helo me for the CPW cover and backing in IE3D

Hello everyone,
I read one paper about adding via-hole and air-bridge to reduce the spurious passband in a single-side CPW.
In this paper, the author put the microstrip air-bridge in the backside of CPW. So the structure became the CPW cover and backing.
I have finished the stimulation of the single-side CPW using IE3D. The result is almost same with the paper's.
However, when I added the backside air-bridge, the spurious passband did not change. I put the air-bridge in the z=0 layer, and define its epsr=1, sigma= (0,0). Then using via-hole to connect it with the CPW.
I thought that my port ( differencial port) and CPW layer sitting is right. As for meshing, I chose 20 per wavelength with AEC opened.
But I still cannot get the same result with the paper. T-T
Did anyone know the right way to stimulate CPW cover and backing? Plz tell me how to correct my IE3D design.
Thanks in advance.
Here is my design. The green one is is the backside structure and orange one is the single-side CPW.