[jee7s]

Mike-

Your slit idea is probably more trouble than it is worth. That slit would have to be pretty large to reliably pass a signal at 400MHz. The physics of EM get funny like that.

You were pretty on track to a potential solution with your multiple antenna idea.

I can understand your intuition about the 10 nanoseconds being too fast to detect reliably. However, you don't need to deal with time, you need to deal with phase. At the carrier's frequency, 433MHz, the wavelength is about 70 centimeters. So, using the phase difference beteen your antennas, you can get an angle to target. Using several antennas in an array (preferably a good distance apart), you can triangulate the location of the transmitters.

This isn't to say that the solution is a piece of cake, but it is possible. I'd suggest looking into ARRL literature (like QEX and QST) for more details about beacon finding. Many in the amateur radio community track beacons as a hobby.

-Jeff, KG2OV