A Compact Wireless Passive Harmonic Sensor for Ammonia Sensing in Packaged Food

Sensing of volatile acidic/basic gases plays a significant role in the early detection of food spoilage. This letter presents a compact low-cost quasi-chipless sensor for monitoring of such gases. The sensor utilizes a dual-band annular ring antenna to receive, modulate, and retransmit an incoming interrogation signal at double the frequency and with an orthogonal polarization. The application of the sensor for monitoring ammonia is presented. The resonant frequency of the receiving mode of the antenna is sensitized to ammonia using a varactor-based transduction scheme utilizing a hydrogel-coated pH electrode pair. The sensor provides a linear response to the logarithm of the concentration of ammonia, a sensitivity of 2.3 MHz per unit log concentration and greater than 1-m range for a signal to noise of better than 30 dB.