Antennas are provided for portable electronic devices. A portable electronic device may have a port that receives a wireless communications adapter. The adapter may be used to provide wireless functionality for the portable electronic device. The adapter may contain a chip antenna that serves as an antenna resonating element. A printed circuit board within the adapter may contain conductor that has been patterned to form a ground plane for the antenna. The portable electronic device may have a conductive structure such as a housing portion. The conductive structure of the portable electronic device serves as a parasitic antenna element that improves antenna efficiency. The portable electronic device may be a handheld electronic device with music player functionality that communicates with a compact portable wireless device in a piece of sports equipment.