I match my top loaded 160m T with switched series capacitors and PCB
relays. I run QRO (SSB QSO's with ZL's yesterday, - yea for conditions).
My match covers most all the band in four steps, no inductors, 3 series
switched 4000pf values. It wouldn't take much top loading to get your
53' vertical resonant at 3.55MHz or so and use the same idea for tuning
the entire band. I'll send off line a note I wrote about this.
CDE CDV16 are higher rated current silver micas.
http://www.cde.com/resources/catalogs/PULSE-MICA.pdf However, for the
low end of that range currents are well below QRO or circulating current
levels. You might try ebay for some ex military transmitter doorknob
silver micas from Eastern Europe.
Grant KZ1W
On 8/14/2017 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.
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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