WG port definition for asymmetric CPW on CST

Hello,
I am trying to simulate an asymmetric CPW which consists of only a ground plane and a strip. The manual suggests that the port size should be about 3-5times wide of strip width. And it is trustable for conventional CPW and microstrip line structures. However, for this asymmetric CPW with the same strip width (W), there is a great difference on port impedance (around 40ohms) between using 3W and 5W port width. And it really matters when designing some line-impedance-sensitive components like couplers, filters .etc. Have you guys got any suggestions for how to objectively choose the port size for this structure? Thank you very much for your help!
Chunwei

Did you choose adaptive mesh? maybe the difference comes from insufficient mesh density.
pl

Chunwei ,
I had the same problem you have and, as plasma told, it was because i needed a higher mesh density. I increased it and that was it!
regards

Thanks very much for your opnions. Actually, I've always done adaptive mesh refinement for the simulation each time with different port widths and that's the reason how come I suspected might it be the port's dimension problem. There is usually a small port impedance difference (within 5ohms) on conventional CPW and MSL. Is there still anything I haven't noticed about? Cheers.
Chunwei