I'm trying to build a proximity sensor that triggers on nearness of the sensor to a perimeter antenna, sort of like an underground invisible fence. I want to feed that input into a microcontroller which will respond when it approaches the perimeter. Most of what I'm finding online is dealing with RFID, which is not exactly what I'm looking for.
Are there components that I can use that will transmit/receive simple signals? If I need to build this with discrete components, are there schematics? The sensor should be sensitive only over a short range (~1m)? Ideally this could be configured by increasing or decreasing the power of the transmitter.
I understand that the question is a bit vague, but I'm not looking for super detailed responses, just a nudge in the right direction.