出现这个情况可能是你需要用Conformal Variant 0。负值是因为过去我们强制选择的正向传播的模式，实际上有反向传播的模式，特别是有SPP时，你可以在分析窗口左下角的Advanced那里勾选CORRECT BACKWARD PROPAGATING MODES：
CORRECT BACKWARD PROPAGATING MODES WHEN PML IS PRESENT: By default, the detection and correction of backward propagating modes, as described above, is not performed when any of the FDE solver boundaries are PML. When PML boundaries are used, you must check this box to apply the correction of backward propagating modes. However, we do not generally recommend checking this box because PML itself can create small amounts of gain in some modes which is typically an indication that the PML is too close to the guiding structure and not that the backward propagating mode has been accidentally selected. In addition, evanescent modes with negative phase velocities are rarely found when PML is used.