Since so much of lightning is pulse related, and I don't have a
verifiable analytical device to measure and take that into account, I
can't really say that it does or doesn't work and why.
However, if it behaves like RF, and due to the monster voltages and
currents present, as a practical
what-are-you-going-to-do-with-what-you-have-today thing, I sure
wouldn't COUNT on it being any level of helpfulness sufficient to
avoid taking other proven anti-lightning measures. I can put a field
strength meter inside a tower and measure RF from an antenna at the
top. This RF is diminished if a balun is used.
I have not had the opportunity to create the tower-by-itself reference
case, because I have to run coax up the tower to get power to the top.
Since I do now have a battery-powered Elecraft K2, I may be able to
try something with the power source at the top. But that also means a
lot of disconnection to set it up, and breaks in the coax where there
presently aren't any, and a new set of sealed splices, and towertop
soldered connectors.
But there's a less risk-intensive way to approach it than hanging off
the top of the mast with a K2...
Think about how we specify percent coverage with a piece of coax
braid. We spend money to get 100% coverage instead of 88%. We call 88%
cheep coax.
Look at a Rohn tower around wires in the middle. What kind of coverage
is that? 3% ? 2% ? Is it even 1% coverage? Certainly will not keep the
shield of coax in the middle from acting as one plate of a capacitor
to something on the outside.
That's UNLESS the coax shield is also grounded to the tower at top and
bottom, and the charge is running to the closest metal, which WOULD be
the tower, not the coax shield inside. That's why pulse dynamics come
into play in a very complex and (for me currently) unknowable way.
Since there is a radiation and impedance penalty to the movement of
charge, and at the frequencies in question the tower is a significant
portion of or multiple wavelengths, the distances and details of
inducement could make the shielding effective or cancel it entirely.
If the tower is always adequate coverage for wires down the middle,
regardless of issues in the prior paragraph, then 100% coverage coax
is a total waste of money. If 100% is useful for coax, then tower
shielding has to be a pipe dream, no?
Without the expensive equipment and time to put that to rest with a
properly buttressed federal legal case, it does seem that sparse
member tower shielding and a presumed need for 100% coax shield
coverage can't fit into the same idea space. One or other of 'em has
problems.
My current money is on tower shielding being the joker. Not going to
find me COUNTING on any lightning help from it. One still needs the
grounding or lightning suppression at the base of the tower, and
grounding shields at the top.
Is it worth the hassle of trying to run one's 1 1/4 hardline INSIDE
the Rohn instead of outside? Having flirted with that idea myself one
time, don't think so. Running the flexible stuff, control lines, small
coax, etc, inside? Maybe nothing gained, but nothing lost either.
If one was talking about coax/wire going down inside one of those
monopole steel tubular cellphone towers (100% coverage), one does not
have the oxymoronic issues, and it's a wide open question.
73, Guy
----- Original Message -----
From: <K7LXC@aol.com>
To: <olinger@bellsouth.net>; <n4zr@contesting.com>;
<towertalk@contesting.com>
Sent: Sunday, June 01, 2003 11:31 AM
Subject: Tower Faraday shield?
> In a message dated 5/31/03 9:25:44 AM Pacific Daylight Time,
> olinger@bellsouth.net writes:
>
> > The tower and the shield are acting like a piece of coax (tower is
the
> > shield, and coax shield is the center conductor). Current from up
top
> > can be found on shield of coax running away from the bottom of
the
> > tower unless the coax shield is grounded at the bottom of the
tower.
> > Even that merely reduces the current unless it's a really good
*RF*
> > ground.
>
> What about the other way around? That is, does the tower act as
a Faraday
> shield to keep lightning transients off of the cables *inside* the
tower? Or
> is this another urban myth?
>
> Cheers,
> Steve K7LXC
> TOWER TECH
>
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