Momentum 仿真 warning怎么处理

--- WARNING -------------------------------------------------------------------
S-parameter results show unphysical behavior for certain frequencies.
Cause        : - inaccurate (high frequency) calibration ;
               - electrically large via using lumped model ;
               - higher order modes at the ports ;
               - slot mode at coplanar ports ;
               - grouped strip-slot ports ;
               - mesh density is too coarse.

我也出现这样的错误了，一直没搞明白  
仿真完了后，S21居然还有正数！
与平时仿真没什么区别，看那英文资料也没弄懂！