Practical designs of this sort of system don't use anything that concentrated - the idea is to build receiver arrays over farmland, since the microwave receiver

Practical designs of this sort of system don't use anything that concentrated - the idea is to build receiver arrays over farmland, since the microwave receivers are really just an array of towers with loops of wire suspended from them. The on-the-ground RF intensity even in the middle of the rectenna would be low - not "advisable to stay in low" but you wouldn't suffer any ill-effects if you strayed in there accidentally for a while while.