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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.
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.
While I am digging trenches (sure a lot easier here than in NH!) I want to
try
to anticipate needs. I have a thought of burying 2 or 3" PVC from the
shack
to the tower to carry the coax and rotor control lines, is that worth the
extra
cost and labor?
Is there any advantage to routing such runs closer or further from the house
when they are parallel for 50 feet or so? I have the option of curving
close to
the foundation or 10-15 feet away as I dig the trench. It doesn't effect
the
length much but I wondered about inductively coupling lightning into the
house
wiring.
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.
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 was advised to extend wires parallel to the foundation out from the shared
ground
to neutralize coupling that way but a concrete porch slab prevents me from
doing so.
I will instead add a couple more ground rods to improve the quality of the
ground in
the sandy soil and then Cadweld them together.
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.
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
Thanks! & 73, DavidC K1YP
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