I have similar poor soil (e.g., rock) conditions at my QTH in south-central
New Mexico. While excavating the foundation for my second tower I hit a
very solid layer at only 18 inches that required a jackhammer and clay spade
attachment to bust through. I'm down to four feet now and ideally have one
more foot to go.
This tower will be 200 ft. away from the house. Ground rods are impossible
to pound in by hand here(although my power company did install one next to
the meter pole). My solution for providing a ground for this tower will be
to bury two radials of 1-1/2 inch wide 26 ga. copper strap about 6 inches
below grade. Each radial will be 50 ft. long and extend in opposite
directions from the tower base. The copper strap attaches to the tower
using Polyphaser TK-4 clamps, which prevents the copper from being in direct
contact with the galvanized steel base (dissimilar metal issue). Ideally,
the strap should also be connected to ground rods spaced every 16 ft. or so,
but that simply is not going to happen in my case. In theory this design
should provide a low impedance path to dissipate electrons into the earth.
Copper strap is more effective than solid copper ground wire because the
large surface area of the strap better exploits the skin effect.
The tower being 200 ft. from the shack is advantageous because the
inductance in the feedline will slow the pulse risetime as it travels down
the coax allowing the ground system to bleed off more charge before it
reaches the shack. In addition, the coax braid will be bonded to the tower
at two points, top and bottom.
I must take exception to your not grounding the coax bulkhead to your house
ground. In my opinion, disconnecting the entry coax from the bulkhead does
not guarantee that strike energy won't get into your house. After traveling
thousands of feet to that point, a few more inches won't make any
difference. Furthermore, if the bulkhead is not bonded to your house ground
then during a strike event there will be a significant difference of
potential between the bulkhead and the house and current will find a way
flow (bad). If instead the bulhead and house ground are tied together, the
two points will be at the same potential and will rise and fall
simultaneously, a much preferred situation.
Best of luck!
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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