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Centering mast on rotator with UST MARB-40

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KR2C, Sep 13, 2017.

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  1. KR2C

    KR2C Premium Subscriber QRZ Page

    I have a UST MA-40 with the MARB-40 rotator mount. It used to have a Hy-gain CD45 for years until high winds spun the antenna around defeating the friction brake.

    I replaces the rotator with a Ham-IV that had the wedge brake thinking this would hold the antenna better. So far so good. However, It seems that this rotator is a bit more sensitive to being centered. I can't seem to get it right. While turning the antenna I can hear the rotator start to struggle at certain points during the rotation.

    I'm looking for suggestions on how to better center the MARB-40, The rotator on the rotator plate, and the short mast to the MA-40.

    Even thought the CD45 turned the mast just fine, I'm still wondering if the Ham-IV is enough to turn the antenna system. The antenna is a Mosley TA-53.

  2. WB6MMV

    WB6MMV Subscriber QRZ Page

    Kurt: I have the MA-40 with the MARB-40 rotator mount with a Ham IV rotor turning the tower. I have had a number of different rotors over the years (Hy-gain and Yaesu) and all work just fine with respect to centering. I use a floating short section of pipe that clamps into the rotor and slips over the outside of the short metal shaft below the bearing and is secured by a pin driven through both the outer section of pipe and the short metal shaft. Its secured by a cotter pin to hold it in place. I am assuming that you have something similar so that you can crank down the tower and tilt it over when you want to service it. My short section of pipe is oversized deliberately to allow the antenna to float relative to the bearing plate and rotor. That way centering is relatively unimportant.

    Hope this helps

    Ken WB6MMV
  3. KR2C

    KR2C Premium Subscriber QRZ Page

    Thank you. My setup is very similar. I do not have the pin/cotter pin. I do have a hardened bolt that a fellow ham gave to me after shearing the Home Depot bolts in heavy winds with the CD45. The difference with my setup is that I cranked this bold tight so that there is no play like you have. I'll try loosening it up to see if I have better results.
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    With the MARB everything must be in perfect alignment to rotate the whole mast from the bottom. My test is simple: If I can't turn it by hand without the rotator, the rotator is going to have problems.
  5. W0AAT

    W0AAT XML Subscriber QRZ Page

    When I had a TT-45(Wilson, ancestor of your tower) I used over size rotor base holes and fender washers. I hand tightened them, ran it both ways in a full circle until it self center, then tightened them down. Worked fine until I decided it was to worn to be safe anymore where the upper strap on the MARB held the mast and rubbed a groove... got deep enough I was worried about snapping.
  6. AI0K

    AI0K Ham Member QRZ Page

    Do you have a thrust bearing? I think it's rather important when you have large antennas to help with just the problem you mention.
    Another possibility is the mast itself has become bent. It doesn't take much bend for it to bind.
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    A <1 degree tilt off vertical will cause it to bind.

    I've installed MA series US Towers with the MARB and it's a real precision process. I really don't like how it's designed. The big ones are worse.
  8. W1VT

    W1VT Ham Member QRZ Page

    When I did portable microwave work on mountaintops I figured it wouldn't hurt to have the mast centered precisely on the rotator, so I found some PVC pipe and sliced it in half to use a shim. You may be able to do something similar.
  9. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    With the CDE/Hy-Gain rotators that is important.

    They're all designed for a mast diameter of 2-1/16" exactly. Any smaller diameter will not rotate concentrically. The instructions clearly state that shimming is required for any mast diameter smaller than 2-1/16".

    The Yaesu rotators and the Alliance HD-73 didn't have that problem because the clamshell mast clamps adjust horizontally and if you adjust them properly the mast will always be concentric no matter what diameter it is.
  10. AI0K

    AI0K Ham Member QRZ Page

    That's true. But so will a mast that is slightly bent. And from the description of what's happened in the past (wind spinning the antennas around) it is the first thing I would be checking. And a thrust bearing will help keep the mast centered (if it's straight).

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