The Theory of Characteristic Modes is proposed as a systematic approach to antenna design to achieve the goal of finding the antenna structure with optimum broadband behavior. This theory provides a physical insight to the radiating nature of microstrip patch antennas and reduces the design optimization time. In this paper, the resonant behavior of different patch slot designs are analyzed using this theory. The modal analysis concludes that the U-slot patch design is the highest radiating structure and hence is a good candidate to achieve the highest impedance bandwidth.