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Sure, but the key there is the short distance. Remember your inverse square law? You might also compare the amount of energy actually captured by the RFID device with the amount output by the reader. The key to the system is the sensitivity of the detector in the reader...
Then remember conservation of energy. Say you have a 50 KW radio transmitter: even if you had a 100% efficient source/receiver coupling, how many people could draw 1 KW from that radio broadcast?
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