Abstract: This paper presents a Ku/K dual-band co-aperture array antenna with a coplanar structure, featuring miniaturization, high aperture utilization, high gain and high isolation. In the design of the unit antenna, the Ku band is a rectangular patch with a corner cut and an internal slot in the shape of a C-shaped gap, and the feeding is achieved through probe coupling feeding to realize single linear polarization. The K band is a rectangular patch with a corner cut and a phase difference of 90°, and the feeding is achieved through a four-element H-shaped gap coupling feeding to realize right-hand circular polarization. The array is composed of 2×2 antenna units. After the two-band antenna units are coplanarly integrated, the feeding networks of each are matched to successfully realize the dual-band common-aperture array antenna, significantly improving the aperture utilization. Simulation and measurement results show that the impedance bandwidth of the Ku band of the coplanar array is 11.10 GHz to 11.78 GHz in simulation and 11.08 GHz to 11.76 GHz in measurement. The impedance bandwidth of the K band is 19.15 GHz to 21.90 GHz in simulation and 19.10 GHz to 21.62 GHz in measurement. At the same time, at 11.5 GHz and 20.4 GHz, the array gains are approximately 12.30 dBi and 15.95 dBi, respectively. The axial ratio bandwidth of the K band array is nearly four times that of the unit, fully verifying the effectiveness of this scheme.