design microstrip patch using feko

i am designing microstrip patch for 1.645GHz using feko,by using feedline at centre i am getting s11 as -3.45dB but i want to get -20dB...which method i have to follow in feko plz help....

First of all verify that your model is correct.Then you can try to change the position of the source feed to change S11.

How did you design the source-port?
Are you working in CadFeko or EditFeko?
Also try changing the length/width a little bit and see what happens.
You can try this aproach

1) design a certain length/width patch.
2)optimize source position to find minimum S11
3)Perform a scan to optimize length in the specified position of the source you found previously.
4)Do the same for width.