Regarding air-variable capacitors:
Most folks refer to air-variable caps by their max C value, and sometimes
include the plate gap and/or rated working voltage (particularly when
selling them!)
But much less frequently mentioned is the minimum C value, which of course
is quite important in Ctune applications.
It's surprising (to me, the uninitiated) how few "vintage" caps (Johnson,
Millen, etc.) are suitable for say a single 3-500z amp without compromising
the loaded Q at 28MHz. Or perhaps I am missing something. (I am purposely
excluding vacuum variables here). To me "suitable..." means around
10-130pF at > 4kV.
So a few questions:
1. What are the primary driving factors behind achieving small minC in an
air-variable? Say I would homebrew one...what are the mechanical/layout
considerations in the capacitor itself that tend towards small (c. 10pF) min C?
2. Are there other sources/manufacturers beyond those I've previously
named? I have several old Johnson catalogs and it seems that only 4 of the
many, many caps they make/made even come close.
3. Or is padding for the low bands more common than I think? There are a
few more Johnson caps that would work if you padded both 40m and 80m.
Thanks,
Scott Townley NX7U
Gilbert, AZ DM43di
http://members.cox.net/nx7u
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