ANTENNA DEVICE AND COMMUNICATION TERMINAL DEVICE

An antenna device includes first and second antennas. The first antenna includes a first radiating element connected to a feed circuit to supply a radio-frequency signal, and a first coil connected between the first radiating element and the feed circuit. The second antenna includes a second coil magnetically coupled to the first coil, and a second radiating element connected to the second coil. An impedance of the first antenna is higher than about 50Ω at a resonant frequency of a fundamental of the second antenna.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to antenna devices and communication terminal devices.

2. Description of the Related Art

In recent years, communication terminal devices have been developed that are caused to operate in a plurality of frequency bands. An antenna device used in such communication terminal devices includes two antennas that are coupled directly or indirectly to each other because the bandwidth of an available frequency band has to be broadened. International Publication No. WO 2019/208297, discloses an antenna device in which two antennas, which are a feed antenna that is supplied with power and a parasitic antenna that is not supplied with power, are coupled.

In the antenna device disclosed in International Publication No. WO 2019/208297, in a case of the use, for example, in Low-Band (about 0.7 GHz to about 0.96 GHz) used in LTE (Long Term Evolution), a resonant frequency is not very high, and thus a sufficient antenna length (length of a radiating element) can be provided.

However, the antenna length is reduced as the resonant frequency increases. For this reason, in a case of the use, for example, in a frequency band (about 3.3 GHz to about 5.0 GHz) used in the fifth generation mobile communication system (5G), in the antenna device, the length of a parasitic antenna that is not supplied with power is reduced, and sufficient radiation efficiency is not achieved in some cases.

SUMMARY OF THE INVENTION

Example embodiments of the present invention provide antenna devices in each of which the bandwidth of a frequency band that is used is broadened and sufficient radiation efficiency is achieved, and communication terminal devices.

An antenna device according to an example embodiment of the present invention includes a first antenna and a second antenna. The first antenna includes a first radiating element connected to a feed circuit that supplies a radio-frequency signal, and a first coil connected between the first radiating element and the feed circuit. The second antenna includes a second coil magnetically coupled to the first coil, and a second radiating element connected to the second coil. An impedance of the first antenna is higher than about 50Ω at a resonant frequency of a fundamental of the second antenna.

A communication terminal device according to an example embodiment of the present invention includes a feed circuit and an antenna device according to an example embodiment of the present invention.

In the antenna devices according to example embodiments of the present invention, since an impedance of a first antenna is higher than about 50Ω at a resonant frequency of a fundamental of the second antenna, a bandwidth of a frequency band that is used is able to be broadened, and sufficient radiation efficiency is able to be achieved.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

Example embodiments of the present invention will be described in detail below with reference to the drawings. The same or corresponding elements or portions in the drawings are denoted by the same reference signs and a repeated description thereof is not provided.

Example Embodiment

FIG.1is a circuit diagram of an antenna device100according to an example embodiment of the present invention. The antenna device100includes a first antenna ANT1and a second antenna ANT2. The first antenna ANT1includes a first radiating element11connected to a feed circuit30, and a first coil L1connected between the first radiating element11and the feed circuit30. The second antenna ANT2includes a second coil L2that magnetically couples to the first coil L1, and a second radiating element12connected to the second coil L2. Thus, the first antenna ANT1is a feed antenna that is supplied with power by the feed circuit30, and the second antenna ANT2is a parasitic antenna that is not connected to the feed circuit30and is not supplied with power by the feed circuit30.

In the first antenna ANT1that is supplied with power, a capacitor13connected in series with the first antenna ANT1, and a coil14with one end connected to the first antenna ANT1are provided. The other end of the coil14is connected to a GND. The capacitor13and the coil14define an LC circuit and function as a filter circuit of the first antenna ANT1. If the filter circuit is unnecessary in the first antenna ANT1, the capacitor13and the coil14do not have to be provided.

In the antenna device100, the first antenna ANT1and the second antenna ANT2are magnetically coupled to each other by the first coil L1and the second coil L2to broaden the bandwidth of an available frequency band. That is, the first coil L1and the second coil L2define an antenna coupling element20.

The antenna coupling element20is a rectangular or substantially rectangular parallelepiped-shaped chip component mounted to a circuit board in an electronic device. For example, in a case where the antenna coupling element20includes a resin multilayer substrate, insulating bases are made of Liquid Crystal Polymer (LCP) sheets. The antenna coupling element20is formed by laminating insulating bases in which copper foil has been patterned into conductive traces of the first coil L1and the second coil L2. Alternatively, for example, in a case where the antenna coupling element20includes a ceramic multilayer substrate, insulating bases are made of Low Temperature Co-fired Ceramics (LTCC). The antenna coupling element20is formed by laminating insulating bases in which copper paste has been printed and patterned into conductive traces of the first coil L1and the second coil L2. Furthermore, the antenna coupling element20is not limited to the ceramic multilayer substrate and may be formed, for example, by repeating application of insulating paste mainly containing glass by screen-printing. Thus, the insulating bases are a non-magnetic material (are not magnetic ferrite), and, as a result, the antenna coupling element20can be used as a transformer with a predetermined inductance and a predetermined coupling coefficient.

As described above, when two radiating elements (the first radiating element11and the second radiating element12) are connected with the antenna coupling element20, the antenna device100can cover a wide band. However, for example, in a case where a frequency band (about 3.3 GHz to about 5.0 GHz) used in the fifth generation mobile communication system (5G), or a frequency band of a 5 GHz band wireless LAN or the like is covered by the antenna device100, the second antenna ANT2has to be caused to resonate at a high resonant frequency, and the length of the second radiating element12is reduced. However, when the length of the second radiating element12is reduced, sufficient radiation efficiency is not achieved in some cases.

Thus, the second antenna ANT2is not caused to resonate at a resonant frequency of its fundamental but is caused to resonate at a resonant frequency of its harmonic, thus making it possible to increase the length of the second radiating element12. That is, when a resonant frequency at which the second antenna ANT2is desired to resonate is, for example, about 4.4 GHz, in a case where the second antenna ANT2is caused to resonate at a fundamental of about 4.4 GHz, the length of the second radiating element12is reduced. However, in a case where the second antenna ANT2is caused to resonate at a third harmonic of about 4.4 GHz, a resonant frequency of the fundamental is about 1.4 GHz. When the length of the second radiating element12is defined by the resonant frequency of the fundamental of about 1.4 GHz, the length of the radiating element can be increased in comparison with a case where the length is defined by a resonant frequency of the fundamental of about 4.4 GHz. In the antenna device100, the second antenna ANT2is caused to resonate at a resonant frequency of its harmonic, thus making it possible to increase the length of the second radiating element12and achieve sufficient radiation efficiency.

Specifically, the lengths of the first radiating element11and the second radiating element12will be described.FIG.2is a schematic diagram illustrating a communication terminal device according to the present example embodiment. The communication terminal device illustrated inFIG.2is a mobile terminal200capable of performing communication in, for example, a band including n78 (about 3.3 GHz to about 3.8 GHz) and a band including n79 (about 4.4 GHz to about 4.9 GHz). For this reason, in the mobile terminal200, the antenna device100including the first antenna ANT1that is excited in the band including n78 and the second antenna ANT2that is excited in the band including n79 is provided. The mobile terminal200is, for example, a mobile phone, smartphone, or tablet.

In the antenna device100, the antenna coupling element20is provided at a back side of a substrate210subjected to patterning of the first radiating element11and the second radiating element12, and the antenna coupling element20couples the first antenna ANT1and the second antenna ANT2. Furthermore, although the first antenna ANT1is electrically connected to the feed circuit30with a line, which is not illustrated, the second antenna ANT2is not electrically connected to the feed circuit30.

As illustrated inFIG.2, the first radiating element11is defined by a linear conductive trace extending leftward inFIG.2from the antenna coupling element20. Furthermore, the second radiating element12is defined by a linear conductive trace extending rightward inFIG.2from the antenna coupling element20and folded back leftward inFIG.2at some point in the trace. Both the first radiating element11and the second radiating element12define and function as a monopole antenna.

Since the first antenna ANT1is excited in the band including n78 and the second antenna ANT2is excited in the band including n79, in a case where the first antenna ANT1and the second antenna ANT2are caused to resonate at resonant frequencies of respective fundamentals, the length of the first radiating element11is longer than the length of the second radiating element12. However, since the second antenna ANT2is caused to resonate at a resonant frequency of its third harmonic, the length of the second radiating element12is longer than the length of the first radiating element11as illustrated inFIG.2.

Even in a case where the second antenna ANT2is caused to resonate at a resonant frequency of its harmonic, the second antenna ANT2resonates at a resonant frequency of its fundamental. If a frequency band covered by the antenna device100does not include the resonant frequency of the fundamental of the second antenna ANT2, when the second antenna ANT2resonates at the resonant frequency of the fundamental, the antenna device100receives an interference wave in a frequency band other than the covered frequency band, resulting in deterioration of communication performance.

Thus, in the antenna device100, an impedance is adjusted so that resonance does not occur at the resonant frequency of the fundamental of the second antenna ANT2. Specifically, in the antenna device100, an impedance of the first antenna ANT1is adjusted so as to exceed, for example, about 50Ω at the resonant frequency of the fundamental of the second antenna ANT2. A figure of about 50Ω is a reference value used in typical antenna design. When the impedance is higher than this, a value of current that flows through circuitry decreases. Furthermore, in typical antenna design, an impedance on an input side of the feed circuit30also corresponds to the vicinity of 50Ω. Thus, a case where “a target impedance is higher than about 50Ω” corresponds to a case where “a target impedance is higher than an impedance on the input side of the feed circuit30”. Furthermore, in the present description, as for comparison between magnitudes of impedances, a comparison is performed by using an absolute value in which a real part and an imaginary part of an impedance are taken into consideration.

Here,FIG.3is a schematic diagram illustrating a configuration in a case where an impedance of the first antenna ANT1according to the present example embodiment is measured. An impedance of the first antenna ANT1is measured in a first measurement state in which, as illustrated inFIG.3, a network analyzer50is connected to the first coil L1in place of the feed circuit30with a measurement cable51. At this time, if the first coil L1and the second coil L2have been integrated into a single element and are difficult to separate, the second radiating element12and the GND are disconnected from the second coil L2to open-circuit both ends of the second coil L2. In the first measurement state, a connection point between the first coil L1and the measurement cable51is used as a measurement point t1, and frequency characteristics of the impedance of the first antenna ANT1are measured by the network analyzer50. As for a phase shift based on the length of the measurement cable51, calibration is performed in advance using the connection point between the measurement cable51and the first coil L1, which is a primary coil of the transformer.

Furthermore,FIG.4is a schematic diagram illustrating a configuration in a case where an impedance of the second antenna ANT2according to the present example embodiment is measured. A resonant frequency of a fundamental of the second antenna ANT2is measured in a second measurement state in which, as illustrated inFIG.4, the network analyzer50is connected to the second coil L2with a measurement cable52instead of being connected to the GND. At this time, if the first coil L1and the second coil L2have been integrated into a single element and are difficult to separate, the first radiating element11and the feed circuit30are disconnected from the first coil L1to open-circuit both ends of the first coil L1. In the second measurement state, a connection point between the second coil L2and the measurement cable52is used as a measurement point t2, and frequency characteristics of the fundamental of the second antenna ANT2are measured by the network analyzer50. Through this measurement, a resonant frequency of the parasitic element can be determined. As for a phase shift based on the length of the measurement cable52, calibration is performed in advance using the connection point between the measurement cable52and the second coil L2, which is a secondary coil of the transformer. Through the measurements in the above-described first and second measurement states, an impedance of the first antenna ANT1at the resonant frequency of the fundamental of the second antenna ANT2can be determined.

In the antenna device100, when the impedance of the first antenna ANT1is higher than about 50Ω at the resonant frequency of the fundamental of the second antenna ANT2, a current that flows through the first coil L1(the primary coil of the transformer) decreases at the resonant frequency of the fundamental of the second antenna ANT2. For this reason, an induced electromotive force generated in the second coil L2(the secondary coil of the transformer) also decreases. Even when the second antenna ANT2itself resonates at the resonant frequency of the fundamental, the entire antenna device100does not resonate at the resonant frequency of the fundamental of the second antenna ANT2.

FIG.5is a graph illustrating frequency characteristics of reflection coefficients of the antenna device100according to the present example embodiment. InFIG.5, the horizontal axis represents frequency, and the vertical axis represents reflection coefficient. Here, a reflection coefficient A is as seen from the feed circuit30toward the antenna coupling element20inFIG.1(that is, a reflection coefficient of the antenna device100). Furthermore, a reflection coefficient B is as seen from the first coil L1toward the first radiating element11inFIG.1(that is, a reflection coefficient of the first antenna ANT1).

In the reflection coefficient A, resonance occurs at a resonant frequency of a fundamental of the first antenna ANT1(a resonant frequency based on the first coil L1and the first radiating element11) in the band including n78, and resonance occurs at a resonant frequency (for example, about 4.8 GHz) of a third harmonic based on the second antenna ANT2in the band including n79. On the other hand, in the reflection coefficient B, resonance occurs at the resonant frequency of the fundamental of the first antenna ANT1in the band including n78. That is, in the antenna device100, when the second antenna ANT2is coupled to the first antenna ANT1with the antenna coupling element20, the bandwidth can be broadened to include the band including n79. When the impedance is adjusted as described above in the antenna device100, no resonance is observed at the resonant frequency (for example, about 1.5 GHz) of the fundamental of the second antenna ANT2in the reflection coefficient A. That is, the antenna device100does not operate at the resonant frequency of the fundamental of the second antenna ANT2.

FIG.6is a graph for explaining radiation efficiency of the antenna device100according to the present example embodiment. InFIG.6, the horizontal axis represents frequency, and the vertical axis represents radiation efficiency. InFIG.6, a characteristic C represents frequency characteristics of radiation efficiency of the antenna device100exhibited when the second antenna ANT2is caused to resonate at a resonant frequency of its third harmonic, and a characteristic D represents frequency characteristics of radiation efficiency of the antenna device100exhibited when the second antenna ANT2is caused to resonate at a resonant frequency of its fundamental. When the second antenna ANT2is caused to resonate at the resonant frequency of the third harmonic, the length of the second radiating element12can be increased, and thus the antenna device100is improved in terms of radiation efficiency in the band including n79 as indicated by the characteristic C.

Next, impedance adjustment will be described in which the antenna device100is not caused to operate at the resonant frequency of the fundamental of the second antenna ANT2.FIGS.7A and7Binclude Smith charts when an impedance of the antenna device100according to the present example embodiment is adjusted to a first state.FIGS.8A and8Binclude graphs for explaining antenna characteristics exhibited when the impedance of the antenna device100according to the present example embodiment is adjusted to the first state.

The Smith charts illustrated inFIGS.7A and7Brepresent a case where a phase shifter (not illustrated) is provided between the first coil L1and the first radiating element11of the antenna device100illustrated inFIG.1to adjust the impedance of the first antenna ANT1to a first state. Here, the first state is a state in which a phase of a reflection coefficient of the first antenna ANT1based on the feed circuit30in the first measurement state is positioned in the vicinity of about 180 degrees by the phase shifter. The phase of the reflection coefficient refers to an angle counterclockwise from an X axis on the right side of the center of a Smith chart to a line connecting a point at a target frequency and the center.

Similarly,FIGS.9A and9Binclude Smith charts when an impedance of the antenna device100according to the present example embodiment is adjusted to a second state.FIGS.10A and10Binclude graphs for explaining antenna characteristics exhibited when the impedance of the antenna device100according to the present example embodiment is adjusted to the second state. Here, the second state is a state in which the phase of the reflection coefficient of the first antenna ANT1based on the feed circuit30in the first measurement state is positioned in the vicinity of about 90 degrees by the phase shifter. Furthermore, the second state is also a state in which a phase difference between a voltage and a current is situated in the vicinity of about 90 degrees.

FIGS.11A and11Binclude Smith charts when an impedance of the antenna device100according to the present example embodiment is adjusted to a third state.FIGS.12A and12Binclude graphs for explaining antenna characteristics exhibited when the impedance of the antenna device100according to the present example embodiment is adjusted to the third state. Here, the third state is a state in which the phase of the reflection coefficient of the first antenna ANT1based on the feed circuit30in the first measurement state is positioned in the vicinity of about 0 (zero) degrees by the phase shifter. Furthermore, the third state is also a state in which a phase difference between a voltage and a current is situated in the vicinity of about 180 degrees.

FIGS.13A and13Binclude Smith charts when an impedance of the antenna device100according to the present example embodiment is adjusted to a fourth state.FIGS.14A and14Binclude graphs for explaining antenna characteristics exhibited when the impedance of the antenna device100according to the present example embodiment is adjusted to the fourth state. Here, the fourth state is a state in which the phase of the reflection coefficient of the first antenna ANT1based on the feed circuit30in the first measurement state is positioned in the vicinity of about −90 degrees by the phase shifter. Furthermore, the fourth state is also a state in which a phase difference between a voltage and a current is situated in the vicinity of about −90 degrees.

InFIGS.7A to14B, to explain the relationship between an impedance of the first antenna ANT1and characteristics of the antenna device100, the phase shifter is provided in the antenna device100to change the impedance of the first antenna ANT1. However, the phase shifter does not necessarily have to be provided in the antenna device100according to the present example embodiment. That is, in the antenna device100according to the present example embodiment, the phase shifter does not have to be provided as long as the impedance is adjusted by a circuit configuration so that the antenna device100does not operate at the resonant frequency of the fundamental of the second antenna ANT2.

Table 1 represents changes in the impedance of the first antenna ANT1at the resonant frequency of the fundamental of the second antenna ANT2. Table 1 represents changes in the impedance of the first antenna ANT1exhibited when the phase (difference) of the reflection coefficient is changed to about 180 degrees, about 90 degrees, about 0 degrees, and about −90 degrees. In Table 1, Re{Z} denotes a real part of the impedance, and Im{Z} denotes an imaginary part of the impedance. Furthermore, in Table 1, all impedances on the input side of the feed circuit30are about 50.0Ω. As for comparison between magnitudes of impedances, a comparison is performed by using an absolute value in which a real part and an imaginary part of an impedance are taken into consideration.

FIGS.7A,9A,11A, and13Arepresent, on the Smith charts, the locus of an impedance as seen from a first radiating element11side looking toward the individual first antenna ANT1to which the second antenna ANT2is not coupled. A mark m1denotes the impedance of the first antenna ANT1at the resonant frequency of the fundamental of the second antenna ANT2. As illustrated inFIGS.7A,9A,11A, and13A, the mark m1moves clockwise as the phase difference of the reflection coefficient is changed to about 180 degrees, about 90 degrees, about 0 degrees, and about −90 degrees. The Smith charts illustrated inFIGS.7A,9A,11A, and13Aare normalized by using about 50Ω, and thus the impedance of the first antenna ANT1is higher than about 50Ω at phase differences of about 90 degrees or less and about −90 degrees or more of the reflection coefficient. In other words, when a point at a target frequency exists in the right half of each Smith chart, the impedance of the first antenna ANT1is higher than about 50Ω.

FIGS.7B,9B,11B, and13Brepresent, on the Smith charts, the locus of an impedance as seen from a feed circuit30side looking toward the entire antenna device100to which the second antenna ANT2is coupled. A mark m2denotes the impedance of the entire antenna device100at the resonant frequency of the fundamental of the second antenna ANT2. As illustrated inFIGS.7B,9B,11B, and13B, the mark m2moves clockwise as the phase difference of the reflection coefficient is changed to about 180 degrees, about 90 degrees, about 0 degrees, and about −90 degrees. The Smith charts illustrated inFIGS.7B,9B,11B, and13Bare normalized by using about 50Ω, and thus the impedance of the entire antenna device100is higher than about 50Ω at phase differences of about 90 degrees or less and about 270 degrees or more. In other words, when a point at a target frequency exists in the right half of each Smith chart, the impedance of the entire antenna device100is higher than about 50Ω.

FIGS.8A,10A,12A, and14Aare graphs illustrating frequency characteristics of the reflection coefficient of the antenna device100exhibited when the phase difference of the reflection coefficient is changed to about 180 degrees, about 90 degrees, about 0 degrees, and about −90 degrees. InFIGS.8A,10A,12A, and14A, the horizontal axis represents frequency, and the vertical axis represents reflection coefficient. A mark m3denotes a peak of the reflection coefficient of the antenna device100at the resonant frequency of the fundamental of the second antenna ANT2. The peak of the reflection coefficient denoted by the mark m3decreases as the phase difference is changed to about 180 degrees, about 90 degrees, and about 0 degrees, and the peak increases as the phase difference is changed to about 0 degrees and about −90 degrees.

FIGS.8B,10B,12B, and14Bare graphs illustrating frequency characteristics of the amount of current that flows through the antenna device100exhibited when the phase difference of the reflection coefficient is changed to about 180 degrees, about 90 degrees, about 0 degrees, and about −90 degrees. InFIGS.8B,10B,12B, and14B, the horizontal axis represents frequency, and the vertical axis represents the amount of current. A characteristic E represents frequency characteristics of the amount of current that flows through the first antenna ANT1, a characteristic F represents frequency characteristics of the amount of current that flows through the second antenna ANT2, and a characteristic G represents frequency characteristics of the amount of current that flows through the entire antenna device100. A mark m4denotes the amount of current that flows through the second antenna ANT2at the resonant frequency of the fundamental of the second antenna ANT2. A region surrounded by a dashed line refers to a range of the resonant frequency of the fundamental of the second antenna ANT2. A mark m5denotes the amount of current that flows through the second antenna ANT2at the resonant frequency of the third harmonic of the second antenna ANT2. The amount of current that flows through the second antenna ANT2denoted by each of the marks m4and m5decreases as the phase difference is changed to about 180 degrees, about 90 degrees, and about 0 degrees, and the amount of current increases as the phase difference is changed to about 0 degrees and about −90 degrees. In particular, when the phase difference of the reflection coefficient is about 0 (zero) degrees, the amount of current that flows through the second antenna ANT2denoted by the mark m4is substantially 0 (zero).

As illustrated inFIGS.8B,10B, and12B, the amount of current (mark m4) that flows through the second antenna ANT2decreases as the phase difference of the reflection coefficient is changed to about 180 degrees, about 90 degrees, and about 0 degrees. On the other hand, as illustrated inFIGS.7B,9B, and11B, the impedance (mark m2) of the entire antenna device100increases as the phase difference is changed to about 180 degrees, about 90 degrees, and about 0 degrees. That is, in the antenna device100, when the impedance is increased at the resonant frequency of the fundamental of the second antenna ANT2, a current that flows through the first coil L1(the primary coil of the transformer) decreases, and thus an induced electromotive force generated in the second coil L2(the secondary coil of the transformer) also decreases.

As illustrated inFIGS.8B,10B, and12B, when the amount of current (mark m4) that flows through the second antenna ANT2decreases as the phase difference of the reflection coefficient is changed to about 180 degrees, about 90 degrees, and about 0 degrees, the peak (mark m3) of the reflection coefficient illustrated inFIGS.8A,10A, and12Adecreases. That is, when an induced electromotive force generated in the second coil L2(the secondary coil of the transformer) at the resonant frequency of the fundamental of the second antenna ANT2decreases, the antenna device100does not resonate at the resonant frequency of the fundamental of the second antenna ANT2.

On the other hand, as illustrated inFIGS.8B,10B, and12B, although the amount of current (mark m5) that flows through the second antenna ANT2also decreases as the phase difference of the reflection coefficient is changed to about 180 degrees, about 90 degrees, and about 0 degrees, the amount of current does not reach substantially 0 (zero) as the mark m4does. For this reason, although an induced electromotive force generated in the second coil L2(the secondary coil of the transformer) at the resonant frequency of the third harmonic of the second antenna ANT2also decreases, the antenna device100resonates at the resonant frequency of the third harmonic of the second antenna ANT2.

As described above, in the antenna device100, the impedance of the first antenna ANT1is adjusted so as to exceed about 50Ω at the resonant frequency of the fundamental of the second antenna ANT2. That is, in the antenna device100, the impedance of the first antenna ANT1is adjusted to be located in the right half of a Smith chart (phase differences of about 90 degrees or less and about 270 degrees or more). Thus, the antenna device100makes it difficult for resonance to occur at the resonant frequency of the fundamental of the second antenna ANT2, enabling an improvement in communication performance.

As described above, the antenna device100according to the present example embodiment includes the first antenna ANT1and the second antenna ANT2. The first antenna ANT1includes the first radiating element11connected to the feed circuit30that supplies a radio-frequency signal, and the first coil L1connected between the first radiating element11and the feed circuit30. The second antenna ANT2includes the second coil L2that magnetically couples to the first coil L1, and the second radiating element12connected to the second coil L2. The impedance of the first antenna ANT1is higher than about 50Ω at the resonant frequency of the fundamental of the second antenna ANT2.

Thus, in the antenna device100according to the present example embodiment, when the impedance of the first antenna ANT1is adjusted, the bandwidth of a frequency band that is used can be broadened, and sufficient radiation efficiency can also be achieved.

The resonant frequency of the fundamental of the first antenna ANT1is preferably lower than a resonant frequency of a harmonic of the second antenna ANT2. This can broaden the bandwidth of a frequency band that is used.

The length of the second radiating element12is preferably longer than the length of the first radiating element11. This can improve radiation efficiency of the second antenna ANT2.

The mobile terminal200(communication terminal device) includes the feed circuit30, and the above-described antenna device100. Thus, the mobile terminal200can perform stable communication in a wide band.

FIG.15is a circuit diagram of an antenna device100A according to Modification 1 of an example embodiment of the present invention. In the antenna device100A illustrated inFIG.15, components that are the same or substantially the same as those in the antenna device100illustrated inFIG.1are denoted by the same reference signs and a detailed description thereof is not repeated.

The antenna device100A includes the first antenna ANT1and the second antenna ANT2. The first antenna ANT1includes the first radiating element11connected to the feed circuit30, and the first coil L1connected between the first radiating element11and the feed circuit30. The second antenna ANT2includes the second coil L2that magnetically couples to the first coil L1, and the second radiating element12connected to the second coil L2. Furthermore, in the antenna device100A, an impedance matching element16is connected between the second coil L2and the second radiating element12.

When the impedance matching element16is provided in the second antenna ANT2, an impedance can be increased at the resonant frequency of the fundamental of the second antenna ANT2. When the impedance is increased by the impedance matching element16, the amount of current that flows through the second antenna ANT2at the resonant frequency of the fundamental of the second antenna ANT2can be further reduced.

As described above, the antenna device100A according to Modification 1 further includes the impedance matching element16(second impedance matching element) connected between the second coil L2and the second radiating element12. Thus, in the antenna device100A according to Modification 1, the amount of current that flows through the second antenna ANT2at the resonant frequency of the fundamental of the second antenna ANT2can be further reduced.

FIG.16is a circuit diagram of an antenna device100B according to Modification 2 of an example embodiment of the present invention. In the antenna device100B illustrated inFIG.16, components that are the same or substantially the same as those in the antenna device100illustrated inFIG.1are denoted by the same reference signs and a detailed description thereof is not repeated.

The antenna device100B includes the first antenna ANT1and the second antenna ANT2. The first antenna ANT1includes the first radiating element11connected to the feed circuit30, and the first coil L1connected between the first radiating element11and the feed circuit30. The second antenna ANT2includes the second coil L2that magnetically couples to the first coil L1, and the second radiating element12connected to the second coil L2. Furthermore, in the antenna device100B, an impedance matching element17is connected between the first coil L1and the feed circuit30.

When the impedance matching element17is provided in the first antenna ANT1, a current that flows through the first coil L1(the primary coil of the transformer) at the resonant frequency of the fundamental of the second antenna ANT2can be shunted. When the current that flows through the first coil L1(the primary coil of the transformer) is shunted by the impedance matching element17, the amount of current that flows through the second antenna ANT2at the resonant frequency of the fundamental of the second antenna ANT2can be further reduced. At this time, the impedance matching element17is a portion of the first antenna ANT1, and an impedance of the first antenna ANT1including the impedance matching element17is measured.

As described above, the antenna device100B according to Modification 2 further includes the impedance matching element17(first impedance matching element) connected between the first coil L1and the feed circuit30. Thus, in the antenna device100B according to Modification 2, the amount of current that flows through the second antenna ANT2at the resonant frequency of the fundamental of the second antenna ANT2can be further reduced.

FIG.17is a circuit diagram of an antenna device100C according to Modification 3 of an example embodiment of the present invention. In the antenna device100C illustrated inFIG.17, components that are the same or substantially the same as those in the antenna device100illustrated inFIG.1are denoted by the same reference signs and a detailed description thereof is not repeated.

The antenna device100C includes the first antenna ANT1and the second antenna ANT2. The first antenna ANT1includes the first radiating element11connected to the feed circuit30, and the first coil L1connected between the first radiating element11and the feed circuit30. The second antenna ANT2includes the second coil L2that magnetically couples to the first coil L1, and the second radiating element12connected to the second coil L2. Furthermore, in the antenna device100C, a capacitor18is connected in parallel with the first coil L1.

In a case where the second antenna ANT2is caused to resonate at a resonant frequency of its third harmonic, when the capacitor18is connected in parallel with the first coil L1, a signal with a resonant frequency of a harmonic equal to or higher than a fifth harmonic can be caused to bypass the transformer (the first coil L1). Thus, resonance can be prevented from occurring at a resonant frequency of a harmonic equal to or higher than an unwanted fifth harmonic in the second antenna ANT2.

As described above, the antenna device100C according to Modification 3 further includes the capacitor18connected in parallel with the first coil L1. Thus, in the antenna device100C according to Modification 3, resonance can be prevented from occurring at a resonant frequency of a harmonic equal to or higher than the unwanted fifth harmonic in the second antenna ANT2.

FIG.18is a circuit diagram of an antenna device100D according to Modification 4 of an example embodiment of the present invention. In the antenna device100D illustrated inFIG.18, components that are the same or substantially the same as those in the antenna device100illustrated inFIG.1are denoted by the same reference signs and a detailed description thereof is not repeated.

In the antenna device100D, the first radiating element11and the second radiating element12are not provided in or on the same substrate. The first radiating element11is provided in or on a sub-substrate70different from a main substrate60where the second radiating element12is provided. In or on the main substrate60, the feed circuit30connected to the first radiating element11, the first coil L1connected between the first radiating element11and the feed circuit30, the second coil L2that magnetically couples to the first coil L1, and the second radiating element12connected to the second coil L2are provided. The first coil L1and the second coil L2define the antenna coupling element20.

The first radiating element11provided in or on the sub-substrate70and the first coil L1provided in or on the main substrate60are connected with a coaxial cable80. The first radiating element11is provided in or on the sub-substrate70and is connected to the main substrate60via the coaxial cable80, thus increasing flexibility in the disposition of the first radiating element11. On the other hand, the second radiating element12, which is a parasitic element, is provided in or on the main substrate60, and thus the second radiating element12can radiate a radio wave from the second coil L2without the radio wave passing through a long transmission line. Consequently, transmission loss can be reduced in the second radiating element12.

A transmission line connecting the main substrate60and the sub-substrate70is not limited to the coaxial cable80and may be defined, for example, by a Printed Circuit Board (PCB), Flexible Printed Circuits (FPC), and a MetroCirc (registered trademark). The MetroCirc (registered trademark) is a resin multilayer substrate including, for example, a plurality of Liquid Crystal Polymer (LCP) sheets that are laminated.

As described above, in the antenna device100D according to Modification 4, the first radiating element11and the first coil L1are connected with the coaxial cable80, and the first radiating element11can be provided in or on the sub-substrate70. The main substrate60and the sub-substrate70are examples, and a substrate where the first radiating element11is provided only has to differ from a substrate where the second radiating element12is provided.

FIG.19is a circuit diagram of an antenna device100E according to Modification 5 of an example embodiment of the present invention. In the antenna device100E illustrated inFIG.19, components that are the same or substantially the same as those in the antenna device100illustrated inFIG.1and the antenna device100D illustrated inFIG.18are denoted by the same reference signs and a detailed description thereof is not repeated.

In the antenna device100E, an impedance matching element19is connected between the first coil L1and a point to which the coaxial cable80is connected. When the impedance matching element19(third impedance matching element) is provided between the first coil L1and the point to which the coaxial cable80is connected, impedance matching between a circuit including the main substrate60and the coaxial cable80can be provided, enabling a reduction in transmission loss in the coaxial cable80, which is a long transmission line.

Other Modifications

The antenna device100according to an example embodiment has been described above in which the first antenna ANT1is caused to resonate at a resonant frequency of its fundamental and the second antenna ANT2is caused to resonate at a resonant frequency of its third harmonic. However, the antenna device100is not limited to this. In the antenna device100, the first antenna ANT1may be caused to resonate at a resonant frequency of its third harmonic, and the second antenna ANT2may be caused to resonate at a resonant frequency of its third harmonic. In a case where the first antenna ANT1is caused to resonate at a resonant frequency of its third harmonic, the first antenna ANT1may also be caused to resonate at a resonant frequency of its fundamental. Thus, in the antenna device100, a frequency band including the resonant frequency of the fundamental of the first antenna ANT1can also be used.

The antenna device100according to an example embodiment has been described above in which the second antenna ANT2is caused to resonate at a resonant frequency of its third harmonic. However, the antenna device100is not limited to this. In the antenna device100, the second antenna ANT2may be caused to resonate at a resonant frequency of a wave other than the third harmonic. The length of a radiating element increases as a resonant frequency decreases. For this reason, if the second antenna ANT2is caused to resonate at a resonant frequency of a harmonic higher than the third harmonic, a resonant frequency of its fundamental further decreases, and thus the length of the second radiating element12can be further increased.

In the antenna device100according to an example embodiment described above, while the first antenna ANT1is excited in the band including n78, the second antenna ANT2is excited in the band including n79 of higher frequencies. However, the antenna device100is not limited to this. In the antenna device100, while the first antenna ANT1is excited in the band including n78, the second antenna ANT2may be excited in a band of lower frequencies. Furthermore, the antenna coupling element20may have additive polarity or subtractive polarity.