1. Field
One embodiment of the invention relates to an antenna device and electronic equipment.
2. Description of the Related Art
Recently, instead of whip antennas that have been widely used, built-in antennas are increasingly being used as wireless communication antennas for electronic equipment capable of wireless communication such as notebook personal computers (PCs). Such a built-in antenna has antenna elements built in the housing, and therefore is easily handled when used or stored compared to the whip antenna. Moreover, the housing can be designed more flexibly.
In electronic equipment having a built-in antenna, due to the miniaturization of the housing, it is often the case that the antenna elements are arranged near metal part such as peripheral circuits mounted on the circuit board. This may lower the input impedance of the antenna and thus may cause an impedance mismatch between the antenna and the feed circuit, resulting in degraded performance.
Accordingly, a folded monopole antenna is used to prevent a decrease in the input impedance of the built-in antenna.
With the folded monopole antenna, the input impedance of the antenna can be increased compared to that of a monopole antenna, and can also be adjusted by the ratio between the diameters of parallel lines. On the other hand, the folded monopole antenna is likely to be larger than a monopole antenna, and therefore requires elaborate arrangement, such as to wire the antenna elements in three dimensions, to be built in small electronic equipment.
Japanese Patent Application Publication (KOKAI) No. 2003-158419 discloses a conventional technology that enables resonance to occur at a plurality of resonant frequencies by adding an antenna to a conventional inverted-F antenna.
FIG. 50 is a schematic diagram of an antenna designed referring to the basic structure of an inverted-F antenna according to the conventional technology. As illustrated in FIG. 50, the inverted-F antenna comprises a radiation conductor (36 mm) 1, a ground conductor 2, a short circuit element (8 mm) 3, a feed line 5, and a feed conductor (80 mm) 6. The radiation conductor 1 is located opposite the ground conductor 2. The short circuit element 3 connects between the radiation conductor 1 and the ground conductor 2. The feed line 5 extends between the radiation conductor 1 and the ground conductor 2 and is connected to a feed point 4 spaced apart by 1 mm from a ground point at which the short circuit element 3 is connected to the ground conductor 2. The feed conductor 6 is connected to the feed line 5. The radiation conductor 1 and the feed conductor 6 are supplied with power via the feed line 5.
FIG. 51 is a Smith chart of the impedance variation of the inverted-F antenna illustrated in FIG. 50. The Smith chart of FIG. 51 represents variations in the input impedance of the inverted-F antenna when the frequency signal fed from the feed point 4 is changed in a range of 700 to 2500 MHz. As illustrated in FIG. 51, the plot of the impedance of the inverted-F antenna deviates upward from the centre of the Smith chart, i.e., 50Ω.
This is because the inductivity of the input impedance increases due to current flowing from the feed point 4 to the ground point as indicated by an arrow in FIG. 50. As a result, an impedance mismatch may occur at a desired frequency band.