Patent Publication Number: US-2022224008-A1

Title: Antenna device and radio communication device including the same

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of International Application No. PCT/JP2020/033117 filed on Sep. 1, 2020 which claims priority from Japanese Patent Application No. 2019-182742 filed on Oct. 3, 2019. The contents of these applications are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND ART 
     Technical Field 
     The present disclosure relates to an antenna device and a radio communication device including the same. 
     For example, Patent Document 1 discloses what is called a dual-band dipole antenna capable of communication at a frequency in a predetermined low-frequency band and at a frequency in a predetermined high-frequency band. To support the dual-band communication, the dipole antenna includes, as a band elimination filter, an LC parallel circuit on the antenna conductor. The LC parallel circuit passes frequencies in the low-frequency band but attenuates frequencies in the high-frequency band. 
     Patent Document 1: U.S. Patent Application Publication No. 2005/0280579 Specification 
     BRIEF SUMMARY 
     A folded antenna such as a folded dipole antenna is known as a downsized antenna. A dual-band antenna can also be downsized likewise. However, the folding has caused the deterioration of antenna efficiency in a high-frequency band on occasions. 
     Hence, the present disclosure addresses reducing the deterioration of antenna efficiency in a high-frequency band in a dual-band antenna device including a folded antenna conductor. 
     To solve the technical problem described above, according to an aspect of the present disclosure, the present disclosure provides an antenna device that is a dual-band antenna device allowed to perform communication at a first frequency in a predetermined frequency band and at a second frequency in a frequency band higher than the predetermined frequency band. The antenna device includes a ground conductor; a folded antenna conductor including a first linear part and a second linear part that are caused to face each other at a distance by folding; an LC resonant circuit that is included in the folded antenna conductor, that passes the first frequency, and that attenuates the second frequency; and a feeding point between the ground conductor and the folded antenna conductor. A narrow gap part is provided between the first linear part and the second linear part of the folded antenna conductor, the narrow gap part measuring a distance shorter than a distance measured in a different portion between the first linear part and the second linear part. 
     According to another aspect of the present disclosure, the present disclosure provides an antenna device that is a dual-band antenna device allowed to perform communication at a first frequency in a predetermined frequency band and at a second frequency in a frequency band higher than the predetermined frequency band. The antenna device includes: a ground conductor; a folded antenna conductor including a first linear part and a second linear part that are caused to face each other at a distance by folding; an LC resonant circuit that is included in the folded antenna conductor, that attenuates the first frequency, and that passes the second frequency; and a feeding point between the ground conductor and the folded antenna conductor. A narrow gap part is provided between the first linear part and the second linear part of the folded antenna conductor, the narrow gap part measuring a distance shorter than a distance measured in a different portion between the first linear part and the second linear part. The LC resonant circuit is included in the narrow gap part. 
     Further, according to another aspect of the present disclosure, the present disclosure provides a radio communication device including: the antenna device; and a feeder circuit that supplies power to the feeding point of the antenna device. 
     According to the present disclosure, the deterioration of antenna efficiency in a high-frequency band can be reduced in the dual-band antenna device including the folded antenna conductor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial top view of a radio communication device including an antenna device according to Embodiment 1 of the present disclosure. 
         FIG. 2  is a graph illustrating the frequency characteristic of the return loss of each of the antenna device according to Embodiment 1 and an antenna device in Comparative Example. 
         FIG. 3  is a graph illustrating antenna efficiency in a high-frequency band of each of the antenna device according to Embodiment 1 and the antenna device in Comparative Example. 
         FIG. 4  is a graph illustrating relationships between a return loss characteristic and branch part widths in each of the antenna device according to Embodiment 1 and the antenna device in Comparative Example. 
         FIG. 5  is a graph illustrating relationships between the return loss characteristic and branch part locations in each of the antenna device according to Embodiment 1 and the antenna device in Comparative Example. 
         FIG. 6  is a partial top view of a radio communication device including an antenna device according to Embodiment 2 of the present disclosure. 
         FIG. 7  is a partial top view of a radio communication device including an antenna device according to Embodiment 3 of the present disclosure. 
         FIG. 8  is a partial top view of a radio communication device including an antenna device according to Embodiment 4 of the present disclosure. 
         FIG. 9  is a partial top view of a radio communication device including an antenna device according to Embodiment 5 of the present disclosure. 
         FIG. 10  is a graph illustrating the frequency characteristic of the return loss of the antenna device according to Embodiment 5. 
     
    
    
     DETAILED DESCRIPTION 
     An antenna device according to an aspect of the present disclosure is a dual-band antenna device allowed to perform communication at a first frequency in a predetermined frequency band and at a second frequency in a frequency band higher than the predetermined frequency band. The antenna device includes a ground conductor; a folded antenna conductor including a first linear part and a second linear part that are caused to face each other at a distance by folding; an LC resonant circuit that is included in the folded antenna conductor, that passes the first frequency, and that attenuates the second frequency; and a feeding point between the ground conductor and the folded antenna conductor. A narrow gap part is provided between the first linear part and the second linear part of the folded antenna conductor, the narrow gap part measuring a distance shorter than a distance measured in a different portion between the first linear part and the second linear part. 
     According to the aspect as above, the deterioration of antenna efficiency in a high-frequency band can be reduced in the dual-band antenna device including the folded antenna conductor. 
     For example, in a case where the first linear part and the second linear part extend parallel to each other, the antenna device may include a branch part that forms a narrow gap part such that one of the first linear part and the second linear part extends toward a different one of the first linear part and the second linear part. 
     For example, the distance between the first linear part and the second linear part can be longer than each of respective line widths of the first linear part and the second linear part. 
     For example, the folded antenna conductor may include a floating-island-like part between the first linear part and the second linear part. The narrow gap part may include a first narrow gap part between the floating-island-like part and the first linear part and a second narrow gap part between the floating-island-like part and the second linear part. 
     For example, the antenna device may further include a capacitor chip included in the narrow gap part and connecting the first linear part and the second linear part. 
     For example, the LC resonant circuit may include the capacitor chip and an inductor chip that are disposed in parallel. 
     For example, the folded antenna conductor may be a folded dipole antenna. 
     For example, the first frequency may be a frequency in a 2.4 GHz band, and the second frequency may be a frequency in a 5 GHz band. 
     An antenna device according to another aspect of the present disclosure is a dual-band antenna device allowed to perform communication at a first frequency in a predetermined frequency band and at a second frequency in a frequency band higher than the predetermined frequency band. The antenna device includes: a ground conductor; a folded antenna conductor including a first linear part and a second linear part that are caused to face each other at a distance by folding; an LC resonant circuit that is included in the folded antenna conductor, that attenuates the first frequency, and that passes the second frequency; and a feeding point between the ground conductor and the folded antenna conductor. A narrow gap part is provided between the first linear part and the second linear part of the folded antenna conductor, the narrow gap part measuring a distance shorter than a distance measured in a different portion between the first linear part and the second linear part. The LC resonant circuit is included in the narrow gap part. 
     According to the aspects as above, the deterioration of the antenna efficiency in the high-frequency band can be reduced in the dual-band antenna device including the folded antenna conductor. 
     A radio communication device according to another aspect of the present disclosure includes the antenna device and a feeding point of the antenna device that supplies power to a feeder circuit. 
     According to the aspect as above, the deterioration of the antenna efficiency in the high-frequency band can be reduced in the dual-band radio communication device including the folded antenna conductor. 
     Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. 
     Embodiment 1 
       FIG. 1  is a partial top view of a radio communication device including an antenna device according to Embodiment 1 of the present disclosure. Note that the X-Y-Z orthogonal coordinate system illustrated in the drawings is provided for easier understanding of the present disclosure and does not limit the disclosure. 
     As illustrated in  FIG. 1 , a radio communication device  50  including an antenna device  10  according to Embodiment 1 is used, being installed in an electronic device capable of radio communication. The antenna device  10  is a dual-band antenna device allowed to perform communication at a first frequency in a predetermined frequency band and at a second frequency in a frequency band higher than the predetermined frequency band. In the case of Embodiment 1, the first frequency is a frequency in a 2.4 GHz band (for example, 2.4 to 2.484 GHz), and the second frequency is a frequency in a 5 GHz band (for example, 5.15 to 5.85 GHz). 
     As illustrated in  FIG. 1 , in the case of Embodiment 1, the antenna device  10  includes a ground conductor  12  that is provided on a base substrate  52  of the radio communication device  50  and a folded antenna conductor  14  that is provided on the base substrate  52  and that is connected to the ground conductor  12 . The antenna device  10  also includes LC resonant circuits  16  included in the folded antenna conductor  14  and a feeding point  18  between the ground conductor  12  and the folded antenna conductor  14 . Note that a feeder circuit (not illustrated) included in the radio communication device  50  is connected to the feeding point  18 . The antenna device  10  receives power from the feeder circuit via the feeding point  18 . 
     In the case of Embodiment 1, the ground conductor  12  of the antenna device  10  is a conductor pattern formed on the base substrate  52  and formed from an insulating material such as copper. 
     In the case of Embodiment 1, the folded antenna conductor  14  of the antenna device  10  is what is called a folded dipole antenna and is a conductor pattern formed from, for example, copper on the base substrate  52 . 
     Specifically, the folded antenna conductor  14  includes a first element part  20  and a second element part  22  in a symmetrical structure (with respect to the Y axis). The folded antenna conductor  14  also includes a parasitic line part  24  and a feeding line part  26  that respectively connect the first element part  20  and the second element part  22  to the ground conductor  12 . 
     The first element part  20  in the folded antenna conductor  14  is connected to an edge  12   a  of the ground conductor  12  (one end in the Y axis) with the parasitic line part  24  interposed therebetween. The first element part  20  also includes a first linear part  20   a  and a second linear part  20   b  that are caused to face each other at a distance by the folding. 
     Specifically, the first element part  20  in the folded antenna conductor  14  extends from the parasitic line part  24  toward an outer side portion (in a negative direction along the X axis) and then extends toward an inner side portion (in a positive direction along the X axis) in such a manner as to change the direction by 180 degrees, that is, being folded. As the result, the first element part  20  includes the first linear part  20   a  and the second linear part  20   b  that face each other at a distance. 
     Note that in the case of Embodiment 1, in the first element part  20 , the first linear part  20   a  and the second linear part  20   b  are parallel to each other, are a distance D 1  spaced, and extend parallel to the edge  12   a  of the ground conductor  12 . The distance D 1  can be longer than each of widths W 1  and W 2  of the respective first and second linear parts  20   a  and  20   b . Unlike this, if the distance D 1  is shorter than each of the widths W 1  and W 2 , a magnetic field generated by current flowing through the first linear part  20   a  hinders the flow of current flowing in an opposite direction through the second linear part  20   b.    
     The second linear part  20   b  of the first element part  20  also includes an open end  20   c . The electrical length of the first element part  20  from the parasitic line part  24  to the open end  20   c  is substantially ¼ the length of the wavelength of the first frequency. 
     The second element part  22  in the folded antenna conductor  14  is connected to the edge  12   a  of the ground conductor  12  with the feeding line part  26  interposed therebetween. The second element part  22  includes a first linear part  22   a  and a second linear part  22   b  that are caused to face each other at a distance by the folding. 
     Specifically, the second element part  22  in the folded antenna conductor  14  extends from the feeding line part  26  toward an outer side portion (in the positive direction along the X axis), then extends toward an inner side portion (in the negative direction along the X axis) in such a manner as to change the direction by 180 degrees, that is, being folded, and terminates. As the result, the second element part  22  includes the first linear part  22   a  and the second linear part  22   b  that face each other at a distance. 
     Note that in the case of Embodiment 1, in the second element part  22 , the first linear part  22   a  and the second linear part  22   b  are parallel to each other, are the distance D 1  spaced, and extend parallel to the edge  12   a  of the ground conductor  12 . The distance D 1  can be longer than each of the widths W 1  and W 2  of the respective first and second linear parts  22   a  and  22   b.    
     The second linear part  22   b  of the second element part  22  includes an open end  22   c . The electrical length of the second element part  22  from the feeding line part  26  to the open end  22   c  is ¼ the length of the wavelength of the first frequency. 
     Further, the first linear part  20   a  of the first element part  20  and the first linear part  22   a  of the second element part  22  are located on one straight line, and the second linear part  20   b  of the first element part  20  and the second linear part  22   b  of the second element part  22  are located on one straight line. 
     Note that in the case of Embodiment 1, the feeding point  18  is provided between the ground conductor  12  and the folded antenna conductor  14 . In the case of Embodiment 1, the feeding point  18  is provided in the connecting part between the ground conductor  12  and the feeding line part  26 . 
     The LC resonant circuits  16  are respectively provided in the first element part  20  and the second element part  22  of the folded antenna conductor  14 . In the case of Embodiment 1, the LC resonant circuits  16  respectively include capacitor chips  28  having predetermined capacitance and inductor chips  30  disposed parallel to the respective capacitor chips  28  and having predetermined inductance. 
     Each LC resonant circuit  16  is an LC parallel circuit that passes the first frequency in the predetermined lower frequency band but attenuates the second frequency in the frequency band higher than the predetermined frequency band, that is, that resonates at the first frequency. The LC resonant circuit  16  is provided in a corresponding one of the first and second element parts  20  and  22  at a position away by ¼ of the wavelength of the second frequency from a corresponding one of the parasitic line part  24  and the feeding line part  26 . 
     According to the antenna device  10  as described above, the first and second element parts  20  and  22  of the folded antenna conductor  14  function as the dipole antenna. In addition, since the first and second element parts  20  and  22  are folded, the antenna device  10  (that is, the radio communication device  50 ) is downsized compared with a case where the first and second element parts  20  and  22  extend on the straight line without necessarily being folded. 
     Further, when communication is performed at the first frequency in the predetermined lower frequency band, current flows through the entire first and second element parts  20  and  22 . In contrast, when communication is performed at the second frequency in the frequency band higher than the predetermined frequency band, current flows through each of portions of the respective first and second element parts  20  and  22  between a corresponding one of the parasitic line part  24  and the feeding line part  26  and the corresponding LC resonant circuit  16 . That is, each LC resonant circuit  16  functions as a band elimination filter for the second frequency. The antenna device  10  functions as the dual-band antenna allowed to perform communication at the first and second frequencies. 
     However, the inventor has found that there is a possibility of deterioration of antenna efficiency at the second frequency in the higher frequency band in the antenna device  10  as described above. The inventor has also identified the cause thereof and found out the following configurations to cope therewith. 
     As illustrated in  FIG. 1 , to reduce the deterioration of the antenna efficiency at the second frequency in the higher frequency band, a narrow gap part  20   d  is provided between the first linear part  20   a  and the second linear part  20   b  of the first element part  20  of the folded antenna conductor  14 , the narrow gap part  20   d  measuring a distance D 2  shorter than the distance D 1  measured in the different portion. Likewise, a narrow gap part  22   d  is provided between the first linear part  22   a  and the second linear part  22   b  of the second element part  22 , the narrow gap part  22   d  measuring the distance D 2  shorter than the distance D 1  measured in the different portion. 
     In the case of Embodiment 1, the first linear part  20   a  of the first element part  20  includes a branch part  20   e  extending toward the second linear part  20   b  and forming the narrow gap part  20   d  between the first linear part  20   a  and the second linear part  20   b . Likewise, the first linear part  22   a  of the second element part  22  includes a branch part  22   e  extending toward the second linear part  22   b  and forming the narrow gap part  22   d  between the first linear part  22   a  and the second linear part  22   b.    
     As illustrated in  FIG. 1 , the branch part  20   e  as described above causes capacitance Cl to be generated between the branch part  20   e  of the first linear part  20   a  of the first element part  20  and the second linear part  20   b . Likewise, the branch part  22   e  causes capacitance Cl to be generated between the branch part  20   e  of the first linear part  22   a  of the second element part  22  and the second linear part  22   b.    
     Advantageous effects exerted by providing the narrow gap parts  20   d  and  22   d  as described above will be described. 
       FIG. 2  is a graph illustrating the frequency characteristic of the return loss of each of the antenna device according to Embodiment 1 and an antenna device in Comparative Example.  FIG. 3  is a graph illustrating antenna efficiency in the high-frequency band of each of the antenna device according to Embodiment 1 and the antenna device in Comparative Example. 
     In  FIGS. 2 and 3 , the antenna device in Comparative Example is substantially the same as an antenna device obtained by removing the branch parts  20   e  and  22   e  from the antenna device  10  according to Embodiment 1. The widths W 1  of the respective first linear parts  20   a  and  22   a  and the widths W 2  of the respective second linear parts  20   b  and  22   b  are each 1 mm, and a width W 3  of each of the branch parts  20   e  and  22   e  is 1.5 mm. In addition, the first linear parts  20   a  and  22   a  are each 26.5 mm long, and the second linear parts  20   b  and  22   b  are each 6 mm long. Further, the distance D 1  between each of the first linear parts  20   a  and  22   a  and a corresponding one of the second linear parts  20   b  and  22   b  is 3 mm, and the distance D 2  of each of the narrow gap parts  20   d  and  22   d  is 0.5 mm. The capacitance of each capacitor chip  28  of the corresponding LC resonant circuit  16  is 0.3 pF, and the inductance of each inductor chip  30  is 2.8 nH. 
     As illustrated in  FIG. 2 , providing the branch parts  20   e  and  22   e  causes frequency shift to a lower frequency in frequencies between a low-frequency band (2.4 GHz band) and a high-frequency band (5 GHz band) (the area surrounded by the broken line circle). Specifically, a harmonic wave at the first frequency (about 2.4 GHz) in the low-frequency band interferes with the fundamental (about 5.7 GHz) at the second frequency in the high-frequency band in the antenna device in Comparative Example without necessarily the branch parts  20   e  and  22   e , but providing the branch parts  20   e  and  22   e  causes the harmonic wave to be shifted to a lower frequency. As illustrated in  FIG. 3 , this improves the antenna efficiency in the high-frequency band, particularly in the lower frequency area in the high-frequency band. As the result, a high antenna frequency is obtained all over the high-frequency band. 
     Note that the shifting degree of the harmonic wave at the first frequency can be controlled by changing the width W 3  and the location of the branch parts  20   e  and  22   e.    
       FIG. 4  is a graph illustrating relationships between a return loss characteristic and branch part widths in each of the antenna device according to Embodiment 1 and the antenna device in Comparative Example.  FIG. 5  is a graph illustrating relationships between the return loss characteristic and branch part locations in each of the antenna device according to Embodiment 1 and the antenna device in Comparative Example. 
     As illustrated in Examples 1 to 3 in  FIG. 4 , increasing the width W 3  of each of the branch parts  20   e  and  22   e  causes each harmonic wave at the first frequency to be shifted to a lower frequency. In addition, as illustrated in Examples 1 and 4 in  FIG. 5 , moving the branch parts  20   e  and  22   e  toward the respective outer side portions (farther from the parasitic line part  24  and the feeding line part  26 ), for example, by only 2 mm also causes each harmonic wave at the first frequency to be shifted to a lower frequency. 
     Thus, as illustrated in  FIGS. 4 and 5 , appropriately changing the width W 3  and the location of each of the branch parts  20   e  and  22   e  enables the shifting degree of the harmonic wave at the first frequency to be controlled desirably. As the result, the interference of the harmonic wave at the first frequency with the fundamental at the second frequency can be reduced more. 
     According to Embodiment 1 as described above, the deterioration of the antenna efficiency in the high-frequency band can be reduced in the dual-band antenna device  10  including the folded antenna conductor  14 . 
     Note that in the case of Embodiment 1, as illustrated in  FIG. 1 , each of the branch parts  20   e  and  22   e  extends from a corresponding one of the first linear parts  20   a  and  22   a  to form a corresponding one of the narrow gap parts  20   d  and  22   d  between a corresponding one of the branch parts  20   e  and  22   e  and a corresponding one of the second linear parts  20   b  and  22   b . Instead of this, branch parts may each extend from a corresponding one of second linear parts to form a narrow gap part between a corresponding one of the branch parts and a corresponding one of first linear parts. 
     Embodiment 2 
     Embodiment 2 is an embodiment improved from Embodiment 1 described above. Embodiment 2 will thus be described with a focus on a point different from Embodiment 1 above. Note that substantially the same components in Embodiment 2 as the components in Embodiment 1 above are denoted by the same reference numerals. 
       FIG. 6  is a partial top view of a radio communication device including an antenna device according to Embodiment 2 of the present disclosure. 
     As illustrated in  FIG. 6 , an antenna device  110  according to Embodiment 2 is included in a radio communication device  150 . A folded antenna conductor  114  of the antenna device  110  includes a first element part  120  and a second element part  122 . The first and second element parts  120  and  122  each includes a corresponding one of first linear parts  120   a  and  122   a  and a corresponding one of second linear parts  120   b  and  122   b . The corresponding one of the first linear parts  120   a  and  122   a  and the corresponding one of the second linear parts  120   b  and  122   b  are caused to face each other at a distance by the folding. 
     Narrow gap parts  120   d  are provided between the first linear part  120   a  and the second linear part  120   b  of the first element part  120 , the narrow gap parts  120   d  each measuring a distance shorter than a distance measured in the other portions therebetween. Likewise, narrow gap parts  122   d  are provided between a first linear part  122   a  and a second linear part  122   b  of the second element part  122 , the narrow gap parts  122   d  each measuring a distance shorter than a distance measured in the other portions therebetween. 
     Unlike Embodiment 1 above, in the case of Embodiment 2, branch parts do not extend from the first linear parts  120   a  and  122   a  and thus do not form the narrow gap parts  120   d  and  122   d.    
     Instead, the first and second element parts  120  and  122  of the folded antenna conductor  114  respectively include floating-island-like parts  120   e  and  122   e  each provided between a corresponding one of the first linear parts  120   a  and  122   a  and a corresponding one of the second linear parts  120   b  and  122   b.    
     The floating-island-like parts  120   e  and  122   e  are not respectively continuous with the first linear parts  120   a  and  122   a  and the second linear parts  120   b  and  122   b  and each have one end forming a corresponding one of the narrow gap parts  120   d  and  122   d  (first narrow gap parts) between the one end and a corresponding one of the first linear parts  120   a  and  122   a  and the other end forming a corresponding one of the narrow gap parts  120   d  and  122   d  (second narrow gap parts) between the other end and a corresponding one of the second linear parts  120   b  and  122   b.    
     Also in Embodiment 2 as described above, like Embodiment 1 above, the deterioration of the antenna efficiency in the high-frequency band can be reduced in the dual-band antenna device  110  including the folded antenna conductor  114 . 
     Embodiment 3 
     Embodiment 3 is an embodiment improved from Embodiment 1 described above. Embodiment 3 will thus be described with a focus on a point different from Embodiment 1 above. Note that substantially the same components in Embodiment 3 as the components in Embodiment 1 above are denoted by the same reference numerals. 
       FIG. 7  is a partial top view of a radio communication device including an antenna device according to Embodiment 3 of the present disclosure. 
     As illustrated in  FIG. 7 , an antenna device  210  according to Embodiment 3 is included in a radio communication device  250 . A folded antenna conductor  214  of the antenna device  210  includes a first element part  220  and a second element part  222 . The first and second element parts  220  and  222  each includes a corresponding one of first linear parts  220   a  and  222   a  and a corresponding one of second linear parts  220   b  and  222   b . The corresponding one of the first linear parts  220   a  and  222   a  and the corresponding one of the second linear parts  220   b  and  222   b  are caused to face each other at a distance by the folding. 
     A narrow gap part  220   d  is provided between the first linear part  220   a  and the second linear part  220   b  of the first element part  220 , the narrow gap part  220   d  measuring a distance shorter than a distance measured in the other portions therebetween. Likewise, a narrow gap part  222   d  is provided between a first linear part  222   a  and a second linear part  222   b  of the second element part  222 , the narrow gap part  222   d  measuring a distance shorter than a distance measured in the other portions therebetween. 
     Unlike Embodiment 1 above, in the case of Embodiment 3, branch parts do not extend from the first linear parts  220   a  and  222   a  and thus do not form the narrow gap parts  220   d  and  222   d . In addition, unlike Embodiment 2 above, any of floating-island-like parts is not formed between a corresponding one of the first linear parts  220   a  and  222   a  and a corresponding one of the second linear parts  220   b  and  222   b  and thus does not form a corresponding one of the narrow gap parts  220   d  and  222   d.    
     Instead, the second linear parts  220   b  and  222   b  extend obliquely with respect to a direction in which the first linear parts  220   a  and  222   a  extend (X-axis direction), in such a manner that portions, of the second linear parts  220   b  and  222   b , closer to open ends  220   c  and  222   c  become closer to the first linear parts  220   a  and  222   a . As the result, the narrow gap parts  220   d  and  222   d  are each formed between a corresponding one of the open ends  220   c  and  222   c  and a corresponding one of the first linear parts  220   a  and  222   a.    
     Also in Embodiment 3 as described above, like Embodiment 1 above, the deterioration of the antenna efficiency in the high-frequency band can be reduced in the dual-band antenna device  210  including the folded antenna conductor  214 . 
     Embodiment 4 
     Embodiment 4 is an embodiment improved from Embodiment 1 described above. Embodiment 4 will thus be described with a focus on a point different from Embodiment 1 above. Note that substantially the same components in Embodiment 4 as the components in Embodiment 1 above are denoted by the same reference numerals. 
       FIG. 8  is a partial top view of a radio communication device including an antenna device according to Embodiment 4 of the present disclosure. 
     As illustrated in  FIG. 8 , an antenna device  310  according to Embodiment 4 is included in a radio communication device  350 . The antenna device  310  according to Embodiment 4 also includes the folded antenna conductor  14  of the antenna device  10  in Embodiment 1 above. The different point is that capacitor chips  332  each connecting a corresponding one of the first linear parts  20   a  and  22   a  and a corresponding one of the second linear parts  20   b  and  22   b  are provided in a corresponding one of the narrow gap parts  20   d  and  22   d  of a corresponding one of the first and second element parts  20  and  22  of the folded antenna conductor  14 . 
     Appropriately selecting capacity value of the capacitor chips  332  enables the capacitance Cl in the narrow gap parts  20   d  and  22   d  to be controlled desirably and easily (for example, compared with the case of changing the shape of the folded antenna conductor  14 ). The shifting degree of the harmonic wave at the first frequency can thereby be controlled desirably. As the result, the interference of the harmonic wave at the first frequency with the fundamental at the second frequency can be reduced more. 
     Also in Embodiment 4 as described above, like Embodiment 1 above, the deterioration of the antenna efficiency in the high-frequency band can be reduced in the dual-band antenna device  310  including the folded antenna conductor  14 . 
     Embodiment 5 
     In the case of Embodiment 1, to function as the dual-band antenna device, the antenna device  10  includes the LC resonant circuits  16 . Each LC resonant circuit  16  is the LC parallel circuit that passes the first frequency in the lower frequency band but attenuates the second frequency in the higher frequency band, that is, that resonates at the first frequency. In contrast, LC resonant circuits of the antenna device in Embodiment 5 perform different operations. Embodiment 5 will thus be described with a focus on a point different from Embodiment 1 above. Note that substantially the same components in Embodiment 5 as the components in Embodiment 1 above are denoted by the same reference numerals. 
       FIG. 9  is a partial top view of a radio communication device including an antenna device according to Embodiment 5 of the present disclosure. 
     As illustrated in  FIG. 9 , an antenna device  410  according to Embodiment 5 is included in a radio communication device  450 . The antenna device  410  includes a folded antenna conductor  414  including a first element part  420  and a second element part  422 . 
     The first element part  420  of the folded antenna conductor  414  includes a first linear part  420   a  and a second linear part  420   b  that are caused to face each other at a distance by the folding. Likewise, the second element part  422  also includes a first linear part  422   a  and a second linear part  422   b  that are caused to face each other at a distance by the folding. 
     In addition, a narrow gap part  420   d  is provided between the first linear part  420   a  and the second linear part  420   b  of the first element part  420 , the narrow gap part  420   d  measuring a distance shorter than a distance measured in the other portions therebetween. In the case of Embodiment 5, the first linear part  420   a  includes a branch part  420   e  extending toward the second linear part  420   b  and forming the narrow gap part  420   d  between the branch part  420   e  and the second linear part  420   b.    
     Likewise, a narrow gap part  422   d  is also provided between the first linear part  422   a  and the second linear part  422   b  of the second element part  422 , the narrow gap part  422   d  measuring a distance shorter than a distance measured in the other portions therebetween. In the case of Embodiment 5, the first linear part  422   a  includes a branch part  422   e  extending toward the second linear part  422   b  and forming the narrow gap part  422   d  between the branch part  422   e  and the second linear part  422   b.    
     In the case of Embodiment 5, LC resonant circuits  434  are respectively provided in the narrow gap parts  420   d  and  422   d  of the respective first and second element parts  420  and  422  and each connect a corresponding one of the first linear parts  420   a  and  422   a  and a corresponding one of the second linear parts  420   b  and  422   b.    
     In addition, in the case of Embodiment 5, LC resonant circuits  434  respectively include capacitor chips  436  having predetermined capacitance and inductor chips  438  disposed parallel to the respective capacitor chips  436  and having predetermined inductance. 
     Further, unlike the LC resonant circuits  16  in Embodiment 1 above, the LC resonant circuits  434  in Embodiment 5 let the second frequency in the higher frequency band pass but let the first frequency in the lower frequency band attenuate, that is, resonate at the first frequency. The capacitance of each capacitor chip  436  of the corresponding LC resonant circuit  434  is 2.1 pF, and the inductance of each inductor chip  438  is 2.0 nH. 
     The antenna device  410  according to Embodiment 5 as described above also provides the same advantageous effects as those in Embodiment 1 above. 
       FIG. 10  is a graph illustrating the frequency characteristic of the return loss of the antenna device according to Embodiment 5. 
     As illustrated in  FIG. 10 , in the antenna device  410  according to Embodiment 5, the harmonic wave (about 2.8 GHz) of the fundamental (about 2.4 GHz) at the first frequency in the low-frequency band (2.4 GHz band) is considerably away from the fundamental at the second frequency (about 5.5 GHz) in the high-frequency band (5 GHz band). The interference of this harmonic wave with the fundamental at the second frequency is thereby reduced. As the result, the high antenna frequency is obtained all over the high-frequency band. 
     Also in Embodiment 5 as described above, like Embodiment 1 above, the deterioration of the antenna efficiency in the high-frequency band can be reduced in the dual-band antenna device  410  including the folded antenna conductor  414 . 
     The present disclosure has heretofore been described by citing embodiments, but the embodiments of the present disclosure are not limited to these embodiments. 
     For example, in the cases of Embodiment 1 and Embodiment 5 above, the LC resonant circuits  16  and  434  each includes the capacitor chip and the inductor chip that are disposed in parallel. The antenna devices are thereby downsized. However, the configuration of the LC resonant circuits is not limited to this configuration. For example, a capacitor element composed of a pair of parallel conductor patterns and an inductor element as a meandering conductor pattern may form an LC resonant circuit on the base substrate. 
     In addition, for example, in the cases of Embodiments 1 to 5 above, each folded antenna conductor is the folded dipole antenna. However, the antenna conductor according to each embodiment of the present disclosure is not limited to this. The folded antenna conductor may be another folded wire antenna such as a folded monopole antenna or a folded inverted-F antenna. 
     The present disclosure has heretofore been described by citing embodiments, it is obvious for those skilled in the art that an embodiment may be combined as a whole or partially with at least one different embodiment to obtain a further embodiment according to the present disclosure. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable to a dual-band antenna device including a linear antenna conductor.