Antenna device and communication terminal device

Abstract

Provided are an antenna device and a communication terminal device that accommodate broadening of a frequency band or accommodate a plurality of frequency bands without any degradation in antenna characteristics. An antenna device (100) according to the present disclosure comprises: a first radiation element (11) to which a power supply circuit (30) is connected; a second radiation element (12); a first coil (L1) connected to the first radiation element (11); a second coil (L2) connected between the second radiation element (12) and a ground electrode, the second coil (L2) being electromagnetically coupled to the first coil (L1); and a capacitor (C2) connected in parallel to the second coil (L2), the capacitor (C2) constituting a parallel resonance circuit with the second coil (L2). The resonance frequency of the parallel resonance circuit is within a band of fundamental wave resonance or harmonic resonance of the first radiation element (11).

Description

Antenna device and communication terminal device

[0001] The present disclosure relates to technologies for an antenna device and a communication terminal device.

[0002] In recent years, communication terminal devices have been using antenna devices with two radiating elements that are directly or indirectly coupled to each other in order to widen the frequency band or to support multiple frequency bands. Specifically, Japanese Patent No. 6760545 (Patent Document 1) discloses an antenna device in which two radiating elements are magnetically coupled using a transformer element.

[0003] Patent No. 6760545

[0004] In the antenna device disclosed in Patent Document 1, a first radiating element connected to a feed circuit and a second radiating element not connected to the feed circuit are magnetically coupled using a transformer element. However, in the antenna device disclosed in Patent Document 1, current from the feed circuit not only flows to the first radiating element but also may flow into the second radiating element via the ground electrode (ground substrate). If current from the feed circuit flows into the second radiating element, there is a risk of a decrease in antenna characteristics (e.g., radiation efficiency).

[0005] The present disclosure has been made to solve such problems, and its purpose is to provide an antenna device and a communication terminal device that can widen the frequency band or support multiple frequency bands without degrading the antenna characteristics.

[0006] An antenna device according to the present disclosure includes a first radiating element connected to a feeder circuit, a second radiating element, a first coil connected to the first radiating element, a second coil connected between the second radiating element and a ground electrode and electromagnetically coupled to the first coil, and a capacitance circuit connected in parallel to the second coil and forming a parallel resonant circuit with the second coil, wherein the resonant frequency of the parallel resonant circuit is a frequency within the fundamental resonance or harmonic resonance band of the first radiating element.

[0007] A communication terminal device according to the present disclosure includes the above antenna device and a power supply circuit.

[0008] The antenna device according to the present disclosure includes a capacitance circuit that forms a parallel resonant circuit with a second coil, and the resonant frequency of the parallel resonant circuit is a frequency within the fundamental or harmonic resonance band of the first radiating element, thereby making it possible to widen the frequency band or accommodate multiple frequency bands without degrading the antenna characteristics.

[0009] FIG. 1 is a circuit diagram of an antenna device according to a first embodiment. FIG. 2 is a schematic diagram showing a communication terminal device according to the first embodiment. FIG. 3 is a diagram showing frequency characteristics of radiation efficiency of the antenna device according to the first embodiment. FIG. 4 is a diagram for explaining the relationship between the polarity of a transformer element and the resonant frequency of the antenna device. FIG. 5 is a diagram for explaining the relationship between the polarity of a transformer element and the resonant frequency of the antenna device. FIG. 6 is a schematic diagram of an antenna device according to a first modification. FIG. 7 is a schematic diagram of an antenna device according to a second modification. FIG. 8 is a schematic diagram of an antenna device according to a third modification. FIG. 9 is a schematic diagram of another antenna device according to the third embodiment.

[0010] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals, and description thereof will not be repeated.

[0011] 1 is a circuit diagram of an antenna device 100 according to Embodiment 1. The antenna device 100 includes a first antenna and a second antenna. The first antenna includes a first radiating element 11 to which a power feed circuit 30 is connected, and a first coil L1 connected in series with the power feed circuit 30 between the first radiating element 11 and a ground electrode.

[0012] The second antenna includes a second radiating element 12, a second coil L2 connected in series with the second radiating element 12, and a capacitor C2 (capacitance circuit) connected in parallel to the second coil L2. The second coil L2 and the capacitor C2 form a parallel resonant circuit 25. The first coil L1 and the second coil L2 are magnetically coupled. In the antenna device 100, the first antenna functions as a power-fed antenna fed by a power feed circuit 30, and the second antenna functions as a parasitic antenna not fed by the power feed circuit 30.

[0013] The first coil L1 and the second coil L2 are mounted on the antenna device 100 as, for example, a transformer element 20. The transformer element 20 is, for example, a rectangular parallelepiped chip component. The transformer element 20 is configured by forming a partial conductor pattern of the first coil L1 and the second coil L2 on each insulating base material (e.g., liquid crystal polymer, low-temperature co-fired ceramics, etc.) and then laminating the insulating base materials. The transformer element 20 may be configured as a separate component from the second coil L2 and the capacitor C2 that constitutes the parallel resonant circuit 25, or may be configured as a single component including the capacitor C2.

[0014] Antenna devices implemented in mobile terminals such as smartphones combine a powered antenna and a parasitic antenna using a transformer element to broaden the frequency band or support multiple frequency bands. In other words, the antenna device constitutes a transformer-coupled multi-band communication antenna. However, in an antenna device in which a first radiating element and a second radiating element are coupled by a transformer element, current from the power feed circuit may flow directly into the second radiating element via the ground electrode. If current from the power feed circuit flows directly into the second radiating element, the current flowing through the first radiating element may decrease, resulting in a deterioration in antenna characteristics (e.g., radiation efficiency).

[0015] Therefore, in the antenna device 100 according to this embodiment, the second coil L2 is connected in parallel to the capacitor C2, and the second coil L2 and the capacitor C2 form a parallel resonant circuit 25. The parallel resonant circuit 25 is configured so that its resonant frequency is within the fundamental resonance or harmonic resonance band of the first radiating element 11. By setting the parallel resonant frequency of the second coil L2 and the capacitor C2 to a frequency within the fundamental resonance or harmonic resonance band of the first radiating element 11, the parallel resonant circuit 25 blocks current of that frequency, suppressing the current flowing from the power feed circuit 30 to the second radiating element 12 and allowing the current from the power feed circuit 30 to flow efficiently to the first radiating element 11.

[0016] Specifically, a case where the antenna device 100 is mounted in a communication terminal device will be described. Fig. 2 is a schematic diagram showing the communication terminal device 1000 according to the first embodiment. The communication terminal device 1000 shown in Fig. 2 is capable of communication in a band including approximately 1.0 GHz and a band including approximately 2.2 GHz, for example. The communication terminal device 1000 is, for example, a smartphone, and has a substrate 10 on which the antenna device 100 is mounted in part of its housing.

[0017] The first radiating element 11, the second radiating element 12, the transformer element 20, and the power feed circuit 30 that constitute the antenna device 100 are mounted on the substrate 10. The first radiating element 11 and the second radiating element 12 are magnetically coupled via the transformer element 20, as shown in Fig. 1. One end of the first radiating element 11 is connected to the ground electrode GND via the transformer element 20 and the power feed circuit 30. One end of the second radiating element 12 is connected to the ground electrode GND via the transformer element 20.

[0018] 1, the transformer element 20 includes the parallel resonant circuit 25, which suppresses the current flowing from the feed circuit 30 to the second radiating element 12 and allows the current from the feed circuit 30 to flow efficiently to the first radiating element 11. Therefore, the antenna device 100 can widen the frequency band or support multiple frequency bands without degrading the antenna characteristics.

[0019] FIG. 3 is a diagram showing the frequency characteristics of the radiation efficiency of the antenna device 100 according to the first embodiment. In FIG. 3, the horizontal axis represents frequency, and the vertical axis represents radiation efficiency. The frequency characteristics of the radiation efficiency shown in FIG. 3 are the results of a simulation performed on the configuration of the antenna device 100 shown in FIG. 2. Specifically, the simulation of the antenna device 100 was performed with the first coil L1 set to 1 nH, the second coil L2 set to 5 nH, the coupling coefficient k set to -0.5, and the capacitor C2 set to 1.5 pF. Note that the simulation was performed with the first antenna having an inductance (17 nH) and a capacitance (2 pF) connected in series to the first radiating element 11, in addition to the first coil L1.

[0020] 3, the solid line graph A shows the frequency characteristics of the radiation efficiency of the antenna device 100, and the dashed line graph B shows the frequency characteristics of the radiation efficiency of an antenna device that does not include capacitor C2. In graph B, the radiation efficiency in the band including approximately 2.4 GHz is reduced due to the influence of the current flowing from the feed circuit 30 to the second radiating element 12. On the other hand, in graph A, the radiation efficiency in the band including approximately 2.4 GHz is not reduced because the current flowing from the feed circuit 30 to the second radiating element 12 is suppressed by the parallel resonant circuit 25.

[0021] This is because the fundamental resonance frequency of the first radiating element 11 shown in Fig. 3 is approximately 0.8 GHz, and the resonant frequency of the parallel resonant circuit 25 of the antenna device 100 is adjusted to approximately 2.4 GHz, which is the harmonic resonance frequency three times that of the first radiating element 11. By adjusting the resonant frequency of the parallel resonant circuit 25 formed by the second coil L2 and capacitor C2 to a frequency within the harmonic resonance band three times that of the first radiating element 11, the radiation efficiency of the antenna device 100 can be maintained high without decreasing in the band including approximately 2.4 GHz as shown in Fig. 3.

[0022] The fundamental resonance frequency of the second radiating element 12 shown in Fig. 3 is adjusted to be within the fundamental resonance band of the first radiating element 11. Therefore, graph A shown in Fig. 3 shows high radiation efficiency in a band including approximately 0.8 GHz. Note that adjusting the fundamental resonance frequency of the second radiating element 12 to be within the fundamental resonance band of the first radiating element 11 is just an example, and the second radiating element 12 may be adjusted to a different frequency. Furthermore, the resonant frequency of the parallel resonant circuit 25 is not limited to a frequency within the triple harmonic resonance band of the first radiating element 11, and may be a frequency within another harmonic resonance band or a frequency within the fundamental resonance band.

[0023] In the antenna device 100, to address the insufficient bandwidth of the first antenna, a second antenna is provided and the two antennas are coupled by a transformer element 20 to broaden the bandwidth. When the antenna device 100 is actually used, the area for forming the radiating elements is limited, as shown in Figure 2, and the first radiating element 11 and the second radiating element 12 are close to each other. In this case, it is necessary to design the device so that the antenna characteristics do not deteriorate due to interference between the first antenna and the second antenna.

[0024] Specifically, to prevent interference between the first antenna and the second antenna, the polarity of the transformer element must be determined so that the phases of the current flowing through the first coil L1 and the current flowing through the second coil L2 of the transformer element 20 are not opposite to each other. Figures 4 and 5 are diagrams for explaining the relationship between the polarity of the transformer element and the resonant frequency of the antenna device.

[0025] 4 and 5 does not include a capacitor C2 for the sake of simplicity, and the first radiating element 11 and the second radiating element 12 are coupled by a transformer element 20. Note that the impedance of the first radiating element 11 is Z １ The current flowing through the first radiating element 11 is I １ The voltage of the first coil L1 is V １ , the voltage of the power supply circuit 30 is V ｓ In addition, the impedance of the second radiating element 12 is Z ２ The current flowing through the second radiating element 12 is I ２ The voltage of the second coil L2 is V２ Furthermore, the mutual inductance between the first coil L1 and the second coil L2 is M, and when the polarity of the transformer element is depolarized, it is +M, and when it is additive, it is −M.

[0026] 4 and 5 , the polarity of the transformer element is depolarized because the conductors of the first coil L1 and the second coil L2 are wound so that the direction of magnetic flux generated in the first coil L1 when current flows from the first radiating element 11 to the feed circuit 30 is opposite to the direction of magnetic flux generated in the second coil L2 when current flows from the second radiating element 12 to the ground electrode. Also, the polarity of the transformer element is additive because the conductors of the first coil L1 and the second coil L2 are wound so that the direction of magnetic flux generated in the first coil L1 when current flows from the first radiating element 11 to the feed circuit 30 is the same as the direction of magnetic flux generated in the second coil L2 when current flows from the second radiating element 12 to the ground electrode.

[0027] When defined as above, the current I of the second radiating element 12 ２ The current I of the first radiating element 11 １ It can be seen from equation (1) that the relationship is determined by the polarity of the transformer element and the impedance of the second coil L2.

[0028]

[0029] The impedance Z of the second radiating element 12, which is a non-power-fed antenna, at the resonant frequency of the first radiating element 11, which is a power-fed antenna, is ２ is the resonant frequency f of the first radiating element 11 １ Therefore, the resonant frequency f ２ When is large (f １ <f ２ ), and has a capacitive property. Therefore, the impedance Z ２ can be expressed as in equation (2). Here, C in equation (2) is the capacitance component in the equivalent circuit of the second antenna, which is a parasitic antenna. Note that the resistance component of the equivalent circuit is ignored.

[0030]

[0031] By applying equation (2) to equation (1), the current I of the second radiating element 12 is calculated as follows: ２ The current I of the first radiating element 11 １ The relationship can be expressed as equation (3).

[0032]

[0033] From equation (3), the current I of the second radiating element 12 is ２ The current I of the first radiating element 11 １ As shown in equation (4), the phase θ of the first radiating element 11 is 0° in the case of depolarization and 180° in the case of additive polarization. １ Therefore, the resonant frequency f ２ When is large (f １ <f ２ ), it is preferable to use a depolarizing transformer element 20 that has the same phase as the first antenna 11 so that the first antenna 11 and the second antenna 21 do not interfere with each other. １ Therefore, the resonant frequency f ２ When is large (f １ <f ２ ) are shown together with the polarities of the transformer elements 20.

[0034]

[0035] On the other hand, the impedance Z of the second radiating element 12 at the resonant frequency of the first radiating element 11 is ２ is the resonant frequency f of the first radiating element 11 １ Therefore, the resonant frequency f ２ When is small (f １ >f ２ ), and has inductive properties. Therefore, the impedance Z ２ can be expressed as in equation (5). Here, L in equation (5) is the inductance component in the equivalent circuit of the second antenna, which is a parasitic antenna. Note that the resistance component of the equivalent circuit is ignored.

[0036]

[0037] By applying equation (5) to equation (1), the current I of the second radiating element 12 is calculated as follows: ２ The current I of the first radiating element 11 １ The relationship can be expressed as equation (6).

[0038]

[0039] From equation (6), the current I of the second radiating element 12 is ２ The current I of the first radiating element 11 １ As shown in equation (7), the phase θ of the first radiating element 11 is 180° in the case of depolarization and 0° in the case of additive polarization. １ Therefore, the resonant frequency f ２ When is small (f １ >f ２ ), it is preferable to use a transformer element 20 with additive polarity that has the same phase so as to prevent interference between the first antenna and the second antenna. １ Therefore, the resonant frequency f ２ When is small (f １ >f ２ ) are shown together with the polarities of the transformer elements 20.

[0040]

[0041] In the above, it is assumed that the first antenna and the second antenna are completely independent, and the current I ２ The current I of the first radiating element 11 １ However, if the first and second antennas are not completely independent and there is coupling between the antennas, the phase relationship can be determined by combining the phase relationship derived above with the phase relationship that occurs due to coupling between the antennas.

[0042] (Modifications) In the above description, the types of the first antenna and the second antenna of the antenna device 100 are not particularly limited. Below, a modification in which the types of the first antenna and the second antenna in the antenna device are limited will be described. Note that the limitations on the types of the first antenna and the second antenna described below can be similarly applied to other embodiments.

[0043] FIG. 6 is a schematic diagram of an antenna device 100a according to Modification 1. Note that in the antenna device 100a shown in FIG. 6, the same components as those in the antenna device 100 shown in FIG. 1 are designated by the same reference numerals, and detailed description thereof will not be repeated. In the antenna device 100a, the first radiating element 11a forms a monopole antenna, and the second radiating element 12a also forms a monopole antenna. The antenna device 100a has a basic antenna configuration, and by coupling two monopole antennas with wide frequency bands, the frequency band can be further widened. Note that the resonant frequency of the parallel resonant circuit 25 formed by the second coil L2 and capacitor C2 is within the frequency band of the power supply circuit 30 that supplies current to the first radiating element 11a (the fundamental resonant frequency of the first radiating element 11a).

[0044] FIG. 7 is a schematic diagram of an antenna device 100b according to Modification 2. In the antenna device 100b shown in FIG. 7, the same components as those in the antenna device 100 shown in FIG. 1 are designated by the same reference numerals and detailed descriptions thereof will not be repeated. In the antenna device 100b, the first radiating element 11b forms an inverted-F antenna (IFA), and the second radiating element 12b forms a monopole antenna. Because the first radiating element 11b is an inverted-F antenna, the antenna device 100b offers improved design flexibility. Furthermore, the first radiating element 11b has a path that connects it to a ground electrode midway from the power feed circuit 30 to the end of the first radiating element 11b, thereby minimizing external influences such as those from the human body. The resonant frequency of the parallel resonant circuit 25, comprised of the second coil L2 and capacitor C2, is within the frequency band of the power feed circuit 30 that supplies current to the first radiating element 11b (the fundamental resonant frequency of the first radiating element 11b).

[0045] FIG. 8 is a schematic diagram of an antenna device 100c according to Modification 3. In the antenna device 100c shown in FIG. 8, the same components as those in the antenna device 100 shown in FIG. 1 are designated by the same reference numerals, and detailed description thereof will not be repeated. In the antenna device 100c, the first radiating element 11c constitutes an inverted-F antenna, and the second radiating element 12c constitutes a monopole antenna. Unlike the first radiating element 11b of the inverted-F antenna shown in FIG. 7, the first radiating element 11c has a first coil L1 connected to a path that is not connected to the power feed circuit 30. In other words, the first coil L1 is connected in parallel to the power feed circuit 30 between the first radiating element 11c and the ground electrode. Therefore, the first coil L1 is not present in the path connecting the first radiating element 11c and the power feed circuit 30, thereby reducing power loss due to the first coil L1.

[0046] The antenna device 100c has an inverted-F antenna for the first radiating element 11c, which allows for greater design flexibility. Furthermore, the first radiating element 11c has a path that is grounded to a ground electrode separate from the path from the feeder circuit 30 to the end of the first radiating element 11c, thereby minimizing external influences such as those from the human body. The resonant frequency of the parallel resonant circuit 25, comprised of the second coil L2 and capacitor C2, is within the frequency band of the feeder circuit 30 that supplies current to the first radiating element 11c (the fundamental resonant frequency of the first radiating element 11c).

[0047] FIG. 9 is a schematic diagram of an antenna device 100d according to a fourth modification. In the antenna device 100d shown in FIG. 9, the same components as those in the antenna device 100 shown in FIG. 1 are designated by the same reference numerals, and detailed description thereof will not be repeated. In the antenna device 100d, the first radiating element 11d forms a monopole antenna, and the second radiating element 12d forms an inverted-F antenna. The second radiating element 12d of the antenna device 100d is an inverted-F antenna, which improves design flexibility. In particular, by providing an inductance element or a capacitance element in the path from the feed circuit 30 to the end of the first radiating element 11b and grounded to the ground electrode, the phase relationship of the current in the first radiating element 11d relative to the current in the second radiating element 12d can be adjusted.

[0048] Furthermore, the first radiating element 11d has a path that is grounded to the ground electrode midway from the power feeder circuit 30 to the end of the first radiating element 11d, thereby suppressing external influences such as from the human body. The resonant frequency of the parallel resonant circuit 25 formed by the second coil L2 and capacitor C2 is within the frequency band of the power feeder circuit 30 that supplies current to the first radiating element 11d (the fundamental resonant frequency of the first radiating element 11b).

[0049] FIG. 10 is a schematic diagram of an antenna device 100e according to Modification 5. In the antenna device 100e shown in FIG. 10, the same components as those in the antenna device 100 shown in FIG. 1 are designated by the same reference numerals, and detailed description thereof will not be repeated. In the antenna device 100e, the first radiating element 11e forms a monopole antenna, and the second radiating element 12e also forms a monopole antenna. By providing a parallel resonant circuit 25 formed by the second coil L2 and the capacitor C2, the antenna device 100e can suppress the influence of the second radiating element 12e on the first radiating element 11e, thereby enabling the second radiating element 12e to be positioned close to the first radiating element 11e, as shown in FIG. 10. The resonant frequency of the parallel resonant circuit 25 formed by the second coil L2 and the capacitor C2 is within the frequency band of the power supply circuit 30 that supplies current to the first radiating element 11d (the fundamental resonant frequency of the first radiating element 11d).

[0050] [Embodiment 2] In the above-described embodiment, the first radiating element 11 is connected to the ground electrode via the first coil L1 and the feed circuit 30, and the second radiating element 12 is connected to the ground electrode via the second coil L2. However, a matching circuit may be provided in one or both of the first antenna including the first radiating element and the second antenna including the second radiating element to match the impedance between them. Fig. 11 is a circuit diagram of an antenna device 200 according to embodiment 2. Note that in the antenna device 200 shown in Fig. 11, the same components as those in the antenna device 100 shown in Fig. 1 are designated by the same reference numerals, and detailed description thereof will not be repeated.

[0051] The antenna device 200 includes a first antenna and a second antenna. The first antenna includes a first radiating element 11 to which a power feed circuit 30 is connected, and a first coil L1 and a matching circuit 41 (first matching circuit) connected in series between the first radiating element 11 and the power feed circuit 30.

[0052] The second antenna includes a second radiating element 12, a second coil L2 and a matching circuit 42 (second matching circuit) connected in series with the second radiating element 12, and a capacitor C2 (capacitance circuit) connected in parallel to the second coil L2. The second coil L2 and the capacitor C2 form a parallel resonant circuit 25. The first coil L1 and the second coil L2 are magnetically coupled.

[0053] In the antenna device 200, the first antenna functions as a powered antenna fed by the power feed circuit 30, and the second antenna functions as a parasitic antenna not fed by the power feed circuit 30. The impedance of the first antenna and the impedance of the second antenna are matched using matching circuits 41 and 42. The matching circuits 41 and 42 are inductors, capacitors, or circuits including these.

[0054] The antenna device 200 can optimize the isolation between the first radiating element 11 and the second radiating element 12 by matching the impedance of the first antenna with the impedance of the second antenna. Note that the antenna device 200 can appropriately employ the configuration described in the first embodiment.

[0055] [Embodiment 3] In the above-described embodiment, the capacitance of the capacitor C2 connected in parallel to the second coil L2 has been described as a fixed value. However, a capacitance circuit capable of varying the capacitance may be employed instead of the capacitor C2. Fig. 12 is a circuit diagram of an antenna device 300 according to the third embodiment. Note that in the antenna device 300 shown in Fig. 12, the same components as those in the antenna device 100 shown in Fig. 1 are designated by the same reference numerals, and detailed description thereof will not be repeated.

[0056] The antenna device 300 includes a first antenna and a second antenna. The first antenna includes a first radiating element 11 to which a power supply circuit 30 is connected, and a first coil L1 connected in series between the first radiating element 11 and the power supply circuit 30.

[0057] The second antenna includes a second radiating element 12, a second coil L2 connected in series with the second radiating element 12, and a variable capacitance element Ca (capacitance circuit) connected in parallel to the second coil L2. The second coil L2 and the variable capacitance element Ca form a parallel resonant circuit 25. The first coil L1 and the second coil L2 are magnetically coupled.

[0058] In the antenna device 300, the first antenna functions as a powered antenna fed by the power feed circuit 30, and the second antenna functions as a parasitic antenna not fed by the power feed circuit 30. The variable capacitance element Ca can vary its capacitance, thereby adjusting the resonant frequency of the parallel resonant circuit 25. By adjusting the resonant frequency of the parallel resonant circuit 25, the antenna device 300 can block the current flowing from the power feed circuit 30 to the second radiating element 12 over a wider band, thereby broadening the frequency band.

[0059] Furthermore, if a control circuit for adjusting the capacitance of the variable capacitance element Ca can be provided, it is possible to actively adjust the resonant frequency of the parallel resonant circuit 25. For example, the antenna device 300 can adjust the frequency band in which no current flows to the second radiating element 12 by temporally adjusting the capacitance of the variable capacitance element Ca using the control circuit.

[0060] The capacitance circuit capable of varying the capacitance is not limited to the variable capacitance element Ca, but may be a capacitance circuit in which a plurality of capacitors with different capacitances are switched using a switch element. Fig. 13 is a circuit diagram of another antenna device 300a according to the third embodiment. In the antenna device 300a shown in Fig. 13, the same components as those in the antenna device 100 shown in Fig. 1 are designated by the same reference numerals, and detailed description thereof will not be repeated.

[0061] The antenna device 300a includes a first antenna and a second antenna. The first antenna includes a first radiating element 11 to which a power supply circuit 30 is connected, and a first coil L1 connected in series between the first radiating element 11 and the power supply circuit 30.

[0062] The second antenna includes a second radiating element 12, a second coil L2 connected in series with the second radiating element 12, and a capacitance circuit Cb connected in parallel to the second coil L2. The second coil L2 and the capacitance circuit Cb form a parallel resonant circuit 25. The first coil L1 and the second coil L2 are magnetically coupled.

[0063] In the antenna device 300a, the first antenna functions as a powered antenna fed by the power feed circuit 30, and the second antenna functions as a parasitic antenna not fed by the power feed circuit 30. The capacitance circuit Cb includes a plurality of capacitors Cb1 with different capacitances and a switch element Cb2 electrically connected to one of the plurality of capacitors Cb1. The capacitance circuit Cb can vary the capacitance by switching the electrically connected capacitor Cb1 using the switch element Cb2. By varying the capacitance, the capacitance circuit Cb can adjust the resonant frequency of the parallel resonant circuit 25. By adjusting the resonant frequency of the parallel resonant circuit 25, the antenna device 300a can block the current flowing from the power feed circuit 30 to the second radiating element 12 over a wider band, thereby broadening the frequency band.

[0064] Furthermore, if a control circuit for adjusting the capacitance of the capacitance circuit Cb can be provided, it is possible to actively adjust the resonant frequency of the parallel resonant circuit 25. For example, the antenna device 300 can adjust the frequency band in which no current flows to the second radiating element 12 by adjusting the capacitance of the capacitance circuit Cb by temporally switching the capacitor Cb1 electrically connected by the switch element Cb2 using the control circuit.

[0065] The antenna devices 300 and 300a may employ the configurations described in the first and second embodiments as appropriate.

[0066] (Aspects) (1) An antenna device according to the present disclosure includes a first radiating element to which a power supply circuit is connected, a second radiating element, a first coil connected to the first radiating element, a second coil connected between the second radiating element and a ground electrode and electromagnetically coupled to the first coil, and a capacitance circuit connected in parallel to the second coil and forming a parallel resonant circuit with the second coil, wherein the resonant frequency of the parallel resonant circuit is a frequency within the fundamental or harmonic resonance band of the first radiating element.

[0067] (2) In the antenna device described in (1), the resonant frequency of the parallel resonant circuit is a frequency within a band of harmonic resonance that is three times that of the first radiating element.

[0068] (3) In the antenna device according to (1) or (2), the frequency of the fundamental resonance of the second radiating element is a frequency within the band of the fundamental resonance of the first radiating element.

[0069] (4) In the antenna device described in any one of (1) to (3), the conductors of the first coil and the second coil are wound so that the direction of the magnetic flux generated in the first coil when a current flows from the first radiating element toward the power supply circuit is opposite to the direction of the magnetic flux generated in the second coil when a current flows from the second radiating element toward the ground electrode.

[0070] (5) In the antenna device described in any one of (1) to (3), the conductors of the first coil and the second coil are wound so that the direction of the magnetic flux generated in the first coil when a current flows from the first radiating element toward the power supply circuit is the same as the direction of the magnetic flux generated in the second coil when a current flows from the second radiating element toward the ground electrode.

[0071] (6) The antenna device according to any one of (1) to (5), further comprising a first matching circuit connected between the first coil and the power supply circuit.

[0072] (7) The antenna device according to any one of (1) to (6) further comprises a second matching circuit connected between the second coil and the ground electrode and connected in series with the second coil and the capacitance circuit.

[0073] (8) In the antenna device according to any one of (1) to (7), the capacitance circuit is a variable capacitance element.

[0074] (9) In the antenna device described in any one of (1) to (7), the capacitance circuit includes a plurality of capacitors having different capacitances and a switch element electrically connected to one of the plurality of capacitors.

[0075] (10) In the antenna device according to any one of (1) to (9), the first coil is connected in series with the feed circuit between the first radiating element and the ground electrode.

[0076] (11) In the antenna device according to any one of (1) to (9), the first coil is connected in parallel to the feed circuit between the first radiating element and the ground electrode.

[0077] (12) A communication terminal device according to the present disclosure includes the antenna device according to any one of (1) to (11) and a power supply circuit that supplies a current to the first radiating element.

[0078] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the description of the above embodiments, and is intended to include all modifications within the meaning and scope of the claims.

[0079] 10 substrate, 11, 11a to 11e first radiating element, 12, 12a to 12e second radiating element, 20 transformer element, 30 power supply circuit, 100, 100a to 100e, 200, 300, 300a antenna device, 1000 communication terminal device.

Claims

1. An antenna device comprising: a first radiating element to which a power supply circuit is connected; a second radiating element; a first coil connected to the first radiating element; a second coil connected between the second radiating element and a ground electrode and electromagnetically coupled to the first coil; and a capacitance circuit connected in parallel to the second coil and forming a parallel resonant circuit with the second coil, wherein the resonant frequency of the parallel resonant circuit is a frequency within the fundamental resonance or harmonic resonance band of the first radiating element.

2. The antenna device according to claim 1, wherein the resonant frequency of the parallel resonant circuit is a frequency within a band of triple harmonic resonance of the first radiating element.

3. The antenna device according to claim 1 or 2, wherein the fundamental resonance frequency of the second radiating element is within the fundamental resonance band of the first radiating element.

4. An antenna device described in any one of claims 1 to 3, wherein the conductors of the first coil and the second coil are wound so that the direction of magnetic flux generated in the first coil when current flows from the first radiating element to the power supply circuit is opposite to the direction of magnetic flux generated in the second coil when current flows from the second radiating element to the ground electrode.

5. An antenna device described in any one of claims 1 to 3, wherein the conductors of the first coil and the second coil are wound so that the direction of magnetic flux generated in the first coil when current flows from the first radiating element to the power supply circuit is the same as the direction of magnetic flux generated in the second coil when current flows from the second radiating element to the ground electrode.

6. An antenna device according to any one of claims 1 to 5, further comprising a first matching circuit connected between the first coil and the power supply circuit.

7. An antenna device according to any one of claims 1 to 6, further comprising a second matching circuit connected between the second coil and the ground electrode and connected in series with the second coil and the capacitance circuit.

8. The antenna device according to any one of claims 1 to 7, wherein the capacitance circuit is a variable capacitance element.

9. An antenna device according to any one of claims 1 to 7, wherein the capacitance circuit includes a plurality of capacitors having different capacitances, and a switch element electrically connected to one of the plurality of capacitors.

10. An antenna device according to any one of claims 1 to 9, wherein the first coil is connected in series with the power supply circuit between the first radiating element and the ground electrode.

11. An antenna device according to any one of claims 1 to 9, wherein the first coil is connected in parallel with the power supply circuit between the first radiating element and the ground electrode.

12. A communication terminal device comprising: the antenna device according to any one of claims 1 to 11; and the power supply circuit that supplies a current to the first radiating element.

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