In recent years, it has become common for cellular phones to have a radiating element for a low frequency band (0.8 GHz to 0.9 GHz) and a radiating element for a high frequency band (1.7 GHz to 2.0 GHz). An antenna device is known in which a variable matching circuit is inserted between a branching point at which one line branches to two radiating elements, and a transmission/reception circuit (for example, refer to Japanese Unexamined Patent Application Publication No. 2010-81370). One of the radiating elements corresponds to a fundamental frequency band and the other radiating element corresponds to a higher-order frequency band.
The variable matching circuit includes a first matching circuit and a variable capacitance element that is connected in series with the first matching circuit. The first matching circuit includes a grounded inductance element and a capacitance element that is connected in parallel with the grounded inductance element. Even if the capacitance of the variable capacitance element of the variable matching circuit is changed, the resonant frequency in the fundamental frequency band can be easily adjusted without greatly affecting the resonant frequency in the higher-order frequency band.
There are plans to introduce carrier aggregation technology to next generation mobile communication systems. Carrier aggregation technology is a technology for forming a single broad band channel by aggregating carriers of a plurality of non-contiguous frequency bands.
In Japan, examples of combinations of frequency bands that are targets for carrier aggregation include the combination of the 1.5 GHz band and the 2.0 GHz band, the combination of the 0.8 GHz band and the 1.5 GHz band, and the combination of the 0.9 GHz band and the 2.0 GHz band. In the United States of America, examples of combinations of frequency bands that are targets for carrier aggregation include combinations of the 0.7 GHz band and a band in a range from the 1.7 GHz band to the 2.0 GHz band. In this specification, sometimes a band in the range of the 0.8 GHz band to the 0.9 GHz band will be referred to as a low frequency band, the 1.5 GHz band will be referred to as a medium frequency band and a band in the range from the 1.7 GHz band to the 2.0 GHz band will be referred to as a high frequency band. However, this does not mean that bands of the low frequency band, the medium frequency band and the high frequency band are limited to these specific bands. In addition, use of higher frequency bands is also being investigated.
In an antenna device of the related art having two radiating elements, one of which is used for a low frequency band and the other of which is for a high frequency band, it is difficult to simultaneously cover all of the combinations of frequency bands that are targets of carrier aggregation. In order to cover all of the combinations of frequency bands, for example, it might be necessary to prepare three or more radiating elements each having an electrical length that is appropriate for the corresponding frequency band.