Research On Compact And Highly Sensitive Microwave Dielectric Sensor

It is of great significance to detect ingredient,status and quality of various mediums both in the fields of scientific research as well as industrial manufacture.Compared to traditional detection methods,the microwave dielectric sensing technology offers many advantages such as fast,reliable,label-free and wide applicability.As a result,it has become one of the major methods for sensing and detection in various areas.However,for the existing microwave dielectric sensing schemes,it is difficult to satisfy the increasing requirements of detection accuracy and the improving complicities of application environment.There has been growing demand for developing novel microwave dielectric sensor with better performance.To solve these problems,this thesis employs substrate integrated waveguide?SIW?re-entrant cavity as the basic structure,and applies it to the microwave dielectric sensing area.Its unique resonant performance and structural advantages are combined to achieve microwave dielectric sensors with high accuracy and sensitivity.The feasibilities and superiorities of the designing schemes are demonstrated in detail by analyzing their operating principles,simulating their sensing characteristics and experimentally testing their performances.The specific achievements of this research are as followed:?1?Based on rectangular SIW re-entrant cavity and microfluidic technology,a novel non-invasive,contactless and reusable microwave sensor for complex permittivity measurement of liquid is designed.The equivalent RLC circuit model of the re-entrant cavity is established using equivalent circuit theory,and its resonant mechanism is revealed through high-frequency electromagnetic simulation.According to the induced electric field distribution of the cavity,a unique winding microfluidic channel is designed.In this manner,the liquid under test?LUT?can be well constrained within the area where induced electric field is highly concentrated,contributing to a strong interaction between them.As a result,the designed sensor has achieved an average sensitivity up to 3.66×10^-1%/??r??.?2?According to the dielectric sensing response of the sensor,the resonant frequency and 3dB bandwidth are chosen as characteristic parameters.Then,their relationships with complex permittivity of LUT are analyzed using nonlinear curve fitting methods.In this way,a quantitative mathematical predictive model for retrieving the complex permittivity of LUT is established.The re-entrant cavity and microfluidic are fabricated using standard printed circuit board and micromachining technology,respectively.Then,the sensor is assembled through cavity and microfluidic integrated manufacturing process.A liquid medium test platform is built,and the performance of sensor is experimentally validated by different standard liquid mediums.Experimental results show that the designed sensor is able to characterize the complex permittivities of various liquids with an accuracy of higher than 96.76%?compared with theoretical values from Debye relaxation equations?.?3?A novel double folded SIW?DFSIW?re-entrant cavity is designed by applying the SIW folding technology to re-entrant cavity.Utilizing its highly concentrated electric field distribution and strong ability in size reduction,an ultra-compact and highly sensitive microwave passive humidity sensor is achieved successfully.Two sensitive regions are introduced into the cavity at where induced electric field is highly concentrated.When moist air enters the sensitive regions through pre-designed air holes,it would cause a strong perturbation to the induced electric field and then result in a large resonant frequency shift of the cavity.This phenomenon is used to achieve the highly sensitive humidity sensing performance of the sensor.Based on first order cavity perturbation theory,the quantitative dependence of the resonant frequency of cavity on ambient relative humidity is derived using quasi-static approximation method.Measured results show that,in relative humidity range of 30%-80%,the absolute sensitivity of the sensor is 135.60kHz/%RH and the corresponding normalized sensitivity is 6.27×10^-3%/%RH.