Free range or coop and enclosed run?
There I really no 100 percent fox deturent, but electric fencing is good. Guardian dogs are helpful. But a secure preditor proof coop and run are the best protection.
I read an article about placing a single strand of electric fence about 8" off the ground. The author said it was effective prevention that is inexpensive to cover a large amount of land. I built an enclosed run with 3 strands of electric fencing.