An electronic device may be provided with a display and wireless circuits. The wireless circuits may include antenna structures and radio-frequency transceiver circuitry that transmits and receives radio-frequency signals using the antenna structures. A ground plane for the antenna structures may be located in the center of the electronic device under the display. A resonating element may be used to reduce signal interference that otherwise arises when simultaneously operating the display and the antenna structures. The resonating element may be implemented using an L-shaped structure have an arm that extends parallel to one of the edges of the display.