how to increase gain, Z parameter?

hi..i'm in the process of manufacture of microstrip antenna using HFSS version 10..

but, i got problem, i can't get the best result of gain, VSWR and Z parameter...they point is always deviated...

for your information, i want to make rectangular microstrip antenna,, array,with gain 11 dBi minimum, VSWR ≤ 1.5 and load impedance 50 Ω...

can you tell me how to get the parameter as my spesification...what should i do?

need your advice soon....please!