Unbalanced-to-Balanced with impedance transformer

Hello,
I am trying to simulate an unit cell of an UHF Arrays using HFSS with reference to:

IEEE Transactions on Antennas and Propagation, Vol 54, No.7, July 2006
-Compact Light Weight UHF Arrays Using Long Slot Apertures

I've drawn out the structure of the unit cell except for the balun transformer. The requirement is a wideband 4:1 (150-600MHz), 180-deg out of phase for the 2 outputs. Can anyone advise me on how to input/design the balun into the HFSS? I've tried to run the simulation and the VSWR result is atrocious. I've attached my HFSS file for reference.

I'm new to HFSS, will appreciate if you can give any advice. Thanks.