On 8/14/17 5:52 AM, Art Greenberg wrote:
I want to build a matching network for a 53-foot vertical for 80/75 and
40. I should be able to cover 80/75 with three LC pairs and 40 with a
single pair. So a set of fixed inductors and capacitors with a few
relays would work.
The design calls for capacitor values in the range 100pF to 300pF.
Assuming an unloaded Q of 1000, TLW says at legal limit the capacitor
will see worst-case peak voltage around 1500V and RMS current around
5.5A.
RF capacitors (doorknobs and plate mica) are built for this application
but they are way too expensive new, and not a whole lot more affordable
used. If they're the only real fixed-value choice, it would be less
expensive to use rotary components and motors, since I have a vacuum
variable and rotary inductor on hand. But I don't think the application
merits that complexity. I'd rather save those parts for a more deserving
application.
tesla coilers use polypropylene dielectric extended foil capacitors (CDE
942 type, specifically) - I don't know about their loss characteristics
at higher frequencies, but they are able to handle significant peak and
average current at hundreds of kHz, and are readily available.
The MFJ legal limit tuner uses a bunch of 62pF high voltage disc ceramic
capacitors in series-parallel combinations to make up the values it
switches in. They are apparently sourced overseas and I haven't found a
local source for the small quantity I need. I can get some very
reasonably priced 1.5kV C0G disc ceramics. But I cannot find current or
dissipation specs for those parts.
Would silver mica capacitors in series-parallel be OK? I haven't found
current or dissipation specs for those either.
What should I expect the unloaded Q of a disk ceramic or a silver mica
capacitor made up that way to be?
Thanks.
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