Antenna device

A small antenna has two or more feeding ports. A radiator is made of a planar conductor having a substantially circular shape having the diameter of a substantially half wavelength or a substantially regularly polygonal shape where the length of a diagonal line passes through the center point is the substantially half wavelength. A ground plate is faced to the radiator. On the radiator, the feeding ports are connected to feeding points on two orthogonal line segments passing through the center of the radiator. This antenna is used as not only a single antenna but also two independent antennas having secured isolation between the feeding ports. A small antenna device used as two independent antennas is thus provided. The radiator is formed in a hat shape having an edge, has an Stepped Impedance Resonator (SIR) structure where the diameter of a crest part is a quarter wavelength, and is shortened.