Transmission line meta-resonator-based sensors for soil hydrosensing and multiphase fluid monitoring

Abstract

Meta-resonator-based transmission line sensors (MRTS) are systems where metamaterials or resonators and a transmission line are arranged closely. This arrangement allows environmental changes to selectively alter the resonating frequency of the transmission line. MRTSs find applications in a wide range of various fields, including chemical sensing, biological sensing, environmental monitoring, and material characterization. The state-of-the-art studies show that electrochemical sensing, piezoelectric sensing, and capacitive sensing are widely used for soil moisture sensing and fluid monitoring; however, they all need direct contact with the test object and have limitations of continuous monitoring. On the other hand, MRTS can monitor the test object remotely and continuously. Also, MRTS is desirable due to its ability to monitor

the test object remotely and consistently, characterized by a low profile, fabrication- friendly design, and repetitive integrative structure. The frequency-tunable

metamaterial or resonators also increase the sensor performance by its frequency- selective nature. Therefore, designing the miniaturized MRTS for soil hydro sensing

and multiphase fluid monitoring is highly desirable, which can provide high sensitivity and portability. This research aims to design two new metamaterial and resonator-based transmission line sensors for monitoring soil moisture and multiphase fluid. Firstly, a new single-band Modified Hilbert (MH) resonator-based transmission line sensor is designed for estimation of soil moisture level. The sensor exhibited measurable linear fluctuations in transmitted signals between 4.5 and 6 GHz for all soil samples. The sensor's sensitivity is quantified at 82 (MHz/pp), indicating its ability to detect moisture fluctuations in different soil samples. Experiments were conducted using three soil

samples with varying moisture levels, from dry to 20%. Secondly, a new metamaterial- inspired sensor employing four split-square resonators (SSRs) has been designed for

multiphase fluid monitoring applications. The sensor's design integrates a microstrip transmission line incorporated four RF resonators, generating distinct resonant frequencies of 3.36 GHz, 3.66 GHz, 4.26 GHz, and 4.69 GHz with a higher quality factor. The average sensitivity of the sensor is 0.75 GHz/ε and 0.025 GHz/ε for two different multiphase fluid samples. The design evolution, test sample preparation, sensor properties, surface current distribution, electric field, and magnetic field for all designed sensors are investigated to understand the sensor's functionality. The simulated results are validated by measurements of the fabricated prototype, and both results agree well, which leads to the designed sensors as potential candidates for soil hydro-sensing and multiphase fluid monitoring applications.