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A microstrip antenna has been proposed to operate at dual frequencies. The microstrip is a single layer patch with nonradiating edges closed with a conducting foil. Resonant frequencies are altered by varying the air gap under the patch. The separation of the resonant frequencies can be nearly zero and has no upper limit in principle. The input impedance is easily matched by shifting the air gap. The radiation patterns are not affected by modification for dual-frequency operation.>
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Lee et al. (1993) studied this question.
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