Mosley Traps - Things I'm Learning

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W1QC, Apr 27, 2020.

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  1. W1QC

    W1QC Ham Member QRZ Page

    I’ve been refurbishing a couple Mosley antennas, a Classic-36 (completed) and what I think is a TA-33. I have also been trying to create a 4NEC2 model of the Mosley TWO-IN-ONE trap. For reasons beyond my control, I don't currently have a tower. Hopefully some day soon...

    Cracking Bobbins

    I have a few bobbins that have cracked and have found a few issues that might affect their performance. When a bobbin does crack the ends of the bobbin spread apart. Often the end of the bobbin that has the screw connecting the coil to the outer tube moves closer to the tap hole. Normally in an undamaged bobbin, the distance between the end of the bobbin and the tap hole is exactly 1 inch. But I have seen some cracked bobbin ends that are less than 0.9 inches away from the tap. See picture.

    To fix this problem put the bobbin in a vice standing up. Put a 1 inch ID 8 inch long piece of pipe over it and tap down the top of the bobbin until its top edge is exactly once inch away from the tap. See picture.

    Some bobbins crack in places where they spread out one or two consecutive turns of the coil, thus changing its value. I think these bobbins should be discarded. If one is going to spend the time and energy taking down the antenna, repairing it and putting it back up, he should spend the money maximizing the chance the coil won’t degrade in short order. However, if the bobbin cracks past the coil (as many do) then you can tap the bobbin back together.

    Mosley Traps Rattle

    I’ve found that if you grab an end of a Mosley trap and wiggle it, you can hear the trap rattle. That’s because the bobbin has some play inside the outer tube. The tolerances are not that tight. One way to reduce the problem is to wrap a layer of electrical tap around the inside end of the bobbin. See picture. I suspect this rattling contributes to the cracking of the bobbin. Imagine all the shaking that goes on during a 50 - 80 MPH wind storm.

    I hope to have more soon.

    Wayne, W1QC

    Attached Files:

    KI4ZUQ and W4GOV like this.
  2. W4GOV

    W4GOV Premium Subscriber QRZ Page

    Thank you so much for this post. I am in the middle of restoring a TA-33 and the tape suggestion on the bobbin was brilliant! Thanks!

    Attached Files:

  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Great writeup!

    If you haven't already read this, you might want to read "HF TRIBANDER PERFORMANCE" which is based on real-life field strength testing including the Mosley beams.

    It's a bit of an eye-opener and based on having installed a lot of HF beams over a lot of years, I agree with everything N0AX and K7LXC wrote.

    There's a compromise using "dual-band" traps, and obviously the main one is it shortens the elements more than using separate traps.:) But people use them, and most seem to like them.

    Only installing beams of a different design at the same place at the same time yields useful data.

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