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BACKGROUND
I have a Wilson MT-61 rotating mast. It
is very similar to the US Tower MA-550. If you ever checked US Tower's
catalog you will find that you can not use their dipole support if you have a
rotating mast with the rotator at the base. So the problem was to come up
with a dipole support that would keep the feed point of the dipole away from the
mast and at the same time keep it in a fixed position so it would not wrap
around the tower.
EXPERIMENTS
With the help of W7AX we tried several
experiments to see if they would work. The objectives were stated above
and one more objective was added. It had to be inexpensive.
We tried to use a rope attached to the tower
and had a pulley suspended from it. The idea was that the dipole would try
to maintain the same position by traveling in the pulley as the tower rotated.
Click on ANY picture for Larger View
This had several problems. First, and in
my opinion most important, the center connector for the dipole had to remain too
close to the mast and affected performance. If we let more out so that it
pulled away from the tower then the coax got caught on the standoff arms.
Next I thought of using a 2" pillow block
bearing. Available for about $21 this would slip over the mast. An
arm would be attached and the dipole supported from it. Two
problems. First, the beam would have to be removed to slip the bearing
down to the correct position. Second, we would lose one of the
objectives....cheap.
SOLUTION
We took a good quality 2x4 with a tight grain
and glued the extra piece shown on the right. Then bored a 2 1/8"
hole through both pieces.
Then the end of the block was split in half so
we could get it on the mast without taking the beam off the mast. 2 extra
screws were used to attach the short block to the long arm. (Just for
safety) The two halves were secured with 4 long screws. We predrilled the
short side but not the long side in order to get a sure attachment without
splitting the wood.
In an attempt to have a smooth surface for the
arm to ride on I took an ordinary 2" PVC slip/slip coupler.
Then we had to split that so we could again
get it on the mast without having to take off the antenna. The clamp was
put around the middle to hold the two sections together.
Here you can see the dipole attached to the
arm and resting on the bearing. The tower is tilted over.
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And finally various views of the installed
arm. It works great! It was cheap! Mission accomplished.
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