When I was first getting on topband, shunt feed looked like the easiest way
to get decent efficiency. Vacuum variables looked pretty expensive so I
looked for cheaper alternatives. I had some coax scraps so I did as I had
done in XV and HS land and used coax stubs, figuring that the dielectric
strength would be more than sufficient. It worked out great, as long as I
was only running 100 watts. But when I put the amp on, there was a bright
flash from the base of the tower and the swr meter hit max. I found a wad
of melted coax where the shunt feed had been. Seems that if there is high
voltage across the shunt feed cap, there is also high current. The coax
withstood the voltage OK, but the current through that dielectric did a
number on it, almost instantly.
Fortunately, there was a hamfest soon thereafter. Vacuum variables were
still expensive, but fixed vacuum capacitors were cheap. I found three that
totalled just about the capacitance I needed. I bought them for about $5
each. They are still in use 23 years later.
I haven't tried this, but another way which might work is to make a high
voltage capacitor out of window glass and aluminum foil. It's workable if
you have some sort of housing at the base of the vertical. The glass will
withstand thousands of volts; I don't know how much dielectric heating would
take place but probably it's very low loss and hence low heating. Glue the
foil to the glass and attach leads using crimp on connectors (Stakons).
Mount the glass on ceramic insulators and you should be good to go.
If anyone tries this, let me know how it works out.
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