i am asking if your antenna has two ports or only one

if it has two ports then

1. the hfss will consider both has impedance Zo=50 ohm and when one is active the other port act as a termination load with 50 ohm impedance and calculate S11

this will be reversed and the second port is active and the first act as 50 ohm termination load and calculate S22

2. if you want to make both ports active please follow link

hfss-sources-edit

a window will pops up in which you can choose which port is active or both to be active by changing the weighting coefficient to 1

In HFSS, when you declare two ports the simulator definitely will give you four parameters (S11, S12, S22, S21).

As suggested by Hend, ports are controlled by fields-Edit sources-Magnitude. When All the input ports magnitude is 1, then you get the gain and efficiency, etc..... of overall design.

When port 1 magnitude is 1, then you will get gain and efficiency, etc...... of antenna associated with the first port by considering remaining  ports are disabled.

thanks all

My question is about terminating one physical port in simulating some antenna when another port is excited, such as, Diversity antenna, or slotted waveguide antenna. and I also found that I can do that by setting 'Magnitude' to zero in source edit.