Yard dogs are good as an alert dog and a deterant. but a friend of my dad's had a German Shepard which was a big lump and would defend its properties the dog was trained as a security dog. But the theiving scum bags who robbed his place malleted the dog, now a good guard dog is no good dead. If someone wants your property bad enough they will get, just make it very difficult for the b@stards.
I think a noisey dog is the best deterrent.