Alternatives to mica capacitors?

Discussion in 'Homebrew and Kit Projects' started by KW4TI, Mar 15, 2019 at 4:37 AM.

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  1. KW4TI

    KW4TI Ham Member QRZ Page

    I have been building my own automatic antenna tuner and noticed that most of the commercial units use mica capacitors.

    I am sure the ESR of the high voltage ceramic cheapies is much higher than the mica capacitors, and the stability of the mica capacitance with temperature is much better.

    However, mica capacitors are expensive, about $3 each, and each of these antenna tuners may have 10 or 15 of them at least. For as cheaply as many automatic antenna tuners are sold, I would think selling $50-$60 of mica capacitors in it would increase the price greatly after the usual mark-up. Probably they don't cost $3 in bulk, but still they look like they are one of the more costly parts of an automatic antenna tuner.

    Are there any alternatives? I was looking at C0G or NP0 capacitors, but these don't generally seem to handle much power. Polypropylene capacitors have low dielectric losses, but they are not as available in small values and are not temperature stable, but perhaps might work well wenough. Polystyrene capacitors are generally unavailable nowadays.

    Finally, I could put several smaller high voltage ceramic capacitors in parallel, so that the load is split over many. For example, 10 100 pF ceramic capacitors in parallel is much cheaper than 1 1000 pF silver mica capacitor, and the ceramic capacitors are rated for 2 kV while the mica is rated for 500 V.

    Will this work well enough to be stable for an automatic antenna tuner? It's not going to be for a high tuned circuit such as a bandpass filter and the L match is a fairly low Q, usually less than 5.

    73,
    Dan
    KW4TI
     
  2. KL7AJ

    KL7AJ Ham Member QRZ Page

    The primary advantage of mica caps is their stability....very important in oscillator circuits and such. And they CAN be capable of tremendous power....but with a price, of course.
     
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    In the RF drivers (kilowatt level) at the world's largest OEM of RF-driven sealed CO2 Lasers, we used a LOT of porcelain dielectric capacitors. Most came from American Technical Ceramics (ATC): http://www.atceramics.com/. In tuned circuits, we used low-drift types. ATC has/had a per-line and per-order minimum but, I see Digi-Key stocks some. :)
     
  4. W1VT

    W1VT Ham Member QRZ Page

    Contact an ebay dealer in the Ukraine and see if he will sell you an assortment of RF doorknob capacitors.
     

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