think resonant radials are going to be very efficient unless quite high!
But to resonate I found it best to tune the radial system against ground! as
though it were an antenna.This needs a ground connection of course but I assume
you already have one.The impedance at the feed point will be quite low (series
resonance).I would then tune the proper ant against ground,and finally replace
the ground with the radial system,without re-adjusting the resonance.I did not
get very high voltages doing this.Of course you could retain the ground
connection as well,and this would no doubt reduce the voltage.further. I
hope this is of some use to you.73s Laurie.