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Choosing antenna tuner capacitor

Discussion in 'Radio Circuits, Repair & Performance' started by 9A5DLZ, Feb 16, 2016.

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  1. 9A5DLZ

    9A5DLZ Ham Member QRZ Page

    OK, I understand. You built it like LCL and I planned on building it like CLC, but I guess it's the same thing.
  2. G4LNA

    G4LNA Ham Member QRZ Page

    Yes, pretty much, the simplified diagram is easier to understand.

    9A5DLZ likes this.
  3. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    Good thing it's in the garden. All that clickety clacking would drive the XYL nuts! The internal tuner in my Ten Tec Jupiter used to wake my wife up, till I built a shack in the garage. :)
  4. G4LNA

    G4LNA Ham Member QRZ Page

    Absolutely! It makes a brilliant bird scarer.
  5. G4LNA

    G4LNA Ham Member QRZ Page

    9A5DLZ likes this.
  6. 9A5DLZ

    9A5DLZ Ham Member QRZ Page

    I am having trouble building two exactly the same inductors for this balanced tuner. Would it be wrong to make unbalanced tuner and put current balun after it, not before like in this image.[​IMG]
    Last edited: Feb 23, 2016
  7. G4LNA

    G4LNA Ham Member QRZ Page

    There is so much wrong with that circuit, firstly the choke Balun isn't the best to use, it should be a 1:1 current Balun like this one. If you use something like an FT 240-43 core it will work fine. Try not to put the Balun on the output, the whole point of that network is to put the Balun on the input so not only is it balancing the whole antenna system, it is getting rid of common mode components, plus putting the balun on the input to the tuner the reactance is less for it to handle.

    The output network doesn't give the full range of impedance that is likely to be matched, it's OK for high impedance, the capacitor needs to be switched to the transmitter side of the L network and it needs to be a larger value, maybe up to 2500pf to handle low impedance as well.

    You don't have to use roller inductor coils, you can get away with using ceramic switching and tapping the coils, you won't get the ultimate L and C ratios, but I've found that they are close enough for practical purposes. My own system I get 1:1 to 1.3:1 across the whole HF apart from 10M where it's 1.5:1. You might have to experiment a bit to find the best taps, but I've not found it a problem if the antenna is permanent, once the tapping points have been found they can be left alone.
  8. 9A5DLZ

    9A5DLZ Ham Member QRZ Page

    G4LNA, of course, I will use relays for coils as I said before. I put this schematic just to give you a general idea what I was thinking of. I should have put this one instead

    In short, I wanted to ask if I could use unbalanced tuner with balun at the output, and you gave me an answer. It's better to use balun at the input and use balanced tuner instead.
  9. G3YRO

    G3YRO Ham Member QRZ Page

    In practice I have found you will need capacitors on the input AND output (ie a pi-network), depending on the impedance the antenna happens to be on a particular frequency . . .

    But I guess you will have those anyway, as you are planning a relay-switched fixed capacitors setup.
  10. G4LNA

    G4LNA Ham Member QRZ Page

    All understood now, as a matter of interest, what problems are you having building the inductors? I've usually found they are the easiest to build.

    If it helps, this is how I built my inductors.

    The pipe is 40mm poly wast pipe, I used Epoxy resin to fix the coils, when the Epoxy sets you just slide the coil off the former, the Epoxy won't stick to the pipe. This is a picture of one of the smaller coils prior to removing it off the former.


    This is a completed 10┬ÁH self supporting coil. The smaller inductances are only about 20mm diameter, so they don't need the former because they are only a few turns.


    Don't forget if you are using this method, mount each of the coils at right angles to each other to stop any inter-reaction to each other.

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