1. Technical Field
The present invention relates to an antenna device, and more particularly, to an antenna device mounted on a vehicle having a body made of resin or glass.
2. Related Art
When an antenna device of a GPS system for a vehicle widely used as a positioning system or an antenna device used for a satellite radio coming into practical use in the United States is mounted on a vehicle such as a car, an antenna element stored in a case is installed generally on the outer rear such as a roof of the vehicle (see Patent Documents 1, 2, etc.). In this case, the case of the antenna device may be attached to a metallic roof of the vehicle, for example, by the use of a permanent magnet or the like.
However, the vehicle having a roof made of resin or glass containing glass fiber resin such as glass fiber has been manufactured in recent years. Accordingly, it is difficult to attach the antenna device to the roof by the use of the magnet or the like. Thus, in case of such a vehicle, the antenna device is attached to the top surface of the roof by the use of adhesives or the like, and a metallic plate or a metallic sheet such as aluminum is installed on the bottom surface, that is, on the indoor surface of the vehicle.
Since the roof of such a vehicle is made of resin or glass, it is possible to install the antenna device on the bottom surface of the roof. Thus, there was proposed an automobile parts with built-in antenna in which a bottom plate of an antenna device is attached to a rib protruding from a bottom surface of a roof (see Patent Document 3).
Patent Document 1: JP-A-2005-109688
Patent Document 2: JP-A-2003-17154
Patent Document 3: JP-A-10-329615
However, in a satellite radio of the United States, a geostationary satellite may be located at a relatively low position on the Pacific. If a low elevation angle directivity gain of the antenna element is not large, electric waves of the satellite radio may not be sufficiently received. Accordingly, there is a case to obtain a sufficient low elevation angle directivity gain of the antenna element while not limited to the satellite radio.
From this point of view, since the roof of the vehicle of the known antenna device is made of metal, the roof of the antenna device serves as a GND of the antenna device. Further, since the roof sufficiently attracts the electric waves, the low elevation angle directivity gain is sufficiently obtained only by installing the antenna element on the top surface of the roof. Accordingly, the low elevation angle directivity gain is sufficient.
However, when the roof of the vehicle is made of resin or glass, the roof does not serve as the GND of the antenna element and does not attract the electric waves. As disclosed in Patent Document 3, even when the bottom plate of the antenna device attached to the rib protruding from the bottom surface of the roof is made of metal, the area thereof is not sufficiently large and it is difficult to obtain a sufficient low elevation angle directivity gain.
In case of the bottom plate of the antenna device in Patent Document 3, or in case of the metallic plate or the metallic sheet installed on the bottom surface of the roof when the above-described antenna element is installed on the top surface of the roof, the area thereof may be widened to sufficiently attract the electric waves. However, it is actually difficult to install the bottom plate or the metallic plate on the entire back surface, which is not preferable in appearance design.