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 firing. 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
