Check impedance changes relative to the distance between the 2 strip

hello friends
im simulating 2 strip lines on a substare that one of the strips is upper area and the other one is in lower area
and i am checking the changes relative to the distance between the strips if i image the lower one to the upper area ,relative to this distance ,im checking the change of impedance of the structure z11 when the distance changes ,
but the problem is that the changes is too much that i didnt expect , example in a small bandwidth over 100 ohms changes but i expect more and more flat changes over 10 12 GhZ bandwidth
any one could help me plz , this is a part of antenna designing so i need to improve it
i can mail u some images of the structure.


thank you for your attention.