CST MWS共面波端口阻抗和理論計算相差較大的疑問
遇到一個疑問，我用CST2012計算一個共面波導的端口阻抗，我把共面波導的參數(shù)按照下面的圖1中的數(shù)據(jù)設計的，
圖1
按照軟件計算出來的，阻抗應51.26.
接著我采用波端口作為激勵，設置端口大?。ǘ丝趯?.5，高2.5）為圖2：
圖2
但是計算出來的線阻抗（line impendence） 只有24點多，如圖3
ps:縫隙0.5mm,已經(jīng)用加密至五個網(wǎng)格了，為什么端口阻抗（線阻抗）和理論計算的差別那么大呢？
附上模型：
樓主為什么要選定Free normal position這個選項？
Free normal position: Please activate this button to define an internal port, i.e. a port location inside the calculation domain. This check button is only enabled for a Free or Full plane transversal range setting (see above).
The value for the normal position can be inserted in the correspondingX/Y/Zpos text field. If the value exceeds the dimension of the calculation domain, the port will be snapped to the boundary of the calculation domain.
Note: Deactivating this button will always locate the port on the lower or upper boundary of the calculation domain, corresponding to the definition of the lower or upper port orientation.
建議樓主把這一項取消，就會得到比較相近的結(jié)果。
另外，cst中的宏計算器計算阻抗是使用近似公式，全波場仿真結(jié)果更準確些。
前面正解---宏計算僅僅是近似而已，具體還得通過端口計算來調(diào)整
因為，我要使用這個共面波導既作為傳輸線，又作為天線的地板，
是這樣的，激勵和天線分別在共面的波導兩側(cè)，
我又看這個天線的遠場圖，必然要將各個方向上的邊界條件設置設置成open或者open（add space),這樣就增加了計算區(qū)域，
如果我不使用Free normal position將激勵調(diào)整到原來的位置（即共面波導的入口出），很明顯這個波端口就要建立在邊界條件一個面處。這樣激勵很明顯是在邊界處開始，在真空的面（介質(zhì)）中的一個波端口激勵，
另外，我用宏計算只是計算一下大概的線寬，縫隙和介質(zhì)及高度的尺寸，只要保證共面波導為50Ohm左右的特性阻抗。
你說的也是把Free normal position去掉嗎？這個我知道，宏計算是近似，更精確是要端口計算調(diào)整。
樓主的要求是這樣啊，那么應該這么辦，先看幫助文件怎么說：
If no symmetry condition is defined in the middle of the coplanar line, both even and odd modes can exist and therefore need to be taken into account. If an electric symmetry condition is specified, only the odd mode can exist. On the other hand, setting a magnetic symmetry condition will consider the even mode only.
If the microstrip line is grounded, another parasitic microstrip mode will exist too as long as no electric symmetry is defined. All this leads us to the following table for the number of modes normally used for the simulation at the coplanar port:
Note that the order in which the port modes are calculated may vary depending on the dimensions of the structure as well as the frequency. Therefore we highly recommend always inspecting the port mode results in order to avoid misinterpreting the S-parameter data.
因此樓主看的是第一個模式，沒有看到真正的奇偶模分布。所以得到的阻抗也就不是真正想看的阻抗了。
Another important aspect in the simulation of coplanar lines is the fact that the mode pattern is frequency dependent (unlike the mode patterns in empty guides or coaxial lines).
The frequency domain solvers automatically recalculate the mode patterns for every frequency point so that this frequency dependency does not constitute a difficulty for analysis.
In contrast, the time domain solver uses the same mode pattern for the entire frequency band which may cause port mode mismatches at frequencies other than the mode calculation frequency. The error increases with increasing distance to the mode calculation frequency.
因此使用時域求解器計算多模問題時要特別注意。
的確是，接地的共面波導，在無對稱的電壁和磁壁下，有三種傳輸模式，我只仿真了其中的一個模式，而且共面線模式的場型是色散的，時域求解器在整個頻帶上使用同一模式的場型，肯定會在某些頻帶失配，時域的求解器不適合共面線的仿真
