Research On On-site Permittivity Measurement Based On Improved Free-space Method

As an important electromagnetic parameter which can characterize the characteristics of the medium,the permittivity is extremely important for simulation modeling and indoor communication system design.Conventional methods for measuring the permittivity include transmission line method,cavity method,probe method,free-space method and so on.Based on the traditional free-space method,the calculation formula of permittivity is deduced respectively under the condition of knife-edge diffraction model and non-infinite double-layered medium model.The accuracy of the theoretical model is verified by the measurement of system S-parameter.Firstly,the background knowledge and measurement principle of free-space method is briefly introduced.For electromagnetic modeling in indoor environment,two models are proposed:knife-edge diffraction model and non-infinite double-layer medium model.The knife-edge diffraction model introduces the edge diffraction in conventional transmission measurement method,which solves the problem of calculation of permittivity under non-infinite conditions of obstacles and enhances the practicability of free-space method in indoor environment;the double-layered medium model considers the situation of dual-layered medium in the building corner.In this paper,the permittivity inversion formulas corresponding to the two models are derived,and experimental measurement systems for verifying the theoretical accuracy of the two models is designed and constructed.Based on the application of metal-plate calibration,time-domain gating and TRL calibration technology,the accuracy of measurement is improved.The reasons for errors of theoretical calculation and field measurement results are also analyzed.For comparison,the permittivity of the same sample is measured by waveguide method with higher accuracy.The experimental results of the two models show good agreements and can be applied to indoor measurement.