Subject: wire guided vehicle

In regard to wire guided vechicles. How are the guding wires 
energized?  Do they use high frequency audio( like 10,000 hertz 
square waves) or do they use low frequency radio waves? What would 
be used for a distance of 500 to 600 feet and a clearance of 4 to 6 
inches?
I thinking of laying a wire on the ground through the woods to haul
wood and stone via wire guided cart.
Are there any www sites that disusss this subject?

Re: wire guided vehicle