Electronic device antennas with multiple parallel plate sectors are provided for handling multiple-input-multiple-output wireless communications. Each antenna sector in a multisector parallel plate antenna may have upper and lower parallel plates with curved outer edges and a straight inner edge. A vertical rear wall may be used to connect the upper and lower parallel plates in each antenna sector along the straight inner edge. Each antenna sector may have an antenna probe. The antenna probe may be formed from a monopole antenna loaded with a planar patch. The planar loading patch may be provided in the form of a conductive disk that is connected to the end of a conductive antenna feed member. The conductive member may be coupled to the center conductor of a transmission line that is used to convey radio-frequency signals between the antenna probe and radio-frequency transceiver circuitry. The antenna sectors may have interplate dielectric structures.