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Succeed in writing UHF RFID Tags - practical tips

Once the decision about adopting RFID to the company’s operations has been made, a number of issues need to be considered. This article discusses the issues related to tags and the tag writing and provides tips to writing and reading tags.

Co-authors: Toni Heijari, Turo Rantanen & Mikko Lähteenmäki

A practical guide to tag writing

This part of the article discusses the actual tag writing. When initializing the writing procedure, the tag in question is singulated against the desired memory bank contents, usually the EPC bank. By singulating a specific tag, you can be sure that you are writing to the correct tag. Without singulation, the write operation is performed in a randomly selected tag. In the writing procedure there are several possible outcomes. The outcome desired of course, is that the writing is successful. But things don't always go as planned. If a tag doesn't have enough power to perform writing it may send back a message indicating that power is not sufficient or that writing was only partially done. It is also possible that a specific tag has left from the reader's field and doesn't reply at all. In such cases the writing must be tried again or for example tag can be moved closer to the reader to improve the communication link.

One practical advice is to keep close only the tag that is being written. Because in that case the user always knows which tag the writing operation is targeting. The output power of the reader can also be limited so that the reader won't notice other tags around, for instance tags lying several meters away from the reader.
A general rule suggests that the maximum writing distance is often only a half of the maximum reading distance. This is because a tag needs more energy to perform a writing operation than a reading operation. The maximum reading and writing distances are affected by the output power, receiver sensitivity, antenna gain, the environment and also tag IC and antenna. We have noticed that there has been some development of the writing sensitivity of the tag ICs, and due to that tags equipped with newer ICs the writing reliability and maximum writing distance have improved significantly.

 

RFID Tag _drawing _UPM

 

The position and direction / orientation of the tag affect the result of the read and write process. More specifically, the reader and tag antenna polarization matching affects the communication link. If using a linear polarized antenna, the tags should be kept in right orientation to be able to read and write the tags. Meaning that if the reader antenna is horizontally polarized the tag should also be horizontally aligned.

The CrossDipole devices of Nordic ID have two separate linearly polarized antennas. When switching between those antennas it is possible to read tags in every orientation and still gain from the benefits provided by the linear polarized antennas.

If the polarization is circular, there is a partial mismatch of polarization. This mismatch causes 3dB (half of power) loss to transmitted energy. In this type of polarization the tag can be in multiple angles and still be read, but the power is not optimally transferred. Typically fixed readers use antennas with circular polarization, since for instance during goods in and at the point of exit the tags can be in any position.

Besides the antenna polarization, also metals, liquids and people between the tag and reader affect the reading performance. In practice they block the RF signals propagation. Furthermore usually a physically larger tag is easier to be read and written from a longer distance compared to a smaller one. That is because increase in antenna size usually improves the performance of the antenna. 
What comes to other equipment in the same area, other readers in use may disturb the performance. In Europe the number of channels is less than in the U.S. for instance. Therefore having multiple readers operating on exactly same channel is more probable in Europe as well. Of course DRM (dense reader mode) helps this problem but it does not solve it completely. It must also be remembered that frequencies that UHF RFID is using may also be used by other radio systems.

Tag -testing -clothes -rack

Instructions for writing tags from a programmers' point of view

To perform tag writing successfully, it is extremely important to make sure that the passive tag is located within a reading area strong enough. This is possible by utilizing the RSSI value (received signal strength indicator) defined by the reader. The best way to make sure that the distance between the reader and tag is sufficient is to call the tag with as low output power as possible and then analyzing the RSSI value received. Reliable tag writing can be started when an RSSI value high enough has been reached with a low output power. For the writing operation itself, the output power should be maximized as high as possible in order to ensure that the passive tag gains enough power.

The reliability of tag writing can be enhanced with RSSI filter in Nordic ID readers, and it can be separately set for both writing and reading operations. RSSI filtering ensures that in reading process tags with a too weak RSSI value will not be shown to the reader. In writing, RSSI filtering has a role to ensure that the tag still is within an optimal distance when the writing operation is started.

 

Read also the previous article by Sini Syrjälä on technology behind tag writing.

2 comments on “Succeed in writing UHF RFID Tags - practical tips”

  1. Posted 24 May 2012 at 23:16:42

    Hello Sini,
    I am working on parking gate access prototypes and I have sample UHF EPC Gen2 inlays from UPM. The university where I will test them requires a 9 digit permit number encoded. The inlays came in with the last 9 digits as zeros. Can I just change those to the permit number?

    Thanks,
    Tom

  2. Gravatar of Sini SyrjäläSini Syrjälä
    Posted 13 June 2012 at 15:32:13

    Hello Tom,
    I assume you are referring to EPC code. The EPC code is changeable, unless the memory bank has been locked. Usually, when the tags leave the factory, the EPC bank is not locked, especially if each of the tags has the same EPC. Hope this information helps!

    Regards,
    Sini

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