How to test for tuning capacitor watts

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

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    I may not have the terminlogy correct, but I hope you can follow.

    I made an endfed coupler:

    and used a capactor from a transister radio. I now find that it doesn't work with 5Watts, and was told it isn't 'man' enough.
    I now have to get one that can cope with higher wattage.

    I have a few tuning caps, somw of which are quite large, and I want to know how to choose one that is just 'strong' enough.

    Can anyone shed light on this please?

    Cheers, Camerart.
  2. VU2NAN

    VU2NAN Ham Member QRZ Page

    Hi Camerart,

    You may calculate the voltage at the antenna feedpoint (at different power levels) considering a feedpoint impedance of 5000 Ω.

    Polyvaricons used in transistor radios can withstand about 100 V.


    Last edited: Oct 13, 2019
    YO3GFH likes this.
  3. YO3GFH

    YO3GFH XML Subscriber QRZ Page

    Hello there!

    For 5 watts, try using the smallest air capacitor (with a similar value) that you have. Those plasticky caps just can't cope with the voltage which appears across them when matching end-feds. Small is beautiful, but you can't have all :)
    Good luck!
    VU2NAN likes this.


    Hi N,
    Thanks for your calculator, very handy, but I'm not skilled enough to be able to use it, as I'm not sure quite what voltage, there will be.
    Hi Y,
    Ok, if I try a different one, how do I know if it's breaking down? If I tried putting a voltage on to one set of plates, while reading the other set, would something happen at it's breakdown voltage. If it does, and I'm still living :) would it have done damage to the cap?

    I used to work in an electric museum, that had Wimshurst machines etc
  5. W9WQA

    W9WQA Ham Member QRZ Page

    ya wonder why some guys never come back huh
    KA0HCP likes this.
  6. YO3GFH

    YO3GFH XML Subscriber QRZ Page

    Air capacitors exhibit "arc-ing" when subjected to more than breakdown voltage. That would be a discharge between the rotor and the stator and may be audible (like when you pull a wool sweater over your head). But, at 5 watts, it should be ok.

    On the other hand, even at low power, if you try to match some crazy reactive load, you may be in for a surprise - for example, I can't use my Z-match with more than 50w in 15m on my crapwire antenna; tried 75w at first and heard arcing, then lowered the power to 50 and it works ok. But to destroy one, you'd have to pump some serious power (hundreds of W).

    For your setup (I assume 5 watts), just replace the plastic with an air cap with 1-2mm distance between the rotor and stator plates, and you should be good. These would take probably hundreds of volts before arcing, so it would be ok.


    The calculator from the link above uses Ohm's law and needs any two components to calculate any other of the I (current), R (resistance), V (voltage) and P (power).
    Last edited: Oct 13, 2019
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    I have now built several EF antennas using a 50:2250 Ohm (1:7 turns ratio) transformer running up to 800W pep through them with no capacitor (therefore no arcing) across the secondary winding of the transformer. I have used both commercial transformers and home-brew ones.

    Without the capacitor, you will have to prune the "dipole" to resonance. Contrary to what some believe, the "dipole" length exists between a CommonModeChoke that should be placed about 0.05 to 0.07 wavelength down the coax from the transformer all the way to the distal end of the wire connected to the transformer secondary. A recent QRZ forum thread dealing with this issue is here. Read the next several posts following the linked one...

    I think that Steve was trying to tune the antenna with the capacitor instead of just pruning the wire. The capacitor is not needed.
    Last edited: Oct 13, 2019
    YO3GFH likes this.
  8. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    You may not have enough or too much capacitance value from the Polyvaricon capacitor. Polyvaricons have several 'gangs" of capacitor plates internally, with different capacitor values which are connected to different terminals.

    You can obtain different capacitance values by connecting the terminals in various series and parallel configurations. You can purchase inexpensive LCR (Inductance, Capacitance, Resistance) meters to measure the capacitance. Some general purpose DVM (Digital Volt Meters) also have a capacitance measuring function. Bill

    -Please ignore Rege, his behavior is inappropriate.
    WA7ARK likes this.
  9. W7UUU

    W7UUU Principal Moderator Lifetime Member 133 Administrator Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    The inane and utterly off-topic posts you are referring to have been removed. Hopefully they will not be repeated.

    Not exactly helpful to folks looking for assistance to have members post silly commentary like that in a technical forum

    WA4SIX, KI4ZUQ, W1TRY and 4 others like this.
  10. KA0HCP

    KA0HCP XML Subscriber QRZ Page

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