If I let my chickens free range, I'd be out of chickens in a hurry. I keep them in a welded wire enclosure and the wire actually runs several feet out on the ground from the enclosure, covered by flat rock to keep the beasts from digging in. I also had to run a smaller mesh hardware cloth on the welded wire at the bottom to keep coons from reaching in and killing chickens through the welded wire.
Best way to protect your flock is to eliminate access to predators with fencing. You can't kill them all. You will never get rid of all the foxes, coyotes, possums, coons, weasels, not to mention predatory birds like hawks, owls, etc which you really aren't supposed to "eliminate" legally, anyhow. I don't generally lose chickens to predators, but I don't free range the girls, either.
Fox's are bad about getting rabies , have read they like skunks and thats where they contract it from, had a buddy bit by a fox when I was a very young man, glad he got the shots as his buddy was faster than him lol. fox was caught and tested positive,