We have an access-control system that uses RFID-enabled cards. Can we determine a tag’s frequency by testing the maximum distance that the tag is held from the door-opening system whilst the door is opening?
No, the distance at which the tag would be read would not determine the frequency. Passive high-frequency (HF) and low-frequency (LF) tags can have the same reading distance. What’s more, it is possible to create a near-field passive ultrahigh-frequency (UHF) tag that can only be read from a short distance, like passive HF and LF tags.
If you can break open the card, you will likely be able to ascertain whether it is an LF, HF or UHF transponder. HF transponders have a coiled copper antenna with a few loops (the number typically varies from about five to 10). LF antennas have many more loops. A passive UHF antenna designed for near-field operation has only a single loop. You could also obtain a spectrum analyzer and use that device to determine the frequency of radio waves emitted by the reader at the door.
—Mark Roberti, Founder and Editor, RFID Journal
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