A rigorous analysis of the features of the modes of an annular sector printed microstrip antenna is presented. General expressions for its radiation field are derived by using the cavity model. The expressions are used to study the radiation patterns, radiation peak in the broadside direction, and beamwidth of different structures for various sector angles, widths, and modes of excitation. This includes the special cases of quarter-, half-, three-quarters-, and ideal-gap open-ring structures. Compared to a closed ring, the results indicate that the TM/sub 12/ mode is superior to the TM/sub 11/ mode with regard to the radiation properties in the annular sector as its angle approaches 2 pi .>
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Sultan et al. (1990) studied this question.