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Can a metal box become a resonator?

Discussion in 'General Electronics Discussion' started by Explorer, Apr 15, 2014.

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  1. Explorer

    Explorer

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    Mar 28, 2014
    I was recently chronicling my explorations with a "Frequency Finder" circuit, a "Dip Adapter" (both of which couple to a signal generator), and a Lodestar Dip Meter—all of which were behaving quite differently, in terms of helping me figure out the resonant frequency of tank circuits ( https://www.electronicspoint.com/threads/need-signal-generator-help.267975/page-2 ).

    I built the Dip Adapter in a plastic box, and then decided to line the plastic box with aluminum tape. Subsequently, I discovered that the display on the signal generator indicates "dips" around the same frequencies (several), no matter what I put the dip adapter next to. (The Dip Adapter has a coil projecting from the top of the box that is meant to inductively couple to tank circuits.)

    Anyway, I'm getting small dips around 360, 430, & 560 kHz. A pretty decent dip around 770 kHz. And a very big dip around 1250 kHz. Depending on what I put the dip adapter next to, those frequencies might shift a little bit. But, I'm definitely not getting the dips that I expect with a couple of test tank circuits that I made up.

    Is it possible that the aluminum lining has caused the box to become a resonator and that it's interfering with the way the thing's supposed to work? Should I rip it all out? Or, could something else be going on?

    Also, does anyone have a Lodestar DM-4061 Dip Meter? If so, does it work???
     
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Yep, it's called cavity resonance. This will occur if any dimension of the enclosure is equal to or greater than 1/2 wavelength of a frequency. What are the dimensions of the tank. I bet ones about 120mm ?
    Adam
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    You can damp the resonances from the housing with ferrite rf absorber polymer foam.
    Lacking such, try by filling the empty space within the housing with a resistive material, e.g. conductive foam from electronics packages. You'll have to ensure that the foam doesn't short circuit parts of your electronics, so put some insulating foil between the foam and the PCB.
     
  4. duke37

    duke37

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    Jan 9, 2011
    A small box such as you have will have resonances in the hundredes of MHz. The effect on sub MHz circuits will be to reduce the tuned frequency due to more capacity and increase the frequency due to less inductance, the inductive effect normally predominates.
     
  5. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    He doesn't give a size does he?
    Adam
     
  6. Explorer

    Explorer

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    Mar 28, 2014
    "Yep, it's called cavity resonance. This will occur if any dimension of the enclosure is equal to or greater than 1/2 wavelength of a frequency. What are the dimensions of the tank. I bet ones about 120mm?"

    Are you asking about the length or diameter of the coil? Or, the dimensions of the box?

    Rather than filling the box with more stuff, shouldn't I just rip out all the aluminum foil tape?

    I had read conflicting advise about metal enclosures and was wondering if I had made a bad choice by building the circuit into a plastic box. But, I have to say, even as I was lining the box with the tape (which took quite a long time), I felt like I was making a mistake.
     
  7. Explorer

    Explorer

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    Mar 28, 2014
    The exterior dimensions of the box are 2-1/8" x 3-7/8" x 5-7/8".
     
  8. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    5-7/8 what's that about 150mm. Just recalculated and dukes right the box is too small for that frequency.
    Adam
     
  9. davenn

    davenn Moderator

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    Sep 5, 2009
    the best dip meters are made in metal boxes eg the Kenwood DM801
    I owned one for many years ... wished I had brought it to Australia when I moved here

    http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Kenwood/trio_dm-801_dipmeter_user.pdf
     
  10. Explorer

    Explorer

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    Mar 28, 2014
    "the box is too small for that frequency." Which frequency? Are you using a formula that a very math-challenged person might understand/use?

    Most of the box is filled with the nice Weston meter I built into the thing. I had to attach a battery case to the exterior, because I couldn't fit the batteries in the box.
     
  11. duke37

    duke37

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    Jan 9, 2011
    The box will act as a screen so that hand effects will be minimised.

    There will be many resonances of the box, I think the lowest frequency will be due to a half wavelength in the longest dimension.
    You could look up Helmholtz resonator for information of acoustic resonances of a volume.

    You are dealing with lowish RF frequencies. Is your box 50m long?
     
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