Simulation of Microstrip Line in HFSS

Hi,

I am trying to simulate a 50 ohm microstrip in HFSS. I am using two waveports at each end of microstrip line and have introduced two coupling capacitors in between the microstrip line. After the introduction of these coupling capacitors, impedance at the waveport has decreased from 50 ohm to 40 ohms. Does anyone know why the real part of impedance has decreased with the introduction of capacitance?

Regards