Just some comments:
Slots in the ground plane of microstrip devices is well known as an EBG structure (Radisic EBG structure). It can be used for reducing radiating element size if well designed. Slow-wave effect in the EBG structure pass-band can be used for such applications.
For slot line printed devices, I recomend another structure :
The dual one of the Radisic Structure consists in adding metallic discs under the slot on the other side of the dielectric substrate.
Then miniaturisation applications have been proposed on annular slot antennas. (see ieee ap-s 2003 article)
---------------------
Littlindian