Hi Gary,
In my case, I use clip on ferrite chokes. Type 73, from
radioworks. Coax shields are grounded at the SPG. Ferrites isolate
the shield exteriors from RF, so the antennas aren't tied together.
When I had the tower up, I used the same scheme, except the ferrites
were in the form of T4G 'line isolators'.
In Doc's case, he can ground the shields at the tower base, and
pretty much go straight into the house from there.
Regardless of whether there's a direct wire connection or not,
you can get induced currents flowing in the "secondary
windings" of the single turn transformer....where the lightning
strike is the primary.
N2TK msg'd me privately. Tony has the best SPG installation I've
ever seen. Really nicely done. His reaction: "one of these days,
I'm gonna get busted for not disconnecting the gear during storms."
So, you do the best you can, and recognize that insurance coverage
is probably worthwhile. :)
Jim Jarvis
410 439 1073 office
443 618 5560 cell
jimjarvis@ieee.org
-----Original Message-----
From: Gary Schafer [mailto:garyschafer@comcast.net]
Sent: Saturday, February 26, 2005 13:35
To: jimjarvis@ieee.org
Cc: kd4e@verizon.net; towertalk@contesting.com
Subject: Re: [TowerTalk] lighting "umbrella"
Good points Jim,
This is where caution should come in when some advocate a coil in the
feed line near the house to act as a "choke" for lightning.
It can just as well act as a multi turn transformer and increase the
pickup of emp radiated from the tower or other objects.
73
Gary k4FMX
Jim Jarvis wrote:
> Folks,
>
> There's an aspect of this that hasn't been touched on,
> that has to be considered.
>
> Let's assume that you have a really good dissipative
> ground system from said tower. Multiple rods, multiple
> radials, smooth strap transitions from tower to gnd, etc.
> And let's further assume that you run 4" strap around the
> perimeter to the AC service entrance ground.
>
> So...you've about done what you can.
>
> Lightning hits the tower. (or a nearby tree, even).
> Current flows to ground.
>
> Some distance away, within the house, you have a
> (telephone(stereo)(TV)(Ham gear).
>
> It's grounded to the power system ground. It also has
> wires dangling off it, for purposes of signal or data
> reception.
>
> Consider that you now have a transformer.
>
> The one-turn primary is the antenna/tower/ground system. (or tree, or
water
> tower)
> The one-turn secondary is the tv cable, tv set, power ground path.
> Or whatever equivalent you have in the shack.
>
> I have measured 1500V between a 10m ground plane and earth ground with a
> dmm,
> when lighting struck a water tower, 300 yds away. It was struck 7 or 8
> times
> in one storm, alone. Imagine what might happen if the distance were much
> shorter?
>
> So...you can take all possible precautions with SPG and good grounding
> techniques,
> and still be done in by induced currents in the "transformer secondary"
> paths.
>
> This is why I disconnect my station completely from antenna, power and
> network,
> during storms. Incidentally, if you leave the radio connected to power
and
> RF GND,
> whatever potentials exist between the RF gnd and the power gnd are
impressed
> across the ground traces in the radio...and power and RF are undoubtedly
> kept separate
> for EMI/EMC/noise reduction purposes.
>
> Just a thought.
>
> n2ea
> jimjarvis@ieee.org
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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