Microstrip to coplanar strips

(to avoid increase bandwidth)
hi use ie3d from zeland

zeland IE3d or sonnet or CFDTD,etc they all can deal with your problem.

U can use Sonnetlite its free to dowload at

www.sonnetusa.com

microstrip to coplanar strips just a plannar problem.so the awr ads sonnet may meet.you.
another you can refer to MMIC maual for non-uniform microstipe line to design it.
the transition often use non-uniform type.

and there has some clsed type formulation to design it.

Also find this in the MTT.