Defining ports on CST ?

Hello

I'm trying to learn CST and I don't understand well how ports are defined.

For example I found a pdf tutorial that simulates a wifi antenna fed through a coaxial cable.
And they define the port like this:

I don"t understand why they just pick the shield of the coaxial and they don't specify the core, neither in a different port.

Another example I can't understand, in this tutorial
http://www.youtube.com/watch?v=k54XWrcI5Bc

Why do they need to pick a port much larger than the microstrip line section?

thanks

I've seen on the official help that coaxial ports are defined like this:

That"s selecting alltogether.
?How can CST figure out where is the core (where we feed the signal) and where it comes back if I don't tell it?
Shouldn't I use a multipin port or two ports?

Dear Skan,

CST (and HFSS) are electromagnetic field solvers. For microstrip lines the waveport needs to be much larger than the width of the line in order to capture the fields so it can make the correct calculations. Try doing a parametric study of a microstrip line where you vary the port size. As the port gets larger you should see the port impedance converge to the theoretical value. A coaxial cable does not have any fields outside of it due to Gauss' Law so the wave port only needs to cover the edges.

I hope this helps.

Thanks you, very good explanation.