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
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