Hi Bob,
For the better part of 17 years, I have had a 105BA (later converted
to 105CA) and 155BA being turned by a Ham IV. (I am the third owner of
this rotator, so I don't know how old it is.) The antennas are spaced six
feet apart.
Since the rotator is going to be 3 inches below the thrust
bearing, I presume you are using the Rohn short top section (2.5 or 3
feet long?) which comes with a flat plate on top for mounting the thrust
bearing. I would not feel comfortable with that arrangement. It seems
to me that with a mast sticking 10 feet out of the tower, and an antenna
on top, and with only a foot or so of mast below the thrust bearing, there
is a chance that the rotator is going to experience a great deal of force
from the bending motion of the mast. (Imagine this arrangement as a lever
with 10 feet extended beyond the fulcrum and 1 foot under the load. The
3 or 4 bolts in the thrust bearing which can adequately keep the weight of
the antennas/mast off the rotator are not likely to absorb all the force
applied by the lever. Of course this comes as my opinion, but I think
you could demonstrate it by setting the tower section sideways on the
ground and applying the force manually after securing the mast in the
thrust bearing. Can you easily make the mast move inside the top section
while lifting the antenna end of the mast? How hard did you have to work
to make that movement?)
What I have done is include more mast inside the tower, sort of
to balance the lengths of the lever on both sides of the fulcrum. For
Rohn 25, I simply took a tower section and had a flat plate welded on top
for the thrust bearing. If 10 feet of mast inside the tower is too much,
simply cut a few feet off the top of the tower section before having the
plate welded on. It does make a manual installation of the mast a little
trickier if the balance point of the mast is at such a location as to
require it to be flipped over to get it into the top of the tower, but I
think it makes for a longer surviving rotator.
Another thing to consider is how difficult it will be to
stabilize the mast inside the 3 foot tower section when it comes time to
replace the rotator. (Yes indeed, you will almost assuredly have to
replace it someday.) It's a lot easier if there is more mast inside the
tower.
73, Dave Clemons K1VUT
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