In the dual frequency operation of microstrip antennas, shorting strips are used to adjust the ratio of frequencies. A multi-port analysis is usually employed to predict the input impedance and resonant frequency of probe-fed microstrip antennas with shorting strips. However, this approach does not provide any information about the field distribution under the patch. In this thesis, a hybrid model, using both the cavity model and point matching, is developed to calculate the field distribution under the patch with shorting pins and strips. In addition, this model also accounts for the conducting nature of the feed and shorting strips, with the help of the point-matching algorithm. Then, to verify the model, the theoretical results obtained from the hybrid method are compared to the experimental results and good agreement is observed. Finally, a genetic algorithm is developed for optimizing the position and width of the shorting strips to achieve desired frequency ratio and input impedances in dual-band operations.