Research On The Mechanism Of Interaction Between Near-field Microwave Probes And Materials

Near-field microwave microscopy is to quantitatively measure the microwave electrodynamic response of materials on a scale much smaller than the wavelength of the radiation free space.The interaction between near-field microwave probes and materials is currently a hot topic in academic research,but few studies involve accurate measurement of high-resolution permittivity.This thesis conducts research on the above-mentioned problems.First,a breakthrough in the smallest dipole model is proposed.The advantage of this model is that it can provide a complete electromagnetic field when the tip interacts with the sample.Secondly,through the study of spatial resolution,the problem of electric field concentration is solved,and the measurement accuracy is further improved.Finally,the microwave nearfield microscopy system and the dielectric constant measurement theory are combined to realize the high-resolution measurement of the dielectric constant in the microwave frequency band.Analyzing the electromagnetic field and spatial resolution of the probe sample interaction is the main research work of this article.This paper establishes an improved model of the smallest dipole instead of the center of the conductive sphere,and uses MATLAB software to analyze the electromagnetic field based on the established model.The results show that as the height of the vertical dipole above the sample decreases,both the electric field and the magnetic field increase.At the same time,the electric field and the dielectric constant of the sample show a negative correlation.This paper also analyzes the electric field distribution when the probe is in contact and non-contact with the sample through the simulation of the conductive sphere model,and verifies the correctness of the minimum dipole model.Based on the conductive sphere model,a finite element analysis was established through COMSOL software,and the accurate measurement of the spatial resolution was realized.At the same time,the measurement of spatial resolution is simplified by the smallest dipole model.The study found that the spatial resolution of the near-field probe mainly depends on the tip geometry of the probe.Through the above theory,the spatial resolution can be estimated in the probe design stage,which has important value for the design of the probe tip details and the control of the distance between the tip and the sample.The design of the dielectric constant measurement method and the construction of the measurement system are the core issues studied in this paper.In combination with the broadband test requirements of the project,the coaxial probe was selected as the research object.Based on the improved equivalent capacitance model and antenna modeling theorem,the relationship between the equivalent load and the dielectric constant is studied.Combining the probe's transmission line model,the dielectric constant of the sample can be obtained through the reflection coefficient.The two measurement principles are verified by HFSS simulation,and the results show that the measurement error by the iterative method using the antenna modeling theorem is smaller.Finally,an experimental system based on the coaxial probe was built.The reflection coefficient measured by the experiment was used to calculate the dielectric constant of the sample to be tested according to the measurement principle,and the experimental system was further improved by error analysis.In summary,this article has conducted a comprehensive study on the interaction between near-field microwave probes and materials.Through the research in this article,we can better understand the electromagnetic field radiation problem of microwave near-field probes,and at the same time provide a set of solutions for the high-resolution measurement of the dielectric constant in the microwave frequency band.It has strong theoretical and practical significance in the field of microwave near-field.