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[Towertalk] Ground wire impedance

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Subject: [Towertalk] Ground wire impedance
From: (Red)
Date: Tue, 30 Jul 2002 08:41:09 -0500
An effective substitute for flashing or other wide conductors is 
parallel wires.  For example, I ground my SPGP with 5 No. 6 copper wires 
spaced across 10 inches (2 1/4" between adjacent wires).  This offers 
nearly the same low inductance as a flat copper 10" wide, and has ample 
cross sectional area to conduct the maximum lightning strike.  They are 
each tied to the ground panel and, at the other end, to a system of 
radial ground wires and the perimeter ground.  The August issue of QST 
has an article describing the grounding.

Incidentally, It has been demonstrated, (reported, if I recall 
correctly, by Uman) that a single #12 copper wire will conduct about 90% 
of lightning strikes.  It cannot dissipate the roughly 10% that exhibit 
a sustained current between strokes.  That sustained current is often 
100 to 200 amperes with durations measured in milleseconds.  While the 
average peak current of lightning strikes is 18,000 amperes, most are of 
microsecond duration for each stroke.

I witnessed a direct strike on a Telex vertical, without damage.  It 
must have been one of the 90% of strikes with no sustained current.  The 
feedline from it was disconnected and outside the house at the time, for 
which I'm grateful.

73 de WO0W

Tom Rauch wrote:

>>are you saying that the small coil of 1/2" copper tubing hanging in my
>>garage would be a better grounding conductor that my heavy gauge solid
>>wire.? (I can never remember if the soft copper tube used for water is
>>K or M. I think M). I think it's 1/2"OD.
>Solid wire has lower resistance than hollow "wire", for the same 
>outer diameter at frequencies where the wall thickness is less than 
>several skin depths. At radio frequencies, it generally makes no 
>difference if the conductor is hollow or solid but near dc or at low 
>AC frequencies it sure does.
>Wide flat thin flashing has less inductance per foot for a given 
>surface area than a round conductor, but it also has more resistance 
>per foot at higher frequencies because current bunches up at the 
>outer edges.
>So I guess it all depends on what you want to do.....and what you 
>have around to use.
>73, Tom W8JI
>Self Supporting Towers, Wireless Weather Stations, see web site: 
>Call 888-333-9041 to place your order, mention you saw this ad and take an 
>additional 5 percent off
>any weather station price.
>Towertalk mailing list

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