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Why Does Our Read Range Drop When We Interrogate Multiple Tags?
When we read many tags simultaneously to capture the EPC numbers and data stored in user memory for inventory purposes, the read range decreases significantly. Why does this happen, and what can we do about it?
Passive ultrahigh-frequency (UHF) transponders receive energy from a reader. The more data stored on the tag that you need to read (or to write to the tag), the more energy will be required to perform these operations and reflect back a signal with the information. When there are many tags in the read field, the reader must also work through an anti-collision algorithm so it can talk to one tag at a time. This also requires energy to be consumed by each tag. If the tags need more energy, then they need to be closer to the reader antenna, which is the source of its energy. This is the reason the read range is shorter.
There have always been two views of passive Electronic Product Code (EPC) tags, in terms of data. The original founders believed that there was no reason to store any data on the tag—all information should be associated with a serial number in the database, they felt, and all that was required on the tag was the unique serial number. End users, however, found that in the real world, it was often helpful to have data stored on the tag. So for cross-docking applications, for example, you could store a number to quickly inform a worker about which store a particular item should be shipped to.
The answer to your problem lies in finding the right balance between using just the serial number (EPC) and using the user memory in the tags. For inventory purposes, it should be okay to simply read the serial numbers, which are associated with specific items (your readers should have an inventorying mode allowing you to do this). Just interrogating the serial numbers will tell you what you have in stock. Any additional data needed on the tags should not be read while conducting inventory counts. This method can be used for other applications as well, such as identifying items that have passed their sell-by date.
—Mark Roberti, Founder and Editor, RFID Journal
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