As mentioned before, RFID tags differ by their communication methods. Active tags communicate with a reader directly because they have their own transmitter that sends a signal to the environment in order to contact the reader or respond to its inquiries.

Passive and semi-passive tags, on the other hand, do not have this option. They can communicate with a reader only by using the same energy sent by the reader. These tags respond by "reflecting" some of the reader's original signal back, modified to carry the tag's data.

The tag uses the reader's signal for data as well as energy. The tag receives the energy from the reader and uses it to power its chip as well as to send a response signal back to the reader. When the tag's chip receives a signal from a reader, it demodulates it to recognize the data from the reader (usually commands or queries). The tag then modulates this signal to include its own data as a response to the reader's inquiry and sends this data to a reader by using the energy provided by the carrier wave received from the reader.

There are two main methods of passive communication, which differ by whether they use a magnetic field or an electric field. These methods are inductive coupling and passive backscatter (or capacitive coupling).

Inductive Coupling

With inductive coupling , the tag knows it is going to be in the magnetic field of the reader and is ready to respond appropriately. Inductive coupling utilizes the magnetic field that is created by changing current in the reader's antenna. When a tag enters this magnetic field, the field induces an electric current in the tag's antenna, which then powers the tag's chip. Data between the reader and a tag are transferred by fluctuations in the magnetic field. This method is used by low- and high-frequency systems (and in some ultra-high-frequency systems) and requires an induction coil within tags (see Figure 4.1).

image from book
Figure 4.1: Examples of various types of tag antennas

Because of the nature of the magnetic field, this communication method allows for only near-field communication and therefore short read ranges, usually less than 1 meter.

Passive Backscatter

Passive backscatter communication is based on an electric field, which is out past the magnetic field. When a tag enters the electric field produced by the reader's antenna, the field creates an electric current in the tag's antenna, which powers the tag's chip. The electric field is used not only for energy transfer, but also for data transfer. The tag's antenna does not have to be a closed loop (such as an induction coil), but can be made into various shapes to achieve the best read rates possible; usually it is a dipole antenna or dual dipole. Passive backscatter is used mainly by ultra-high-frequency and microwave tags. Because of the nature of the electric field, these tags can achieve longer read ranges than tags of the same size utilizing a magnetic field.

Table 4.2 compares the attributes of inductive coupling and passive backscatter.

Table 4.2: Differences between Inductive Coupling and Passive Backscatter
Open table as spreadsheet

Inductive Coupling

Passive Backscatter

Field Used



Tag Antenna

Induction coil (conductive loop)

Antenna of any shape (single dipole, dual dipole)

Read Range



Performance around Liquids and Metals




LF, HF (UHF possible)

UHF, microwave

CompTIA RFID+ Study Guide Exam RF0-101, includes CD-ROM
CompTIA RFID+ Study Guide Exam RF0-101, includes CD-ROM
Year: 2006
Pages: 136 © 2008-2017.
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