A Performance Evaluation System Of Microwave Anechoic Chamber

With the discovery and wide application of electromagnetic waves,the electromagnetic environment in space is becoming more and more complicated.At the same time,a microwave anechoic chamber that can simulate the electromagnetic environment for testing has emerged.It can provide a stable electromagnetic environment and separate external electromagnetic waves.Therefore,it has gradually becoming a new venue for antenna measurement and electromagnetic testing.The construction process of the microwave anechoic chamber is very complicated,and the cost is very expensive.Once completed,it is difficult to modify.If the microwave anechoic chamber can be simulated and analyzed to evaluate the performance of it when building it.The construction cost of the microwave anechoic chamber can be greatly reduced,and the performance of the microwave anechoic chamber can reach the expected requirements.This paper based on the geometric optical method and the finite difference time domain method,uses MATLAB to simulate a microwave anechoic chamber suitable for the frequency range of 400 MHz to 40 GHz.In the high frequency band,the chamber model was established by geometrical optics.Transform the source horn antenna into coordinates to obtain the expression of the electric field in the far zone under the rectangular coordinate system.Equivalent the source horn antenna to a point source and symmetrically it about the six walls of the chamber.Then the path from the symmetry point to the quiet zone is equal to the reflection path of the source antenna to the quiet zone.Decomposed a incident electromagnetic wave along the x-axis,y-axis,and z-axis directions,and its incidence angle to different planes is not equal,using the reflection principle of the pyramid interface,the equivalent incident angle of the same incident angle to different planes is obtained,and then the reflection coefficient of the incident electromagnetic wave to different planes is calculated,the reflected electric field reaching the quiet zone is calculated.Add the reflected electric fields of the six walls to obtain the total reflected electric field.Finally,compare the amplitude of the total reflected electric field with the direct electric field and take the logarithm to get the expression of the reflected level.Incorporated the formula of the reflection level derived by the geometric optical method into MATLAB,and set the size of the chamber,the operating frequency,the position of the source antenna,the size of the quiet zone and the reflection coefficient of the absorbing material.The magnitude of the reflection level in the quiet area of the chamber at different operating frequencies at high frequencies would be calculated.In the low frequency band,the chamber model was established by the finite difference time domain method.Discuss from the aspects of meshing method,stability of solution and absorption boundary.Divide the chamber with the Yee grid(electric field and magnetic field are staggered in space by half a step).Use the time harmonics to excite the transmitting antenna,derive the time step and space step to stabilize the reflected electric field and the total electric field.Set the absorption boundary(ferrite tile in the total field).Finally,the iterative equation of the finite difference method in the time domain is derived.Get the reflected electric field and the direct electric field inside the microwave anechoic chamber,compared the amplitude of the two field and take the logarithm to get the expression of the reflected level.Incorporated the formula of the reflection level derived by the finite-difference time domain method into MATLAB,and set the size of the chamber,the operating frequency,the relative permittivity and permeability of the absorbing material at different frequencies,the number of time iteration steps and the number of PML grid layers.The magnitude of the direct field,total field,reflection field and reflection level in the chamber at low frequencies would be calculated.Finally,in order to input the parameters of the chamber more intuitively and evaluate the performance of the chamber more quickly,a software operation interface was designed with MATLAB,link the complex electromagnetic field algorithm behind the operation interface.