针对农田灌溉土壤含水量的监测需求，设计开发一种基于频域反射（frequency domain reflectometry, FDR）法的环式土壤水分传感器，并选取砂土、砂壤土、粉壤土和黏土4种类型的土壤对FDR传感器进行实验室标定。标定过程中将土壤体积含水量θV与FDR传感器电路的振荡频率SF采用指数函数θV=aSBF进行拟合来确定a、b参数，从而建立SF与θV对立关系。实验室标定结果表明：当FDR传感器安装深度为15 cm时，振荡频率SF与与土壤体积含水量θV的拟合效果较好；在5 cm深度处，FDR传感器离土表距离较小，电磁波受到表层空气的干扰，导致数据测量不稳定，两者数据拟合效果稍差。在此基础上，将在实验室标定后FDR传感器安装于玉米田进行野外精度检定，与取土烘干法对比分析发现FDR传感器可以较好地反映土壤水分的动态变化，且精度符合行业标准的要求。

Dynamic monitoring of soil moisture is of great significance for guiding timely and appropriate agricultural irrigation. Currently, the water content measurement instruments used in domestic field experiments are mainly imported from instrument companies in Australia, the Netherlands, and other countries. However, these instruments do not open the bottom-level sensor calibration program to ordinary users, resulting in significant differences in water monitoring accuracy when used in different soil types. Frequency domain reflectometry (FDR) is the most widely used soil moisture measurement method in China. In response to the monitoring needs of soil moisture in agricultural irrigation fields, a ring-shaped soil moisture sensor based on the FDR method was designed and developed. The function relationship between soil volume moisture content and FDR sensor circuit oscillation frequency was established by preparing standard soil samples in the laboratory, and the accuracy of the sensor was verified through field measurement data. Seven treatments were set up in the indoor experiment, with soil volume moisture content of 13.21%, 16.28%, 20.46%, 27.45%, 31.20%, 34.54%, and 39.80%, respectively. FDR sensors were installed at two depths of 5 cm and 15 cm from the soil surface, and the oscillation frequency SF of the sensor and the soil volume moisture content θv obtained by the drying method were fitted using the exponential function relationship θV=aSBF to determine the coefficients a and b corresponding to the sensor oscillation circuit. The field validation experiment was conducted at the irrigation test station in Baoding. FDR sensors were buried at depths of 15 cm and 60 cm in the irrigation district for soil moisture observation, and conventional observations were conducted using the soil drying method at the same time. The laboratory calibration results showed that at a depth of 15 cm, the fit between the oscillation frequency SF and the soil volume moisture content θv was good, and the R2 value of the fitting curve was 0.876 4. In contrast, at a depth of 5 cm, the data measurement was unstable due to the small distance between the FDR sensor and the soil surface, and the electromagnetic wave was interfered with by the surface air, resulting in slightly worse fitting results. The field validation experiment results showed that the FDR soil moisture sensor calibrated with a and b coefficients prepared in the laboratory with soil samples can accurately reflect the actual soil moisture content ranging from the crop wilting coefficient to the soil saturation moisture content, and can be used for guidance of precision irrigation. In response to the needs of agricultural irrigation, a ring-shaped soil moisture sensor based on the FDR principle was developed, and its accuracy was tested in the laboratory and field. The laboratory soil column test of the sensor found that the accuracy of the sensor is affected by the burial depth in the soil. When the burial depth of the sensor is 15 cm from the soil sample surface, the R2 value of the fitting curve between the FDR sensor measurement data and the volume moisture content obtained by the drying method is 0.876 4, and the fitting effect is better. At a depth of 5 cm from the soil sample surface, the fitting curve effect between FDR data and drying method data is not ideal, mainly because the FDR sensor is too close to the soil surface, and the electromagnetic wave is interfered with by the surface air, resulting in unstable data measurement. To ensure the reliability of the data, it is recommended that the ring-shaped soil moisture sensor be used for soil moisture monitoring below 15 cm from the soil surface in practical applications. Field calibration is usually more cumbersome than laboratory calibration, and requires much more time than laboratory calibration. The FDR sensor developed is calibrated with soil samples prepared from the field measurement area, and its observed soil volume moisture content and the error range of the soil drying method are [0.5%, 1.1%], which meets the accuracy requirements of the "Automatic Soil Moisture Observation Instrument" (QX/T 567-2020). Therefore, the method of laboratory calibration by taking soil samples in the measurement field can be used to improve the calibration efficiency.