A Compact, Zero-Power and Low-Noise Harmonic-Transponder for Liquid and Moisture Sensing

Harmonic radar has recently gained increasing popularity for its capability in suppressing clutters and cross-talks between the transmitting and receiving antennas. Here, we proposed a compact and passive harmonic-transponder sensor (or harmonic sensor) that can nonlinearly backscatter the second harmonic, with the frequency conversion efficiency being modulated by the soil moisture content. The wireless moisture sensor is composed of a dual-resonance and dual-polarization microstrip antenna integrated with a microfluidic tank, whose liquid level detunes the output second harmonic power. We envision that the proposed noise-immune, passive and low-profile wireless moisture sensor may find broad applications in the industrial internet-of-thing (IoT), environmental research, smart farming, and smart cities.