An ultrawide-band circularly polarized microstrip antenna loaded with arc-shaped branches was proposed for satellite communication in integrated sensing and communication. By cutting the square loop antenna and adjusting the surface transverse and longitudinal currents, circularly polarized waves were generated. The axial ratio bandwidth was expanded by loading two quarter arc-shaped branches. By designing an adjacent coupled feed structure that resonated with the top layer of the antenna, antenna miniaturization and impedance bandwidth expansion were achieved. The simulation and experimental results show that the antenna emits right-handed circularly polarized radiation; the impedance bandwidth is 9.06~22.55 GHz (85.4%); the 3 dB circular polarization axis ratio bandwidth is 9.91~21.54 GHz (74%); the peak gain is 3.5 dBi; and the dimensions of the antenna are 11.7 mm×11.7 mm×0.127 mm (0.38λ mm×0.38λ mm×0.127 mm, where λ is the free space wavelength corresponding to 9.91 GHz). The designed antenna structure is simple, compact, and has a wide operating frequency that can cover the Ku-band.