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.
>
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>
>
_______________________________________________
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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