Field of the Invention
The present invention relates to an antenna device and, more particularly, to an antenna device suitable for NFC (Near Field Communication).
Description of Related Art
In recent years, a mobile electronic device such as a smartphone is equipped with an RFID (Radio Frequency Identification: individual identification by radio waves) system and further equipped with, as a communication means of the RFID, an antenna for performing near field communication with a reader/writer and the like.
Further, the mobile electronic device is provided with a metallic shield so as to protect a built-in circuit from external noise and to prevent unnecessary radiation of noise generated inside the device. Particularly, recently, a housing itself of the mobile electronic device is made of metal instead of resin, considering thinness, light weight, durability against drop impact, design, and the like. Cases where the metallic housing doubles as the metallic shield have been increasing. However, since generally the metallic shield shields electric waves, when an antenna needs to be provided, it is necessary to arrange the antenna at a position not overlapping the metallic shield. When the metallic shield is arranged over a wide range, arrangement of the antenna becomes a serious problem.
To solve the above problem, in antenna devices disclosed in, e.g., Japanese Patent No. 4,687,832, an opening is formed in a metal layer, a slit connecting the opening and an outer edge is formed, and an antenna coil is arranged such that an inner diameter portion of the antenna coil overlaps with the opening of the metal layer. In this configuration, current flows in the metal layer so as to shield a magnetic field generated by flowing of current in a coil conductor, and the current flowing around the opening of the metal layer passes around the slit, with the result that current flows also around the metal layer by edge effect. As a result, a magnetic field is generated also from the metal layer, and the metal layer makes a large loop of a magnetic flux, thereby increasing a communication distance between the antenna device and an antenna of an apparatus at a communication partner side. That is, it is possible to allow the metal layer to function as an accelerator for increasing a communication distance of the antenna coil.
While the above-described antenna device is so-called a planar antenna coil, there is also known a three-dimensional wound-type coil antenna. As compared with the planar coil antenna, the coil antenna of such a type can be increased in inductance more easily, can generate more magnetic lines of force, and can be miniaturized more easily.
However, the wound-type coil antenna has excessively strong directivity. In addition, when the wound-type coil antenna is reduced in size, it cannot generate a large loop magnetic flux that can be interlinked with an opposing antenna. Thus, the wound-type coil antenna has problems for practical use as an antenna for near field communication.