Wireless Chipless System for Humidity Sensing.

This work describes a fully wireless sensory system where a chipless strategy is followed in the sensor part. Alternatively, to characterize only the sensing element, we present the response of the reader antenna when the sensing element is placed in its vicinity: changes in the parameter of interest are seen by the reader through inductive coupling, varying its frequency response. The sensing part consists of a LC circuit manufactured by printing techniques on a flexible substrate, whose electrical permittivity shows dependence with the moisture content. The measurement distance show significant differences in the frequency response: a change of 700 kHz is observed when the measurement is performed directly on the wireless chipless sensor between 20% and 80%RH, while this variation in frequency is reduced more than three times when measuring at the reader antenna with 5 mm distance between elements. Furthermore, we demonstrate the importance of the separation between reader and sensor to get a reliable measuring system.