Are you trying to design the transition or an antenna?
For:
a) transition: you should model the transmission line with two connectors.
dont need to worry about radiation unles you want to. I think you should do it
even if you want to design an antenna, so you can have an idea about the
parameters of importance that affect the transition alone, for example: the
radius ratio.
b) antenna: you should have already a model or a way for the feeding of the
antenna. In others words, the transition should be chosen and you should be
confident what you get. After that, the variation of the pattern and/or return loss
will be mostly due to model size, this imply aspect as closer distance to the radiation
boundary, number of iterations, meashing, etc. there is a lot of information in
this board about it.
You model is very simple, if you post the file someone can help you better.