Radar Cross Section-Based Chipless Tag With Built-In Reference for Relative Humidity Monitoring of Packaged Food Commodities

Dry food commodities like grains and pulses can be stored safely for several years under controlled storage conditions. The equilibrium moisture content of the packaged grain is one of the most important parameters required to be monitored and controlled for extended safe storage. This paper presents a single element dual-polarized sensing tag based on an annular slot antenna operating at two closely spaced resonant frequencies for radar cross-section-based monitoring of relative humidity in hermetically packaged food commodities. One of the resonant frequencies is functionalized for sensing while the other acts as a built-in reference, mitigating the effects of environmental loading. A polyvinyl alcohol coated interdigitated capacitor, integrated onto the antenna is used as a capacitive transducer for the sensing element. Laboratory scale measurements were carried out using the saturated salt solution method. Results show that the proposed sensor can be used for monitoring a wide range of humidity conditions.