CST毫米波雷達模擬（二）

本期我們將詳細介紹CST毫米波雷達天線模擬中的天線陣子設計。

CST毫米波雷達模擬（一）中介紹過CST專用的天線庫Antenna Magus，工程師只需要輸入天線的規格要求，軟體就會自動幫您產生3D模型。這裡省略掉Antenna Magus的設定過程，下圖是Antenna Magus產生的天線模型，已經導入CST進行下一步模擬。

A 3D model showing a rectangular channel with a series of square obstacles placed along its centerline. The channel is colored light blue, and the obstacles are beige. There are coordinate axes labeled u, v, and w at the right end of the channel, indicating the orientation of the model. The background consists of a grid pattern.

模擬計算

對於這種尺度的模型，既可以選擇CST的時域（T-solver）求解器，也可以選擇頻域（F-solver）求解器，都能得到非常精確的結果。

常常會聽到讀者說，CST的時域演算法精確度不夠，結果和頻域對不上，真是這樣嗎？下面是我用CST的T和F求解器模擬的S參數對比，可以看出兩個演算法的一致性非常好。

A graph titled 'S-Parameters [Magnitude]' showing the magnitude of S-parameters (S1,1T and S1,1F) in dB versus frequency in GHz. The frequency range is from 68 GHz to 82 GHz, and the magnitude range is from -20 dB to 10 dB. Two curves, one in brown (S1,1T) and one in magenta (S1,1F), are plotted and closely follow each other, indicating similar behavior across the frequency range. The graph shows notable dips around 72 GHz and 78 GHz.

這裡教大家一些小技巧，當我們做天線模擬時，一定要用不同的解算器來做對比，如果不同演算法都能拿到相同的結果，這才能說明模擬結果是準確的。以下幾個設定要點，是天線模擬時要特別注意的。

首先，boundary的設置，reflection level的值要設定1e-6或更小，邊界尺寸適度加大。

The image shows two dialog boxes from a software interface. The first dialog box is titled 'Boundary Conditions' and has options for setting boundaries in different directions (Xmin, Xmax, Ymin, Ymax, Zmin, Zmax) with all set to 'open (add space)'. There is also a checkbox for 'Apply in all directions' and a field labeled 'Cond.' set to 1000 S/m. The second dialog box is titled 'Settings for PML Boundary' and includes properties such as 'Estimated reflection level' set to 1e-06, 'Automatic minimum distance to structure' set to 'Fraction of wavelength' with a value of 4, and 'At frequency' set to 'User' with 'frequency_min'.

其次，F和T求解器做比較時，其中材料Fit as in Time Domain要勾選上，求解器的精度設定1e-6甚至更高，勾選網格自適應加密。

The image shows two dialog boxes from a software interface, likely related to electromagnetic simulation or computational physics. The left dialog box is titled 'Frequency Domain Solver Parameters' and includes settings for method, mesh type, excitation, frequency samples, adaptive mesh refinement, and sensitivity analysis. The right dialog box is titled 'Special Frequency Domain Solver Parameters' and includes settings for the equation system solver, results, open boundaries, materials, solver order, and waveguide ports. Both dialog boxes contain various checkboxes, dropdown menus, and input fields for configuring the simulation parameters.

第三，時域T求解器的boundary設定與F相同，重點是網格設定。在天線陣子周圍畫一個空氣盒子，用來設定local mesh。如果模擬結果和F不一致，就需要對local mesh做進一步的加密，直到兩個解算器拿到完全相同的結果。

A high-frequency mesh simulation is displayed with a highlighted green path. The mesh has 18,196,920 mesh cells and a mesh plane at Z of 0.048 mm. The mouse coordinates are X, Y, Z: 18.376, -0.737, 0.048 mm and iX, iY, iZ: 815, 69, 41. The dX, dY, dZ values are 0.025, 0.023, 0.019 mm. There are no symmetry planes.

最後，時域解算器要勾選Hardware acceleration選項，可以大幅提升時域模擬效率。

設計最佳化

從前面的S參數結果可以看出，天線的諧振點並不在77G，這就需要天線設計工程師對天線的結構尺寸進行調整。

通常情況下，天線需要調整的參數非常多，如果每次調整都重新模擬S參數，那麼效率會非常低。為了更有效率地完成天線最佳化設計，工程師可以利用CST自帶的最佳化器對參數進行自動調優。此外，還可以使用另一個非常好用的功能：敏感度分析sensitivity analysis，詳細設定可參考FAQ088：如何使用敏感度分析sensitivity analysis — 天線F-ROM求解器。

這裡在天線的長度方向上，設定一個參數offset，在解算器Sensitivity Analysis裡勾選offset ，然後再計算。