please guide me in my design of antenna

hello all
i have designed an antenna and simulated it. i m getting all the parameter very gud but except gain. so please anybody guide me in improving the gain. i m giving the details of my antenna and i have pasted the design of my antenna. in the design the black portion has been cutted out.

Er=4.3
h=1.5
tangent loss=0.004
feeding is coaxial x=0,y=4.57
return loss = -33.56
resonant frequency= 6.50
mismatch loss 0.00
vswr=1.06
directivity =7.56 dbi
but gain is too low that is 2.35 dbi
so plz guide me how can i improve the gainuntitled.bmp

Hard to believe that you can get more than 2.35 dBi of gain from this kind of antenna using FR4 (I assume the resonant frequency is 6.5GHz)