Mutual coupling of stacked UHF RFID antennas in NFC applications

As RFID deployment moves from pallet level to item level, UHF RFID has gained more and more momentum in NFC (Near Field Communication) applications recently. Similar to HF RFID tags, UHF RFID tags in NFC also use inductive coupling antennas to interact with the readers. However, in item-level deployment, items with UHF RFID tags are often stacked closely and interrogated by the reader simultaneously. Closely stacked RFID antennas have strong mutual coupling effects, which often lead to significant performance degradation and even failure. In this paper, we first present a mathematical model of the coupling effect of stacked RFID antennas and calculate the performance degradation at different locations of the stack. We then use Agilent® AMDS to model a stack of 20 RFID labels and simulate the degradation effects. Further more, we perform lab tests on the 20-label stack and measure the sensitivity at different locations. The results from mathematical calculation, AMDS simulation and lab tests agree with each other nicely and demonstrate that the weak spots are not distributed monotonically along the stack.