How to choose impedance from s2p file to design power divider

Hi all
I'm design the microstrip antenna for 0.694-0.96 ghz band and I want to design power divider 2 way for my system. I simulate the microstrip antenna then export s2p file. I put s2p file on the output of power divider.
People tell me how I should choose reference impedance to match impedance.
Please help me!
Thank you

Is this antenna not optimized for 50 Ohm Characteristic Impedance ?If it's not, you should use Simple Matching circuit between Power Divider and and the Antenna.
Because designing a Power Divider/Combiner for different Characteristic Impedance rather than 50 Ohm ( or real Zc) is pretty difficult,time consuming.