How to test for tuning capacitor watts

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

ad: L-HROutlet
ad: l-rl
ad: l-BCInc
ad: L-MFJ
ad: Left-3
ad: Subscribe
ad: Left-2
  1. KM3F

    KM3F Ham Member QRZ Page

    Look at the cap a different way.
    Much simpler to consider an example.
    If your transmitter is putting out 50 watts into a 50 ohm antenna load, the capacitor has to pass 1 amp of current.
    Pwr = I sqed times R or 1 x1 x 50 ohms.
    That little receive only tuning cap has to pass 1 amp of RF current.
    The plates are so close together and the insulation between them cannot handle the current alone without heating to destruction let alone the effects of voltage punching through the insulation between the plates.
    You really don't need to consider anything else about it on a practical basis.
    The current heats the insulation, the voltage pulls electrons out of their orbit (in the insulation) that causes the physical destruction.
    These parameters are not present (to such intensity) in a receive only application.


    Thanks for the feedback.

    I remember it working when I made it, and the LED came on at best SWR, when the Cap was turned. This was only on 1W, but now I need 5W.

    I'm puzzled why the Cap was added if so many of you say it isn't needed.

    I'm not much of an expert, but I don't think the Cap runs at 50Ohm, but has a transformer to 50Ohm from the endfed wire, which I assume to be more that 50Ohm.

    I have a slightly larger air Cap which I will try as an experiment, then see if I can see sparks in the dark.
    If it isn't any good, then I'll add the counterpoise wire as suggested.
  3. N8FVJ

    N8FVJ Ham Member QRZ Page

    Using an old transistor radio tuning capacitor is a recipe for disaster. These are for receive only at a volt or so. The smallest spaced air capacitor will handle 5 watts power. A 500 volt air space capacitor on the load side will handle 500 watts without arcing (some use it for 800 watts PEP, but....) and a 1200 volt spaced air load capacitor will handle the legal limit power. Plate tuning capacitors are different and require a minimum of twice the tube plate voltage IMO.
  4. N7EKU

    N7EKU Ham Member QRZ Page

    Hi Cam?

    What LED are you talking about here? Are you using a bridge SWR indicator? In the usual of this type, you want the LED to go out; a lit LED indictates high SWR.

    Idealistically the cap is not needed. So if you are erecting the antenna permanently, and there will be no changes in the antenna or in the ground situation, then, using an antenna analyser, you can adjust the wire length so that you get an exact resistive match. However AA5TB made this matcher for portable use so that he didn't have to exactly adjust the wire length everytime hi put the antenna up.

    Yes you are correct that the cap is not seeing 50 ohms. AA5TB checked that his antenna showed about 4700 ohms. So the cap will see pretty high voltages using that. However, he was having success with it at 5W. I think if the matcher is not well adjusted, it may experience higher voltages or higher currents then when well matched and that could be the problem.

    However, for reliability maybe an air spaced capacitor or a mica compression cap may be better. Both can often be found in older radios and CB antenna matchers.





    Hi N,
    This is quite a small aircap, I'm about to try, where I can see the gaps, if I look into the vanes, and bearing in mind that the transistor one, did work on 1W.


    Hi M,
    I thought it was in the link on POST#1, but another look doesn't show it, so I must have a modified version. Press the button, and transmit, move the cap and when lit it's tuned.
    As mentioned I have an old airgap type, given to me by a silent key, who liked valves.

    Yes, it was for different effects on the endfed.

    I'll post how I get on. (Weather permitting)
  7. N7EKU

    N7EKU Ham Member QRZ Page

    Hi C,

    Post if you can, that LED circuit. I've never seen one like that before and it would be interesting to what they did to make it work that way.




    This is a better link than #1 and the schematic has another link. Let me know you can see the SCH. Thanks to Stephen R Yates AA5TB
  9. N7EKU

    N7EKU Ham Member QRZ Page


    Ah if you made that one, you are supposed to tune to make the LED dim or go dark completely. If you did the opposite, I suspect that's what caused the cap to burn out.


  10. KM3F

    KM3F Ham Member QRZ Page

    If marginal in power handling capacity, a CAP could withstand the RF voltage and current under matched conditions when the values may be lower.
    But take it out of match, those values can go beyond the CAP ability to handle the condition.
    If this condition might be present, best to do the match with an Analyzer then put power to it and leave it alone.
    Most Tuners are designed with some reserve capability.

Share This Page

ad: ARR