Dave:
A couple of points:
o Your 180-foot plus long ground wire won't even be "seen" by the SPG
(assuming the SPG is at the house/shack entrance.) Polyphaser suggests that
a buried ground wire 70 feet long (plus or minus a bit, I'm sure) is about
the max length that will be effective in dumping lightning charges into the
earth. The lightning-instigated current is nearly fully dissipated into the
earth within that length; longer lengths are a waste of money. I'd suggest
keeping the entire ground wire in the trench, but use ground rods only out
to 70 feet away from the tower and from the house.
o You should pound ground rods into the earth at intervals that are twice
the rod length, i.e., ten foot rods spaced twenty feet apart. Closer
spacing simply saturates the earth between the two rods with charge
(according to PP), reducing the earthing effect of the individual ground
rods.
o Others on TT have disagreed with me in the past on this, but it's
unlikely you'll be affected by currents induced in the earth by nearby
strikes. (A direct hit on your tower/antennas will be dissipated into the
earth by your ground system.) The effect of having a ground wire in the
trench along with the coaxes and control cables is that the lightning
current will be shared by another conductor, minimizing the current that
each has to carry for that brief lightning stroke period (typically
microseconds.) In any case, the coax shield will be grounded at the SPG
near the shack and the center conductor won't see any current from the
shield. Your control cables might pick up some current from the ground
conductor, so...
o ... don't put your ground conductor in the same trench. (You didn't
want to read that, I know.) You might consider digging another trench a
dozen or more feet away from the comms cable trench so as to minimize the
induction effect on your control cables. I dug a ground-wire-only trench
for my tower ground wire for just this reason (
j282/ersmar/?action=view¤t=Trenchtowardshack.jpg ). BTW: It's only
50 cable feet from my tower to the SPG outside my shack, so I connected the
tower ground to the SPG.
o If you dig a separate trench for your ground wire you won't have to
concern yourself with burying this wire above the coax and control cables.
73 de
Gene Smar AD3F
(Actually, five points.)
----- Original Message -----
From: "Dave - AB7E" <xdavid@cis-broadband.com>
To: "Towertalk e-Goups" <towertalk@contesting.com>
Sent: Saturday, August 08, 2009 3:40 PM
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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