'thedarkavenger',
bogdanfirst is right, please re-consider the use of radio to do this. If you want it to be invisible, use infrared light or ultrasonics.
You can try to make two antennas as suggested and compare the signals, but this will be extremely inaccurate with commonly available materials. There are too many factors such as antenna construction, impedance mismatching, component tolerances, multipath, etc.. to make it possible for you to do.
You could try to measure the phase difference of the two signals, but you will some of the same problems as above. If you get a highly directional antenna and rotate it, this could be your best route. This would eliminate the tough comparing involved with two antennas, and it would give you a peak signal at a certain point. This would involve a bit of processing, but who said this was easy???
Goodluck,
Steve