Antennas for electronic devices such as portable computers are provided. An antenna may be formed from a conductive cavity and an antenna probe that serves as an antenna feed. The conductive cavity may have the shape of a quartered rectangular cavity and may have first and second side walls, top and bottom walls, and first and second openings. The first and second openings may be planar in shape and may meet at a right angle along an axis. The antenna probe may be disposed along the axis. The axis at which the first and second openings of the cavity meet may be located at the corner of an electronic device housing. The portable computer may have upper and lower housing portions that meet at a gasket. The gasket may be placed adjacent to the cavity face openings so that radio-frequency signals may enter and exit the cavity through the gasket.