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Thread: Idea for variable capacitor

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  1. #11
    Join Date
    Jul 2009
    Location
    Hurricane, WV
    Posts
    192

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    I'm intrigued at the thought of building a transmatch using all hand built components. I have a few questions I've not been able to really find the answers for. Hopefully someone with more smarts than I can put me on the right path...
    1. I assume from reading that up to 250 pf per cap would be sufficient? Is that too much or would more be better?
    2. I wonder what would happen if the aluminum plates were annodised? The aluminum oxide formed is non-conductive or so it seems by the annodising article.
    3. A value for inductance for the coil to match the capacitance?
    4. Plate spacing for the cap if used with an amplifier capable of 1000 watts output?
    5. Would the enclosure need to be metal? Being somewhat of a woodworker also I was thinking of a nice looking wooden box for it.
    I have almost all the material needed...don't have the allthread. My strengths lean way more to the mechanical rather than electrical. Thanks in advance.
    Never do anything you don't want to explain to the ambulance technicians.

  2. #12
    Join Date
    Jul 2009
    Location
    Hurricane, WV
    Posts
    192

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    I appreciate the response, loaded with info and looks like what I need. Thank You and good night.
    Never do anything you don't want to explain to the ambulance technicians.

  3. #13

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    Regarding the wood enclosure, we used to use Masonite with aluminum foil glued to it to make chassis for our projects. We'd use Elmer's glue and roll the foil into place with a small wall paper brayer.

    And if one can find large sheets of surplus unetched double-sided PCB, they make excellent capacitor plates that don't flex like sheet copper. Here's one described in Lindsay Books "Impoverished Radio Experimenter, Vol #1
    Last edited by W4HAY; 04-05-2012 at 08:25 PM.
    "A Republic, if you can keep it" -- Ben Franklin

  4. #14
    Join Date
    Jan 2008
    Location
    Buenos Aires - ARGENTINA
    Posts
    5

    Default

    You can "Slighty Variable" the coil of your ATU if you put a rotable shorted loop on the cold side of the coil. When the shorted loop is coplanar to the coil, decrease the inductance ald when is perpendicula,r increase it.
    The C and the L couls be switchable.
    I hope this will be usefull.
    Regards.
    Edgardo Maffia LU1AR - Buenos Aires - Argentina

  5. #15
    Join Date
    Jan 2000
    Location
    Phoenix AZ
    Posts
    730

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    When in the islands we had to get innovative. Aluminum sheet cut to size to make plate(s), insulators from nylon bushing stripped from fans and other junk and a piece of brass tubing to cut up to sandwich plates for the variable part and you had the basic. After that was alternate sandwiching of the plates and I found that a stainless clip cut a fastened to the main frame to keep constant contact on the variable shaft worked for keeping everything solid.
    I ran close to a KW with this and it held up fine. I admit it was not the pretty thing from a store but it worked!
    73
    Dick

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