The microstrip patch antenna is a popular printed resonant antenna for narrow-band microwave wireless links that require semi-hemispherical coverage. Due to its planar configuration and ease of integration with microstrip technology, the microstrip patch antenna has been heavily studied and is often used as elements for an array. However, they suffer from the disadvantage of low operating bandwidth [1]-[3]. Cavity model has been somewhat successful for calculating input impedance and radiation pattern [4]. Although proven to be useful for predicting input impedance, this method does not rigorously account for the presence of surface waves on the antenna substrate. Furthermore, it does not take into account existence of other modes with a dominant mode which deforms the surface currents and hence deforms the pattern lobs, and does not include feed effects. This paper presents the use of cavity model to analyze the rectangular microstrip antenna RMPA because this method can give a good pre-estimation on rectangular patch performance [5]. RMPA is characterized by its length L, width W, and thickness h, as shown in Fig. 1. The microstrip antenna treated as a cavity bounded by a magnetic wall along the edge, and by electric wall from above and below suggest and the E has only the z component and H only the xy-components [4,6].