> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Gene Smar
> Sent: Saturday, August 08, 2009 3:56 PM
> To: Dave - AB7E; Towertalk e-Goups
> Subject: Re: [TowerTalk] "Faraday Shield" for Coax and Control Lines
>
> 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.
That is not exactly why PP suggests a maximum of 70 feet of ground wire. The
reason is that the long wire becomes too inductive to be of much use when
longer. But that will vary with the type of soil involved.
>
> 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.
Ground currents can indeed induce currents in other buried conductors. It is
not uncommon for a power line strike to induce currents into completely
buried phone lines as an example.
(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...
The center conductor of your coax can pick up current from current flowing
on the shield of the coax. (look up "transfer impedance") this is why you
still need protection devices on your coax line even if you have an antenna
with a DC grounded center conductor, common with VHF and UHF antennas.
>
> o ... don't put your ground conductor in the same trench. (You didn't
> want to read that, I know.)
There is no difference from having your antenna cables run in the same
trench with or without a ground wire. The coax cables are going to carry as
much or more current than the ground wire as all the coax cables are
connected to the same tower and hopefully all grounded to the base of the
tower.
The trick is to have an adequate ground system at the tower base to
dissipate most of the energy there, keeping the base of the tower at a low
energy point.
73
Gary K4FMX
>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.)
>
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