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I have read of people tuning a Yagi beam antenna, (in this case a 6 Meter 3 element Yagi), with a variable air spaced capacitor.. Then removing the variable capacitor measuring the capacitance and fitting a fixed capacitor in it's place. The voltage is zero in the centre of the beam, current is maxium, with a 100 watts drive , voltage at the gamma match is what ??. I experimented with this and used a 50 volt ceramic capacitor, OOPS, problems. TOO low a voltage. WHAT is the current and voltage present at the capacitor??.When the capacitor was inspected, it had little black holes in it. Looked second hand. OK I needed a higher voltage capacitor. How high?? What type of capacitor ??. Ceramic, silver mica, multi layer ceramic, Door knob. IS it the current causing the problem. Variable capacitor has been refitted. NO further problems. DON'T touch, ok, agreed. My question is what type of capacitor could have been used, please

As a followup, yes, 70 Volts would explain the destroyed capacitor. Perhaps something in the 500 Volt or 1000 Volt range would been better. Mylar / polyester capacitors ??.(500 to 1000V).

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    $\begingroup$ Hi John, and welcome to the site! You only need a capacitor if you're measuring an inductive reactance at the feed point. What are you seeing? $\endgroup$
    – webmarc
    Commented Apr 2, 2023 at 1:25
  • $\begingroup$ About 33pf was measured on the variable capacitor and a fixed capacitor was fitted in its place. Inductive reactance was about 28 to 30 ohms. (wide spacing of both elements. Gamma rod is set at about 8 ins. Beam worked exceptionally well until low voltage capacitor failure. Never thought to, NOT fit a capacitor, Google always tells you to tune with a capacitor?? could be interesting to try. Q. which type of capacitor is suitable for RF. Too many questions and NOT enough room. , Either question was too silly and simple or no one knew the answer. Thank you for your reply, appreciated $\endgroup$
    – john
    Commented Apr 3, 2023 at 0:58
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    $\begingroup$ John, the voltage isn't 0 at your capacitor/feed-point, because your feed-point is actually at the 50 Ohm point of the radiating element; so you likely had >70v going through a 50v capacitor. Use a higher voltage value; have you looked at Mylar/polyester caps? $\endgroup$
    – Louis Seaman
    Commented Apr 3, 2023 at 12:48
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    $\begingroup$ Welcome, John! I am including the remainder of your comment below. If you have followup questions, please post a new question rather than posting it as an answer to your question. :) Ceramic, silver mica, multi layer ceramic, Door knob capacitor's. Have also been suggested. I have drawn a blank, where do I look to find the answer. Which one will best handle RF. I have seen Door Knob capacitors in Linear Amps. E-Bay sell those in 33pF, Your thoughts please. $\endgroup$
    – David Hoelzer ♦
    Commented Apr 5, 2023 at 0:51

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John, use Ohm's Law to calculate https://ohmslawcalculator.com/ohms-law-calculator your voltage given your other parameters e.g. 100 watts output into a 50 Ohm load. Remember, the voltage necessary to have enough current to get to 100 watts of power through a 50 Ohm load is a discrete value, and not a random combination of voltage/current values that happen to multiply to 100 watts. So at 100 watts, you are at ~70 volts, so there is no way you need a 500 or 1000 volt capacitor.

I found a capacitor digikey.com/en/products/filter/film-capacitors/62 at Digikey; they have 250v 33pF polypropylene capacitors. Use the product filter tool to filter for your pF value and then you will see what I'm talking about.

Sounds like a fun project, good luck!

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    $\begingroup$ 70 Volts in a 50 ohm system at 1:1 SWR. But if $X_L$ or $X_C$ is 30 ohms, then you have a series resonant circuit with a Q of about 2, which might double the voltage and current. Then if SWR is ~2:1, pushing your luck at the band edges, one of them could be higher again. So a few hundred volts rating feels better. $\endgroup$
    – tomnexus
    Commented Apr 4, 2023 at 15:10
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    $\begingroup$ Hi,Antenna reads on VNA R=49 -51 ohms, X = close to zero We seem to be drifting OFF the subject. $\endgroup$
    – john
    Commented Apr 5, 2023 at 9:24
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    $\begingroup$ Hi Louis,I have looked at Digikey site, Nothing to loose so will order and try a 250Volt polyproplene capacitor, be interesting to see what transpires. Antenna reads on VNA R=49 -51 ohms, X = close to zero We seem to be drifting OFF the subject, What I wanted to know was WHICH TYPE OF CAPACITOR TO USE. Ceramic, silver mica, multi layer ceramic, Door knob capacitor, Polyproplene. Would Polyproplene be the best. Agreed a higher working voltage would be best. $\endgroup$
    – john
    Commented Apr 5, 2023 at 9:36
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    $\begingroup$ I believe you want a capacitor that is fairly immune to weather, John; so I would go with a style that is epoxy sealed like polypropylene. Read this document on film capacitors and harsh "climatic" resistance: vishay.com/docs/26033/gentechinfofilm.pdf then read the datasheets for the item you choose. If you're still concerned about weather/water getting into the cap., mix-up some epoxy and coat it further. Choosing your voltage value will dictate your choices, and then it's just a question of weather proofing if necessary. If my main response helped, give it Up vote. Thanks! $\endgroup$
    – Louis Seaman
    Commented Apr 5, 2023 at 10:42
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    $\begingroup$ These capacitors are so cheap, you might even consider buying a few different values in case you want to tune the circuit even further. $\endgroup$
    – Louis Seaman
    Commented Apr 5, 2023 at 10:50

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