I would caution against using Russian military surplus doorknobs, or
Chinese doorknobs, in a tuned circuit without understanding their
dielectrics. I tried a couple and was very disappointed.
With HEC 7.5kV doorknobs, you would want to make it out of values of 170pF
or below. Below 170pF they have NPO characteristics.
Tim N3QE
On Tue, Oct 16, 2018 at 1:46 PM Herbert Schoenbohm <
herbert.schoenbohm@gmail.com> wrote:
> Unless you use LMR 600 or Andrew heliax a cap made from standard RG-8 or
> 213 has its limitations. eBay has a ton of high current fixed caps very
> close to the value you need. Some are from Ukraine (Russian military
> surplus) and are very inexpensive. You might also consider a home-brewed
> sandwich cap made from aluminum plates and Teflon insulation between them.
> This method of heavy duty fixed caps seems to be the vogue of many present
> amplifiers and tuners. Another alternative is to get a handful of 5KV
> doorknob caps at 100pf and add them in parallel or series until you come up
> with the value you need.
>
> Good luck
>
> Herb Schoenbohm, KV4FZ
>
> On Mon, Oct 15, 2018 at 5:18 PM Richard (Rick) Karlquist <
> richard@karlquist.com> wrote:
>
> >
> >
> > On 10/15/2018 10:43 AM, Tree wrote:
> >
> > > I replaced the variable cap with a home made cap using RG8. I had one
> >
> > Sometimes, capacitors made from coax are lossy. I modeled your
> > coaxial capacitor using Simsmith (very easy to do) and the Q turns out
> > to be 340, if I did it correctly. So the coaxial capacitor gets a clean
> > bill of health and probably doesn't have anything to do with
> > your broad bandwidth.
> >
> > Rick N6RK
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