W5DXP's IC-706 Screwdriver Antenna Tuning Circuit
The circuitry could not be more simple. There is one 10k ohm pullup
resistor and one ~12vdc relay. Any small ~12vdc relay will work but I used
the Radio Shack 275-248a relay priced at about $3. The operation of the
circuit is as follows: If the present SWR needs to be known, the tuner
button on the IC-706 is depressed. For a moment, the transmitter is put
into 10 watt CW transmit mode and the SWR can be read from the display.
Otherwise, the IC-706 need not be touched. When the screwdriver antenna
tuning switch is depressed in either direction, the 12vdc relay is enabled
thus supplying GND to the TKEY terminal. Anytime the screwdriver motor has
12vdc applied, the IC-706 turns on automatically in 10 watt CW transmit
mode. The SWR can be observed anytime the screwdriver motor is powered
and the tuning switch can be released when minimum SWR is achieved. W5DXP
searched the web for similar designs and decided to roll his own. For
about $5 worth of parts, this is the most convenient design available.
Save about $25 and roll your own. W5DXP mounted the relay on a small PC
board that plugs into the IC-706. The four-pin Molex plug is also available
at Radio Shack.