Which begs the question: Is it safe to disconnect your transmission line
and lay it out on the floor? If the transmission line connects to an
ungrounded antenna (a dipole, for instance), then it can build up very high
voltages even if not directly hit by lightning.
But if the antenna is hit, do we think that lightning which has jumped
hundreds of feet is going to respect the six feet between the operator and
that coax on the floor?
Seems that might risk turning an otherwise useful ham into a krispy kritter.
I'd rather lose a rig than a life.
73, Mike N4NT@wireco.net
Johnson City, Tennessee
----- 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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