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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. N7EKU

    N7EKU Ham Member QRZ Page

    The circuit was designed for use in tuning a wire that is one half wavelength on the band in use. Is that how you used it? Or did you use it to try and tune a half wavelength wire on another band? If you did the latter, that is probably why the cap failed as the circuit was not designed as a random wire tuner...


    Hi N,
    Ok, good.
    I should be able to put a couple of 100V through it somehow,


    Hi N2,
    I've only tried it on 40Mtr, but thanks for explaining how to change wires, and range.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    That tuner will not tune a random wire to any of several bands!

    It is intended that you swap the wire length to one that is ~1/2 wavelength long as you go from band to band.

    The required halfwave wire length (ft) on each band is about 490/f. (69ft for 7.1MHz, not 468/f)

    If you make the wire the right length, you dont need the capacitor, anyway.

    Tens of thousands of EFHW antennas are deployed without a capacitor!

    The antenna should be operated at resonance (achieved by pruning the overall wire length). At resonance, with 10W applied to the antenna, the peak voltage at the transformer primary is 1.414*sqrt(10*50) = 31.6V. Step that up through a 1:7 t.r. transformer, and the secondary voltage is 221V, which you applied to a polyvaricon which I would guess is rated for tens of V!

    @CAMERART, I thought that you understood how an EFHW works. Here is a presentation that goes beyond what AA5TB has on his website:

    There are two links; one for the slides, and the other is for a movie that I showed halfway through the slide show.

    End Fed HF Antennas – SLIDES - by Mike Mladejovsky WA7ARK

    End Fed HF Antennas – GIF Fileby Mike Mladejovsky WA7ARK
    Last edited: Oct 14, 2019
  5. N7EKU

    N7EKU Ham Member QRZ Page


    It could be then that the capacitor didn't have enough range the way you had it hooked up. Someone posted a picture/diagram of a typical transistor radio capacitor and the way that the leads are arranged. Do you have a meter that can measure capacitance down to pF? If so, you can measure each section and then combine them to get the capacitance needed. Also, it might need a low minimum too, so you can get that by measuring the capacitance once the leads are chosen and turning the trimmer caps on the back to the lowest value (sometimes you can see through the plastic on the back and just adjust the trimmers so their plates are all unmeshed).

    It may still work because it could be only one set of plates is damaged. If you don't have a capacitance meter you can use an ohmmeter to check sections and make sure they are not shorted.

    How did you adjust the circuit when you were using it? Do you have an SWR meter? How did you know your output level?

    Do you have a radio license yet? Most exams will cover Ohm's Law and other material needed for understanding better circuits like this.



    Hi N,
    I 'did' Ohms law 60 year ago, and past my Ham exam nearly 50 years ago, but didn't get my licence till a couple of years ago, and while I knew 'stuff' once, I have to play it back a few times to recall it.

    I have a capacitance meter and an antenna analyser, so when I made it, it was tested this way, and the Cap was in range, and in the field, the LED switched when tuned.

    I'll use all of the information given and get it going again.


    Hi W,
    Thanks for that, I did understand it all when I made it, but it's lost a bit over time.

    Regarding the Mike Mladejovsky explanation, I think this uses a coax, where the Yates one tunes.
  8. AA5MT

    AA5MT Ham Member QRZ Page

    It's the voltage, not the power. I arced a 300 watt tuner at 5 watts, and a 1500 watt tuner at 30 watts. I opened the top to look at it, and every component was arcing, not just the capacitors. No damage, thankfully. It was just this particular antenna at this particular frequency. All equipment worked afterward, even the antenna. I just have to stay away from that combination.

  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    All transformer-fed, near-end-fed wires require a counterpoise, which needs to be a minimum of about 0.05 to 0.07 wavelengths long. You can either use a "real" short wire counterpoise, or you can allow some common-mode current to flow down the outside of a short piece of coax which will act as the required "missing part of the dipole", as Rege likes to say.

    This means that when you take your antenna portable, you will always have to have an actual piece of coax between the rig and the transformer.

    Yates did not seem to understand that the feedline to this type of antenna is the counterpoise, and is a required part of the system. The capacitor is, and always was, a red herring!
  10. N7EKU

    N7EKU Ham Member QRZ Page


    Reading through all of Yates' material, I think he understands quite well all the aspects of this antenna (and need for, and what actually functions as, the counterpoise).



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