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Editor's Note: Miss Part I? Part I Part II, Part III and Part IV.
The readers and tags in an RFID system communicate with each other through RF waves.
Encoding the data (to be communicated) into an RF wave (carrier signal), the emission of the RF wave by antennas, and the propagation of the RF waves between the antennas are governed by underlying physics principles. The data is encoded into the RF wave using modulation techniques. From performance viewpoint, there are two main factors in the
RFID communication: the strength of the signal and the direction of the signal. In other words, you must understand all the characteristics that result in either the loss of power in the signal or the change of direction of the signal. For example, cable loss, impedance, and voltage standing wave ratio (VSWR) are important factors that affect how strong a signal antenna gets from the source through the transmission line. Because readers are directing their signal at the tags, the antennas used in RFID systems are directional antennas. Therefore, directivity, antenna gain, and polarization are important physical quantities that impact the performance of antennas.
Once the antenna radiates the RF waves into the free space, performance indicates how intact it will reach its destination. This part of the performance depends on factors such as absorption, refection, refraction, and scattering. Water is a good absorber, and metals are good reflectors. RFID systems typically use two kinds of communication technique: inductive coupling to communicate within the near fi eld and backscattering to communicate in the far field. Inductive coupling is used by RFID systems operating at LF and HF because the high wavelengths corresponding to these high frequencies will require ridiculously large antennas.
Most of the physics behind RFID relates to how readers and tags communicate with each other. In the next chapter, we discuss tags in greater detail.
Antenna The device used to transmit and receive signals such as radio waves. Both a reader and a tag have their own antennas through which they communicate with each other.
Antenna gain Ratio of energy radiated at a point of maximum radiation from an antenna to the energy radiated at the same point by some reference antenna.
Attenuation Decrease in the amount of something. In RF physics, it means the decrease in amplitude (strength) of the RF signal (wave).
Backscattering The process of collecting an inbound signal (energy), changing the signal (the data it carries), and reflecting it back to where it came from.
Beamwidth The angle between the two half-power points around the point
(the main lobe) that has the peak effective radiated power.
Cable loss The amount of signal power lost in the cable being used as a transmission line.
Characteristic impedance The impedance of the transmission line when it's assumed to be lossless and of infinite length.
Carrier signal The wave that carries the data signal.
Data signal The wave that actually contains the information that needs to go to the receiver.
Diffraction The bending of an EM wave when it strikes sharp edges or when it passes through a narrow gap (slit).
Directivity The ability of an antenna to focus in a particular direction to transmit or receive energy. It is calculated as the ratio of the maximum value of power transmitted
(or received) per unit of solid angle to the average power transmitted (or received) per unit of solid angle.
Effective radiated power The power that will need to be supplied to a reference antenna to produce the same power as this antenna is radiating in a specific direction.
Far field The EM radiations beyond the antenna's near field. In the far field, the signal power decreases as square of the distance from the antenna.
Impedance Resistance to the flow of current in a circuit element, measured as a ratio of voltage across the element and current through the element.
Interference The interaction between two waves. The signal wave can interact with other waves that it meets on the way to its destination. A resultant wave is produced as a result of interference, and the receiver receives the resultant wave.
Link margin Refers to the ratio of maximum effective signal strength received to the minimum signal strength received. In RFID, it means the amount of power that a tag can extract from the RF signal before the communication between the tag and the reader weakens.
Modulation The process that encodes the data signal into the carrier signal and creates the radio wave that the antenna actually transmits to propagate.
Near field The EM radiations within the distance of the order of one wavelength from the antenna. In the near field, the signal power decreases as a cube of the distance from the antenna.
Noise An unwanted electrical wave (or energy) present in a circuit or in a signal.
Polarization Refers to the direction of oscillations in the EM waves transmitted by the antenna.
Reflection The abrupt change in direction of a wave at an interface between two dissimilar media so that the wave returns into the medium from which it hit the interface.
Refraction The change in direction of a wave at an interface between two dissimilar media, but the wave does not return to the medium from which it hit the interface.
Resonance The characteristic of a system to absorb more energy when the frequency of its oscillations matches the system's natural frequency (resonant frequency) than it does at other frequencies.
Scattering The phenomenon of absorbing a wave and reradiating it, thereby changing its direction.
Standing wave A pattern of waves produced from the interference of two waves of the same frequency traveling in opposite directions on the same transmission line.
Voltage standing wave ratio (VSWR) The ratio of maximum voltage to minimum voltage along the transmission line.
Wavefront Refers to the geometrical shape of the space occupied by a traveling wave. For example, an EM wave from an isotropic antenna travels in the free space in all directions, making spherical wavefronts.
About the Authors
Frank Thornton, Owner, Blackthorn Systems, New Hampshire, USA; and Paul Sanghera, Educator, technologist, and an entrepreneur, California, USA
Printed with permission from Syngress, a division of Elsevier. Copyright2008. "How to Cheat at Deploying and Securing RFID" by Frank Thornton and Paul Sanghera. For more information about this title and other similar books, please visit ElsevierDirect.