Abstract: Dielectric properties are of important value in the application of microwave hot processing of agricultural products. A scheme for measuring the dielectric properties of agricultural products based on uncelebrated coaxial transmission reflection method and modified general effective medium formula (MGEM) was proposed, in which the granular agricultural products were taken as the research object. Dielectric property data of rapeseeds, coarse cereals and grass seeds in the microwave frequency band were measured. Relevant key issues were then analyzed and discussed. These results demonstrated that the measurement scheme could be used to measure the dielectric properties of particulate agricultural products at high frequency, and the dielectric properties of granular agricultural products under the room temperature were related with microwave frequency and water (oil) content. Such relationship met the corresponding relational equation. Given the microwave frequency 2.45 GHz for civil use and the water content M∈(2%,25%), the dielectric constant was between 3.5 and 9, and the loss factor was between 0.1 and 2. This study provided a reference method for  measurement of dielectric properties of granular agricultural products, and the basis for the design of microwave drying and nondestructive testing devices for agricultural products. The technological method was suitable to measurement and analysis of dielectric properties of other small granular substances.

Key words: granular agricultural products, dielectric property, measurement, microwave frequency band

摘要： 介电特性在农产品微波热加工应用研究中具有重要价值。提出一种基于无校准同轴传输反射法和修正通用有效介质公式（modified general effective medium，MGEM）的农产品介电特性测量方案，以颗粒状农产品为对象，实验测量获得微波频率段内17种菜籽颗粒、杂粮颗粒和草籽颗粒的介电特性数据，对相关关键问题进行了分析和讨论。研究结果表明，该测量方案可用于测量和分析高频波段下颗粒状农产品的介电特性；室温下颗粒状农产品的介电特性与微波频率、含水（油）率之间具有相互依存关系,且满足相应的关系方程；在民用微波频率2.45 GHz和含水率M∈(2%,25%)条件下，不同种类颗粒状农产品的介电常数介于3.5～9.0之间，损耗因子介于0.1～2.0之间。相关研究为颗粒状农产品的介电特性测量提供参考，实验数据为农产品的微波干燥和无损检测装置设计提供依据，技术方法适用于小颗粒物质的介电特性测量和分析。

关键词: 颗粒状农产品, 介电特性, 测量, 微波频段