[TenTec] Pegasus & lightning

[Michael O. Hyder]

----- Original Message ----- 
From: Dr. Gerald N. Johnson, P.E. <geraldj@ames.net>
To: Robert <bigmack@nortexinfo.net>
Cc: <tentec@contesting.com>
Sent: Tuesday, October 19, 1999 11:38 PM
Subject: Re: [TenTec] Pegasus & lightning


> 
> 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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