[Amps] air variable min C?

[Scott Townley]

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