9:1 unun

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W3JWP, May 31, 2017.

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  1. W3JWP

    W3JWP Ham Member QRZ Page

    I am trying to build a 9:1 unun for my endfed dipole. I am having trouble with my swr. It is built with a T 200-2 ring and 14ga magnet wire with 10 turns. Long wire is about 53ft. Based on the design from what i have read it should have an acceptable swr mostof the way from 80-10m... with 20 being about the sweet spot. Instead i am seeing it above 3 and 4 swr except for 80m where it dips just below 3 unless you crank the power to say 50w and then its all terrible. See pics below. Can someone point me in the right direction? I want to build the antenna rather than buy it for the sake of accomplishment.

    73's
    W3JWP IMG_3928.JPG IMG_3929.JPG IMG_3932.JPG
     
  2. W6OGC

    W6OGC Ham Member QRZ Page

    Is that bare wire, shorting out?
     
  3. W3JWP

    W3JWP Ham Member QRZ Page

    No it is enamel coated wire which i took great care to protect during th winding process.
     
  4. W6OGC

    W6OGC Ham Member QRZ Page

  5. KT6A

    KT6A Ham Member QRZ Page

    Without an impedance tester handy, it could be difficult to detect an actual short with enameled wire in those windings. With an impedance tester, you would have decreasing amounts of impedance if measured from the following pairs of points: Antenna wire to Antenna ground (three inductors in series), solder point on the toroid to antenna ground (two inductors in series), and center pin of SO-238 to antenna ground (one inductor). If you don't see decreasing values, or if two or more measures values are very close, then you have a short.
     
    WB5YUZ likes this.
  6. W3JWP

    W3JWP Ham Member QRZ Page

    Is the design i have sound based on the info given? Or am I barking up the wrong tree? I will check impedance when i get home. Just want to know that what i built is sound
     
    KD6RF likes this.
  7. W1VT

    W1VT Ham Member QRZ Page

    I'd suggest testing it with a 470 ohm 1/4W or 1/2W resistor and measuring its frequency response. If wired correctly, you should get a reasonably low SWR somewhere. I've had good luck with type 2 iron powder in transformers for single band antennas. But, the SWR performance could have been better. I chose to ignore this issue and just adjusted the antenna lengths to tune that out. My rig sees a low SWR on my favorite 80M frequencies. Can't be all that bad--my 80M country count is now over 200 countries on a short top loaded vertical!

    http://www.karinya.net/g3txq/unun/
    This web page says a 9:1 transformer on a T-200-2 works well. But, if you look carefully, the SWR performance, by conventional amateur standards, is terrible.

    A ferrite core with a higher permeability typically gives better SWR performance with the tradeoff of higher loss.
     
    Last edited: May 31, 2017
    WB5YUZ likes this.
  8. KT6A

    KT6A Ham Member QRZ Page

    Your unun looks good, and the windings and solder connections appear to be in the right place, but if any of the enamel coating wore or chipped off where the wires touched, it may be difficult to tell if there is a short with a visual inspection. As W6OGC said, insulated wire is a preference. I used that in mine without fail.
     
    NQ9L likes this.
  9. W3JWP

    W3JWP Ham Member QRZ Page

    I will have to change it up and redo it. Does gauge wire make a difference? The original design stated 1mm/ 18ga wire. I am using 14ga. Is that a problem? Or say i went with 16ga? I want it to be beefy. As i would like it to handle up to 600w
     
  10. KF5FEI

    KF5FEI Ham Member QRZ Page

    I believe the one I built was with a mix 43 2.4" toroid. I did use the #18 magnet wire. Your construction technique looks good -- the only possible error could be in the phasing. With wire all the same color, it is hard to keep up with windings. As mentioned previously, you can use a ~450 ohm resistor and an analyzer to check it.
     

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