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1.1 Overview
Radio frequency identi?¬?cation (RFID) is rooted in discoveries made by Faraday during the
mid-nineteenth century and discoveries made between 1900 and 1940 in radio and radar
technologies. Faraday discovered the concept of mutual induction, which forms the basis
for powering passive tags operating in the near ?¬?eld. Technological advances enabling the
development of far-?¬?eld tags occurred during the ?¬?rst half of the twentieth century. This
chapter focuses on passive tags operating in the far ?¬?eld.
Far-?¬?eld tags must harvest energy to operate and backscatter the interrogation signal
transmitted by the reader to communicate with the reader. Two discoveries form the
basis for far-?¬?eld passive RFID tags. First, the development of crystal set radios provides
the basis for a tag to power itself. Crystal set radios used energy contained in the radio
frequency (RF) signal to move a diaphragm in the headset enabling those without
electricity (most people outside a major city during the early twentieth century) to listen
to radio broadcasts. Second, discoveries in the ?¬?eld of radar during the Second World
War form the basis for the backscatter communication employed by passive RFID tags.
All objects re?¬‚ect radio waves, and the tag can change the characteristics of the radio
waves it re?¬‚ects by changing the matching at the connection between the chip and the
antenna making up the tag.
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