Have you trawled up previous discussion on this matter? One of the great things you get to quickly learn about Arduino is that, no matter what your problem is, that chances are pretty good that somebody else has had it too. I'm sure people have gone well over 10m with CAT5 cable, so all of his can't be hard, for chrissakes, but, before you get blinded by science and double schottky-diodes, maybe a solution lies here: