I am building a magnet loop antenna for 20 meters. I want an antenna that handle 20 to 30 watts of power. I am using the KI6GD magnetic loop calculator. I have an 8-200 pf air variable capacitor ...
I've never seen (or heard of) the KI6GD loop calculator and so cannot comment on it. But I'd like to ask, What are you making the loop from? Most STLs of reasonable efficiency use large copper or aluminum pipe and a butterfly capacitor as a minimum. Wire --even large wire-- is out of the question for transmitting because the radiation resistance is often in the milliohms, and that means you have to pay careful attention to minimizing the resistance everywhere. That includes the short connections between the loop itself and the capacitor.
As Dale says, that includes the wipers on an ordinary air variable, which can turn red hot at even modest power levels due to the enormous current flowing through the loop. I second his suggestion on the butterfly, which has no wiper, and has been discussed here before in this context.
My own STL project is on hold, but I think you will find some very useful info on a page I created at http://www.w0btu.com/magnetic_loops.html
. There's a link there to AA5TB's loop calculator, which I used, and a screenshot of it with the STL I want to build (someday).
Using a section of coax as a capacitor has also been discussed here at length. I forget the details, but the butterfly is quite a bit superior, though you cannot beat a proper vacuum variable. One person made a variable capacitor using paralleled sections of homemade hardline which move in-and-out trombone style. The link to it is on my page.
Anyway, I wouldn't be quick to rule out a vacuum variable. I bought a very nice NOS Russian surplus vacuum variable very cheap. EBay is your friend.
I heard a guy on 17 meters today with a small loop who always has a good signal.
I wish you well on your project, Ron.
EDIT: copy the line below into google.com for a partial list of previous discussions here on this subject:
site:eham.net transmitting loop capacitor efficient