[TowerTalk] Need Capacitors for Matching Network

[jimlux]

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.
>