In general, a monopole antenna where a wire element having a length of ¼ of an antenna operating wavelength is disposed on a base plate is used as a line-shaped antenna. In addition, in order to obtain the monopole antenna having a small size and a low profile, an inverted L-shaped antenna has been developed by folding and bending a middle portion of the monopole antenna.
However, in the inverted L-shaped antenna, since a reactance section defined by a length of a horizontal portion of the antenna element parallel to the base plate has a large capacitive value, it is difficult to obtain matching at a feed line of 50Ω. Therefore, in order to facilitate the matching between the antenna element and the feed line having 50Ω, there is proposed an inverted F-shaped antenna. The inverted F-shaped antenna includes a stub for connecting the base plate to a radiation element in the vicinity of the feed point disposed at a middle portion of the antenna element. By doing so, the capacitive value caused from the reactance section, it is possible to easily obtain matching to the feed line having 50Ω (see, for example, “Illustrated Antenna System”, by Hujimoto Kyohei, October 1996, p. 118-119, Sougou Denshi Publishing Company).
In addition, for example, in a communication apparatus such as a mobile phone, a communication control circuit is disposed in an inner portion of a case, and an antenna device is disposed in an inner portion of an antenna receiving portion provided to protrude from the case.
However, recently, a mobile phone coping with multi-band has been provided, so that a characteristic for multiple frequencies is required for a built-in antenna device used for the mobile phone. As a general provided one, there are a dual band mobile phone for GSM (Global System for Mobile Communication) using a band of 900 MHz and DCS (Digital Cellular System) using 1.8 GHz in Europe and a dual band mobile phone for AMPS (Advanced Mobile Phone Service) using a band of 800 MHz and PCS (Personal Communication Services) using a band of 1.9 GHz band. As a built-in antenna device used for the mobile phone coping with the dual bands, antennas manufactured by modifying a planar inverted F-shaped antenna or an inverted F-shaped antenna are widely used.
Conventionally, as such an antenna device, there is proposed an antenna device constructed by forming a slit in a radiation plate on a plate of a planar inverted F-shaped antenna and dividing the radiation plate into first and second radiation plates, thereby performing resonance with a frequency corresponding to a wavelength which is about ¼ of path lengths (see, for example, Japanese Unexamined Patent Application Publication No. 10-93332 (FIG. 2)).
In addition, there is proposed an antenna device constructed by disposing an non-excitation electrode in the vicinity of an inverted F-shaped antenna disposed on a conductor plane and generating even and odd modes, thereby performing resonance with a frequency corresponding to a wavelength which is about ¼ of lengths of radiation conductors (see, for example, Japanese Unexamined Patent Application publication No. 9-326632 (FIG. 2)).
In addition, there is proposed an antenna device using line-shaped first inverted L-shaped antenna element and second inverted L-shaped antenna element, thereby performing resonance with two different frequencies (see, for example, Japanese Unexamined Patent Application publication No. 2002-185238 (FIG. 2)). In the antenna device, a length of a radiation conductor needs to be about ⅛ to ⅜ with respect to the resonance frequency.
In addition, in an antenna device, there is the following Formula 1 as a relation between a size of an antenna element and antenna characteristics (see “New Antenna Engineering”, by Hiroyuki, September 1996, p. 108-109, Sougou Denshi Publishing Company).(Electrical Volume of Antenna)/(Band)×(Gain)×(Efficiency)=Constant Value  (Formula 1)
In Formula 1, the constant value is a value defined according to a type of an antenna.