[TowerTalk] lighting "umbrella"

[Gary Schafer]

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 at ieee.org
> 
> 
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