A wideband S-band F-shaped patch antenna is proposed for CubeSats communications. To broaden bandwidth, it uses two arms with different lengths to generate a second resonant frequency. The effect of the arm length and width on the return loss, resonant frequency and impedance bandwidth on a 3U CubeSat is studied. The simulation results show that the antenna achieves a wideband of 1121 MHz (1.606–2.727 GHz) with a return loss below −10 dB over the entire frequency band from 1.606 to 2.727 GHz. The antenna has a high gain of 8.51 dB and a small return loss of −32.85 dB at 2.45 GHz.