To be honest you could probably get by with pyrolectric infra red sensors near the perimeter of the detection threshold using an rf module link to overcome the distance problem. There is a product on the gadget shop website that works in this manner. If the length of the perimeter is not to great a capacitive proximity senosr(s) could be used to detect a person crossing or near the "fence" line.
Scanning I/R sensors are one solution to obtaining a range to target but this involves building hardware similiar to that used in heat seeking missiles and the like.
( Oscillating or Rotating Mirror in front of PIR sensor ) similair to the sort of thing used in simple laser displays
Full blown radar requires very high speed counters to implement, the gear is out there but VERY expensive and none of the commercial maufacturers are interested in talking to private individuals on a hobby electronics basis
A heat seeking "robot" based on an an off road R/C model car carrying a paint grenade might be interesting though, you could not outrun it, you will not see it coming, you might hear it, but by then it would be to late!
DO NOT DO THIS
IT WILL BE FATAL
NOT EVEN AS A JOKE FOR A "JACKASS" VIDEO
I (bleep)THEE NOT
IF YOU THINK I AM KIDDING , PUT A HENS EGG IN A MICROWAVE FOR FIVE MINUTES AT FULL POWER!
(CLEAN MICROWAVE OVEN AFTERWARDS OR MOTHER WILL KILL YOU)
If I bypassed the door switch on a microwave oven and invited some poor sap to put his head in while I switched it on...
THE CORONERS / C.S.I. REPORT WOULD GO SOMETHING LIKE THIS:
The the water in the cranium vapourised and forced the the brain tissue out of the skull via the optic nerve, ear canal and the base of the skull where the spinal cord enters. :shock:
Would rather go feet first into a bark chipper!
And your contact lenses would be be stuck to the cornea of the eye, in normal life they "float" in place using the surface tension.This fluid qiuckly boils away under certain conditions , not just microwave RF but ArcWelding as well.