Thursday, December 8, 2011

RFID Terms and Definitions

Radio Frequency Identification Tag (RFID)

RFID Tags applications consist in a radio frequencies wave (signal) exchanged between the RFID Tag and the RFID Reader. The information coded and stored in the RFID Tag transmits via RF waves to the RFID Reader for decoding and retrieving.

Electronic Product Code (EPC)

Modern replacement of the Universal Product Code (UPC), the EPC RFID chip stored data is from 32 to 256 bits long, with a 96-bit unique identifier.

Write Once, Read Many (WORM)

WORM is part of the protocol Generation 1.0 and 2.0 - Class 1, referring to the RFID tag chip properties to have data written once but readable as often as needed.


Inlay is another name for the RFID tag chip where data is stored. For more details on how an inlay works, see Backscattering Editorial.

Impedance (Z)

Impedance is a complex number and defines one of the characteristics of the RFID antennas.

Impedance has a Real part (representing the Ohm resistance and a Reactive part (representing the capacitive/inductive behavior).

The real part R is independent of frequency, but the imaginary part is highly depending on frequency, with (capacitive behavior) and (inductive behavior) competing with each other.

Regarding how important is the reactive impedance for antenna power dissipation see Backscattering Editorial.

Plane wave and the Free Space

Electro-Magnetic (EM) field propagation that can be approximated with a plane wave in a free space if the measurement point is located in a plain flat field with no reflective surrounding objects, and the distance to the emitting source is long enough that the wave spherical curvature can be considered as a straight line at the measuring probe.

Free Space impedance

The free space impedance is equal with 377 ohms.

Open Area Test Site (OATS)

Measurements performed in a free space environment.

Anechoic Chamber

A radiofrequency "anechoic chamber" is a shielded room whose walls are covered with a material that scatters or absorbs so much of the incident energy that it can simulate free space.


Salisbury Sheet is a sheet of paper that had been coated with a substance to give it a surface resistivity of 377 ohms per square. When placed at exactly one-quarter wavelength away from the metal wall, the reflected signals virtually disappear.

It only works at one frequency

Jaumann Sandwich

More Salisbury sheets - tapered both in resistance and in the distance from the metal wall - can achieve a 20 dB reduction in reflection over a 5 to 1 bandwidth.

The resistive Salisbury sheets (the Jaumann Sandwich) are tapered over one-wavelength distance. "Jaumann Sandwich" is also the theoretic originator of the Pyramidal Absorbers materials for the anechoic chamber walls.

Below 100 MHz, due to the anechoic effects, the absorbent material is the ferrite tile.

Transverse Electro-Magnetic (TEM) Cell

Invented by Crawford, the TEM Cell is a bench replica of an anechoic chamber and generates accurate electromagnetic waves over a vide frequency range from DC (0 Hz) to several MHz.

The disadvantage is that the Cell dimensions decreases when operating frequency increases, and imposes constraint on the size of the Equipment Under test (EUT).

Voltage Standing Wave Ratio (VSWR)

The VSWR is the ratio between the maximum to minimum voltage in a standing wave pattern and it is a measure of the mismatch between the transmission line and the load.

VSWR = 1 indicates perfect matched load.

Effective Radiated Power (ERP)

ERP is the power radiated by antenna (like reader antenna) measured in Watts in its direction of maximum gain under specified conditions of measurement and in the absence of modulation.

Isotropic antenna

Antenna considerate that evenly distributes power in all directions.

Effective Isotropically Radiated Power (EIRP)

EIRP is the amount of power emitted by an isotropic antenna to produce the peak power density in the direction of maximum gain.

The EIRP takes in consideration all the losses into transmission line and connectors besides the gain and power of the antenna.


When an inlay antenna is stimulated by an external radiation (electromagnetic wave), it responds through a straightforward mechanism by sending the same electromagnetic wave back to the source without the need of an onboard power supply or synthesizer.

Listen-Before-Talk (LBT) Communication Technique

LBT (also known as Listen-Before-Transmit) is a communication technique imposed in Europe and Japan for the Reader to find an unoccupied RFID regulated channel before transmitting.

When a reader (i.e. its own receiver) detects that another device (any other radio transmitters) occupies a regulated channel, it will automatically switched to another channel and check if it is unused before transmitting its carrier.

European ETSI regulations impose as mandatory a listen period of 5 milliseconds, followed by a 4 seconds communication transmission, and than immediately switch to an other channel where the protocol stars all over again.

Frequency Hopping Spread Spectrum (FHSS) Technique

FHSS is a communication technique imposed in USA, China and India for the Reader to bounce from regulated channel to regulated channel in a pre-assigned, pseudo-random sequence to avoid bumping into other readers.

USregulations allow the Reader to transmit for 0.4 seconds before it jumps to another regulated channel.

Industrial, Scientific, and Medical (ISM) bands

ISM is the unlicensed radio frequencies band where RFID tags operate. Each country designates its own ISM where operating devices do not require a license but are still subject to signal-regulated emissions limits (power levels).

European Telecommunications Standard Institute (ETSI)

Under the Council Directive 98/34/EC mandate issued by European Commission in regard to RFID regulation in Europe, the ETSI produced asset of technical requirements under: EN 302 208-1 Electromagnetic compatibility and Radio spectrum matters (ERM)

The ETSI regulates the European RFID frequencies range into 15 channels with a width of 200 KHz, where only channels (4 ~ 13) are allowed a maximum Reader antenna power of 2 Watts ERP.

Triboelectrification (Tribocharging)

Tribocharging is the process of two (different susceptible polarity) materials that contact, exchange electrons, than separate, and create electrostatic charges on their surfaces.

Triboelectrification is yet another way besides friction that creates electrostatic charges on material surfaces.


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