There are these round plastic balls that you can throw in a ditch and later
come back and use a locator to find the ball. Always wondered how they work so
I bought one and took it apart.
It was partially full of some liquid, I presume antifreeze. Probably to keep
it from floating.
Inside there was a hockey puck type of plastic enclosure.
Inside that was a coil and capacitor. Nothing else. No RFID chip or
transponder or anything else. Just a simple LC resonant tank circuit. A bit
of a let down.
Then I realized I probably did not need to go to all this effort:
https://patents.google.com/patent/US5699048A/en
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