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Inexpensive wide-range variable capacitor?

Discussion in 'Electronic Basics' started by Ken, Jan 20, 2005.

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  1. Ken

    Ken Guest

    I need to be able to vary the capacitance of tuning capacitor for a
    [huge] ferrite antenna from 3 pF to 0.3 uF -- a range of five orders
    of magnitude.

    It does not appear that there is an air variable cap that can do the
    job, at least one smaller than my house.

    I was thinking of using 18 or so fixed caps each with an SPST switch
    to include or exclude each one, with the first being 3pF, the second
    being 6pF, the third being 12pf, etc. -- like a binary number. Then I
    could set any capacitance in the range to within 3 pF.

    Since SPST switches are around 70 cents, I was instead going to use a
    18-pin strip double pin pin-header strip ($2.04 at Mouser) and 18
    shunts (15 cents per) to switch each cap.

    Is there a better way of doing this?

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  2. john jardine

    john jardine Guest

    I was hoping by now some bright spark would have thrown in a clever standard
    solution. This need must come up many times but for the life in me I can't
    figure an easy arrangement down at the bottom end.
    The binary idea will work down to say 100pF but beyond that the stray C's
    become significant circuit components. There's maybe 2pF across each link
    when open and a variable amount of distributed pF to ground (dependant on
    component sizing). Say a total of 50pF with all links open.
    It looks like 5 or 6, small graded capacitors, need switching in
    series-parallel *before* the main binary elements, just to handle say the
    3pF to 50pF area.
    Even if the low end is simplified using a standard 10pF to 350pF variable
    cap there would still need to be some kind of preamble switched network
    (unless someone's already been there and dunnitright :).
  3. Rethink the concept.

    You are talking about a massive capacitor, or you've misread something.

    Decide that you don't need that range.

    Decide that you can make do with multiple antennas.

    The problem is that you have too much of a minimum to maximum frequency
    range. YOu don't state frequency range, but at the upper end that
    loopstick is likely not going to be very efficient, and stray capacitance
    will be significant.

    But then you also want it to go low in frequency, and for the inductance
    that's suitable, sort of, for the upper frequency, you have to swamp
    it with a lot of capacitance for the low frequency end. Raise the
    inducatance, and the capacitor won't be so unwieldy, but of course
    you lose the top end.

    Form follows function, but you've only told us what you think you need,
    rather than what you are trying to do. More solutions can come from
    knowing the end game.

    Traditionally, one would start with a decent variable capacitor, and then
    put fixed capacitors in parallel to get lower ranges. It cuts the tuning
    range per segment, though this is important because tuning the range in
    a single swoop of a really large variable capacitor would mean tuning
    becomes a problem.

    If you don't need full range, then you don't even need a bunch of
    fixed capacitors, just some for the specific frequency ranges.

    But likely the best solution is to go to two or more antennas.
    There likely is a point where it makes sense to do that, even apart
    from the unwieldy "variable capacitor".

  4. Ken

    Ken Guest

    Very well. I have modified an HF receiver so that it can receive VLF
    and LF and even ELF down as low as 1kHz.

    I am looking for an antenna that will fit in a small space that will
    receive 9kHz to 55kHz signals (VLF & LF) and feed them to a radio
    expecing a 50 ohm impedance antenna.

    I decided on a ferrite loop, using a VFR-12C Stormwise ferrite, See:

    Now I am trying to decide what number of turns, whether to use taps,
    what additional components, etc.

    One design calls for 500 turns, no taps, one cap in parallel with the
    loop for tuning and a second in series (10% of the first one) to
    couple with the radio. I like this arrangement and would use the
    "binary box" to set almost any capacitance in a
    six-orders-of-magnitude range.

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