Patent Publication Number: US-8525732-B2

Title: Antenna device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an antenna device and, more particularly, to a technique for adjusting antenna characteristics. 
     2. Description of Related Art 
     A chip-like antenna element incorporated in a small radio terminal such as a mobile phone is formed by printing a radiation electrode and a feeding electrode on the surface of a block dielectric body. The radiation electrode and feeding electrode are capacitively coupled to each other through a gap (hereinafter, referred to as “feeding gap”). When AC current is fed to the feeding electrode to generate an electric field, the AC current flows also in the radiation electrode and thereby radio waves are generated from the radiation electrode. 
     CITATION LIST 
     Patent Document 
     
         
         [Patent Document 1] Jpn. Pat. Appln. Laid-Open Publication No. 10-13138 
       
    
     Antenna characteristics such as resonance frequency and impedance change depending on the capacitance of the feeding gap (hereinafter, referred to as “feeding coupling capacitance”). Typically, the feeding gap is often formed on the upper surface or side surface of a dielectric body (refer to, e.g., Patent Document 1). In this case, it is necessary to reduce the width of the feeding gap in order to increase the feeding coupling capacitance. However, with the current fabrication technology (thick film electrode printing technology), it is difficult to reduce the width of the feeding gap to 0.3 mm or less. 
     The present invention has been made in view of the above problem, and a main object thereof is to realize an antenna device capable of easily adjusting its antenna characteristics and capable of being easily manufactured. 
     SUMMARY 
     An antenna device according to the present invention includes an antenna element and a printed board. The antenna element has a dielectric body having substantially a rectangular solid shape, on the surface of which a radiation electrode, a feeding electrode, and a ground electrode are printed. The printed board includes a mounting area in which the antenna element is mounted and a ground pattern area formed around the mounting area. Both the feeding electrode and ground electrode are formed only on the bottom surface of the dielectric body. The radiation electrode is formed on the upper surface of the dielectric body, a first side surface thereof, and bottom surface thereof in a folded configuration. A second side surface opposite to the first side surface is configured as a non-electrode formation area. 
     The surface on which the feeding electrode is formed is opposite to the upper surface on which the radiation electrode is formed across the dielectric body. A capacitance is formed through the dielectric body, so that it is easy to increase feeding coupling capacitance formed between the feeding electrode and radiation electrode. The same can be said for a capacitance (hereinafter, referred to as “ground coupling capacitance”) formed between the ground electrode and radiation electrode. Further, by changing the area of the feeding electrode, ground electrode, or radiation electrode, the ground coupling electrode or feeding electrode is changed to change antenna characteristics such as resonance frequency or impedance, making it easy to adjust the antenna characteristics. 
     Both or one of third and fourth side surfaces adjacent to the first and second side surfaces may be configured as a non-electrode formation area. Alternatively, apart of the radiation electrode may be formed on the upper part of both or one of the third and fourth side surfaces adjacent to the first and second side surfaces. 
     The radiation electrode may be formed on the entirety of the upper surface. The radiation electrode may be formed on the entirety of the first side surface. This configuration simplifies the electrode pattern of the antenna device, enhancing manufacturability. 
     The ground electrode may be formed as to be opposed to the radiation electrode with a gap having a predetermined width interposed therebetween on the bottom surface. 
     It is to be noted that any arbitrary combination of the above-described structural components and expressions changed between an apparatus, a system, etc. are all effective as and encompassed by the present embodiments. 
     The present invention is effective for realizing an antenna device capable of easily adjusting antenna characteristics and capable of being easily manufactured. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above features and advantages of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a view illustrating an outer appearance of an antenna device in a first embodiment of the present invention; 
         FIG. 2  is a development view of an antenna element in the first embodiment; 
         FIG. 3  is an equivalent circuit diagram of the antenna device; 
         FIG. 4  is an outer appearance of an antenna device in a comparative example; 
         FIG. 5  is a graph illustrating a comparison between a first type and side-gap type in terms of a relationship between return loss and frequency; 
         FIG. 6  is a graph illustrating a comparison between the first type and side-gap type in terms of a relationship between radiation efficiency and frequency; 
         FIG. 7  is an outer appearance of an antenna device in a second embodiment; 
         FIG. 8  is an outer appearance of an antenna device in a third embodiment; 
         FIG. 9  is a graph illustrating a comparison between the first, second, third types in terms of a relationship between return loss and frequency; 
         FIG. 10  is a graph illustrating a comparison between the first, second, and third types in terms of a relationship between radiation efficiency and frequency; 
         FIG. 11  is an outer appearance of the antenna device in which a mounting area is surrounded by a metal body; 
         FIG. 12  is a top view of the antenna device in which the mounting area is surrounded by the metal body; and 
         FIG. 13  is a graph illustrating a comparison between the first and second types in terms of relationships between the radiation efficiency and frequency exhibited when the metal body is installed and when not. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the following embodiments, an antenna element incorporated in a mobile phone is used as an example. An antenna device is formed as a mobile phone incorporating the antenna element. 
       FIG. 1  is a view illustrating an outer appearance of an antenna device  100  in a first embodiment of the present invention. The antenna device  100  is formed by mounting an antenna element  124  on a printed board  122  of the mobile phone. As illustrated in  FIG. 1 , x-axis is set along the short length direction of the antenna element  124 , y-axis is set along the longitudinal direction thereof, and z-axis is set along the thickness direction thereof. 
     The printed board  122  has a rectangular plate-like shape of a size of 40 mm×100 mm (x×y). The printed board  122  includes a ground pattern area  103  formed on substantially the entire surface of the printed board and a mounting area  104  formed at a part of the same. The mounting area  104  is formed at a peripheral portion of the printed board  122  such as a side portion or a corner portion. More specifically, in the present embodiment, the mounting area  104  is formed at the center of the long side portion of the printed board  122  (refer also to  FIG. 12 ). The size of the mounting area  104  is 5.0 mm×3.0 mm (x×y). The x-direction length of the mounting area  104  is preferably 1.5 times or more longer than the y-direction length of the same. 
     The resonance frequency of the antenna device  100  is set to about 2.45 GHz which is the frequency band of Bluetooth®. The size of the antenna element  124  is 1.25 mm×2.0 mm×0.8 mm (x×y×z). 
     There are formed in the mounting area  104  three electrode patterns: a feeding pattern  106 ; a first ground electrode connection pattern  108 ; and a second ground electrode connection pattern  110 . The feeding pattern  106  receives AC power through a feeding line  112  which is a transmission line having a characteristic impedance of 50Ω. The antenna element  124  is bonded onto these patterns. That is, all or some of these patterns function as a land of the antenna element  124 . 
       FIG. 2  is a development view of the antenna element  124  in the first embodiment. A structure of the antenna element  124  will be described with reference to  FIGS. 1 and 2 . The antenna element  124  has a dielectric body having substantially a rectangular solid shape as a base body, on the surface of which a feeding electrode  130 , a radiation electrode  132 , and a ground electrode  134  are printed. A rectangular bottom surface  138  (1.25 mm×2.0 mm) is bonded to the mounting area  104  and, thereby, the antenna element  124  is fixed to the printed board  122 . The four side surfaces of the antenna element  124  are referred to respectively as a first side surface  140  (1.25 mm×0.8 mm), a second side surface  142  (1.25 mm×0.8 mm), a third side surface  144  (2.0 mm×0.8 mm), and a fourth side surface  146  (2.0 mm×0.8 mm). The fourth side surface  146  is a surface situated on the peripheral side of the printed board  122 , and the third side surface  144  is a surface situated on the inner side of the same. 
     The radiation electrode  132  is printed on an upper surface  136 , the first side surface  140 , and the bottom surface  138  in a folded configuration. Hereinafter, the parts of the radiation electrode  132  that are printed on the upper surface  136 , first side surface  140 , and bottom surface  138  are referred to respectively as “radiation electrodes  132   a ,  132   b , and  132   c ”. The radiation electrode  132   a  is printed on the entire upper surface  136 . The radiation electrode  132   b  is printed on the entire first side surface  140 . The radiation electrode  132   c  is printed only on a part of the bottom surface  138 . 
     The feeding electrode  130  and ground electrode  134  are also printed on the bottom surface  138 . The feeding electrode  130  and ground electrode  134  extend partially in parallel to each other. The ground electrode  134  is formed into an L-shape that surrounds the feeding electrode  130  and extends partially in parallel to the radiation electrode  132   c . The second side surface  142 , third side surface  144 , and fourth side surface  146  are each a non-electrode formation area. The open end (voltage point) of the ground electrode  134 , that is, the leading end of the short arm of the L-shape faces the outer periphery of the printed board  122  (refer to  FIG. 1 ). The reason for this is to keep the open end far away from the ground pattern area  103  so as to increase radiation resistance as much as possible. 
     The feeding electrode  130  is connected to the feeding pattern  106  and receives AC power from the feeding line  112 . The ground electrode  134  is connected to the ground pattern area  103  having a ground potential through the second ground electrode connection pattern  110 . The radiation electrode  132   c  is connected to the ground pattern area  103  through the first ground electrode connection pattern  108 . 
     The ground electrode  134  and radiation electrode  132   a  face each other in terms of planes, so that ground coupling capacitance C 1  is formed between the ground electrode  134  and radiation electrode  132   a . Similarly, feeding coupling capacitance C 2  is formed between the feeding electrode  130  and radiation electrode  132   a . That is, the dielectric body itself forms a feeding gap. The resonance frequency of the antenna device  100  changes depending on the ground coupling capacitance C 1 . The impedance matching of the antenna device can be adjusted mainly by the feeding coupling capacitance C 2 . 
     Upon receiving power, the feeding electrode  130  generates an electric field, and AC current is generated in the radiation electrode  132   a  through the feeding coupling capacitance C 2 . This leads the radiation electrode  132  to generate radio waves. 
     The ground electrode  134  and radiation electrode  132   a  are opposed to each other as parallel planes, so that it is easy to ensure larger ground coupling capacitance C 1  as compared to a case where the gap is formed in the upper surface or side surface of the dielectric body. Similarly, the feeding electrode  130  and radiation electrode  132   a  are opposed to each other as parallel planes, so that it is easy to increase the feeding coupling capacitance C 2 . The feeding electrode  130  and ground electrode  134  or ground electrode  134  and radiation electrode  132   c  are included on the same plane, so that influence of the capacitance generated between the above electrodes is negligibly small as compared to the ground coupling capacitance C 1  and feeding coupling capacitance C 2 . 
     The magnitude of the ground coupling capacitance C 1  can be adjusted depending on the area of the ground electrode  134  or height of the antenna element  124 . For example, a procedure may be adopted in which the ground coupling capacitance C 1  is roughly adjusted depending on the height of the antenna element  124  first, and then it is finely adjusted depending on the area of the ground electrode  134 . Similarly, the magnitude of the feeding coupling capacitance C 2  can be adjusted depending on the area of the feeding electrode  130  or height of the antenna element  124 . For example, a procedure may be adopted in which the feeding coupling capacitance C 2  is roughly adjusted depending on the height of the antenna element  124  first, and then it is finely adjusted depending on the area of the feeding electrode  130 . 
     The ground coupling capacitance C 1  or feeding coupling capacitance C 2  is changed by changing the area (shape) of the ground electrode  134  or feeding electrode  130 , so that it is easier to extend the adjustable range of the coupling capacitance than in the case where the width of the feeding gap is adjusted to adjust the coupling capacitance. As a result, it is possible to adjust antenna characteristics only by means of the antenna element  124  precisely without excessively depending on so-called a (external) matching element for use in adjustment of impedance or resonance frequency. 
     The mounting area  104  is often formed in the corner portion of the printed board  122 . This is because that it is easier to suppress return loss than in the case where the antenna element  124  is formed in the side edge portion of the printed board  122 . In the case of the antenna element  124  in the first embodiment, the ground coupling capacitance C 1  and feeding coupling capacitance C 2  can be adjusted widely, making it easy to realize low return loss and high radiation efficiency. As a result, even when the mounting area  104  is formed in the side edge portion, e.g., the center of the long side of the printed board  122 , practical performance can be achieved. 
     In the antenna element  124 , the electrode patterns of the surfaces other than the bottom surface  138  are extremely simple. Further, electrode pattern of the bottom surface  138  is not so complicated. This allows easy production of the antenna element  124 , easily leading to quality stabilization. 
       FIG. 3  is an equivalent circuit diagram of the antenna device  100 . AC power source  150  is a feeding source that feeds AC current to the feeding pattern  106  and feeding electrode  130 . 
       FIG. 4  is an outer appearance of an antenna device  105  in a comparative example. The antenna device  105  illustrated in the comparative example is obtained by a simulation based on the assumption that an antenna element having a configuration illustrated in FIG. 4 of Patent Document 1 is used in 2.45 GHz frequency band. Unlike the antenna element  124  (hereinafter, referred to also as “first type”) of the antenna device  100  in the first embodiment, an antenna element  125  (hereinafter, referred to also as “side-gap type”) of the antenna device  105  of the comparative example has gaps G 1  and G 2  formed on a second side surface  142 . As a result of a simulation performed with the x-direction size and y-direction size of the antenna device  125  fixed to the same dimensions as those of the antenna element  124 , the obtained size of the antenna element  125  was 1.25 mm×2.0 mm×2.0 mm (x×y×z). That is, the height of the antenna element is increased as compared to the first type. The width of the gap G 1  was 0.01 mm, and the width of the gap G 2  was 0.04 mm, making the actual production of the antenna element difficult. 
       FIG. 5  is a graph illustrating a comparison between the first type and side-gap type in terms of a relationship between the return loss and frequency. It is assumed here that the relative permittivity of the base bodies (dielectric bodies) of both the antenna elements  124  and  125  be 37. Further, it is assumed that the feeding electrode  130 , radiation electrode  132 , and ground electrode  134  be made of copper (Cu). As illustrated in  FIG. 5 , in the Bluetooth® frequency band, the return loss of the first type is considerably smaller than that of the side-gap type. 
       FIG. 6  is a graph illustrating a comparison between the first type and side-gap type in terms of a relationship between the radiation efficiency and frequency. The radiation efficiency is considerably improved in the first type as compared to in the side-gap type. The maximum radiation efficiency of the first type was 78.6(%), and maximum radiation efficiency of the side-gap type was 23.2(%). That is, in the case where the first type is used, the maximum radiation efficiency is improved by 55.4(%) (=78.6−23.2). 
       FIG. 7  is an outer appearance of an antenna device  101  in a second embodiment. Unlike the case of first type, in an antenna element  126  (hereinafter, referred to as “second type”) of the antenna device  101  in the second embodiment, the radiation electrode  132  is formed also in the upper portions (upper surface  136  side) of the third and fourth side surfaces  144  and  146 . The area of the radiation electrode  132  is thus increased in effect, which easily leads to reduction of VSWR (Voltage Standing Wave Ratio). 
       FIG. 8  is an outer appearance of an antenna device  102  in a third embodiment. In an antenna element  127  (hereinafter, referred to also as “third type”) of the antenna device  102  in the third embodiment, the radiation electrode  132  is formed in the upper portion (upper surface  136  side) of the fourth side surface  146  and the outer peripheral side (fourth side surface  146  side) of the upper surface  136 . When the distance (hereinafter, referred to as “ground distance”) between the radiation electrode  132  and ground pattern area  102  is small, electromagnetic coupling (hereinafter, referred to as “ground coupling”) between the radiation electrode  132  and ground pattern area  102  easily becomes obvious. When the ground coupling becomes large, radiation resistance becomes smaller, easily leading to degradation of the radiation efficiency. Since the main part of the radiation electrode  132  is formed in the outer peripheral side of the upper surface  136  and upper portion of the fourth side surface  146  in the third type, a sufficient ground distance can be ensured. 
       FIG. 9  is a graph illustrating a comparison between the first, second, third types in terms of a relationship between the return loss and frequency. The return loss of the second and third types is smaller than that of the first type, but the difference therebetween is very small. 
       FIG. 10  is a graph illustrating a comparison between the first, second, and third types in terms of a relationship between the radiation efficiency and frequency. The maximum radiation efficiency of the first to third types is about 79(%), which means there is no significant difference. From the above, it was found that the first, second, and third types have equivalent antenna performance. The electrode pattern of the radiation electrode  132  is the simplest in the first type, so the first type can be said to be the most excellent of the three types in the viewpoint of manufacturability. 
       FIG. 11  is an outer appearance of the antenna device  100  in which the mounting area  104  is surrounded by a metal body  114 , and  FIG. 12  is a top view of  FIG. 11 . In the second type, the third and fourth side surfaces  144  and  146  are each partly covered by the radiation electrode  132 , so that shielding effectiveness is expected to be achieved in the second type. That is, an assumption is made for the second type that the radiation electrode  132  of the side surface portion effectively protects the feeding electrode  130  or ground electrode  134  from external influences. In order to verify this assumption, a simulation was carried out to estimate antenna characteristics obtained in the configuration in which the metal body  114  is installed around the mounting area  104 . The use of the metal body  114  is based on the assumption that a battery, an LCD (Liquid Crystal Display), a shield case, a metal frame, or other electronic components are installed in that portion. The size of the metal body  114  is 36 mm×45 mm×5.0 mm (x×y×z). As a distance between the mounting area  104  and metal body  114 , a distance of 3.0 mm is ensured in both x- and y-directions. 
       FIG. 13  is a graph illustrating a comparison between the first and second types in terms of relationships between the radiation efficiency and frequency exhibited when the metal body  114  is installed and when not. Unlike the case of  FIG. 10 ,  FIG. 13  represents frequency characteristics around the maximum radiation efficiency. In the case where the metal body  114  was not installed, the maximum radiation efficiencies in the first and second types were 78.6(%) and 78.9(%) respectively, which means there is little difference. In the case where the metal body  114  was installed, the maximum radiation efficiencies in the first and second types were 75.7(%) and 76.8(%) respectively. That is, in the case of the first type, installation of the metal body  114  reduces the maximum radiation efficiency by 2.9(%) (=78.6−75.7), while in the case of the second type, the maximum radiation efficiency is reduced by 2.1(%) (=78.9−76.8). That is, the second type is less subject to the influence of the metal body than the first type, which verifies the above assumption. 
     The antenna devices  100 ,  101 , and  102  have been described based on the respective embodiments. In every embodiment, the feeding electrode  130  and ground electrode  134  of the bottom surface  138  are opposed to the radiation electrode  132   a  of the upper surface  136  as parallel planes, so that it is possible to easily increase the ground coupling capacitance C 1  and feeding coupling capacitance C 2 . The second side surface  142  is made to act as a fully open end, and no feeding gap is formed on any of the surfaces of the antenna element  124 . The ground coupling capacitance C 1  or feeding coupling capacitance C 2  changes depending on the area of the feeding electrode  130  or ground electrode  134 . Therefore, it is possible to significantly change antenna characteristics only by means of the electrode patterns formed on the antenna element. Further, the simplicity of the electrode patterns enhances manufacturability and contributes to cost reduction and quality stabilization. 
     In the case where antenna characteristics are adjusted using an inductor, the radiation efficiency can be reduced by the resistance component of the inductor. However, in the case of the antenna element according to the present embodiment, both the resonance frequency and impedance can be adjusted by a capacitance (ground coupling capacitance C 1 , feeding coupling capacitance C 2 ), eliminating the need of forming the inductance using the electrode pattern. 
     The present invention has been described based on the above embodiments. It should be understood by those skilled in the art that the above embodiments are merely exemplary of the invention, various modifications and changes may be made within the scope of the claims of the present invention, and all such variations may be included within the scope of the claims of the present invention. Thus, the descriptions and drawings in this specification should be considered as not restrictive but illustrative.