I used the cable
 

My back yard must be 30% copper from all the various types of radial wire I've laid out over the past 18 years. Just checked and I'm currently working from a 500' roll #14 thhn cable green insulation,

With all the rain my QTH got this summer, my grass is thick and long. Never looked healthier. Trees too.

Radials should be fastened to the TOP of the soil (I use the ends of wire coat hangers to tack them down). They bury themselves in a few months under a tough web of thhn building cable. With all the rain we've had, I can't find some that I laid down just a month ago.