Patent Publication Number: US-9843085-B2

Title: Directional coupler

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a directional coupler, and more particularly, to a directional coupler which includes a main line and a sub-line that are electromagnetically coupled to each other. 
     2. Description of the Related Art 
     For example, a known directional coupler is disclosed in Japanese Patent No. 3203253. In the directional coupler, a first coupling line in a spiral or substantially spiral shape faces a second coupling line in the same shape as the first coupling line with a dielectric layer therebetween. With this configuration, the first coupling line and the second coupling line are electromagnetically coupled to each other and form a directional coupler. 
     With the directional coupler described in Japanese Patent No. 3203253, in order to make a fine adjustment of the degree of coupling between the first coupling line and the second coupling line, adjusting the thickness of the dielectric layer provided between the first coupling line and the second coupling line is considered. However, since the first coupling line is provided on one dielectric layer and the second coupling line is provided on another dielectric layer, adjusting the thickness of the dielectric layer provided between the first coupling line and the second coupling line causes the entire first coupling line and the entire second coupling line to be closer to or farther away from each other. Therefore, the degree of coupling between the first coupling line and the second coupling line will greatly vary. As described above, it is difficult for the directional coupler described in Japanese Patent No. 3203253 to make a fine adjustment of the degree of coupling between the first coupling line and the second coupling line. 
     SUMMARY OF THE INVENTION 
     Accordingly, preferred embodiments of the present invention provide a directional coupler which is capable of making a fine adjustment of the degree of coupling between a main line and a sub-line. 
     According to a preferred embodiment of the present invention, a directional coupler includes a multilayer body including a plurality of stacked dielectric layers; a main line that includes a first main line portion and a second main line portion which are connected in series to each other in this order and that is provided in the multilayer body; and a sub-line that includes a first sub-line portion and a second sub-line portion which are connected in series to each other in this order, the first sub-line portion being electromagnetically coupled to the first main line portion, the second sub-line portion being electromagnetically coupled to the second main line portion, and the sub-line being provided on one side in a stacking direction with respect to the main line in the multilayer body. The second main line portion is provided on a dielectric layer that is different from a dielectric layer on which the first main line portion is provided and/or the second sub-line portion is provided on a dielectric layer that is different from a dielectric layer on which the first sub-line portion is provided. 
     According to various preferred embodiments of the present invention, a fine adjustment of the degree of coupling between a main line and a sub-line is achieved. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an equivalent circuit diagram of directional couplers according to first to fifth preferred embodiments of the present invention. 
         FIG. 2  is an external perspective view of the directional couplers according to the first, second, and fourth preferred embodiments of the present invention. 
         FIG. 3  is an exploded perspective view of a multilayer body of the directional coupler according to the first preferred embodiment of the present invention. 
         FIG. 4  is an exploded perspective view of a multilayer body of the directional coupler according to the second preferred embodiment of the present invention. 
         FIG. 5  is an exploded perspective view of a multilayer body of the directional coupler according to the third preferred embodiment of the present invention. 
         FIG. 6  is an exploded perspective view of a multilayer body of the directional coupler according to the fourth preferred embodiment of the present invention. 
         FIG. 7  is an external perspective view of a directional coupler according to a fifth preferred embodiment of the present invention. 
         FIG. 8  is an exploded perspective view of a multilayer body of the directional coupler according to the fifth preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, directional couplers according to preferred embodiments of the present invention will be described. 
     First Preferred Embodiment 
     Hereinafter, a directional coupler according to a first preferred embodiment will be described with reference to drawings.  FIG. 1  is an equivalent circuit diagram of directional couplers  10   a  to  10   e  according to the first to fifth preferred embodiments of the present invention. 
     A circuit configuration of the directional coupler  10   a  will be described below. The directional coupler  10   a  is used in a specific frequency band. A specific frequency band is, for example, a frequency band (for example, about 698 MHz to about 3800 MHz) in which a Long Term Evolution (LTE) is used. 
     The directional coupler  10   a  includes outer electrodes  14   a  to  14   j , a main line M, a sub-line S, and capacitors C 1  to C 4  as a circuit configuration. The main line M is connected between the outer electrodes  14   a  and  14   b , and includes main line portions M 1  to M 3 . The main line portions M 1  to M 3  are connected in series in this order between the outer electrodes  14   a  and  14   b.    
     The sub-line S is connected between the outer electrodes  14   c  and  14   d , and includes sub-line portions S 1  to S 3 . The sub-line portions S 1  to S 3  are connected in series in this order between the outer electrodes  14   c  and  14   d.    
     Furthermore, the main line portion M 1  and the sub-line portion S 1  are electromagnetically coupled to each other. The main line portion M 2  and the sub-line portion S 2  are electromagnetically coupled to each other. The main line portion M 3  and the sub-line portion S 3  are electromagnetically coupled to each other. The main line portion M 2  and the sub-line portion S 2  are, as will be described later, in closer proximity than the main line portion M 1  and the sub-line portion S 1  and than the main line portion M 3  and the sub-line portion S 3 . 
     The capacitor C 1  is connected between the outer electrode  14   a  and the outer electrodes  14   e  to  14   j . The capacitor C 2  is connected between the outer electrode  14   b  and the outer electrodes  14   e  to  14   j . The capacitor C 3  is connected between the outer electrode  14   c  and the outer electrodes  14   e  to  14   j . The capacitor C 4  is connected between the outer electrode  14   d  and the outer electrodes  14   e  to  14   j.    
     In the directional coupler  10   a , the outer electrode  14   a  is used as an input port, and the outer electrode  14   b  is used as an output port. Furthermore, the outer electrode  14   c  is used as a coupling port, and the outer electrode  14   d  is used as a terminating port which terminates in about 50Ω, for example. Furthermore, the outer electrode  14   e  to  14   j  are used as ground ports being connected to the ground. When a signal is input to the outer electrode  14   a , the signal is output from the outer electrode  14   b . Furthermore, since the main line M and the sub-line S are electromagnetically coupled to each other, a signal having a power proportional to the power of the signal output from the outer electrode  14   b  is output from the outer electrode  14   c.    
     Next, a specific configuration of the directional coupler  10   a  according to the first preferred embodiment will be described with reference to drawings.  FIG. 2  is an external perspective view of the directional couplers  10   a ,  10   b , and  10   d  according to the first, second, and fourth preferred embodiments.  FIG. 3  is an exploded perspective view of a multilayer body  12  of the directional coupler  10   a  according to the first preferred embodiment. Hereinafter, a stacking direction of the multilayer body  12  is defined as a vertical direction, a long-side direction of the directional coupler  10   a  when viewed in plan from above is defined as a longitudinal direction, and a short-side direction of the directional coupler  10   a  when viewed in plan from above is defined as a horizontal direction. 
     As illustrated in  FIGS. 2 and 3 , the directional coupler  10   a  includes the multilayer body  12 ; the outer electrodes  14   a  to  14   j ; the main line M; the sub-line S; lead conductors  18   a ,  18   b ,  20   a , and  20   b ; ground conductors  22  and  24 ; capacitor conductors  26   a  to  26   d ; and via-hole conductors v 1 , v 4 , v 5 , and v 8 . 
     As illustrated in  FIG. 2 , the multilayer body  12  preferably has a rectangular or substantially rectangular parallelepiped shape, and as illustrated in  FIG. 3 , the multilayer body  12  is configured by stacking dielectric layers  16   a  to  16   k  each having a rectangular or substantially rectangular parallelepiped shape and made from dielectric ceramic materials, in this order from the top to the bottom. Hereinafter, an up-side main surface of the multilayer body  12  will be referred to as an upper surface, and a down-side main surface of the multilayer body  12  will be referred to as a lower surface. A front-side end surface of the multilayer body  12  will be referred to as a front surface, and a back-side end surface of the multilayer body  12  will be referred to as a back surface. A right-side side surface of the multilayer body  12  will be referred to as a right surface, and a left-side side surface of the multilayer body  12  will be referred to as a left surface. The bottom surface of the multilayer body  12  is a mounting surface which faces a circuit board when the directional coupler  10   a  is mounted on the circuit board. Furthermore, upper surfaces of the dielectric layers  16   a  to  16   k  will be referred to as first surfaces, and lower surfaces of the dielectric layers  16   a  to  16   k  will be referred to as second surfaces. 
     The outer electrodes  14   b ,  14   e ,  14   f , and  14   c  are provided on the left surface of the multilayer body  12  so as to be aligned in this order from the back side to the front side. The outer electrodes  14   b ,  14   e ,  14   f , and  14   c  extend in the vertical direction and are bent onto the upper surface and the bottom surface of the multilayer body  12 . 
     The outer electrodes  14   d ,  14   g ,  14   h , and  14   a  are provided on the right surface of the multilayer body  12  so as to be aligned in this order from the back side to the front side. The outer electrodes  14   d ,  14   g ,  14   h , and  14   a  extend in the vertical direction and are bent onto the upper surface and the bottom surface of the multilayer body  12 . 
     The outer electrode  14   i  extends in the vertical direction on the back surface of the multilayer body  12  and is bent onto the upper surface and the bottom surface of the multilayer body  12 . The outer electrode  14   j  extends in the vertical direction on the front surface of the multilayer body  12  and is bent onto the upper surface and the bottom surface of the multilayer body  12 . 
     The main line M is provided within the multilayer body  12 , and includes the main line portions M 1  to M 3  and via-hole conductors v 2  and v 3 . The main line portion M 1 , which is a first main line portion, is a linear conductor provided on a front half portion of the first surface of the dielectric layer  16   d . When viewed in plan from above, the main line portion M 1  inner-circumferentially extends with only substantially one turn in a counterclockwise direction from a start point located at the center of the front half portion of the dielectric layer  16   d  towards an end point located on the right side against the center (intersection of the diagonals) of the dielectric layer  16   d . The main line portion M 1  is in the form of substantially one turn. However, the main line portion M 1  may be configured to inner-circumferentially extend with multiple turns. Hereinafter, the start point of the main line portion M 1  will be referred to as an upstream end, and the end point of the main line portion M 1  will be referred to as a downstream end. 
     The main line portion M 3  is a linear conductor provided on a back half portion of the first surface of the dielectric layer  16   d . When viewed in plan from above, the main line portion M 3  inner-circumferentially extends with only substantially one turn in a clockwise direction from a start point located on the left side against the center (intersection of the diagonals) of the dielectric layer  16   d  towards an end point located at the center of the back half portion of the dielectric layer  16   d . The main line portion M 3  is in the form of substantially one turn. However, the main line portion M 3  may be configured to inner-circumferentially extend with multiple turns. 
     The main line portion M 3  has the same shape as the main line portion M 1 . In more detail, when rotating the main line portion M 3  by about 180 degrees around the center of the dielectric layer  16   d , the shape of the main line portion M 3  matches the shape of the main line portion M 1 . That is, the main line portion M 1  and the main line portion M 3  are point-symmetric to each other with respect to the center of the dielectric layer  16   d . Hereinafter, the start point of the main line portion M 3  will be referred to as an upstream end, and the end point of the main line portion M 3  will be referred to as a downstream end. 
     The main line portion M 2 , which is a second main line portion, is provided on the first surface of the dielectric layer  16   e , which is different from the dielectric layer  16   d  on which the main line portions M 1  and M 3  are provided. In the directional coupler  10   a , the main line portion M 2  is provided at a position lower than the main line portions M 1  and M 3 . The main line portion M 2  is a linear conductor which extends in the horizontal direction at the center of the longitudinal direction of the dielectric layer  16   e , and electrically connects the downstream end of the main line portion M 1  with the upstream end of the main line portion M 3 . The length of the main line portion M 2  is shorter than each of the lengths of the main line portions M 1  and M 3 . When viewed in plan from above, the start point of the main line portion M 2  and the downstream end of the main line portion M 1  overlap. When viewed in plan from above, the end point of the main line point M 2  and the upstream end of the main line portion M 3  overlap. Hereinafter, the start point of the main line portion M 2  will be referred to as an upstream end, and the end point of the main line portion M 2  will be referred to as a downstream end. The main line portions M 1  to M 3  are preferably formed by applying conductive paste mainly composed of metal, such as Cu or Ag, onto the first surfaces of the dielectric layers  16   d  and  16   e.    
     The via-hole conductor v 2  penetrates through the dielectric layer  16   d  in the vertical direction, and connects the downstream end of the main line portion M 1  with the upstream end of the main line portion M 2 . The via-hole conductor v 3  penetrates through the dielectric layer  16   d  in the vertical direction, and connects the downstream end of the main line portion M 2  with the upstream end of the main line portion M 3 . With this configuration, the main line portions M 1  to M 3  are connected in series in this order via the via-hole conductors v 2  and v 3 . The via-hole conductors v 2  and v 3  are preferably formed by filling conductive paste mainly composed of metal, such as Cu or Ag, into via-holes provided in the dielectric layer  16   d.    
     The lead conductor  18   a  is provided at a position above the main line M, and more specifically, the lead conductor  18   a  is a linear conductor in a straight or substantially straight line shape provided on the first surface of the dielectric layer  16   c . When viewed in plan from above, one end portion of the lead conductor  18   a  and the upstream end of the main line portion M 1  overlap. The other end portion of the lead conductor  18   a  is led to the long side on the right side of the dielectric layer  16   c , and is connected to the outer electrode  14   a.    
     The via-hole conductor v 1  penetrates through the dielectric layer  16   c  in the vertical direction, and connects one end portion of the lead conductor  18   a  with the upstream end of the main line portion M 1 . 
     The lead conductor  18   b  is provided at a position above the main line M, and more specifically, the lead conductor  18   b  is a linear conductor in a straight or substantially straight line shape provided on the first surface of the dielectric layer  16   c . When viewed in plan from above, one end portion of the lead conductor  18   b  and the downstream end of the main line portion M 3  overlap. The other end portion of the lead conductor  18   b  is led to the long side on the left side of the dielectric layer  16   c , and is connected to the outer electrode  14   b.    
     The lead conductor  18   b  has the same shape as the lead conductor  18   a . In more detail, when rotating the lead conductor  18   b  by about 180 degrees around the center of the dielectric layer  16   c , the shape of the lead conductor  18   b  matches the shape of the lead conductor  18   a . That is, the lead conductor  18   a  and the lead conductor  18   b  are point-symmetric to each other with respect to the center of the dielectric layer  16   c.    
     The via-hole conductor v 4  penetrates through the dielectric layer  16   c  in the vertical direction, and connects the one end portion of the lead conductor  18   b  with the downstream end of the main line portion M 3 . With this configuration, the main line M is connected between the outer electrodes  14   a  and  14   b . The via-hole conductors v 1  and v 4  are preferably formed by filling conductive paste mainly composed of metal, such as Cu or Ag, into via-holes provided in the dielectric layer  16   c.    
     The sub-line S is provided within the multilayer body  12 , and includes the sub-line portions S 1  to S 3  and via-hole conductors v 6  and v 7 . The sub-line portion S 1 , which is a first sub-line portion, is a linear conductor provided on a front half portion of the first surface of the dielectric layer  16   g , and is electromagnetically coupled to the main line portion M 1 . When viewed in plan from above, the sub-line portion S 1  has the same shape as the main line portion M 1 , and the sub-line portion S 1  and the main line portion M 1  overlap in such a manner that they correspond to each other. More specifically, when viewed in plan from above, the sub-line portion S 1  inner-circumferentially extends with only substantially one turn in a counterclockwise direction from a start point located at the center of the front half portion of the dielectric layer  16   g  towards an end point located on the right side against the center (intersection of the diagonals) of the dielectric layer  16   g . Hereinafter, the start point of the sub-line portion S 1  will be referred to as an upstream end, and the end point of the sub-line portion S 1  will be referred to as a downstream end. 
     The sub-line portion S 3  is a linear conductor provided on a back half portion of the first surface of the dielectric layer  16   g , and is electromagnetically coupled to the main line portion M 3 . When viewed in plan from above, the sub-line portion S 3  has the same shape as the main line portion M 3 , and the sub-line portion S 3  and the main line portion M 3  overlap in such a manner that they correspond to each other. More specifically, when viewed in plan from above, the sub-line portion S 3  inner-circumferentially extends with only substantially one turn in a clockwise direction from a start point located on the left side against the center (intersection of the diagonals) of the dielectric layer  16   g  towards an end point located at the center of the back half portion of the dielectric layer  16   g.    
     The sub-line portion S 3  has the same shape as the sub-line portion S 1 . In more detail, when rotating the sub-line portion S 3  by about 180 degrees around the center of the dielectric layer  16   g , the shape of the sub-line portion S 3  matches the shape of the sub-line portion S 1 . That is, the sub-line portion S 1  and the sub-line portion S 3  are point-symmetric to each other with respect to the center of the dielectric layer  16   g . Hereinafter, the start point of the sub-line portion S 3  will be referred to as an upstream end, and the end point of the sub-line portion S 3  will be referred to as a downstream end. 
     The sub-line portion S 2 , which is a second sub-line portion, is provided on the first surface of the dielectric layer  16   f , which is different from the dielectric layer  16   e  on which the main line portion M 2  is provided and the dielectric layer  16   g  on which the sub-line portions S 1  and S 3  are provided. In the directional coupler  10   a , the sub-line portion S 2  is provided at a position above the sub-line portions S 1  and S 3 . With this configuration, the space between the main line portion M 2  and the sub-line portion S 2  is smaller than each of the space between the main line portion M 1  and the sub-line portion S 1  and the space between the main line portion M 3  and the sub-line portion S 3 . 
     The sub-line portion S 2  is a linear conductor which extends in the horizontal direction at the center of the longitudinal direction of the dielectric layer  16   f . When viewed in plan from above, the sub-line portion S 2  has the same shape as the main line portion M 2 , and the sub-line portion S 2  and the main line portion M 2  overlap in such a manner that they correspond to each other. The length of the sub-line portion S 2  is shorter than each of the lengths of the sub-line portions S 1  and S 3 . When viewed in plan from above, the start point of the sub-line portion S 2  and the downstream end of the sub-line portion S 1  overlap. When viewed in plan from above, the end point of the sub-line portion S 2  and the upstream end of the sub-line portion S 3  overlap. Hereinafter, the start point of the sub-line portion S 2  will be referred to as an upstream end, and the end point of the sub-line portion S 2  will be referred to as a downstream end. The sub-line portions S 1  to S 3  are preferably formed by applying conductive paste mainly composed of metal, such as Cu or Ag, onto the first surfaces of the dielectric layers  16   f  and  16   g.    
     The via-hole conductor v 6  penetrates through the dielectric layer  16   f  in the vertical direction, and connects the downstream end of the sub-line portion S 1  with the upstream end of the sub-line portion S 2 . The via-hole conductor v 7  penetrates through the dielectric layer  16   f  in the vertical direction, and connects the downstream end of the sub-line portion S 2  with the upstream end of the sub-line portion S 3 . With this configuration, the sub-line portions S 1  to S 3  are connected in series in this order via the via-hole conductors v 6  and v 7 . The via-hole conductors v 6  and v 7  are preferably formed by filling conductive paste mainly composed of metal, such as Cu or Ag, into via-holes provided in the dielectric layer  16   f.    
     The lead conductor  20   a  is provided at a position lower than the sub-line S, and more specifically, the lead conductor  20   a  is a linear conductor in a straight or substantially straight line shape provided on the first surface of the dielectric layer  16   h . When viewed in plan from above, one end portion of the lead conductor  20   a  and the upstream end of the sub-line portion S 1  overlap. The other end portion of the lead conductor  20   a  is led to the long side on the left side of the dielectric layer  16   h , and is connected to the outer electrode  14   c . Furthermore, the lead conductor  20   a  has the same length as the lead conductor  18   a . With this configuration, when viewed in plan from above, connecting the right end of the lead conductor  18   a  and the left end of the lead conductor  20   a  with a straight line defines an isosceles triangle. 
     The via-hole conductor v 5  penetrates through the dielectric layer  16   g  in the vertical direction, and connects one end portion of the lead conductor  20   a  with the upstream end of the sub-line portion S 1 . 
     The lead conductor  20   b  is provided at a position lower than the sub-line S, and more specifically, the lead conductor  20   b  is a linear conductor in a straight or substantially straight line shape provided on the first surface of the dielectric layer  16   h . When viewed in plan from above, one end portion of the lead conductor  20   b  and the downstream end of the sub-line portion S 3  overlap. The other end portion of the lead conductor  20   b  is led to the long side on the right side of the dielectric layer  16   h , and is connected to the outer electrode  14   d . Furthermore, the lead conductor  20   b  has the same length as the lead conductor  18   b . With this configuration, when viewed in plan from above, connecting the left end of the lead conductor  18   b  and the right end of the lead conductor  20   b  with a straight line defines an isosceles triangle. 
     The lead conductor  20   b  has the same shape as the lead conductor  20   a . In more detail, when rotating the lead conductor  20   b  by about 180 degrees around the center of the dielectric layer  16   h , the shape of the lead conductor  20   b  matches the shape of the lead conductor  20   a . That is, the lead conductor  20   a  and the lead conductor  20   b  are point-symmetric to each other with respect to the center of the dielectric layer  16   h . The lead conductors  18   a ,  18   b ,  20   a , and  20   b  are preferably formed by applying conductive paste mainly composed of metal, such as Cu or Ag, onto the first surfaces of the dielectric layers  16   c  and  16   h.    
     The via-hole conductor v 8  penetrates through the dielectric layer  16   g  in the vertical direction, and connects one end portion of the lead conductor  20   b  with the downstream end of the sub-line portion S 3 . With this configuration, the sub-line S is connected between the outer electrodes  14   c  and  14   d . The via-hole conductors v 5  and v 8  are preferably formed by filling conductive paste mainly composed of metal, such as Cu or Ag, into via-holes provided in the dielectric layer  16   g.    
     The ground conductor  22  is provided in the multilayer body  12 , and is provided at a position above the main line M, the sub-line S, and the lead conductors  18   a ,  18   b ,  20   a , and  20   b . In more detail, the ground conductor  22  is arranged so as to cover substantially the whole first surface of the dielectric layer  16   b , and is in a rectangular or substantially rectangular parallelepiped shape. Furthermore, the ground conductor  22  is led to each side of the dielectric layer  16   b , and is connected to the outer electrodes  14   e  to  14   j . Moreover, the ground conductor  22  and the main line portions M 1  to M 3  overlap when viewed in plan from above. 
     The ground conductor  24  is provided in the multilayer body  12 , and is provided at a position lower than the main line M, the sub-line S, and the lead conductors  18   a ,  18   b ,  20   a , and  20   b . In more detail, the ground conductor  24  is arranged so as to cover substantially the whole first surface of the dielectric layer  16   i , and is in a rectangular or substantially rectangular parallelepiped shape. Furthermore, the ground conductor  24  is led to each side of the dielectric layer  16   i , and is connected to the outer electrodes  14   e  to  14   j . Moreover, the ground conductor  24  and the sub-line portions S 1  to S 3  overlap when viewed in plan from above. The ground conductors  22  and  24  are preferably formed by applying conductive paste mainly composed of metal, such as Cu or Ag, onto the first surfaces of the dielectric layers  16   b  and  16   i.    
     The capacitor conductors  26   a  to  26   d  are provided in the multilayer body  12 , and are provided at positions lower than the ground conductor  24 . In more detail, the capacitor conductors  26   a  to  26   d  are conductors in a rectangular or substantially rectangular shape provided on the first surface of the dielectric layer  16   j . The capacitor conductor  26   a  is led to the long side on the right side of the dielectric layer  16   j , and is connected to the outer electrode  14   a . Furthermore, the capacitor conductor  26   a  defines the capacitor C 1  by facing the ground conductor  24  with the dielectric layer  16   i  therebetween. With this configuration, the capacitor C 1  is connected between the outer electrode  14   a  and the outer electrodes  14   e  to  14   j.    
     The capacitor conductor  26   b  is led to the long side on the left side of the dielectric layer  16   j , and is connected to the outer electrode  14   b . Furthermore, the capacitor conductor  26   b  forms the capacitor C 2  by facing the ground conductor  24  with the dielectric layer  16   i  therebetween. With this configuration, the capacitor C 2  is connected between the outer electrode  14   b  and the outer electrodes  14   e  to  14   j.    
     The capacitor conductor  26   c  is led to the long side on the left side of the dielectric layer  16   j , and is connected to the outer electrode  14   c . Furthermore, the capacitor conductor  26   c  forms the capacitor C 3  by facing the ground conductor  24  with the dielectric layer  16   i  therebetween. With this configuration, the capacitor C 3  is connected between the outer electrode  14   c  and the outer electrodes  14   e  to  14   j.    
     The capacitor conductor  26   d  is led to the long side on the right side of the dielectric layer  16   j , and is connected to the outer electrode  14   d . Furthermore, the capacitor conductor  26   d  defines the capacitor C 4  by facing the ground conductor  24  with the dielectric layer  16   i  therebetween. With this configuration, the capacitor C 4  is connected between the outer electrode  14   d  and the outer electrodes  14   e  to  14   j . The capacitor conductors  26   a  to  26   d  are preferably formed by applying conductive paste mainly composed of Cu or Ag onto the first surface of the dielectric layer  16   j.    
     With the directional coupler  10   a  configured as described above, a fine adjustment of the degree of coupling between the main line M and the sub-line S is achieved. In more detail, in the directional coupler  10   a , the main line M is configured by connecting the main line portions M 1  to M 3  in series to each other. Furthermore, the main line portion M 2  is provided on the dielectric layer  16   e , which is different from the dielectric layer  16   d  on which the main line portions M 1  and M 3  are provided. Similarly, the sub-line S is configured by connecting the sub-line portions S 1  to S 3  in series to each other. Furthermore, the sub-line portion S 2  is provided on the dielectric layer  16   f , which is different from the dielectric layer  16   g  on which the sub-line portions S 1  and S 3  are provided. With this configuration, the space between the main line portion M 2  and the sub-line portion S 2  can be changed without changing the space between the main line portion M 1  and the sub-line portion S 1  and without changing the space between the main line portion M 3  and the sub-line portion S 3 . More specifically, by reducing the thickness of the dielectric layer  16   e  and increasing the thicknesses of the dielectric layers  16   d  and  16   f , the space between the main line portion M 2  and the sub-line portion S 2  is significantly reduced without changing the space between the main line portion M 1  and the sub-line portion S 1  and without changing the space between the main line portion M 3  and the sub-line portion S 3 . With this configuration, the degree of coupling between the main line M and the sub-line S is slightly increased. In contrast, by increasing the thickness of the dielectric layer  16   e  and reducing the thicknesses of the dielectric layers  16   d  and  16   f , the space between the main line portion M 2  and the sub-line portion S 2  is significantly increased without changing the space between the main line portion M 1  and the sub-line portion S 1  and without changing the space between the main line portion M 3  and the sub-line portion S 3 . With this configuration, the degree of coupling between the main line M and the sub-line S is slightly reduced. As described above, with the directional coupler  10   a , a fine adjustment of the degree of coupling between the main line M and the sub-line S is achieved. 
     Furthermore, the length of the main line portion M 2  is shorter than each of the lengths of the main line portions M 1  and M 3 , and the length of the sub-line portion S 2  is shorter than each of the lengths of the sub-line portions S 1  and S 3 . Therefore, in the case where the space between the main line portion M 2  and the sub-line portion S 2  is changed, the amount of change in the degree of coupling between the main line M and the sub-line S is small. Accordingly, with the directional coupler  10   a , a fine adjustment of the degree of coupling between the main line M and the sub-line S is achieved. 
     Furthermore, since the main line portion M 1  and the sub-line portion S 1  overlap in such a manner that they correspond to each other, the main line portion M 2  and the sub-line portion S 2  overlap in such a manner that they correspond to each other, and the main line portion M 3  and the sub-line portion S 3  overlap in such a manner that they correspond to each other, the degree of coupling between the main line M and the sub-line S may be increased. 
     Furthermore, when viewed in plan from above, the main line portions M 1  to M 3  have the same shape, and the main line portions M 1  to M 3  and the sub-line portions S 1  to S 3  respectively overlap in such a manner that they correspond to each other. With this configuration, the structure of the main line M and the structure of the sub-line S are closer to each other. As a result, electrical characteristics, such as characteristic impedance, of the main line M, and electrical characteristics, such as characteristic impedance, of the sub-line S, are closer to each other. Therefore, a difference between the phase of a signal output from the outer electrode  14   b  and the phase of a signal output from the outer electrode  14   c  decreases. That is, phase difference characteristics of the directional coupler  10   a  is improved. 
     Furthermore, the main line portion M 1  and the main line portion M 3  inner-circumferentially extend in opposite directions. With this configuration, for example, in the case where a magnetic flux passes through the center of the main line portion M 1  in an upward direction, a magnetic flux passes through the center of the main line portion M 3  in a downward direction. Therefore, the magnetic flux passing through the center of the main line portion M 1  makes a U-turn on the upper side of the main line M and passes through the center of the main line portion M 3 , and the magnetic flux passing through the center of the main line portion M 3  makes a U-turn on the lower side of the main line M and passes through the center of the main line portion M 1 . That is, a closed magnetic path is provided in the main line M. With this configuration, a situation in which the magnetic flux generated by the main line M is disturbed by external influences is prevented. The same may be applied to the sub-line S. 
     Furthermore, the lead conductor  18   a  and the lead conductor  20   a  have the same length. Therefore, resistances and phase changes of the lead conductor  18   a  and the lead conductor  20   a  are equal or substantially equal to each other. Thus, electrical characteristics, such as, characteristic impedance between the outer electrodes  14   a  and  14   b , and electrical characteristics, such as characteristic impedance between the outer electrodes  14   c  and  14   d , are closer to each other. Moreover, the phase difference characteristics of the directional coupler  10   a  are improved. The same may be applied to the lead conductor  18   b  and the lead conductor  20   b.    
     Furthermore, since the lead conductors  18   a ,  18   b ,  20   a , and  20   b  are each in a straight or substantially straight line shape, connection with the outer electrodes is achieved with the shortest distance. Therefore, the resistances of these lead conductors are reduced, and unnecessary magnetic coupling and capacity coupling are reduced. Thus, insertion loss of the directional coupler  10   a  is decreased. 
     Furthermore, in the directional coupler  10   a , the capacitor C 1  is provided between the outer electrode  14   a  and the outer electrodes  14   e  to  14   j , the capacitor C 2  is provided between the outer electrode  14   b  and the outer electrodes  14   e  to  14   j , the capacitor C 3  is provided between the outer electrode  14   c  and the outer electrodes  14   e  to  14   j , and the capacitor C 4  is provided between the outer electrode  14   d  and the outer electrodes  14   e  to  14   j . With this configuration, by adjusting the capacitances of the capacitors C 1  to C 4 , the characteristic impedance between the outer electrodes  14   a  and  14   b  and the characteristic impedance between the outer electrodes  14   c  and  14   d  are adjusted. Accordingly, by making these characteristic impedances closer to each other, the phase difference characteristics of the directional coupler  10   a  are improved. 
     Furthermore, the ground conductor  22  is provided at a position above the main line M, the sub-line S, and the lead conductors  18   a ,  18   b ,  20   a , and  20   b . With this configuration, noise input to the directional coupler  10   a  from the top is absorbed by the ground conductor  22 . As a result, input of noise to the main line M, the sub-line S, and the lead conductors  18   a ,  18   b ,  20   a , and  20   b  is significantly reduced or prevented. 
     Furthermore, the ground conductor  24  is provided at a position lower than the main line M, the sub-line S, and the lead conductors  18   a ,  18   b ,  20   a , and  20   b . With this configuration, noise input to the directional coupler  10   a  from the bottom is absorbed by the ground conductor  24 . As a result, input of noise to the main line M, the sub-line S, and the lead conductors  18   a ,  18   b ,  20   a , and  20   b  is significantly reduced or prevented. 
     Furthermore, the ground conductor  24  is provided at a position between the main line M, the sub-line S, the lead conductors  18   a ,  18   b ,  20   a , and  20   b , and the capacitor conductors  26   a  to  26   d . With this configuration, formation of unnecessary capacitance between the main line M, the sub-line S, the lead conductors  18   a ,  18   b ,  20   a , and  20   b , and the capacitor conductors  26   a  to  26   d  is significantly reduced or prevented. 
     Second Preferred Embodiment 
     Hereinafter, a specific configuration of the directional coupler  10   b  according to a second preferred embodiment of the present invention will be explained with reference to drawings.  FIG. 4  is an exploded perspective view of the multilayer body  12  of the directional coupler  10   b  according to the second preferred embodiment. Since the circuit configuration of the directional coupler  10   b  is the same as the circuit configuration of the directional coupler  10   a , explanation of the circuit configuration of the directional coupler  10   b  will be omitted.  FIG. 2  will be used as an external perspective view of the directional coupler  10   b.    
     The directional coupler  10   b  differs from the directional coupler  10   a  in the shapes of the main line portions M 1  to M 3  and the sub-line portions S 1  to S 3 . The directional coupler  10   b  will be explained below with focus on these differences. 
     When viewed in plan from above, the main line portion M 1  has a spiral or substantially spiral shape which inner-circumferentially extends with plural turns in a counterclockwise direction from a start point located at the center of a front half portion of the dielectric layer  16   d  towards an end point located near the center of the short side on the front side of the dielectric layer  16   d.    
     When viewed in plan from above, the main line portion M 3  has a spiral or substantially spiral shape which inner-circumferentially extends with plural turns in a counterclockwise direction from a start point located near the center of the short side on the back side of the dielectric layer  16   d  towards an end point located at the center of a back half portion of the dielectric layer  16   d . The main line portion M 3  arranged as described above and the main line portion M 1  are line-symmetric to each other with respect to a straight line horizontally passing through the center in the longitudinal direction of the dielectric layer  16   d.    
     The main line portion M 2  is provided on the first surface of the dielectric layer  16   e . The main line portion M 2  extends in the longitudinal direction, and both ends of the main line portion M 2  are bent to the left. However, when viewed in plan from above, the main line portion M 2  and the main line portions M 1  and M 3  do not overlap in portions other than the upstream end and the downstream end. The upstream end of the main line portion M 2  is connected to the downstream end of the main line portion M 1  via the via-hole conductor v 2 . The downstream end of the main line portion M 2  is connected to the upstream end of the main line portion M 3  via the via-hole conductor v 3 . 
     When viewed in plan from above, the sub-line portion S 1  has a spiral or substantially spiral shape which inner-circumferentially extends with plural turns in a counterclockwise direction from a start point located at the center of a front half portion of the dielectric layer  16   g  towards an end point located near the center of the short side on the front side of the dielectric layer  16   g.    
     When viewed in plan from above, the sub-line portion S 3  has a spiral or substantially spiral shape which inner-circumferentially extends with plural turns in a counterclockwise direction from a start point located near the center of the short side on the back side of the dielectric layer  16   g  towards an end point located at the center of a back half portion of the dielectric layer  16   g . The sub-line portion S 3  arranged as described above and the sub-line portion S 1  are line-symmetric to each other with respect to a straight line horizontally passing through the center in the longitudinal direction of the dielectric layer  16   g.    
     The sub-line portion S 2  is provided on the first surface of the dielectric layer  16   f . The sub-line portion S 2  extends in the longitudinal direction, and both ends of the sub-line portion S 2  are bent to the left. However, when viewed in plan from above, the sub-line portion S 2  and the sub-line portions S 1  and S 3  do not overlap in portions other than the upstream end and the downstream end. The upstream end of the sub-line portion S 2  is connected to the downstream end of the sub-line portion S 1  via the via-hole conductor v 6 . The downstream end of the sub-line portion S 2  is connected to the upstream end of the sub-line portion S 3  via the via-hole conductor v 7 . 
     The directional coupler  10   b  configured as described above achieves the same effects as those achieved by the directional coupler  10   a.    
     Furthermore, in the directional coupler  10   b , the main line M and the lead conductors  18   a  and  18   b ; and the sub-line S and the lead conductors  20   a  and  20   b  are line-symmetric to each other with respect to a straight line horizontally passing through the center in the longitudinal direction of the dielectric layers  16   d  and  16   g . With this configuration, electrical characteristics, such as characteristic impedance, of the main line M and the lead conductors  18   a  and  18   b , and electrical characteristics, such as characteristic impedance, of the sub-line S and the lead conductors  20   a  and  20   b , are closer to each other. As a result, the phase difference characteristics of the directional coupler  10   b  are improved. 
     Furthermore, in the directional coupler  10   b , the main line portions M 1  and M 2  and the sub-line portions S 1  and S 2  each have a spiral or substantially spiral shape. Therefore, in the case where the length of the main line portions M 1  and M 2  and the sub-line portions S 1  and S 2  of the directional coupler  10   b  and the length of the main line portions M 1  and M 2  and the sub-line portions S 1  and S 2  of the directional coupler  10   a  are the same, the area occupied by the main line portions M 1  and M 2  and the sub-line portions S 1  and S 2  in the directional coupler  10   b  is smaller than the area occupied by the main line portions M 1  and M 2  and the sub-line portions S 1  and S 2  in the directional coupler  10   a . Accordingly, the size of the directional coupler  10   b  is made smaller than the size of the directional coupler  10   a . In addition, with the sub-line portions S 1  and S 2  each having a spiral or substantially spiral shape, the lengths of the lines are increased. Therefore, lower frequencies may also be coped with. As a result, the directional coupler  10   b  which is capable coping with a wide frequency range from lower frequencies to higher frequencies is attained. 
     Furthermore, in the directional coupler  10   b , the main line portions M 1  and M 2  and the sub-line portions S 1  and S 2  each have a spiral or substantially spiral shape. Therefore, in the case where the area occupied by the main line portions M 1  and M 2  and the sub-line portions S 1  and S 2  in the directional coupler  10   b  and the area occupied by the main line portions M 1  and M 2  and the sub-line portions S 1  and S 2  in the directional coupler  10   a  are the same, the length of the main line portions M 1  and M 2  and the sub-line portions S 1  and S 2  of the directional coupler  10   b  is longer than the length of the main line portions M 1  and M 2  and the sub-line portions S 1  and S 2  of the directional coupler  10   a . Accordingly, the directional coupler  10   b  is capable of being used in frequencies lower than the directional coupler  10   a.    
     Furthermore, when viewed in plan from above, the main line portion M 2  and the main line portions M 1  and M 3  do not overlap in portions other than the upstream end and the downstream end. Therefore, the main line portion M 2  does not interrupt a magnetic flux generated by the main line portions M 1  and M 3 . Similarly, when viewed in plan from above, the sub-line portion S 2  and the sub-line portions S 1  and S 3  do not overlap in portions other than the upstream end and the downstream end. Therefore, the sub-line portion S 2  does not interrupt a magnetic flux generated by the sub-line portions S 1  and S 3 . 
     Third Preferred Embodiment 
     Hereinafter, a specific configuration of the directional coupler  10   c  according to a third preferred embodiment of the present invention will be explained with reference to drawings.  FIG. 5  is an exploded perspective view of the multilayer body  12  of the directional coupler  10   c  according to the third preferred embodiment. Since the circuit configuration of the directional coupler  10   c  is the same as the circuit configuration of the directional coupler  10   a , explanation of the circuit configuration of the directional coupler  10   c  will be omitted. 
     The directional coupler  10   c  differs from the directional coupler  10   a  in that the directional coupler  10   c  further includes a ground conductor  28  and via-hole conductors v 10  to v 21 . The directional coupler  10   c  will be explained below with focus on these differences. 
     The ground conductor  28  is provided at the center of the bottom surface of the multilayer body  12 , that is, at the center of the second surface of the dielectric layer  16   k . The ground conductor  28  has a cross-shaped or a substantially cross-shaped configuration. More specifically, the ground conductor  28  includes a longitudinally-extending band-shaped conductor and a horizontally-extending band-shaped conductor which pass through the center of the dielectric layer  16   k . Furthermore, by being led to the short side in the longitudinal direction of the dielectric layer  16   k  and to the long side in the horizontal direction of the dielectric layer  16   k , the ground conductor  28  is connected to the outer electrodes  14   e  to  14   j . However, the ground conductor  28  is not in contact with portions of the outer electrodes  14   a  to  14   d  that are bent onto the bottom surface. 
     The via-hole conductors v 10 , v 14 , and v 18  penetrate through the dielectric layers  16   i  to  16   k  in the vertical direction. The via-hole conductors v 10 , v 14 , and v 18  are connected to each other to define a via-hole conductor, and connect the ground conductor  24  with the ground conductor  28 . 
     The via-hole conductors v 11 , v 15 , and v 19  penetrate through the dielectric layers  16   i  to  16   k  in the vertical direction. The via-hole conductors v 11 , v 15 , and v 19  are connected to each other to define a via-hole conductor, and connect the ground conductor  24  with the ground conductor  28 . 
     The via-hole conductors v 12 , v 16 , and v 20  penetrate through the dielectric layers  16   i  to  16   k  in the vertical direction. The via-hole conductors v 12 , v 16 , and v 20  are connected to each other to define a via-hole conductor, and connect the ground conductor  24  with the ground conductor  28 . 
     The via-hole conductors v 13 , v 17 , and v 21  penetrate through the dielectric layers  16   i  to  16   k  in the vertical direction. The via-hole conductors v 13 , v 17 , and v 21  are connected to each other to define a via-hole conductor, and connect the ground conductor  24  with the ground conductor  28 . 
     The directional coupler  10   c  configured as described above achieves the same effects as those achieved by the directional coupler  10   a.    
     Furthermore, the directional coupler  10   c  achieves a high heat dissipation. In more detail, when the directional coupler  10   c  is mounted on a circuit board, the ground conductor  28  is disposed in contact with the circuit board. The ground conductor  28 , which is made of metal, has a thermal conductivity higher than the dielectric layer  16   k , which is made from dielectric ceramic materials. Therefore, heat generated by the directional coupler  10   c  is efficiently transmitted to the circuit board via the ground conductor  28 . Consequently, the heat dissipation of the directional coupler  10   c  is greatly improved. 
     Furthermore, since the ground conductor  24  and the ground conductor  28  are connected through the via-hole conductors v 10  to v 21 , the ground conductor  24  is reliably maintained at the ground potential. 
     Fourth Preferred Embodiment 
     Hereinafter, a specific configuration of the directional coupler  10   d  according to the fourth preferred embodiment will be explained with reference to drawings.  FIG. 6  is an exploded perspective view of the multilayer body  12  of the directional coupler  10   d  according to the fourth preferred embodiment. Since the circuit configuration of the directional coupler  10   d  is the same as the circuit configuration of the directional coupler  10   a , explanation of the circuit configuration of the directional coupler  10   d  will be omitted.  FIG. 2  will be used as an external perspective view of the directional coupler  10   d.    
     The directional coupler  10   d  differs from the directional coupler  10   a  in that the directional coupler  10   d  does not include the dielectric layer  16   f  and that the sub-line portion S 2  of the directional coupler  10   d  is provided on the first surface of the dielectric layer  16   g . The directional coupler  10   d  will be explained below with focus on these differences. 
     The sub-line portion S 2  is connected to the sub-line portion S 1  and the sub-line portion S 3  on the first surface of the dielectric layer  16   g.    
     Also with the directional coupler  10   d  having the configuration described above, by adjusting the thicknesses of the dielectric layers  16   d  and  16   e , the space between the main line portion M 2  and the sub-line portion S 2  is capable of being adjusted without changing the space between the main line portion M 1  and the sub-line portion S 1  and without changing the space between the main line portion M 3  and the sub-line portion S 3 . Accordingly, also with the directional coupler  10   d , a fine adjustment of the degree of coupling between the main line M and the sub-line S is achieved. 
     Furthermore, the number of dielectric layers of the directional coupler  10   d  is reduced by one compared to the number of dielectric layers of the directional coupler  10   a.    
     In the directional coupler  10   d , the main line portions M 1  and M 3  are provided on the first surface of the dielectric layer  16   d , the main line portion M 2  is provided on the first surface of the dielectric layer  16   e , and the sub-line portions S 1  to S 3  are provided on the first surface of the dielectric layer  16   g . However, the main line portions M 1  to M 3  may be provided on the first surface of the dielectric layer  16   d , the sub-line portions S 1  and S 3  may be provided on the first surface of the dielectric layer  16   g , and the sub-line portion S 2  may be provided on the first surface of the dielectric layer  16   f.    
     Fifth Preferred Embodiment 
     Hereinafter, a specific configuration of a directional coupler  10   e  according to a fifth preferred embodiment will be explained with reference to drawings.  FIG. 7  is an external perspective view of the directional coupler  10   e  according to the fifth preferred embodiment.  FIG. 8  is an exploded perspective view of the multilayer body  12  of the directional coupler  10   e  according to the fifth preferred embodiment. Since the circuit configuration of the directional coupler  10   e  is preferably the same or substantially the same as the circuit configuration of the directional coupler  10   a , explanation of the circuit configuration of the directional coupler  10   e  will be omitted. 
     As illustrated in  FIGS. 7 and 8 , the directional coupler  10   e  differs from the directional coupler  10   a  in the following four points. 
     First difference: the outer electrodes  14   f  and  14   h  are not provided. 
     Second difference: a dielectric layer  16   l  is provided between the dielectric layer  16   c  and the dielectric layer  16   d , and a dielectric layer  16   m  is provided between the dielectric layer  16   g  and the dielectric layer  16   h.    
     Third difference: via-hole conductors v 31  and v 32  are provided in the dielectric layer  16   l , and via-hole conductors v 33  and v 34  are provided in the dielectric layer  16   m.    
     Fourth difference: a ground conductor  40   a  is provided on a first surface of the dielectric layer  16   l , and a ground conductor  40   b  is provided on a first surface of the dielectric layer  16   m.    
     The via-hole conductor v 31  penetrates through the dielectric layer  16   l  in the vertical direction, and the via-hole conductor v 31  and the via-hole conductor v 1  configure a single via-hole conductor. The via-hole conductors v 1  and v 31  connect one end of the lead conductor  18   a  with the upstream end of the main line portion M 1 . 
     The via-hole conductor v 32  penetrates through the dielectric layer  16   l  in the vertical direction, and the via-hole conductor v 32  and the via-hole conductor v 4  configure a single via-hole conductor. The via-hole conductors v 4  and v 32  connect one end of the lead conductor  18   b  with the downstream end of the main line portion M 3 . 
     The ground conductor  40   a  is provided at a position higher than the main line portions M 1  to M 3  and lower than the ground conductor  22 , and more specifically, the ground conductor  40   a  is a linear conductor having a straight line or substantially straight line shape provided on the first surface of the dielectric layer  16   l . The ground conductor  40   a  connects the center of the right-hand long side with the center of the left-hand long side of the dielectric layer  16   l . Accordingly, the ground conductor  40   a  is connected to the outer electrodes  14   e  and  14   g . Furthermore, the ground conductor  40   a  and the main line portion M 2  overlap when viewed in plan from above. 
     The ground conductor  40   b  is provided at a position lower than the sub-line portions S 1  to S 3  and higher than the ground conductor  24 , and more specifically, the ground conductor  40   b  is a linear conductor with a straight line or substantially straight line shape provided on the first surface of the dielectric layer  16   m . The ground conductor  40   b  connects the center of the right-hand long side with the center of the left-hand long side of the dielectric layer  16   m . Accordingly, the ground conductor  40   b  is connected to the outer electrodes  14   e  and  14   g . Furthermore, the ground conductor  40   b  and the sub-line portion S 2  overlap when viewed in plan from above. 
     Also with the directional coupler  10   e  having the configuration described above, by adjusting the thicknesses of the dielectric layers  16   d  and  16   e , the space between the main line portion M 2  and the sub-line portion S 2  is capable of being adjusted without changing the space between the main line portion M 1  and the sub-line portion S 1  and without changing the space between the main line portion M 3  and the sub-line portion S 3 . Accordingly, also with the directional coupler  10   e , a fine adjustment of the degree of coupling between the main line M and the sub-line S is capable of being achieved. 
     Furthermore, the directional coupler  10   e  achieves improved transmission characteristics and coupling characteristics, compared to the directional coupler  10   a . More specifically, in the directional coupler  10   a , the main line portion M 2  is provided at a position lower than the main line portions M 1  and M 3 . Therefore, the distance in the vertical direction between the main line portion M 2  and the ground conductor  22  is larger than the distance in the vertical direction between the main line portions M 1  and M 3  and the ground conductor  22 . Thus, the capacitance generated between the main line portion M 2  and the ground conductor  22  is smaller than the capacitance generated between the main line portions M 1  and M 3  and the ground conductor  22 . Accordingly, the characteristic impedance of the main line portion M 2  is higher than the characteristic impedance of the main line portions M 1  and M 3 . Consequently, reflection of a high-frequency signal is generated between the main line portions M 1  and M 3  and the main line portion M 2 , and the transmission characteristics and coupling characteristics of the directional coupler  10   a  are thus decreased. 
     Thus, in the directional coupler  10   e , the ground conductor  40   a  is provided at a position higher than the main line portions M 1  to M 3  and lower than the ground conductor  22 , and the ground conductor  40   a  and the main line portion M 2  overlap when viewed in plan from above. Accordingly, a capacitance is generated between the main line portion M 2  and the ground conductor  40   a . Consequently, the characteristic impedance of the main line portions M 1  and M 3  and the characteristic impedance of the main line portion M 2  are made closer to each other. As a result, reflection of a high-frequency signal is prevented from being generated between the main line portions M 1  and M 3  and the main line portion M 2 , and the transmission characteristics and coupling characteristics of the directional coupler  10   e  are thus improved. The same effects as those of the main line portions M 1  to M 3  and the ground conductor  40   a  are achieved by the sub-line portions S 1  to S 3  and the ground conductor  40   b.    
     Other Preferred Embodiments 
     A directional coupler according to the present invention is not limited to the directional couplers  10   a  to  10   e  according to the foregoing preferred embodiments. Various changes may be made to the present invention within the scope of the gist of the present invention. 
     The configurations of the directional couplers  10   a  to  10   e  may be combined together. 
     In the directional couplers  10   a  to  10   e , the main line portion M 2  and the sub-line portion S 2  may be provided on the same dielectric layer. In this case, the main line portion M 2  and the sub-line portion S 2  are arranged on the dielectric layer in such a manner that they are different in position in the longitudinal direction and/or horizontal direction. By adjusting the space between the main line portion M 2  and the sub-line portion S 2  or adjusting the lengths of the main line portion M 2  and the sub-line portion S 2 , a fine adjustment of the degree of coupling between the main line M and the sub-line S may be made. 
     In the directional couplers  10   a  to  10   e , by changing the positions of the main line portion M 2  or the sub-line portion S 2  in the longitudinal direction and/or horizontal direction on an insulating layer, the space between the main line portion M 2  and the sub-line portion S 2  may be adjusted to make a fine adjustment of the degree of coupling between the main line M and the sub-line S. 
     Furthermore, in the directional couplers  10   a  to  10   e , the line width of the main line portion M 2  may be different from the line width of the sub-line portion S 2 . Similarly, the line width of the main line portion M 1  may be different from the line width of the sub-line portion S 1  or the line width of the main line portion M 3  may be different from the line width of the sub-line portion S 3 . By adjusting the line widths of the main line portions M 1  to M 3  and the line widths of the sub-line portions S 1  to S 3 , the characteristic impedance of the main line M and the characteristic impedance of the sub-line S are adjusted. 
     In the directional couplers  10   a ,  10   b ,  10   d , and  10   e , it is preferable that, when viewed in plan from above, the portions of the outer electrodes  14   a  to  14   d  that are bent onto the bottom surface (hereinafter, bent portions  15   a  to  15   d  (see  FIG. 3 )) are smaller than the capacitor conductors  26   a  to  26   d , respectively, and are accommodated within the capacitor conductors  26   a  to  26   d  (that is, do not extend outside the capacitor conductors  26   a  to  26   d ), respectively. With this configuration, formation of unnecessary capacitance between the bent portions  15   a  to  15   d  and the ground conductor  24  is significantly reduced or prevented. 
     In the directional couplers  10   a  to  10   e , the main line portion M 1  or the main line portion M 3  may not be provided. In this case, the main line portion M 2  is connected to the lead conductor  18   a  or the lead conductor  18   b . Similarly, the sub-line portion S 1  or the sub-line portion S 3  may not be provided. In this case, the sub-line portion S 2  may be connected to the lead conductor  20   a  or the lead conductor  20   b.    
     The main line portion M 1  and the main line portion M 3  may be provided on different dielectric layers. 
     The sub-line portion S 1  and the sub-line portion S 3  may be provided on different dielectric layers. 
     The shape of the main line portion M 1  may be different from the shape of the sub-line portion S 1 . The shape of the main line portion M 2  may be different from the shape of the sub-line portion S 2 . The shape of the main line portion M 3  may be different from the shape of the sub-line portion S 3 . 
     The space between the main line portion M 2  and the sub-line portion S 2  may be greater than each of the space between the main line portion M 1  and the sub-line portion S 1  and the space between the main line portion M 3  and the sub-line portion S 3 . 
     Preferred embodiments of the present invention are useful for a directional coupler, and more particularly, are excellent in that a fine adjustment of the degree of coupling between a main line and a sub-line is achieved. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.