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Old 5th Oct 2022, 2:28 pm   #1
kellys_eye
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Default High voltage RF capacitors

Can anyone point me to a supplier (in small quantities) of high voltage RF capacitors suitable to be used in an ATU? The MLCC variety seem in common use these days but other than getting reels of 5000 I can't seem to find 1-off quantities.

TIA
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Old 5th Oct 2022, 2:44 pm   #2
G6Tanuki
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Default Re: High voltage RF capacitors

What sort of power rating are you thinking of?

For high-pass/low-pass filters up to 100 Watts or so I use the Cornell-Dubilier CD17/18 type

https://www.cde.com/resources/catalogs/CD-1718.pdf

or the CD15 series, which are available from RS.

Rated for voltages between 250 and 2500V.
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Old 5th Oct 2022, 4:26 pm   #3
kellys_eye
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Default Re: High voltage RF capacitors

My intention is for QRP use but would prefer to add a reasonable margin in case I decide to run a linear too - max 100W so that range you suggest could fit the bill.

The variable capacitor I have is rated for 1kW though and it seems to be 'wasteful' to not build the ATU to its full potential.

Now I'm 'undecided'!
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Old 5th Oct 2022, 5:05 pm   #4
G6Tanuki
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Default Re: High voltage RF capacitors

The ones I linked to are fine for properly-designed LPF/HPF networks and matching networks, though being physically small they could have issues if used in a circuit with inappropriately-high loaded 'Q' where voltages/currents can easily get out of hand - to the point where you can melt soldered joints.

So don't use them in applications where a match is achieved by twiddling-knobs-while-transmitting-significant-amounts-of-RF-power. Like I said, they'll do for up to 100 Watts in properly-matched situations. Get your match using a noise-bridge or something, *then* switch to the proper transmitter.

Of course the survivorship of 'circuits with components selected for a properly-matched condition' can be problematic when something fails [where did that falling tree come from? It's broken my dipole!] so a fast-acting SWR-sensing-and-lockout circuit is always worth having.
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Old 6th Oct 2022, 4:25 pm   #5
kellys_eye
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Default Re: High voltage RF capacitors

Thanks for that. As ever, simple desire overrides practicalities and when thought about (clear headed) I would most unlikely to exceed 100W or even 25W as the primary concern is for maintaining QRP levels (<10W).

Your recommendations have been taken onboard and I thank you for that insight.
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