A broadband antenna system includes a ground plane, a metal plate parallel to the ground plane, and constituting a capacitance load against the ground plane, and a radiation structure connected perpendicularly to the ground plane and the metal plate. The radiation structure includes a feed conductor to supply an electric signal, a short-circuit stub to transfer the supplied electric signal to the ground plane, a conducting bridge to interconnect the feed conductor and the short-circuit stub, which is separated from the metal plate, and a radiating conductor connected to the ground plane the metal plate, and coupled to the supplied electric signal to thereby radiate electromagnetic waves.