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A dual-frequency circularly polarised printed antenna is obtained by modifying the printed antenna composed of strips and slots proposed in previous work. The fundamental structure of the dual-frequency antenna consists of two different length strips and a slot, which are excited through a microstrip feed line. Before discussing the dual frequency operation, simplified calculations are made for a single pair of a strip and a slot in order to investigate the basic properties of the antenna. Based on the calculations, the configuration of a desired dual-frequency antenna can be determined, and some of the characteristics may be predicted. Finally, calculated results are compared with experimental data, and the agreement between them is found to be good.