1. Field of the Invention
The present invention relates to an antenna device capable of transmitting and receiving radio signals in a plurality of frequency bands, and a communication terminal apparatus including such an antenna device.
2. Description of the Related Art
In a communication terminal apparatus including a mobile phone, for example, such a loop antenna as disclosed in Japanese Patent Laying-Open No. 2002-43826 may be utilized. This loop antenna is configured by a looped-shaped conductor having one end as a power feed end and the other end as a ground end, and having an entire length of one wavelength. This loop antenna suppresses gain reduction even when being used in proximity to a human body, and exhibits excellent radiation characteristics.
In recent years, there is a need for a communication terminal apparatus to accommodate a plurality of frequency bands. For example, a communication terminal apparatus accommodating a penta-band of GSM (registered trademark; Global System for Mobile communication) 850, GSM900, GSM1800, GSM1900, and UMTS (Universal Mobile Telecommunications System) is required to accommodate a relatively wider band of 824 to 960 MHz (Low Band) and 1710 to 2170 MHz (High Band).
According to the loop antenna for accommodating such a relatively wider band, as shown in FIG. 1A, three resonances (resonance 1, resonance 2 and resonance 3) are used to cover a plurality of frequency bands. In other words, resonance 1 forms a passband in a Low Band while resonance 2 and resonance 3 form a band in a High Band.
As shown in FIG. 1B, resonance 1 is caused by fundamental waves in the odd mode, and shows a resonance mode having monopole-type current distribution in which the intermediate point of loop antenna 101 is defined as an electric field maximum point. Resonance 2 occurs in the even mode, and shows a resonance mode having dipole-type current distribution in which there are two electric field maximum points on loop antenna 101. Resonance 3 is caused by harmonics in the odd mode, and shows a resonance mode having current distribution as shown in the figure in which there are three electric field maximum points on loop antenna 101. In this case, the “odd mode” represents a mode in the state where the current direction from the power feed end to the radiation element and the current direction from the ground end to the radiation element are aligned with each other. The “even mode” represents a mode in the state where the current direction from the power feed end to the radiation element and the current direction from the ground end to the radiation element are opposite to each other.
The resonance frequency of each resonance can be determined by the size of loop antenna 101. On the other hand, when this resonance frequency is controlled in a matching circuit, it is conceivable to implement a configuration in which an inductance element L1 and an inductance element L2 are loaded at the power feed end and the ground end, respectively, of the antenna, as shown in FIG. 1C.
However, when inductance elements are loaded in this way to adjust the frequency, the amount of change in each resonance frequency is increased as the frequency is higher. In other words, by the method of simply loading an inductance element, it is difficult to independently control the resonance frequency for each resonance mode.