[TowerTalk] "Faraday Shield" for Coax and Control Lines

[Dave - AB7E]

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 at cis-broadband.com>
To: "Towertalk e-Goups" <towertalk at 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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