question about simulating Sievenpiper's structure in HFSS

I am trying to simulate Sievenpiper's structure in HFSS, but no matter how hard I try I still can not get right results. Especially when I am calculating 2 modes between the light lines, the frequency results of the first mode and the second mode are very close to each other. I suspect it's my pml setting problem. I don't know how to calculate PML in HFSS so I just used the default results for the free space termination.

Could anybody help me with this please. I appreciate it a lot!
Thanks!

Hi, Maybe this tutorial can help.

http://www.emtalk.com/tut_3.htm

If not you can ask the author.

Thanks a lot for your help!

First of all, you should apply the PML on the end face of air box. ABC is not a good radiation boudary because when propogating direction of EM wave is >45 degree with norm direction of the boundary face, ABC will not function well. When you solve dispersion problem, PML must be applied.
If the mesher of the software mesh the model in the way of axis-symmetry, you will not get two very close eigenmode. Unfortunately, you can't interfere the mesher's work, so you get the degenerated eigenmode, i.e., two identical generated eigen mode with same frequency exist, the non-symmetric meshing way degenerated them, then you get two different but very close solutions.