Hence, this reduces the read range of the tag.
Matching and deliberate mismatching are used for backscatter communication. The tag
can alter the matching by adding or removing an impedance, typically a capacitor, by
means of a switch. When the capacitance is included in the circuit, the matching is not
optimal and the tag re?¬‚ects an amount of energy B. When the capacitance is not in the
circuit, the matching is optimal and the tag re?¬‚ects an amount of energy A. The energy
amounts A and B are not equal. Using this difference, the tag can modulate data onto the
re?¬‚ected radio waves and communicate with the reader. Amplitude shift keying (ASK) or
phase shift keying (PSK) are possible using this difference in backscatter.
1.2.1 General Overview of the Physics of RFID
The reading of an RFID tag is more than recognizing the backscatter by a simple RF
receiver. In order for a passive tag to operate, the reader=interrogator must supply the
operating energy to the tag through the transmission of an RF continuous wave (CW). The
magnitude of the transmission of this energy is determined by the Friis equation as shown
in Figure 1.1. There are three key elements involving physical aspects of antennas in energy
harvesting: (1) antenna gains, (2) re?¬‚ection coef?¬?cients, and (3) polarization.
Consider ?¬?rst the antenna gains, GT and GR, for the transmitter and receiver, respectively.
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