A Zero-Power Harmonic Tag for Real-Time Wireless Food Quality Monitoring

This letter proposes a compact, low-profile, and battery-free wireless sensor, capable of continuously monitoring spoilage gases, such as ammonia, in meat and seafood products. The wireless sensor consists of a high-Q microstrip antenna loaded with a varactor diode, whose capacitance is reconfigured by an electrochemical ammonia sensor such that the resonance frequency of the antenna can be tuned by ammonia levels. This (receiving) antenna sensor operating at the fundamental frequency f(0) is connected to a passive frequency doubler and a wideband monopole (transmitting) antenna operating at the second harmonic 2f(0). By receiving and retransmitting radio waves with orthogonal frequencies under the frequency-hopping spread spectrum framework, electromagnetic interferences, such as clutters and jamming, can be filtered out, thus ensuring robust wireless food sensing with absolute accuracy in noisy environments. The effectiveness and robustness of the proposed sensor are demonstrated through remote monitoring of the spoilage process of packaged fish within 48 h. Our measurement results show that the ammonia concentration can be continuously monitored by tracking the peak frequency of the received strength signal indicator at harmonic frequencies. This passive and chipless radio frequency identification sensor with minimal footprint, low complexity, and low cost may be readily deployed into the food package/container for real-time assessment and forecasting of food quality and safety.