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Thread: Practical Bias T examples for HF usage?

  1. #31
    Join Date
    Jul 2005
    Safford,  AZ


    Quote Originally Posted by 4L4AGU View Post
    Currently I have no other SWR measurement equipment, except SWR meter built in FT450AT. It shows perfect flat SWR up to 50mhz, as it will do when directly hooked to dummy load. Regarding the insertion loss, it was not in measurable range from 1 to 30 mhz, it showed up itself only below 200khz. Below is attached schematics of my bias-t.

    Attachment 91595
    Did you try it with just the single .68uF cap and a single coil? Isolation to the DC port may be a little less, but that is really not that critical. I have never seen these things that had to use two coils.

    Usually, there is one cap in series, or maybe multiple parallel caps working as a single high current series capacitor, then a single non toroidial inductor of about 150uF or so. Then a couple of caps at the DC side of the inductor.

    We cannot tax our way to prosperity.

  2. #32
    Join Date
    Feb 2004
    Bakersfield, CA

    Default used two inductors. With the circuit you have it is possible to control two devices just by using reverse polarity. If you're looking to have voltage applied to a pre-amp mounted on the tower then you would use a bridge rectifier and it will always have the proper voltage regardless of polarity.
    It would be better to parallel C1 and C2, but if it works for you then don't mess with it.
    Just a thought.

  3. #33
    Join Date
    Feb 2003
    SW Missouri


    Actually, at least four control states are possible: Off, +, -, and AC.
    73, Mike

  4. #34


    LOL all what I need is to deliver 12V, 500mah current to MFJ-925 ATU

    The double stage, 2 capacitor design was choosen because with one inductor, leakage started to increase as frequency goes higher, and was showing -19dB on 24mhz.

    The component values were selected based on what I had on hands, no special calculations, just try & fry

  5. #35


    Quote Originally Posted by 4L4AGU View Post special calculations, just try & fry
    I like it but "empirical research" reads better on a resume.
    In the words of Al Einstein, "If we knew what it was we were doing, it would not be called research, would it?"

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