CPW design

Hi,
I am very new at designing CPW. I just simply want a 50-ohm CPW structure, taking Si as substrate material.
I used line calculation of ADS and synthesized it 50 ohms. But the S11 is not coming 50 ohm, it is because of the resistivity I guess. How to perfectly design a CPW structure for 50-ohm characteristics impedance?

Can you post your cpw with dimensions? For cpw you should use silicon oxide as substrate. Silicon is the substrate but the cpw should be in silicon oxide.