Free Space Test Technology Of THz Complex Permittivity

Terahertz(THz)wave is widely used in high-speed wireless communication,public safety inspection,non-destructive high-resolution flaw detection and other fields due to its special frequency band,wide frequency band,strong penetration and high resolution.However,in the terahertz frequency band,the complex permittivity of the material is different from that in the microwave frequency band.The heating situation during the application of the material will also lead to the change of the complex permittivity of the material.Accurately obtaining the complex permittivity of the material is the key to achieving terahertz applications.Therefore,it is necessary to design a terahertz material complex permittivity test device at room temperature and variable temperature.Based on the principle of free space method,thesis studies the testing technology of complex permittivity of materials in terahertz frequency band,which is carried out from five aspects:dynamic research,theoretical analysis,simulation modeling,system construction and material testing.Firstly,the existing terahertz band material complex permittivity test technology is introduced,and the free space method is determined.Secondly,the principle of free space method is analyzed.Based on the theory of plane wave spectrum and plane wave scattering matrix,the Gaussian beam model is derived,and the physical model of complex permittivity of materials tested by free space method is improved.The relationship between Gaussian beam reflection and transmission coefficient and uniform plane wave reflection and transmission coefficient is established,and the difference between the two models is compared and analyzed theoretically.Then,important devices such as transmitting and receiving antennas,ellipsoid reflectors,and rectangular-circular transitions are developed.With vector network analyzers,infrared thermometers,temperature control boxes,heating coils and other devices,a Gaussian beam waist radius test system,terahertz complex permittivity room temperature and variable temperature test system are built.Based on TRL calibration technology,the test system is used to test the sample to be tested,and the test data of normal temperature and variable temperature are given.The test performance of permittivity test by using uniform plane wave model is as follows:in the frequency range of 110～170 GHz and the temperature range of room temperature～150℃,the test error of relative permittivity of the sample to be tested is |Δε’r/ε’r|≤7%,and the test error of loss tangent is |Δ tan δ|≤15%tan δ+0.05.Finally,according to the test results at room temperature,the test results of the horn antenna and the dual-mode horn antenna are compared.The measured beam waist radius value is substituted into the Gaussian beam model for permittivity inversion,and the results are compared with the results calculated by the uniform plane wave model.The results show that the permittivity accuracy of the Gaussian beam model inversion is higher than that of the uniform plane wave model.