CHEAP DIPOLE SUPPORT

FOR

ROTATING MASTS

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.

And finally various views of the installed arm.  It works great!  It was cheap!  Mission accomplished.