'How to Cheat': The Physics of RFID--Part II

Editor's Note: Miss Part I? Click here.

RFID Communication Techniques
Communication is basically the transfer of information--that is, to send information from one location and to receive it at another. In the RF world, this is accomplished by the transfer of energy (which contains the information coded in it) through RF waves. There are two main communication techniques that the RFID readers and tags use to communicate with each other. These techniques are coupling and backscattering.

Communication Through Coupling
Coupling, in general, is the transfer of energy from one medium, such as a metallic wire or an optical fiber, to another similar medium. Some examples of coupling include capacitive (electrostatic) coupling and inductive (magnetic) coupling.

As explained in Chapter 1, inductive coupling is the process of transferring energy from one circuit to another through a shared magnetic field by virtue of the mutual inductance between the two circuits. Note the following points about inductive coupling:

• Inductive coupling is used by low-frequency or high-frequency RFID systems. This way the tag and the reader can use a loop-style coil for an antenna because the traditional antenna would need to be too long due to the long wavelengths of the low-frequency waves.
• Inductive coupling works only in the near field of the RF signal.
• Sometimes inductive coupling is further subdivided into two kinds of coupling:


1. Close coupling within a range of about 1cm
2. Remote coupling within a range of about 1 cm to 1m

The power transfer between the two coils depends on the following quantities:

• Operating frequency of the system
• Number of turns/windings in the coils
• Area enclosed by each coil
• Angle the coils make with each other; for maximum power transfer, the coils should be aligned in the same plane
• Distance between the two coils

The magnetic field can be used to transfer energy only within the short range. For long-range communication, you need to send information through EM waves (radiation). This technique used in RFID systems is called radiative coupling or backscattering.

Communication Through Backscattering
Backscattering is the process of collecting an inbound signal (energy), changing the signal (the data it carries), and reflecting it back to where it came from. The long-range RFID systems operating at ultra-high frequency (UHF) or microwave frequencies use this communication technique. The reader sends out the information in the form of an EM wave at a specific frequency; the tag receives the wave, encodes the information into the wave (changes the wave), and scatters it back to the reader.

When you design and install a system, there is always a set of performance requirements that could differ from customer to customer. The antenna is an important component of an RFID system. Therefore it's important to understand what constitutes and affects the performance of an antenna.

Understanding Performance Characteristics of an RFID System

Radio devices communicate using antennas for transmitting and receiving the signals. Just like any other radio device, RFID tags and readers can also communicate with each other using antennas. The information is encoded into an RF wave and sent to the antenna through a transmission line. So, antennas play a vital role in an RFID system, and it is important to understand the characteristics of the transmission line and antennas that impact the performance. These characteristics are discussed in the following sections.

Cable Loss
RFID systems typically use 50-Ω coaxial cable as a transmission line. Cable loss is the amount of signal power lost in the cable. The longer the cable, the greater the loss.

Impedance
Impedance is defined as resistance to the flow of current in a circuit element and is measured as a ratio of voltage, say V, across the element and current, say I, through the element:

The antenna receives power (in terms of current) from the source through the transmission line. The input impedance, Z _i, for the antenna is the following: