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An improvement to a recently reported theory for the analysis of the pattern and impedance loci of microstrip antennas is developed. It yields a theory which is simple and inexpensive to apply. The fields in the interior of the antennas are characterized in terms of a discrete set of modes. The poles corresponding to these modes are complex and depend on the losses in the antenna. The representation of the fields in terms of these modes is rigorous only for a bona fide cavity with no copper loss. The proper shift in the complex poles due to the addition of copper and radiative losses is approximated by lumping the latter two together with the dielectric loss to form an effective loss tangent. By so doing, it is found that the resulting expressions for impedance of the microstrip antenna are in good agreement with measured results for all modes and feed locations. The theory is applied to the evaluation of impedance variation with feed location, to multiport analysis, and to the design of circularly polarized microstrip antennas.
Date of Publication: Jan 1981