1. Field of the Invention
This invention relates to a flat antenna, and particularly to a so-called multi-band antenna effective in a plurality of different frequency bands.
2. Description of the Related Art
Antennas in related art include multi-band antennas that handle a UHF signal and an LF signal. (For example refer to JP-A-7-30316, Page 3, 4 and FIG. 2)
As shown in FIG. 2 of JP-A-7-30316, an inner circular antenna element 7 and an outer annular antenna element 8 disposed on the same region as the inner circular antenna element surrounding the inner circular antenna element, and are provided on a circular plate 6 made of a dielectric material. Both of the antenna elements 7 and 8 are used for transmitting UHF signals, and the outer annular antenna element 8 is used for receiving LF signals. Accordingly, the transmission of UHF signals and reception of LF signals, which are in mutually different frequency bands, are possible.
Now, in a case where this kind of idea for multi-band idea is applied to a patch antenna shown in FIGS. 5A and 5B of the drawings accompanying the present specification and a plurality of antenna patterns having different frequency bands are formed on the same plane region on a dielectric substrate in the form of a flat plate having a uniform thickness, it is not possible to obtain a good multi-band antenna showing characteristics corresponding to the required frequency bands.
That is, in flat antennas like the patch antennas shown in FIG. 5A and FIG. 5B, in a case where the desired frequencies to which the antenna is applied are high, lengths A, A′ of the antenna patterns 2A, 2B become correspondingly shorter. Consequently, for example in a case where on a flat plate dielectric substrate 3 having a uniform thickness B suited to the antenna pattern 2A for a low frequency, the antenna pattern 2B for a higher frequency is formed, the length A′ of the high frequency antenna pattern may be smaller than the plate thickness B of the dielectric substrate.
In a case where the thickness B of the dielectric substrate 1 is sufficiently smaller than the length A of the antenna pattern 2A as shown in FIG. 5A, because an electric field E in the plate thickness direction created by received electric waves acts effectively upon the antenna pattern 2A, the electric waves can be efficiently received by the antenna pattern 2A. However, in a case where the thickness B′ of the dielectric substrate 3 is greater than the length A′ of the antenna pattern 2B as shown in FIG. 5B, the electric field E readily deviates from the plate thickness direction, i.e. the direction toward the antenna pattern 2A, radiation losses arise, and efficient reception becomes difficult.
Accordingly, a multi-band flat antenna, which shows excellent radio characteristics in each of multiple different frequency bands, has been awaited.