Harmonic Tag with Probe-Fed Patch Antennas

This paper presents a multi-layer passive harmonic tag design. The tag is made of three layers: the top layer features the antenna units, the second layer consists of a plain ground surface, and the bottom layer houses a Schottky diode and impedance-matching circuit operating within this framework. There are two probe-fed rectangular patch antennas, one serves as the 2.5 GHz receiving antenna, and the other serves as the 5 GHz transmitting antenna, respectively. The specifically designed L-shaped matching networks are deployed at the input terminal and output terminal of the Schottky diode to achieve impedance matching. This approach leads to an enhancement in conversion gain, attributed to the combined gains of the antennas and the diode matching circuit. The harmonic tag is fabricated and evaluated in outdoor field tests. In line-of-sight scenarios, the tag demonstrates a maximum detection range of 12.2 m with 15 dBm transmitting power. Additionally, an underground test of the harmonic tag is conducted which achieves 10 cm of sensing depth in wet soil conditions.