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Tapped Toroid ATU

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by GW0HDY, Jun 10, 2021.

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  1. K8JD

    K8JD Ham Member QRZ Page

    I never though of using a tuner in a mobile. I just stopped and changed the Hustler coils to change bands !!
    KU3X likes this.
  2. GW0HDY

    GW0HDY Ham Member QRZ Page

    Way back I used a mono band mobile antenna and this type of step down transformer worked great for me.
    I wanted to make another to go mobile on 40 metres again. and kindly WA7ARK provided all the excellent
    information required.
  3. OK2TAP

    OK2TAP Ham Member QRZ Page

    What ferrites will you use ... Amidon FT125-43?
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    I would not call this a "tuner", because the mobile antenna first has to be brought to resonance by adjusting the loading-coil inductance and/or the length of the short whip above the loading coil. The "tuner" is the antenna itself ;)

    Once you get "resonance", with most mobile antennas, you will like be left with a feedpoint impedance (Z = R +- jX) with R between 3 to 15 Ohms and small X Ohms.

    This results in a Swr50 that is much too high for the transceiver's Swr protection circuitry, and it will throttle the transmitted power to a small fraction of its (100W?) rating.

    It is the job of the tapped transformer to step-up the Z closer to 50 Ohms to reduce the Swr as seen by the transceiver's transmitter Swr protection.
    WB5YUZ likes this.
  5. HB9PJT

    HB9PJT Ham Member QRZ Page

  6. WA7ARK

    WA7ARK Ham Member QRZ Page


    Not a tapped transformer like being discussed.

    This is for a different method of matching a mobile whip. Here you have to purposely "misadjust" the antenna loading coil so that the Zfp is has a high inductive reactance (+jX), which typically also raises the R term.

    Then you use the shunt capacitor in the box to cancel the +jX of the antenna with the shunt -jX provided by the capacitor, leaving the elevated R, which is hopefully closer to 50 Ohms than the intrinsic resonant whip would be.

    The problem comes if there is not sufficient adjustment range in the antenna loading coil.
    Last edited: Jul 22, 2021
    WB2UAQ likes this.
  7. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    And I think you need good continuous inductance adjustment for the shunt C or the shunt L method to work. This is because the slight misadjustment above or below resonance is pretty touchy but, I don't know this for a fact.
    I was following along with a group that was using the Hi Q antenna and the Hi Q can adjust the inductance continuously unlike a tapped coil. The screw driver antenna version should be OK as well and probably easier to tune with the lower Q. What do you think?

    I have been trying to explain the slight off resonance method to the guys close by me. Yes the resistive part of the feed point Z increases I worked out a few examples guessing at the feed point Z. It might even work with some of the tapped version configurations because many of them have pretty lossy inductors.

    I don't drive long and far enough to bother with mobile HF but one of these days I'll do it for the experience. No one wants to stop by and let make my own measurements on their lash-ups.
  8. N8TGQ

    N8TGQ Ham Member QRZ Page


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