one or two hundred feet in diameter. Within that
area, positive charges from the ground move up and make "streamers"
from tall objects. One of the streamers eventually connects with the
stepped leader, creating a cloud-to-ground conductive path, and then
the main stroke happens. All this takes place very fast, in
milliseconds. But from this description, it seems like what you do
will not much change the chance of lightning striking the house - if
you're in the target area then you may very well get struck, but
otherwise not. -- H

Posted by JRStern on October 17, 2009, 3:23 pm

On Thu, 15 Oct 2009 09:26:29 -0700 (PDT), Heathcliff

>From what I have read, for cloud-to-ground lightning, the sequence
>generally goes like this. A "stepped leader" comes down from the
>cloud base in a series of steps or jumps. When it gets within a
>couple hundred feet of the ground, now there is kind of a target area
>on the ground, maybe one or two hundred feet in diameter. Within that
>area, positive charges from the ground move up and make "streamers"
>from tall objects. One of the streamers eventually connects with the
>stepped leader, creating a cloud-to-ground conductive path, and then
>the main stroke happens. All this takes place very fast, in
>milliseconds. But from this description, it seems like what you do
>will not much change the chance of lightning striking the house - if
>you're in the target area then you may very well get struck, but
>otherwise not. -- H

I believe that charge builds up on the ground over a fairly large area
in advance of even the beginning of a strike, so the question is what
kind of tall objects conduct it effectively up to where the charges in
the cloud can "see" it. Does it matter if you run a copper rod from
ground to steeple, or give it an air gap or insulator gap of an inch
or three? I wonder. My guess is, it doesn't matter much, that the
fields and charges are actually fuzzy and distributed.

Too bad someone can't figure out a way to actively attract - and
capture! - all that energy.

J.

Posted by mike on October 15, 2009, 1:08 pm

> sure what to do. =A0Anyone have any input on this topic? =A0Ideas are gra=

tefully

> received...
> Thanks,
> Dave

This link seems to have a reasonable analysis:

http://www.weatherimagery.com/blog/lightning-rods-attract-lightning/

Since strikes are so rare (unless you live atop a skyscraper), it
makes it hard to do a cost-risk analysis even without the lightning
attraction issues.

Posted by mm on October 15, 2009, 1:14 pm

AIUI, it's the other way around. During a lightning storm, a
negative** charge can build up on (or near?) the tops of buildings,
attracted by the positive** charge repeatedly building up in the sky.

**I might have these two backwards, at least for some/most of the
time.

The lightning is attacted to places with the opposite charge. The
grounding allows the antenna to reach equilibrium with the earth,
basically neutrality, so the charge and thus the attraction isn't
present, and it's LESS likely to get struck when it's grounded.

Same with lightning rods. The notion that the relatively small wires
grounding your antenna or a lightning rod are there to carry the
lightning to ground is false. They would melt in an instant if
lightning actually struck.

I"m still confused about the needles. Typical rods, last I looked,
have balls with iiuc store a charge, and needles maybe a foot long
pointing upwards. I heard a story about some farmers who would break
off the needles because they thought things were prettier that way,
but that ruins the rod.

> Now I am not
>sure what to do. Anyone have any input on this topic? Ideas are gratefully
>received...

The American Radio Relay League, www.arrl.com might have stuff on
this. In the 50's and 60's the ham radio license exam included
antenna construction, but had a lot to do with how oscillators work
and other electronic internals. Based on the exam I took last year,
they are no concentrating on antenna construction, avoiding
interference with other radios, and safety. I guess because radios
are too complicated now for anyone to build one at home, unless he
just assembles a kit. So the exam used to be very hard, at least for
me as a 14 year old. This time I passed without even studying, 10%
because I remembered things I learned for the first exam, 10% because
of things I'd learned along the way, and 80% becaue it's easier.

>Thanks,
>Dave

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? on grounding TV antenna - Page 4