This antenna is controlled by a DC voltage on the coax. If I remember
right, more than 8.5 volts raises the tap and less than that lowers
it. Information on building your own control box and RF/DC splitter
is at
http://members.aol.com/wa8lmf/ham/.
I was researching this to go the opposite direction -- I want to
make my FT-100D automatically operate some other screwdriver
antenna.