Research And Design Of Dielectric Constant Measurement Method Based On Resonance Characteristics Of Microwave Structures

As one of the important physical properties of microwave dielectric materials,complex dielectric constant is an important index to reflect the dielectric properties of materials.It represents the ability of dielectric materials to store and dissipate energy.Permittivity is often measured to infer other parameters such as temperature and humidity of the measured system.Whether for the purpose of improving the design performance of microwave devices or detecting and judging the biological performance of the system,the accurate measurement of the dielectric constant has important practical significance and application background.Microwave resonance method is widely used in the field of dielectric constant measurement because of its high sensitivity.Using microwave resonance method to achieve high accuracy measurement of dielectric constant has become one of the important research topics for researchers at home and abroad.Aiming at the problem of measuring the solid and liquid substances’dielectric constant,based on the microwave resonance structure,this thesis focuses on designing the high sensitivity dielectric constant sensor.The main research contents are as follows:Firstly,the resonator sensor based on substrate integrated waveguide-complementary open resonator ring（SIW-CSRR）structure is designed to measure the dielectric constant of solid material.By loading three complementary open resonator ring（CSRR）with different opening directions in the ground plane,the measurement of complex dielectric constants in a wide range are achieved.Simulation results show that a resonance frequency offset of 101 MHz for the per unit dielectric constant is achieved.The sensitivity of measuring is 1.25%.By calculating the material under test with a known dielectric constant,the measurement results of four plates match well with the standard values,with a relative error of the real part of the dielectric constant of less than 2%and an error of less than0.0099 for the imaginary part.The sensor can be used to measure the dielectric constant of solid material.Secondly,a liquid dielectric constant sensor based on cubic container structure is proposed to measure the dielectric constant of liquid material.The cubic container which consists of dielectric substrate with open resonator ring（SRR）and microstrip lines can enhance the electric field intensity in the measuring area.High sensitivity can be obtained from measuring dielectric constant with the characteristics of the structure resonate.The simulation results show that when the dielectric constant of the sample varied from 1 to 10,the frequency offset of 199 MHz for the per unit dielectric constant is achieved.The sensitivity of measuring is 3.45%.the permittivity of oil with different metal content is measured by using the relation between the fitted permittivity and the resonant frequency.The relative error is less than 1.5%.Finally,in order to solve the problem of the poor performance of the traditional microwave resonance method in multi-parameter fitting data processing,a permittivity measurement method based on Back Propagation Neural Network algorithm is proposed.The designed solid and liquid sensors are used as microwave resonators.The simulated S parameters of different material samples are used as the training data of the neural network.The amplitude and phase of S₁₁ and resonant frequency f are taken as the input layer of the Neural Network,respectively.The dielectric constant and dielectric loss are taken as the output to establish the Neural Network model.The measurement results show that,for solid materials,the relative error of the dielectric constant is less than 0.6%and the error of the dielectric loss is less than 0.00003.The relative error of the oil dielectric constant is less than 0.3%.Compared with the traditional data processing of microwave resonance method,the introduction of BP Neural Network algorithm can significantly improve the accuracy of dielectric constant measurement.