Patent Publication Number: US-9843095-B2

Title: Antenna element and antenna device

Description:
TECHNICAL FIELD 
     The present invention relates to an antenna technology, and more specifically to an antenna technology using a detachable external antenna. 
     BACKGROUND ART 
     Patent Literature 1 discloses one of antenna technologies using a detachable external antenna. An antenna device disclosed in Patent Literature 1 uses an earphone cable as an external antenna. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1 
     Japanese Patent Application Publication, Tokukai, No. 2008-92265 (Publication Date: Apr. 17, 2008) 
     SUMMARY OF INVENTION 
     Technical Problem 
     The antenna device disclosed in Patent Literature 1 can secure excellent antenna characteristics in a state where the earphone cable is engaged, but cannot receive a radio signal in a state where the earphone cable is not engaged, that is, in the absence of an antenna element. 
     The present invention is attained in view of the above problem. It is a main object of the present invention to provide a technology for securing excellent antenna characteristics even in a case where an external antenna is not engaged. 
     Solution to Problem 
     In order to solve the above problem, an antenna element in accordance with an aspect of the present invention includes: a feeding point; a connector in which an external antenna is detachably engaged; a first partial element connecting the feeding point and the connector; and a second partial element branching from the first partial element and having an open end at a different position from the connector. 
     Advantageous Effects of Invention 
     According to an aspect of the present invention, while the external antenna is not engaged, an electric current dominantly flows between the feeding point and the open end via the second partial element. Thus, it is possible to secure excellent antenna characteristics even in a case the external antenna is not engaged. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view illustrating overviews of antenna elements in accordance with embodiments of the present invention. 
         FIG. 2  is a view illustrating an appearance of a front side of a mobile phone terminal in accordance with embodiments of the present invention. 
         FIG. 3  is views each illustrating an appearance of a back side of a mobile phone terminal (from which a battery cover is being removed) in accordance with embodiments of the present invention, in a state where an external antenna is not engaged. 
         FIG. 4  is a graph showing an example return loss in embodiments of the present invention, in a case where an external antenna is not engaged. 
         FIG. 5  is views each illustrating an appearance of a back side of a mobile phone terminal (from which a battery cover is being removed) in accordance with embodiments of the present invention, in a state where an external antenna is engaged. 
         FIG. 6  is a graph showing an example return loss in embodiments of the present invention, in a case where an external antenna is engaged. 
         FIG. 7  is a view illustrating an example main configuration of a connector in accordance to embodiments of the present invention. 
         FIG. 8  is a view illustrating an antenna device configured differently from an antenna device of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the drawings. Note that members having the same function in the embodiments are given the same reference numerals. 
     (Basic Configuration) 
       FIG. 1  is a view illustrating overviews of antenna elements  10  in accordance with Embodiments 1 through 3 of the present invention. (a) of  FIG. 1  illustrates the overview of the antenna element  10  in accordance with Embodiment 1, (b) of  FIG. 1  illustrates the overview of the antenna element  10  in accordance with Embodiment 2, and (c) of  FIG. 1  illustrates the overview of the antenna element  10  in accordance with Embodiment 3. Note that  FIG. 1  illustrates the antenna element  10  being incorporated into an antenna device  30  and being connected to a circuit section  20  which is provided in the antenna device  30 . 
     The antenna device  30  is not limited to any specific device, provided that it performs at least one of (i) transmission of radio signals and (ii) reception of radio signals via the antenna element  10 . For example, the antenna device  30  can be applied to not only terminals such as mobile phone terminals (including smart phones), mobile information terminals (including tablet terminals) but also radio devices in general, such as image display devices capable of radio transmission and/or radio reception and audio output devices capable of radio transmission and/or radio reception. 
     The circuit section  20  includes a radio circuit section  21  and a matching circuit section  22 , and transmits and receives a radio signal by feeding the antenna element  10 . Note that the matching circuit section  22  can be provided as necessary or can be omitted. Further, the matching circuit section  22  is adjusted so as to have an optimum matching constant in a state in which any external antenna is not engaged. However, the arrangement of the matching circuit section  22  is not limited to such an arrangement. For example, the matching circuit section  22  can be arranged such that it detects engagement of the external antenna and then switches the matching so that the matching has the optimum matching constant in a state in which the external antenna is engaged, when the external antenna is engaged. 
     The antenna element  10  is constructed by a feeding point  16 , a connector  15 , and a plurality of partial elements ( 11  through  14 ). The antenna element  10  is placed in a fully off-ground area. Note that, the fully off-ground area does not mean a transmission line having characteristic impedance of, for example, 50Ω or 75Ω, but means an area where the antenna element  10  is placed at a distance, from the ground, enough to operate as a so-called radiating element. 
     The feeding point  16  is a part that is fed by the circuit section  20 , and the feeding point  16  can be, for example, a junction of a feeding line extending from the circuit section  20 . A method for connecting the feeding point  16  and the feeding line extending from the circuit section  20  is not limited to any specific method, provided that the feeding point  16  and the feeding line extending from the circuit section  20  are connected directly to each other or connected to each other in a high-frequency manner. For example, the connection may be made by, for example, pressing or soldering a plate spring, a gasket, or the like provided at the end of the feeding line to the feeding point  16 . Alternatively, conductors may be faced in close proximity to each other and connected by a capacitor. 
     The connector  15  is configured such that the external antenna is detachably engaged in the connector  15 . As described later, the connector  15  can be constructed as an earphone jack in a case where the external antenna is an earphone cable. Note, however, that the external antenna is not limited to the earphone cable, provided that the external antenna is an attachable/detachable antenna and that the connector  15  is connected directly to the external antenna or connected to the external antenna in a high-frequency manner. 
     The partial elements ( 11  through  14 ) are parts that constitute an antenna element and are not limited to any specific ones, provided that the partial elements are conductors constructed to be capable of emitting radio waves. The partial elements can be each, for example, a conductive pattern formed on a substrate, a conductive pattern formed on a flexible printed circuit board, a conductive pattern formed on a dielectric such as a resin by LDS (Laser Direct Structuring) method, an MID (Molded Interconnect Device), a DPA (Direct Printed Antenna), or the like. Alternatively, the partial elements may be a combination of any of the exemplified ones (For example, the conductive pattern formed on the substrate is provided in an area close to a feeding end, while the pattern formed by the LDS method is provided in the other area). 
     In each of the embodiments, the antenna element  10  includes: a first partial element  11  connecting the feeding point  16  and the connector  15 ; and a second partial element  12  branching at a branch point  11   a  from the first element  11  and having an open end  12   a  at a different position from the connector  15 . Note that no switch is provided at the branch point  11   a , and the first partial element  11  and the second partial element  12  are connected to the feeding point  16  regardless of an engagement state of the external antenna. 
     In Embodiment 1, the branch point  11  is provided at about a midpoint position of the first partial element  11  (see (a) of  FIG. 1 ). The first partial element  11  constructs (i) a path from the branch point  11   a  to the connector  15  and (ii) a path from the branch point  11   a  to the feeding point  16 . The second partial element  12  constructs a path from the branch point  11   a  to the open end  12   a . Note that the path from the branch point  11   a  to the feeding point  16  can be referred to as a third partial element  14 , and the path from the feeding point  16  to the open end  12   a  (in other words, a path formed by a combination of the second element  12  and the third element  14 ) can be referred to as a fourth partial element  13 . 
     According to Embodiment 1, while the external antenna is engaged, an electric current dominantly flows through the external antenna and the first partial element  11 , and it is therefore possible to secure antenna characteristics. While the external antenna is not engaged, an electric current dominantly flows through the second partial element  12  and the third partial element  14  (i.e. the fourth partial element  13 ), and it is therefore possible to secure antenna characteristics. 
     On the other hand, in a case where the second partial element  12  is absent, antenna characteristics can be degraded while the external antenna is not engaged. That is, in the absence of the second partial element  12 , an electric current dominantly flows through the first partial element  11  which has an open end corresponding to the connector  15 , while the external antenna is not engaged. In this case, the connector  15  can be located at a position that is unfavorable in serving as the open end of the antenna element (e.g. at a position around which a metallic component is present) because the connector  15  can be provided in a limited location in the antenna device in order to allow the external antenna to be detachably engaged in the connector  15 . Particularly, in a case where the connector  15  is an earphone jack, a signal wire for audio signals approaches the open end of the first partial element  11 . This causes degradation in antenna characteristics of the first partial element  11 . 
     In contrast, according to Embodiment 1, the open end  12   a  is located at a different position from the connector  15 , and the open end  12   a  is provided at a distance from the metallic component which is present around the connector  15 . This facilitates an electric current flowing through the second partial element  12 . Accordingly, while the external antenna is not engaged, an electric current dominantly flows between the feeding point  16  and the open end  12   a  via the second partial element  12 . This allows the path connecting the feeding point  16  and the opening end  12   a  (i.e. the fourth partial element  13 ) to operate as a monopole antenna, and therefore makes it possible to prevent degradation in antenna characteristics. In particular, the open end  12   a , unlike the connector  15 , has a high degree of placement flexibility. Therefore, as compared to the connector  15 , the open end  12   a  can be provided under conditions advantageous to an antenna, including a secure distance from a peripheral metallic component, and length, thickness, and the like appropriate for emission of radio waves. Accordingly, it is possible to attain more excellent antenna characteristics. 
     In Embodiment 2, the branch point  11   a  is provided at a connection part of the first partial element  11  at which part the first partial element  11  is connected to the connector  15  (see (b) of  FIG. 1 ). The first partial element  11  constructs a path from the branch point  11   a  to the feeding point  16 , and the second partial element  12  constructs a path from the branch point  11   a  to the open end  12   a . Note that the path from the feeding point  16  to the branch point  11   a  (i.e. the first partial element  11 ) can be referred to as the third partial element  14 , and the path from the feeding point  16  to the open end  12   a  (i.e. the path formed by a combination of the second partial element  12  and the first partial element  11 ) can be referred to as the fourth partial element  13 . 
     According to Embodiment 2, as in Embodiment 1, while the external antenna is engaged, an electric current dominantly flows through the external antenna and the first partial element  11 , and it is therefore possible to secure antenna characteristics. Further, as in Embodiment 1, the open end  12   a  is located at a different position from the connector  15 , and the open end  12   a  is provided at a distance from the metallic component which is present around the connector  15 . This facilitates an electric current flowing through the second partial element  12 . Accordingly, while the external antenna is not engaged, an electric current dominantly flows through the second partial element  12  and the first partial element  11  (i.e. the fourth partial element  13 ), and it is therefore possible to secure antenna characteristics. Further, as in Embodiment 1, the opening end  12   a  has a high degree of placement flexibility, Therefore, the open end  12   a  can be provided under conditions advantageous to an antenna, including a secure distance from a peripheral metallic component, and length, thickness, and the like appropriate for emission of radio waves. Accordingly, it is possible to attain more excellent antenna characteristics. 
     In Embodiment 3, the first partial element  11  and the second partial element  12  are branched at the feeding point  16  (see (c) of  FIG. 1 ). The first partial element  11  constructs a path from the feeding point  16  to the connector  15 , and the second partial element  12  constructs a path from the feeding point  16  to the open end  12   a . Note that the path from the feeding point  16  to the open end  12   a  can be referred to as the fourth partial element  13 . 
     According to Embodiment 3, as in Embodiment 1, while the external antenna is engaged, an electric current dominantly flows through the external antenna and the first partial element  11 , and it is therefore possible to secure antenna characteristics. Further, as in Embodiment 1, the open end  12   a  is located at a different position from the connector  15 , and the open end  12   a  is provided at a distance from the metallic component which is present around the connector  15 . This facilitates an electric current flowing through the second partial element  12 . Accordingly, while the external antenna is not engaged, an electric current dominantly flows through the second partial element  12  (the fourth partial element  13 ), and it is therefore possible to secure antenna characteristics. Further, as in Embodiment 1, the opening end  12   a  has a high degree of placement flexibility, Therefore, the open end  12   a  can be provided under conditions advantageous to an antenna, including a secure distance from a peripheral metallic component, and length, thickness, and the like appropriate for radiation of radio waves. Accordingly, it is possible to attain more excellent antenna characteristics. 
     (Suitable Electrical Lengths of the Partial Elements] 
     In each of the embodiments, the partial elements are preferably arranged such that an electrical length of the fourth partial element  13  (a sum of a second electrical length L 2 , an electrical length of the second partial element  12 , and an electrical length of the third partial element  14 ) is closer to a ¼ wavelength of an applied frequency of the antenna element  10  than to an electrical length of the first partial element  11  (first electrical length L 1 ). 
     According to the arrangement as described above, while the external antenna is not engaged, an electrical length of the path from the open end  12   a  to the feeding point  16  (second electrical length L 2 ) is closer to the ¼ wavelength of the applied frequency of the antenna element  10  than to an electrical length of the path from the connector  15  to the feeding point  16  (first electrical length L 1 ). This generates a greater resonance in the path from the open end  12   a  and the feeding point  16  (fourth partial element  13 ) and allows a larger amount of electric current to flow over the path from the open end  12   a  to the feeding point  16  (fourth partial element  13 ). This makes it possible to further improve antenna characteristics. 
     Especially, the electrical length of the fourth partial element  13  (second electrical length L 2 ) is preferably not less than a ⅛ wavelength of the applied frequency of the antenna element  10  but not more than a ⅜ wavelength of the applied frequency of the antenna element  10 , and is further preferably approximately a ¼ wavelength of the applied frequency of the antenna element  10 . With such an arrangement, the fourth partial element  13  suitably operates as a monopole antenna. 
     Further, the partial elements are preferably arranged such that an electrical length of the third partial element  14  (third electrical length L 3 ) is longer than one fourth of the second electrical length L 2 , or that the electrical length of the third partial element  14  (third electrical length L 3 ) is longer than whichever shorter one of (i) one fourth of the second electrical length L 2  or (ii) a 1/16 wavelength of the applied frequency of the antenna element  10  (In other words, the third electrical length L 3  is preferably longer than whichever shorter one of (i) one fourth of the second electrical length L 2  or (ii) the 1/16 wavelength of the applied frequency of the antenna element  10 .). 
     With such an arrangement, while the external antenna is engaged, an electric current is hard to flow through the second partial element  12  (resonance is hard to occur in the second partial element  12 ), and a larger amount of electric current flows through the external antenna. This makes it possible to further improve antenna characteristics. 
     More specifically, an electrical length of the second partial element  12  (fourth electrical length L 4 ) is determined by subtracting the electrical length of the third partial element  14  (third electrical length L 3 ) from the electrical length of the fourth partial element  13  (second electrical length L 2 ). With an arrangement in which the second electrical length L 2  is closer to the ¼ wavelength of the applied frequency of the antenna element  10 , and the third electrical length L 3  is longer than one fourth of the second electrical length L 2  or longer than the 1/16 wavelength of the applied frequency of the antenna element  10 , the electrical length of the second partial element  12  (fourth electrical length L 4 ) becomes sufficiently shorter than the ¼ wavelength of the applied frequency of the antenna element  10  (for example, in a case where the second electrical length L 2  is the ¼ wavelength of the applied frequency of the antenna element  10 , the fourth electrical length L 4  becomes not more than a 3/16 wavelength of the applied frequency). Therefore, resonance in the second partial element  12  occurs at a frequency falling outside a band of the applied frequency (at a frequency higher than the applied frequency). With this arrangement, while the external antenna is engaged, an electric current is hard to flow through the second partial element  12 , and a larger amount of electric current flows through the external antenna. This makes it possible to further improve antenna characteristics. 
     Note that the above-described conditions are not satisfied in Embodiment 3. This is because, in Embodiment, the third partial element  14  is absent, and the third electrical length L 3  is therefore taken as 0. 
     [Embodiment in which the Connector  15  Serves as an Earphone Jack] 
     Next, more detailed embodiments will be described below. In Embodiments 4 through 6, the antenna devices  30  in Embodiments 1 through 3 respectively are arranged such that each of the antenna devices  30  is a mobile phone terminal having a function of receiving a digital television broadcast (one-segment broadcast or full-segment broadcast), and that each of the connectors  15  is an earphone jack for allowing an external antenna (earphone cable)  40  detachably engaged therein. The other arrangements in Embodiment 4, 5, and 6 are similar to those in Embodiment 1, 2, and 3, respectively. 
     In the above arrangement, the earphone cable is used as the external antenna  40 , and the earphone jack is used as the connector  15 . This arrangement eliminates the need to separately provide another connector  15  for allowing the external antenna to be engaged in that connector  15 . Further, any earphone can be used as the external antenna  40 . 
     Further, in Embodiments 4 through 6, the antenna element  10  is an antenna for receiving a digital television broadcast. It is considered that a user often views a digital television broadcast with the external antenna (earphone cable)  40  engaged in the connector  15 . Therefore, by using the external antenna (the earphone cable)  40  as an antenna for receiving a digital television broadcast, it is possible to suitably receive a digital television broadcast in order for the user to view the digital television broadcast. 
       FIG. 2  is a view illustrating an appearance of a front side of the antenna device (mobile phone terminal)  30  in accordance with Embodiments 4 through 6.  FIG. 2  illustrates a state where the external antenna (earphone cable)  40  is not engaged. As illustrated in  FIG. 2 , the connector (earphone jack)  15  is provided on a right end of a top end part of the antenna device  30  when viewed from the front. Further, a liquid crystal panel  31  is provided in a central part of the antenna device  30 . Further, an operation button  32  is arranged in the lower end part of the antenna device  30 . 
       FIG. 3  is views each illustrating an appearance of a back side of the antenna device (mobile phone terminal)  30  in accordance with Embodiments 4 through 6. (a) of  FIG. 3  illustrates an appearance of a back side of the antenna device  30  in accordance with Embodiment 4, (b) of  FIG. 3  illustrates an appearance of a back side of the antenna device  30  in accordance with Embodiment 5, and (c) of  FIG. 3  illustrates an appearance of a back side of the antenna device  30  in accordance with Embodiment 6.  FIG. 3  illustrates a state where the external antenna (earphone cable)  40  is not engaged.  FIG. 3  also illustrates a state where a battery cover (not shown) to be placed on the rearmost back face of the antenna device  30  (mobile phone terminal)  30  is removed so that the antenna element  10  is exposed to view. 
     As illustrated in  FIG. 3 , in Embodiments 4 through 6, the antenna element  10  is provided on a back-side surface of a housing of the antenna device (mobile phone terminal)  30 . Further, a part where the feeding point  16  of the antenna element  10  is provided passes through the housing. The antenna element  10  is fed by the circuit section  20  via a spring, a gasket or the like, inside the housing. 
     In Embodiment 4, the open end  12   a  is provided at a different position from the connector  15 . In other words, the open end  12   a  is provided apart from, for example, a signal wire for audio signals which signal wire is provided in the connector  15 . With this arrangement, an electric current dominantly flows over a path connecting the feeding point  16  and the open end  12   a , via the second partial element  12  and a part of the first partial element  11  (see (a) of  FIG. 3 ). Similarly, in Embodiment 5, the open end  12   a  is provided at a different position from the connector  15 , so that an electric current dominantly flows over the path connecting the feeding point  16  and the open end  12   a , via the first partial element  11  and the second partial element  12  (see (b) of  FIG. 3 ). In Embodiment 6, the open end  12   a  is provided at a different position from the connector  15 , and an electric current dominantly flows over the path connecting the feeding point  16  and the open end  12   a , via the second partial element  12  (see (c) of  FIG. 3 ). 
     Accordingly, as illustrated in  FIG. 4 , it is possible to obtain a suitable return loss at an applied frequency. Note that,  FIG. 4  is a graph showing an example return loss of the antenna element  10  in a case where the external antenna (earphone cable)  40  is not engaged, in Embodiments 4 through 6. 
       FIG. 5  is a view illustrating an appearance of a back side of the antenna device (mobile phone terminal)  30  in accordance with Embodiments 4 through 6.  FIG. 5  illustrates a state where the external antenna (earphone cable)  40  is engaged (a state where an earphone plug  41  is inserted into the connector (earphone jack)  15 ). Further, similarly to  FIG. 3 ,  FIG. 5  illustrates a state where the battery cover (not shown) to be placed in the rearmost back face of the antenna device (mobile phone terminal)  30  is removed so that the antenna element  10  is exposed to view. 
     In Embodiment 4, as shown in (a) of  FIG. 5 , the external antenna (earphone cable)  40  is connected to the feeding point  16  via the earphone plug  41 , the connector (earphone jack)  15 , and the first partial element  11 . In this arrangement, the external antenna (earphone cable)  40  is provided apart from a metallic component of the antenna device (mobile phone terminal)  30 . This facilitates an electric current to flow through the external antenna (earphone cable)  40 . Accordingly, an electric current hardly flows through the second partial element  12 , but dominantly flows through the external antenna (earphone cable)  40 . Similarly, in Embodiment 5, the external antenna (earphone cable)  40  is connected to the feeding point  16  via the earphone plug  41 , the connector (earphone jack)  15 , and the first partial element  11 , so that an electric current dominantly flows through the external antenna (earphone cable)  40  (see (b) of  FIG. 5 ). Similarly, in Embodiment 6, the external antenna (the earphone cable)  40  is connected to the feeding point  16  via the earphone plug  41 , the connector (earphone jack)  15 , and the first partial element  11 , so that an electric current dominantly flows through the external antenna (earphone cable)  40  (see (c) of  FIG. 5 ). 
     Accordingly, as illustrated in  FIG. 6 , it is possible to obtain a highly suitable return loss at an applied frequency. Note that,  FIG. 6  is a graph showing an example of return loss of the antenna element  10  in Embodiments 4 through 6, in a case where the external antenna (earphone cable)  40  is engaged. 
     Note that, the external antenna (earphone cable)  40  is not limited to any specific antenna and can be any antenna irrespective of, for example, length and type. From one viewpoint, however, the electrical length of the external antenna (earphone cable)  40  is more preferably such that the electrical length of the external antenna  40  is sufficiently longer than the fourth partial element  13 . In addition, the electrical length of the external antenna (earphone cable)  40  is further preferably not less than a ⅜ wavelength of an applied frequency of the antenna element  10 , and particularly preferably not less than a ½ wavelength of the applied frequency. Using the external antenna (the earphone cable)  40  sufficiently longer in electrical length than the fourth partial element  13  generates no resonance of a ¼ wavelength system. This makes it possible to reduce an effect caused while the external antenna (earphone cable)  40  is attached to a human body (ears). 
       FIG. 7  is a view illustrating an example main configuration of the connector (earphone jack)  15  in Embodiments 4 through 6. With reference to  FIG. 7 , the description below deals with an arrangement in which an external conductor (ground wire) of the earphone cable is used as an antenna. The present invention, however, is not limited to such an arrangement. Alternatively, a signal wire for audio signals or the like of the earphone cable can be used as an antenna. 
     As shown in  FIG. 7 , the connector (earphone jack)  15  includes: a signal wire  15   a  connecting to a signal wire for sound, microphone input, and the like of the external antenna (earphone cable)  40 ; and a ground terminal  15   b  for connecting to the external conductor (ground wire). 
     The ground terminal  15   b  has branches. One branch of the ground terminal  15   b  is connected to the ground of the antenna device (mobile phone terminal)  30 , and the other branch is connected to the first partial element  11  via a connecting part  15   d  such as a gasket. A filter  15   c  is provided between the ground terminal  15   b  and the ground and between the connecting part  15   d  and the ground. 
     When the earphone plug  41  is inserted into the connector (earphone jack)  15 , the external conductor (ground wire) of the external antenna (earphone cable)  40  is connected to the first partial element  11  via the ground terminal  15   b  and the connecting part  15   d , so that the external conductor (ground wire) of the external antenna (earphone cable)  40  can be used as an antenna. 
     Note that the filter  15   c  is constructed by, for example, a choke coil and is arranged to interrupt (release) an electric current of an applied frequency of the antenna element  10  in a high-frequency manner. Accordingly, it is possible to interrupt (release) between the ground of the antenna device (mobile phone terminal)  30  and the first partial element  11  in a high-frequency manner. Therefore, it is possible (i) to avoid a high-frequency connection between the feeding point  16  and the ground of the antenna device (mobile phone terminal)  30 , and (ii) to successfully use the external conductor (ground wire) of the external antenna (earphone cable) as an antenna. 
     Similarly, in a case where a signal wire of the external antenna (earphone cable)  40 , such as a signal wire for audio signals, is to be used as an antenna, the connecting part  15   d  connecting to the first partial element  11  is electrically connected to any one of the signal wires  15   a  which one is intended to be used as the antenna, and the filter  15   c , which is arranged so as to interrupt (release) an electric current of the applied frequency of the antenna element  10  in a high-frequency manner, is placed at a position closer to the circuit than to the connecting point between the signal wire  15   a  and the connecting part  15   d  (at a position opposite the connector  15 ). Such an arrangement allows an existing signal wire, a ground wire, or the like to be used as an antenna, while eliminating a dedicated antenna wire. 
     In the above-described arrangement, a ground wire, a signal wire for audio signals or the like signal which signals are used in the external antenna (earphone cable)  40  are individually used as an antenna. The present invention is not limited to such an arrangement. Alternatively, a ground wire and a signal wire for audio signals or the like signals can be tied in a bundle to be used as an antenna. Further alternatively, only a plurality of signal wires for audio signals or the like signals can be tied in a bundle to be used as an antenna. 
     (Comparison with Another Configuration] 
     Another antenna device configured differently from the antenna device of the present invention will be described below in comparison with the antenna device  30  in accordance with the foregoing embodiments.  FIG. 8  is a view illustrating an antenna device configured differently from the antenna device of the present invention. 
     In an antenna device  90  illustrated in (a) of  FIG. 8 , connection between the feeding point  16  and the connector  15  is made without any intermediate antenna element, as in the technique disclosed in Patent Literature 1. 
     Such an arrangement brings about excellent antenna characteristics in a case where the external antenna  40  is engaged. However, in a case where the external antenna  40  is not engaged, anything that can operate as an antenna does not exist. This disables transmission and reception. 
     In an antenna device  91  illustrated in (b) of  FIG. 8 , the feeding point  16  and the connector  15  are connected to each other via an element  92 . Further, the feeding point  16  and the connector  15  are connected to both ends of the element  92 . 
     Such an arrangement brings about excellent antenna characteristics in a case where the external antenna  40  is engaged. However, in a case where the external antenna  40  is not engaged, the open end of the element  92  corresponds to the connector  15 . Therefore, the open end approaches a metallic component, such as a signal wire for audio signals, provided in the connector  15 . Thus, it is difficult to secure excellent antenna characteristics. 
     In contrast, the antenna device  30  in accordance with the foregoing embodiment can secure excellent antenna characteristics even in a case where the external antenna  40  is not engaged. 
     [Recap] 
     An antenna element  10  in accordance with Aspect 1 of the present invention includes: a feeding point  16 ; a connector  15  in which an external antenna  40  is detachably engaged; a first partial element  11  connecting the feeding point  16  and the connector  15 ; and a second partial element  12  branching from the first partial element  11  and having an open end  12   a  at a different position from the connector  15 . 
     According to the above arrangement, while the external antenna  40  is not engaged, an electric current dominantly flows between the feeding point  16  and the open end  12   a  via the second partial element  12 . This makes it possible to secure antenna characteristics. 
     On the other hand, in a case where the second partial element  12  is absent, antenna characteristics can be degraded while the external antenna  40  is not engaged. That is, in the absence of the second partial element  12 , an electric current dominantly flows through the first partial element  11  whose open end corresponds to the connector  15 , while the external antenna is not engaged. In this case, the connector  15  can be located at a position that is unfavorable in serving as the open end of the antenna element (e.g. at a position around which a metallic component is present) because an available location of the connector  15  can be provided in a limited location in the antenna device in order to allow the external antenna to be engaged in the connector  15  detachably. Particularly in a case where the connector  15  is an earphone jack, a signal wire for audio signals approaches the open end of the first partial element  11 . This causes degradation in antenna characteristics of the first partial element  11 . 
     In contrast, according to the above arrangement, the open end  12   a  is located at a different position from the connector  15 , and the open end  12   a  is provided at a distance from the metallic component which is present around the connector  15 . This facilitates an electric current flowing through the second partial element  12 . Accordingly, while the external antenna  40  is not engaged, an electric current dominantly flows between the feeding point  16  and the open end  12   a  via the second partial element  12 . This allows the path connecting the feeding point  16  and the open end  12   a  to operate as a monopole antenna, and therefore makes it possible to prevent degradation in antenna characteristics. 
     As described above, the above arrangement makes it possible to secure excellent antenna characteristics even in a case where the external antenna  40  is not engaged. 
     The antenna element  10  in accordance with Aspect 2 of the present invention is preferably arranged in Aspect 1 such that a second electrical length L 2  from the feeding point  16  to the open end  12   a  is closer to a ¼ wavelength of an applied frequency of the antenna element  10  than a first electrical length L 1  from the feeding point  16  to the connector  15  is. 
     According to the above arrangement, a greater resonance is generated over the path from the open end  12   a  to the feeding point  16  via the second partial element  12 , and a larger amount of electric current flows over the path from the open end  12   a  to the feeding point  16 . Therefore, it is possible to further improve antenna characteristics. 
     The antenna element  10  in accordance with Aspect 3 of the present invention is preferably arranged in Aspect 2 such that a third electrical length L 3  from the feeding point  16  to a branch point  11   a  at which and the second partial element  12  is branched from the first partial element  11  is longer than whichever shorter one of (i) one fourth of the second electrical length L 2  or (ii) a 1/16 wavelength of the applied frequency of the antenna element  10 . 
     According to the above arrangement, the electrical length of the second partial element  12  is sufficiently shorter than the ¼ wavelength of the applied frequency of the antenna element  10 . This causes the second partial element  12  to resonate at a frequency falling outside a band of the applied frequency (at a frequency higher than the applied frequency). With this arrangement, while the external antenna is engaged, an electrical current is hard to flow through the second partial element  12 , and a larger amount of electric current flows through the external antenna. This makes it possible to further improve antenna characteristics. 
     The antenna element  10  in accordance with Aspect 4 of the present invention can be arranged in Aspects 1 through 3 such that the external antenna  40  is an earphone cable while the connector  15  is an earphone jack. 
     According to the above arrangement, the earphone cable is used as the external antenna  40 , while the earphone jack is used as the connector  15 . This arrangement eliminates the need to separately provide another connector  15  for allowing the external antenna to be engaged in that connector  15 . 
     The antenna device  30  in accordance with Aspect 5 of the present invention is provided with the antenna element  10  of Embodiments 1 through 4 described earlier. 
     According to the above arrangement, the antenna device  30  can secure excellent antenna characteristics even in a case where the external antenna  40  is not engaged. 
     The antenna device  30  in accordance with Aspect 6 of the present invention may be arranged in Aspect 5 such that the antenna device  30  includes a metallic component (such as signal wires  15   a ), wherein the open end  12   a  is provided farther from the metallic component than the connector  15  is. 
     According to the above arrangement, it is possible to secure more excellent antenna characteristics in a case where the external antenna  40  is not engaged. 
     The present invention is not limited to the descriptions of the embodiments, but can be altered by a person skilled in the art within the scope of the claims. An embodiment derived from a proper combination of technical means disclosed in different embodiments is also encompassed in the technical scope of the present invention. Moreover, a novel technical feature can be created by a combination of technical means disclosed in these embodiments. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to radio devices in general. 
     REFERENCE SIGNS LIST 
     
         
           10  Antenna element 
           11  First partial element 
           11   a  Branch point 
           12  Second partial element 
           12   a  Open end 
           13  Fourth partial element 
           14  Third partial element 
           15  Connector 
           15   a  Signal wire 
           15   b  Ground terminal 
           15   c  Filter 
           15   d  Connecting part 
           16  Feeding point 
           20  Circuit section 
           21  Radio circuit section 
           22  Matching circuit section 
           30  Antenna device 
           40  External antenna