Balanced tuners

Discussion in 'Homebrew and Kit Projects' started by EI6GXB, Sep 13, 2008.

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  1. EI6GXB

    EI6GXB Ham Member QRZ Page


    Built the balanced-balanced tuner, which performs pretty well, albeit with some preliminary RF-in-the-shack problems.

    Question(s), though :

    I've heard that this tuner mustn't be mounted in a metal enclosure due to coupling.
    How then, to prevent stray RF?

    The leads for the inductors - SWR varies massively if they are moved, I suspect mounting them vertically may overcome that one, so that the wires leave at right angles to the coils as here :

    And twisting the leads around each other, an old trick from building valve amps to keep the heater hum down.



    Last edited: Sep 13, 2008
  2. KB1KIX

    KB1KIX Ham Member QRZ Page

    Sorry I can't help you on this one... but I had to comment and say....

    That is really nice lookin!!!!

    Real nice workmanship.

  3. EI6GXB

    EI6GXB Ham Member QRZ Page

    That;s not mine lol!!

  4. W4HAY

    W4HAY XML Subscriber QRZ Page

    The rule-of-thumb for air-core inductors is to keep the metal at least one coil diameter away from the inductor(s).
  5. EI6GXB

    EI6GXB Ham Member QRZ Page

    WA-HAY?! Cool call!

    Will do, need to do some rearrangement anyhow.
  6. K7NNO

    K7NNO Moderator QRZ Page

    I built that tuner on a piece of plywood with the coils in the horizontal position. Works great. I plan on using a plexiglas front with a rotary switch since I have all that on hand. I see no problem putting it in a metal box and would if I had one laying around.
  7. W2VW

    W2VW Ham Member QRZ Page

    If that tuner is working correctly it doesn't need to be in a metal box unless you are concerned with someone getting an R.F. burn.

    If R.F. in the shack is a problem it could be a sign of antenna system imbalance. Antenna system meaning "tuner" feedline and doublet.

    Start with the tuner. Layout and lead lengths are important. The tuner pictured is electrically correct but will have problems with certain loads due to wiring. It should be constructed using a symmetrical layout with no leads close to other leads. The coil pair should be kept away from the input BalUn so as not to couple any energy.

    Another R.F. in the shack problem is when the antenna system presents a very high impedance to the "tuner's" output terminals. Tuner layout will not fix this. Adding or subtracting a few feet of balanced feeder OR doublet length should help. 2 feet at 10 meters can make quite a difference. Lower bands require proportionately juggling longer lengths to trim the system away from a high voltage feed situation.

    Another thing to look at would be the input BalUn. Certain commonly found types aren't all they are cracked up to be. The "ugly" coil of coax BalUn works very well at certain frequencies but won't cover all H.F. bands by itself. You can put these in series to get better common mode rejection. As an example 1 might have 25 turns and the second might have 6 turns of coax on a similar form. As an alternative, there are excellent toroidal BalUns available for little money these days.

    My experience with a number of installations using the same tuner design as you is that the doublet itself can be installed in a compromise situation and be relatively forgiving of imbalance. Small city lot installations can sometimes be OK. Same could be said for any good coupler design.
  8. EI6GXB

    EI6GXB Ham Member QRZ Page

    I'm wondering where my RF is coming from, what I'll do is have a few feet off the feedline first.

    I'd erroneously placed the two coils very close together but now, a lot better. Is it good practise, as in valve amp circuits , to twist all the cables together, ie. the coil clip cables twisted together, and the capacitor to output also twisted, but not near the coil clips?

  9. W2VW

    W2VW Ham Member QRZ Page

    Steer clear of twisting wires together. This practice will leave the circuit with unnecessary capacitance and limit it's tuning range.

    Twisted pairs work well for relatively low impedance signal transmission. The tuner in question can be called on to deal with much higher impedance depending on the system load and generator frequency.
  10. AI3V

    AI3V Ham Member QRZ Page

    Be aware that sometimes "RF in the shack" is the normal, and correct (albeit undesired) result of, well, the antenna radiating!

    It this case the only cure is to move the antenna farther away.

    Also, unless the "tuner" is built absolutely symmetrical. it will not be "Balanced", and there will be more or less radiation from it.

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