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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

    Hi all!

    Trying my hand at a DIY mag loop antenna and having some major issues getting it to resonate.

    Main loop diameter is ~5.5ft, coupling loop is ~1ft, and I'm using a Jennings UCSXF-1500-10 vacuum capacitor (10kV, 25-1500pf) that I got for a steal on eBay. Right now it's sitting in my living room on an umbrella stand, but at least a couple feet from anything metallic.

    My problem is this: when running through a VNA, I'm reading basically infinite SWR from 1-30 MHz. There's a small dip way up at ~150MHz which is likely just some resonance on one of the elements, but otherwise it seems like my antenna is a dud. I can tune the capacitor from full-out to full-in and I get no response on the VNA sweeps. Just a nice flat line at the maximum SWR reading of ~200:1.

    First check was DC continuity, and everything looks okay there. Coupling loop is connected to the BNC just fine, and the main loop has perfectly good resistance all the way around and into the capacitor terminals.

    I'm wondering what the next steps would be in troubleshooting here. Mag loops are not mechanically complex, so I really can't figure out what's going wrong. There must be something stupidly simple I'm missing here.

    Attached are three pictures of the capacitor feedpoint connections and one of the coupling loop. Hopefully something jumps out to someone here. Otherwise I'm worried I've dumped my money into a useless design.

    Help is appreciated!


    Attached Files:

  2. AK5B

    AK5B Ham Member QRZ Page

    No reduction drive? How are you turning the cap's shaft? Either the VVC is a dud (lost it's seal) or you are skipping right through the point of resonance without a reduction drive to slow things way down. MFJ sells 6:1 reduction drives as I recall and also a Crystal Radio Society outfit in Kansas is where I got the ones I use for my loops.

    Hope this helps---don't give up just yet---a 5.5' STL with that big VVC ought to play very well on 20-40 meters once you figure it out.


    KK4NSF likes this.
  3. K6CLS

    K6CLS Ham Member QRZ Page

    yeah, I'd suspect the big cap. do you have a way to measure capacitance? LCR meters are so handy, and $cheep these days.

    just for tuning, can you put like 50pf ceramic on it? just for tuning, don't TX ha ha... anything you have in your parts drawer between say 15pf to 300pf will work: it'll make a dip somewhere, .you'll find when you sweep SWR.
    PU2OZT likes this.
  4. K6CLS

    K6CLS Ham Member QRZ Page

    BTW last year I picked up 4 vacuum caps for free. Put them on a high pot tester. none of them blew up (darn!), but several broke down at low voltages, like 200v or 500v, on 10kv rated parts... others simply did not adjust their capacitance. oh well. in to the electronics recyclers bin.
    PU2OZT likes this.
  5. WA1UIL

    WA1UIL Ham Member QRZ Page

    Try connecting a 1.8 to 30 MC receiver directly to the coupling loop and start tuning slowly. Receiver should peak somewhere in that range. Even if the VVC lost its seal it still would act as a capacitor while receiving.Only other thing would be if the coupling loop was not constructed correctly. Keep at it.
    KK4NSF likes this.
  6. PU2OZT

    PU2OZT Ham Member QRZ Page

    Except for the bargain buy, OP should question his VV range, a bit off for HF and proper tuning. Nonetheless...
  7. NK7Z

    NK7Z XML Subscriber QRZ Page

    I was teaching a class on loop building, and built a test loop of the same design we would be using... I ran the analyzer on it, and got the same results you did, no dip anywhere... This scared me greatly as the class was only a few days away. Something like 30 people had purchased the same loop building kit.

    For testing, I first hooked up a radio to the loop, and ran through tuning. I heard a distinct peak, that tested the loop as working, and relaxed me greatly... The next question was why does it show flat on my test device?

    After a bit of thinking it hit me... The loop is so narrow in resonance, that when I had my test device set to 1-30 MHz., the steps the test device took were so large they were stepping over the resonant point of the loop, hence the lack of a resonant point being located, and a flat curve with no dip showing. The testing device, was just missing the narrow dip in the loop.

    Try using a radio, and tuning the VV cap, if you hear a peak, your loop is working, or at least it probably is working. Next set your device to a far smaller range, (say 1 MHz wide), centered on the resonant point. I bet you find the dip...
    KJ7RDV, NG1H, AK5B and 1 other person like this.
  8. PU2OZT

    PU2OZT Ham Member QRZ Page

    WA9UAA and AK5B like this.
  9. NQ8J

    NQ8J Ham Member QRZ Page

    ^^^ This!!! ^^^

    When I first built a mag loop, I had my NanaVNA set to sweep from 3.5-30MHz. The NanoVNA will only sweep 101 points, or every 262KHz with the sweep settings I was using. I found using the band scope on my SDR a good way to find out where it's tuned, and then use the VNA to sweep a 20Khz span for fine tuning.
    KA0HCP likes this.
  10. W3AXL

    W3AXL XML Subscriber QRZ Page

    Thanks for all the super helpful replies everyone!

    I sure do hope the capacitor isn't bad, because I'll be out a decent chunk of change if it is. It's definitely showing some age, but mechanically seems relatively sound. Like some others have said, even if it blew its seal it should still be capacitive, so at least for preliminary testing I would assume it would be fine. We'll find out if the seal is still intact once I start putting some wattage through it.

    RE: capacitor value, like I stated earlier the reason I went with this cap is because I won it for a steal online. Yeah the low end is a little low for my uses but I'm mostly concerned with 40-20 so it should do the business for now. I guess if it ends up being dead I can start shopping for a slightly more suitable range.

    The dip being too small to see was something I didn't even consider but that absolutely makes sense too. The NanoVNA in particular only has 100 or so points across whatever span you're looking at, so if the dip is as narrow as people say there's no surprise I'm not seeing anything.

    I'll give the RX test a shot today and we'll see how that goes.
    PU2OZT likes this.

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