[TowerTalk] Need Capacitors for Matching Network

[Grant Saviers]

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.
>