Band L microstrip antenna

hello guys!

i design a stacked patch antenna who works in L band and i would like to increase the banwith to work between 1.5 Ghz to 1.7 Ghz .
i tried many to decrease the Er and increase the substrate height but it doesn't work .

is someone could give me a help or and idea please. i work with CST design studio 2012

here's my design below

thanks

you can increase the bandwidth by applying the air gap or by cutting the slots in the patch or by partially removal of the ground plane.