Accurate Acquisition Method Of Soil Electromagnetic Reflection Coefficient Based On De-embedding Technology

The accurate determination of soil water content is very important for effective water management,irrigation measures,crop growth,and pollutant monitoring;and the physical properties of the soil,such as structure,plasticity,compressibility,adhesion,cohesion,etc.,are all compatible with each other.The soil moisture status is closely related.The open-end coaxial probe method uses frequency domain reflection technology,combined with a vector network analyzer,to measure the complex permittivity of the soil in a wide frequency band,and has the characteristics of fast,non-destructive,and high-precision.However,a problem will inevitably be encountered in the measurement process.A passive probe must be inserted between the soil medium to be tested and the vector network analyzer to achieve the measurement.In order to remove the probe’s influence on the soil electromagnetic reflection coefficient during measurement,this The subject researches the de-embedding technology of single-port coaxial probes and the application of specific soil moisture measurement.The specific content is as follows:(1)In the process of solving the three-load de-embedding,it is first necessary to obtain the theoretical reflection coefficient of the medium to be measured,convert the threedimensional problem of the electromagnetic field into a one-dimensional problem in the spectral domain,establish the equations of the electric field and the magnetic field in the medium,according to the inside and outside of the coaxial probe Electromagnetic field continuity boundary conditions,eliminate the electric field and magnetic field parameters,establish the quasi-static model of the coaxial probe,and establish the measurement model in the full-wave simulation software for comparison.The result is that the average absolute error of the real part of the reflection coefficient is 0.001,and the imaginary part is average.The absolute value error is 0.022,indicating that the quasi-static model has high accuracy;then the theoretical calculation value and the actual measurement value of the quasi-static model of air,deionized water and isoacetone are combined to calculate the probe’s Scattering parameters;finally,the three-load de-embedding effect is verified by comparing the calculated value of the measured absolute ethanol with the calculated value of the Cole-Cole formula.The average absolute value error of the real part of the complex permittivity is 0.375,and the average absolute value error of the imaginary part is 0.279.Which shows the higher accuracy of the three-load de-embedding method.(2)In order to solve the problems of three-load de-embedding,simplify the de-embedding steps,and compare the full-wave simulation de-embedding and phase compensation method de-embedding.When the open coaxial probe matches the impedance of the calibration surface and the coaxial line,full-wave de-embedding and traditional phase compensation method deembedding have better de-embedding effects.The effectiveness of single-port full-wave deembedding is verified.When the impedance characteristics of the probe and the coaxial line do not match due to process errors or other reasons,electromagnetic waves will be reflected on the calibration surface,and the de-embedding effect will not be achieved by phase compensation alone,while the single-port full-wave de-embedding still has a higher deembedding effect.Embedding accuracy.(3)Time-domain de-embedding only needs to open a load state,and three de-embedding error parameters can be extracted through time window gating under time-domain conditions.The accuracy of time domain de-embedding is verified from three aspects.First,the reflection coefficient is verified under simulation conditions.The average absolute errors of the real and imaginary parts of the time-domain de-embedding and the ideal value are 0.0025 and 0.0021,respectively.Then the complex permittivity of absolute ethanol calculated by actual measurement,within the effective frequency range,compared with the theoretical value,the average absolute error of the real and imaginary parts are 0.0025 and 0.0021 respectively.Finally,by measuring different concentrations of sodium chloride solution,compared with the three-load de-embedding method,the maximum absolute error of the real part is 2.7,and the maximum error of the imaginary part is 3.1.It shows the reliability of the time-domain deembedding method,and provides technical support for the open-end coaxial probe method to measure dielectric.(4)De-embedding and quasi-static calculation models are used to measure and calculate the complex permittivity of yellow clay,medium loam and loess soil with different moisture content.The complex permittivity of soil samples with different moisture content increases with the increase of moisture content.The real part is more distinguishable than the imaginary part.100 frequency points are selected at equal intervals in the 10MHz～4GHz measurement frequency band.Through third-order polynomial fitting,the coefficients of determination of each selected frequency point are all above 0.92,and the root mean square error is all less than0.55.And in the 1.83GHz～3.03 GHz frequency band,the model fitting effect is better than other frequency bands.The maximum coefficient of determination value at the frequency of1.837 GHz is 0.984,and the minimum root mean square error value is 0.219.It shows that the soil dielectric measurement model proposed in this paper is suitable for characterizing soil moisture content.