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How to test for tuning capacitor watts

Discussion in 'General Technical Questions and Answers' started by CAMERART, Oct 13, 2019.

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  1. WA7ARK

    WA7ARK Ham Member QRZ Page

    Most hams just transmit into a capacitor to see if it arcs. If it does, it didn't have sufficient spacing between the plates.

    Arcing usually pitts and burns the plates of an air dielectric capacitor to where you have to buff out the burned spots. With a film dielectric, you are going to destroy the capacitor by blowing a hole through the film.
     
  2. KL7AJ

    KL7AJ Ham Member QRZ Page

    This is true....I've sanded down a number of sizzled plates in my career.
     
  3. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    I would advise against testing a polyvaricon with high voltage. Any breakdown damages the dielectric, probably fatally. Test with an Ohmeter. As long as you get an 'open" then the cap should still be good.

    Yes, I would consider 5v to be low power. Follow the project specifications and you should be good.
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Except for really big and really good ones, which use vacuum-variable capacitors, usually motor-driven.:)
     
  5. VU2NAN

    VU2NAN Ham Member QRZ Page

    Hi Camerart,

    You just need to enter the power and the feedpoint impedance (assume 5000 Ω for the end fed antenna) to get the voltage at the feedpoint.

    Voltage breakdown would result in overheating or arcing.

    73,

    Nandu.
     
  6. CAMERART

    CAMERART QRZ Member

    Hi K,
    If you check again the link in post#1, you will see that the cap does tune between 40 and 17Mtrs.
    C
     
  7. CAMERART

    CAMERART QRZ Member

    Hi N,
    I recall that in 'normal' conditions a rule of thumb is 1mm for 1000V for an arc.

    5W------------7:1 transformer---------- Here's the result from your suggestion?

    So approx 1/5 MM plate gap. Does this seem correct?
    C
     

    Attached Files:

  8. SM0AOM

    SM0AOM Ham Member QRZ Page

    It was quite common for the larger ceramic capacitor types ("beaker" or "saucer" sizes) once found in
    MF and LF antenna matching networks.

    73/
    Karl-Arne
    SM0AOM
     
  9. CAMERART

    CAMERART QRZ Member

    Hi K and W,
    Re High voltage testing. I have a megger motor with no gauge, but could borrow one. How would I use one?
    C.
     
  10. VU2NAN

    VU2NAN Ham Member QRZ Page

    Hi Camerart,

    Yes, that's correct.

    73,

    Nandu.
     

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