空间波与长试样法测量介电常数的结果与对照

Results and Contrasts of Space Wave and Long Sample Methods Measuring the Dielectric Constants

用空间波法测量无限大半空间模拟介质板的复介电常数 ,包括金属板定标法与双极化法 ,测量中同时接收前向散射的振幅和相位 ,实验在X波段进行。该文的金属板定标法本质上是自由空间中的长试样法 。

The dielectric constant of a board simulating the medium in half space is measured in the laboratory using the space wave method including the metal-scaling method and the dual polarization method The experiment is performed in X frequency range,the amplitudes and the phases of the received fields are measured simultaneously The dielectric board consists of paraffin,plaster and graphite First,the scattered fields by different Fresnel zones in the board are calculated by Kirchhoff formula It is concluded that the area larger than 10 Fresnel zones can be approximately regarded as infinitely large The thick of the board is determined by the attenuation coefficient of the elctromagnetic waves After repeatedly measuring the dielectric constants in the waveguide for different prescriptions,the weight ratio in the board is chosen as paraffin:plaster:graphite=2 38:1:1 8,in which case the attenuation is desired The thickness and the area of the board are chosen as 10cm and 135cm×105cm The experiment frequency is 9 965GHz,the 3dB half width of the wave bean is 9°。The microwave receiver(Atlanda 1780) receives the phases and amplitudes of the reflected fields in the specular direction In the metal-scaling method,transmitting and receiving are performed in a same polarization,the reflection coefficient of the medium is obtained by comparing the phases and amplitudes of the fields reflected by the medium board and a metal board,then the permittivity is deduced from the reflection coefficient The incident angle is adopted as 15°in the metal-scaling method,the measurements indicate that the results using parallel polarization is better than that using vertical polarization In the dual polarization method,however,the incident angle is taken to be 60°,a wave with both vertical and parallel polarizations is transmitted by an inclined horn,the amplitude and the phase of the reflected field in the specular direction is received for each polarization respectively A simple formula to deduce the dielectric constant from the amplitude ratio and phase difference of these two polarizations is given in the paper The measurements indicate that the result of dual polarization method agree well with that of the matal-scaling method using parallel polarization The measured result of the long sample method in the waveguide has a little difference(less than 20%)with their results,which originates from the inevitable difference between the sample part and the whole board From the contrast with the long sample method in the waveguide,it is found that our metal-scaling method is intrinsically a long sample method in the free space The paper also gives the corresponding formulas for the waveguide to the free space The paper analyses the errors of the phase measurement in the metal-scaling method which mainly come from the flatness of the metal board and the stability of the instruments In the experiment the real part of the permittivity is not sensitive to the phase error,the imaginary part only have a relative error of the same order with the phase error,thus the results of the experiment are rather stable,which verifies the feasibility of the phase method Both the metal-scaling and the dual polarization methods have the advantages that the measurements and the inverse formulas are simple and the results are stable But for the purpose of on-site measuements,the dual polarization method is better