Monopole antennas are widely used in communication systems since they can provide omni-directional radiation pattern in the horizontal plane with vertical polarization. However, the profile of the conventional monopole antenna is about 1/4 of the wavelength in the resonant frequency of the antenna which is too large for some communication systems like airplane and satellite communication systems. In these systems, it is desired to replace the monopole antenna with a low profile antenna. Meanwhile, microstrip antennas have been extensively used in communication systems because of their advantages such as low profile, low weight, planar structure, easy fabrication, and so on. Therefore, it is expected to consider the inherently low profile microstrip antenna as a candidate for replacing the monopole antenna. However, the conventional microstrip antenna provides a radiation pattern with maximum near to the zero elevtion angles in its eigenmode while the monopole antenna radiates endfire pattern with maximum near to zero azimuth angles. It has been known that the circular microstrip antenna can radiate monopole-like radiation pattern in TM 0m modes. However, these are higher order modes and the size of antenna must be large enough to stimulate these modes. One way to overcome this issue is to load the microstrip antenna with one or multiple shorting pins. It has been shown that Shorted Pins Circular Microstrip Antenna (SPCMA) can provide monopole-like radiation pattern in the fundamental mode. However, no comprehensive analytical model for the SPCMA has been presented yet. In this thesis, an analytical approach to analyze SPCMAs is proposed. The method is based on the cavity model in cunjuction with image and scattering theorems. It is shown numerically and experimentaly that the method can accurately analyze the circular microstrip antenna loading with a number of shorting pins. Some of the SPCMA with monopole-like radiation pattern are investigated by the method. The resonant frequency, electromagnetic fields, radiation pattern, and input impedance of these antennas are developed as analytical expressions based on the model. Some of the analytical results are compared with the experimental results to confirm the analytical approach. The approach may also be used to analyze other shorting pins microstarip antennas as well.
