Because of the rapid expansion of the use of wireless communications apparatuses such as cell phones, etc., more frequency band ranges have become used for communications systems. Particularly, increasing numbers of cell phones handling pluralities of transmitting/receiving bands, such as dual-band, triple-band and quad-band cell phones, have recently got used. For example, quad-band cell phones for communications systems in a GSM (registered trademark) 850/900 band, a DCS band, a PCS band and a UMTS band need antennas (multi-band antennas) capable of handling these frequency bands, because the GSM (registered trademark) 850/900 band uses a frequency band of 824-960 MHz, the DCS band uses a frequency band of 1710-1850 MHz, the PCS band uses a frequency band of 1850-1990 MHz, and the UMTS band uses a frequency band of 1920-2170 MHz.
An antenna element (radiation element, radiation electrode, or radiation line, which may be called simply “line”) constituting an antenna usually has resonance in a fundamental frequency (fundamental mode), and resonance in higher frequencies (higher mode). For example, the fundamental mode has a ¼ wavelength, and the higher mode has a ¾ wavelength. When fundamental-mode resonance is obtained, for example, in a GSM (registered trademark) 850/900 band in a multi-band antenna constituted by one antenna element, a DCS band, etc. correspond to higher-mode resonance. However, because the DCS band, the PCS band and the UMTS band have frequencies about 2-2.5 times that of the GSM (registered trademark) band, failing to meet the condition that pluralities of frequency bands have a 1:3 relation, they are not simply applicable to higher-mode resonance. Also, in higher-mode resonance, a bandwidth providing a proper VSWR (voltage standing wave ratio) is narrow.
Because the GSM (registered trademark) 850/900 band has a frequency bandwidth of 136 MHz and a center frequency of 892 MHz, its relative bandwidth is about 15.3% [136 MHz/892 MHz]. Also, because the DCS band, the PCS band and the UMTS Band 1 band have a frequency bandwidth of 460 MHz and a center frequency of 1940 MHz, their relative bandwidth is about 23.7% [460 MHz/1940 MHz]. In such frequency bands, impedance matching is difficult to achieve by resonance with one antenna element, and its bandwidth is insufficient.
Against such problems, JP 10-107671 A proposes an antenna shown in FIG. 35. This antenna comprises a feeding cable 7, a flat radiation plate 4 (antenna element) disposed in parallel to a ground electrode GND, connected to the feeding cable 7 at a feeding point A, and grounded via a short-circuiting pin 8, and a frequency-adjusting means 30 disposed between an open end of the flat radiation plate 4 and the ground electrode GND. As the equivalent circuit of FIG. 36 shows, the frequency-adjusting means 30 comprises a variable capacitance diode CR1, and the control of bias current to the variable capacitance diode CR1 makes it possible to adjust the resonance frequency of the antenna in different frequency bands. The variable capacitance diode may be called “varicap diode” or “varactor diode.”
JP 2002-232232 A discloses, as shown in FIGS. 37 and 38, a multi-band antenna comprising a first antenna element 3 for a first frequency band and a second antenna element 4 for a second frequency band sharing a feeding point A and grounded at one end via a short-circuiting path 8; a metal plate 2 opposing the antenna elements 3, 4 via an insulator 6 and a variable capacitance diode CR1 connected to the metal plate 2, which are disposed between the first and second antenna elements 3, 4 and a ground electrode GND. Because grounded capacitance can be changed by controlling bias current supplied to the variable capacitance diode CR1, this multi-band antenna can be used in pluralities of frequency bands.
The antennas disclosed in JP 10-107671 A and JP 2002-232232 A can be used in pluralities of frequency bands with grounded capacitance changed by a variable capacitance diode disposed in series between the antenna element and the ground electrode. The variable capacitance diode has electrostatic capacitance continuously changing by the application of reverse bias voltage. However, because power consumption and battery voltage have been reduced in mobile communications apparatuses such as cell phones, etc., resulting in smaller change width of voltage applied to variable capacitance diodes, the mere arrangement of a variable capacitance diode between an antenna element and a ground electrode restricts the variation range of electrostatic capacitance, so that tuning in a desired range is likely difficult. Also, the change of electrostatic capacitance is not inversely proportional to voltage applied, making the adjustment of resonance frequency also difficult.
Further, the antenna disclosed in JP 2002-232232 A comprising pluralities of antenna elements arranged on a plane and a metal plate 2 opposing the antenna elements via an insulator 6 suffer the problem of a large size.
As another example of multi-band antennas comprising pluralities of antenna elements, JP 2005-150937 A discloses, as shown in FIG. 39, an antenna comprising an antenna element 4 connected to a feeding point, a parasitic antenna element 5 electromagnetically-coupled to the antenna element 4, a ground-side electrode 21 between an open end K of the antenna element 4 and a ground electrode GND, and a switch means 22 for switching the connection of the ground-side electrode 21 to the ground electrode GND. With a resonance frequency in a fundamental frequency band based on the operation of the antenna element 4 variable depending on electrostatic capacitance between the ground-side electrode 21 and the open end K of the antenna element 4, higher frequency bands are expanded by multi-resonance with the parasitic antenna element 5. Also proposed is the adjustment of a resonance frequency according to a frequency used, by changing the capacitance of a variable capacitance diode disposed between the open end K of the antenna element 4 and the ground electrode GND. Thus, this antenna is operable as a multi-band antenna by the action of an antenna element and a parasitic antenna element electromagnetically-coupled to the antenna element, with a resonance frequency variable by changing electrostatic capacitance between the open end of the antenna element and a ground electrode. However, this antenna comprising an antenna element electromagnetically coupled to a parasitic antenna element suffers the problem that its VSWR characteristics are likely to deteriorate because the change of the resonance frequency of a low-frequency band leads to the change of the resonance frequency of a higher frequency band. Also, because the antenna element and the parasitic antenna element are arranged on the same plane, the antenna is disadvantageously large.