buzzp is right. Say a transmitter will accept a 500 ohm max load. If you drop the signal across a 250 ohm resistor (creating a 1-5 VDC internal signal), that leaves you with 250 ohms to work with. If 18 ga. copper wire was the only resistance, you could install the transmitter over 3 miles away!
However, intermediate connections, bends in the wire, internal resistance (control system and transmitter), power supply, etc. all contribute to resistance.
A more practical max distance is 1 mile, and that's only under ideal circumstances.
