This study presents a broadband, circularly polarized millimeter-wave four-port multiple-input–multiple-output
(MIMO) antenna design featuring enhanced performance attributes. The MIMO antenna structure comprises four
monopole elements with a common ground. Simulated on a Rogers RT5880 substrate with dimensions 25 mm ×
31 mm and a thickness of 1.575 mm, the antenna demonstrates excellent characteristics. The MIMO achieves a
high impedance bandwidth of 21.5 to 45 GHz, a wide 3-dB axial ratio bandwidth (ARBW) of 23.5 to 30.7 GHz,
covering critical frequencies such as 28 GHz, and exhibits high isolation with S21 < -20 dBi. The axial ratio
bandwidth (ARBW) and peak gain (8.5 dBi) are enhanced, and the MIMO performance is greatly improved by
adding parasitic elements between the MIMO elements. Massive MIMO can be constructed using an eight-port
from the same monopole antenna. The compact size and simplified fabrication process make this antenna design
a promising solution for demanding ultra-wideband communication scenarios and 5G millimeter-wave
applications.