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how to test pf of a Variable Capacitor

Discussion in 'Homebrew and Kit Projects' started by 2E0OKC, Jan 16, 2013.

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  1. 2E0OKC

    2E0OKC Ham Member QRZ Page

    hi all im doing an antenna project,

    and having a few probs , i would like to test a Variable air Capacitor this is ment to be rated at well 1 to 500pf what i was told when it was sold to me . i have got a pf meter now , is there any way i can test it,

    my Variable Capacitor looks same as this but with 22 segment,s

    see im only getting 2 bands, i do have many other people helping me in the antenna part but as this wasnt realy antenna , i wonted to try here , hope all you guys understand, thanks for any info and help best regards, andy
  2. K8ERV

    K8ERV Ham Member QRZ Page

    Connect the ground test lead to the most massive part of the cap, usually the stator. Put the + test lead
    near but not touching the rotor connection.
    Measure this lead capacity.
    Connect the + lead to the rotor, measure again.
    Subtract the lead capacity reading.
    Do the above for the cap rotated to both ends of it's travel.

    Some cap meters have a zeroing adjust that you can use to cancel the test lead capacity.

    TOM K8ERV Montrose Colo
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    "1 to 500 pF" with a capacitor of that design seems very improbable.

    The "500 pF" part sounds plausible, but the "1 pF" part doesn't. Most air variable caps looking like that and having a max capacitance of 500 pF would have a "min" of about 15-20 pF (possibly a bit more).
  4. K8ERV

    K8ERV Ham Member QRZ Page

    I saw that too but figured it was a typo.

    TOM K8ERV Montrose Colo
  5. KA9JLM

    KA9JLM Platinum Subscriber QRZ Page

    Forgot to say bought on ebay.

    A good cap meter that will work in the pf range would be in order, and keep the leads short.

    A grid or gate dipper is good also, but requires math.

    What bands are you shooting for ?
  6. 2E0OKC

    2E0OKC Ham Member QRZ Page

    hi and thanks to all you guys that replyed, well it all started out just making a mag loop antenna, i used 22mm copper tube with 6 side, the loop is 3ft 2 inch across , i got this Variable air Capacitor from ebay , one of my fav shops haha, the add said , 0-500pf and i know this wasnt to be true aswell, but it was wide space sooooooo i got it hoping it would do the job, ,, 40m is the main band and it does rez there, but you know with all this reading on loops i should get hummmm 4 bands. i do peak on 7mhz and 10mhz. thats it, was hoping for 14mhz band at least more, but with in antenna and feeder in qrz i dont wont to go to much on that here as its in the wrong place but , mainly was sent away to do tests and to try and work out why my loop not working, well i dont think it is, i can seem to tune on 7mhz in too places of the Variable air Capacitor not sure if this is odd, i just tryed what k8erv said , im not getting a reading near it but on it my meter is set to 2000pf and im getting around 1200 but im not sure if im doing this right, it doesnt stay on 1200 it jumps around 1150 1200 1250 give or take, this Variable air Capacitor could well be the wrong type, just saying aswell , once tuned on 40m compair with my vertical this loop is very hissy, anyway , i feel a cuppa tea coming on , so have a nice evening to all, andy
  7. K8ERV

    K8ERV Ham Member QRZ Page

    Jumping may be a bad meter, have you tried it on a fixed cap?

    Yes, I would not expect the lead cap to read on that setting. But you may need a more
    sensitive range to measure the minimum. The lead cap may not be important for your use.

    TOM K8ERV Montrose Colo
  8. 2E0OKC

    2E0OKC Ham Member QRZ Page

    on a fix cap it not jumping my meter wasnt lot of money i do agree , this is the make of the meter honeytek a6013L think its a china one but i did get it from the uk ,
  9. KE3WD

    KE3WD Ham Member QRZ Page

    That jumping around is a common situation when the brushes that contact the rotor at either end get dry, corroded, dirty.

    Applying a small amount of Deoxit but without spraying it into the variable capacitor, but spraying a wee amount of the good juice into something like the aerosol cap first, then using a toothpick to dab very small amound on the brush, maybe repeating a few times to get enough of the liquid only on that brush to rotor contact space and not on any plates, then rotate the capacitor full travel ten to twenty times while still wetted to mechanically clean things should establish good electrical contact again.

    Then try running your test with the capacitance meter again.

    It likely is not a problem with the meter, the capacitor is exhibiting the same problem both on the antenna and on the meter.

  10. KO6WB

    KO6WB Premium Subscriber QRZ Page

    If you are using a DMM that can measure pfd then make the test leads as short as possible. Having leads just long enough to reach from the DMM to the capacitor. No more then 3 maybe 4 inches and the connections should not be in contact with anything else including your fingers. Again, this type of measurement will not tell you the ESR (Equivalent Series Resistance) of the capacitor. The ESR will tell you the quality of the capacitor.
    According to my calculations a 3.2 foot diameter loop can tune from 60 meters to 10 meters with a 3 pfd to 350 pfd variable capacitor. Getting a capacitor to cover that range would be very difficult and even a vacuum variable would have difficulty covering that range.
    The use of the capacitor you have will make a magnetic loop efficiency a gamble. You can make a homebrew capacitor that would work so much better then the one you have.
    As a test you can use a length of coax as a set value capacitor for your loop. Pick a frequency on 20 meters and get the capacitance value you need to operate there. Hint: about 40 pfd will work.
    Here's the information for this test;
    The coax isn't really that high in Q but it's better then most of the ordinary variable capacitors you can get.
    Hope this helps
  11. 2E0OKC

    2E0OKC Ham Member QRZ Page

    thanks for all your replys , and i will look into maybe making my own vari cap, but i have done a test and wonted to let you guys know.befor i started i was getting a jumpy 1150-1250pf reading, after looking at the vari cap i see theres an arm from earth pressing on the middle shaft i have taken this out and now my readings are,
    55pf-550pf and stable just saying is there anyway i can lower the bottom end 55pf so i can get it lower. then i could maybe use this vari cap for 20m-40m these are the bands i maily use but its still fun having a bash of making this antenna project im doing, have a great day all,
  12. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Normally anything you do to decrease the minimum capacitance will also decrease the maximum capacitance.

    What range do you actually need?
  13. WB5WPA

    WB5WPA Ham Member QRZ Page

    Need to go lower in capacitance? Put two caps in series! Could also use a short length of coaxial cable to do this ... somewhere around 24 pF per foot with standard RG type cables ...

    BTW, I assume you have read the caveats about using air-variable caps in small transmitting loops on account of the mechanical method used 'contact' the rotor (moving part) of the cap .. at the HIGH currents seen during transmit this will cause very noticeable loss (and generate heat!!!!).

    I have seen the diff between air-vars and vac-var on larger loops (e.g. 1/4 WL loop) where the Q is not that high to start with ...

  14. KO6WB

    KO6WB Premium Subscriber QRZ Page

    To tune a 3.2 foot diameter loop to 20 meters the capacitance needs to be about 40 pfd and the capacitance needed for 40 meters is 189 pfd. That means you have too much capacitance for 20 meters and not enough to make it down to the 80 meter band. The 60, 40 and 30 meter band are covered. With the current values you have it is impossible to use a single capacitor to obtain the tuning range of 40 pfd to 190 pfd. To get 55 pfd down to 40 pfd requires a series capacitor of 140 pfd. With the maximum capacitance of the variable of 550 pfd in series with a 140 pfd fixed capacitor the value you would have is about 110 pfd. So now the range of tuning for the loop is about 9 MHz to 14.35 MHz. Using series capacitors in a magnetic loop isn't a good idea unless it has high Q and can handle the voltage. Same thing with a parallel arrangement.
    What you can do is either make the loop smaller or you can start removing plates from the capacitor until it has the range needed.
    The Q of the variable capacitor is highly suspect as are the losses in the contacts. Use a short piece of coax to get the magnetic loop operations confirmed.
    If you still wish to stick with the variable capacitor then making the loop smaller will bring the 20 meter band into the 55 pfd range of the capacitor and still enable operations on 40 meters.
    With a loop of 7.3 feet in circumference the loop will tune from 4.8 MHz to 14.35 MHz. Efficiency with good components is from 1.6% up to 42.4%. So that'll cover 60, 40, 30 and 20 meter bands. This is not a good example of a magnetic loops capability. You'll still make contacts but not as well as you could or should.
    You may want to rethink your approach for better efficiency on the bands you want to work.
    Hope this helps.
  15. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Or, just remove every second rotor plate from the existing cap and it will tune about the range required for 20 through 40m.
  16. KA9JLM

    KA9JLM Platinum Subscriber QRZ Page

    1 at a time, then test. They are hard to add back sometimes.

    But that may work.
  17. KE3WD

    KE3WD Ham Member QRZ Page

    Fixed capacitance in series almost always made more sense to me than removal of plates.

    Toggle switch with the cap soldered directly across its N.O. terminals allows shorting it out of the circuit easily.

    Removing plates is non-reversible for the most part.

  18. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Well thinking about it it is possible for a single series capacitor to lower the capacitance of the variable to the range desired. It's easy, the series capacitor also needs to be variable. So if you had a matching capacitor then the capacitance would always be half of a single variable. It would go from 27.5 pfd to 275 pfd. the tuning range would then be 5.84 MHz to 16.25 MHz. Efficiency with good components rises up to 7.5 to 75% instead of the 1.6 to 42.4% because the loop can still be a 10 foot circumference instead of 7.3 foot circumference.
    There are many issues that need to be resolved for this to work the way envisioned.
    Toggle switches are not up to the task in a magnetic loop. They'll have too much loss in them and they will easily arc over even at QRP power.
    All this is speculation until the OP returns and lets us know what he plans to do next.
    Have fun
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