Study on the Influence of Liquid Level Height in Containers on RFID System Performance

Focusing on the issue of how variations in liquid level height within a container affect the performance of passive ultrahigh frequency (UHF) radio frequency identification (RFID) tags, this study derives a link budget model for a passive UHF RFID system based on RFID operational principles and electromagnetic wave propagation theory. Using power transmission coefficients, the study analyzes how impedance mismatch caused by liquid in the container affects system performance. To validate the theoretical model, a combination of simulations and indoor experiments was employed, establishing segmented models of the tag response signal power (RSSI) as a function of liquid level height in both vertical and horizontal tag orientations. The RSSI of two tags, Alien9662 and Alien9640, was tested in an open indoor environment across varying liquid levels from 0 mm to 140 mm, measuring signal strength variations under different liquid levels. Theoretical analysis and experimental results reveal that when the liquid level changes along the antenna’s bent arm, RSSI decreases significantly (e.g., from –43.4 dBm to –75.6 dBm for the Alien9662 tag in vertical deployment). when the liquid level changes along the small electrical loop, RSSI first increases and then decreases (e.g., from –52.8 dBm to –43.4 dBm for L < 20 mm), exhibiting a nonlinear variation with liquid level height. The RSSI changes observed in both tags align with the segmented models, validating the model’s accuracy. These findings not only provide a theoretical basis for understanding the impact of liquid environments on RFID system performance but also offer a reference for optimizing RFID tag placement in liquid containers, which could support practical applications such as inventory management and liquid level monitoring.