HFSS...Problem with simulating differential traces.

Hi all,

I want to find out the variations in the impedance of a FPC which is run closely and parallel to a stainless steel shield.

The FPC contains a 3 layered stackup as given below.

1. Coverlay
2. TOP Signal Layer
3. Core
4. GND
5. FRClad
6. Core
7. Bottom Signal Layer
8. Coverlay
Below which SS shield is present with an air gap between them.

i have created two models to measure Zoc and Zsc from which i get characteristic differential impedance of the traces.

The problem: when i increase the separation between the shield and flex the differential impedance increases till a separation (15mil) after which it decreases.

But the impedance should increase with the increase in air gap between FPC and Shield and be a constant after say 50mils.

I have also attached my model. HFSS users please help...