Time Domain Reflectometry (TDR) is a familiar sounding principle used in soil moisture transmitters. Time domain reflectometry is a method to determine soil moisture content by using the difference in the propagation velocity of electromagnetic waves in different media. The time-domain reflectometry model was established as early as 1939 and was originally used in the telecommunications industry to find cable breaks. It was first proposed by Canada scientists in 1980 and used for farmland moisture determination in 1985. However, any kind of time-domain reflectometry cannot directly measure the soil moisture content, and the soil moisture content is calculated by measuring the dielectric constant of the soil. The dielectric constant ε of the medium can be calculated using the following formula: ε = { ( L a / V p ) / L} 2 where V p is the relative velocity relative to the propagation velocity of the electromagnetic wave in a vacuum, L is the length of the cable, and L a is the length of the surface cable as indicated on the cable tester. The Vp value is 0 in the case of polyethylene coaxial cables that are generally used. 66, but in order to maximize the demarcation energy of the reflected wavetype tester (cable test) when measuring soil moisture, the transfer switch is usually set at 0. 99 。 Since the relationship between the soil dielectric constant ε and the volume moisture content θ (m3 / m3) is not affected by the contact state, temperature, solute presence, dissolution weight, soil structure, soil properties and hysteresis between the soil and the probe electrode, the following relationship always exists (Topp et al., 1980): θ = – 5. 3 ×10- 2 + 2. 92 ×10- 2ε-5. 5 ×10- 4ε2 + 4. 3 ×10- 6ε。 The soil moisture transmitter is suitable for scientific experiments, water-saving irrigation, greenhouses, flowers and vegetables, grassland pastures, soil rapid measurement, plant culture, sewage treatment and measurement of various particulate matter water content. The soil moisture transmitter has high measurement accuracy, reliable performance, and is less affected by soil salt content, and can be suitable for a variety of soil qualities.

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