Abstract : This report consists of four parts. Part I is on an Efficient Analysis of Infinite Microstrip Arrays on Electricity Thick Substrates. The emphasis is placed on a numerically efficient method. The analysis also includes the case of infinite arrays with printed dipoles. Part II is an extension of the previous work on multiple port microstrip antennas. Experiments were carried out, which showed that this improved theory could predict the loading effect at the second port at the resonant frequency of the input port very accurately. Part III is devoted to the theory and experiment of single element dual-frequency microstrip antennas. The main objective is to achieve a dual-frequency operation for the same linear polarization with a single element fed at one port. This is discussed in Part IV. Keywords: Periodic Green's Functions, and Multipole-Port Microstrip Antennas.