To: <towertalk@contesting.com>
> Date: Mon, 01 Jun 1998 08:42:49 -0600
> From: Dave D'Epagnier <DAVED@ctilidar.com>
> Subject: RE: [TowerTalk] Re: Lightning ground "Holy Wars"
> To: "'w8ji.tom@MCIONE.com'" <w8ji.tom@mcione.com>,
> 'Towertalk' <towertalk@contesting.com>
Hi Dave,
There's a few things below that are generally useful to
towertalkians, since they apply to both lightning and RF.
First, I appreciate the info. I e-mailed you before this arrived and
asked a few questions about the formulas.
> By the way Tom, if you play with these formulas, the units are
> MKS,
> the inductance is in nH, I left a paranthesis out of the ribbon
> equation (tough to write equations here), and the wire equation is
> for solid wire, not tubing.
I need to get a copy of that book and will do that, unless someone
can FAX me a copy of the pages covering the formulas. I hate spending
money unless it's on a toy, like more wire and rope. I'm sure you
know this, but let me bring it up again...
1.) Tubing is no different than solid wire when skin depth is much
less than the wall thickness. At higher frequencies (and lightning is
certainly not steady state dc) the conductor in the middle becomes
less and less important.
That's why copper weld antenna wire works as well as solid copper,
the thickness of tubing matters very little at RF, and why double
shielded cable does nothing a solid thin shield won't do at RF.
2.) The impedance of a flat strip also becomes higher at higher
frequencies, because of current "bunching" (that is a handle I
hang on the effect where time-varying currents are forced to the
outer edges of the strip). Thickness of the strip is also unimportant
at a high enough rate of current change, because of skin effect.
That's why a smooth round conductor often has less loss resistance
than the same surface area flat conductor at radio frequencies.
One thing missing is frequency and skin depth in the formulas. I
think we are heading for the unanswered question about the
frequency/energy content of lightning. I disagree completely with the
any notion lightning behaves like dc. It makes it easy to give
answers and solve problems, but I suspect the answers are tainted by
the view lightning is dc or behaves like dc.
> The main point is that copper strip like the
> stuff you can get from Polyphaser (1.5"wide by 0.015" thick) is a lot
> lower inductance than the standard "wimpy" AWG#2 wire used for
> grounding, and a heck of a lot more flexible than copper pipe (and
> more expensive than either!).
I'm not sure about that yet because I have no idea what a Fourier
analysis of a typical (if there is such a thing) lightning strike
would reveal for energy content vs frequency. I do know I would never
trust a *long* #2 lead, and my gut instincts tell me a 1.5 inch wide
smooth ribbon is probably better. But sometimes instinct isn't
always correct......
Someone posted something about most of the energy being in the
several hundred kilohertz region. Can anyone verify that, or
the percentage of energy at different frequencies?
My own personal rule, lacking solid data, is to treat lightning more
like RF rather than dc. I do that because any time I've seen damage
from a hit, its behavior was more like RF energy than dc or low
frequency ac.
This seems to be a topic surrounded by conflicting information,
typical when there is a lack of solid data. I'm in the process of
collecting as much data as I can, and appreciate the help. When I get
enough info, I'll make it available to anyone who needs the
references. Maybe someone who is a good author will write a nice
factual article without any bias or sales pitches.
73, Tom W8JI
w8ji.tom@MCIONE.com
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