From: medin@catbyte.b30.ingr.com (Dave Medin)
Subject: Re: Television Antennas and Grounding
Reply-To: medin@catbyte.b30.ingr.com
Organization: Intergraph Corporation, Huntsville AL
Date: Wed, 30 Sep 1992 15:27:35 GMT

In article <1992Sep30.012014.26543@cis.ohio-state.edu>, rubin@skiing.cis.ohio-state.edu (Daniel Rubin) writes:
|> I recently installed a rooftop television antenna.  I grounded the mask as
|> was recommended in the instruction and I got to thinking.  Why is it a good
|> idea to ground the antenna?  I would think that this would invite lighting
|> to stike the mask!  I mean the mask is definitely the tallest point with the
|> lowest resistance.  Wouldn't this be inviting trouble.  I would think a
|> lightining strike would fry the grounding wire which is next to the house.
|> This seems like a fire hazard not to mention the jolt to the TV tuner etc...
|> Wouldn't you want to deter lighting instead of providing a ideal path for it?

The ground, as you say, provides a means to shunt the majority
of the lightning bolt's energy directly to ground. Isolating the
antenna really doesn't help decrease its attractiveness,
as you're going to get enough charge leakage through even an
"isolated" system that attractive charge will still migrate
from the ground and appear on the antenna when the ground
charge is in the vicinity (ever try to perfectly isolate a system
to several tens of thousands of volts?). Being that until the
bolt there is very little current flow, even a high resistance
isolation simply forms a resistive voltage divider with the
discharge path to the surrounding air which is usually much higher.

Attractive lightning targets are enhanced by providing a strong
charge gradient on the target surface. This is usually more
of a function of sharp points on the object (of which a TV antenna
has many). Oil refineries and other high-risk targets typically
do two things--either provide as uniform a target as possible
by making all protruding surfaces smooth, or by using a needle
umbrella which contains a multitude of needle points (several
per sq in) on a metallic fabric that is stretched out like an umbrella.
The idea, interestingly enough, is to create such strong local charge
gradients that the charge difference bleeds off through corona
discharge rather than a sudden bolt. The devices, from broadcast
experience, seem to work very well...

Some points to ponder on grounding and installation:

	1. The ground wire should be run in a direct line to the
	grounding rod or network. Avoid sharp bends in the wire.

	2. Don't use aluminum ground wire. It: vaporizes faster,
	has a greater tendency to throw molten bits of metal which
	start fires, has greater bulk resistance, and corrodes
	in contact with most ground clamps.

	3. Braid is much better than wire (lower skin effect resistance).

	4. Don't tie in with your house's electrical grounding system!

	5. 1/2" or 3/4" type M copper pipe makes a wonderful, cheap
	ground rod.

	6. Use a telco (two piece) grounding clamp on the mast and
	the ground rod. Don't just wrap the wire around the mast
	and figure lightning will jump to it. We're talking about
	thousands of peak amps here, and contact resistance is
	important.

	6. Run about 15 feet of extra coax, and before it enteres the
	house leave it in a coil with about a 1' radius. Stretching
	out the turns of the coil (use a PVC form) helps, too. This
	forms a common mode inductor to discourage lightning from
	following the lead into your house.

Back when I used to work broadcast and install MATV systems on the side,
I saw some interesting things lightning did...

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