It is necessary to realize the multitude of available RFID solutions. Taking into account the operating frequency of the tags, we can distinguish the following types:
– low frequency (10-500 kHz), near-field system,
– high frequency (10-15 MHz), near-field system,
– ultra-high frequency (860-960 MHz), far-field system using modulated backscatter,
– microwave frequency (2.4-5.0 GHz), far-field system using modulated backscatter.
Passive tags, operating at frequencies up to 100 MHz, are usually powered by magnetic induction, according to the same laws of physics that are applied in power transformers (tags with inductive coupling). The current flowing in the reader’s antenna coil creates a magnetic field. Therefore, if a sample tag is located in the area bounded by the lines of force of this field, a current will be induced in its antenna coil, which will charge a capacitor and, as a result, provide the energy necessary to power its electronics. The information contained in the tag is transmitted to the reader by powering (loading) the tag’s coil, according to a pattern that changes accordingly over time. As a result, the tag’s antenna (coil) causes the creation of a modulated magnetic field, which is received by the reader. This process is called load modulation. The data contained in the tag is transmitted by changing the parameters of the magnetic field, which causes a current to be induced in the reader’s antenna (a change in field parameters implies a change in current parameters) (Fig. 1). In the far-field area, however, we are dealing with propagating waves, which are colloquially referred to as radio waves. […] For a wave with a frequency of 13.56 MHz, this limit is therefore 3.6 m, while for a UHF signal with a frequency of 900 MHz, this value drops drastically to the level of a few centimeters. It should be noted that even a lower frequency does not guarantee that the tag will be read, even if it is at a distance less than or equal to the limit value.
The article comes from the magazine “e-Fakty” 5/2006.