[TowerTalk] lighting "umbrella"

[Jerry Keller - K3BZ]

Hi, Jim...

Which is it... a lightning GAP at the tower base?  Or grounded coax at the tower base?  Or didn't I 
understand you?

And did you mean that as little as one loop in the coax between the tower base and the shack will 
act as enough of a choke to induce a strike to dissipate through the tower grounds?

73, Jerry K3BZ
----- Original Message ----- 
From: "Jim Jarvis" <jimjarvis at comcast.net>
To: <garyschafer at comcast.net>; <jimjarvis at ieee.org>
Cc: <kd4e at verizon.net>; <towertalk at contesting.com>
Sent: Saturday, February 26, 2005 2:03 PM
Subject: Re: [TowerTalk] lighting "umbrella"


>
> 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
>>
>>
>> _______________________________________________
>>
>> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
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> 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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