is an antenna a vltage source?

i wnat to simulate a array of two identic rectangular microstrip antenna , each one is excited by a voltage source , the boundaries must be perfectly electric and the ground plane is finite.
when i simulate the project with hfss v9.2 i didn't find S21 in the list of outputs parameters,
can any one help me!
i attached my project

Hi,

I have checked your file, you have to use two waveports instead.

change the size of the air box too

thanks for yo,r help, but i have a question: i want to compute the S21 of this two antennas by moment method and compare result with simulation, the source were planar voltage source in moment method, is it possible to replace them by waveport in simulation.
Concernig the box, i can just modify its height(in z direction) because all the structure is considered embeded in a weveguide