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RFID Interrogation Zone Basics

This chapter will help you prepare for the CompTIA RFID+ Exam, focusing on interrogation zone basics, with sample questions and detailed answers for you to practice.
This chapter is from the book

This chapter is from the book

Terms you need to understand:




Hybrid interrogator


Tethered interrogator


Interrogation signal


Read range


Write range


COM ports


Host computer




Tag's UID




Binary tree/tree-walking






Circular polarization


Linear polarization


Bi-static antenna


Mono-static antenna


Time in beam/dwell time


Faraday cage


Antenna field hole


Antenna gain


Effective radiated power (ERP)


Effective isotropically radiated power (EIRP)


Front-back ratio




Free space loss


Field density/power density

Techniques and concepts you need to master:


Dwell time


Interrogation zone


Dense interrogator/dense reader mode


Frequency hopping


Listen Before Talk


Software synchronization






Anticollision methods


AB Symmetry


Calculation of radiated power output


Calculation of free space loss


Calculation of field (power) density

An RFID interrogator (also called an RFID reader) and RFID antennas are two of the major hardware components that make up an RFID system. Depending on the specific application, interrogators and antennas are configured differently to optimize tag read rates. Typical configurations consist of portals, arrays, and tunnels. Power limits can be enhanced by optimizing the radiated signal in a focused manner, to allow for more accuracy and reliability. Various materials can also aid the radiated signal by giving it a better coverage area to enhance the readability of tags. Antenna configurations can also aid the interrogator in reading or writing to tags. When writing to tags, accuracy and time are the main issues.

RFID Interrogator Functionality

An RFID interrogator is an electronic device that generates and receives an interrogation signal (in fact this signal is in its nature a radio signal). These radio signals are radiated or received by an antenna or antennas that are attached to the interrogator. Depending on their type, interrogators can have various capabilities, which include

  • Reading and writing data to tags.
  • Operating on either a single or on multiple frequencies.
  • Performing anticollision processing.
  • Reading bar code or other Automatic Data Collection (ADC) capabilities; these are called hybrid interrogators.

RFID Interrogator Components and Their Function

Figure 3.1 shows the components of a basic interrogator. They include


Figure 3.1 Basic RFID interrogator anatomy.

  • A receiver that holds an amplifier and a demodulator
  • A transmitter that holds a modulator and a power amplifier
  • An oscillator
  • A controller/processor
  • An input/output port to an antenna


The amplifier "expands" the signal received from the tag through the interrogator's antenna for processing, and the demodulator extracts the information from the signal.

The controller/processor performs the data processing functions and manages the communications with the external network.


The oscillator provides the carrier signal to the modulator and a reference signal to the demodulator circuits. The modulator adds information to the signal to be transmitted to a tag. Then the power amplifier amplifies the modulated signal and routes it to the antenna. The antenna radiates the signal to a tag.

Communication Methods

RFID interrogators use different methods to communicate with RFID tags depending on whether the RFID system is active or passive.

Passive and semi-passive tags use passive backscatter to communicate. The signal is generated by the interrogator and radiated through the antenna. The signal is then demodulated by the tag to decode the reader's commands. The reader requested data is reflected back to the interrogator through the modulated signal. This method is also discussed in detail in Chapter 2 on tags.

Active tag communication differs from passive methods in that the tag does not reflect the signal from the interrogator. Because an active tag has its own power source and transmitter, the tag does not have to wait for the interrogator to transmit a signal. This tag can send its data or "beacon" at certain intervals as defined by the system.

Read and Write Range

The capability of an RFID interrogator to communicate successfully with a tag is heavily dependent on two factors:

  • The distance between the interrogator and the tag
  • The tag and the tag's dwell time

The tag's dwell time is the time a tag is in the interrogator's RF field.

An RFID interrogator's read range is the distance between the interrogator and the RFID tag at which the signals from the tag can be read properly.

Similarly, an RFID interrogator's write range is the maximum distance at which information within the RF signal from the interrogator can be received correctly and stored within the memory of the tag's microchip.

More power is needed to write to a tag than to read it. As a result, the tags need to be closer to the antenna to write than to read. The general rule is that the write range is 50–70% of the read range of a particular interrogation zone.

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