If you were talking about trees a basic copper wire Lacovsky coil at the base of each one wouldn't take too long, though by the sounds of things that's not what we are talking about.
For long distances the magnetised cables are unquestionably the best because one set of magnets will keep the cable magnetised for miles and miles, as long as it heads towards magnetic north. How deep do you till? I think these cables can be up to 50cms deep to maintain best effect
The way I plant my garden the rows are spaced 3 feet apart and plants 1 foot apart in each row. I till the entire space as deep as I can which right now is about 1 foot or 30cm. I think initially I can do a coil on each of my two cherry trees as I don't mess with the ground around them. I have a bunch of copper wire, tubing, and even braided from shielded coax to work with.