HFSS 模擬波導阻抗

我想了解波導的阻抗是否隨者公式 Zte = (377)/(1-(fc/f)^2)^0.5 變化
                                                                                
模擬X band (8-13 GHz),WR 90 (波導) 400 mil * 900mil
                                                                                
趨勢大致上與公式是一致的,
                                                                                
但是實際上的阻抗值,計算值和模擬值會落差50ohm
                                                                                
以中心頻10.5GHz 來說,模擬值為430ohm,
                                                                                
但是計算值卻為482 ohm
                                                                                                                                                
是waveport設定有問題,還是其他問題?
                                                                                
不知道有沒有大大知道問題出在哪裡 謝謝.