Microstrip antenna design using ADS tool

Hello
I am working on microstrip antenna. How to calculate feed line impedance ?
After simulation I am getting linear graph of S11. How could I solve this ?

You mean input impedance at the antenna port?

Just the scalar return loss (or the VSWR) doesn't give information about the complex impedance of the antenna. Can give information how close (or far) is the resistive part of antenna impedance to the impedance of your network analyzer port (usually 50 ohms).

The most common approach is to use a 50 ohms feed line and use a matching network (if is necessary) between the line and the antenna.
To design the matching network you have to find first the complex impedance of the antenna.