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[TenTec] Pegasus & lightning

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Subject: [TenTec] Pegasus & lightning
From: geraldj@ames.net (Dr. Gerald N. Johnson, P.E.)
Date: Tue, 19 Oct 1999 22:38:28 -0500
Clean glass is good insulation. I fear the points of the coax connector
might cause corona to break the glass though. In any case I figure there
needs to be FEET of separation between the coax and the rigs. To get
that I mounted my antenna patch panel up high and when I disconnect the
cables the interior cables end up on the floor a long ways from the
patch panel that's grounded ONLY outside through the tower and lightning
arrestor. I've been hit while disconnected with the cables dangling from
feedthroughs, some of the radios haven't worked since and I had to
replace a light switch or two in the other end of the house. I connect
the shack ground to the antenna grounds only through the coax or with a
ground wire for those occasions where feeding the wire against ground or
I have RF feedback problems. Otherwise when disconnected I keep a
several foot gap from outside to inside.

While the National Electrical Code depends on grounds you can't get
enough grounds to withstand a direct lighting hit until you have them
connected with a band of copper at least a foot or two wide, and you
can't still keep from getting a significant voltage drop between ground
rods. That wide strap has to connect to the water pipe, and the
electrical service too, to be of any benefit.

I believe in separation during storms.

Think of it this way:

A really good ground rod might have a resistance of 10 ohms, an ordinary
one, maybe 50 ohms. If a lightning current is a mere Kilo amp, how much
voltage can you get between ground rods 30 feet apart? 10 to 50 KV. That
will jump an inch or so. And a good lightning strike could easily be 10
Kilo amps. Its really hard to deflect damaging currents from the radio
by shunting alone. Its easier to deflect them as much as possible, then
introduce series impedance to protect the radio, such as an air gap that
50 or 100 KV won't jump.

Polyphaser protection gaps have been doing well protecting our local
repeater antenna on top a water tower with nothing protecting it from a
direct hit for better than a decade now. They should do well protecting
an HF radio too, but if the radio gets 10% of a Kilo amp through the
grounds, they won't take it either so theres lots of damage.

I say grounding plus isolation is necessary for radio protection.

73, Jerry, K0CQ

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