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A novel dual-frequency design of a single-layer single-feed circular microstrip antenna with an offset open-ring slot is demonstrated. For the proposed design, an offset open-ring slot is embedded in the circular patch. By selecting a suitable radius of the circular patch enclosed by this offset open-ring slot, a dual-frequency operation with its two operating modes excited with the same polarization planes is obtained. The frequency ratio of the two frequencies is within a range of about 1.22 to 2.17. The measured results show that similar broadside radiation patterns are obtained and the variations of antenna gain are small for frequencies within the two resonant modes. Details of the antenna design and the experimental results obtained for the dual-frequency performance are presented.