This paper presents an X-band circularly polarized patch antenna with a simulated high gain of 12.5 dBi and a simulated side-lobe-level of below -20 dB, within the bandwidth of 5.2%. This single element antenna achieves high gain by exploiting the mode superposition of TM 11 + TM 13 (excited by a circle of vias loading the cavity) and TM 12 + TM 14 (excited by the cavity itself) radiated from the two apertures, respectively. A simple feeding network, consisting of a quadrature hybrid and a couple of 90° bent branches, is used in the design to excite RHCP or LHCP mode in the antenna cavity. A capacitive disk feeding is implemented so that the antenna can be matched to the feeding network, and the super-positioned fields inside the cavity are not disturbed by the feeding pins. This approach makes the design of a dual-sense circularly polarized antenna with high gain easier since the array configuration is not needed. High gain, in combination with a dual-sense circular polarization, makes the proposed antenna particularly attractive for space applications involving small satellites. With the proposed design, it is possible to employ only one antenna rather than two (RHCP and LHCP), thus saving space on the aircraft, which is clearly a limited resource.