Experimental evaluation and upper-bounds of cross-sensitivity in stacked RFID sensors

The dense distribution of wireless sensors based on Ultra-High-Frequency (UHF) Radio-Frequency Identification (RFID) technology, in the food market or drug cold chains, raises issues regarding the effects of mutual electromagnetic coupling on sensing. In the case of stacked items, in fact, inter-antenna coupling can cause disturbance to sensor measurements, thus affecting the specificity and reliability of the collected data. This paper experimentally investigates the effects of coupling for some configurations of antenna size and alignments by exploiting capacitive sensing based on the emerging auto-tuning integrated circuit (IC) architectures. The results revealed that electromagnetic coupling typically induces cross-sensitivity and instability so that the variation of any sensor's output will also be indirectly captured by adjacent devices. However, this disturbing effect vanishes after a threshold decoupling distance of the order of 4 mm for a small-footprint loop (15mm x 15 mm), and 15 mm in the case of a card-like footprint (C-dipole, 54mm x 16 mm). Moreover, experiments revealed that the above distances can be halved by resorting to a 180 degrees rotation of the adjacent items.