UHF RFID TAG WRITING 101
After you have decided to adopt RFID for your company’s operations, there are a number of things that must be considered related to UHF RFID tags and tag writing. This article discusses them and provides some tips for successful writing and reading of UHF RFID tags.
PRACTICAL GUIDE TO UHF RFID TAG WRITING
Before a tag can be read, it must be written. When initializing the writing procedure, the target tag 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 on the correct tag. Without singulation, the write operation is performed on a randomly selected tag.
Ideally, the writing procedure is successful. If the tag doesn't receive enough power to be written to, it can return a message indicating that the power level is insufficient or that writing was only partially successful. A tag can also be lost from the reader’s field and not reply at all. In these situations, you can move the tag closer to the reader to improve the communication link and try the write operation again.
One practical tip is to hold close to the reader only the tag that is being written. This way, the user knows exactly which tag the writing operation is targeting. The output power of the reader can also be limited to avoid detecting other tags, such as those several meters away from the reader.
As a general rule, the maximum writing distance is often 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, the tag IC, and the antenna. We have noticed that there has been development in tag IC writing sensitivity. Tags equipped with newer ICs have better writing reliability and significantly improved maximum writing distance.
Figure 1: Example UHF RFID tag
The position and direction (orientation) of a tag affects the effectiveness of its read and write processes. In particular, matching the reader and tag antenna polarization improves the communication link. When using a linear polarized antenna, tags should be kept in the correct position for reading and writing. For example, if the reader antenna is horizontally polarized, the tag should be horizontally aligned.
The CrossDipole devices by Nordic ID have two separate linearly polarized antennas. By switching between those antennas it is possible to read tags in any orientation and still gain the benefits provided by linear polarized antennas.
Circular polarization creates partial polarization mismatch, which causes 3dB loss to the transmitted energy, meaning a loss of half the power. Consequently, although the antenna can read tags at various angles, power is not optimally transferred. Typically, antennas with circular polarization are used for fixed readers in scenarios where tags can be in any position, such as for goods-in reading and at the point of exit.
Besides antenna polarization, materials including metals, liquids and people between the tag and reader affect reading performance by blocking RF signal propagation. Tag size also matters. Usually a physically larger tag is easier to read and write from a longer distance compared to a smaller one, because antenna performance usually increases with antenna size.
Other readers nearby can also impact performance. Fewer channels are available for European devices than there are for US devices, so it is more likely that multiple readers are operating on the same channel in the European context. Of course, DRM (dense reader mode) can reduce the impact of this problem but cannot solve it completely. It is also worth remembering, that frequencies used for UHF RFID may also be used by other radio systems.
Tag writing FROM the PROGRAMMER perspective
To perform UHF RFID tag writing successfully, it is extremely important to make sure that the passive tag is located within a sufficiently strong reading area. 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 reader and tag is sufficient is to call the tag with as low an output power as possible and then analyze 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 much as possible to ensure that the passive tag gains enough power.
On Nordic ID readers, tag writing reliability can be enhanced with a RSSI filter which can be set separately for writing and reading operations. RSSI filtering ensures that, in reading mode, tags with a weak RSSI value are not detected by the reader. In writing mode, RSSI filtering ensures that the tag is within an optimal distance when the writing operation starts.