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Comparison of HFSS and CST simulation of patch antenna

Release date:2021-12-28Author source:KinghelmViews:302

Learn antenna is a dry cargo antenna technology column focusing on antenna simulation and debugging, supplemented by theoretical principles, including introduction to antenna, principle introduction of various antennas, simulation software modeling, design, debugging process and ideas. If you want to see the content or technical problems, you can write a message at the end of the text.





01、Brief introduction


     
At present, the built-in solution algorithms in HFSS include: finite element algorithm (FEM), integral equation algorithm (ie), high frequency algorithm (SBR + Solver), hybrid algorithm (febi, ie region), domain decomposition algorithm (DDM, fa-ddm), time domain algorithm (transient), eigenmode algorithm (CMA), eigenmode solver, etc
https://zhuanlan.zhihu.com/p/113897875  

In fact, most people simulate simple antennas and filters. Using the finite element algorithm of HFSS and the adaptive mesh generation and encryption technology of the software itself, setting the convergent Max MAG delta S (default 0.02) is enough to meet their simulation requirements.



The use of software and the setting of other algorithm solvers will not be described here.

When most engineers use HFSS software, they will find that it has high requirements for computer configuration, especially memory. Moreover, it is difficult to meet the computational power of the simulation requirements of electrically large size and UWB.

CST just makes up for the short board of HFSS simulation UWB, but its simulation accuracy in small size, circular and other structures is not high. As shown in the figure below, the triangular mesh generation of HFSS is particularly delicate at the edge, especially the circular structure accessories, while the hexahedral mesh generation of CST is too regular.



Although the local mesh of CST can be used to encrypt the local mesh near structures such as gaps and circles, beginners may still be the fool adaptive subdivision of HFSS, which is more humanized.

CST software uses the full wave time-domain simulation algorithm of electromagnetic field - finite integral method (FIT) to discretize and iteratively solve the Maxwell integral equation. Due to the time-domain algorithm fit, it only needs to be solved step by step without matrix inversion. This inherent characteristic determines that its suitable simulation structure covers small, medium and large, and can achieve good performance. The calculation amount of volume moment method, finite element method and finite integration method (reflected in CPU time and required memory) is respectively proportional to the 3rd, 2nd and 1.1 ~ 1.2nd power of the number of grids n. It can be seen that the computational force requirements of finite integration method are lower than those of HFSS finite element method.

For CST software, time domain solver is also commonly used. In addition, it also has frequency domain solver, eigenmode solver, integral equation method, asymptotic calculation and multi-layer medium algorithm.



In the next section, we will compare the simulation accuracy of the two software, mainly focusing on the FEM + automatic mesh generation encryption simulation of HFSS and the time domain solver and frequency domain solver of CST.


0 2Script construction of back fed patch antenna

     

The common feeding methods of rectangular patch antenna include side feeding and back feeding. This tweet uses back feeding for simulation analysis.





Firstly, Rogers 4350b with substrate thickness of 0.762mm is selected, and the resonant frequency is 5.8GHz. (slide left and right to see the complete formula)

According to the above formula, the width and length of the patch antenna are 16.9mm and 13.3mm respectively.


After the last two tweets, the first bullet of hfss-api introduction: draw a box and the second bullet of hfss-api introduction: basic shape and operation, you can now directly roll out the HFSS vbs script of a back fed patch antenna (see the end of the text for the download link, in the examples folder):


clear;clc;path = mfilename('fullpath');i=strfind(path,'');path=path(1:i(end));cd(path);addpath(genpath(strcat(path,'hfssapi-by-Jianhui Huang')));try    % 填写路径    % tmpPrjFile:生成的aedt或者hfss(安装hfss15以下的后缀名为.hfss)项目文件的路径名    % tmpScriptFile:生成的vbs脚本文件的路径名    tmpPrjFile = 'F:vbsScriptPatch_Probe_Feed.aedt';    tmpScriptFile = 'F:vbsScriptauto_code.vbs';    % hfssExePath:HFSS软件的路径    hfssExePath = 'D:softwareHFSS15AnsysEM18.2Win64ansysedt.exe';    % 创建一个可读写vbs脚本文件.    fid = fopen(tmpScriptFile, 'wt');    %创建一个新的HFSS项目并[敏感词]一个新的设计文件.    hfssNewProject(fid);    Design_name='element';    hfssInsertDesign(fid, Design_name);     Patch_W=16.9;Patch_L=13.3;    Sub_W=35;Sub_L=30;Sub_H=0.762;copper_H=0.035;    Probe_dy=-4;Probe_dx=0;    Inner_R=0.5;Diel_R=exp(50/60*sqrt(1))*Inner_R;Outer_R=1.5;L0=2;    % hfssVariableInsert(fid,DesignName,variableName, value, units,flag)    hfssVariableInsert(fid,Design_name,'Patch_W', Patch_W, 'mm',1);    hfssVariableInsert(fid,Design_name,'Patch_L', Patch_L, 'mm',1);    hfssVariableInsert(fid,Design_name,'Sub_W', Sub_W, 'mm',1);    hfssVariableInsert(fid,Design_name,'Sub_L', Sub_L, 'mm',1);    hfssVariableInsert(fid,Design_name,'Sub_H', Sub_H, 'mm',1);    hfssVariableInsert(fid,Design_name,'copper_H', copper_H, 'mm',1);    hfssVariableInsert(fid,Design_name,'Probe_dx', Probe_dx, 'mm',1);    hfssVariableInsert(fid,Design_name,'Probe_dy', Probe_dy, 'mm',1);    hfssVariableInsert(fid,Design_name,'L0', L0, 'mm',1);    hfssVariableInsert(fid,Design_name,'Inner_R', Inner_R, 'mm',1);    hfssVariableInsert(fid,Design_name,'Diel_R', 'exp(50/60*sqrt(1))*Inner_R', 'mm',2);    hfssVariableInsert(fid,Design_name,'Outer_R', Outer_R, 'mm',1);     % 画基板    % hfssBox(fid, BoxName, Start, Size, Units, Color, Material, Transparency, flag)    hfssBox(fid, 'Sub1', {'-Sub_W/2', '-Sub_L/2', '0mm'}, {'Sub_W', 'Sub_L', 'Sub_H'}, 'mm',...        "(0 128 128)", "Rogers RO4350 (tm)", 0, 2);    % 画贴片    hfssBox(fid, 'Patch', {'-Patch_W/2', '-Patch_L/2', 'Sub_H'}, {'Patch_W', 'Patch_L', 'copper_H'}, 'mm',...        "(255 128 0)", "copper", 0, 2);    % 画GND    hfssBox(fid, 'GND', {'-Sub_W/2', '-Sub_L/2', '0mm'}, {'Sub_W', 'Sub_L', '-copper_H'}, 'mm',...        "(128 128 128)", "copper", 0, 2);     % 画同轴部分    % 画同轴内芯    % hfssCylinder(fid, CylinderName, Axis, Center, Radius, Height, Units, Color, Material, Transparency, flag)    hfssCylinder(fid, 'Inner', 'Z', {'Probe_dx', 'Probe_dy', 'Sub_H+copper_H'}, 'Inner_R','-(Sub_H+copper_H*2+L0)', 'mm',...       "(128 128 128)", "copper", 0, 2);    hfssCylinder(fid, 'Diel', 'Z', {'Probe_dx', 'Probe_dy', '-copper_H'}, 'Diel_R','-L0', 'mm',...       "(0 128 128)", "vacuum", 0, 2);    hfssCylinder(fid, 'Outer', 'Z', {'Probe_dx', 'Probe_dy', '-copper_H'}, 'Outer_R','-L0', 'mm',...       "(128 128 128)", "copper", 0, 2);     % 地板开过孔    hfssCylinder(fid, 'GND_hole', 'Z', {'Probe_dx', 'Probe_dy', '0mm'}, 'Diel_R','-copper_H', 'mm',...       "(255 128 0)", "vacuum", 0, 2);    % 布尔操作    hfssSubtract(fid, {'Outer'}, {'Diel'}, true);    hfssSubtract(fid, {'Sub1','Patch','Diel'}, {'Inner'}, true);    hfssSubtract(fid, {'GND'}, {'GND_hole'}, false);    % 保存项目文件到指定路径    hfssSaveProject(fid, tmpPrjFile,1);    % Close the HFSS Script File.    fclose(fid);    disp('vbs脚本已生成!');catch    disp('程序出现异常!');    fclose(fid);end


The above code modifies the paths tmpprjfile, tmpscriptfile, hfssexepath and design according to the individual situation_ Name, the vbs script generated by MATLAB The m file and the downloaded hfssapi by Jianhui Huang are placed in the same general folder. Click Run to generate the vbs script (under the path of the self assigned tmpscriptfile). Vbs script can be run directly or run script in HFSS software.

After the modeling is completed, add the region by yourself, set the radiation boundary conditions and the setup of analysis, and then the simulation can be carried out (after the subsequent boundary and analysis are synchronized, they can be established in the script).


Analysis设置


At this time, the simulation results can be seen that the antenna resonant frequency is biased to low frequency, and the input impedance deviates from 50 ohms.



At this time, someone will say that adjusting the antenna is metaphysics. How can I know which variables to adjust and how many variables to adjust? Can I directly use optimization? In fact, those who have understood the relevant principles of patch antenna know that at this time, they only need to adjust the length of antenna and the position of feed deviation from the center. The former affects the resonant frequency and the latter affects the matching of antenna.

It can be seen from the above figure that the impedance matching is better when the feed point is 2.5mm away from the center of the patch antenna.




However, at this time, the resonant frequency of the antenna is still biased to the low frequency of 5.6ghz, so the setting of 5.8GHz back fed patch antenna can be completed by appropriately shortening the antenna length.



矩形贴片天线长度扫参结果


  


0 3Comparison of CST and HFSS simulation results   

     

Select modeler - > export in the menu bar above HFSS and save it in step format.

Then open CST, select and import the above step file under export, delete irrelevant models such as region, and set material properties and boundary conditions.

Using the time domain solver and the default mesh generation setting, the simulated resonant frequency is 5.759ghz, which is 40MHz different from the HFSS simulation results.


CST time domain meshproperties and S11 results


The solver of the above model is directly changed to the frequency domain solver, and the grid division is set according to the figure below. The simulated resonant frequency is 5.825ghz, which is about 25MHz different from the HFSS simulation result, which is very close.



CST frequency domain meshproperties and S11 results





In general, the simulation results of electrically small microstrip patch antenna under the FEM + automatic mesh generation encryption simulation of HFSS and the time and frequency domain solver of CST are acceptable. After all, antenna design belongs to the category of engineering. In fact, processing and welding tolerance should be considered, so it is still necessary to hit PCB several times for test analysis, debugging and optimization. There is no significance to make a rigid simulation difference.


It takes a lot of time and energy to write basic code and make comments. I hope you will like to share more!


Code sharing area

hfssapi-by-Jianhui Huang

Download link (follow-up code is continuously updated in the following link):

https://pan.baidu.com/s/1N0EE3Uv7krkypfzi9vxCvg

Extraction code:o5p5

The code has been encapsulated and packaged as a p file, which cannot be modified. Each time you download and overwrite it, you can swap it according to the function comments!



Note: MATLAB generates vbs script The m file is placed in the same general folder as hfssapi by Jianhui Huang. Do not run in the examples folder M file!



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