Usually, RFID tags are passive, i.e., they do not have onboard
power supplies and harvest the energy required for
transmitting their ID from the query signal transmitted
by a RFID reader in the proximity. In fact, this signal generates
a current into the tag antenna by induction and such a
current is utilized to supply the microchip which will
transmit the tag ID. Usually, the gain (power of the signal
received by the reader divided by the power of the signal
transmitted by the same reader) characterizing such systems
is very low. However, thanks to the highly directive
antennas utilized by the readers, tags ID can be correctly
received within a radio range that can be as long as a
few meters. Transmission may occur in several frequency
bands spanning from low frequencies (LF) at 124–
135 kHz up to ultra high frequencies (UHF) at 860–
960 MHz that have the longest range.