Seems to me that amount of cantilevered weight (you don't mention how
far above the rotator is the 200# antenna) is risky for most rotators.
The tail-twister has a nice spacing between the bearings, but how strong
are the aluminum casting and the bottom 5/16-18 threads in it, a die
casting?
I've built a couple of mast supports for similar situations.
For a friend I made a "C" shaped extension for his 4x4" 6061 mast so he
could mount a tree of VHF/UHF antennas. The bottom of the C was 1/4"
plate bolted to the existing mast top plate and had enough offset to
mount a rotator, standard hole pattern. Then one 2 1/2" sq tube about
3' to another horizontal plate for a "thrust" bearing. Thus, the
rotator load was that of one mounted inside the tower.
So I would build a stub tower extension large enough to fit the
rotator. Mount a triangular 1/4" thick steel plate to the top plate of
the tower using the existing rotator holes. For 200 lbs of antenna,
three 3' to 4' long vertical 1" diameter (1" sch 40 pipe is 1.32" od)
supports (spaced enough to get in the rotator and plate large enough to
weld to) to a same size as bottom top plate that holds a polymer radial
bearing. Then the limit is the tower spec not the rotator. Most any
welding shop could knock this out in an hour or two if you are able to
do the finishing & painting.
Or maybe Chris KF7P.com might make a product for rotators that don't fit
or masts larger in diameter than the tower top tube. Known problems.
For telephone poles, I made a "C" shaped frame that bolted to the pole,
4' tall vertical and had a rotator pattern drilled plate two horizontals
welded to the vertical. Rotator at the bottom, "thrust" bearing at the
top (I admit it was a Yaesu bearing, this was before I became
evangelical about plastic ones). I had side straps to the sides of the
pole to minimize the twist on the thru the pole threaded rods. This
held an A4S in a high wind ridge top environment. The 35' tall pole was
guyed.
Grant KZ1W
On 7/21/2017 9:46 AM, Jeff AC0C wrote:
I am using a mobile crank-up / tilt tower for some antenna testing that has a
top section too small to get a ham4 rotor in. Unfortunately that means I need
to mount the rotor on a pole that extends from the top of the tower.
The good thing about this setup is the tower nests and the tilts down
essentially parallel to the ground, allowing you to mount the antenna while
standing on the ground. The bad thing about that setup is the rotor has got to
be stout enough to hold the antenna weight while the rotor is temporarily
horizontal.
I have used a ham4 in the past but that was with antennas which were pretty light weight
(under 75 lbs). This time the antennas will be somewhat more heavy – up to about 200
lbs – and I have a tailtwister available.
Would the tailtwister would be happy holding that much weight off the nose when
it’s temporarily horizontal?
Higain says the load capacity for a mast mounted configuration is half of a
inside-tower-mount case but I have no idea if that is applicable in the case of
the rotor being mounted horizontally. Seems like it would because the rotor is
going to feel a lot of torque in the externally mounted case with serious wind.
In my case wind is not an issue. I don’t leave the antenna on the tower when
I’m not testing it. So the only question is how the tailtwister would do with a
couple hundred pounds pulling down on the top when the tower is tilted over?
Appreciate any comments or insights.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
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