1. Field of the Invention
The present invention relates to a patch antenna, and particularly to a technology for shifting the directivity of a patch antenna.
2. Description of Background Art
Recently, patch antennas, which are compact and slim circular polarization antennas, are commercially available. FIG. 7 shows a conventional example of such patch antennas. The conventional example has a main body 1000, where a conductive ground electrode 1002 is formed on an entire back surface of a rectangular dielectric substrate 1001, and a conductive antenna electrode 1003 is formed in the center of the front surface of the dielectric substrate 1001. This type of patch antenna is disclosed by Japanese Laid-open patent application No. 2002-11367, for example.
In this prior art, two modes different in phase by 90 degrees are driven by: supply of a high-frequency signal power to the antenna electrode 1003; and grounding of the ground electrode 1002, thereby emitting circular polarization waves. The supply path of high-frequency signal power is a coaxial cable, for example.
There is an array antenna in which a plurality of the aforesaid patch antenna, arranged in lines, are provided as a compact and slim vertical polarization antenna. There is a prior art in which one patch antenna, being one element of such an array antenna, has a main body 1010 whose both ends of a ground electrode 1004 are extended and bent to form bent portions 1002a, as FIG. 8 shows. The bent portion 1002a has an object of preventing electromagnetic wave interference among the patch antennas. This type of patch antenna is disclosed by Japanese Laid-open patent application No. H09-172321, for example.
In another prior art, in a multiple-layer circuit board, only the top layer is formed to have the same structure as the main body 1000 of the aforesaid patch antenna. Japanese Laid-open patent application No. 2001-94336, for example, discloses that the patch antenna and the circuit board are formed into an integral body.
With all the above-mentioned patch antennas, the orthogonal direction to the main surface of the antenna electrode corresponds to a direction in which the antenna advantage is the largest. In other words, the above-mentioned patch antennas have the directivity in the direction orthogonal to the main surface of the antenna electrode.
An apparatus to which such a patch antenna is applied is a vehicle-mounted GPS (global positioning system), which obtains the position of the vehicle using the radio waves received from a plurality of satellites.
Usually, a patch antenna used for such a vehicle-mounted GPS is set on a place parallel to the earth, such as on a flat dashboard of a vehicle.
In such a case, the directivity of the patch antenna will be substantially immediately above the vehicle, which is desirable for receiving the radio waves from a satellite traveling 20,000 kilometers above from the earth.
Another example of the apparatus to which the patch antenna is applied is an ETC (electronic toll collection) apparatus also used by being mounted in a vehicle and performing transmission/reception of information to/from an external device.
Usually, just as in the case of the vehicle-mounted GPS, such a vehicle-mounted ETC apparatus is equipped with a patch antenna, and the setting place of the patch antenna is also on a dashboard of the vehicle.
However, prior to passing a tollgate, the ETC apparatus performs wireless communication with a road antenna provided at 5 meters height in the vicinity of the tollgate. This means that an ETC apparatus has to perform transmission/reception of information with a road antenna deviated from the directivity of its patch antenna, and so has a problem of having less antenna advantages than originally intended, as well as having reduced transmission/reception performance in the intended direction.
One means to solve this problem is to incline the attitude of the patch antenna toward the front, thereby bringing the actual directivity closer to the intended transmission/reception direction.
Although being an inexpensive means, this causes another problem, as a tradeoff, that the height of the ETC apparatus becomes large because of the inclining of the patch antenna, which leads to reduction of compactness and slimness of the ETC apparatus.