This paper presents a passive wireless sensing paradigm based on the harmonic transponder comprising an antenna covered with a metamaterial-inspired helical structure, which is sensitive to the liquid-level within.

An interrogator transmits frequency hopped UHF RFID signals to the transponder, and a sniffer detects the harmonic signal from the transponder and decodes its RSSI array. A μL-resolution absolute accuracy sensing is demonstrated to support the low-cost ubiquitous liquid monitoring.