Roy Koeppe wrote:
> About:
> My 160m/80m/40m/20m amp design calls for a 250pF Ctune and 1500pF Cload.
>
> "I notice some designs use capacitors with the full capacitance range
> variables, while others use lower-value variables and fixed caps
> switched into parallel for 80m and/or 160m operation.
>
> Rather than using the full range air variables I built (very large), I
> also have commercial 25-95pF and 100-800pF air variables I could use for
> Ctune and Cload respectively (with appropriate voltage/plate spacing).
>
> Space, layout, and component availability is certainly easier with the
> second option, but what are the disadvantages or pitfalls of doing so?"
>
>
> Steve, KK7UV
>
> Hi Steve,
>
> I will discuss what you already instinctively know! First, when needing
> to cover a wide range of bands, most large-value variables often do not
> have a small enough minimum value to be suitable for use at the highest
> frequencies. An exception may be vacuum variables, which tend to have a
> very small minimum value. Second, physical size may be a consideration
> as you made note of. Third, big-value caps tend to tune very "fast,"
> especially on the higher bands, and require much gearing down with
> vernier drives. Fourth, air variables and vac variables are the most
> stable of the bunch. Fixed padders usually DRIFT with heat (from any
> source, including internal heating). The worst in this category are
> ceramics to varying degrees. Micas are good, but hard to find at high
> current ratings these days. The use of padders requires more complicated
> switching arrangements and wiring. Fifth, prices may dictate which to
> use.
>
> 73, Roy K6XK
>
>
>
I agree a vacuum variable is the better solution. Another possibility is
to use two
dissimilar caps ganged together. In your case, say a 100pf and a 150pf.
You use
them in parallel for 160m, the 150pf for 80m, and the 100pf for 40m and
higher.
Another possibility is to use a 250pf cap for 160m and 80m and switch a cap
in series with it for the higher bands. The downside is that the
switching adds
stray capacitance which may make it hard to hit 10m.
73, Roger
--
Remember the USS Liberty (AGTR-5)
http://ussliberty.org/
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