what is the role of slots and air gap in enhance the gain in microstrip antenna.

Hello
i have designed, simulated a high gain microstrip antenna, it is basically a stacked form antenna which has some slots with airgap. i got very good gain but i do not know the basic reason of high gain after cutting the slot into the patch and introducing the air gap.
how did air gap and slots put the effect on this gain paramters so that it has enhanced?