Feeding doublet to an unbalanced tuner

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by G5TM, Mar 21, 2019.

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  1. G5TM

    G5TM Premium Subscriber QRZ Page


    Hi all

    If I ran 300 ohm ladderline into one of these (see link above) and double male connected it to my existng unbalanced LDG auto tuner - would this do without the need for a 1:1 or 4:1 current balun (with a very short run of coax) which I would otherwise need, in order to ccomplete the feed from antenna centre to the tuner?

    Before anyone suggests - I am unable to run ladderline straight to the shack so this is my option as I can place the autotuner in an out-house and run coax to the shack.

  2. AI3V

    AI3V Ham Member QRZ Page


    You want a balun at any balanced/unbalanced junction.

    Please be aware that for any balun to work properly the balanced part of the antenna system has to actually be balanced.

    G5TM likes this.
  3. W5DXP

    W5DXP Ham Member QRZ Page

    Also be aware that a 1:1 50 ohm transmission line transformer only does a 1:1 transformation when the load is 50 ohms. For instance, the following graph shows the impedance looking into the feedline at the choke output (broad line) vs the impedance at the input (narrow line) for a typical 1:1 TLT on 10m. The impedance at the output on 28 MHz is around 150 ohms while the impedance at the input is around 20 ohms. That's a transformation ratio of 7.5:1, not 1:1, and results in higher losses in the tuner than if the ratio was actually 1:1.
    G5TM likes this.
  4. G5TM

    G5TM Premium Subscriber QRZ Page

    Thank you both for taking the time to reply. It's appreciated. 73
  5. WB2JIX

    WB2JIX Ham Member QRZ Page

    As a side note worth mentioning, most every customer of mine that calls or emails and says they can't tune ALL the bands on their doublet, are using an LDG tuner.
    In my opinion and my observation, they just can't handle the range of high SWR's present on a doublet. Certainly so when we hope to tune 6-160M


    G5TM likes this.
  6. G5TM

    G5TM Premium Subscriber QRZ Page

    Interesting observation.
    If using a balanced tuner do you need to retune if moving to another frequency in the same band? Say moving to 14250 from 14180 for instance?
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    And this is avoidable if you take the trouble to calculate the impedance looking into the ladder line from the shack end as a function of ladder-line length, doublet length and frequency.

    Using a random-length ladder-line with a random-length doublet does not insulate you from the having to figure it out, and possibly change the length of one or other to avoid impossible-to-match combinations, or combinations that arc over if using QRO.
    WN1MB likes this.
  8. AA5CT

    AA5CT Ham Member QRZ Page

    Hmmm ... dart-board planning? One of our instructors had a technique he called the "four-paw puppy dog" system and it involved paper on the floor and counting footprints ...
  9. K2CAJ

    K2CAJ XML Subscriber QRZ Page


    I have a doublet and an LDG IT-100 tuner. The tuner is definitely not balanced---it's just an L network---but I already have it and I want to spend zero dollars for a while, so this is the tuner I'm using. It's also neatly compatible with my IC-7100.

    What I did was make a 1:1 Guanella current balun by wrapping 11 turns bifilar around a toroid, one end connecting to the ladder line and one end going to an SO-239 with a very short stub right to the tuner's antenna output. That provides an interface from the ladder line to the coax tuner input, and balances the output of the unbalanced tuner.

    I should add, however, that I designed the whole thing so that balun+ladder line tunes the doublet to 50Ω on 20m, so on my main band of interest I'm not using the tuner at all. However, it tunes up alright on other bands, albeit not all. If I wanted to tune up on other bands I'd choose a different length of ladder line, but I optimized mine for tunerless operation on 20m.
  10. AI3V

    AI3V Ham Member QRZ Page

    If you find your tuner Dosn't like to tune a certain band, try hanging another pair of wires in parallel with your existing wire, cut to be a center fed halfwave on the trouble band.

    This will keep the automatic tuning without the bother of manually switching line in and out.


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