Batteryless Radio and Ambient-Powered Receivers

Can useful information be extracted from an electromagnetic wave without any external electrical power?

That deceptively simple question was answered more than a century ago by the crystal detector. The same physical principle—passive rectification of radio-frequency energy—underpins today’s passive RFID tags, ambient RF energy harvesters, and battery-free sensor nodes. Between those two eras lies a continuous engineering thread that has rarely been examined as a single narrative.