This paper presents a design of 250MHz single-ended digital transmitter (DTX) with switched-capacitor power amplifier (SCPA) for communication between mobile phones and satellites. The implementation of SCPA enhances linearity and efficiency. For compact die area and high efficiency, 50%-duty-cycled clocks are used in the DTX eliminating second-order harmonic without differential architecture. Aiming at higher efficiency compared to conventional quadrature DTX, the technique of digital-domain in-phase (I) and quadrature phase (Q) cell sharing is applied in this work. Moreover, the combination of three signals with specific amplitude and phase eliminates third-order harmonic, improving the output spectrum significantly. The DTX is designed in 22nm standard CMOS process and reaches a peak output power of 8.87dBm at 250MHz and power-added efficiencies (PAE) of 57.4%. When modulating a 4 MHz 64-QAM signal, the error vector magnitude (EVM) is 0.61%, while the average output power and PAE are 1.93dBm and 32.9%, respectively.