Fundamental TM11 mode of circular microstrip antennas has a limitation of low broadside gain. This thesis explores and investigates the possibility of designing high gain circular microstrip antennas by using higher zeros of the first order mode, or the TM1m modes. Deficiencies in the existing methods of gain enhancement, motivates the need to look for new methods. This is done by first investigating the radiation characteristics of TM1m modes, and then providing techniques for sidelobe suppression in their radiation patterns. Several approaches are proposed and corresponding to each developed technique, an example of high gain antenna is designed, fabricated and experimentally evaluated. First, through the radiation characteristics of a magnetic loop over an infinite ground plane, we explain the occurrence of high sidelobes in the E-plane radiation patterns of the TM1m modes. By noting the peak directivity and sidelobe variation with loop size, we propose, investigate and demonstrate the use of high permittivity substrates to reduce the sidelobes in the TM12 mode. Second, to remove the dependence of the radiation characteristics on high permittivity substrates, another technique to suppress the high sidelobes of TM12 mode, is presented. It is found that introducing a narrow nonresonant rectangular slot at the patch center, sidelobes of the TM12 mode can be effectively suppressed. Sidelobe level (SLL) suppression is demonstrated by both simulation and measurement, using various configurations. Third, it is proposed that linearly superimposing the radiation fields of either odd or even zeros of the first order mode, can achieve both high gain and low sidelobes. To show this, stacked antenna configurations of TM11 and TM13 modes are thoroughly investigated and SLL suppression is demonstrated. Finally, to leverage further advantage of the linear superposition of modes, a single layer annular slot loaded antenna configuration is proposed. The proposed new configurations are based on higher order TM1m modes, which, for excitation, requires electrically large conducting discs. This property offers one more advantage at high frequencies, where antenna size becomes too small for the fundamental mode to fabricate and feed. The proposed electrically large antennas eliminate these problems, and become more practical to fabricate. / February 2017