This study proposes a design of a wideband shark-fin antenna that operates in the sub-6 GHz band for 5G communication. A wideband shark-fin antenna is a multiple-input-multiple-output antenna consisting of two antenna elements. For operation in the 4G and 5G operation bands, wideband characteristics were implemented using a coupler and a half-lambda loop pattern. The manufactured wideband shark-fin antenna satisfied the condition VSWR<3.5:1 in the 4G and 5G operating bands. The average gains over the operating bandwidth were greater than −1.97 dBi, indicating that the shark-fin antenna had excellent radiation characteristics.