I'm thinking about how I can make matching networks for the center of a
40 foot doublet for field day. I have a couple of those 20 footer
fiberglass poles and a center mount and want to be able to match on
20m, 15m, and 10m for now, and there are lots of ways I could do this,
ladderline to the base and a manual or auto tuner, autotuner at the
feedpoint, etc, etc, but what I'd really like to do is have switchable
fixed-tuned networks as my intent for this antenna is to be set up in
the middle of a field in the clear at the same height all the time, and
I want to have FAST bandswitching.

An autotuner really isn't in the cards financially, and needs power.

So, inductors are easy. I've got 500 feet of #10 copper wire.

I need capacitors. Preferably very very cheap ones, and I guess I need
a number of values.

So it seems like 3/4" copper tubing (type L) slipped into 5/8" copper
tubing with an air dielectric has a capacitance of 30pF per inch and a
breakdown voltage of 1300V, ballpark in the *ideal* case.

Two questions: It seems like having a small gap in a cylindrical
capacitor like this probably makes it act like more of a lumped-element
capacitor than, say, a length of RG-58. I guess the latter is more
like 30pF per FOOT, and so you need to use a much longer line in terms
of a wavelength to get the desired capacitance. Sound right to you
folks?

Second, I know the actual breakdown voltage of such an object is going
to be much less than 30kV/cm*the gap width. Anyone using copper tubing
caps like this (I saw a magloop "trombone" unit some years back like
this) and what are the dimensions and actual breakdown voltages of real
units?

I guess polishing the tubes, making the cut edges rounded and so forth
will help some, but maintaining the gap will be tough. I can easily
derate for concentricity errors, but what about the ends? What is a
good overlap of the two sections? Is it better to have the tubes be
exactly the same length, flush with each other, or maybe have one
longer than the other one so that the ends of one are far away from the
ends of the other, or is it likely no difference? I can always go to a
bigger gap; 0.035in diametral difference is probably pushing it for
100W use but I haven't yet calculated likely voltages across these
things, so here I'll take any general comments on making coaxial caps
that stand high voltage. I'd probably prefer air dielectric to teflon
heat-shrink or something, I don't know how available that is.

73,
Dan
N3OX