Study Of A Novel Sensor Using Microstrip Resonant Structure For Permittivity Measurements Of Liquids

The permittivity of dielectrics has extremely important physical meanings sinceit is one parameter of the constitute relations in Maxwell's equations. Thepermittivity describes the characteristics of the electric polarization in materials,which is a key parameter in electromagnetics. Therefore, the accurate permittivitymeasurements are important fundamental subjects.Microstrip transmission lines are widely applied in radio frequency andmicrowave circuit design because it is low cost, compact, light weight and easilyconformal. At present, various structures of permittivity measurements have beenproposed by using microstrip transmission lines, which can be categorized to eitherresonant or non-resonant type. When the permittivity of a material is non-sensitivewith respect to the frequency, the measurement accuracy of the resonant structure isgenerally higher than that of the non-resonant structure, but it is difficult to measuremedium or high loss materials. Thus, the thesis aims at solving the problem.An improved half-lambda open-circuit microstrip resonator with interdigitalcoupling capacitors has been designed and fabricated on RT/Duroid 5880 substrate.A 3mm height metallic cover is welded on the top side of the substrate for protectingthe resonator from directly contacting to materials under test. On the bottom side, aslot is etched on the ground, which is just located beneath the resonant microstripline on the top layer. The permittivity of materials under test will influence theresonant frequency and the quality factor of the microstrip resonator through the etched slot. Experimental results showed that the proposed sensor could be appliedto measure the permittivity of medium or high loss liquids.The equivalent circuit model and description equation of the sensor areobtained. The weighted least-square curve fitting procedure is applied to extract theresonant frequency, the loaded/unloaded quality factor and the coupling coefficientfrom the measured S parameters of the sensor. Results showed that the algorithmprovided high calculation accuracy and could automatically suppress the noise inmeasurements. The weighted function could extract the most sensitive part of themeasured data.The proposed sensor is analyzed by a two dimensional finite difference timedomain (2D-FDTD) method, which is suitable for calculating the intrinsicparameters of a transmission line. Numerical simulations showed that the 2D-FDTDmethod was fast and required less memory. The resonant frequency of the sensordecreases when the permittivity or conductivity of materials under test increases.The unloaded quality factor of the sensor increases when the permittivity ofmaterials under test increases, and it also decreases when the conductivity ofmaterials under test increases.Combined with the 2D-FDTD method and the weighted least-square curvefitting procedure for quality factor calculation, a calibration procedure and apermittivity reconstruction algorithm based on iterative cubic spline interpolationare proposed in this thesis. The complex permittivity can be obtained from themeasured reflection coefficients of the sensor immerged in liquid materials undertest at different frequencies. The proposed sensor and corresponding algorithms arevalidated by measuring the equivalent complex permittivity of a CH₃OH/C₂H₅OHmixture. Results showed that the novel sensor was Suitable for the on-linepermittivity measurements of medium or high loss liquids whose permittivity werenon-sensitive with respect to the frequency.