correct port distance cst

Hi,
I am trying to simulate some published papers on metamaterials in CST MWS llike "G.Dolling,M.Wegner,C.M.Soukoulis,S.Linden-Negative index metamaterials at 780nm wavelength,Optics Letter,Vol 32 No 1,2007". I am having problem with the waveguide ports. The problem is that when i change the distance of the waveguide ports from the unit cell boundary, the s-parameter plot is shifting in frequency. how to decide which is the right distance of waveguide ports from the unit cell boundary.

I have uploaded the paper i tried to simulate.

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