Hhmmm ... the answers I've received here so far don't necessarily combine to a
clear conclusion, possibly because some people think I'm talking about
conducted currents from a strike on the tower. I'm not ... I think the tower
is reasonably well grounded at the base and I'm more interested in the chance
for induced currents on the runs to the shack due to a nearby lightning burst
(not necessarily a direct hit).
First off, here's why I think the tower is well grounded, as a couple of folks
have questioned. It's a heavy duty freestanding tower with the three legs
embedded about five feet in a 9' by 9' by 6' foundation. The foundation is
loaded with LOTS of rebar (see the details at www.ab7e.com) all wired together,
and each piece of rebar in the entire bottom grid is cadwelded together using
#4 solid copper wire. The bottom of the embedded tower legs are cadwelded to
the bottom grid of the rebar cage with one #4 solid copper wire for each leg.
The bottom of each tower leg is also cadwelded to its own ground rod buried a
few inches under the foundation with #4 solid copper wire (no, I don't buy into
the exploding foundation myth). Lastly, each tower leg has two #4 solid ground
wires (six wires total) running about thirty feet away from the tower (every 60
degrees radially) with cadwelded ground rods spaced every few feet long them.
The shields of the coax, Heliax, etc are bonded to the tower both at the top of
the tower and at the base. The control lines currently have surge suppressors
at the top of the tower, and soon will have additional ones both at the base
and at the shack. The coax and Heliax have grounded differential mode surge
suppressors at the shack. All feedlines are DC shorted either at the antenna
or the shack (the ICE suppressors have a shunt coil), or both.
So, to reiterate, my concern was mainly about induced common mode currents on
the feedline shields or control lines, and I have a hard time understanding why
induced currents on the ground wire from the tower to the SPG would in turn
induce greater currents on the feedline shields or control wires than would
already be induced on them of their own accord from the field energy associated
with the lightning. So why run the ground wire in a separate trench as so many
have advised? Doesn't make sense to me unless the concern was about CONDUCTED
ground currents from the tower.
I also still do not understand why having a ground wire above the feedlines
would not provide some measure of shielding from INDUCED currents if that
ground wire was connected to ground rods every few feet along it's length. The
concern about induced common mode currents is mostly with respect to ground, is
it not? Possibly I am wrong on that assumption. And if such a grounded shield
wire doesn't perform a useful function, why do the electric utilities use it on
telephone poles? The answer isn't critical to my installation one way or the
other, but I'd like to understand.
73,
Dave AB7E
------Original Mail------
From: "Dave - AB7E" <xdavid@cis-broadband.com>
To: "Towertalk e-Goups" <towertalk@contesting.com>
Sent: Sat, 08 Aug 2009 12:40:09 -0700
Subject: [TowerTalk] "Faraday Shield" for Coax and Control Lines
I asked this question over a year ago and didn't get any responses, so I'm
going to try it again.
The trench from my shack to my tower is roughly 180 feet long, and since it was
dug with a backhoe it is a couple of feet deep. The ground wire to the SPG,
the Heliax, and the various control lines will all be in that same trench. The
tower itself is extremely well grounded.
I live in an area with the potential for violent summer lightning storms, and
I'm wondering if the ground wire to the SPG near the shack might provide a
shielding effect to minimize induced common mode surges on the Heliax and
control lines, much like the top wire on electrical utility lines is used. I
can position the ground wire a foot or so above the other lines and Cadweld it
to ground rods spaced regularly (every ten feet?) along the run.
Anyone have a thought on whether or not that would provide any extra protection
from induced surges?
73,
Dave AB7E
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