DIY Mag Loop Infinite SWR

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W3AXL, Nov 5, 2020.

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  1. W3AXL

    W3AXL XML Subscriber QRZ Page

    Alright, quick morning update. Much better success this morning, thanks to my non-tired brain and some finer tuning.


    Turns out I had the capacitor way off tuning-wise. And, I think the comments above about this capacitor being a little too large in range are probably true. Every 1/8-turn moves me a good 100KHz or so, if not more, so while the stepper motor I'm planning on adding should be alright, I'll probably look at investing in a gearbox reduction of some kind just to make things a little more precise.

    When I have some more time this afternoon, I'll actually set it up on the balcony and try some HF. These initial results are promising though! Definitely happy that my design isn't a total flop.
    K6CLS, AK5B and PU2OZT like this.
  2. AK5B

    AK5B Ham Member QRZ Page

    Think of the resonance point of a well-built STL as a one-horse town in the middle of nowhere. If you so much as blink when you pass through it you'll miss it completely.;)

    FWIW I use a 6:1 reduction drive with a 0.6 RPM DC gearmotor controlled by a PWM speed and direction module---slow as a turtle with a hangover but works FB for fine-tuning once in range.

    Have fun---and you will---loop building can be addicting!



    (The loopy guy in South Texas)
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  3. W3AXL

    W3AXL XML Subscriber QRZ Page

    Another small update: looks like my current loop size and capacitor combo cap out at around 13.6 MHz. Just shy of my 20m target. Thinking since my loop is oversized to begin with (I was going for 5ft diameter) I should be able to lop some of the pipe off the ends to shrink it down a bit. Hopefully that's enough to get me safely to a workable 20m antenna.
  4. NQ8J

    NQ8J Ham Member QRZ Page

    Shouldn't be a problem, reducing the size of the loop by 10% should more than get you there on 20M.

    You should have no problem tuning 80M either. I had a 3' loop and a 1200pF-20pF cap and could go from about 3MHz to 18.5MHz
    W3AXL likes this.
  5. AK5B

    AK5B Ham Member QRZ Page

    Yes, that is exactly what you need to do to get to 20m. At 17.25' circumference (5.5' diameter) you need only 13.2pf to resonate at 14.2MHz---so if you shorten the element to 15 to 16 feet you might get it with the cap you have (@16 to 17.9pf needed). 15 feet equates to a 4.8 foot diameter loop, FWIW.

    You might want to check out the KI6GD loop calculator (it is downloadable from Simone, IW5EDI's site) to figure out what will work best for you, too.

    EDIT: Oops! I just re-read your first post and saw that your min. capacitance is 25pf---so scratch the lengths I suggested above---for 20 meters capability you will either have to reduce the pfs with a different cap OR shorten the element further---12.1' to be exact which would allow 25pf to tune 14.2MHz. You'll still be able to tune 40 meters but not quite at the same efficiency.


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  6. W3AXL

    W3AXL XML Subscriber QRZ Page

    Alright, now we're in business.

    Took about 8 or 9 inches off of each end of the loop using my laser eyeball. Even made it slightly more circular too as a bonus. Hooked everything back up and ran another sweep on 20m:


    That's definitely well within "good enough" territory for me. Minimum SWR is a 1.3 right now, and I could probably make it even better with some adjustment of the coupling loop, but I'm very happy with how this has turned out so far.
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  7. W3AXL

    W3AXL XML Subscriber QRZ Page

    More plots for 20m, 40m, and 80m.


    Things get real tight at 80m. 2:1 SWR bandwidth is just about 4kHz. Usable, but I doubt the efficiency is anything to write home about either. Still, could be useful for local/state nets. Overall, I'm pleased as punch.

    Now to put some power into it and see if we get any sparks.
    WB5YUZ, NQ8J, WA9UAA and 4 others like this.
  8. WA2COP

    WA2COP Premium Subscriber QRZ Page

    Nice work!
    I am interested how you made your connections. Curious what you did and why you went that route? Can you talk about it a little more?
    PU2OZT likes this.
  9. BILLYBOB560

    BILLYBOB560 QRZ Member

    If a Vacuum Variable loses it's seal, the shaft gets REALLY easy to turn.

    But I doubt this has happened in your case. Those Jennings units are *beautifully* designed and constructed (Works Of Art, IMO). They're quite Robust.

  10. K8JD

    K8JD Ham Member QRZ Page

    I believe the magloops derive efficiency by limiting bandwidth drastically.
    I have a full wave loop that works low with SWR (below 1.5:1) over most of the 40 M band . (Actually measured from 6.9 to 7.2 MHz)
    That has a circunference of 140 ft, I used a "Q" section of RG6 for matching.

    If I make a mag loop of 6 ft diameter it would probably only work over a few KHz before you needed to retune it.
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