Radio Basics For UHF RFID

Every object in the world that has an electric charge creates an electrostatic potential, which falls inversely as the distance. Most electric currents flow in a balanced loop: the potential from current flowing up cancels that from current flowing down, and no net effect results on distant observers. This chapter introduces the basics of radio technology: transmission, modulation, bandwidth, signal voltage, and power. A periodic signal that persists indefinitely, without changing its amplitude, frequency, or phase—a continuous wave (CW) signal—carries no information other than the fact that it is present. Unmodified on–off keying (OOK) is admirably simple and seems promising as a method of modulating a reader signal. Smoothing the signals also makes the receiver's problem harder. To transmit to a tag, a reader uses amplitude modulation to send a series of digital symbols. The symbols are coded to ensure that sufficient power is always being transmitted regardless of the data contained within in. Most RFID equipment operates in spectrums set aside for unlicensed use by the governmental body that regulates radio operation in a given jurisdiction.