Re: Antenna And Lightening Hits?
- From: "Mr Ed" <ecamin@xxxxxxxxxxxxx>
- Date: Wed, 1 Oct 2008 10:39:53 -0400
Excellent Jer.
We don't listen to AM radio much anymore but how many remember when driving
under a storm and hearing a static hiss crescendo on the radio, becoming
very loud and then all of a sudden it disappears only to slowly build up
again? That is because there is a ball on the end of the antenna. The
charge builds up and then discharges as you drive down the road. Of course
the purpose of the ball is to protect from piercing an eye. If the ball was
not there the charge would probably not build up near as much. It kinda
lends credibility to Jer's discussion.
--
Mr Ed
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"Jer" <gdunn@xxxxxxxxxxx> wrote in message
news:hMudnUlJ38Rb_X_VnZ2dnUVZ_tHinZ2d@xxxxxxxxxxxxxxxxxxxxxxxxx
Wes Newell wrote:
On Tue, 30 Sep 2008 02:02:39 -0500, Jer wrote:
Wes Newell wrote:
On Sun, 28 Sep 2008 18:37:52 -0700, Richard C. wrote:It may help to know that all tall buildings have a lightning protection
Look it up in the NEC!After having my antenna hit twice by lightning in the first year I out
Grounding is required.
up an Antenna in 1960 something, I've never grounded another one. And
I've not had another strike since then. And while grounding is
considered safer, I'm not convinced it really is after having a TV I
was watching literally explode the picture tube on one hit. Luckily, I
didn't get injured except for slight burns throwing the burning TV out
of the front door.
system in their roofs. They're sharp pointy metal spikes scattered
about and they're all bonded together and to the building's metal
superstructure. It's a modern version of what's used on the roofs of
barns all over rural areas.
Exactly. And take a look at all the transmission towers. Just like tall
buildings (the Empire State building is a good example) they have a
lightning rod *above* the transmission antenna connected directly to
ground using very large cable. And the actual antenna is isolated from
this ground. Putting up a grounded house antenna is nothing more than
putting up a lightning rod. I don't care what the codes call for. At my
house my rooftop antenna is not grounded. And I haven't had a hit in the
last 40 something years. And get get a lot of lightning in my area.
For broadcast facilities (Empire building) it depends on what type of
protection is possible with respect to what type of structure it used.
Some antennas are isolated, some aren't. If they're isolated, some form
of additional protection is necessary separate from the antenna, ie.
additional structures like overhead grids. If they're not isolated, then
the antenna (and tower) is part of the protection system, but there's
still an air gap protector somewhere, usually close to a building entrance
point. These are all instances where the antenna is expected to be
energized 24x7 from a transmitter.
For a home TV antenna, there's no expectation of an energized antenna,
unless it becomes energized from a lightning strike. If that happens,
there needs to be some way of dissipating that energy safely to ground,
ie. a ground block (a small air gap device) inserted into the coax line
near the building entrance.
Here's what happens... when a thunderstorm floats by overhead, the air
becomes positively charged with respect to the negative ground. At some
moment, when the difference of potential is great enough to jump the air
gap between, a lightning strike is highly probable. Now, these sharp
spike things on building roofs aren't an 'attractant' for lightning
intended to give the strike some place to hit, it's a 'drain point' to
give the excess electrons of the building some place to go. The sharp
points are very effective in allowing electrons to drain off the points
into the air, minimizing the difference of potential between the negative
charged building and the positive charged air overhead, thereby
significantly reducing the likelihood of a strike in the first place. Do
they always work? No, of course not. And when they don't prevent a
strike, the energy still has some place to go, ie. through the building's
internal steel structure, through the foundation bonds, to ground. And
under this condition, energy presented to any appliances inside the
building is significantly reduced, preferably reduced enough to be
mitigated by existing surge protection built in to each appliance, or a
common surge protector device on it's mains.
We all know most homes these days don't have a handy internal metallic
structure for easy roof bonds, and that's a shame because if I was going
to erect a TV antenna (typically isolated from ground), I'd certainly want
to protect the house with a well-bonded protection system (spikes on the
high peaks), and include a ground block on the TV coax line - the coax
line for my sat antenna has one. For urban neighborhoods, roof spikes are
persona non grata, and since there are many other structures nearby, it's
a 'safety in numbers' type of thing, so house designers are hedging their
bets on the house being a target. But, if it is targeted, it, and
anything sticking up over the roof needs to be well protected. Given your
stated history of presumed safety, I'd figure there are other nearby
structures that are more likely targets than you are, or there's some
other condition we're unaware of that reduces your chances of being a
target. When it comes to lightning protection, there are no guarantees -
one can only do one's best to influence the issue to one's presumed
advantage - which is a little better than a crap shoot.
If anyone wants to dispute this recollection from my early days, feel free
to point out my own clouds.
--
jer
email reply - I am not a 'ten'
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