Patent Publication Number: US-2009237309-A1

Title: Radio apparatus and antenna device including anisotropic dielectric material

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2008-75228 filed on Mar. 24, 2008; the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a radio apparatus and an antenna device, and in particular to an antenna device including dielectric material for isolation and a radio apparatus having the antenna device. 
     2. Description of the Related Art 
     Radio apparatuses such as mobile phones are required to be downsized, and thus, e.g., a mobile radio apparatus having an antenna loaded with dielectric material so as to be downsized due to a wavelength shortening effect is disclosed in Japanese Patent Publication of Unexamined Applications (Kokai), No. 2007-201633. 
     The mobile radio apparatus disclosed in JP 2007-201633 has a water-tight packing made of dielectric material and arranged in contact with an antenna. According to JP 2007-201633, the water-tight packing made of dielectric material may contribute to downsizing of the antenna by shortening a wavelength of an electromagnetic wave sent or received by the antenna. The water-tight packing may have an effect of preventing an alien substance such as a drop of water from entering a casing of the mobile radio apparatus. 
     Meanwhile, a radio apparatus including a dielectric material of a high permittivity value is disclosed in Japanese Patent Publication of Unexamined Applications (Kokai), No. 2004-208084. The radio apparatus of JP 2004-208084 is configured to control directivity of an antenna loaded with the dielectric material by using an effect of concentrating more energy of an electric field as the permittivity value is higher. 
     According to JP 2004-208084, the antenna is loaded with dielectric material of relatively high permittivity and extremely low loss on the opposite side to a human body. The antenna may concentrate the energy of the electric field of an electromagnetic wave sent or received by the antenna on the portion loaded with the dielectric material, thereby. In some cases, the antenna may have directivity in an opposite direction to the human body by forming a curvature on a surface of the dielectric material so that the electromagnetic wave may pass through the curvature. 
     In a case where dielectric material is arranged in the casing such as in the mobile radio apparatus of JP 2007-201633, the electric field concentration between a ground conductor, a metallic portion or material arranged in the casing and the antenna through the dielectric material may cause a stronger coupling. Thus, there may be a problem in that a value of antenna impedance is likely to decrease, or that a resonant frequency is likely to deviate. 
     The mobile phone of JP 2004-208084 has an antenna element loaded with the dielectric material that is half-sphere or half-cylinder shaped towards the outside of the casing. Such a configuration may limit a location of the dielectric material, thus causing limitation on an arrangement design. Thus, there may be a problem in that the configuration is not necessarily useful for downsizing of the mobile phone. 
     SUMMARY OF THE INVENTION 
     Accordingly, an advantage of the present invention is to have a wavelength shortening effect of an antenna of a radio apparatus loaded with dielectric material, and to simultaneously suppress a coupling between the antenna and circuits or portions surrounding the antenna with less limitation on the arrangement design. 
     To achieve the above advantage, one aspect of the present invention is to provide a radio apparatus having a printed board and an antenna element. The antenna element is configured to be fed at a feed portion provided in the printed board. The antenna element includes a portion arranged parallel to the printed board. The anisotropic dielectric material is arranged in such a way that a direction of a maximum permittivity value of the anisotropic dielectric material equals a direction of the portion of the antenna element arranged parallel to the printed board. The anisotropic dielectric material is arranged in contact with the portion of the antenna element arranged parallel to the printed board. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of main portions of a radio apparatus of a first embodiment of the present invention. 
         FIG. 2  is a perspective view showing a configuration and dimensions of a model used for an experiment. 
         FIG. 3  is a perspective view showing a configuration and dimensions of another model used for the experiment. 
         FIG. 4  is a graph of VSWR-frequency characteristics of an antenna element of each of the models shown in  FIG. 2  and  FIG. 3  estimated by the experiment. 
         FIG. 5  is a graph of VSWR-frequency characteristics of the antenna element of each of the models shown in  FIG. 2  and  FIG. 3  estimated by a simulation. 
         FIG. 6  is a perspective view of main portions of a radio apparatus of a second embodiment of the present invention. 
         FIG. 7  is a perspective view of main portions of a radio apparatus of a third embodiment of the present invention. 
         FIG. 8  is a perspective view of main portions of a radio apparatus of a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail. In following descriptions, terms such as upper, lower, left, right, horizontal or vertical used while referring to a drawing shall be interpreted on a page of the drawing unless otherwise noted. A same reference numeral given in no less than two drawings shall represent a same member or a same portion. 
     A first embodiment of the present invention will be described with reference to  FIGS. 1-5 .  FIG. 1  is a perspective view of main portions of a radio apparatus  1  of the first embodiment. The radio apparatus  1  has a printed board  10  contained in a casing (not shown) of the radio apparatus  1 , and an antenna element  11 . The printed board  10  includes a conductive pattern that is not shown and given a ground level voltage while the radio apparatus  1  is working. The conductive pattern is hereafter called the ground conductor. 
     As shown in  FIG. 1 , the antenna element  11  is depicted as nearly L-shaped. The antenna element  11  may be fed, at an end connected to a feed portion  12 , by a radio circuit that is provided in the printed board  10  and is not shown. Another end of the antenna element  11  is, e.g., open ended as shown in  FIG. 1 . A portion of the antenna element  11  including the open end is arranged parallel to the printed board  10 . The term “parallel” used here and hereafter means not exactly but almost parallel as well as exactly parallel, and so is the term “equal” hereafter. 
     The radio apparatus  1  has an anisotropic dielectric material  13  made of material having a relatively high permittivity value in a specific direction in three-dimensional space, and a relatively low permittivity value in remaining directions (i.e., anisotropic permittivity). The anisotropic dielectric material  13  is arranged in contact with a portion of the antenna element  11  arranged parallel to the printed board  10 . In other words, the antenna element  11  is loaded with the anisotropic dielectric material  13 . The anisotropic dielectric material  13  is arranged in such a way that a direction of a maximum permittivity value (as shown by a block arrow in  FIG. 1 ) is equal to the direction of the portion of the antenna element  11  arranged parallel to the printed board  10 . 
     If the antenna element  11  is fed at the feed portion  12 , a resonant wavelength of a radio frequency voltage or current distributed along a line of the antenna element  11  may be shortened to a value determined by the maximum permittivity value of the anisotropic dielectric material  13 . As the (maximum) permittivity value is higher, the wavelength shortening effect is more obvious. 
     Meanwhile, the ground conductor of the printed board  10  and the open end portion of the antenna element  11  arranged parallel to each other may produce capacitance between each other so as to be coupled through the capacitance with each other. As the permittivity value of the anisotropic dielectric material  13  in a direction from the antenna element  11  to the printed board  10  (i.e., perpendicular to the direction of the block arrow shown in  FIG. 1 ) is lower, the above coupling is weaker. The above coupling may reduce the impedance of the antenna element  11  so as to shift the resonant frequency to a lower frequency range. Thus, as the above coupling is weaker, such a shift of the resonant frequency may be suppressed more. 
     Qualitatively speaking, as the anisotropy of the anisotropic dielectric material  13  is more obvious (i.e., a ratio of the permittivity in the direction of the block arrow shown in  FIG. 1  to the permittivity in the remaining directions (called a high-to-low ratio of the permittivity) is greater), the cost of the wavelength shortening effect such as a decrease in impedance or a shift of the resonant frequency may be reduced. 
     An effect of the first embodiment estimated by an experiment and by a simulation will be described with reference to  FIGS. 2-5 .  FIG. 2  is a perspective view showing a configuration and dimensions of a model (called a radio apparatus  1   a ) used for the experiment.  FIG. 3  is a perspective view showing a configuration and dimensions of another model (called a radio apparatus  1   b ) used for the experiment. 
     The radio apparatus la shown in  FIG. 2  has the printed board  10  and the anisotropic dielectric material  13  which are same as shown in  FIG. 1 . The anisotropic dielectric material  13  is arranged in an end portion of the printed board  10 . The direction of the maximum permittivity value is shown by a block arrow, as in  FIG. 1 . An area of a ground conductor  10   a of the printed board  10  is indicated by cross-hatching. 
     The antenna device la has an antenna element that is slightly different from the antenna element  11  shown in  FIG. 1  with respect to a shape and a connection, and thus given a reference numeral  11   a . The antenna element  11   a  may be fed at the feed portion  12  (same as the corresponding one shown in  FIG. 1 ) provided around an upper side of the ground conductor  10   a  that is closer to the anisotropic dielectric material  13 . The antenna element  11  a is arranged in contact with the anisotropic dielectric material  13  and to turn back. The antenna element  11   a  has a grounded end short-circuited to the ground conductor  10   a.    
     As shown in  FIG. 2 , the ground conductor  10   a  has a portion of an upper end shaped sticking out towards the anisotropic dielectric material  13 . The shape of the above portion imitates a configuration that a metallic component of a mobile phone, e.g., a speaker, is arranged close to an antenna. In this case, the direction of the maximum permittivity value shown by the block arrow may be parallel to a portion of the metallic component facing the antenna element  11   a.    
     As shown in  FIG. 2 , there is a space of 1 millimeters (mm) between the anisotropic dielectric material  13  and the above sticking-out portion of the ground conductor  10   a . There is a space of 10 mm between the anisotropic dielectric material  13  and the remaining portion of the upper end of the ground conductor  10   a . The ground conductor  10   a  is 90 mm long between the remaining portion of the upper end and a lower end (i.e., a lower end of the printed board  10 ). The ground conductor  10   a  (and the printed board  10 ) is 45 mm wide. 
     The radio apparatus  1   b  shown in  FIG. 3  has the printed board  10 , the anisotropic dielectric material  13  and the antenna element  11  a which are same as shown in  FIG. 1 . The anisotropic dielectric material  13  is arranged in an end portion of the printed board  10 . The direction of the maximum permittivity value is shown by a block arrow, as in  FIG. 1 . An area of a ground conductor  10   b  of the printed board  10  is indicated by cross-hatching. 
     The ground conductor  10   b  is shaped differently from the ground conductor  10   a  without a sticking-out portion of an upper end. The antenna element  11   a  may be fed at the feed portion  12  (i.e., a same as shown in  FIG. 1  or  FIG. 2 ). The antenna element  11   a  has a grounded end short-circuited to the ground conductor  10   b.    
     As shown in  FIG. 3 , there is a space of 10 mm between the anisotropic dielectric material  13  and the upper end of the ground conductor  10   b . The ground conductor  10   b  is 90 mm long between the upper end and a lower end (i.e., a lower end of the printed board  10 ). The ground conductor  10   b  (and the printed board  10 ) is 45 mm wide. 
       FIG. 4  is a graph of voltage standing wave ratio (VSWR) vs. frequency characteristics of the antenna element  11   a  of the radio apparatuses  1   a  and  1   b  estimated by experiment.  FIG. 4  has a horizontal axis and a vertical axis representing frequencies (in megahertz (MHz)) and the VSWR, respectively. 
     In  FIG. 4 , a solid curve on a left side represents a characteristic of the radio apparatus  1   a  shown in  FIG. 2 . A dashed curve on the left side represents a characteristic of the radio apparatus  1   a  shown in  FIG. 2  on an assumption that the anisotropic dielectric material  13  is replaced by an isotropic dielectric material. 
     In  FIG. 4 , a solid curve on a right side represents a characteristic of the radio apparatus  1   b  shown in  FIG. 3 . A dashed curve on the right side represents a characteristic of the radio apparatus  1   b  shown in  FIG. 3  on an assumption that the anisotropic dielectric material  13  is replaced by an isotropic dielectric material. 
     The anisotropic dielectric material  13  used for the experiment has, e.g., a relative permittivity value of  12  in the direction of the maximum permittivity value, and a relative permittivity value of  9  in the remaining directions. The isotropic dielectric material used for the experiment has, e.g., a relative permittivity value of  7 . The antenna element  11   a  is given a length so that, e.g., a resonant frequency of the configuration shown in  FIG. 3  loaded with the isotropic dielectric material (corresponding to the dashed curve on the right side of  FIG. 4 ) is 520 MHz. 
     A comparison of the solid and dashed curves on the right side of  FIG. 4  shows that the resonant frequency changes by a few MHz depending on whether the dielectric material that the antenna element  11   a  is loaded with is isotropic or anisotropic, and that the change is smaller than a change of the resonant frequency between the solid and dashed curves on the left side of  FIG. 4  (nearly 10 MHz). What is described above may be explained as follows. 
     As the space between the antenna element  11   a  and the ground conductor  10   b  in the configuration of the radio apparatus  1   b  shown in  FIG. 3  is relatively great, there is a relatively weak coupling through the capacitance produced between the antenna element  11   a  and the ground conductor  10   b . Thus, the resonant frequency of the radio apparatus  1   b  may be relatively less affected by a degree of concentration of the energy of the electric field in the direction perpendicular to the block arrow (i.e., by the difference of the permittivity in the above direction). 
     Meanwhile, as the space between the antenna element  11   a  and the ground conductor  10   a  in the configuration of the radio apparatus  1   a  shown in  FIG. 2 , corresponding to the solid and dashed curves on the left side of  FIG. 4 , is relatively small, there is a relatively strong coupling produced between the antenna element  11   a  and the ground conductor  10   a  through the capacitance. Thus, the resonant frequency of the radio apparatus  1   a  may be more affected by the degree of concentration of the energy of the electric field in the direction perpendicular to the block arrow (i.e., by the difference of the permittivity in the above direction). 
     In the configuration of the radio apparatus  1   a  shown in  FIG. 2 , the permittivity value is smaller in the direction perpendicular to the block arrow so that the coupling is made weaker in a case where the anisotropic dielectric material  13  is used (represented by the solid curve on the left side of  FIG. 4 ) than in a case where the isotropic dielectric material is used (represented by the dashed curve on the left side of  FIG. 4 ). Thus, the use of the anisotropic dielectric material  13  may suppress a downward shift of the resonant frequency. 
     A comparison between the use of the anisotropic dielectric material  13  (represented by the solid curve on the right side of  FIG. 4 ) and the use of the isotropic dielectric material (represented by the dashed curve on the right side of  FIG. 4 ) may produce a similar result in the configuration of the radio apparatus  1   b.    
     Both in  FIG. 2  and in  FIG. 3 , the permittivity of the dielectric material that the antenna element  11   a  is loaded with in the direction of the block arrow does not change depending on whether the dielectric material is anisotropic or isotropic, and neither does the wavelength shortening effect. That is, producing the same wavelength shortening effect as the use of the isotropic dielectric material, the use of the anisotropic dielectric material  13  may suppress the coupling between the antenna element and the ground conductor of the printed board or metallic components, and may suppress the resultant downward shift of the resonant frequency. 
     Both in  FIG. 2  and in  FIG. 3 , it is proved by the experiment that a measured value of radiation efficiency less changes depending on whether the dielectric material is anisotropic or isotropic. 
     The permittivity value of the anisotropic dielectric material  13  used for the experiment illustrated in  FIG. 4  has a high-to-low ratio (i.e., a ratio of the maximum value in the direction of anisotropy to the value in the remaining directions) of 7 to 3.  FIG. 5  is a graph of voltage standing wave ratio (VSWR)-frequency characteristics of the antenna element  11   a  of the radio apparatus  1   a  and  1   b  estimated by a simulation given several values of the high-to-low ratio as a variable parameter.  FIG. 5  has a horizontal axis and a vertical axis representing frequencies (in MHz) and the VSWR, respectively. 
     In  FIG. 5 , a dashed curve without plots is a same as the solid curve on the left side of  FIG. 4  representing the experiment data for reference. Three curves with filled square plots represent characteristics of the radio apparatus  1   a  shown in  FIG. 2 . A right-hand one of the above curves (showing a resonant frequency of nearly 520 MHz) is obtained given the relative permittivity value of 12 in the direction of the maximum permittivity value and the relative permittivity value of 3 in the remaining directions. 
     A middle one of the above curves (showing a resonant frequency of nearly 500 MHz) is obtained given the relative permittivity value of 12 in the direction of the maximum permittivity value and the relative permittivity value of 6 in the remaining directions. A left-hand one of the above curves (showing a resonant frequency of nearly 490 MHz) is obtained given the relative permittivity value of 12 in the direction of the maximum permittivity value and the relative permittivity value of 9 in the remaining directions. 
     In  FIG. 5 , three curves with blank square plots represent characteristics of the radio apparatus  1  b shown in  FIG. 3 . A right-hand one of the above curves (showing a resonant frequency of nearly 580 MHz) is obtained given the relative permittivity value of 12 in the direction of the maximum permittivity value and the relative permittivity value of 3 in the remaining directions. 
     A middle one of the above curves (showing a resonant frequency of nearly 570 MHz) is obtained given the relative permittivity value of 12 in the direction of the maximum permittivity value and the relative permittivity value of 6 in the remaining directions. A left-hand one of the above curves (showing a resonant frequency of nearly 560 MHz) is obtained given the relative permittivity value of 12 in the direction of the maximum permittivity value and the relative permittivity value of 9 in the remaining directions. 
     Differences of the resonant frequencies among the three curves with the filled plots are greater than differences of the resonant frequencies among the three curves with the blank plots, for a same reason (the width of the space between the antenna element  11   a  and the ground conductor  10   a  or  10   b ) as explained with reference to  FIG. 4 . 
     A comparison of the three curves with the filled plots with one another shows that the coupling between the antenna element  11   a  and the ground conductor  10   a  is made weaker as the high-to-low ratio of the permittivity is higher, and thus the downward shift of the resonant frequency may be suppressed. A comparison of the three curves with the blank plots with one another shows that the coupling between the antenna element  11   a  and the ground conductor  10   b  is made weaker as the high-to-low ratio of the permittivity is higher, and thus the downward shift of the resonant frequency may be suppressed. 
     According to the first embodiment of the present invention described above, producing almost a same wavelength shortening effect as loaded with an isotropic dielectric material, the antenna device having the antenna element loaded with the anisotropic dielectric material may suppress a coupling with a printed board and so forth and a downward shift of a resonant frequency. 
     A second embodiment of the present invention will be described with reference to  FIG. 6 , a perspective view of main portions of a radio apparatus  2  of the second embodiment. The radio apparatus  2  has a printed board  10  contained in a casing (not shown) of the radio apparatus  2 , and an antenna element  11  that may be fed at a feed portion  12  (each of these portions is a same as the corresponding one of the first embodiment given the same reference numeral). 
     The radio apparatus  2  has an anisotropic dielectric material  23  and an isotropic dielectric material  24 . The anisotropic dielectric material  23  is arranged in contact with a portion of the antenna element  11  arranged parallel to the printed board  10  towards the inside of the printed board  10 . The anisotropic dielectric material  23  is arranged in such a way that a direction of a maximum permittivity value (as shown by a block arrow in  FIG. 6 ) is equal to the direction of the portion of the antenna element  11  arranged parallel to the printed board  10 . 
     Meanwhile, the isotropic dielectric material  24  is arranged in contact with the portion of the antenna element  11  arranged parallel to the printed board  10  towards the outside of the printed board  10 . In other words, the portion of the antenna element  11  arranged parallel to the printed board  10  is arranged between the anisotropic dielectric material  23  and the isotropic dielectric material  24 . 
     In  FIG. 6 , being fed at the feed portion  12 , the antenna element  11  may have a wavelength shortening effect determined by permittivity of the anisotropic dielectric material  23  in the direction of the block arrow and permittivity of the isotropic dielectric material  24 . 
     Meanwhile, as the permittivity value of the anisotropic dielectric material  23  in the direction perpendicular to the direction of the block arrow shown in  FIG. 6  is lower, a coupling between the ground conductor of the printed board  10  and the open end portion of the antenna element  11 , which are parallel to each other, is weaker. That is, due to the anisotropic permittivity of the anisotropic dielectric material  23 , the antenna element  11  may have a wavelength shortening effect and may suppress a shift of the resonant frequency caused by the coupling with the ground conductor of the printed board  10  and so forth. 
     As the portion of the antenna element  11  arranged parallel to the printed board  10  is loaded with the isotropic dielectric material  24  arranged towards the outside of the printed board  10 , the antenna element  11  may concentrate energy of an electric field going to outside space of the radio apparatus  2  and may enforce electromagnetic field radiation to the outside space of the radio apparatus  2 , thereby. 
     According to the second embodiment of the present invention described above, the radio apparatus may obtain an additional effect of the enforced electromagnetic field radiation to the outside space. 
     A third embodiment of the present invention will be described with reference to  FIG. 7 , a perspective view of main portions of a radio apparatus  3  of the third embodiment. The radio apparatus  3  has a printed board  10  contained in a casing (not shown) of the radio apparatus  3 , and an antenna element  11  that may be fed at a feed portion  12 . The radio apparatus  3  has an anisotropic dielectric material  13  that the antenna element  11  is loaded with. 
     Each of these portions is a same as the corresponding one of the first embodiment given the same reference numeral. As the antenna element  11  is loaded with the anisotropic dielectric material  13  arranged towards the inside of the printed board  10 , a portion of the antenna element  11  located behind the anisotropic dielectric material  13  in  FIG. 7  is shown by dashed lines. The direction of the maximum permittivity value of the anisotropic dielectric material  13  is indicated by the same block arrow as shown in  FIG. 1 . 
     The radio apparatus  3  has an additional antenna element  31  in addition to the antenna element  11 . The additional antenna element  31  may be fed, at an end connected to a feed portion  12 , by a radio circuit (provided for a system that is different from the radio circuit configured to feed the antenna element  11 ) that is provided in the printed board  10  and is not shown. Another end of the antenna element  11  is, e.g., open ended as shown in  FIG. 7 . 
     The additional antenna element  31  has a portion including the open end, and arranged parallel to and to face a portion of the antenna element  11  including the open end. In other words, the anisotropic dielectric material  13  is arranged in such a way that the direction of the maximum permittivity value (as shown by a block arrow in  FIG. 7 ) is equal to the direction of the portion of the antenna element  11  facing the antenna element  11 . In  FIG. 7 , being fed at the feed portion  12 , the antenna element  11  may have a wavelength shortening effect determined by the permittivity of the anisotropic dielectric material  13  in the direction of the block arrow. 
     Meanwhile, as the permittivity value of the anisotropic dielectric material  13  in the direction perpendicular to the direction of the block arrow shown in  FIG. 7  is lower, a coupling between the open end portion of the additional antenna element  31  and the open end portion of the antenna element  11 , which are parallel to each other, is weaker. That is, producing a wavelength shortening effect of the antenna element  11 , the use of the anisotropic dielectric material  13  may suppress a coupling between different systems through the coupling between the antenna element  11  and the additional antenna element  31 . 
     According to the third embodiment of the present invention described above, an additional effect of suppressing interference between different systems may be obtained. 
     A fourth embodiment of the present invention will be described with reference to  FIG. 8 , a perspective view of main portions of a radio apparatus  4  of the fourth embodiment. The radio apparatus  4  has a printed board  10  contained in a casing (not shown) of the radio apparatus  4 , and an antenna element  11  that may be fed at a feed portion  12  (each of these portions is a same as the corresponding one of the first embodiment given the same reference numeral). 
     The radio apparatus  4  has an additional antenna element  41  in addition to the antenna element  11 . The additional antenna element  41  is a parasitic element having, e.g., an open end and an opposite end short-circuited to the ground conductor of the printed board  10 . A portion of the additional antenna element  41  including the open end is arranged parallel to the antenna element  11 . 
     The radio apparatus  4  has an anisotropic dielectric material  43  arranged in contact with a portion of the antenna element  11  arranged parallel to the printed board  10 . The anisotropic dielectric material  43  is arranged in such a way that a direction of a maximum permittivity value (as shown by a block arrow in  FIG. 8 ) is equal to the direction of the portion of the antenna element  11  arranged parallel to the printed board  10 . 
     The radio apparatus  4  has an anisotropic dielectric material  44  arranged in contact with the antenna element  11  around the additional antenna element  41 . 
     In  FIG. 8 , being fed at the feed portion  12 , the antenna element  11  may have a wavelength shortening effect determined by the permittivity of the anisotropic dielectric material  43  in the direction of the block arrow. 
     Due to the anisotropic permittivity of the anisotropic dielectric material  43 , the antenna element  11  may suppress a shift of the resonant frequency caused by the coupling with the ground conductor of the printed board  10  and so forth. 
     Meanwhile, as the isotropic dielectric material  24  is arranged in contact with and loaded onto the antenna element  11 , the antenna element  11  and the additional antenna element  41  may concentrate energy of an electric field between each other and may enforce a mutual coupling thereby. 
     The radio apparatus  4  may have an antenna element of a different system arranged close to the anisotropic dielectric material  43 . In such a configuration, while the isotropic dielectric material  43  may enforce the coupling between the antenna element  11  and the additional antenna element  41 , the anisotropic dielectric material  43  may suppress a coupling between the antenna element  11  and the antenna element of the different system. Strength of each of the couplings may be optionally selected thereby. 
     According to the fourth embodiment of the present invention described above, an additional effect of an enforced coupling with an additional antenna element may be obtained. 
     In the above description of the embodiments, the configurations, shapes, dimensions, connections or positional relations of the antenna devices, the materials such as the dielectric materials, the printed boards, etc. are considered as exemplary only, and thus may be variously modified within the scope of the present invention. 
     The particular hardware or software implementation of the present invention may be varied while still remaining within the scope of the present invention. It is therefore to be understood that within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described herein.