see below
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From: DavidC[SMTP:eDoc@netzero.net]
Sent: Wednesday, October 27, 1999 2:49 PM
To: Bob Wanderer; Paul Finch; Richard Spindler; TowerTalk Reflector
Subject: Re: [TowerTalk] Grounding Wire/Strip
> The water table here in west central Florida is 18-19 feet, or so I am
told.
> Is your understanding that I should drive a rod down to that depth to
> which to ground the tower?
> Not necessarily. You don't really need to hit the water table with the
> ground rod although it probably would help in your case. What is your
soil?
> If it is sandy or other poorly conductive material, you would be better
off
> hitting the water table.
> Realize that one of the purposes of the ground rod is to get some of the
> surge current into other (deeper) conductive layers.
I am told by informed sources that the soil in this immediate area is mostly
sand just about down to the water table (18-19 feet down), though there are
traces of clay starting to mix in down below 9 feet or so.
Clay usually is conductive (especially compared to sand); however, it
could be a different pH. Be careful for both the ground system and mounting
the tower base and guy anchors. Dave K6LL had various soil layers with
differing pH's and had dissimilar metal (due to pH changes) corrosion
cause him to lose a tower guy at the anchor. Corrosion can occur to an
anchor guy at the point it goes from air or soil into the concrete ped. That's
why a good inspection program is an absolute necessity.
> My tower will be about 100 feet from the shared ground for everything in
> the house & shack. If I run one of my lines as 1" 75 ohm CATV coax and
> ground the heavy shell at the tower base and at the shared ground point
> do I really need to run an additional ground-only wire to the house?
> No. Once your tower and the house single point ground are separated
> by more than about 75' they can be treated as separate systems and tieing
> them together is unnecessary unless you want to do so anyway. Nothing is
> really gained or lost. However, you must establish a good ground system
> at the tower as well as one at the single-point ground location.
Understood, thanks!
> There is a common fallacy that underground cables are somehow immune
> to lightning surge currents. I would suggest keeping the parallel cables
as far
> apart as possible.
My concern is if I have 100 plus feet of coax and rotor control line running
outside, above or below ground, and at least 50 feet of that is run in close
proximity (less than 3 feet) parallel to the foundation of my
concrete-block-
on-a-slab-home (all wiring is above ground but some plumbing, at least
drains,
are beside and under the slab) that there *may* be a possibility of
lightning
energy that gets into the antenna and rotor lines coupling into the house
wiring.
I do intend to protect all antenna and rotor lines at the tower and
shack-entry points.
It could, but if you protect the house wiring that should solve that
problem.
> Another common error is to have protected and unprotected lines in
parallel
> and close together after the protection. Surge current from the
unprotected line
> can be induced into the protected line under these circumstances.
I intend to protect every single line and have some of the Polyphaser
devices
already. A request to them for clarification as to the additional specific
devices
I need went unresponded to ... though it could have been lost when my E-mail
Inbox self-deleted ... I will resend it to them. There sure are lots of
pieces to
the lightning protection puzzle when done correctly!
For a number of reasons, PolyPhaser has become less responsive to ham
radio ops.
Steve K7LXC is a good source of assistance as is Ron Block of W.R. Block &
Associates in Clarksboro, NJ. In addition to being Roger's brother (who founded
PolyPhaser), he is a ham and has in the past given seminars on lightning
protection
at Dayton. I can assist too.
> For lightning protection purposes, the ground rods should be about twice
> the distance apart that they are in good conductive soil. So, if you have
an 8'
> rod and 4' is in good conductive soil, the spacing should be 8'; if all 8'
were in
> good conductive soil, the spacing would be 16'. I personally have
advocated
> a 1.5:1 rather than 2:1 spacing since it is usually difficult to know how
much of
> the rod is in conductive soil unless [extensice] soil samples have been
taken.
> CADWeld(TM) is a trademark and should be noted as such. However, it
> has taken the place of the more generic exothermic welding and, like
Xerox(TM)
> for photocopying and Kleenex(TM) for tissues, is used to mean that even if
you
> are using another brand of welding, photocopier, or tissue.
OK, will take a closer look at that ... and will use copper strip or 2/0 to
bond them.
> A last suggestion. Sandy soil can be glassified (petrified) by a
lightning surge.
> Then you've lost the ability to get the surge current into the ground.
Periodically
> mechanically test your system by pulling up on the rods. I'd suggest
quarterly and
> not less than semi-annually in your case. Hopefully, you've got "useful"
soil under
> the sand. 73, Bob AA0CY
Sure ... down 19 feet at the water table ...! ;-)
Perhaps I need a single 20 foot ground rod at the entry point as well as at
the tower?
The main problem with that is while it establishes a nice ground, you
have
but one path for the surge to go. You may be better off with several 20 footers
about 15 feet apart or as many as you can afford to install. You may be able
to get rods at a lower cost from either a utility or a junk yard. I once
picked up
several 8' copper-clad 5/8"-dia rods for a few bucks each at a salvage yard.
73, Bob AA0CY
Thanks again! & 73, DavidC K1YP
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