JIMMIE wrote:

Try some 3mil Teflon, Teflon has a very high capacitive coefficent.

I'm afraid you're misinformed. The dielectric constant of PTFE Teflon is
2.1, or just about twice that of air. Quite a few plastics are higher
(e.g. Mylar at 3.2 and PVC around 3.5), and many materials, such as
those used for capacitors, have dielectric constants that are a lot
higher (e.g., barium titanate at 1500 - 2000). Of course, a lot of the
latter aren't physically suited for this application. A long time ago, I
had trouble with microstrip line dispersion in a high speed delay line
compensation network design. So I chose Teflon for the substrate
material because of its *low* dielectric constant. The previous design
was on an alumina substrate having a dielectric constant of about 10.

The capacitance of two parallel plates is directly proportional to the
dielectric constant and the plate surface area, and inversely
proportional to the plate spacing. So putting 0.1 inch of Teflon between
the plates gives you the same capacitance as putting the plates 0.05
inch apart with air between.

This isn't to say that Teflon might not be a good choice. It's a very
low loss material, and chemically very inert. It's soft so won't
scratch, but it's slippery which might be a disadvantage. It's also
subject to cold flow, but there probably won't be enough pressure for
that to be a problem.

Roy Lewallen, W7EL