Interesting - effect background material extent in CST MWS

Hi all,
I am simulating a normal CPW ( No Ground plane ) in CST MWS. I am using open boundaries in all directions. Port size is enough to accomodate the field patter ( 5* height and 3*( Width_center_conductor + 2* Gap)). I get the designed line impedance of 92 Ohm. I notice a curious thing. Background matrerial is "Normal" type with Epsilonr=1 and Mur=1. If I set the backgrouind material -> Surrounding space->Lower Z and upper Z to say 3 mm each. I get the line impedance as 82 Ohm. Again changing these distances alters the line impedance. However such a thing does not happen ( too small variations to note) for a microstrip. How does this surrounding space affect the soultion of wave port in MWS for a CPW ? Thanks for listerning.
-svarun

Interesting.
May be it is the definiton of the line impedance.
CST calculates Zline from power and current, this differs from
the voltage, current calculation.
The power takes all E and H vectors into account, whereas the UI-Definition is based on a E-integration from one metal to the other and a integration of H in a surrounding curve of a metal.