I used 20 gage copper plated steel wire feed welding wire. I bought a big spool of it and laid out 125 radials 30 to 36 feet long (BIG spool of wire!). I also soldered the wire into a crimp ring terminal with six wires per terminal, after putting in the six wires, I would crimp, then solder the terminal. You can see it on my blog, if you are interested, it is on my space, under N8CMQ.
In a few years that wire will falling apart.
Welding wire is not copper plated. It is a light copper flash that almost rubs off. It typically lasts about a year or less in open air, let alone in contact with soil.
