waveport size hfss and lumped port

Hi everybody,

I am going to make an antenna that is fed by diffrentail CPS line with ground and I'm trying to find the best way to define the port to get the most accurate results. I've modelled the lines to give 100 ohm impedance, but when I defined a waveport, I found that it gives the correct field distribution but with different value for the port impedance. I'm really very surprised and I tried many other lengths and widths for the port but the maximum value I got for the impedance was 60 ohm instead of 100 ohm. I don't know what is the problem really and I dont know in my case is it better to use to waveport or use lumped port and force it to be 100 ohm. They give me different results for the S-parameters and I'm totally lost. Could anyone help me in that?

thanks a lot