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How do you tune a multi-section helical bandpass filter?

Discussion in 'Electronic Design' started by mike, Apr 13, 2013.

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  1. mike

    mike Guest

    A friend has asked me for help re-tuning a 4-section helical bandpass
    filter for the 2-meter ham band. It has no adjustments, so there
    won't be too many chances to recover from botched cutting.

    As I recall, there's considerable interaction between the sections.
    And since he's been bending on the resonators, the bandpass is
    quite out of whack. He wants to move it up 2 MHz. and restore
    the shape.

    A couple of decades ago, I had a procedure that involved measuring
    return loss.
    You disable (short out) all but the first resonator and set the dip
    of the curve here.
    Then enable the second section and put that dip there.
    So on till you get to the end.

    AS I recall, it got you very close in one pass.
    The second dip moved the first one, but the end result
    bandpass came out close to what you want.

    Problem is that I've misplaced the details of where "here"
    and "there" are. I've also misplaced much of my memory.

    Anybody remember the details of that technique?
    Or any technique that might be practical on a filter
    with
    no tuning adjustments and little chance to recover from
    mistakes?

    Thanks, mike
     
  2. Jamie

    Jamie Guest

    Service monitor? an all in one tool!

    Jamie
     
  3. Joerg

    Joerg Guest

    http://www.macomtech.com/static/PDFs/TechnicaArticles/Bandpass_Filter_tutorial2.pdf

    Design instructions but no tuning info per se. You could contact them
    and see if they have more. Then there is a book by Zverev. The best
    "McGyver style" alignment instruction I ever saw was in an ARRL handbook
    but I can't recall what year. You might want to ask in a ham radio group
    if someone still has it.

    http://www.rfcafe.com/references/electrical/helical-resonator.htm

    Not sure how far the Zverev book goes into tuning, I don't have it here.
     
  4. Joerg

    Joerg Guest

    In case Mike has IEEE library access:

    http://www.rfcurrent.com/wp-content/uploads/2010/12/Milton-Dishal-Publications-on-Filters.pdf

    Unfortunately I only have access to ultrasound and to aerospace
    publications.
     
  5. Joerg

    Joerg Guest

    I tuned my last one using the "wet finger and a beer" method. Worked :)
     
  6. Guest

    I dunno if it counts as an official method, but I've used a spectrum
    analyzer+tracking generator. Each resonator makes a "bump" in the
    passband; you can locate any given bump by grossly mistuning it (with
    a hand-held cap, for example), and watching the bump jump back and
    forth. Tune, repeat.
     
  7. mike

    mike Guest

    Thanks, guys for the inputs.
    Dishal's method was what I was looking for.

    I'd found the HP article very interesting. I'll try the time-domain
    stuff next time I get an opportunity.

    I'm trying to do this remotely with someone with limited equipment.
    We'll see how it goes... ;-)


    The filter is
    http://www.dci.ca/pdf/DCI-145-2H.pdf

    Thanks, again, mike
     
  8. Yeah, the (Dishal) method is the classic one for coupled resonator
    filters.

    Vizmuller has a book on helical filters, if you want something in
    addition to the Zverev material.
     
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