RHCP microstrip patch antenna using HFSS

I am designing a RHCP microstrip patch antenna by cutting the diagonal slot of antenna.
The operating frequency is 1575.42MHz.
I am able to get the Fc through my design, but the problem is I can't get a good polarized gain and cross polarized gain.
My lecturer told me the different between RHCP and LHCP gain should be in the range 10-15 dB. However, I can get the different of 3 dB only. (I am tuning the dimension of the slot to get the gain )

Can anyone help me to see what is the problem of my design?


Thank You
RHCP_patch_antenna.rar

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Besides that, I have tried many values of slot dimensions to optimize my design. I notice that when the slot width become smaller, the RHCP gain will increase, however, it leads to increasing of LHCP gain also. Hence, I can't enlarge the difference between the RHCP and LHCP gain.

Can anyone help me to see whether I'm wrong in the design or concept?
Really thank a lot.