Passive low-frequency (LF) and high-frequency (HF) RFID tags operate in the near field, while ultrahigh-frequency (UHF) tags typically operate in the far field. You can not read tags that operate at different frequencies using a single interrogator. Passive UHF tags can operate in either the near or far field, or both, depending on the design of the reader antenna and the transponder antenna.
Near-field tags always have a loop antenna, and the reader antenna also has to have a loop antenna. The two loops form an electromagnetic field, and changes in this field (created by changing the electrical load on the tag antenna) are picked up by the reader antenna and converted to ones and zeros that computers can understand.
Far-field tag antennas backscatter the signal, sort of like a mirror reflecting back light. A near-field tag can not communicate with a far-field reader antenna, and vice versa. However, it is possible to design a tag antenna that can communicate with either a near- or far-field reader antenna. The tag antenna has both a loop and a dipole, or some other far-field antenna.
To my knowledge, no near-field UHF tags are currently on the market, nor are any tags that can operate in both the near and far fields. If any of our readers know of someone who is making such a tag, please post information about it below.
—Mark Roberti, Editor, RFID Journal
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