关于CST超宽频仿真的警告

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最近在做一个超宽频的天线设计，设计带宽是3-40 GHz，我仿真频率设置的是2-40 GHz。仿真的时候报错：some higer order propagation modes at port 1 are not considered in time domain caculation. This possibly leads to an inaccurate energy balance. consider to increase the number of modes or decrease the upper frequency limit. 请问大家这是怎么回事啊?我把port的mode order 由1改为3后还是报相同的警告。谢谢大家！