The “weak spots” in stacked UHF RFID tags in NFC applications

In near-field (NF) applications, stacked UHF RFID tags are known to be less readable than the stand-alone tags. It is also intuitively known that “weak spots” exist when more tags are stacked together, especially when the tags are placed closer to each other. However, “weak spots” in NF-RFID had not been theoretically analyzed in the past, nor had the phenomena been quantitatively measured. In this paper, we show that the “weak spots” are mainly the results of mutual coupling between the tag antennas in NF, we also demonstrate that the profiles of the weak spots are strongly determined by the separation between tags, and they are not monotonically distributed along the stack, i.e., weakest spots are not necessarily on the far end or the center of the stack. To verify our analysis, EM tool simulation and lab measurements are conducted, the results from theoretical calculation, simulation and experiments agree with each other nicely.