How are you feeding it and terminating it? You should be using waveguide ports. They need to go down to the ground, about 3 to 5 line-widths each side and at least that much in the air above.

What is your dielectric and what are your conductors? How long is the line? Your insertion loss is not just unacceptable, it is ridiculous, unless the line is 100s of metres long.

The impedance is about right - very close to 50 ohms.

Thank you dear Malcolm your comment "They need to go down to the ground, about 3 to 5 line-widths each side and at least that much in the air above." solves this problem. I didn't see your tip in CST documentation. Could you let me know how you understand this dimensions?

This is standard in microstrip- design. It is the region where the microstrip fields are. It may be in the help somewhere, but CST is a simulation tool, it does not tell you how to design microstrip. You need to find out how to understand and design microstrip, then use CST to check or improve your microstrip designs.

Hello,

I agree with White Sir. CST does not tell you about how to design but design of antenna like microstrip or any other should be properly understood then design is possible.

Some references are attached.

Thanks,