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My 9:1 un-un has developed a dead short.. ughh

Discussion in 'Amateur Radio Antennas' started by PA770, Aug 18, 2018.

  1. PA770

    PA770 W9WDX Member

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    I have a 9:1 I bought on EB about a year ago and I guess it got moisture in it.. I got a dead short at the SO-239..



    This sucks and I only.found when running new coax to the new shack.. and yes I tested the coax 3 times before I realized and tested the 9:1.. ughh.. glad my meter has a tone on it..

    IMG_20180715_112557153.jpg
     

  2. The DB

    The DB Sr. Member

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    Depending on the design, many if not most 9:1 ununs have a DC short between all connections. This is not necessarily a problem as RF is not DC and does not follow DC rules.


    The DB
     
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  3. PA770

    PA770 W9WDX Member

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    So your saying I may be ok??

    How would I know?? I mean I get a short on the opposing sides of the unit as well.. one on the antenna side and one on the counterpoise lug as well..

    I can't open it's sealed.
     
  4. The DB

    The DB Sr. Member

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    The first thing I would do is put a 450 ohm resistor across the antenna side of the unun and then see how well it matches up with an SWR meter. This is essentially putting a dummy load across the output of said unun matched for what the unun is supposed to convert the impedance to. I doubt SWR will be a perfect 1:1 for various reasons, but it should still be close. Just don't exceed the power of the resistor by to much during this test.


    The DB
     
  5. nomadradio

    nomadradio Analog Retentive

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    Your'e measuring DC resistance.

    The impedance of a balun only applies to the RF signals that pass through it.

    Not the same thing. It's a transformer, of a sort. And DC measurements of a transformer winding can tell you only one thing. That none of the internal connections inside it are broken.

    That's a good thing, right?

    Takes an RF instrument like a SWR meter or antenna analyzer to see what it is or isn't doing.

    73
     
  6. 543_Dallas

    543_Dallas Sr. Member

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    The 450 ohm resistor test will show you if something is wrong with it. Your new a99 antenna also has a dead short. ;)
     
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  7. nomadradio

    nomadradio Analog Retentive

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    For DC measurement, but not for RF.

    You hope.

    73
     
  8. 543_Dallas

    543_Dallas Sr. Member

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    Yes. Thanks for clearing that up again.
     
  9. PA770

    PA770 W9WDX Member

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    Ok. After some thought I thru the coax back on and fired up the HF.. all is well.. the deadshort thing I guess was what you folks said.. a DC thing, not RF. Got my normal SWR tunings for 80/40/20m bands.

    Thanks to all
     
    The DB likes this.
  10. PA770

    PA770 W9WDX Member

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    Just checked.. yes A99 has dead short using an ohm metee.. so guessing that is normal??
     
  11. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    Yes. It’s normal. Mine showed the same thing before I put it up, was fine when I checked it with the analyzer though.
     
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  12. PA770

    PA770 W9WDX Member

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    All is well. Just checked swr.. thanks
     
  13. M0GVZ

    M0GVZ Sr. Member

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    And on the Imax 2000 there's a capacitor in the middle section so testing with an ohm meter it shows open circuit. There are a lot of people who have needlessly thrown away a perfectly working Imax 2000 or bought a replacement middle section because they just went on the fact the multimeter showed no connection between the ends. However if you used an analogue meter and knew what you were looking at it would be immediately obvious there was a capacitor there as you saw the needle spike on initial contact.
     
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