On Sat, 04 Oct 2003 16:27:16 -0700 Dennis Ashworth <K7FL@ARRL.NET>
writes:
> I need to insert a 1000pf of capacitance in series with my 80M vertical
to
> move the resonant frequency up to the phone band. I have been using a
> vacuum variable, but would like to use it in another project - and
change
> to a fixed capacitor. The logical choice would be a fixed 1000 pf
vacuum
> capacitor. Failing to locate a fixed vacuum cap, what low loss
dielectric
> materials should I be considering given the frequency, voltage and
current
> involved. The driving impedance is 40 ohms and I run the legal limit.
> Therefore, currents will be in the 6-8A range ... voltages probably
less
> than 500 V.
>
> Thoughts on dielectric materials and sources would be most appreciated.
> Although this is not AMP related, I suspect you folks know the
> answer(s).
>
> Dennis, K7FL
>
Hi Dennis,
3 suggestions.
1. Find an old mica transmitting cap. The kind with a ceramic body
(a couple inches in diameter) and metal end caps.
2. Parallel up several of the smaller old brown cased mica parts, the
ones that are about 1½" wide. These are typically good for a couple
amps each. Five 200pF in parallel would be a good option.
3. Get one of the new ceramic transmitting caps made by ATC, Temex or
Murata. They have some .001's rated at 1600V and good for better than
10A.
73 & Good afternoon,
Marv WC6W
*
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