I've seen rotators at the bottom of towers that worked fine and I've seen
some that scare me.
Some things to remember about mounting the rotator low.
The tower base still supports all the weight.
The tower base receives more of a shearing movement than twisting as it's
over a shorter distance.
A tube or even rod running up through a tower and supporting the antennas is
a torque rod/spring/torsion bar
and acts like one.
Depending on the mass of the system it may put a lot of twisting moment on
the rotator it wouldn't normally recieve. That whole works swinging back
and forth has a *lot* of momentum.
A thrust bearing at the top of the tower serves no purpose in this case
except to keep the mast centered, otherwise you are tranaserring the
additional vertical load of the mast and antennas to the top of the tower.
PVC or thrust bearings used for alignment at several points are a good idea.
The best system I saw had the rotator mounted independently of the tower.
It was on a steel plate which mounted on heavy angle iron which was bolted
to the concrete in which the tower was set. The concrete was poured a bit
wider to allow for this.
I once saw the plans for 120' of 25G with the rotator between one and two
feet off the ground and mounted to the tower. The torque tube up the center
was 1 1/2" chrome molloy tube with 1/2" wall. (It looked like an inch and a
half rod with a half inch hole in the center) I could not lift a 24'
section. Some one probably has the chart for weight, but I'll swear the
stuff was over 200# per length. I can still press 100# over my head even at
my age (which would put my back on a heating pad for a few days and my Dr
who reads this list would kick my ahh...butt. Ask me about it next time I'm
in the office), but I could barely lift one end of a section. There were 6
sections to reach the top and it extended about 15' above the top so it
wasn't a full 7 sections. At 6 sections I'd figure 1200#, plus antennas.
The base of that tower was supporting 12 sections of 25G at roughly 30# per
section or 360#, plus 1200# for mast and maybe another 60# for rotator and a
large VHF array.
All that said, a well designed system taking into account the loads involved
can be a very handy set up.
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
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