This article presents a broadband circularly polarized four-port multiple-input–multiple-output (MIMO) antenna with high isolation is presented in this article. The proposed MIMO antenna consists of four octal-shaped monopole antennas with common ground is realized and fabricated on FR-4 (εr = 4.3) substrate with a thickness of 1.6 mm and a size of 140 mm × 140 mm. Measurement results of the presented MIMO antenna show features of a wide impedance bandwidth (IBW) of 139.39% (2–11.2 GHz) and a 3-dB axial ratio bandwidth (ARBW) of 91.95% (2.25–6.4 GHz) with simple and economical fabrication. Additionally, the investigated MIMO antenna has high isolation (S21