[time-nuts] Lightning arrestors for GPSDO antenna

Bill Hawkins bill at iaxs.net
Fri Oct 17 00:46:31 EDT 2014

Years ago, I visited an installation that used a large triggered spark
gap to discharge a large bank of caps at 100 KV into the primary of an
air wound (about 8 feet in diameter) 1:10 step up pulse transformer
connected to two aluminum plates about 30 feet square separated by
several feet of water (the dielectric). At the center of the plates was
a spark gap that was used to study EMP phenomena.

There certainly was high power RF associated with the discharge, a tiny
fraction of what a lightning bolt produces.

All of the electronic measuring equipment was contained in a room-sized
double-walled copper screen Faraday cage, so that the jumping lines on
Tektronix 545 oscilloscopes didn't just show noise from the 100 KV gap

Even though grounding couldn't neutralize the effects of the pulse, the
Faraday cage isolated the contents from the fields. The next best thing
to a Faraday cage is a ground plane that has one connection to Earth
ground. And if you can't get a great metal plate, a single point for all
instrument grounds is the last best thing - if you have to have an
elevated antenna.

When I had a mast with two HP conical GPS antennas to a pair of HP
Z3801A receivers, I chose a single Earth-grounded lightning rod for the
plastic pipe mast. Nothing else connected to that ground. It was
intended to discharge local induced static electricity, not take a
direct hit. The equipment (and the antennas via the RG-8 cables) was all
connected to a common ground point connected to the house electrical
ground. The computer connections to the house network were wireless. No
lightning arrestors were used.

This system got one test when the neighbor took a direct hit to a tree
close to his house. An arc jumped to a nearby outdoor floodlight and did
considerable damage in his house. My antennas were about 100 feet away.
The one nearest the strike died, but there were no other effects. Sadly,
I was not motivated to examine the effect on the GPS receivers, although
I still have the GPSCon data here somewhere.

I gave the dead antenna to the man who bought my GPS setup (downsizing
for the next stage of old age). He later told me that he fixed the
antenna by replacing one of the amplifier chips. Sounds like EMP damage
to me, the kind no arrestor could have helped.

But I would not dissuade anyone from employing lightning arrestors, for
the peace of mind it brings -- until a direct hit occurs.
However, I'm in Minnesota with less than a tenth of the probability of a
hit in, say, Florida.

While there's not much about precision time in this posting, I hope it
was useful to those who probe the sky with antennas.

Bill Hawkins

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