the simple way to determine the voltage at the top of your top loaded
vertical is to take the antenna current at the loading coil and multiply it
by the loading coil reactance. This gives the voltage relative to the
segment just below the coil.
Another method in NEC is to remove the loading coil and place the source in
its place. Adjust the source voltage until you have the same antenna current
as in the original configuration (coil at top, source at the bottom).
With the antenna you describe and running 1500 W the voltage across the coil
will be around 10 kV peak. The RMS current is around 7 amps. For that a
vacuum relay is the thing to use. Let's say the relay coil power cable is a
length of coax. Bond the shield to the tower at the relay location. At the
tower base bond it again. Run the cable thru ferrite bead or turns thru a
ferrite core to give a common-mode impedance of 500 ohms or more.
A suitable relay is the CYNERGY 3 #DAR71210. Digkey.com has these in stock
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