slot in patch antenna

The patch itself provides one resonant point and the slot is another resonant point.
The slot forces current to flow in a different path on the patch and each path of current flow corresponds to a resonant point. So, S11 will change.

Slot is generally applied for Broadbanding the response .. all uWB applications have slot ..So u can use slot in microstrip along with cpw feed ....

With little tuning ur S11 will also be gr8