[time-nuts] GPS antenna selection — lightning

Gary E. Miller gem at rellim.com
Fri Aug 5 17:47:38 EDT 2016

Yo Attila!

On Fri, 5 Aug 2016 22:51:06 +0200
Attila Kinali <attila at kinali.ch> wrote:

> While it is true, that an indoor antenna is suceptible to surges like
> an outdoor antenna, it is not true that an outdoor antenna is
> equivalent to an indoor antenna when it comes to lightning
> protection. 

I agree there are differences.  But not as mmuch as you think.

> Because an outdoor antenna can be _directly_ hit by a lightning.

And so can an indoor antetnna.  I live it lightning country, and it is
common for a lightning bolt to travel right through an asphalt roof to
hit metal pipes and/or wire inside a house.

I have seen this many times, it has happened to my next door neighbor and
to my son.  If you are lucky your homeowners insurance will cover a lot of
the damage.

> Please be aware that the grounding of the antenna is not to protect
> the equipment from surges, but to prevent conduction of the lightning
> into the house that could cause electrocution and fires. 

A direct hit on an antenna will laugh at your surge protector.  Nothing
at all can protect your electrical system from a direct hit.

I have seen 440V main switchboards exploded from lightning hits.  The
mess is incredible.  The switchboard case looks like a large bomb went off
inside and the cover leaves a dent on the far wall.

The surge protector on your antenna coax will try to limit the static
voltage on the center conductor to about 1,500V.  Now you have turned
your antenna into a passable lightning rod.

> To protect the house and its inhabitants from the lightning strike,
> an external antenna needs to be either lower than any lightning rod
> and within its 45m ball or needs its own conductor and grounding
> to discharge any lightning energy and thus preventing it from following
> the antenna cable into the house.

If you have any doubt about lightning you need to get some lightning rods
on your roof.  A usually passable solution is to run #8 wire from your
offical building ground directly up to an antenna mast or two on your
roof.  Best if it can be done with zero splices.

The point of the lightning rod is not to dissipate a lightning strike,
nothing can do that.  Instead it bleeds away static before it becomes
a lightning strike.

In the midwest in the winter the humidity in a house can get well below
5% and 1,500V of statis is quite common.  So indoor surge protectors
can also be useful.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588
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