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Re: [TowerTalk] Grounding Coax at Tower

Subject: Re: [TowerTalk] Grounding Coax at Tower
From: Jim Lux <>
Date: Wed, 27 Feb 2008 19:11:29 -0800
List-post: <">>
David Robbins K1TTT wrote:
>> K4SAV wrote:
>>> Having the cables exit the tower above ground is a really bad idea.
>>> Voltage drop down the tower is very high during a strike.  Anything
>>> exiting the tower above ground will see a very large voltage impressed
>>> on the cable at the point it exits the tower.
>> But how high?  And what needs to withstand that potential?
>> The voltage would depend on whether resistance or inductance dominates.
>> A 30kA strike on a 1 ohm tower would only be 30kV over the whole length
>> of the tower.. call it on the order of 5kV over a 10 foot length out of
>> a 60 foot tower.
>> A 30kA strike with a rise time of 2 microseconds on a 20 microhenry
>> tower (1 uH/meter) would result in a voltage across 10 feet of about
>> 3E-6*30E3/2E-6 or 45kV.. so for the direct strike scenario, the
>> inductance dominates.
> Well, I ran this through the software that I write for EPRI (
> (yes, I do get paid for this software work, though not for trying it for ham
> tower experiments)  The software is for lightning protection design on power
> lines, not for radio towers, but it will do some things on a single tower
> that are interesting and that I trust.  For this case I set it up for a 120'
> tall pole to approximate a tower.  With a 25 ohm driven rods ground in
> average soil.  Then I hit it with a 30ka 2usec risetime stroke.
> Drum roll please....
> At 10' up from the ground I get a voltage of 450kv... now, this is of course
> relative to the infinitely distant ground (or your power line entrance).
> Relative to the earth at the base of the tower it is only about 100kv.

Fascinating.  I'm guessing the 350kV difference is the 30kA through the 
25 ohm ground impedance or something like that? And my back of the 
envelope was only off by a factor of 2 (which is better than I'd 
expect.. I'd be happy if I was within a factor of 5-10)

> For those interested in such things the estimated surge impedance (similar
> to coax characteristic impedance) for this tower works out to 240ohms, most
> of it is pure inductance of course.  To get an idea of the total possible
> voltage, if there were no ground that would make a voltage of about 7MV at
> the top of the tower, with the ground connected that is reduced to 1.7MV at
> .5usec into the stroke, well before the peak current is reached.  With the
> insulators flashed over that peak voltage is only 800kv, this is because
> lots of the current is drained away from the tower on the wires.

1.7 MV at the top of 120 feet. Is the voltage profile reasonably uniform 
down the tower (e.g. at 12 feet it's 170kV, etc.) or is it distinctly 

On tesla coils, where I (and others) have done lots of modeling and 
measurements, you get a funny distribution.  It's not linear because the 
C from a section of the secondary to the ground plane is larger at the 
bottom than at the top. Sort of like a tapered transmission line.

That's for the fundamental excitation. There's also other effects from 
fast transients (when you get a spark, the rise time is tens of 
nanoseconds).  The folks killing their garage door openers and other 
consumer electronics with tesla coils kill them from the induced voltage 
from the field from the sparks hitting the ground.

Even worse are small Marx banks.  2 or 3 nanosecond rise times aren't 
unusual, and a 300kV spark a couple feet long from low inductance 
capacitors.... that's a solid state equipment zapper.

> Some things that mitigate the situation... the coax/control cable bundles
> leaving the tower are usually much skinnier than the tower, unless you have
> bundles like mine of course.  This gives them a much higher impedance than
> the tower so more current goes down than out the cable bundle.  For a single
> cable this can be something like a 10:1 ratio in currents.  Intermediate
> supports that are conductive, like the wood poles I use, can drain more
> current along the way to the shack.  And good grounding outside the shack to
> your house perimeter ground should equalize the cable bundle with the shack
> ground at the entrance... yes there is a big inductive loop, but that should
> be shorted out at the shack end if you do it right.
> Fun stuff this lightning, isn't it.

It is that...
Any high power transient system is interesting.


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