An electronic device may be provided with antenna structures. The antenna structures may be formed using a dielectric carrier structure such as a speaker enclosure, so that interior space within the electronic device that is occupied by a speaker can be used in forming an antenna. A speaker driver may be mounted in the speaker enclosure. Openings in the speaker enclosure may allow sound from the speaker driver to be emitted from the speaker enclosure. The antenna structures may have first and second loop antenna resonating elements. The first loop antenna resonating element may indirectly feed the second loop antenna resonating element. The second loop antenna resonating element may be a distributed loop element formed from a strip of metal with a width that loops around the speaker enclosure. Openings in the second loop antenna resonating element may be aligned with the speaker enclosure openings.