Electronic devices may be provided with antenna structures and antenna isolation element structures. An antenna array may be located within an electronic device. The antenna array may have multiple antennas and interposed antenna isolation element structures for isolating the antennas from each other. An antenna isolation element structure may have a dielectric carrier with a longitudinal axis. A sheet of conductive material may extend around the longitudinal axis to form a conductive loop structure. The loop structure in the antenna isolation element may have a gap that spans the sheet of conductive material parallel to the longitudinal axis. Electronic components may bridge the gap. Control circuitry may adjust the electronic components to tune the antenna isolation element.