Antenna

An antenna including a rectangular ground plane, a feed point disposed in a vicinity of a corner of the ground plane, an antenna element coupled to the feed point, and a parasitic element coupled to the ground plane. In this antenna, the shortest length from the feed point to a distant narrow side of the ground plane and a shortest length from the feed point to a tip of the parasitic element via the ground plane have substantially the same electrical length. Such a configuration can correspond to a small and thin size and reduce SAR easily.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an antenna mainly used in a wireless device for mobile communication and the like.

2. Background Art

Recently, demand for wireless devices for mobile communication such as a portable telephone has increased rapidly. An antenna mounted on a wireless device is one of the important devices affecting the performance of the wireless device. Antennas have been required to be small and thin in size and have a reduced effect on a human.

Herein, such a conventional antenna mounted on a portable telephone as a wireless device is described with reference to a drawing.

FIG. 5is a schematic view showing a conventional antenna mounted on a portable telephone. InFIG. 5, conventional antenna6includes rectangular ground plane3, feed point4disposed in the vicinity of a corner of ground plane3, antenna element1coupled to feed point4, variable reactance element5disposed on ground plane3, and parasitic element2coupled to variable reactance element5.

The above-mentioned conventional antenna6switches values of variable reactance element5by using a switching processing circuit and the like, between a value of variable reactance element5showing omnidirectional radiation characteristics during a standby time and a value of variable reactance element5showing directional radiation characteristics by coupling antenna element1and parasitic element2to each other so that the radiation of electric waves to the side of a human is reduced during a talk time.

As mentioned above, the conventional antenna6changes the radiation characteristics of antenna6between during a standby time and during a talk time by changing the value of variable reactance element5. Thus, an excellent radiation characteristic is kept in each state. An example of prior art information related to the invention of this application includes Japanese Patent Application Unexamined Publication No. 2005-295002.

In the conventional antenna6, by changing the value of variable reactance element5during a talk time, antenna element1and parasitic element2are coupled to each other so as to strengthen the directivity of antenna6, thereby reducing the radiation of electric waves to the human side that is brought to the ear during a talk time. As a result, it is possible to reduce SAR (Specific Absorption Rate) that is an index showing the degree of the effect of electromagnetic wave on the human.

However, in order to reduce SAR, it is necessary to change values of variable reactance element5between a standby time and a talk time. Therefore, it is necessary to add a switching processing circuit and the like.

SUMMARY OF THE INVENTION

The present invention provides an antenna having a configuration in which a variable reactance element is not used and the adjustment thereof is not required and which is capable of corresponding to a small and thin size of a wireless device and reduces SAR as an index showing the degree of the effect of electromagnetic waves on a human.

The antenna of the present invention includes a substantially rectangular ground plane, a feed point disposed in the vicinity of a corner of this ground plane, an antenna element coupled to this feed point, and a parasitic element coupled to the ground plane. In this antenna, the shortest length from the feed point to a distant narrow side of the ground plane and the shortest length from the feed point to a tip of the parasitic element via the ground plane have substantially the same electrical length. Since a route from the feed point to the distant narrow side of the ground plane and a route from the feed point to the tip of the parasitic element via the ground plane have the same length, a high-frequency current mainly excited on the ground plane is distributed into two directions. As a result, the peak value of the high-frequency current mainly excited on the ground plane is lowered, and SAR can be reduced. Therefore, since a variable reactance element is not used, it is possible to provide an antenna capable of corresponding to a small and thin size and reducing SAR easily.

DETAILED DESCRIPTION OF THE INVENTION

First Embodiment

Hereinafter, the embodiment of the present invention is described with reference to drawings. Note here that the same reference numerals are given to portions of the same configuration described in the conventional technology and the detailed description thereof is omitted herein.

FIG. 1is a schematic view showing an antenna in accordance with a first embodiment of the present invention. InFIG. 1, antenna15includes substantially rectangular ground plane3, feed point4disposed in the vicinity of a corner of ground plane3, antenna element11coupled to feed point4, and parasitic element12coupled to ground plane3. The length of parasitic element12is adjusted and coupled so that the shortest length from feed point4to a distant narrow side of ground plane3and the shortest length from feed point4to a tip of parasitic element12via ground plane3have substantially the same electrical length.

Herein, an operation state of antenna15is described. When antenna15is operated, with the excitation by antenna element11and ground plane3, a high-frequency current flows in ground plane3. At this time, in antenna15in accordance with the first embodiment, high-frequency current13excited in the direction from feed point4to the distant narrow side of ground plane3and high-frequency current14excited from feeding point4to parasitic element12via ground plane3have substantially the same electrical length. Consequently, a main high-frequency current excited on ground plane3is distributed into two directions.

In this way, since the main high-frequency current excited on ground plane3is distributed into two directions, the peak value of the high-frequency current excited on ground plane3is lowered, so that SAR can be reduced.

Therefore, unlike a conventional technology, since antenna15does not need to use a variable reactance element, it is possible to realize an antenna that can correspond to small and thin size of equipment and can reduce SAR easily.

FIG. 2is a schematic view showing a configuration in which an inductor is disposed between a parasitic element and a ground plane of an antenna in accordance with the first embodiment of the present invention. As shown inFIG. 2, inductor17may be disposed between ground plane3and parasitic element16. With such a configuration, an apparent high-frequency electrical length is increased by inductor17. Therefore, even when parasitic element16is made to be shorter, the distance of the route from feed point4to a tip of parasitic element16via ground plane3can be electrically equalized to the direct distance from feed point4to the distant narrow side of ground plane3. Thus, antenna18can be miniaturized. In this configuration, inductor17is disposed on ground plane3. However, the similar effect can be obtained when inductor17is disposed in the middle of parasitic element16or at the tip of parasitic element16.

FIG. 3is a schematic view showing a configuration in which a switch is disposed between a parasitic element and a ground plane of an antenna in accordance with the first embodiment of the present invention. InFIG. 3, switch19is disposed on ground plane3coupled to parasitic element12. With such a layout, when switch19is turned off, a high-frequency current excited on ground plane3is one. When switch19is turned on, a high-frequency current excited on ground plane3is distributed because a high-frequency current flowing from ground plane3to parasitic element12is added. That is to say, since the high-frequency current excited on ground plane3can be changed by switch19, not only reduction of SAR but also control of the directivity can be realized.

FIG. 4is a perspective view showing an antenna in which an antenna element and a parasitic element are held on a holding portion made of insulating resin in accordance with the first embodiment of the present invention. InFIG. 4, caulking protrusions are provided on the surface of holding portion23made of insulating resin and the caulking protrusions are crushed and caulked. With such a configuration, antenna element21and parasitic element22corresponding to the above-mentioned antenna element11and parasitic element12can be attached and held on holding portion23. Therefore, antenna24can be treated as one component and easily mounted on the wireless device. Note here that a method of attaching each element to holding portion23may include a method of adhesively fixing each element to the surface of holding portion23with an adhesive agent, and a method of fixing antenna element21and parasitic element22by insert molding with insulating resin such as ABS. The fixing method is not particularly limited.

Since the antenna of the present invention has a configuration in which a variable reactance element and the like is not used, it has an advantageous effect that it can correspond to a small and thin size of a wireless device and reduce SAR easily. It is useful mainly for a wireless device for mobile communication and the like.