Soil Permittivities In LF-UHF Frequency Range And Soil Moisture Measurement In Frequency Domain

Soil moisture accurate measurement is one of important research subject for agriculturalengineering. There are many methods to measure soil water content. Frequency domaindielectric sensor is characterized by low cost, high degree of automation features and becomethe important sensor for automation monitoring equipment. But, Precision and adaptability ofthe soil frequency domain dielectric sensors is not so good. The main reason is the complexityof the soil dielectric property. So the study on soil dielectric permittivities will be significanceto enrich soil dielectric permittivity theories, reveals the mechanism of soil dielectricpolarization, increase the precision and adaptability of frequency domain dielectric sensor. Itis good for developing soil specific elements measuring device.The mechanism of dielectric polarization was researched. The probe structure willinfluence the soil dielectric permittivity measurement. A7-needles probe with length of28mm and needle-to-needle spacing of10mm was determined. Four kinds of soil dielectricpermittivity was researched in the LF～UHF (1Mhz～3GHz) frequency range with vectornetwork analyzer. The spectrum characteristics of complex dielectric permittivity andapparent dielectric constant was studied at different quality of a soil, different water contentunder different temperature and different salinity. The soil relaxation process and themechanism of polarization was studied and the ideal frequency range of soil water dielectricmeasurement is determined. According to the effect of soil mixed polarization and conductioncurrent by temperature, salt and so on, the soil polarization in different frequency range anddielectric noise of completely dry soil was analyzed. Based on the research of the dielectricpermittivity of soil in frequency domain,combined with the principle of electromagneticmeasurement, a75MHZ new soil moisture sensor was developed. The accuracy andadaptability of the new sensor is perfect, it will has a good production potential.(1) The probe structure influence soil dielectric permittivity measurement. The spacing ofneedle is not the key factor to the soil dielectric property measurement within a certainspacing, and the length of the probe had a greater influence on the soil dielectric propertymeasurement. The electromagnetic field distribution of multi-needle is similar with that ofcoaxial line. multi-needle probe will improve the measurement accuracy of soil dielectric.(2) Soil dielectric permittivity changed with soil water content, and changed withfrequency. At lower frequencies, the soil dielectric measurements decreases rapidly with theincrease of frequency. With the further increase of frequency, the dielectric value changinggradually slow with frequency. Dielectric value had been little effected by salt, temperatureand so on in the61MHz～130MHz range for complex permittivity and62～130MHz rangefor apparent dielectric constant.(3) The research show that the clay content and the interfacial polarization affect the soildielectric properties. Soil texture has a smaller influence on the soil dielectric properties in the43.5MHz to237MHz frequency range. (4) Through the four oven-dry soil dielectric properties research, the interfacialpolarization has greater influence on the soil dielectric properties. Due to the structurecharacteristics of soil particles, it have larger dielectric noise in the21MHz～1.18GHzfrequency range. The reason is mainly caused by the interfacial polarization. The effects ofdielectric noise should be avoided in practice.(5) Soil temperature affects soil polarization rate and thermal motion of ions, which one isstrong or weak under different temperature will affected the dielectric properties of soil.Lower a range, soil dielectric value increase with temperature rise, and the dielectric valuedecrease with temperature increasing greater the range.47MHz～146MHz for the real part ofpermittivity,50MHz～110MHz for the apparent permittivity, the dielectric value are littleaffected by the temperature.(6) The dielectric imaginary part increases with the soil conductivity increasing for moresoil salt content. The real part of dielectric permittivity is only related to the amount of soilwater molecules, so it is less affected by soil salinity. The real part of dielectric permittivity isideal way for saline soil moisture measurement. the apparent permittivity value are littleaffected by the soil salt content.(7) According to the research,61MHz～130MHz for the real part of permittivity and62MHz～110MHz for the apparent permittivity, it is optimal range for soil moisturemeasurement. For the soil water content measurement,75MHZ is the best frequency for soildielectric measurement. The value of determination coefficient of the real part dielectricpermittivity with soil water content is0.95557, the root mean square error RMSE is0.013.The value of determination coefficient of the real part dielectric permittivity with soil watercontent is0.9223, the root mean square error RMSE is0.021. They have a good relationshipwith the soil water content.(8) Including extreme cases such as saturated and dry state, the dielectric dispersionfrequency have a good relationship with the soil water content in the condition of differentsoil type, temperature and salinity. The decision coefficient values R2is0.9693, The rootmean square error RMSE is0.021.So,dielectric dispersion frequency is another ideal methodto improve the accuracy of soil water content measurement.(9) A new FDR sensor, be operated in75MHz,was developed. It has small volume, goodstability, high accuracy of measurement, etc.it is suitable for different soil moisturemeasurement.