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. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Here is an example of one Polyvaricon. This example is more complex and has [Four] sections/gangs and contacts/solder leads [and Two Trimmers]. A very basic polyvaricon may have only two sections (each with a Trimmer).

    Note the different values of the sections. Normally you will find that sections are in pairs with the same value. The center post (brass color) is the main tuning control which rotates all of the sections at the same time.

    [​IMG]
     
  2. K6BSU

    K6BSU Ham Member QRZ Page

    Antenna tuners should have only air variable capacitors. As noted by others, high voltages can be produced by tuners if the antenna SWR is high. For an air variable, I use the typical breakdown voltage of 40,000 volts/inch. For a 100 Watt tuner, that might require at least 0.1 inch plate spacing. Plastic dielectric caps are questionable, except for very low power.
     
  3. CAMERART

    CAMERART QRZ Member

    Hi Y,
    Yes, I understand Ohms law, but I don't quite understand the higher voltage that maybe there with an endfed wire transformer.

    The original cap is as mentioned a small plastic type from a little transistor radio, and I'm guessing here, but as it's only a receiver, may be the problem when transmitting. It work previously with a Foxx3 1W transceiver,but didn't with 5W, and it seems the most probable answer.
    C
     
  4. CAMERART

    CAMERART QRZ Member

  5. CAMERART

    CAMERART QRZ Member

    Hi again K,
    This is like the one already on the coupler, that may be giving trouble.
    C
     
  6. CAMERART

    CAMERART QRZ Member

    Hi K,
    Is 5W what you call 'very low power'?
    C.
     
  7. CAMERART

    CAMERART QRZ Member

    Hi,
    Thanks for all of the replies.

    I would like to know how to test any capacitor, to breakdown voltage. How would I do this, and if one breaks down, would it damage it?
    C
     
  8. CAMERART

    CAMERART QRZ Member

    Hi K,
    Don't worry about Rege, if it keeps him happy, there's room for him here, it bothers me not, I've had worse :)

    Re: the coupler in #1. When working, the LED switches when the SWR is correct. Perhaps, on the fail test, I simply had the wire in a bad position, so far out of SWR, that it didn't switch the LED, or transmit.

    I have an analyser, that I can try with it. I'll report back.
    C
     
  9. KL7AJ

    KL7AJ Ham Member QRZ Page

    Capacitors are rated in volts and (for higher power RF applications) in amperes. Theoretically, watts would be irrelevant, since capacitors are presumably totally reactive. (Although you could probably come up with an Apparent Power rating for capacitors...never seen that.)
     
    K8AI likes this.
  10. KL7AJ

    KL7AJ Ham Member QRZ Page

    p.s. Non destructive breakdown testing of a capacitor is done with a "megger" or equivalent, which is a high voltage source with an EXTREMELY high impedance ...usually on the order of a gigohm or more, to limit the current to save levels when the capacitor breaks down.
     

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