[TowerTalk] lighting "umbrella"

[Jim Jarvis]

Actually, Gary, a SINGLE TURN lightning choke
will work quite effectively.

They're commonly used on AM bcdst towers, to
increase the dV at the tower base, and trigger an
arc at the tower base lightning gap.

Multiple turn chokes CAN work, coming off the tower.
But you have to look upon the coax as sacrificial, in
event of a strike.  You want to induce an arc out at the
tower, to prevent it from coming into the house.

What is dumb...but often done....is to make those chokes,
but NOT to ground the coax to the tower base first.

Jim Jarvis
410 439 1073 office
443 618 5560 cell
jimjarvis at ieee.org

-----Original Message-----
From: Gary Schafer [mailto:garyschafer at comcast.net]
Sent: Saturday, February 26, 2005 13:35
To: jimjarvis at ieee.org
Cc: kd4e at verizon.net; towertalk at 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 at ieee.org
>
>
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>
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