Patent Publication Number: US-7589687-B2

Title: Antenna apparatus provided with antenna element excited through multiple feeding points

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an antenna apparatus provided with a single antenna element which is excited through a plurality of feeding points, and a wireless communication apparatus including this antenna apparatus. More particularly, the present invention relates to an antenna apparatus, e.g., for mobile communication, and relates to a wireless communication apparatus including this antenna apparatus. 
     2. Description of the Related Art 
     The size and thickness of portable wireless communication apparatuses, such as mobile phones, have been rapidly reduced. Portable wireless communication apparatuses have been transformed from apparatuses to be used only as conventional telephones, to data terminals for transmitting and receiving electronic mails and for browsing web pages of WWW (World Wide Web). Further, since the amount of information to be handled has increased from that of conventional audio and text information to that of pictures and videos, a further improvement in communication quality is required. In such circumstances, an antenna apparatus capable of switching among directivities has been proposed. 
     PCT International Publication WO02/39544 discloses an antenna device including a rectangular conductive board, and a flat plate antenna mounted on the board with a dielectric interposed therebetween. The antenna device is characterized by exciting the antenna in a certain direction so as to flow a current through the board in one diagonal direction, and exciting the antenna in a different direction so as to flow a current through the board in the other diagonal direction. As such, in the antenna device disclosed in PCT International Publication WO02/3 9544, the directivity and polarization direction of the antenna device can be changed by varying the direction of a current flowing through the board. 
     Japanese Patent Laid-Open Publication No. 2005-130216 discloses a mobile radio apparatus that is foldable and that has a mechanism joining a first case and second case at a hinge part allowing said mobile radio apparatus to open and close. The mobile radio apparatus includes: a first flat conductor placed on a first plane inside the first case along a longitudinal direction of the first case, and a second flat conductor and third flat conductor placed on a second plane opposing a first plane inside the first case along the longitudinal direction of the first case, and feeding means for feeding the first flat conductor and feeding selectively the second flat conductor or the third flat conductor at a phase different from a phase with which the first flat conductor is fed. The mobile wireless apparatus disclosed in Japanese Patent Laid-Open Publication No. 2005-130216 can improve communication performance by switching between the second and third flat conductors in response to a reduction in reception level. 
     PCT International Publication WO01/97325 discloses a portable radio unit including a dipole antenna, and two feeding means each connected to one of two antenna elements that compose the dipole antenna. 
     Recently, an antenna apparatus has appeared that adopts MIMO (Multi-Input Multi-Output) technology for simultaneously transmitting and/or receiving radio signals of a plurality of channels by space division multiplexing, in order to increasing communication capacity and achieve high-speed communication. The antenna apparatus that performs MIMO communication needs to simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other, each having a different directivity, polarization characteristics, or the like, in order to achieve the space division multiplexing. The antenna device disclosed in PCT International Publication WO02/39544 can switch over to a different directivity, however, this antenna device cannot simultaneously implement a plurality of states, each having a different directivity. The mobile radio apparatus disclosed in Japanese Patent Laid-Open Publication No. 2005-130216 requires a plurality of antenna elements (flat conductors), and results in a complicated structure. Furthermore, in a similar manner to that of the antenna device disclosed in PCT International Publication WO02/39544, although this mobile radio apparatus can switch over to a different directivity, this mobile radio apparatus cannot simultaneously implement a plurality of states, each having a different directivity. The portable radio unit disclosed in PCT International Publication WO01/97325 cannot switch between directivities, and also cannot simultaneously implement a plurality of states, each having a different directivity. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is therefore to solve the above mentioned problems, and provide an antenna apparatus capable of simultaneously transmitting and/or receiving a plurality of radio signals with low correlation to each other, while having a simpler configuration than that of the prior art devices, and provide a wireless communication apparatus including this antenna apparatus. 
     According to a first aspect of the present invention, an antenna apparatus includes an antenna element having at least one slit, a first feeding point provided at a position on the antenna element, and a second feeding point provided along the slit. The antenna element is excited as an electric current antenna through the first feeding point, and at the same time, the slit is excited as a magnetic current antenna through the second feeding point. 
     In the antenna apparatus, the slit has an open end on a periphery of the antenna element. 
     Moreover, in the antenna apparatus, when the antenna element is excited as an electric current antenna, a radio signal is fed to the antenna element through a capacitor. 
     Further, in the antenna apparatus, the first and second feeding points are provided on the antenna element so as to be spatially spaced apart from each other by an odd multiple of ¼ wavelength of radio signals transmitted and/or received by the antenna apparatus. 
     Furthermore, in the antenna apparatus, the antenna apparatus transmits and/or receives a plurality of different radio signals by exciting the antenna element through the first and second feeding points simultaneously. 
     Moreover, in the antenna apparatus, the plurality of different radio signals are a plurality of channel signals transmitted and received using a MIMO communication method. 
     The antenna apparatus further includes a ground conductor connected to the antenna element. 
     According to a second aspect of the present invention, a wireless communication apparatus transmits and/or receives a plurality of radio signals using an antenna apparatus, the antenna apparatus includes an antenna element having at least one slit, a first feeding point provided at a position on the antenna element, and a second feeding point provided along the slit. The antenna apparatus excites the antenna element as an electric current antenna through the first feeding point, and at the same time, excites the slit as a magnetic current antenna through the second feeding point. 
     As described above, according to the antenna apparatus and wireless communication apparatus of the present invention, an antenna apparatus and a wireless communication apparatus can be provided that are capable of simultaneously transmitting and/or receiving a plurality of radio signals with low correlation to each other, while having a simple configuration. 
     According to the present invention, while reducing the number of antenna elements to one, it is possible to excite this antenna element as multiple antenna portions, and also to ensure isolation between these multiple antenna portions. The most important effects provided by the present invention include that the isolation between multiple antenna portions is ensured even when exciting a single antenna element through a plurality of feeding points simultaneously so that the antenna element operates as the multiple antenna portions; that the correlation coefficient between radio signals (electromagnetic waves) transmitted and/or received by the respective antenna portions can be reduced because the radio signals transmitted and/or received by the respective antenna portions have different polarizations; and that no degeneration occurs even when the antenna element has a symmetric structure, because different feeding methods (current feeding and voltage feeding) are used; and accordingly, each antenna portion operates well. 
     According to the antenna apparatus and wireless communication apparatus of the present invention, the isolation between the antenna portions can be improved, by further including an electromagnetic coupling adjuster. 
     Thus, in an antenna apparatus including a single antenna element, it becomes possible, for example, to transmit and/or receive radio signals of a plurality of channels using a MIMO communication method, to simultaneously perform wireless communications for a plurality of applications, or to simultaneously perform wireless communications in a plurality of frequency bands. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various objects, features, and advantages of the present invention will be disclosed as preferred embodiments which are described below with reference to the accompanying drawings. 
         FIG. 1  is a perspective view showing a schematic configuration of an antenna apparatus according to a first preferred embodiment of the present invention; 
         FIG. 2  is a block diagram showing a detailed configuration of a circuit of the antenna apparatus in  FIG. 1 ; 
         FIG. 3  is a block diagram showing a detailed configuration of a circuit of an antenna apparatus according to a modified preferred embodiment of the first preferred embodiment of the present invention; 
         FIG. 4A  is a front view of a mobile phone showing a first exemplary implementation of the antenna apparatus in  FIG. 1 ; 
         FIG. 4B  is a side view of the mobile phone showing the first exemplary implementation of the antenna apparatus in  FIG. 1 ; 
         FIG. 5A  is a front view of a mobile phone showing a second exemplary implementation of the antenna apparatus in  FIG. 1 ; 
         FIG. 5B  is a side view of the mobile phone showing the second exemplary implementation of the antenna apparatus in  FIG. 1 ; 
         FIG. 6  is a perspective view showing a schematic configuration of an antenna apparatus according to a second preferred embodiment of the present invention; 
         FIG. 7  is a block diagram showing a detailed configuration of a circuit of the antenna apparatus in  FIG. 6 ; 
         FIG. 8A  is a front view of a mobile phone showing a first exemplary implementation of the antenna apparatus in  FIG. 6 ; 
         FIG. 8B  is a side view of the mobile phone showing the first exemplary implementation of the antenna apparatus in  FIG. 6 ; 
         FIG. 8C  is a perspective view showing a left hinge portion  103   a  of the mobile phone showing the first exemplary implementation of the antenna apparatus in  FIG. 6 ; 
         FIG. 8D  is a perspective view showing a position at which an inner conductor  103   ad  is inserted into the left hinge portion  103   a  of the mobile phone showing the first exemplary implementation of the antenna apparatus in  FIG. 6 ; 
         FIG. 9A  is a front view of a mobile phone showing a second exemplary implementation of the antenna apparatus in  FIG. 6 ; 
         FIG. 9B  is a side view of the mobile phone showing the second exemplary implementation of the antenna apparatus in  FIG. 6 ; 
         FIG. 10  is a perspective view showing a schematic configuration of an antenna apparatus according to a third preferred embodiment of the present invention; 
         FIG. 11  is a block diagram showing a detailed configuration of a circuit of the antenna apparatus in  FIG. 10 ; 
         FIG. 12A  is a front view of a mobile phone showing a first exemplary implementation of the antenna apparatus in  FIG. 10 ; 
         FIG. 12B  is a side view of the mobile phone showing the first exemplary implementation of the antenna apparatus in  FIG. 10 ; 
         FIG. 13A  is a front view of a mobile phone showing a second exemplary implementation of the antenna apparatus in  FIG. 10 ; 
         FIG. 13B  is a side view of the mobile phone showing the second exemplary implementation of the antenna apparatus in  FIG. 10 ; 
         FIG. 14  is a perspective view showing a schematic configuration of an antenna apparatus according to a fourth preferred embodiment of the present invention; 
         FIG. 15  is a block diagram showing a detailed configuration of a circuit of the antenna apparatus in  FIG. 14 ; 
         FIG. 16A  is a front view of a mobile phone showing a first exemplary implementation of the antenna apparatus in  FIG. 14 ; 
         FIG. 16B  is a side view of the mobile phone showing the first exemplary implementation of the antenna apparatus in  FIG. 14 ; 
         FIG. 16C  is a top view showing a detailed configuration of a slit S 2  of the mobile phone showing the first exemplary implementation of the antenna apparatus in  FIG. 14 ; 
         FIG. 17A  is a front view of a mobile phone showing a second exemplary implementation of the antenna apparatus in  FIG. 14 ; 
         FIG. 17B  is a side view of the mobile phone showing the second exemplary implementation of the antenna apparatus in  FIG. 14 ; 
         FIG. 17C  is a top view showing a detailed configuration of a slit S 2  of the mobile phone showing the second exemplary implementation of the antenna apparatus in  FIG. 14 ; 
         FIG. 18  is a perspective view showing a schematic configuration of an antenna apparatus according to a first modified preferred embodiment of the fourth preferred embodiment of the present invention; 
         FIG. 19  is a perspective view showing a schematic configuration of an antenna apparatus according to a second modified preferred embodiment of the fourth preferred embodiment of the present invention; 
         FIG. 20  is a graph showing an intra-antenna coupling coefficient S 21  versus frequency, in the antenna apparatus in  FIG. 19 ; 
         FIG. 21  is a perspective view showing a schematic configuration of an antenna apparatus without a slit S 2 , which is a comparative example of the antenna apparatus in  FIG. 19 ; 
         FIG. 22  is a graph showing an intra-antenna coupling coefficient S 21  versus frequency, in the antenna apparatus in  FIG. 21 ; 
         FIG. 23  is a perspective view showing a schematic configuration of an antenna apparatus according to a fifth preferred embodiment of the present invention; 
         FIG. 24  is a perspective view showing a schematic configuration of an antenna apparatus according to a sixth preferred embodiment of the present invention; 
         FIG. 25  is a block diagram showing a detailed configuration of a circuit of the antenna apparatus in  FIG. 24 ; 
         FIG. 26A  is a front view of a mobile phone showing an exemplary implementation of the antenna apparatus in  FIG. 24 ; 
         FIG. 26B  is a side view of the mobile phone showing the exemplary implementation of the antenna apparatus in  FIG. 24 ; 
         FIG. 27  is a perspective view showing a schematic configuration of an antenna apparatus according to a seventh preferred embodiment of the present invention; 
         FIG. 28  is a perspective view showing a schematic configuration of an antenna apparatus according to an eighth preferred embodiment of the present invention; 
         FIG. 29A  is a front view of a mobile phone showing an exemplary implementation of the antenna apparatus in  FIG. 28 ; 
         FIG. 29B  is a side view of the mobile phone showing the exemplary implementation of the antenna apparatus in  FIG. 28 ; 
         FIG. 30  is a perspective view showing a schematic configuration of an antenna apparatus according to a modified preferred embodiment of the third preferred embodiment of the present invention; 
         FIG. 31  is a perspective view showing a schematic configuration of an antenna apparatus according to a third modified preferred embodiment of the fourth preferred embodiment of the present invention; 
         FIG. 32  is a perspective view showing a schematic configuration of an antenna apparatus according to a fourth modified preferred embodiment of the fourth preferred embodiment of the present invention; 
         FIG. 33  is a perspective view showing a schematic configuration of an antenna apparatus according to a first modified preferred embodiment of the fifth preferred embodiment of the present invention; 
         FIG. 34  is a perspective view showing a schematic configuration of an antenna apparatus according to a second modified preferred embodiment of the fifth preferred embodiment of the present invention; 
         FIG. 35  is a perspective view showing a schematic configuration of an antenna apparatus according to a first modified preferred embodiment of the eighth preferred embodiment of the present invention; and 
         FIG. 36  is a perspective view showing a schematic configuration of an antenna apparatus according to a second modified preferred embodiment of the eighth preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments according to the present invention will be described below with reference to the drawings. Note that in the drawings the same reference numerals denote like components. 
     First Preferred Embodiment 
       FIG. 1  is a perspective view showing a schematic configuration of an antenna apparatus according to a first preferred embodiment of the present invention. The antenna apparatus of the present preferred embodiment is characterized in that it includes a rectangular antenna element  1  having two different feeding points P 1  and P 2 , and makes the single antenna element  1  operate as two antenna portions, by exciting the antenna element  1  as a first antenna portion through the feeding point P 1 , and at the same time, exciting the antenna element  1  as a second antenna portion through the feeding point P 2 . 
     In  FIG. 1 , the antenna apparatus includes the antenna element  1  made of a rectangular conductive plate with a horizontal length L 1 ×a vertical length L 2 , and a ground conductor  2  made of a rectangular conductive plate with a horizontal length L 1 ×a vertical length L 3 . The antenna element  1  and the ground conductor  2  are juxtaposed so as to be spaced from each other by a certain distance, so that one side of the antenna element  1  and one side of the ground conductor  2  (in the present preferred embodiment, the sides with the length L 1 ) are opposed to each other. On the antenna element  1 , the two feeding points P 1  and P 2  are provided close to a side opposing to the ground conductor  2  (a bottom side of the antenna element  1 ), such that these feeding points P 1  and P 2  are spaced apart from each other by a distance L 4 . The feeding point P 1  is connected to a radio signal processor circuit  3  through a feed line F 1 , and similarly, the feeding point P 2  is connected to the radio signal processor circuit  3  through a feed line F 2 . Each of the feed lines F 1  and F 2  can be made of, for example, a coaxial cable with an impedance of 50Ω, and in this case, respective inner conductors of the coaxial cables connect the radio signal processor circuit  3  to the feeding points P 1  and P 2 , and on the other hand, respective outer conductors of the coaxial cables are connected to the ground conductor  2 . Although  FIG. 1  shows that the radio signal processor circuit  3  is integrated with the ground conductor  2 , the radio signal processor circuit  3  and the ground conductor  2  may be separately provided. The shape of the antenna element  1  is not limited to rectangular, but may be, e.g., polygonal, circular or elliptic. 
     The distance L 4  between the feeding points P 1  and P 2  satisfies the following relation of expression (1):
 
 L 4=(¼ +n/ 2)λ  (1),
 
where λ denotes a wavelength of radio signals transmitted and/or received by the antenna apparatus, and n denotes an integer greater than or equal to 0.
 
     In other words, the distance L 4  between the feeding points P 1  and P 2  is an odd multiple of ¼ wavelength of radio signals transmitted and/or received by the antenna apparatus. 
     In the antenna apparatus of the present preferred embodiment with the above-described configuration, it is possible to make the single antenna element  1  operate as two antenna portions such that the antenna element  1  is excited as the first antenna portion through the feeding point P 1 , and at the same time, the antenna element  1  is excited as the second antenna portion through the feeding point P 2 . As such, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals. 
       FIG. 2  is a block diagram showing a detailed configuration of a circuit of the antenna apparatus in  FIG. 1 . The feeding points P 1  and P 2  of the antenna element  1  are respectively connected, through the feed lines F 1  and F 2 , to switches  11 - 1  and  11 - 2  of a switch circuit  11  in the radio signal processor circuit  3 . The switch circuit  11  switches, under control of an antenna controller and modulator/demodulator circuit  16 , to either a state in which the antenna element  1  is directly connected to the antenna controller and modulator/demodulator circuit  16 , or a state in which the antenna element  1  is connected to the antenna controller and modulator/demodulator circuit  16  through an amplitude and phase controller circuit  12 . When the antenna element  1  is directly connected to the antenna controller and modulator/demodulator circuit  16 , the antenna controller and modulator/demodulator circuit  16  operates as a MIMO modulator/demodulator circuit, and transmits and/or receives, through the antenna element  1 , radio signals of a plurality of channels (in the present preferred embodiment, two channels) using a MIMO communication method. The antenna controller and modulator/demodulator circuit  16  may perform modulation or demodulation of two independent radio signals, instead of performing a MIMO modulation or demodulation, and in this case, the antenna apparatus of the present preferred embodiment can simultaneously perform wireless communications for a plurality of applications, or simultaneously perform wireless communications in a plurality of frequency bands. On the other hand, when the antenna element  1  is connected to the antenna controller and modulator/demodulator circuit  16  through the amplitude and phase controller circuit  12 , the amplitude and phase controller circuit  12  performs adaptive control on transmitted and/or received radio signals under control of an adaptive controller circuit  15 . The amplitude and phase controller circuit  12  includes amplitude adjusters  13 - 1  and  13 - 2 , and phase shifters  14 - 1  and  14 - 2 . Upon reception, each of signals received and respectively passed through the switches  11 - 1  and  11 - 2  is inputted to the amplitude and phase controller circuit  12  and inputted to the adaptive controller circuit  15 . Preferably, for the purpose of maximum ratio combining, the adaptive controller circuit  15  determines the amounts of changes in amplitudes and amounts of phase shifts of the signals based on the inputted received signals, changes the amplitude and phase of the signal passed through the switch  11 - 1 , by means of the amplitude adjuster  13 - 1  and the phase shifter  14 - 1 , and changes the amplitude and phase of the signal passed through the switch  11 - 2 , by means of the amplitude adjuster  13 - 2  and the phase shifter  14 - 2 . The received signals whose amplitudes and phases have been changed are combined with each other, and the combined signal is inputted to the antenna controller and modulator/demodulator circuit  16 . Upon transmission, in order to direct a beam in a desired direction, the adaptive controller circuit  15  determines the amounts of changes in amplitudes and amounts of phase shifts of signals to be transmitted under control of the antenna controller and modulator/demodulator circuit  16 , and according to this determination, makes the amplitude and phase controller circuit  12  change the amplitudes and phases of the signals to be transmitted. The antenna controller and modulator/demodulator circuit  16  is connected, through an input/output terminal  17  of the radio signal processor circuit  3 , to further circuits (not shown) in a wireless communication apparatus including the antenna apparatus of the present preferred embodiment. 
       FIG. 4A  is a front view of a mobile phone showing a first exemplary implementation of the antenna apparatus in  FIG. 1 , and  FIG. 4B  is a side view thereof. In  FIGS. 4A and 4B , the mobile phone of the present exemplary implementation includes an upper housing  101  and a lower housing  102 , each being shaped in a substantially rectangular parallelepiped. The upper housing  101  and the lower housing  102  are connected to each other in a foldable manner through a cylindrical hinge portion  103 . The upper housing  101  includes a first upper housing portion  101   a  located on a side close to a user during a telephone call using the mobile phone (in the following description, referred to as the “inner side” of the mobile phone), and a second upper housing portion  101   b  located on a side away from the user (hereinafter, referred to as the “outer side” of the mobile phone). The first upper housing portion  101   a  and the second upper housing portion  101   b  are secured at a left bottom portion of the inner side of the upper housing  101  by a screw  107  and a screw receiving portion (not shown), and secured at a right bottom portion of the inner side of the upper housing  101  by a screw  108  and a screw receiving portion  108   a . In the present exemplary implementation, each of the first upper housing portion  101   a  and the second upper housing portion  101   b  is made of a conductor, and thus the upper housing  101  operates as the antenna element  1  in  FIGS. 1 and 2 . On the other hand, the lower housing  102  is made of a dielectric (e.g., plastic). The hinge portion  103  includes a left hinge portion  103   a  and a right hinge portion  103   b  which are mechanically connected to the first upper housing portion  101   a , and includes a central hinge portion  103   c  which is integrally formed with the lower housing  102  and fits between the left hinge portion  103   a  and the right hinge portion  103   b . The upper housing  101  and the lower housing  102  can be rotated about the hinge portion  103  by a rotating shaft (not shown) extending through the left hinge portion  103   a , the central hinge portion  103   c  and the right hinge portion  103   b , and thus can be folded. In addition, a display  106  is disposed at substantially the center of the first upper housing portion  101   a , and a speaker  104  is disposed above the display  306 . Furthermore, a microphone  105  is disposed on the inner side of the mobile phone and in the vicinity of a bottom end of the lower housing  102 , and a rechargeable battery  110  is disposed on the opposite side of the microphone  105  (i.e., the outer side of the mobile phone) in the lower housing  102 . A rectangular printed wiring board  109  is disposed within the lower housing  102  and at substantially the center in a thickness direction of the lower housing  102  (for ease of illustration, the representation of the thickness of the printed wiring board  109  is omitted). On the entire outer side surface of the printed wiring board  109  is formed a conductive pattern which acts as the ground conductor  2  in  FIG. 1 , on the other hand, on an inner side surface of the printed wiring board  109  is provided a radio signal processor circuit  3 . A feed line F 1  is made of a coaxial cable, extends from the radio signal processor circuit  3  to the upper housing  101  through the left hinge portion  103   a , and is electrically connected to the left bottom portion of the first upper housing portion  101   a  by the screw  107 . This connection point acts as the feeding point P 1  of the antenna element  1 . Similarly, a feed line F 2  is also made of a coaxial cable, extends from the radio signal processor circuit  3  to the upper housing  101  through the right hinge portion  103   b , and is electrically connected to the right bottom portion of the first upper housing portion  101   a  by the screw  108 . This connection point acts as the feeding point P 2  of the antenna element  1 . The lower housing  102  may be made of a conductor, and in this case, the lower housing  102  instead of the printed wiring board  109  acts as the ground conductor  2 . 
       FIG. 5A  is a front view of a mobile phone showing a second exemplary implementation of the antenna apparatus in  FIG. 1 , and  FIG. 5B  is a side view thereof. The mobile phone of the present exemplary implementation is characterized in that each of a first upper housing portion  101   a  and a second upper housing portion  101   b  is made of a dielectric (e.g., plastic), and an antenna element  1  made of a rectangular conductive plate is provided within an upper housing  101 . A feed line F 1  is electrically connected to a feeding point P 1  at a left bottom portion of the antenna element  1 , and similarly, a feed line F 2  is electrically connected to a feeding point P 2  at a right bottom portion of the antenna element  1 . 
       FIG. 3  is a block diagram showing a detail configuration of a circuit of an antenna apparatus according to a modified preferred embodiment of the first preferred embodiment of the present invention. In the case that the antenna apparatus of the present preferred embodiment is provided to a foldable type mobile phone, such as those shown in  FIGS. 4A ,  4 B,  5 A and  5 B, a left hinge portion  103   a  and a right hinge portion  103   b  of the mobile phone may be made of a conductive material such as aluminum or zinc, the left hinge portion  103   a  may be used as part of a feed line F 1 , and the right hinge portion  103   b  may be used as part of a feed line F 2 . 
     As described above, according to the antenna apparatus of the present preferred embodiment, it is possible to make the single antenna element  1  operate as two antenna portions, and accordingly, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals. 
     Second Preferred Embodiment 
       FIG. 6  is a perspective view showing a schematic configuration of an antenna apparatus according to a second preferred embodiment of the present invention. Although in the antenna apparatus of the first preferred embodiment the radio signal is directly fed to the antenna element  1  at both of the feeding points P 1  and P 2 , the antenna apparatus of the second preferred embodiment is characterized by that a radio signal is capacitively fed (fed through a capacitor) to a antenna element  1  at one of the feeding points P 1  and P 2  in  FIG. 1 , i.e., at a feeding point P 1 . 
     In  FIG. 6 , the antenna apparatus is provided with an electrode E 1 , which is made of a conductive plate and provided in parallel to an antenna element  1  at a position where the feeding point P 1  is provided in  FIG. 1 . The electrode E 1  is spaced from the antenna element  1  by a certain distance L 11  through air or a certain dielectric material. Thus, in the antenna apparatus of the present preferred embodiment, a capacitor is formed by the electrode E 1  and the antenna element  1 , a feeding point P 1  is provided on the electrode E 1 , and a radio signal is fed to the antenna element  1  through this capacitor. In the following description, the feeding point P 1 , the electrode E 1 , and the capacitor formed by the electrode E 1  and the antenna element  1  are also referred to as a “capacitive feeding portion” of a first antenna portion. A point on the antenna element  1  that is closest to the feeding point P 1  is regarded as a reference point P 1   a  for the capacitive feeding, and a distance L 4  between the reference point P 1   a  and a feeding point P 2  satisfies the expression (1), in a similar manner to that of the first preferred embodiment. The size of the electrode E 1  is appropriately determined according to the frequency of radio signals transmitted and/or received by the antenna apparatus. Preferably, the size is determined such that the length in at least one direction of the electrode E 1  (e.g., in the case of a rectangular electrode E 1 , the direction of a longitudinal side thereof) is (¼+n/2) λ, where λ denotes a wavelength of radio signals transmitted and/or received by the antenna apparatus, and n denotes an integer greater than or equal to 0. 
     In the antenna apparatus of the present preferred embodiment with the above described configuration, the feeding point P 1  at which a radio signal is fed through a capacitor acts as a voltage feeding point, and the feeding point P 2  at which a radio signal is fed directly acts as a current feeding point, and therefore, isolation between the first antenna portion and the second antenna portion improves as compared with the case of the first preferred embodiment. As such, in the antenna apparatus of the present preferred embodiment, it is possible to make the single antenna element  1  operate as two antenna portions such that the antenna element  1  is excited as the first antenna portion through the feeding point P 1 , and at the same time, the antenna element  1  is excited as the second antenna portion through the feeding point P 2 . Accordingly, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other. Note that in prior art circular polarization antennas, a single antenna element is simultaneously excited through two feeding points provided on the antenna element by two signals having a 90° phase difference relative to each other, on the other hand, the antenna apparatus of the present preferred embodiment does not have a constant phase difference between signals. Since the antenna apparatus of the present preferred embodiment can improve the isolation according to the distance L 4 , it is possible to simultaneously excite a plurality of feeding points by different signals, and thus achieve a MIMO operation, while having a simple configuration. 
       FIG. 7  is a block diagram showing a detailed configuration of a circuit of the antenna apparatus in  FIG. 6 . A capacitive feeding portion of the first antenna portion is preferably provided within a left hinge portion  103   a , as will be described in detail later with reference to  FIGS. 8A ,  8 B,  8 C and  8 D.  FIG. 7  shows that a space between conductive components  103   ac  and  103   ad  which configure the left hinge portion  103   a  (e.g., a space between the conductive components  103   ac  and  103   ad  spaced apart from each other by means of a dielectric) acts as a capacitor C 1 . The reference point P 1   a  for the capacitive feeding of the antenna element  1  is connected to the left hinge portion  103   a , and the feeding point P 1  provided on the left hinge portion  103   a  is connected to a radio signal processor circuit  3  through the feed line F 1 . 
       FIG. 8A  is a front view of a mobile phone showing a first exemplary implementation of the antenna apparatus in  FIG. 6 ,  FIG. 8B  is a side view thereof,  FIG. 8C  is a perspective view showing a left hinge portion  103   a  in  FIG. 8A , and  FIG. 8D  is a perspective view showing a position at which an inner conductor  103   ad  is inserted into the left hinge portion  103   a  in  FIG. 8C . In the mobile phone of the present exemplary implementation, the left hinge portion  103   a  is made of a conductive material such as aluminum or zinc, and has, as shown in  FIG. 8C , an integral structure including a blade portion  103   ab  and a cylindrical portion  103   ac . The blade portion  103   ab  has a screw hole  103   aa  for receiving a screw  107 , by which the left hinge portion  103   a  is electrically and mechanically connected to a left bottom portion of an upper housing  101 . As shown in  FIG. 8D , a cylindrical inner conductor  103   ad  made of a conductive material is inserted into the cylindrical portion  103   ac  of the left hinge portion  103   a  in a rotatable manner. At least one of the inner side of the cylindrical portion  103   ac  and the outer side of the inner conductor  103   ad  is coated by a dielectric, and thus, when the inner conductor  103   ad  is inserted into the cylindrical portion  103   ac , a capacitor C 1  of  FIG. 7  is formed between the inner side surface of the cylindrical portion  103   ac  and the outer side surface of the inner conductor  103   ad . The inner conductor  103   ad  is connected to a radio signal processor circuit  3  through a feed line F 1  made of a coaxial cable or the like. In the present exemplary implementation, in a similar manner to that of the exemplary implementation in  FIGS. 4A and 4B , each of a first upper housing portion  101   a  and a second upper housing portion  101   b  is made of a conductor, and thus an upper housing  101  operates as the antenna element  1  in  FIGS. 6 and 7 . In the present exemplary implementation, a point at which the feed line F 1  is connected to the inner conductor  103   ad  is regarded as a feeding point P 1 , and a point at which the left hinge portion  103   a  is connected to the upper housing  101  by the screw  107  is regarded as a reference point P 1   a  for the capacitive feeding. In the mobile phone of the present exemplary implementation, a right hinge portion  103   b  also has an integral structure including a blade portion and a cylindrical portion, and the blade portion has a screw hole (not shown) for receiving a screw  108 , by which the right hinge portion  103   a  is mechanically connected to the upper housing  101 . A feed line F 2  extends from the radio signal processor circuit  3  to the upper housing  101  through a pass-through hole (not shown) provided in the right hinge portion  103   b , and is electrically connected to a right bottom portion of the first upper housing portion  101   a  by the screw  108 . This connection point acts as a feeding point P 2  of the antenna element  1 . 
       FIG. 9A  is a front view of a mobile phone showing a second exemplary implementation of the antenna apparatus in  FIG. 6 , and  FIG. 9B  is a side view thereof. The present exemplary implementation is characterized in that, in a similar manner to that of the exemplary implementation in  FIGS. 5A and 5B , each of a first upper housing portion  101   a  and a second upper housing portion  101   b  is made of a dielectric, and an antenna element  1  made of a rectangular conductive plate is provided within an upper housing  101 . In the present exemplary implementation, a left hinge portion  103   a  and a right hinge portion  103   b  themselves are configured in the same manner as in the exemplary implementation in  FIGS. 8A ,  8 B,  8 C and  8 D. The left hinge portion  103   a  is mechanically connected to the upper housing  101  at a screw hole of a blade portion thereof, and is electrically connected to a left bottom portion of the antenna element  1 . A point at which the left hinge portion  103   a  is connected to the antenna element  1  by a screw  107  is regarded as a reference point P 1   a  for the capacitive feeding. On the other hand, the right hinge portion  103   b  is mechanically connected to the upper housing  101  at a screw hole of a blade portion thereof. A feed line F 2  is electrically connected to a right bottom portion of the antenna element  1  by a screw  108 , and this connection point acts as a feeding point P 2  of the antenna element  1 . 
     The exemplary implementations in  FIGS. 8A ,  8 B,  8 C,  8 D,  9 A and  9 B describe the case in which the feed line F 2  is made of a coaxial cable or the like, alternatively, as shown in  FIG. 3 , the right hinge portion  103   b  may be used as part of the feed line F 2 . In this case, the right hinge portion  103   b  is made of a conductive material, in a similar manner to that of the left hinge portion  103   a , and a cylindrical inner conductor made of a conductive material is inserted into a cylindrical portion of the right hinge portion  103   b  in a rotatable manner. An electrical connection is established between the inner side surface of the cylindrical portion and the outer side surface of the inner conductor without providing dielectric coating to any of the inner side of the cylindrical portion and the outer side of the inner conductor, and furthermore, in a similar manner to that of the inner conductor  103   ad  of the left hinge portion  103   a , an inner conductor of the right hinge portion  103   b  is connected to the radio signal processor circuit  3  through a coaxial cable or the like. In this configuration, a point at which the right hinge portion  103   b  is connected to the upper housing  101  or the antenna element  1  by the screw  108  acts as a feeding point P 2 . 
     As described above, according to the antenna apparatus of the present preferred embodiment, it is possible to make the single antenna element  1  operate as two antenna portions, and accordingly, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other. 
     Third Preferred Embodiment 
       FIG. 10  is a perspective view showing a schematic configuration of an antenna apparatus according to a third preferred embodiment of the present invention, and  FIG. 11  is a block diagram showing a detailed configuration of a circuit of the antenna apparatus in  FIG. 10 . Although in the antenna apparatus of the second preferred embodiment the antenna element  1  operates as an electric current antenna (i.e., an antenna in which the antenna element  1  acts as an electric current source) in each of both cases in which the radio signal is capacitively fed to the antenna element  1  through the feeding point P 1  and in which the radio signal is fed directly to the antenna element  1  through the feeding point P 2 , the antenna apparatus of the third preferred embodiment is characterized in that the antenna apparatus further has a slit S 1 , and when a radio signal is fed to the slit S 1  through a feeding point P 2 , the slit S 1  is made to operate as a magnetic current antenna (i.e., an antenna in which the slit S 1  acts as a magnetic current source), or a slit antenna. 
     Referring  FIG. 10 , the antenna apparatus of the present preferred embodiment is provided with, on an antenna element  1 , a capacitive feeding portion configured in the same manner as in  FIG. 6 , and with a slit S 1  having a certain width and a length L 21  and having an open end at one end thereof. The slit S 1  has, as its open end, an opening at a side of the antenna element  1  that is opposed to a ground conductor  2 , and the opening of the slit S 1  is located apart from a reference point P 1   a  for the capacitive feeding of a first antenna portion by a distance L 22 . In addition, a feeding point P 2  is provided along the slit S 1  at a position apart from the opening of the slit S 1  by a distance L 23 , and the feeding point P 2  is connected to a radio signal processor circuit  3  through a feed line F 2  made of a coaxial cable or the like, in a similar manner to those of the first and second preferred embodiments. 
     A distance L 22 +L 23  between the feeding points P 1  and P 2  satisfies the following relation of expression (2):
 
 L 22 +L 23=(¼ +n/ 2)λ  (2),
 
where λ denotes a wavelength of radio signals transmitted and/or received by the antenna apparatus, and n denotes an integer greater than or equal to 0.
 
     Although  FIG. 10  shows that the feeding point P 2  is positioned apart from the opening of the slit S 1  by the distance L 23 , the present invention is not limited so, and the feeding point P 2  can be provided at a desired position along the slit S 1 , as long as the position satisfies the expression (2) (i.e., 0≦L 23 &lt;L 21 ). 
     According to the antenna apparatus of the present preferred embodiment, the antenna element  1  is made to operate as an electric current antenna (first antenna portion) by feeding at the feeding point P 1  the antenna element  1  made of a conductive plate with voltage through a capacitor, on the other hand, the slit S 1  is made to operate as a magnetic current antenna (second antenna portion) by directly feeding the slit S 1  with current at the feeding point P 2 . Accordingly, in the antenna apparatus of the present preferred embodiment, the distance between the feeding points P 1  and P 2  is configured according to the expression (2), and additionally, polarization directions each formed when the antenna element  1  is excited through the feeding point P 1  and when the antenna element  1  is excited through the feeding point P 2  are different from each other, by the differences of the planar antenna and slit antenna, the capacitive feeding and direct feeding, the voltage feeding and current feeding, and the electric current antenna and magnetic current antenna. Therefore, in the present preferred embodiment, isolation between the first antenna portion and the second antenna portion is improved as compared with the case of the first and second preferred embodiments, and −10 dB or better isolation can be achieved. As such, in the antenna apparatus of the present preferred embodiment, it is possible to make the single antenna element  1  operate as two antenna portions such that the antenna element  1  is excited as the first antenna portion through the feeding point P 1 , and at the same time, the slit S 1  is excited as the second antenna portion through the feeding point P 2 . Accordingly, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other. 
       FIG. 12A  is a front view of a mobile phone showing a first exemplary implementation of the antenna apparatus in  FIG. 10 , and  FIG. 12B  is a side view thereof. In the present exemplary implementation, each of a first upper housing portion  101   a  and a second upper housing portion  101   b  is made of a conductor in a similar manner to those of the exemplary implementations in  FIGS. 4A ,  4 B,  8 A,  8 B,  8 C and  8 D, and a slit S 1  is formed in a right side surface of an upper housing  101  and between the first upper housing portion  101   a  and the second upper housing portion  101   b . For the purpose of configuring a lower end of the slit S 1  (i.e., an end of the slit S 1  on the side close to a hinge portion  103 ) as an open end, a portion of the hinge portion  103  opposing the lower end of the slit S 1  is preferably made as empty space or made of a dielectric material. A feeding point P 2  is provided at a position upward from the lower end of the slit S 1  by a certain distance, and in a similar manner to those of the exemplary implementations in  FIGS. 8A ,  8 B,  8 C and  8 D, the feeding point P 2  extends to a lower housing  102  through a feed line F 2 , and then, is connected to a radio signal processor circuit  3 . Thus, the upper housing  101  operates as an antenna element  1  having the slit S 1  in  FIGS. 10 and 11 . The space inside the slit S 1  is preferably filled by a dielectric material, for mechanical reinforcement. 
       FIG. 13A  is a front view of a mobile phone showing a second exemplary implementation of the antenna apparatus in  FIG. 10 , and  FIG. 13B  is a side view thereof. The present exemplary implementation is characterized in that each of a first upper housing portion  101   a  and a second upper housing portion  101   b  is made of a dielectric, an antenna element  1  made of a rectangular conductive plate is provided within an upper housing  101 , in a similar manner to those of the exemplary implementations in  FIGS. 5A ,  5 B,  9 A and  9 B, and furthermore, a slit S 1  is formed in the antenna element  1 . A lower end of the slit S 1  is configured as an open end, and a feeding point P 2  is provided at a position upward from the lower end by a certain distance. As with the exemplary implementation in  FIGS. 8A ,  8 B,  8 C and  8 D, the feeding point P 2  extends to a lower housing  102  through a feed line F 2 , and then, is connected to a radio signal processor circuit  3 . 
     As described above, according to the antenna apparatus of the present preferred embodiment, it is possible to make the single antenna element  1  operate as two antenna portions, and accordingly, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other. 
     Fourth Preferred Embodiment 
       FIG. 14  is a perspective view showing a schematic configuration of an antenna apparatus according to a fourth preferred embodiment of the present invention and  FIG. 15  is a block diagram showing a detailed configuration of a circuit of the antenna apparatus in  FIG. 14 . The antenna apparatus of the present preferred embodiment is characterized by the configuration of the antenna apparatus of the third preferred embodiment, and additionally, by having a slit S 2  for adjusting electromagnetic coupling, between a first antenna portion and a second antenna portion, so that a certain amount of isolation between the first antenna portion and the second antenna portion is ensured. 
     Referring  FIG. 14 , the antenna apparatus of the present preferred embodiment is provided with, on an antenna element  1 , the configuration of the antenna apparatus in  FIG. 10 , and additionally, a slit S 2  having a certain width and a length L 31  and having an open end at one end thereof. The slit S 2  has, as its open end, an opening at a side of the antenna element  1  opposing a ground conductor  2 , and between a reference point P 1   a  for the capacitive feeding of a first antenna portion and an opening of a slit S 1  of a second antenna portion. The opening of the slit S 2  is located apart from the reference point P 1   a  for the capacitive feeding by a distance L 32 , and apart from the opening of the slit S 1  by a distance L 33 . 
     A distance L 32 +2×L 31 +L 33 +L 23  between feeding points P 1  and P 2  satisfies the following relation of expression (3):
 
 L 32+2 ×L 31 +L 33 +L 23=(¼ +n/ 2)  (3),
 
where λ denotes a wavelength of radio signals transmitted and/or received by the antenna apparatus, and n denotes an integer greater than or equal to 0.
 
     Although  FIG. 14  shows that the feeding point P 2  is positioned apart from the opening of the slit S 1  by the distance L 23 , the present invention is not limited so, and the feeding point P 2  can be provided at a desired position along the slit S 1 , as long as the position satisfies the expression (3) (i.e., 0≦L 23 &lt;L 21 ). 
     According to the antenna apparatus of the present preferred embodiment, isolation between the first antenna portion and the second antenna portion is further improved over that of the antenna apparatus of the third preferred embodiment, by virtue of the slit S 2  for adjusting electromagnetic coupling between the first antenna portion and the second antenna portion. As such, in the antenna apparatus of the present preferred embodiment, it is possible to make the single antenna element  1  operate as two antenna portions such that the antenna element  1  is excited as the first antenna portion through the feeding point P 1 , and at the same time, the slit S 1  is excited as the second antenna portion through the feeding point P 2 . Accordingly, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other. 
       FIG. 16A  is a front view of a mobile phone showing a first exemplary implementation of the antenna apparatus in  FIG. 14 ,  FIG. 16B  is a side view thereof, and  FIG. 16C  is a top view showing a detailed configuration of a slit S 2  in  FIG. 16A . The present exemplary implementation is provided with the configuration of the exemplary implementation in  FIGS. 12A and 12B , and additionally, a slit S 2  within a first upper housing portion  101   a  below a display  106 . In the present exemplary implementation, for ensuring the length L 31  of the slit S 2 , the slit S 2  is configured as a T-shaped slit as shown in  FIG. 16C , which consists of a horizontal slit S 2   a  and a vertical slit S 2   b , and in which the sum of the length of the slit S 2   a  and the length of the slit S 2   b  is the length L 31 . For the purpose of configuring a lower end of the slit S 2  as an open end, it is preferable that a central hinge portion  103   c  opposing a lower end of the vertical slit S 2   b  is preferably made of a dielectric material. Thus, an upper housing  101  operates as the antenna element  1  having the slit S 2  in  FIGS. 14 and 15 . The space inside the slit  2  is preferably filled by a dielectric material, for mechanical reinforcement. The slit S 2  is not limited to be configured in T-shape, and an arbitrary shape having the length L 31  can be employed. 
       FIG. 17A  is a front view of a mobile phone showing a second exemplary implementation of the antenna apparatus in  FIG. 14 ,  FIG. 17B  is a side view thereof, and  FIG. 17C  is a top view showing a detailed configuration of a slit S 2  in  FIG. 17A  The present exemplary implementation is provided with the configuration of the exemplary implementation in  FIGS. 13A and 13B , and additionally, a slit S 2  on an antenna element  1 . In the present exemplary implementation, in a similar manner to that of the exemplary implementation in  FIGS. 16A ,  16 B and  16 C, the slit S 2  is configured as a T-shaped slit as shown in  FIG. 17C , which consists of a horizontal slit S 2   a  and a vertical slit S 2   b , and in which the sum of the length of the slit S 2   a  and the length of the slit S 2   b  is the length L 31 . A lower end of the slit S 2  (i.e., a lower end of the vertical slit S 2   b ) is configured as an open end. The slit S 2  is not limited to be configured in T-shape, and an arbitrary shape having the length L 31  can be employed. 
       FIG. 18  is a perspective view showing a schematic configuration of an antenna apparatus according to a first modified preferred embodiment of the fourth embodiment of the present invention. Although in the antenna apparatus in  FIG. 14  the first antenna portion is made to operate as an electric current antenna and the second antenna portion is made to operate as a magnetic current antenna, the present modified preferred embodiment is characterized in that the antenna apparatus has a slit S 3  instead of the electrode E 1  in  FIG. 14 , and a first antenna portion is also made to operate as a magnetic current antenna (or a slit antenna). 
     Referring  FIG. 18 , the antenna apparatus of the present modified preferred embodiment is provided with, on an antenna element  1 , a slit S 3  having a certain width and a length L 41  and having an open end at one end thereof, instead of the capacitive feeding portion including the electrode E 1  of the first antenna portion in  FIG. 14 . The slit S 3  has, as its open end, an opening at a side of the antenna element  1  opposing a ground conductor  2 , and the opening of the slit S 3  is located apart from an opening of a slit S 2  by a distance L 43 , and the opening of the slit S 2  is located apart from an opening of a slit S 1  by a distance L 44 . In addition, a feeding point P 1  is provided along the slit S 3  at a position apart from the opening of the slit S 3  by a distance L 42 , and the feeding point P 1  is connected to a radio signal processor circuit  3  through a feed line F 1  made of a coaxial cable or the like, in a similar manner to that of a feeding point P 2  of the slit S 1 . 
     A distance L 42 +L 43 +2×L 31 +L 44 +L 23  between the feeding points P 1  and P 2  satisfies the following relation of expression (4):
 
 L 42 +L 43+2 ×L 31 +L 44 +L 23=(¼+n/2)λ  (4),
 
where λ denotes a wavelength of radio signals transmitted and/or received by the antenna apparatus, and n denotes an integer greater than or equal to 0.
 
     Although  FIG. 18  shows that the feeding point P 1  is positioned apart from the opening of the slit S 3  by the distance L 42  and the feeding point P 2  is positioned apart from the opening of the slit S 1  by the distance L 23 , the present invention is not limited so, and the feeding point P 1  can be provided at a desired position along the slit S 3 , as long as the position satisfies the expression (4) (i.e., 0≦L 42 &lt;L 41 ), and similarly, the feeding point P 2  can be provided at a desired position along the slit S 1  (i.e., 0≦L 23 &lt;L 21 ). 
     According to the antenna apparatus of the present modified preferred embodiment, the distance between the feeding points P 1  and P 2  is configured according to the expression (4), and additionally, the slit S 2  is provided for adjusting electromagnetic coupling between the first antenna portion and the second antenna portion, thus even in the case that both of the first antenna portion and the second antenna portion are excited as magnetic current antennas, sufficient isolation between the first antenna portion and the second antenna portion (e.g., −10 dB or better isolation) can be achieved. As such, in the antenna apparatus of the present preferred embodiment, it is possible to make the single antenna element  1  operate as two antenna portions such that the slit S 3  is excited as the first antenna portion through the feeding point P 1 , and at the same time, the slit S 1  is excited as the second antenna portion through the feeding point P 2 . Accordingly, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other. 
       FIG. 19  is a perspective view showing a schematic configuration of an antenna apparatus according to a second modified preferred embodiment of the fourth preferred embodiment of the present invention. Although in the antenna apparatus in  FIG. 14  the first antenna portion is made to operate as an electric current antenna and the second antenna portion is made to operate as a magnetic current antenna, the present modified preferred embodiment is characterized in that the antenna apparatus is provided with an electrode E 2  instead of the slit S 1  in  FIG. 14 , and a second antenna portion is also made to operate as an electric current antenna. 
     Referring  FIG. 19 , the antenna apparatus of the present modified preferred embodiment is provided with, on an antenna element  1 , an electrode E 2  made of a conductive plate and provided in parallel to the antenna element  1 , instead of the slit S 1  in  FIG. 14 . The electrode E 2  is spaced from the antenna element  1  by a certain distance through air or a certain dielectric material (in the antenna apparatus in  FIG. 19 , the electrode E 2  is spaced by the same distance L 1  as the electrode E 1 ). Thus, in the antenna apparatus of the present modified preferred embodiment, a capacitor is formed by the electrode E 2  and the antenna element  1 , a feeding point P 2  is provided on the electrode E 2 , and a radio signal is fed to the antenna element  1  through this capacitor. As such, the feeding point P 2 , the electrode E 2 , and the capacitor formed by the electrode E 2  and the antenna element  1  configure a capacitive feeding portion for a second antenna portion. A point on the antenna element  1  that is closest to the feeding point P 2  is regarded as a reference point P 2   a  for the capacitive feeding. The size of the electrode E 2  is appropriately determined according to the frequency of radio signals transmitted and/or received by the antenna apparatus, in a similar manner to that of the electrode E 1 . Preferably, the size is determined such that the length in at least one direction of the electrode E 2  (e.g., in the case of a rectangular electrode E 2 , the direction of a longitudinal side thereof) is (¼+n/2)λ, where λ denotes a wavelength of radio signals transmitted and/or received by the antenna apparatus, and n denotes an integer greater than or equal to 0. In the following description, each of the electrodes E 1  and E 2  is made of a rectangular conductive plate with a horizontal length L 52 ×a vertical length L 51 , the electrode E 1  is located such that the left and bottom sides thereof are close to the left and bottom sides of the rectangular antenna element  1 , and the electrode E 2  is located such that the right and bottom sides thereof are close to the right and bottom sides of the antenna element  1 . A feeding point P 1  is provided at a left bottom end of the electrode E 1 , and the feeding point P 2  is provided at a right bottom end of the electrode E 2 . Accordingly, a reference point P 1   a  for the capacitive feeding is located at a left bottom end of the antenna element  1 , and the reference point P 2   a  is located at a right bottom end of the antenna element  1 . A slit S 2  has a length L 31  and a width L 53 , and is provided in parallel to left and right sides of the antenna element  1 . An opening at a lower end of the slit S 2  is located apart from the reference point P 1   a  for the capacitive feeding toward the right by a distance L 54 , and apart from the reference point P 2   a  toward the left by a distance L 55 . The antenna element  1  and a ground conductor  2  are in the same plane, and are spaced from each other by a distance L 56 . 
     A distance L 54 +2×L 31 +L 55  between the feeding points P 1  and P 2  satisfies the following relation of expression (5):
 
 L 54+2 ×L 31 +L 55=(¼ +n/ 2)λ  (5),
 
where λ denotes a wavelength of radio signals transmitted and/or received by the antenna apparatus, and n denotes an integer greater than or equal to 0.
 
     According to the antenna apparatus of the present modified preferred embodiment, the distance between the feeding points P 1  and P 2  is configured according to the expression (5), and additionally, the slit S 2  is formed for adjusting electromagnetic coupling between the first antenna portion and the second antenna portion, thus even in the case that both of the first antenna portion and the second antenna portion are excited as electric current antennas, sufficient isolation between the first antenna portion and the second antenna portion (e.g., −10 dB or better isolation) can be achieved. As such, in the antenna apparatus of the present preferred embodiment, it is possible to make the single antenna element  1  operate as two antenna portions such that the antenna element  1  is excited as the first antenna portion through the feeding point P 1 , and at the same time, the antenna element  1  is excited as the second antenna portion through the feeding point P 2 . Accordingly, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other. 
     Referring  FIGS. 20 to 22 , effects of forming the slit S 2  in the antenna apparatus of the present preferred embodiment will be described below. In a simulation conducted by the inventors of the present invention, it is examined how isolation between the first antenna portion and the second antenna portion varies while changing the length L 31  of the slit S 2  in the antenna apparatus in  FIG. 19 . In order to identify the isolation between the first antenna portion and the second antenna portion, a parameter S 21  of a transfer coefficient (hereinafter, referred to as the intra-antenna coupling coefficient S 21 ) is used which is defined from a first port of the radio signal processor circuit  3  connected to the feeding point P 1  through the feed line F 1  of 50Ω, to a second port of the radio signal processor circuit  3  connected to the feeding point P 2  through the feed line F 2  of 50Ω. 
       FIG. 20  is a graph showing the intra-antenna coupling coefficient S 21  versus frequency, in the antenna apparatus in  FIG. 19 . In the simulation of  FIG. 20 , the antenna element  1  is configured with the following dimensions (in millimeter). 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 L1 = 45 
               
               
                   
                 L2 = L3 = 90 
               
               
                   
                 L11 = 1 
               
               
                   
                 L31 = 30, 35, 40 
               
               
                   
                 L51 = 43 
               
               
                   
                 L52 = 10 
               
               
                   
                 L53 = 1 
               
               
                   
                 L54 = L55 = 22.5 
               
               
                   
                 L56 = 5 
               
               
                   
                   
               
            
           
         
       
     
     According to  FIG. 20 , it can be seen that isolation characteristics is improved depending on the length L 31  of the slit S 2 . Comparing to the cases in which the length L 31  of the slit S 2  is 30 mm and 40 mm, it can be concluded that when the length L 31  of the slit S 2  is 35 mm, an optimum value of isolation characteristics is obtained. 
     On the other hand, for comparison, simulation results for the case in which the slit S 2  is omitted are shown.  FIG. 21  is a perspective view showing a schematic configuration of an antenna apparatus without a slit S 2 , which is a comparative example of the antenna apparatus in  FIG. 19 , and  FIG. 22  is a graph showing the intra-antenna coupling coefficient S 21  versus frequency, in the antenna apparatus in  FIG. 21 . The structure of the antenna apparatus in  FIG. 21  is the same as that of the antenna apparatus used for the simulation in  FIG. 20 , except that a slit S 2  is omitted. According to  FIG. 22 , when a slit S 2  is omitted, isolation between the first antenna portion and the second antenna portion is insufficient. 
     As described above, according to the antenna apparatus of the present preferred embodiment, it is possible to make the single antenna element  1  operate as two antenna portions, and accordingly, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other. 
     Fifth Preferred Embodiment 
       FIG. 23  is a perspective view showing a schematic configuration of an antenna apparatus according to a fifth preferred embodiment of the present invention. The configuration for improving the isolation between a first antenna portion and a second antenna portion is not limited to the one provided with a slit S 2 , as in the fourth preferred embodiment, and alternatively, stub conductors ST 1 , ST 2  and ST 3  such as those shown in  FIG. 23  may be provided. 
     Referring  FIG. 23 , the antenna apparatus of the present preferred embodiment is provided with the configuration of the antenna apparatus of the third preferred embodiment, and additionally, a stub conductor ST 1  close to a first antenna portion, a stub conductor ST 2  close to a second antenna portion, and/or a stub conductor ST 3  provided between the first antenna portion and the second portion. In the present preferred embodiment, each of the stub conductors ST 1 , ST 2  and ST 3  is configured as a strip-shaped conductor. The stub conductor ST 1  has a certain length L 61 , and is provided on a left side of a rectangular antenna element  1  so as to be located apart from a left bottom vertex of the antenna element  1  by a certain distance L 62 . The stub conductor ST 2  has a certain length L 63 , and is provided on a right side of the rectangular antenna element  1  so as to be located apart from a right bottom vertex of the antenna element  1  by a certain distance L 64 . As shown in  FIG. 23 , for preventing the stub conductors ST 1  and ST 2  from protruding and being obstacle, the longitudinal direction of the stub conductor ST 1  may be provided close to the left side of the antenna element  1  such that the longitudinal direction of the stub conductor ST 1  is substantially parallel to the left side of the antenna element  1 , and the longitudinal direction of the stub conductor ST 2  may be provided close to the right side of the antenna element  1  such that the longitudinal direction of the stub conductor ST 2  is substantially parallel to the right side of the antenna element  1 . Furthermore, the stub conductor ST 3  has a certain length L 65 , and is provided at a position on a side of the antenna element  1  opposing a ground conductor  2  (bottom side of the antenna element  1 ), where the stub conductor ST 3  is positioned apart from a reference point P 1   a  for the capacitive feeding of the first antenna portion by a distance L 66 , and apart from an opening of a slit S 1  of the second antenna portion by a distance L 67 . For preventing the stub conductor ST 3  from protruding and being obstacle, in a similar manner to that of the stub conductors ST 1  and ST 2 , the longitudinal direction of the stub conductor ST 3  may be provided close to the bottom side of the antenna element  1  such that the longitudinal direction of the stub conductor ST 1  is substantially parallel to the bottom side of the antenna element  1 . The respective lengths L 61 , L 63  and L 65  of the stub conductors ST 1 , ST 2  and ST 3  are preferably determined to be equal to (¼+n/2)λ, where λ denotes a wavelength of radio signals transmitted and/or received by the antenna apparatus, and n denotes an integer greater than or equal to 0. 
     In the present preferred embodiment, when taking into account effect by the stub conductors ST 1 , ST 2  and ST 3 , the electrical distance between feeding points P 1  and P 2  is a length of (¼+n/2)λ, where λ denotes a wavelength of radio signals transmitted and/or received by the antenna apparatus, and n denotes an integer greater than or equal to 0. 
     Although  FIG. 23  shows that the feeding point P 2  is positioned apart from the opening of the slit S 1  by a distance L 23 , the present invention is not limited so, and the feeding point P 2  can be provided at a desired position along the slit S 1 , as long as the feeding point P 2  is provided at a position where the electrical distance between the feeding points P 1  and P 2  is the length of (¼+n/2)λ. 
     The configuration, in which the stub conductors ST 1 , ST 2  and ST 3  are provided instead of the slit S 2 , may be applied to an antenna apparatus having two slits S 1  and S 2 , such as the antenna apparatus in  FIG. 18 , alternatively, may be applied to an antenna apparatus having two capacitive feeding portions, such as the antenna apparatus in  FIG. 19 . 
     According to the antenna apparatus of the present preferred embodiment, at least one of the stub conductors ST 1 , ST 2 , and ST 3  is provided for adjusting electromagnetic coupling between the first antenna portion and the second antenna portion, and accordingly, isolation between the first antenna portion and the second antenna portion is further improved over the case of the antenna apparatus of the third preferred embodiment. As such, in the antenna apparatus of the present preferred embodiment, it is possible to make the single antenna element  1  operate as two antenna portions such that the antenna element  1  is excited as the first antenna portion through the feeding point P 1 , and at the same time, the slit S 1  is excited as the second antenna portion through the feeding point P 2 . Accordingly, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other. 
     Sixth Preferred Embodiment 
       FIG. 24  is a perspective view showing a schematic configuration of an antenna apparatus according to a sixth preferred embodiment of the present invention. According to the present invention, it is possible to make a single antenna element  1  operate as not only two antenna portions, but operate as three or more antenna portions. The present preferred embodiment is characterized in that an antenna element  1  is provided with three feeding points P 1 , P 2  and P 3 , and the single antenna element  1  is made to operate as three antenna portions, by exciting the antenna element  1  as a first antenna portion through the feeding point P 1 , exciting the antenna element  1  as a second antenna portion through the feeding point P 2 , and at the same time, exciting the antenna element  1  as a second antenna portion through the feeding point P 3 . 
     Referring  FIG. 24 , the antenna apparatus of the present preferred embodiment is provided with, on an antenna element  1 , an electrode E 3  which is made of a conductive plate and provided in parallel to the antenna element  1 , instead of the slit S 2  in  FIG. 18 . The electrode E 3  is spaced from the antenna element  1  by a certain distance L 71  through air or a certain dielectric material. Thus, in the antenna apparatus of the present preferred embodiment, a capacitor is formed by the electrode E 3  and the antenna element  1 , a third feeding point P 3  is provided on the electrode E 1 , and the feeding point P 3  is connected to a radio signal processor circuit  3   a  through a feed line F 3 . As with feed lines F 1  and F 2 , the feed line F 3  can be made of a coaxial cable having an impedance of 50Ω, and in this case, an inner conductor of the coaxial cable connects the feeding point P 3  to the radio signal processor circuit  3   a , and on the other hand, an outer conductor of the coaxial cable is connected to a ground conductor  2 . The feeding point P 3 , the electrode E 3 , and a capacitor formed by the electrode E 3  and the antenna element  1  configure a capacitive feeding portion for a third antenna portion. A radio signal is fed to the antenna element  1  through this capacitor, and thus, operates as the third antenna portion. The size of the electrode E 3  is appropriately determined according to the frequency of radio signals transmitted and/or received by the antenna apparatus. Preferably, the size is determined such that the length in at least one direction of the electrode E 3  (e.g., in the case of a rectangular electrode E 3 , the direction of a longitudinal side thereof) is (¼+n/2)λ, where λ denotes a wavelength of radio signals transmitted and/or received by the antenna apparatus, and n denotes an integer greater than or equal to 0. A point on the antenna element  1  that is closest to the feeding point P 3  is regarded as a reference point P 3   a  for the capacitive feeding. 
     The antenna apparatus of the present preferred embodiment further has a slit S 4  for adjusting electromagnetic coupling, in the antenna element  1  and between the second antenna portion and the third antenna portion. The slit S 4  has a certain width and a length L 72 , and one end of the slit S 4  has, as an open end, an opening on a side of the antenna element  1  opposing the ground conductor  2 . The antenna apparatus of the present preferred embodiment further has a slit S 5  for adjusting electromagnetic coupling, in the antenna element  1  and between the first antenna portion and the third antenna portion. The slit S 5  has a certain width and a length L 73 , and one end of the slit S 5  has, as an open end, an opening on a side of the antenna element  1  opposing the ground conductor  2 . The opening of the slit S 4  is located apart from a reference point P 3   a  for the capacitive feeding of the third antenna portion by a distance L 76 , and apart from an opening of a slit S 1  of the second antenna portion by a distance L 77 . The opening of the slit S 5  is located apart from an opening of a slit S 3  of the first antenna portion by a distance L 74 , and apart from the reference point P 3   a  of the third antenna portion of the capacitive feeding by a distance L 75 . 
     A distance L 42 +L 74 +2×L 73 +L 75  between the feeding points P 1  and P 3  satisfies the following relation of expression (6):
 
 L 42 +L 74+2 ×L 73 +L 75=(¼ +n 1/2)λ  (6),
 
where λ denotes a wavelength of radio signals transmitted and/or received by the antenna apparatus, and n1 denotes an integer greater than or equal to 0.
 
     Although  FIG. 24  shows that the feeding point P 1  is positioned apart from the opening of the slit S 3  by the distance L 42 , the present invention is not limited so, and the feeding point P 1  can be provided at a desired position along the slit S 3 , as long as the position satisfies the expression (6) (i.e., 0≦L 42 &lt;L 41 ). 
     A distance L 23 +L 77 +2×L 72 +L 76  between the feeding points P 2  and P 3  satisfies the following relation of expression (7):
 
 L 23 +L 77+2 ×L 72 +L 76=(¼ +n 2/2)λ  (7),
 
where λ denotes a wavelength of radio signals transmitted and/or received by the antenna apparatus, and n2 denotes an integer greater than or equal to 0.
 
     Although  FIG. 24  shows that the feeding point P 2  is positioned apart from the opening of the slit S 1  by the distance L 23 , the present invention is not limited so, and the feeding point P 2  can be provided at a desired position along the slit S 1 , as long as the position satisfies the expression (7) (i.e., 0≦L 23 &lt;L 21 ). 
     According to the antenna apparatus of the present preferred embodiment with the above-described configuration, while isolations between the antenna portions are ensured by the slits S 4  and S 5 , the slit S 3  is excited as the first antenna portion through the feeding point P 1 , the slit S 1  is excited as the second antenna portion through the feeding point P 2 , and at the same time, the antenna element  1  is excited as the third antenna portion through the feeding point P 3 , and thus, it is possible to make the single antenna element  1  operate as three antenna portions. Accordingly, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other. 
       FIG. 25  is a block diagram showing a detailed configuration of a circuit of the antenna apparatus in  FIG. 24 . The configuration of a radio signal processor circuit  3   a  is substantially the same as that of the radio signal processor circuit  3   a  in  FIG. 2  and the others, except that the radio signal processor circuit  3   a  processes signals transmitted and/or received by three antenna portions. In  FIG. 25 , the feeding points P 1  and P 2  of the antenna element  1  are respectively connected, through feed lines F 1  and F 2 , to switches  11 - 1  and  11 - 2  of a switch circuit  11   a  in the radio signal processor circuit  3   a . A capacitive feeding portion of the third antenna portion is preferably provided within a central hinge portion  103   c , as will be described in detail later with reference to  FIGS. 26A and 26B .  FIG. 25  shows that a space between conductive components  103   ca  and  103   cb  which configure the central hinge portion  103   c  (e.g., a space between the conductive components  103   ca  and  103   cb  spaced from each other by a dielectric) acts as a capacitor C 3 . A feed line F 3  includes a first feed line F 3   a , the central hinge portion  103   c , and a second feed line F 3   b . The reference point P 3   a  for the capacitive feeding of the antenna element  1  is connected to the central hinge portion  103   c  through the second feed line F 3   b , and the feeding point P 3  provided on the central hinge portion  103   c  is connected, through the first feed line F 3   a , to a switch  11 - 3  of the switch circuit  11   a  in the radio signal processor circuit  3   a . The switch circuit  11   a  switches, under control of an antenna controller and modulator/demodulator circuit  16   a , to either a state in which the antenna element  1  is directly connected to the antenna controller and modulator/demodulator circuit  16   a , or a state in which the antenna element  1  is connected to the antenna controller and modulator/demodulator circuit  16   a  through an amplitude and phase controller circuit  12   a . When the antenna element  1  is directly connected to the antenna controller and modulator/demodulator circuit  16   a , the antenna controller and modulator/demodulator circuit  16   a  operates as a MIMO modulator/demodulator circuit, and transmits and/or receives radio signals of a plurality of channels (in the present preferred embodiment, three channels) using to a MIMO communication method through the antenna element  1 . The antenna controller and modulator/demodulator circuit  16   a  may perform modulation or demodulation of three independent radio signals, instead of performing a MIMO modulation or demodulation, and in this case, the antenna apparatus of the present preferred embodiment can simultaneously perform wireless communications for a plurality of applications, or simultaneously perform wireless communications in a plurality of frequency bands. On the other hand, when the antenna element  1  is connected to the antenna controller and modulator/demodulator circuit  16   a  through the amplitude and phase controller circuit  12   a , the amplitude and phase controller circuit  12   a  performs adaptive control on transmitted and/or received radio signals under control of an adaptive controller circuit  15   a . The amplitude and phase controller circuit  12   a  includes amplitude adjusters  13 - 1 ,  13 - 2  and  13 - 3 , and phase shifters  14 - 1 ,  14 - 2  and  14 - 3 . Upon reception, each of signals received and respectively passed through the switches  11 - 1 ,  11 - 2  and  11 - 3  is inputted to the amplitude and phase controller circuit  12   a  and inputted to the adaptive controller circuit  15   a . Preferably, for the purpose of maximum ratio combining, the adaptive controller circuit  15   a  determines the amounts of changes in amplitudes and amounts of phase shifts of the signals based on the inputted received signals, changes the amplitude and phase of the signal passed through the switch  11 - 1 , by means of the amplitude adjuster  13 - 1  and the phase shifter  14 - 1 , changes the amplitude and phase of the signal passed through the switch  11 - 2 , by means of the amplitude adjuster  13 - 2  and the phase shifter  14 - 2 , and changes the amplitude and phase of the signal passed through the switch  11 - 3 , by means of the amplitude adjuster  13 - 3  and the phase shifter  14 - 3 . The received signals whose amplitudes and phases have been changed are combined with each other, and the combined signal is inputted to the antenna controller and modulator/demodulator circuit  16   a . Upon transmission, in order to direct a beam in a desired direction, the adaptive controller circuit  15   a  determines the amounts of changes in amplitudes and amounts of phase shifts of signals to be transmitted under control of the antenna controller and modulator/demodulator circuit  16   a , and according to this determination, makes the amplitude and phase controller circuit  12   a  change the amplitudes and phases of the signals to be transmitted. The antenna controller and modulator/demodulator circuit  16   a  is connected, through an input/output terminal  17  of the radio signal processor circuit  3   a , to further circuits (not shown) in a wireless communication apparatus including an antenna apparatus of the present preferred embodiment. 
       FIG. 26A  is a front view of a mobile phone showing an exemplary implementation of the antenna apparatus in  FIG. 24 , and  FIG. 26B  is a side view thereof. In the present exemplary implementation, in a similar manner to those of the exemplary implementations in  FIGS. 4A ,  4 B,  8 A,  8 B,  8 C,  8 D,  12 A,  12 B,  16 A,  16 B and  16 C, each of a first upper housing portion  101   a  and a second upper housing portion  101   b  is made of a conductor, and a slit S 1 , a feeding point P 2 , and a feed line F 2  of a second antenna portion are configured in the same manner as in the exemplary implementations in  FIGS. 12A ,  12 B,  16 A,  16 B and  16 C. A slit S 3  of a first antenna portion is configured in the same manner as the slit S 1 , and is provided in a left side surface of an upper housing  101  and between the first upper housing portion  101   a  and the second upper housing portion  101   b . For the purpose of configuring a lower end of the slit S 3  (i.e., an end of the slit S 3  on the side close to a hinge portion  103 ) as an open end, a portion of the hinge portion  103  opposing the lower end of the slit S 3  is preferably made as empty space or be made of a dielectric material. A feeding point P 1  is provided at a position upward from the lower end of the slit S 3  by a certain distance, and the feeding point P 1  extends to a lower housing  102  through a feed line F 1 , and then, is connected to a radio signal processor circuit  3   a . A central hinge portion  103   c  includes a cylindrical portion  103   ca  mechanically connected to the lower housing  102 , and a cylindrical inner conductor  103   cb  inserted into the cylindrical portion  103   ca  in a rotatable manner. Each of the cylindrical portion  103   ca  and the inner conductor  103   cb  is made of a conductive material such as aluminum or zinc. At least one of the inner side of the cylindrical portion  103   ca  and the outer side of the inner conductor  103   cb  is coated by a dielectric, and thus, when the inner conductor  103   cb  is inserted into the cylindrical portion  103   ca , a capacitor C 3  of  FIG. 25  is formed between the inner side surface of the cylindrical portion  103   ca  and the outer side surface of the inner conductor  103   cb . The radio signal processor circuit  3   a  is connected to the cylindrical portion  103   ca  through a first feed line F 3   a  made of a coaxial cable or the like, and the inner conductor  103   cb  is connected to the first upper housing portion  101   a  through a second feed line F 3   b  made of a coaxial cable or the like. In the present exemplary implementation, a point at which the first feed line F 3   a  is connected to the cylindrical portion  103   ca  is regarded as a feeding point P 3   a , and a point at which the second feed line F 3   b  is connected to the first upper housing portion  101   a  is regarded as a reference point P 3   a  for the capacitive feeding. Furthermore, a slit S 5  is formed in a portion of the first upper housing portion  101   a  opposing the hinge portion  103  so as to be located between the first antenna portion (i.e., the slit S 3  and the feeding point P 1 ) and a point at which the second feed line F 3   b  is connected to the first upper housing portion  101   a  (the reference point P 3   a  for the capacitive feeding). Similarly, a slit S 4  is formed in a portion of the first upper housing portion  101   a  opposing the hinge portion  103  so as to be located between the second antenna portion (i.e., the slit S 1  and the feeding point P 2 ) and a point at which the second feed line F 3   b  is connected to the first upper housing portion  101   a  (the reference point P 3   a  for the capacitive feeding). In the present exemplary implementation, for ensuring the length L 72  of the slit S 4  and the length L 73  of the slit  5 , each of the slits S 4  and S 5  is configured as an L-shaped slit, but is not limited to this shape. For the purpose of configuring a lower end of each of the slits S 4  and S 5  is an open end, a portion of the hinge portion  103  opposing the lower ends of the slits S 4  and S 5  is preferably made as empty space or be made of a dielectric material. The spaces inside the slits S 1 , S 3 ,  84  and S 5  are preferably filled by a dielectric material, for mechanical reinforcement. Thus, the upper housing  101  operates as an antenna element  1  in  FIGS. 24 and 25 . 
     As an exemplary implementation of the present preferred embodiment, a mobile phone may be configured such that, in a similar manner to those of the exemplary implementations in  FIGS. 5A ,  5 B,  9 A,  9 B,  13 A,  13 B,  17 A,  17 B and  17 C, each of a first upper housing portion  101   a  and a second upper housing portion  101   b  is made of a dielectric, an antenna element  1  made of a rectangular conductive plate is provided within an upper housing  101 , the antenna element  1  has slits S 1 , S 3 , S 4  and  5 , and an inner conductor  103   cb  is connected to the antenna element  1  through a feed line F 3   a.    
     As shown in  FIGS. 24 ,  25 ,  26 A and  26 B, instead of a configuration including one capacitive feeding portion (electric current antenna) and two magnetic current antennas, it is also possible to adopt a configuration including two capacitive feeding portions and one magnetic current antenna, or a configuration including the other combination of other numbers of electric current antennas and magnetic current antennas. Further, for the purpose of adjusting electromagnetic coupling, stub conductors such as those described in the fifth preferred embodiment may be provided, instead of the slits S 4  and S 5 . 
     As described above, according to the antenna apparatus of the present preferred embodiment, it is possible to make the single antenna element  1  operate as three antenna portions, accordingly, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other. 
     Seventh Preferred Embodiment 
       FIG. 27  is a perspective view showing a schematic configuration of an antenna apparatus according to a seventh preferred embodiment of the present invention. The slit S 1  of the second antenna portion is not limited to a configuration in which, as in the third and fourth preferred embodiments, the slit S 1  has an opening at a side opposing the ground conductor  2 , and alternatively, the slit S 1  may have an opening at a different position of an antenna element  1 . The antenna apparatus of the present preferred embodiment is characterized in that an L-shaped slit S 1   a  is formed as a second antenna portion, instead of a linear slit S 1  in the fourth preferred embodiment, and the slit S 1   a  has an opening on a left side of an antenna element  1 . 
     In  FIG. 27 , the slit S 1   a  is configured as an L-shaped slit having a first portion with a length L 81  extending in an up/down direction in the drawing, and a second portion with a length L 82  extending in a left/right direction in the drawing. The opening of the slit S 1   a  is provided at a position proceeding upward from a right bottom end of the antenna element  1  by a distance L 84 . A feeding point P 2  of the slit S 1   a  is provided at a position proceeding by a distance L 85  from the bend between the second portion (left/right direction portion) and the first portion (up/down direction portion) of the slit S 1   a . In the present preferred embodiment, an opening of a slit S 2  is located at a position proceeding leftward from a right bottom end of the antenna element  1  by a distance L 83 . 
     A distance L 32 +2×L 31 +L 83 +L 84 +L 82 +L 85  between the feeding points P 1  and P 2  satisfies the following relation of expression (8):
 
 L 32+2 ×L 31 +L 83 +L 84 +L 82+85=(¼ +n/ 2)λ  (8),
 
where λ denotes a wavelength of radio signals transmitted and/or received by the antenna apparatus, and n denotes an integer greater than or equal to 0.
 
     It is also possible to adopt a configuration in which a slit S 2  is not formed, as in the third preferred embodiment, and in this case, a distance L 32 +L 83 +L 84 +L 82 +L 85  between the feeding points P 1  and P 2  satisfies the following relation of expression (9):
 
 L 32 +L 83 +L 84 +L 82 +L 85=(¼ +n/ 2)λ  (9),
 
where λ denotes a wavelength of radio signals transmitted and/or received by the antenna apparatus, and n denotes an integer greater than or equal to 0.
 
     Although  FIG. 27  shows that the feeding point P 2  is positioned apart from the opening of the slit S 1   a  by a distance L 85 +L 82 , the present invention is not limited so, and the feeding point P 2  can be provided at a desired position along the slit S 1   a , as long as the position satisfies the expression (8) or (9). 
     Accordingly, in the antenna apparatus of the present preferred embodiment, the distance between the feeding points P 1  and P 2  is configured according to the expression (8) or (9), and thus, it is possible to make the single antenna element  1  operate as two antenna portions such that the antenna element  1  is excited as the first antenna portion through the feeding point P 1 , and at the same time, the slit S 1   a  is excited as the second antenna portion through the feeding point P 2 . Accordingly, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other. 
     Eighth Preferred Embodiment 
       FIG. 28  is a perspective view showing a schematic configuration of an antenna apparatus according to an eighth preferred embodiment of the present invention. In the antenna apparatuses according to the above-described first to seventh preferred embodiments, it is also possible to adopt a configuration in which an antenna element  1  is electrically connected to a ground conductor  2 . The antenna apparatus of  FIG. 28  is characterized in that in the antenna apparatus of the fourth preferred embodiment, the right bottom end of the antenna element  1  and the right top end of the ground conductor  2 , which are opposed to each other, are connected by a short-circuit conductor T 1 . 
       FIG. 29A  is a front view of a mobile phone showing an exemplary implementation of the antenna apparatus in  FIG. 28 , and  FIG. 29B  is a side view thereof. The short-circuit conductor T 1  is made of, for example, a coaxial cable or conductive wire, and extends from a ground conductor  2  formed on one side of a printed wiring board  109  within a lower housing  102  to an upper housing  101  through a right hinge portion  103   b , and then, is electrically connected to a right bottom portion of a first upper housing portion  101   a  by a screw  108 . In the case that the lower housing  102  is made of a conductor, the short-circuit conductor T 1  is connected to the lower housing  102 , instead of connected to the printed wiring board  109 . 
     According to the present preferred embodiment, an effect equivalent to r-matching can be obtained by connecting the antenna element  1  with the ground conductor  2 , and therefore, the radiation characteristics of the antenna apparatus can be improved. Furthermore, since the antenna element  1  is connected with the ground conductor  2  by the short-circuit conductor T 1 , the ground for, e.g., a display  106  and/or a camera (not shown) disposed in the upper housing  101  can be enhanced, therefore, for example, an effect of preventing malfunction of a mobile phone due to static electricity can be expected. 
     As described above, according to the antenna apparatus of the present preferred embodiment, it is possible to make the single antenna element  1  operate as two antenna portions, accordingly, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other. 
     Modified Preferred Embodiment 
     The exemplary implementations of the antenna apparatuses according to the preferred embodiments of the present invention are not limited to a mobile phone, and any other apparatus having a wireless communication function can be configured. For example, it is possible to configure a laptop personal computer, a handheld personal computer, a mobile phone which is not of a foldable type, other portable terminal apparatuses, or the like, having the antenna apparatuses according to the preferred embodiments. In the case that the antenna apparatuses according to the respective preferred embodiments are provided on a laptop personal computer, the personal computer includes an upper housing and a lower housing which are connected to each other by a hinge portion, and in such personal computer, it is possible to make the upper housing of a conductive plate such that the upper housing operates as an antenna element  1 . When providing a laptop personal computer with a capacitive feeding portion, the configuration is not limited to one in which a capacitor is provided within a hinge portion of the personal computer, and alternatively, an electrode spaced from an upper housing by a certain distance may be provided (e.g., see  FIG. 10 ). The size of the electrode is preferably determined such that the length in at least one direction of the electrode (e.g., in the case of a rectangular electrode, the direction of a longitudinal side thereof) is (¼+n/2)λ, where λ denotes a wavelength of radio signals transmitted and/or received by the antenna apparatus, and n denotes an integer greater than or equal to 0. 
     A further configuration may be adopted in which the configurations in the above-described respective preferred embodiments are combined. For example, when a first antenna portion is made to operate as an electric current antenna and a second antenna portion is made to operate as a magnetic current antenna, a radio signal may be fed directly to the first antenna portion, instead of being capacitively fed, as shown in  FIGS. 30 ,  31 ,  33  and  35  without through a capacitor.  FIG. 30  is a perspective view showing a schematic configuration of an antenna apparatus according to a modified preferred embodiment of the third preferred embodiment of the present invention.  FIG. 31  is a perspective view showing a schematic configuration of an antenna apparatus according to a third modified preferred embodiment of the fourth preferred embodiment of the present invention.  FIG. 33  is a perspective view showing a schematic configuration of an antenna apparatus according to a first modified preferred embodiment of the fifth preferred embodiment of the present invention.  FIG. 35  is a perspective view showing a schematic configuration of an antenna apparatus according to a first modified preferred embodiment of the eighth preferred embodiment of the present invention. Furthermore, when a slit for adjusting electromagnetic coupling is formed between a first antenna portion and a second antenna portion, different antenna radiation methods (an electric current antenna and a magnetic current antenna) need not to be used, and accordingly, both antenna portions may be made to operate as magnetic current antennas as shown in  FIG. 18 , or operate as electric current antennas (capacitive feeding) as shown in  FIG. 19 , or alternatively, operate as electric current antennas to which radio signals are directly fed as shown in  FIGS. 32 ,  34  and  36 .  FIG. 32  is a perspective view showing a schematic configuration of an antenna apparatus according to a fourth modified preferred embodiment of the fourth preferred embodiment of the present invention.  FIG. 34  is a perspective view showing a schematic configuration of an antenna apparatus according to a second modified preferred embodiment of the fifth preferred embodiment of the present invention.  FIG. 36  is a perspective view showing a schematic configuration of an antenna apparatus according to a second modified preferred embodiment of the eighth preferred embodiment of the present invention. According to the modified preferred embodiments in  FIGS. 30 to 36 , in a similar manner to that of the configuration described earlier, it is possible to make the single antenna element  1  operate as two antenna portions, accordingly, while having a simple configuration, the antenna apparatus can simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other. As further modified preferred embodiments, the short-circuit conductor T 1  of the eighth preferred embodiment may be provided to the antenna apparatus of the first preferred embodiment etc. 
     As described above, the antenna apparatuses of the preferred embodiments according to the present invention can simultaneously transmit and/or receive a plurality of radio signals with low correlation to each other, while having a simple configuration. Accordingly, it becomes possible, for example, to transmit and/or receive radio signals of a plurality of channels using a MIMO communication method, to simultaneously perform wireless communications for a plurality of applications, or to simultaneously perform wireless communications in a plurality of frequency bands. 
     According to the antenna apparatuses of the preferred embodiments according to the present invention, while achieving a thin and small-sized antenna apparatus by reducing the number of antenna elements, it is possible to improve the spatial correlation by ensuring the isolation between a plurality of antenna portions, and implementing the polarization diversity in radio signals transmitted and/or received by the plurality of antenna portions. In addition, according to the present antenna apparatuses, even when using a single antenna element, it is possible to simultaneously transmit and/or receive a plurality of radio signals without the need for a time division process or the like. 
     As described above, in the antenna apparatuses according to the preferred embodiments of the present invention, a MIMO operation is enabled by exciting a single antenna element  1  through a plurality of feeding points simultaneously, as well as ensuring the isolation between antenna portions (or isolation between the feeding points). As specific methods adapted for ensuring the isolation include: to adjust an electric length such that the spatial phase difference between the feeding points is an odd multiple of ¼ wavelength, to use voltage feeding and current feeding, and to use an electric current antenna system and a magnetic current antenna system. Furthermore, a MIMO operation with higher performance is enabled by ensuring the isolation between the feeding points by means of a slit formed between the feeding points for adjusting electromagnetic coupling. 
     The antenna apparatus and wireless communication apparatus of the present invention can be implemented, for example, as a mobile phone or as a wireless LAN apparatus. The present antenna apparatus can be mounted, for example, on a wireless communication apparatus for performing a MIMO communication, but not limited to MIMO, and can be mounted on a wireless communication apparatus capable of simultaneously performing communications for a plurality of applications (multi-applications). 
     As described above, although the present invention is described in detail with reference to preferred embodiments, the present invention is not limited to such embodiments. It will be obvious to those skilled in the art that numerous modified preferred embodiments and altered preferred embodiments are possible within the technical scope of the present invention as defined in the following appended claims.