You may want to check out Dakota Alert, they specialize in wireless security equipment.

Their website is: Dakota Alert, Wireless Alarms and Security

They make a wireless MURS system that is quite nice. The sensors are totally weatherproof, have an 80' max detection range, and a 1 mile transmission range over flat ground. (It's more like 1/2 a mile or less if you live in a hilly area though.) The range of the sensor can even be adjusted, and multiple sensors can be used. The handheld receivers can also be used for two-way communication is a base unit is purchased. The base unit can also be set up to turn on lights, set off an alarm, etc. when an alert is triggered.

They are somewhat pricey, and it can take a while to fine tune their alignment but they work very well. One drawback is that something as large as a cat or a skunk or even a large bird like an owl can set them off, so you will get some false alarms. However, the peace of mind is often worth the monetary expense and occasionally getting waken up in the middle of the night by a damn skunk.