A circularly polarized active integrated transmitter antenna based on a switched capacitor power amplifier (SCPA) is designed to meet the increasing demand for high performance in modern wireless communication systems. In the active integrated transmitter antenna, the antenna not only provides the optimal load impedance for the SCPA, but also compensates for the parasitic inductance introduced by the bonding wires, reducing circuit losses and achieving a compact and integrated designing result. Due to the optimal load impedance provided by the antenna, the SCPA die removes the output matching network, and the die is designed with polar and quadrature multiplexing to support more complex modulation while maintaining system compactness. The planar slot antenna introduces a section of elongated stub in the annular slot structure so that the base mode of the annular slot structure is decomposed into two orthogonal degenerate resonant modes, and both right-hand circular polarization and left-hand circular polarization are achieved in the same frequency band. The experimental results show that the designed SCPA operates at 1.92 GHz with a maximum output power of 10.25 dBm. The equivalent isotropic radiated power (EIRP) of the active integrated antenna at the operating frequency reaches 12.5 dBm.