A Group-Delay-Based Chipless RFID Humidity Tag Sensor Using Silicon Nanowires

This letter proposes, for the first time, a wireless passive chipless RFID tag for identification and humidity sensing applications. The proposed sensor tag, based on time domain, consists of cascaded group of transmission-line sections (C-sections) coupled at alternative ends capable of producing group delay peaks at a particular frequency. C-sections allow producing a unique ID for a tag. Sensing is achieved by depositing silicon nanowires on the strips of the C-sections group. Silicon nanowires are very sensitive to environmental humidity variation, which in turn changes the radar cross section (RCS), phase, and group delay of the tag. This feature is utilized for humidity sensor applications. The proposed sensor tag is experimentally verified. An RCS change of 30 dB and group delay variation of nearly 22.3 ns was observed near the fundamental frequency over a bandwidth of 40 MHz for a relative humidity variation of 60.2%-88%. Measurements were performed in a real environment. The obtained results confirm the very good potential of nanowires in humidity sensor applications.