How do I tune return loss in hfss?

[Shirpo]

Hi, I'm working on a small project to practice using hfss, my project is to redraw and simulate the antenna in the picture to give the same result as a antenna in an article. but when I finish the result of the return loss is quite different, in the example the return loss can reach to -27dB while mine only reach -12dB, the size of the antenna I made is quite identical to the one in the article too but the return loss is very different.
So I want to ask how can I tune to make the return loss to be better? Thanks in advance