This paper presents a detailed analysis of microstrip antennas excited in the $ TM_{mn}^{z} $ mode at X-band. The study focuses on the near field analysis, radiation pattern calculation using the angular spectrum of plane waves, and the investigation of input impedance, gain, and radiation efficiency as functions of substrate thickness and strip width. The primary objective is to develop a new theory based on the angular spectrum of plane waves and the principle of images to explain the radiation characteristics of microstrip antennas and verify it experimentally. The analysis of the near field confirms the feasibility of exciting the $ TM_{mn} $ mode, as the quasi-TEM mode does not exist for typical microstrip configurations. The geometry of the microstrip structure and the field distributions are discussed, with the wave equation reduced to Laplace's equation for certain conditions. The scalar potential is derived for the $ TM_{mn}^{z} $ mode, and the propagation constants are calculated based on the dispersion relation. The paper also presents the estimation of the reflection coefficient $ R_{mn} $ and its determination using a rigorous analytical method. The scalar functions $ \psi_{mn}^{(1)} $, $ \psi_{mn}^{(2)} $, and $ \psi_{mn}^{(3)} $ are derived for the three regions of the microstrip structure, and the boundary conditions are applied to determine the constants $ C_1 $, $ C_2 $, and $ C_3 $. The field components for the $ TM_{mn}^{z} $ mode are calculated, and the experimental study of the near field is conducted using field sampling probes. The radiation characteristics are analyzed using the angular spectrum of plane waves, and the radiation pattern is derived by multiplying the strip's radiation pattern by the array factor. The effect of the ground plane is considered, and the radiation pattern in the $ \phi_0 = 0^\circ $ plane is discussed. The paper also presents the directivity, gain, radiation efficiency, and input impedance characteristics of the microstrip antennas. The results show that the radiation efficiency is higher for smaller values of the shape factor $ a/b $, and the 3-dB beam width is generally high. The study concludes that the radiation theory based on the current strip and its image is well justified, and the coaxial method of exciting pure TM waves is valid. The findings contribute to the understanding of microstrip antenna behavior and provide insights into their design and performance.This paper presents a detailed analysis of microstrip antennas excited in the $ TM_{mn}^{z} $ mode at X-band. The study focuses on the near field analysis, radiation pattern calculation using the angular spectrum of plane waves, and the investigation of input impedance, gain, and radiation efficiency as functions of substrate thickness and strip width. The primary objective is to develop a new theory based on the angular spectrum of plane waves and the principle of images to explain the radiation characteristics of microstrip antennas and verify it experimentally. The analysis of the near field confirms the feasibility of exciting the $ TM_{mn} $ mode, as the quasi-TEM mode does not exist for typical microstrip configurations. The geometry of the microstrip structure and the field distributions are discussed, with the wave equation reduced to Laplace's equation for certain conditions. The scalar potential is derived for the $ TM_{mn}^{z} $ mode, and the propagation constants are calculated based on the dispersion relation. The paper also presents the estimation of the reflection coefficient $ R_{mn} $ and its determination using a rigorous analytical method. The scalar functions $ \psi_{mn}^{(1)} $, $ \psi_{mn}^{(2)} $, and $ \psi_{mn}^{(3)} $ are derived for the three regions of the microstrip structure, and the boundary conditions are applied to determine the constants $ C_1 $, $ C_2 $, and $ C_3 $. The field components for the $ TM_{mn}^{z} $ mode are calculated, and the experimental study of the near field is conducted using field sampling probes. The radiation characteristics are analyzed using the angular spectrum of plane waves, and the radiation pattern is derived by multiplying the strip's radiation pattern by the array factor. The effect of the ground plane is considered, and the radiation pattern in the $ \phi_0 = 0^\circ $ plane is discussed. The paper also presents the directivity, gain, radiation efficiency, and input impedance characteristics of the microstrip antennas. The results show that the radiation efficiency is higher for smaller values of the shape factor $ a/b $, and the 3-dB beam width is generally high. The study concludes that the radiation theory based on the current strip and its image is well justified, and the coaxial method of exciting pure TM waves is valid. The findings contribute to the understanding of microstrip antenna behavior and provide insights into their design and performance.