Gene Smar wrote:
> 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.
>
This is one where I strongly disagree with PP. My tower is around 90
feet from the station. About 80 feet from the entrance grounding
bulkhead. I had one of the cables to the rig disconnected at the
selector switch. All of these cables are about 8 to 10' long with the
shield grounded to the bulkhead. Although tied to all the other cables,
the disconnected cable flashed over to the selector switch housing about
10" away with a bang, loud enough to leave my ears ringing.
How long that cable has to be before the tower no longer sees the house
varies considerably with the rise time, duration, and fall time of the
lightning stroke. So, I think PP did every one a great disservice by say
the tower no longer sees the shack from more than 75 feet.
> 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.
Agreed
> Closer
> spacing simply saturates the earth between the two rods with charge
> (according to PP),
Let's just say they are less effective than two ground rods twice the
rods length apart.
> 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.)
More or less.
> 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.)
It's more than that. A heavy ground cable in the same trench or worse,
conduit will carry much more current than the coax shields and can
inductively couple current into the coax and control cables. But I do
agree that a good ground *system* will dissipate most or at least a good
portion of the strike into the ground. Typically a nearby strike will
*directly* induce current into cables and wiring within the house rather
than coming in on a well grounded and designed ground system.
> 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.
Agreed.
> (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.
>
Due to variations I'd at least keep the ground wire out to 200 feet.
It's true that with some strikes 75 feet would be more than enough and
in some rare cases you'd want more than 200. I figure the extra cost
and time is worth it as an extra 125 feet isn't all that expensive and
can be buried with a trencher faster and cheaper than a back hoe with a
whale of a lot less mess.
My tower is as I said a tad over 75' from the SPG. The tower has taken
at least 15 (Visually verified) direct hits. I don't know how many it's
actually taken. Since I finished up the ground system those 15 hits
apparently did no damage, but that one disconnected lean on this side of
the SPG Flashed over 10" to switch housing that was tied to the same
SPG ground with LMR-400 of approximately the same length.
> 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 have over 600' of bare #2 and 32 or 33 ground rods cad welded to the
system.
>> 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?
>>
Actually your lines are capacitively coupled to the ground so they in
essence bleed off more steeply rising (and falling) pulses into the
ground. AS I mentioned above, I'd want the ground line in a seperate trench.
73,
Roger (K8RI)
>> 73,
>> Dave AB7E
>>
>>
>>
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