1. Field of the Invention
The present invention relates to a dual-band antenna having small size and low height which is capable of transmitting and receiving respective signal waves of high and low frequency bands, and which is suitable for using as a vehicle antenna or a portable antenna.
2. Descriptions of the Related Art
As an antenna having a suitable small size and low height, an inverted F type antenna has been suggested, in which a notch is provided in a radiating conductive plate such that the radiating conductive plate is excited in two types of high and low frequency bands (for example, see Japanese Unexamined Patent Application Publication No. 10-93332, pages 2 to 3, FIG. 1).
FIG. 10 is a perspective view of a conventional inverted F type dual-band antenna. In the inverted F type dual-band antenna 1, a radiating conductive plate 2 is arranged to oppose a surface of a grounding conductor 5 in an approximately parallel manner, and a shorting conductive plate 3 extends approximately orthogonally from an outer edge of the radiating conductive plate 2. The radiating conductive plate 2 has a L-shaped conductor piece 2a which is excited at a first frequency f1, a rectangular conductor piece 2b which is excited at a second frequency f2 higher than the first frequency f1, and a rectangular notch 4. A lower end of the shorting conductive plate 3 is connected to the surface of the grounding conductor 5. Further, to a predetermined position of the radiating conductive plate 2, a feeding pin 6 is connected by means of soldering, and a lower end of the feeding pin 6 is connected to a feeding circuit (not shown) without contacting the surface of the grounding conductor 5.
In the conventional dual-band antenna 1 having such a schematic configuration, the length along an extended direction of the L-shaped conductor piece 2a is set to be about a quarter of a resonant length λ1 corresponding to the first frequency f1, and the length of the rectangular conductor piece 2b is set to be about a quarter of a resonant length λ2 corresponding to the second Frequency f2 (but λ2<λ1). Therefore, by selectively supplying high frequency powers corresponding to two types of high and low frequency bands (high band and low band) to the radiating conductive plate 2 via the feeding pin 6, it is possible to allow the respective conductor pieces 2a and 2b to be excited at a different frequency from each other, and to transmit or receive signal waves of two types of high and low frequency bands.
Further, in such an inverted F type dual-band antenna, a technique in which a radiating conductor is formed in a meandering form is widely adopted. In such a manner, since a path of current flowing through the radiating conductor is made along the meandering form to prolong an electric field, it is possible to easily miniaturize the entire antenna.
Recently, although the miniaturization of vehicle dual-band antennas is in high demand, in the case of miniaturizing the above-mentioned conventional dual-band antenna 1, there is a problem in that a desired bandwidth cannot be secured when using the low band, since antenna devices generally have a feature that a resonant bandwidth is typically narrowed due to the miniaturization. In particular, the resonant bandwidth tends to be narrowed in the case of the low band in which the resonant length is long. Here, the bandwidth is a frequency range at which a return loss (the amount of reflection attenuation) is less than −10 dB, and the dual-band antenna must secure a bandwidth wider than an using frequency band with respect to the respective waves of the high band and the low band, which results in obstructing the miniaturization.
Further, in the case in which the radiating conductor for the low band is formed in the meandering form in an effort to miniaturize the dual-band antenna, a reverse current is generated at a close range in a way of a current path of the radiating conductor, and an electric field due to the reverse current is easily offset, such that the radiation efficiency is unavoidably lowered. In addition, since the antenna device generally has a narrower bandwidth following the miniaturization along with lowered radiation efficiency, it is difficult to secure a desired bandwidth if the miniaturization of the dual-band antenna is implemented by making the radiating conductor in the meandering form.
In addition, in the conventional dual-band antenna 1, electric waves having a polarized wave direction parallel to the radiating conductive plate 2 (for example, horizontal polarized wave), as well as electric waves having a polarized wave direction orthogonal to the radiating conductive plate 2 (for example, vertical polarized wave), are radiated, such that the uniformity of polarized wave is lowered. Therefore, gains of electric waves of a specific polarized wave direction are proportionally lowered, which results in a problem in that it is difficult to obtain a high gain.