how to increase bandwidth of a Microstip antenna array

Hi,

I'm designing a 4x1 microstrip array (attached a snapshot) that suppose to work in the 2.4-2.5 GHZ band, i chose my resonance frequency at 2.45GHZ, Er=2.2.
I'm getting pretty good results, both gain and S11 - (-38 dB), but my bandwidth is too narrow.. how can i increase it so it'll cover most of the 2.4-2.5 GHZ range (-3dB)?

thanks!

increase substrate thickness

@theone23
Could plz tell me the spacing between ur patch elements in the array and the impedances u used for ur corporate feed network? like which is 50ohm and which is 100ohm or 70 ohm etc in ur array structure...
Awaitng