Slot antenna literature

Hi, everyone. Can anyone suggest any literature on slot antennas. I'm designing slot antenna with ground plane reflector (to get unidirectional pattern and high gain) and need physical interpretation for simulated results, cause the gain of my system increased by 4 times, not by 2 as i thought it would.

Hi,

what i know is that the slot antenna has some "hole" (is a PIFA antenna with holes), that makes the current flow to generate another resonance and good antenna gain.Be atentive that the geometry of the antenna influence the gain and the resonance.

Hope i was clear
Regards