Patent Publication Number: US-2020292654-A1

Title: Antenna apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to an antenna apparatus. 
     BACKGROUND ART 
     When an antenna apparatus is mounted on conveyances such as cars or buildings, there is a risk of impairing an external appearance thereof because antenna elements project outward from a surface of the conveyance or the building. Therefore, Patent Literature 1 and 2 disclose a configuration where a recess is formed on a surface of a conveyance or a building and antenna elements are arranged in the recess. 
     For purposes of monitoring a radio wave environment etc., there is a demand for means for detecting an incoming direction of radio waves over a wide frequency band with high precision. Therefore, there is a method using an array antenna where a plurality of antenna elements are arranged, in which information on a phase and the like of radio waves received through each of the plurality of antenna elements is used. Patent Literature 3 discloses an antenna apparatus that detects an incoming direction of radio waves. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent No. 2692253 
     Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2003-309409 
     Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2002-71776 
     SUMMARY OF INVENTION 
     Technical Problem 
     An embedded antenna for cars disclosed in Patent Literature 1 is an antenna for a specific frequency band which has a fractional bandwidth of about 20% and a gain is not taken into consideration. Therefore, it is difficult to apply this embedded antenna to uses in which a gain needs to be secured over a wide band (e.g., detection of an incoming direction in monitoring of the radio wave environment). 
     In a configuration disclosed in Patent Literature 2, a plurality of antenna elements are encapsulated with molded resin and the molded resin encapsulating the plurality of antenna elements is fixed in a recess with an adhesive, a double-faced tape, or a bolt, etc. Thus, a task for mounting an antenna apparatus on a conveyance or a building, especially a task for fixing antenna elements, is complicated. Further, in Patent Literature 2, no method for obtaining stable characteristics over a wide frequency band and no measure against variations in the characteristics caused by an approach of persons or objects are taken into consideration. 
     There is a possibility that an antenna apparatus that detects a radio direction (incoming direction of radio waves) by using a plurality of antenna elements as described in Patent Literature 3 may significantly impair an external appearance of a conveyance or a building. An array antenna for precisely detecting an incoming direction of radio waves needs to have stable frequency characteristics over a wide band, be less susceptible to variations in these characteristics caused by persons or objects present inside a conveyance or a building, and optimize distances between antenna elements according to a frequency band where a detection is performed. Therefore, there are some restrictions on the size and/or the position of the antenna elements, and as a result it is difficult to arrange the antenna elements so that they are inconspicuous in appearance. Thus, for example, as shown in FIG. 22 of Patent Literature 3, an external appearance is significantly impaired since a large mounting space is necessary to mount a plurality of antenna elements for detecting a radio direction on a car. 
     In order to reduce the possibility of impairing an external appearance of a conveyance or a building, a glass antenna which is embedded in or printed on a window pane has been in practical use. However, there is a problem that characteristics of a glass antenna are easily varied due to an approach of persons or objects. Further, since there are considerable restrictions on the installation place, it has been difficult to arrange a plurality of glass antennas at arbitrary intervals. 
     Therefore, an object of the present invention is to solve the problem described above, and provide an antenna apparatus capable of being easily mounted on a conveyance or a building and detecting an incoming direction of radio waves with high precision without significantly impairing an external appearance. 
     Solution to Problem 
     An antenna apparatus according to the present invention includes an array antenna including a plurality of antenna elements and configured to detect an incoming direction of a radio wave, including: a recess formed in a part of a conveyance or a building, an antenna element disposed in the recess, and a cover or an insulating material closing a mouth of the recess, in which the recess includes a bottom, the mouth and a side wall connecting the bottom and the mouth, and a shape of the recess widens from the bottom toward the mouth, and the bottom and the side wall are made of a conductor. 
     Advantageous Effects of Invention 
     According to the present invention, an antenna is easily mounted on a conveyance or a building, and an incoming direction of radio waves can be precisely detected without significantly impairing an external appearance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view schematically showing the antenna apparatus according to an embodiment of the present invention; 
         FIG. 2  is a perspective view showing the antenna apparatus including conical-shaped antenna elements according to other embodiments of the present invention; 
         FIG. 3  is a cross-sectional view schematically showing a comparative example of the antenna apparatus shown in  FIG. 2 ; 
         FIG. 4  is a cross-sectional view schematically showing an example of the antenna apparatus shown in  FIG. 2 ; 
         FIG. 5  is a cross-sectional view schematically showing other examples of the antenna apparatus shown in  FIG. 2 ; 
         FIG. 6  is a graph showing a calculation result of a horizontal gain at each frequency of the antenna apparatuses shown in  FIGS. 3 to 5 ; 
         FIG. 7  is a perspective view showing an example of a car with the antenna apparatus mounted thereon according to the present invention; 
         FIG. 8  is a cross-sectional view schematically showing the antenna apparatus according to other embodiments of the present invention; 
         FIG. 9  is a cross-sectional view schematically showing a modified example of the antenna apparatus shown in  FIG. 1 ; 
         FIG. 10  is a cross-sectional view schematically showing other modified examples of the antenna apparatus shown in  FIG. 1 ; 
         FIG. 11  is a perspective view showing an example of a car with other antenna apparatuses mounted thereon according to the present invention; 
         FIG. 12  is a schematic view showing an inside of a car mounted with the antenna apparatus shown in  FIG. 11 ; 
         FIG. 13A  is a cross-sectional view showing an example of the antenna apparatuses according to other embodiments of the present invention; and 
         FIG. 13B  is a cross-sectional view showing other examples of the antenna apparatuses according to other embodiments of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention are explained hereinafter with reference to the drawings.  FIG. 1  shows a basic configuration of an antenna apparatus  1  according to the present invention. In this example, the antenna apparatus  1  is mounted on a roof part made of a conductor, which is used as a mounting surface  2 , of a conveyance such as a car. In other words, a recess  3  is formed on a part of the roof part. The shape of this recess  3  widens from a bottom  3   a  toward a mouth  3   b , and a side wall  3   c  connecting the bottom  3   a  with the mouth  3   b  is inclined at an angle of 120° or larger, preferably at an angle of 135° or larger, with respect to the bottom  3   a . The bottom  3   a  and the side wall  3   c  are electrically connected with each other and constitute a ground part (embedded ground plate). The ground part is made of a conductor and is electrically connected with the mounting surface  2  where the recess  3  is formed. An antenna element  4  (e.g., monopole antenna) is positioned in the recess  3 . A feeding point  5  is provided on the ground part and is connected with the antenna element  4 . The mouth  3   b  is closed by a cover  6 . 
     The side wall  3   c  of this antenna apparatus  1  is inclined and hence a reflected wave generated on the side wall  3   c  travels toward the upper mouth  3   b . Therefore, there is an effect that a horizontal gain is stabilized. Further, since the antenna element  4  is separated from the space below the ground part by the ground part, the antenna element  4  is less likely to be affected by a person or an object in the space below the ground part. Therefore, there is an advantageous effect that antenna characteristics are stabilized. Further, since the mounting surface  2  is made of a conductor and is electrically connected to the ground part, the ground part substantially becomes very large. Therefore, it is less likely to be affected by a person or an object indoors. 
       FIG. 2  shows an antenna apparatus  1  including a conical-shaped antenna element  4  having a wide band property as an application example of the antenna apparatus  1  shown in  FIG. 1 . The cover  6  is omitted for an illustration purpose in this  FIG. 2  and in  FIGS. 3 to 5, 7, 8 and 11  described later.  FIGS. 3 to 5  show three examples of the antenna apparatus  1  each of which is composed of a conical-shaped antenna element  4  and a ground part. In these antenna apparatuses  1 , the inclination angles of the side wall  3   b  with respect to a horizontal direction (which is parallel to the bottom  3   a  in these examples) are 90°, 130° and 160°, respectively.  FIG. 6  shows a result of horizontal gains in these antenna apparatuses  1  calculated by carrying out electromagnetic field simulations. A height of the side wall  3   b  is one half of a wavelength at a calculated upper-limit frequency fl. It can be confirmed from  FIG. 6  that the more the inclination angle is increased as the side wall  3   b  is inclined, the smaller the variations caused by changes in the frequencies become, and hence these characteristics can be made more stable. 
     The number of antenna elements  4  arranged in one recess  3  is not limited to one and a plurality of antenna elements  4  can be arranged in the recess  3 .  FIG. 7  shows a configuration in which a circular array antenna with a plurality of antenna elements  4  arranged in one recess  3  is embedded in a roof part of a conveyance. The installation place of a glass antenna is limited to a glass part and therefore flexibility in constructing an array antenna is low. In contrast, in the present invention, a plurality of antenna elements  4  are arranged at arbitrary intervals and hence it is possible to provide an array antenna that is less conspicuous in appearance. 
     The shape of the antenna element  4  is not limited to the linear shape as shown in  FIG. 1  or the conical shape as shown in  FIG. 2 . That is, the antenna element  4  may have a variety of arbitrary shapes such as a folded-line shape, a planer shape, and a three-dimensional shape. Further, a matching element may be attached to the feeding point  5 . Moreover, a short-circuit part may be provided between the antenna element  4  and the ground part as in an inverted-F antenna. In the case where electric power is supplied by using a coaxial cable, an influence of the cable can also be reduced by providing a ferrite core around the coaxial cable. 
     The ground part composed of the bottom  3   a  and the side wall  3   c  of the recess  3  may be a variety of conductors such as a conductor plate, a conductor thin film, a punching metal, or a conductor net. Further, it may have a structure in which the conductor is held by an insulator. Moreover, the shape of the ground part is not limited to a truncated-cone shape as shown in  FIG. 2  and may instead be an arbitrary shape in which the side wall  3   c  is inclined. The inclination angle of the side wall  3   c  does not have to be constant and the bottom  3   a  may be inclined as well. The recess  3  may also have such a shape that the bottom  3   a  and the side wall  3   c  are seamlessly formed as one piece. For example, the recess  3  has an inverted conical shape. Further, the side wall  3   c  may have a step shape shown in  FIG. 8  or a curved-surface shape, though it is not shown. 
     The cover  6  covering the recess is not limited to a plate or the like which is placed only over the ground part. That is, the cover  6  may be a plate or the like that is placed so as to cover an area wider than the ground part, including the mounting surface  2  as shown in  FIG. 9 . A structure serving as a support may be provided between the ground part and the cover  6  in order to support the cover  6 . Further, instead of using the cover  6 , it may have a structure in which the recess  3  is filled with an insulating material  7  as shown in  FIG. 10 . Further, both the cover  6  and the insulating material  7  may be used in combination. 
     The number of antenna elements  4  arranged in the recess  3  is not limited to one and a plurality of antenna elements can be arranged in the recess  3 . In the present invention, a plurality of antenna elements  4  may be arranged in one recess  3  as shown in  FIG. 7  in order to form an array antenna for detecting an incoming direction of radio waves. However, the antenna array may instead have a configuration in which a plurality of recesses  3  in each of which one antenna element  4  is disposed are formed as shown in  FIG. 11 . Further, an arbitrary number of recesses  3  in which an arbitrary number of antenna elements  4  are arranged may be formed. Regarding the electrical connection between the side wall  3   c  and the mounting surface  2 , and that between the side wall  3   c  and the bottom  3 , a method in which they are electrically connected to each other over the whole circumference, a method in which they are electrically connected to each other at a plurality of points, or a method in which they are connected to each other through a capacitive coupling in a high-frequency manner may be used. Further, depending on a desired performance, they may not be connected to each other. The antenna apparatus according to this embodiment detects an incoming direction of radio waves by using a well-known MUSIC method etc. in which a direction is determined by processing amplitudes and phases of radio waves received through a plurality of antenna elements  4 . However, an incoming direction of radio waves may be detected by other methods. In this embodiment, as described above, a plurality of antenna elements  4  constitute an array antenna that detects an incoming direction of radio waves. 
     The antenna element  4  may be a ground-type antenna element  4  such as a monopole type, or other types of antenna elements  4  such as a dipole-type antenna element  4  etc. In the case of using a dipole-type antenna element  4 , the bottom  3   a  and the side wall  3   b  are also preferably made of a conductor. In the case of disposing the above-described dipole-type antenna element  4  between the bottom  3   a  and the side wall  3   b , and the cover  6 , the advantageous effect that this antenna element  4  is less susceptible to reflection in the side wall  3   b  can be obtained, as in the case of the monopole-type antenna element  4 . In the antenna apparatus  1  according to this embodiment, the entire recess  3  can be removed. Therefore, a maintenance can be easily performed and the antenna apparatus can be easily installed or replaced when necessary. 
     Functions and effects of the antenna apparatus  1  according to the present invention will be described hereinafter. The antenna apparatus  1  that detects an incoming direction of radio waves often needs to cover a wide frequency band such as when monitoring a radio wave environment. When there is an object or the like which causes a reflection around the antenna apparatus  1 , a false image tends to be generated at a specific frequency according to a distance between the antenna apparatus  1  and the object. Therefore, it has been difficult to mount the antenna apparatus on a conveyance or the like so that it is inconspicuous in appearance. In contrast to this, the antenna apparatus  1  according to the present invention can reduce an influence of a reflection in a wide frequency range and detect a direction over a wide band by providing the recess  3  in the mounting surface  2  such as a roof part of a conveyance and disposing the antenna element  4  in the recess  3 . The inclination angle of the side wall  3   c  with respect to the bottom  3   a  (horizontal surface) is at least 120° or larger, preferably about 135° or larger. 
     Further, it is possible to obtain a stable gain over a wide band by forming a shape of the antenna element  4  into a three-dimensional shape such as a conical shape. Thus, it is possible to obtain good characteristics as the antenna apparatus  1  that detects an incoming direction of radio waves by a combination of the side wall  3   c  having an inclination and the bottom  3   a  with the three-dimensional shape antenna element  4 . 
     The configuration in which a plurality of small recesses  3  is formed as shown in  FIG. 11  is effective especially when a mechanical strength is weakened by forming a large recess  3 . For example, the recesses  3  can be formed in gaps between frames  8  of a roof part of a conveyance while leaving the frames  8  as they are as shown in  FIG. 12  showing a state in which an interior panel in a conveyance is removed, and hence an array antenna for detecting a direction can be formed while maintaining the strength of the conveyance. 
     The antenna apparatus  1  according to the present invention has a feature that its characteristics hardly changes even when persons or objects approach the antenna apparatus since the antenna element  4  is separated, by the bottom  3   a  and the side wall  3   c  which are made of a conductor, from the space below them. In addition, as shown in  FIG. 13A , it is possible to make the characteristics more resistant to changes by adding an inside cover  9  made of an insulator that covers the bottom  3   a  and the side wall  3   c  to isolate them from the space below (space inside the conveyance or the building). The inside cover  9  covers the surface of the bottom  3   a  and the side wall  3   c  on the side opposite to the mouth  3   b . The reason for this covering is as follows. When the bottom  3   a  and the side wall  3   c  made of a conductor come into contact with a human body or the like, a high frequency current flows into them and their characteristics are changed. However, it is possible to prevent this change by preventing the contact between them. Further, a capacitive coupling slightly causes an adverse effect even when there is no such contact. Therefore, the above change can be more effectively prevented by moving the inside cover  9  away from the bottom  3   a  and the side wall  3   c  which are made of a conductor as shown in  FIG. 13B . Thus, a structure may be provided in which the inside cover  9  covers the surface of the bottom  3   a  and the side wall  3   c  on the side opposite to the mouth  3   b  with an interval therebetween. Further, a similar effect can be obtained by increasing the thickness of the inside cover  9 , instead of moving it away. It is effective to form the inside cover  9  by using a material having a small relative dielectric constant to reduce its capacitance. 
     Although the present invention is explained above with reference to embodiments, the present invention is not limited to the above-described embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the invention. 
     This application is based upon and claims the benefit of priority from Japanese patent applications No. 2016-68848, filed on Mar. 30, 2016, the disclosure of which is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS LIST 
     
         
           1  ANTENNA APPARATUS 
           2  MOUNTING SURFACE 
           3  RECESS 
           3   a  BOTTOM 
           3   b  MOUTH 
           3   c  SIDE WALL 
           4  ANTENNA ELEMENT 
           5  FEEDING POINT 
           6  COVER 
           7  INSULATING MATERIAL 
           8  FLAMES 
           9  INSIDE COVER