Active and Passive Transponders
An important distinction criterion of different RFID systems is how the energy supply of the transponder works. Here we distinguish between passive and active transponders. Passive transponders do not have any power supply. Through the transponder antenna, the magnetic or electromagnetic field of the reader provides all the energy required for operating the transponder. In order to transmit data from the transponder to the reader, the field of the reader can be modulated (e.g. by load modulation or modulated backscatter; see Section 3.2) or the transponder can intermediately store, for a short time, energy from the field of the reader (see Section 3.3). That means that the energy emitted by the reader is used for data transmission both from the reader to the transponder and back to the reader. If the transponder is located outside the reader’s range, the transponder has no power supply at all and, therefore, will not be able to send signals.

Active transponders have their own energy supply, e.g. in form of a battery or a solar cell. Here the power supply is used to provide voltage to the chip. The magnetic or electromagnetic field received by the reader is therefore no longer necessary for the power supply of the chip. That means that the field may be much weaker than the field required for operating a passive transponder. This condition can substantially increase the communication range if the transponder is capable of detecting the weaker reader signal. But even an active RFID transponder is not able to generate a high-frequency signal of its own, but can only modulate the reader field in order to transmit data between transponder and reader, similar to the procedure in passive transponders. Thus, the energy from the transponder’s own power supply does not contribute to data transmission from the transponder to the reader! In the literature, this type of transponder is often called ‘semi-passive’ transponder (Kleist et al., 2004), which refers to the fact that this transponder is not able to generate a high-frequency signal.

As both passive and active (semi-active) RFID transponders need the reader’s magnetic or electromagnetic field for transmitting data, there are physical limitations that substantially restrict the achievable reading ranges. Taking into account the permitted transmitting power of RFID readers, the maximum achievable range is 15 m, depending on the frequency band.

The circuit design of another class of active transponders corresponds to that of a classic radio device. These transponders have an active transmitter (TX) and often also a high-quality receiver (RX). In order to transmit data to a reader, a transmitter is switched on and the antenna emits a high-frequency electromagnetic field. A local energy source, e.g. a battery, supplies the transponder with power.

These transponders emit a high-frequency electromagnetic field instead of modulating the reader’s field. From a pure technical perspective, these transponders are not genuine ‘RFID’ transponders, but short-range radio devices (SRD). For several decades, similar devices have been used for data transmission from remote places, for instance. Due to other physical mechanisms and taking into account the permitted transmitting power, short-range devices can have a range of up to several hundred metres. The larger the transmitting power, the larger the ranges that can be achieved, in comparison with conventional radio equipment.

In order to benefit from the continuing RFID boom, short-range devices are marketed as RFID devices. From a marketing perspective, this is a feasible approach. However, a technician should be always aware of the differences between RFID and short-range devices, as well as of the reasons behind the large range of SRD.

The RFID handbook does not include short-range devices as there is a large number of specialist literature on this topic. For an introduction, we recommend Bensky (2000).