I am looking at some UHF RFID readers for a project and I have a few questions about the overall concept of detecting passive RFID tags.

The system is concerned with keeping track of people passing through a specific point (doorway, staircase, etc.). This is not an access-control application; ideally the people walking through a certain checkpoint would have RFID tags on them but wouldn't take them out for scanning. Sort of like a car passing through an automated barrier after RFID tag detection, only in this case there is no barrier involved.

Think of it as a passive personnel locating system. The system would only keep track of "Person A was last seen walking through stairwell 4, half an hour ago", that's about it.

A person with RFID tag passing through a door above which an RFID reader is mounted

(Image source)

I have looked at readers like this one with circular polarization antenna and a range of 3-5 meters. I'll be using passive RFID cards. My questions are:

  • My assumption is that the readers like this work only in a straight line-of-sight. If that is correct, on what factor does the "width" of the scannable area depend? If my assumption is incorrect, does the range of 5 meters mean it's actually the radius of the area that is covered by this reader?
  • If a scanner like this were to be placed on the ceiling above a door, would it effectively be able to scan the tags passing below it through that door? How big a problem would "shadowing" be in this case (when a person has the tag in their pocket or two people walk next to each other so that the one in front is blocked by the one in back)?

If you could place a wire loop antenna around the door you could do this with low frequency RFID. Three turns of 10 AWG wire surrounding a doorway makes a nice passageway detector. TIRIS readers were recently discontinued by Texas Instruments but ProtagD in Germany makes clones of them.

Shadowing will be a challenge regardless of technology. You want a large read zone to have good detection efficiency while a smaller read zone would limit shadowing.

Low frequency RFID has a better defined read zone while UHF has longer read range which could increase shadowing. UHF also will sometimes pick up a rogue tag that is quite far away (in another room).

Low frequency RFID without anti-collision will only detect the strongest tag and block others. UHF has anti-collision which means it will detect all tags in the vicinity, again increasing the opportunity for shadowing.

Some readers can measure the tag signal strength but this is a poor indication of distance since the strength changes as the tag changes orientation relative to the antenna. A tag further away from the antenna could have a stronger signal than one closer that is not oriented as well.

The primary advantage of UHF in this application would be that the antenna is a small box you can mount above the door as opposed to LF which would need a wire loop around the door. By pointing the UHF antenna down it might work to limit the zone to just the doorway area.

HF RFID probably would not be a good fit for this due to short read range and limited antenna sizes for scanning large areas.


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