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A design for dual-band circularly polarized (CP) antennas is presented. The antenna is composed of two concentric shorted annular slots, and each slot can generate circular polarization radiation at its one wavelength resonant mode or one-and-a-half wavelength resonant mode by varying the position of the shorted section. When the one wavelength mode of the outer slot and the one-and-a-half wavelength mode of the inner slot are simultaneously excited, the antenna has two CP operating frequencies with a large frequency ratio. A small frequency ratio can also be achieved when the outer and inner annular slots are operated at the one-and-a-half wavelength mode and one wavelength mode, respectively. Two examples with the frequency ratios of 1.05 and 2 are designed and implemented. Details of the experimental results are shown.