How Many Tags Can Be Read By an RFID Reader at One Time?

Let me take the second part of the question first. Technically, all RFID readers can interrogate only one tag at a time. If two tags are sending signals to a reader simultaneously, there is no way for the device to distinguish one from another. However, there are special anti-collision algorithms that enable interrogators to "singulate" on specific tags—that is, a reader can talk to one tag at a time, but in very rapid succession. This happens so quickly that it appears the reader is interrogating many tags at once.

There are, of course, many different types of RFID systems—passive low-frequency (LF), high-frequency (HF) and ultrahigh-frequency (UHF), as well as active tags that operate at 433 MHz, 915 MHz, 2.45 MHz and 5.6 GHz. Lower-frequency means less data can be transmitted during a given period of time. So with passive LF tags, which operate at 125 KHz or 134 KHz, it would take slightly longer for 100 tags to communicate with a reader than with an HF system that operates at 13.45 MHz, or a UHF system that operates at 860 to 960 MHz.

The amount of time that tags spend in the read field is critical. You will not be able to read 1,000 tags moving through a dock door, but if you put the same 1,000 tags in a tunnel reader, in which there are antennas on all four sides, you might be able to read all 1,000 tags with no problem for a few seconds.

To summarize, RFID tags can be read one after another very quickly, but for a very dense tag population, the tags would need to be in the read field for a few seconds. For more information on how to set up a UHF system to read dense tag populations, see the answer to a recent Ask the Experts question, How Can I Read 1,000 Tagged Apparel Items Within a Small Area?