Half a mile will need 80~100 milliWatts
A passive/reflective RFID-style device will not work at that distance
Tell me where you are...to determine a legal frequency-band (if wanted)
More details needed:
How long should the device be active (TX)
Should it only TX when asked (by other TX) that will need a receiver on
each as well
Continious or pulses (TX every few sec's ?)
Maximal size
Where is the tag/tracker put (hidden ?)
How will you DF just with signal-strength ?