From how far away can an interrogator read a tag?
There are many different types of radio frequency identification technology, each with its own read range. Passive low-frequency (LF) and high-frequency (HF) tags can be read from a distance of about 3 feet (0.9 meter), whereas some active systems can read tags from 1,000 feet away or more.
If you are asking about passive ultrahigh-frequency (UHF) RFID, the range depends on several factors. One is the reader antenna's power output, which is regulated in most countries. Typically, the output is limited to 2 watts of effective radiated power (ERP). But in Pakistan, the government is working to establish regulations that would limit output to just 0.1 ERP, so the read range will be more limited than in other countries.
Another factor is the type of antenna you use. If you employ a linear-polarized antenna with a dipole tag oriented in the same direction as the antenna, the energy from the reader antenna will be concentrated in a beam and will give you a longer read range than a circular-polarized antenna—which emits energy in a spiral, so that a dipole tag can be read in any orientation to the reader—would.
Another factor is transponder design. If you have a very small antenna on the tag for use on, say, a pharmaceutical pill bottle, the read range will be much shorter than if you have a tag designed for a longer read range on a larger asset. I have seen Omni-ID's Ultra tag, for instance (see Omni-ID Launches New High-Performance UHF Gen 2 Tags), read at a distance of more than 80 feet, which is the longest read range I have witnessed using a conventional passive UHF reader.
One last factor is the nature of the antenna system. Phased-array antennas can focus on very small segments of the read field for a moment, and then proceed to the next segment. This ability to focus enables these systems to detect fainter signals than can be read via a conventional patch antenna, and they can also provide some relative location information.
If you separate the reader's send and receive functions, you can increase the read range even further. The tag receives energy from the reader, and the energy emitted by that reader is limited by regulation. So if you put a device that emits energy (known as an exciter) near a tag to power that tag, then a phased-array antenna can pick up a signal from as distant as 600 feet or more.
I apologize for the long answer, but I wanted to provide a full sense of the complexities involved, and not just offer a simplistic number that might be misleading.
—Mark Roberti, Founder and Editor, RFID Journal
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