There is a control box that goes inside with the rig.

The coax from the tuner connects to this box, as does a short jumper from 
the output of the rig.

Inside the control box is a power supply for the tuner.

Part of the power supply circuit is a capacitor which allows the DC voltage 
and current to travel up the coax to the outdoor portion of the tuner. At 
the same time, this capacitor prevents the voltage from going into the 
radio.

Thus, you use one cable for both RF and DC.

It's like so many other mysteries of electronics. They don't work on paper, 
but work perfectly in actual practice.


Mike Duke, K5XU
American Council of Blind Radio Amateurs