I thought the good thing about living in the U.S.A. was that if someone tresspassed on your property, you had a legal right to blow their head off with the highest caliber automated rifle you own.
I did a search and found this:
Electronic Yard Fence
It's a motion detector that emits light and ear piercing noise when set off. It's only got a 15 foot range though, so you might need quite a few if your property is big.
Better yet, put in some Electric Fencing:
Electric Fencing
So far the highest voltage I've found is in the above link (10,000 Volts), although most seem to average out around 4,000 to 5,000 Volts.