You got the input impedance if you change _ by + but not the TDR impedance. The S-parameters being a frequency response, I dont think its should be correct but I cant help you any further, sorry.
David
Yep, just change the upper part of the formula, use + sign insetead of minus.
For more information look for the paper from Daniel Wu. There was a post recently about Ansoft presentations you can download from the web site.
I can't remember about the name of the post, but it was within last 2 weeks.
The paper was written for HFSS 8.5, but should not make any difference.
Hope this helps.