Abstract:
A three-axis antenna containing: a bobbin for housing a core, made of a resin having an top flange and a bottom flange both of which include four flange pieces at both ends of the winding column in the thickness direction of the core; a first coil and a second coil wound in the spaces between the flange pieces to cross each other at the upper and lower surfaces of the core; and a third coil wound at the side surface of the core and between the top flange and the bottom flange.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2013-247171, filed on Nov. 29, 2013, 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 small-sized three-axis antenna which is used in a receiving system of a keyless entry system or a security system, etc. 
     2. Description of the Related Art 
     In recent years, a three-axis antenna, which is omni-directional and can be installed in a miniaturized receiving system, has been used widely as an antenna for LF band which is used in the receiving set, called as a fob, of a keyless entry system or of a security system for vehicles. 
       FIG. 4  is a perspective view of a conventional three-axis antenna  1 . The three-axis antenna  1  includes an X axis coil  4   x , a Y axis coil  4   y  and a Z axis coil  4   z , the coils being orthogonally wound around a ferrite core  2  which is configured as a flat octangular body having fan-shaped auricles. 
     The core  2  is set on a resin base  3  to which a plurality of metal terminals are implanted, and the terminals of the X axis coil  4   x , the Y axis coil  4   y  and the Z axis coil  4   z  are wound around winding portions  5   a  of metal terminals  5  and soldered to be electrically connected. 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     Due to general demands for miniaturization and thinning of receiving sets, a three-axis antenna is required to be smaller and thinner. 
     However, conventional three-axis antennas have had to put up with the problem that a smaller core provides insufficient inductance, and with the problem that a complexly shaped core requires higher processing costs and thus raises the cost of an antenna coil. 
     To compensate for the insufficient inductance, the apparent solution is to increase the number of windings of a coil. To fit within available space for such a winding, one option is to use a thinner core, and the other is to use thinner wire. However, since the ferrite which makes the core is brittle, the thinner the core is, the brittler it is. Thus, the manufacturing process becomes difficult and the processing costs increase. Further, use of thin wire to increase the number of winding results in the increase of the DC resistance and of the capacity between the wires. Consequently, the Q value and the self-resonant frequency dropped resulting in lower the characteristics of antenna coils. Therefore, the miniaturization of a three-axis antenna has met substantial obstacles. 
     Means for Solving the Problem 
     The three-axis antenna according to the present invention is characterized by: 
     a three-axis antenna comprising: 
     a bobbin for housing a core, said bobbin being made of a resin and having a top flange and a bottom flange both of which include four flange pieces at both ends of the winding column in the thickness direction of the core; 
     a first coil and a second coil wound in the spaces between the flange pieces to cross each other at the upper and lower surfaces of the core; and 
     a third core wound at the side surface of the core and between the top flange and the bottom flange. 
     Effect of the Invention 
     According to the three-axis antenna of the present invention, even if miniaturization and space saving are carried out, it is possible to provide a three-axis antenna which is manufacturable at a low cost and has stable characteristics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view from above of a three-axis antenna according to the present invention; 
         FIG. 2  is an exploded perspective view of the three-axis antenna according to the present invention; 
         FIG. 3  is a perspective view of a bobbin of the three-axis antenna according to the present invention; and 
         FIG. 4  is a perspective view of a conventional three-axis antenna. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The three-axis antenna according to the present invention will be described below, referring to  FIGS. 1-3 . 
       FIG. 1  is a perspective view from above of a three-axis antenna according to the present invention.  FIG. 2  is an exploded perspective view thereof. 
     As shown in  FIG. 1 , a three-axis antenna  10  comprises a ferrite core  20 , a resin bobbin  30 , and an X axis coil  41 , a Y axis coil  42  and a Z axis coil  43 , on which insulation coated wires are provided respectively. 
     As shown in  FIG. 2 , the core  20  is flat and parallelepiped-shaped, and has an X recess  21  and a Y recess  22  which cross orthogonal to each other at the corresponding positions on the top surface and bottom surface thereof. The thickness of the core  20  around the X recess  21  is tx, and the thickness of the Y recess  22  is ty, with tx&lt;y. 
     A through hole  39  penetrating the core  20  in the thickness direction, a top flange  31  having four flange pieces  31   a - 31   d  on the upper end of the Z winding axis, and a bottom flange  32  having four flange pieces  32   a - 32   d  are provided on a bobbin  30 . 
     Designating the space between the flange pieces  31   a ,  32   a  and the flange pieces  31   d ,  32   d  as a space  34   ad , the space between the flange pieces  31   b ,  32   b  and the flange pieces  31   c ,  32   c  as a space  34   bc , the space between the flange pieces  31   a ,  32   a  and the flange pieces  31   b ,  32   b  as a space  34   ab  and the space between the flange pieces  31   c ,  32   c  and the flange pieces  31   d ,  32   d  as a space  34   cd , the height of the Z winding axis at the spaces  34   ad ,  34   bc  is equal to the thickness tx of the X recess  21  of the core  20 , and the height of the Z winding axis at the spaces  34   ab ,  34   cd  is equal to the thickness ty of the Y recess  22  of the core  20 . 
     The bobbin  30  houses the core  20  in the through hole  39  so that the thicknesses tx, ty of the recesses  21 ,  22  match the height of the Z winding axis. The X axis coil  41  and the Y axis coil  42  are wound around the core  20  orthogonally to each other at the upper surface and the lower surface, as the X axis coil  41  is wound around the space  34   ad ,  34   bc  and the recess  21  as the X winding axis, and the Y axis coil  42  is wound around the space  34   ab ,  34   cd  and the recess  22  as the Y winding axis. Further, the Z axis coil  43  is wound around the Z winding axis in the space between the top flange  31  and the bottom flange  32  to weave around and orthogonally to each of the X winding axis and the Y winding axis. 
       FIG. 3  is a perspective view of the bobbin  30  to show the detailed structure thereof. As shown in  FIG. 3 , there are intermediate flanges  33   x ,  33   y  and  33   z  around the X winding axis, the Y winding axis and the Z winding axis between the divided flanges  31 ,  32 . 
     Namely, the X axis coil, the Y axis coil and the Z axis coil are divided and wound as described below: 
     the X axis coil  41  is divided by the intermediate flange  33   x  into the coils  41   a  and  41   b;    
     the Y axis coil  42  is divided by the intermediate flange  33   y  into the coils  42   a  and  42   b ; and the Z axis coil  43  is divided by the intermediate flange  33   z  into the coils  43   a  and  43   b.    
     The respective coils are wound in divided manner thus the capacities between the wires are lowered. The coils can be divided into three or more by providing plural intermediate flanges. 
     Since the sectional height tx of the X axis coil  41  and the sectional height ty of the Y axis coil  42  are different from each other, the decline of the three-axis antenna&#39;s characteristics by the mutual contact of the X axis coil  41  and the Y axis coil  42  is avoided. 
     A plurality of metal terminals  50  having winding portions  51  are implanted into the bottom flange  32 . The terminals of the X axis coil  41 , the Y axis coil  42  and the Z axis coil  43  are wound around the respective winding portions  51  and soldered to be connected electrically. 
     Around the X winding axis and the Y winding axis, grooves  38  for guiding the respective terminals of the X axis coil  41  and the Y axis coil  42  are provided to prevent wires thereof from disconnection due to stress when winding. 
     The three-axis antenna  10  is molded in resin to expose a portion of the metal terminal  50 , and the exposed portion is adaptively bent to be mounted on a printed circuit board (not shown). 
     Without the auricular portions of the conventional three-axis antenna, simplified structure of the three-axis antenna  10  means that the main processing costs are low. As the bobbin is made of tough resin, it is easily possible to decrease the thickness of the bobbin so as to secure a space for winding. 
     As a result, a three-axis antenna of low manufacturing cost, and a miniaturized and space saving profile will be provided. The three coils  41 ,  42  and  43  are wound in divided manner respectively so that the capacities between the wires of the coils can be decreased to provide a three-axis antenna of consistent characteristics. 
     Although conventional antennas can be modified to divide the coils into more than two by providing protrusions on a core, it will result in brittle structure due to the complicated shape and in high costs of processing. 
     The present invention is preferable to conventional antennas since the flanges on a bobbin of resin are sturdy. Although in the abovementioned embodiment the cores are shown as parallelepipeds, a flat cylindrical shape is also employable. Also, a mixture of magnetic powder and the resin material can be used as the resin for the bobbins. 
     EXPLANATIONS OF CODES 
     
         
           1 ,  10  three-axis antenna 
           2 ,  20  core 
           21  X recess 
           22  Y recess 
           3  base 
           30  bobbin 
           31  top flange 
           32  bottom flange 
           31   a ,  31   b ,  31   c ,  31   d ,  32   a ,  32   b ,  32   c ,  32   d      flange piece     
           33   x ,  33   y ,  33   z      intermediate flange     
           34   ab ,  34   bc ,  34   cd ,  34   ad      space     
           38  groove 
           39  through hole 
           4   x ,  41  X axis coil 
           4   y ,  42  Y axis coil 
           4   z ,  43  Z axis coil 
           5 ,  50  metal terminal 
           5   a ,  51  winding portion 
         tx, ty thickness of core (sectional height of coil)