Design of a Differential FDR Microwave Soil Moisture Sensor Using an Exponential Tapered Line

Abstract

This paper proposes a differential frequency domain reflectometry (FDR) microwave soil moisture sensor employing an exponential tapered line to obtain improved impedance matching characteristics in a non-uniform microstrip line structure. In the proposed sensor, the width of the central microstrip line is set to 2 mm to obtain a distinct peak value in the sensing-band, while the feed microstrip line width is designed as 5 mm to achieve high skirt characteristics in the reference-band. To improve the impedance matching characteristics of the non-uniform microstrip lines, an exponential tapered line is applied to the feeding line structure. As a result, the proposed sensor simultaneously achieves the distinct peak value and high skirt characteristics. In addition, triangular spiral resonator and rectangular spiral resonator structures are employed to implement the differential FDR method, making the sensing- and reference-bands, respectively. The fabricated sensor is experimentally evaluated under various volumetric water content (VWC) conditions. When the VWC (%) varies from 0% to 20%, the frequency difference between the sensing- and reference-bands $f_d$ shifts from 3150 MHz to 1925 MHz with a coefficient of determination R² of 0.926. These results demonstrate that the proposed sensor is suitable for smart farming applications requiring accurate soil moisture monitoring.