Phil,
I modeled your antenna, and if I did it correctly, the 350 uH lighting shunt
inductor value can be substantially reduced. Even 35 uH looks good. This would
allow 10X the lightning induced di/dt before the vacuum cap is subjected to
enough voltage to arc. 1 kA/us will put 35 kV across the 35 uH inductor/cap
versus 100 A/us with the 350 uH inductor. If you go this route I can calculate
the physical stress on the inductor as the magnetic field pulls on the tower.
We wouldn't want the inductor ripping to pieces. Also more attention to
insulation is needed with a coil having less turns. #12 AWG THHN house wiring
should be good for a transient voltage of 10 kV or more between turns. I have a
15 kV hipot power supply and can confirm this.
Dave WX7G
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