I know, its dual or quad ridged horn antenna.
@ what frequency you are making broad band horn? Which application?
Its not easy if you try to implement exponential taper with equation based curve.
I will suggest following easy way:
Based on impedances fix the ridge thickness for regions of horn.
Use graph paper to decide co-ordinates & draw single points in HFSS.
Then connet them to form the complete curve.
And duplicate it to form structure.
I also tried using equation based curve, it works but you need to play with it.