Good Questions Dennis!
I believe Rohn makes a screw-in anchor with a 4 ft shaft and 6 inch dia
plate.
I use screw anchors exclusively on my 7 towers, 2 each in each direction
for the
taller towers (100 ft and up). Mobile home supply outlets, CATV
companies,
and electric utility supply houses are also useful sources for guying
materials.
IMHO, thrust bearings are over used by the amateur community. Rotors are
designed to carry vertical loads of several hundred pounds, and in fact
last
longer when they do so. This is because the vertical load helps keep the
ball bearings centered in their races. If a single thrust bearing is
used at
the top, the weight is taken off the rotor but there is considerable side
load
due to the lever action of the mast pivoting at the thrust bearing. The
solution
is to use two thrust bearings, one at the top, and the other just above
the
rotor. This is gross overkill for most amateur installations, especially
with
modest to small antennas. A "pointy top" section and rotor works just
fine.
For long masts, I like to keep at least 1/3 inside the tower, moving the
rotor
down inside. On Rohn 25, it is easier to place the rotor shelf on the
very
top of the last straight section, cutting ONE horizontal step at the
bottom of
the top section for easy insertion / removal of the rotor. This will
have
minimal impact on the tower strength.
I have never heard of anyone coating tthe shafts on their anchors but it
seems like a worthwhile idea. After 20+ years in the ground, the bottom
section on some of my (TV) towers are showing heavy rust and some metal
flaking just below ground level. Dunno how well Rohn 25 holds up under
ground.
Please post your answers, or at least send me a copy.
Thanks,
Tom N4KG
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