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A novel 2 2 array of microstrip antennas was developed that generates orthogonal circularly polarized waves at two frequencies simultaneously. This true shared aperture can be used as a building block in full-duplex wireless communication applications such as focal-plane arrays for satellite internet (VSAT) and wireless connectors. An alternative implementation of the sequential rotation technique was applied to create dual-frequency operation using only a single aperture. The concept was verified with experimental results from a prototype consisting of a 2 2 array of aperture coupled microstrip antennas (ACMAs) with corresponding feed network operating at 4 and 6 GHz. Measurements show that the axial ratio (AR) is below 1.2 dB for both the low-band as well as the high-band mode of operation. The isolation between both modes is larger than 26 dB, eliminating the need of high-performance duplexers.