Idea for variable capacitor

Discussion in 'Homebrew and Kit Projects' started by KF5JAK, Mar 15, 2012.

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  1. KF5JAK

    KF5JAK Ham Member QRZ Page

    Not wanting to order a variable capacitor for a DIY tuner, I had an idea of use a 6+ pole rotary switch wired starting with a direct connection, bypassing the capacitor, then a 100pf cap, 200pf on the 3rd pole, etc. Maxing at 500pf. Now, I know this won't be as precise as a "real" air variable capacitor, but, I would think it's better than nothing. Just wanted to get some opinions on it from some people a bit more experienced than myself. Thanks!
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Will only work if the tuner's inductor is continuously variable, like a roller inductor.

    Otherwise I think this is a waste of time.
  3. KF5JAK

    KF5JAK Ham Member QRZ Page

    Why a roller inductor, why not tapped?
  4. K7JEM

    K7JEM Ham Member QRZ Page

    The idea is that something has to be nearly continuously variable in order to properly tune. If your caps are fixed, then the inductor needs to be variable. If the inductor is tapped, then the caps need to be variable, or have at least a good number of positions.

    You could probably put in toggle switches with cap values of 5, 10, 20, 40, 80, 160, and 320pF. That would allow you to pick any capacitance from 5 to 635pF in 5pF steps, but it would use 7 switches. That would probably be small enough steps to work with a switched tapped inductor.

  5. AB9LZ

    AB9LZ Ham Member QRZ Page

    A lot of autotuners work this way, using switches (relays) to swap values around until it finds something close to a match, the problem is that a computer can manage that kind of switching complexity pretty easily, humans not so much...

    Spring for a variable cap, it's worth it.

    73 m/4
  6. K7JEM

    K7JEM Ham Member QRZ Page

    Exactly. But if he wanted to do it, it would be possible. Perhaps if he had a few spots to tune up on, and the antenna wasn't changing much, he could write down the switch settings and make it work for him.

    In order to tune up on a random frequency, it might take a while to go through all the possibilities manually. A computer controlling relays can do that in a few seconds.

    Variable capacitors are not all that expensive, and for a manual tuner would be a better choice.

  7. K8ERV

    K8ERV Ham Member QRZ Page

    Another possibility would be to use a small variable cap to fill in the voids in the switched system.

    TOM K8ERV Montrose Colo
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Very true and some commercial amplifiers did that. My old Johnson Thunderbolt amp actually did that, although the variable cap wasn't exactly "small," since it had to handle a kW at 200 Ohms load. So, it was big, but not a lot of range. The range was all by fixed, switched caps (also fairly large transmitting micas). But the idea certainly works.
  9. W9GB

    W9GB Ham Member QRZ Page

  10. K5UOS

    K5UOS Premium Subscriber QRZ Page

    My opinion is buy a variable capacitor. New and used capacitors are easy to find.

    You did not say what tuner you will be building. A QRP vs QRO and/or long wire vs some other type could have different requirements.
  11. KB5HAB

    KB5HAB Ham Member QRZ Page

    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.
  12. KB5HAB

    KB5HAB Ham Member QRZ Page

    I appreciate the response, loaded with info and looks like what I need. Thank You and good night.
  13. W4HAY

    W4HAY Ham Member QRZ Page

    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: Apr 5, 2012
  14. LU1AR

    LU1AR Ham Member QRZ Page

    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.
    Edgardo Maffia LU1AR - Buenos Aires - Argentina
  15. KH2G

    KH2G Ham Member QRZ Page

    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!
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