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I've been working on some RFID stuff lately and I'm wondering if someone could help me understand how the beam direction works?

I'm having problems with my RFID reader picking up signals from tags that are behind it. From what I understand, the beam that powers the passive tags should be a cone and therefore it shouldn't be able to reach tags that are behind the reader, but this is being done by both of my RFID readers (the readers are Chafon UHF high performance integrated reader, model CF-RU5309). I did some more digging into my RFID reader and it's apparently omnidirectional but I'm not sure if this is possible. Does this mean it can read any tag within a circular area around it?

I've tried to put up some aluminum shielding on the wall behind where the reader is installed but that doesn't seem to have done much other than stopping tags that were really far back behind it from being read, while close ones can still be read. The shielding is a 3ft square placed at the center of the reader, with another rectangle of aluminum shielding that covers up to 2 ft above where the tags sit behind the reader. The tags being read are xerafy microX ii and are set up so that there's two tags on a large aluminum piece of metal. I've included a quick sketch of my setup and a link to the products I'm using. The tags being read are about 3 feet off the ground while the reader is about 10 feet above ground.

Sketch of Setup

http://chafonrfid.com/productdetails.aspx?pid=692

https://www.atlasrfidstore.com/xerafy-microx-ii-rfid-tag/

The antenna is claimed to be a 9dbi circular antenna. Not sure if this means it can't be omnidirectional but I'm pretty sure that I found online that the reader is stated to be omnidirectional.

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  • \$\begingroup\$ Your diagram is not making any sense. What frequency are you using? Are the tags pasive? \$\endgroup\$
    – Andy aka
    Mar 9, 2021 at 16:52
  • \$\begingroup\$ The tags are passive, on-metal tags. The reader is UHF and works at 902-928 MHz I believe. The diagram is just to show that the reader is mounted on a wall, angled slightly downwards and is reading tags that are behind it, on the other side of the wall which should be outside of its range. \$\endgroup\$
    – Spitfire
    Mar 9, 2021 at 17:26
  • \$\begingroup\$ @spitfire The manufacturer site doesn't offer any details or pattern for the internal antenna. Can you read the FCC approval ID# from the device? A search on that sometimes offers internal photos that will help. \$\endgroup\$ Mar 9, 2021 at 17:59
  • \$\begingroup\$ Until you get more info, I'd guess it's a circularly polarized, flat patch antenna, and has very little front-to-back ratio (i.e. back-side rejection of signals). Your shielding attempts won't work because signals diffuse around edges. I'll wait for FCC info. \$\endgroup\$ Mar 9, 2021 at 18:03
  • \$\begingroup\$ @MarkLeavitt I'll try to get the ID# ASAP. I'm in Canada so I'm not sure if it'd have an FCC ID but since it mentions USA freq bands I think it should. When you mention signals diffusing around edges, does that mean the shielding isn't working because of the gap I have? So if I were to cover that wall entirely with aluminum shielding it might work? \$\endgroup\$
    – Spitfire
    Mar 9, 2021 at 18:15

2 Answers 2

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The radiation pattern from an antenna this size at 900 MHz isn't going to have a sharp beam cutoff, like a cone. It's more like a soft, blurry bubble. Also radio waves will diffract (bend) at metallic edges, and scatter (reflect) from metallic surfaces.

You're relying too much on the RFID antenna's directivity. Use distance more. Three recommendations:

  1. Mount the antennas as close as possible to the monitored zones. Maybe you can suspend them more directly over the cart path as it enters the door. The face of the device should point directly downward at the desired read zone. This gives the shortest possible distance to the target.

  2. Turn down the power output. Remember 0 dBm is not zero power. Can you set it to -10 or -20 dBm.

  3. The link indicates this device can report received signal strength (RSSI). This should be significantly lower when pings return from the stray device area. Use it to discriminate between desired and undesired readings.

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  • \$\begingroup\$ Thanks Mark, I can't set the dBm below 0, but setting it to 3 seems to have stopped it from picking up signals behind it. I'll take a look at the RSSI as you mentioned. \$\endgroup\$
    – Spitfire
    Mar 10, 2021 at 16:00
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For RFiD readers, Circular polarization antennas allow the reader to read tags in various orientations within the sensing range of RFID reader. This does not mean the RFID is generating RF power from all directions like a dipole antenna.

What you are seeing is potentially caused by the RFID reader set to a higher output power, which can cause several reflections and cause the reader to detect tags which are not supposed to be detected.

Some vendors set the RFID reader RF power at maximum by default. The accompanying configuration software(which varies by vendor) can allow you to reduce the power output and that should help you limit the field of view of the reader.

If reducing the power limits your reader's range equally in front and back direction. the reader's antenna could have an obscure propagation pattern focusing the radiated power in a certain direction which would not be an easy fix.

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  • \$\begingroup\$ Hi, based on what you said I just ran some tests on what happens when I adjust the gain. It appears to have had no effect for either of the readers, even when set to 0 dbm, because the maximum read range is the same when the power is set to max or lowered to different values. I think there's an issue with either the reader quality or the software so I may have to try resolving this through shielding or some other method. \$\endgroup\$
    – Spitfire
    Mar 9, 2021 at 18:29
  • \$\begingroup\$ Hello, I have experience with RFID deployment. If changing power of the reader has no affect on the range at all in any direction, there may be a firmware issue preventing the power to actually change. \$\endgroup\$ Mar 10, 2021 at 8:36
  • \$\begingroup\$ The range of RFID readers MUST change when you change the radiated power because as the power is reduced, the RF energy density reduces as you go farther from the antenna and at certain point the energy is too low to energize a passive tag. So yes this could be a manufacturing issue with the RFID reader. \$\endgroup\$ Mar 10, 2021 at 8:37
  • \$\begingroup\$ And Mark made a good point, many RFID readers can return RSSI value for every tag and you can do some filtration in software. But do note that the RSSI value is not necessarily a function of only the distance between tag and antenna. It is linked to tag orientation, material behind tag aswell. \$\endgroup\$ Mar 10, 2021 at 8:40
  • \$\begingroup\$ Apologies, I was mixing up the read range and the beam width. The width of the beam did not change very much with changes in power settings, I'm assuming because I could only test close to the antenna so not at a range where it would be noticeable. The read range does appear to have shortened after setting it to 3 dBm. I'll take a look at the RSSI values today. \$\endgroup\$
    – Spitfire
    Mar 10, 2021 at 16:04

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