Coplanar strip line-HFSS

Hello.
i wish to design a co planar strip line on an artificial dielectric slab. the same i have already desinged for a parallel plate stripline, but as it is known that the width and height of the waveport is expreseed as times of width and sapcing of stripline and height of the substrate, i wish to know the guidelines on how to design when the distance between the two striplines is smaller than the width of the waveport.

Thanks in advance.

-Ashish