If I get what you mean, is that just sensing the presence of a body through its interference with the RF field? If so, neat idea.
I wonder whether vegetation might need to be kept clear though. Then again, in a desert, vegetation is usually either really small or really slow-growing, except maybe in the rainy season. Then again, I don't know what effect vegetation would have on it.
You could hook the thing up to loudspeakers and an audio playback device. Scare the heck out of someone who hadn't even touched the fence yet.
"Please step away from the fence. You are being monitored."
What if he dosn't speak English.?
Torben