John,

I designed some automatic tuners on the market. It was a very lengthy
process in selecting components and laying out circuit boards. It
really is a much better value to just buy one, unless you want to learn
how to design a tuner.

You probably want powdered iron mixes for low frequencies (larger
inductor values) and air core coils for small values.

The capacitors would have to be silver mica types although you might
find some high current high Q multi-layer ceramics that will work.
There is no reason to go much over the contact voltage breakdown of the
relays when picking capacitor voltage, since the relay would fail
first. Current is another issue. You must obtain capacitors that handle
high current, meaning the capacitor must have low ESR. As a general
rule that would mean a snubber mica capacitor or a multi-layer HV chip
capacitor.

Relay selection is critical. I cut dozens of relays open looking for
good candidates, and then HV tested and RF tested the relays for
breakdown voltage. A DC test will not work.

It's pretty tough to build an automatic tuner that will handle 100
watts or more into a wide range of load impedances. Most of the problem
is parts selection and board layout.

73 Tom