| Microstrip antenna has a lot of merits such as little volume, light in weight, easily conformaity with carrier, good compatibility with integrate circuit, easy batch production, so it is connected with broad application in several decades. But it is a mostly disadvantage that has rather narrow frequency strip. In order to solve the problem, this paper adopts a simple and feasible method that air gaps are introduced between patch and ground plane. This method not only extends bandwidth of antennas effectively, but also optimizes its performance.First, we deduce theoretical formulae of calculating equivalent dielectric constant and resonance frequency of rectangular microstrip antennas with air gaps from electromagnetic field theory based on the modified theory of cavity model. Then, we discusses the radiant principle of microstrip antennas on the base of transmission line theory and analyzes the problem of air gaps how to influence pattern and other idiocratic parameters of a microstrip antenna in detail. Besides those work mentioned above, the feature of our method able to both extend bandwidth of frequency and optimize performance is demonstrated by a design example of a rectangle microstrip antenna with air gaps. In the paper, we give simulation results of some characteristic parameters as well. Experimental data obtained through Matlab show that all design demand is met basically. In the final, the input impedance of the microstrip antenna are plotted through Smith Chart V1. 91.
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