An antenna assembly includes first and second antennas each generating a resonant mode to cover an operating bandwidth, and a transmission line. The first includes a first radiation unit with a feed-in portion coupled to a first feed portion in contact with a core wire of a coaxial cable and a first grounding portion. The second antenna includes a second radiation unit with a second feed-in portion coupled to a second feed portion in contact with a conductive shielding layer of the coaxial cable and a second grounding portion. The transmission line includes first and second connecting portions coupled respectively to the second feed portion of the second feed-in portion. When a signal within the operating bandwidth is transmitted through the coaxial cable, the energy of the signal is distributed among the first and second antennas.