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Tapped Toroid ATU

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

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  1. GW0HDY

    GW0HDY Ham Member QRZ Page

    About 30 years back I came across an article on how to construct a simple atu for a moble vertical. It used a rotary 12 position switch connected to 12 taps from a toroid. It worked quite well with my G Whip. I no longer have the article or ATU but have searched for the article with out any luck, Does anyone have any information on these?
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Adding a series loading coil to mobile vertical can only cancel the capacitive reactance of a very short (relative to wavelength) whip antenna. Since the real part of feedpoint Impedance is much too low (typically 3 to 5 Ohms) to drive from a 50 Ohm rig, just canceling the -jX with a series loading inductor leaves the impossible-to-feed 3-10 Ohms of Resistance. It takes a two component matching network, like a "series L/shunt L" or a "series L/shunt C" to do the job without a transformer.

    Alternatively, put a loading coil in the whip, adjust it to get rid of reactance, leaving 3 to 10 Ohms resistive. Then use a step-down ferrite auto-transformer with multiple taps to match 50 Ohms to ~3 to 10 Ohms.

    I have an old Siltronix MMBX-1 here. The rotary switch is marked 3, 7, 12, 19, 28, 38, and 50 Ohms. SO-239 in/out. Weighs about 2 lb...
    upload_2021-6-10_14-4-34.jpeg
     
    Last edited: Jun 10, 2021
    WB5YUZ likes this.
  3. GW0HDY

    GW0HDY Ham Member QRZ Page

    Thanks for the reply.
    The Siltronix you have shown is similar to what I was looking for.
    Now I have a name I will try googling it.
     
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    It was also marketed by others: Try SWAN

    It would be easy to make one.
     
    K0UO likes this.
  5. GW0HDY

    GW0HDY Ham Member QRZ Page

    Yes I looked at some SWAN information.

    I made one back in the day and I was searching for info to make another.
    Thought I may come across a diagram somewhere.
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    You want me to take the lid off the one I have and post a picture of its innards?
     
    GW0HDY likes this.
  7. GW0HDY

    GW0HDY Ham Member QRZ Page

    That would be really helpful if you could. It would be a good starting point for me.
     
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    Here you go:
    upload_2021-6-12_9-34-27.png
    upload_2021-6-12_9-37-51.png
    upload_2021-6-12_9-39-14.png
    upload_2021-6-12_9-40-4.png
    My best guess:
    upload_2021-6-12_10-7-5.png
     
    K0UO, OK2TAP and GW0HDY like this.
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    Edit after time expired:

    I missed one turn on the secondary and one pole on the switch when I drew the schematic. It really does have eight turns and seven switch positions, with taps at 2,3,4,5,6,7 and 8 turns. The seven switch positions are marked 3, 7, 12, 19, 28, 38, and 50 Ohms.

    If the output tap is at 2turns, then 50*(2^2)/(8^8) = 50*4/64 = 3.125 Ohms.
    ...
    If the output tap is at 5turns, then 50*(5^2)/(8^8) = 50*25/64 = 19.53 Ohms.
    ...
    If the output tap is at 6turns, then 50*(6^2)/(8^8) = 50*36/64 = 28.1 Ohms.
     
    GW0HDY likes this.
  10. GW0HDY

    GW0HDY Ham Member QRZ Page

    That is excellent, thank you very much.
    I will order the parts I haven't got then
    have a go at putting it together.
    I will photograph it and post how I got on.
    Thank you again.
     

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