Yagi-uda high SWR on 991a

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KN6KKW, Feb 2, 2021.

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  1. KN6KKW

    KN6KKW Ham Member QRZ Page

    I built a yagi tape measure antenna and checked the swr with my nanovna. The swr is under 2 but when I try to transmit on my 991a I get a high swr warning.

    I checked the pl259 to make sure there wasn’t a short but it looks like there is. It makes sense considering the hairpin match or whatever it is called is attached across the driven elements.

    I read that the infinite impedance is to be expected with this type of antenna. So why is the swr showing as high on the 991a but not the nanovna?
     
  2. SA4MDN

    SA4MDN Ham Member QRZ Page

  3. KN6KKW

    KN6KKW Ham Member QRZ Page

  4. KN6KKW

    KN6KKW Ham Member QRZ Page

    The antenna is 2m. I forgot to add that to the question.
     
  5. G8FXC

    G8FXC XML Subscriber QRZ Page

    I would expect a 2m Yagi to show a DC short across the feed - the driven element is often a folded dipole which will measure as a DC short.

    Martin (G8FXC)
     
    KN6KKW likes this.
  6. K2XT

    K2XT Premium Subscriber QRZ Page

    Tell us a bit more, please. Did you calibrate the nanovna over a relatively small range such as 10 mhz? (140-150 mhz)
    Then what did the swr curve look like? i.e. what was the freq where it was lowest?
     
    KN6KKW and KP4SX like this.
  7. KN6KKW

    KN6KKW Ham Member QRZ Page

    I calibrated 144-148 and on the nanovna it shows a good SWR of 1.5:1 and it is consistent along the band. Nice and flat. But when I put a wattmeter inline with it and check the swr it shows 99.9 (infinite). Also on my radio it was saying high SWR.

    I guess I'm just curious how the SWR can be accurately measured with an antenna that has a short circuit. I'm also curious why the wattmeter would show terrible swr while the nanovna shows almost perfect swr.
    I also don't want to fry my new radio and this is the first yagi i've built so I'm being cautious. Thanks!
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Many antenna designs would show a "short circuit" for DC. That has nothing to do with a "short circuit" for RF.

    Actually, most of the best antenna designs show a DC short circuit, which is a good thing because it can reduce static and improve lightning immunity.

    But if the VNA shows it's good and the "rig" shows it isn't, that would point to something being different in those measurements, like a different coax jumper, different connector, different something.
     
    KN6KKW likes this.
  9. K9WW

    K9WW Ham Member QRZ Page

    Please refer to the post above by SA4MDN regarding a possible problem with the autotuner board in your 991a. Click on the link. It might also explain the problems you were having with your 10 meter dipole.

    Kirk K9WW
     
    KN6KKW likes this.
  10. PE1OUW

    PE1OUW Ham Member QRZ Page

    I doubt the internal AT is also used for 2m (and 70cm for that matter). I might be wrong, but please check. I would switch it off and then check the SWR again.

    Edit: the internal AT will only 'do' 160 - 6m. Not 2m and 70cm.
     
    Last edited: Nov 5, 2021

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