Patent Publication Number: US-2022231390-A1

Title: Mode converter, rf module, and mobile terminal

Description:
TECHNICAL FIELD 
     The present invention relates to a mode converter that converts between a mode of a post-wall waveguide and a mode of a line including a strip-shaped conductor. The present invention also relates to an RF module and a mobile terminal both of which include such a mode converter. 
     BACKGROUND ART 
     Non-patent Literature 1 discloses a mode converter that carries out mutual conversion between a guide mode of a post-wall waveguide and a guide mode of a microstrip line. 
     An RF module  101  including such conventional mode converters  110   a  and  110   b  is illustrated in  FIGS. 16 and 17 .  FIG. 16  is an exploded perspective view illustrating the RF module  101 , and  FIG. 17  is a cross-sectional view illustrating the RF module  101 . 
     In the mode converters  110   a  and  110   b , a post-wall waveguide PW includes: a dielectric substrate  111 ; a pair of conductor layers  112  and  113  formed respectively on a pair of main surfaces of the substrate  111 ; and a post wall  114  formed inside the substrate  111 . In the pair of conductor layers  112  and  113 , a region surrounded by the post wall  114  in a plan view functions as a pair of wide walls between which a rectangular-parallelepiped-shaped waveguide region is sandwiched from two directions (for example, up and down directions), and the post wall functions as a pair of narrow walls and a pair of short walls surrounding the waveguide region from four directions (for example, front, rear, left, and right directions). The post wall  114  is constituted by a plurality of through-vias which are provided in a fence-like manner inside the substrate  111  and which short-circuit the pair of conductor layers. 
     Further, in the mode converters  110   a  and  110   b , the microstrip line MS includes: strip-shaped conductors  116   a  and  116   b  which functions as a signal line; a ground layer constituted by the conductor layer  112 ; and a dielectric layer  115  which separates the signal line and the ground layer. The microstrip line MS is formed directly on one of the main surfaces of the post-wall waveguide. 
     In addition, to carry out mutual conversion between the guide mode of the post-wall waveguide PW and the guide mode of the microstrip line MS, the mode converters  110   a  and  110   b  include blind vias BVa and BVb that are connected to first ends of the strip-shaped conductors  116   a  and  116   b  which constitute the microstrip line MS. The blind vias BVa and BVb function as excitation pins. The excitation pins may be blind vias or may be through-vias. 
     In the mode converters  110   a  and  110   b , the strip-shaped conductors  116   a  and  116   b  can be used respectively as an input port and an output port of the post-wall waveguide PW. For example, as illustrated in  FIGS. 16 and 17 , the RF module  101  is obtained by forming the mode converters  110   a  and  110   b  respectively at both ends of the post-wall waveguide PW (near the short walls), mounting a radio frequency integrated circuit (RFIC)  121  on the strip-shaped conductor  116   a  of the mode converter  110   a , and mounting an antenna  122  on the strip-shaped conductor  116   b  of the mode converter  110   b . The RF module  101  is an RF module in which the RFIC  121  and the antenna  122  are mounted directly on one of the main surfaces of the post-wall waveguide PW. 
     Regarding such an RF module, there is a demand for manufacturing the RF module by mounting an RFIC and an antenna in advance on another substrate (referred to as a mounting substrate) which differs from the substrate constituting the post-wall waveguide, and then joining the mounting substrate and the post-wall waveguide. An RF module  101 A having such a configuration is illustrated in  FIG. 18 .  FIG. 18  is a cross-sectional view illustrating the RF module  101 A. 
     To realize such a demand, it is possible to employ an aspect in which, as illustrated in  FIG. 18 , strip-shaped conductors  116 Aa and  116 Ab are drawn to a back side of the mounting substrate (a side closer to the post-wall waveguide PW) using through-vias, and then the strip-shaped conductors  116 Aa and  116 Ab drawn to the back side of the mounting substrate and blind vias BVa and BVb are joined with use of bumps Ba and Bb. 
     CITATION LIST 
     Non-Patent Literature 
     
         
         [Non-Patent Literature 1] 
         Yusuke Uemichi, et al. “A ultra low-loss silica-based transformer between microstrip line and post-wall waveguide for millimeter-wave antenna-in-package applications,” IEEE MTT-S IMS, Jun. 2014. 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     As a dielectric constituting the substrate  111  of the post-wall waveguide PW, quartz, ceramics, sapphire, silicon, etc. are often employed. On the other hand, as a dielectric constituting a mounting substrate  115 A, a fluorine resin (for example, Teflon (registered trademark)), a glass epoxy resin, a liquid crystal polymer, a polyimide resin, cycloolefin, etc. are often employed. In such a case, the linear expansion coefficient of the dielectric constituting the substrate  111  and the linear expansion coefficient of the dielectric constituting the mounting substrate  115 A differ greatly. Thus, a stress is repeatedly added to the bumps Ba and Bb with a change in environmental temperature at which the RF module  101 A is used, and eventually causes cracks in either one or both of the bumps Ba and Bb. This may cause transmission failure either between the RFIC  121  and the post-wall waveguide PW or between the antenna  122  and the post-wall waveguide PW or both between the RFIC  121  and the post-wall waveguide PW and between the antenna  122  and the post-wall waveguide PW (see  FIG. 18 ). 
     To strengthen joining between the mounting substrate  115 A and the post-wall waveguide PW, it is also possible to employ an aspect in which a solder  118  is used to join a main surface of the mounting substrate  115 A and a main surface of the post-wall waveguide PW, both of which main surfaces face each other (see  FIG. 18 ). However, since the linear expansion coefficient of the dielectric constituting the substrate of the post-wall waveguide PW and the linear expansion coefficient of the dielectric constituting the mounting substrate differ greatly, it is difficult to prevent the occurrence of cracks even though the solder  118  is used in addition to the bumps Ba and Bb. 
     An aspect of the present invention has been made in view of the above problems. It is an object of the aspect of the present invention to provide a mode converter that converts between a mode of a post-wall waveguide and a mode of a line in which a strip-shaped conductor formed on another substrate different from a substrate of a post-walled waveguide serves as a signal line, wherein the mode converter is less likely to cause a transmission failure resulting from a change in environmental temperature than the conventional mode converter. Further, it is an object of the aspect of the present invention to provide an RF module and a mobile terminal both of which include such a mode converter. 
     Solution to Problem 
     A mode converter in accordance with an aspect of the present invention includes: a post-wall waveguide in which an opening is provided in a first wide wall; a dielectric substrate having a first main surface and a second main surface on at least one of which a strip-shaped conductor is formed; and a joining member joining directly or indirectly the first wide wall and the substrate, wherein at least a portion of the opening and at least a portion of the strip-shaped conductor overlap each other in a plan view of the post-wall waveguide. 
     An RF module in accordance with an aspect of the present invention includes: a post-wall waveguide in which a first opening is provided in a vicinity of a first short wall of a first wide wall, and a second opening is provided in a vicinity of a second short wall of the first wide wall; a dielectric substrate having a first main surface and a second main surface, wherein a first strip-shaped conductor is formed on at least one of the first main surface and the second main surface, a second strip-shaped conductor is formed on at least one of the first main surface and the second main surface, and an antenna is formed on at least one of the first main surface and the second main surface; a joining member joining directly or indirectly the first wide wall and the substrate; and an RFIC being mounted on the first main surface of the substrate and having terminals any of which is electrically connected to the first strip-shaped conductor, wherein the antenna is electrically connected to the second strip-shaped conductor, and in a plan view of the post-wall waveguide, at least a portion of the first opening and at least a portion of the first strip-shaped conductor overlap each other, and at least a portion of the second opening and at least a portion of the second strip-shaped conductor overlap each other. 
     A mobile terminal in accordance with an aspect of the present invention includes: an RF module in accordance with an aspect of the present invention as at least one of a transmission module, a reception module, and a transmission/reception module. 
     An RF module in accordance with an aspect of the present invention includes: a post-wall waveguide having a waveguide region formed by a pair of wide walls, narrow walls, and a pair of short walls, wherein (1) a first opening is provided in a first end region which is in a first wide wall of the pair of wide walls and includes a first short wall, (2) a second opening is provided in a second end region which is in the first wide wall of the pair of wide walls and includes a second short wall, and (3) the first end region and the second end region are arranged so as to be parallel to and close to each other; a dielectric substrate having a first main surface and a second main surface on at least one of which a third strip-shaped conductor and a fourth strip-shaped conductor are formed; a joining member joining directly or indirectly the first wide wall and the substrate; and an RFIC being mounted on the first main surface of the substrate and having an output terminal and an input terminal which are electrically connected to the third strip-shaped conductor and the fourth strip-shaped conductor, respectively, wherein in a plan view of the post-wall waveguide, at least a portion of the first opening and at least a portion of the third strip-shaped conductor overlap each other, and at least a portion of the second opening and at least a portion of the fourth strip-shaped conductor overlap each other. 
     Advantageous Effects of Invention 
     According to an aspect of the present invention, it is possible to provide a mode converter that converts between a mode of a post-wall waveguide and a mode of a line in which a strip-shaped conductor formed on another substrate different from a substrate of a post-walled waveguide serves as a signal line, wherein the mode converter is less likely to cause a transmission failure resulting from a change in environmental temperature than the conventional mode converter. Further, according to an aspect of the present invention, it is possible to provide an RF module and a mobile terminal both of which include such a mode converter. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a plan view illustrating a mode converter in accordance with Embodiment 1 of the present invention. 
         FIG. 2  is a cross-sectional view illustrating the mode converter illustrated in  FIG. 1 . 
         FIG. 3  is a cross-sectional view illustrating a first variation of the mode converter illustrated in  FIG. 1 . 
         FIG. 4  is a plan view illustrating a second variation of the mode converter illustrated in  FIG. 1 . 
         FIG. 5  is a cross-sectional view illustrating a third variation of the mode converter illustrated in  FIG. 1 . 
         FIG. 6  is an exploded perspective view illustrating an RF module in accordance with Embodiment 2 of the present invention. 
         FIG. 7  is a cross-sectional view illustrating the RF module illustrated in  FIG. 6 . 
         FIG. 8  is a cross-sectional view illustrating a mode converter showing results of simulation of a conversion characteristic of the mode converter in accordance with Example 1 of the present invention. 
         FIG. 9  is a plan view illustrating a mode converter showing results of simulation of a conversion characteristic of the mode converter in accordance with Example 1 of the present invention. 
         FIG. 10  is a graph showing respective reflection characteristics and respective transmission characteristics of mode converters in accordance with Example 1 and Reference Example of the present invention. 
         FIG. 11  is an enlarged graph showing respective transmission characteristics of the mode converters in accordance with Example 1 and Reference Example of the present invention. 
         FIG. 12  is a graph showing a reflection characteristic and transmission characteristic of a mode converter in accordance with Example 2 of the present invention and a reflection characteristic and transmission characteristic of a mode converter in accordance with Example 3 of the present invention. 
         FIG. 13  is an enlarged graph showing a transmission characteristic of the mode converter in accordance with Example 2 of the present invention and a transmission characteristic of the mode converter in accordance with Example 3 of the present invention. 
         FIG. 14  is a graph showing a reflection characteristic of a mode converter in accordance with Example Group 4 of the present invention. 
         FIG. 15  is a graph showing transmission characteristics of the mode converter in accordance with Example Group 4 of the present invention. 
         FIG. 16  is an exploded perspective view illustrating an RF module including the conventional mode converter. 
         FIG. 17  is a cross-sectional view illustrating the RF module including the conventional mode converter. 
         FIG. 18  is a cross-sectional view illustrating an RF module that includes the conventional mode converter and is in a form which differs from the form of the RF module illustrated in  FIG. 17 . 
         FIG. 19  is a graph showing a reflection characteristic of a mode converter in accordance with Example 5 of the present invention. 
         FIG. 20  is a graph showing a transmission characteristic of the mode converter in accordance with Example 5 of the present invention. 
         FIG. 21  is a graph showing a reflection characteristic of a mode converter in accordance with Example Group 6 of the present invention. 
         FIG. 22  is a graph showing a transmission characteristic of the mode converter in accordance with Example Group 6 of the present invention. 
         FIG. 23  is a cross-sectional view illustrating a fourth variation of the mode converter illustrated in  FIG. 1 . 
         FIG. 24  is a graph showing respective reflection characteristics of the mode converters in accordance with Example Group 7 of the present invention. 
         FIG. 25  is a graph showing respective reflection characteristics of the mode converters in accordance with Example Group 8 of the present invention. 
         FIG. 26  is a graph showing a reflection characteristic of the mode converter in accordance with Example 9 of the present invention. 
         FIG. 27  is a graph showing a transmission characteristic of the mode converter in accordance with Example 9 of the present invention. 
         FIG. 28  is a plan view illustrating a fifth variation of the mode converter illustrated in  FIG. 1 . 
         FIG. 29  is a graph showing respective reflection characteristics of the mode converters in accordance with Example Group 10 of the present invention. 
         FIG. 30  is a graph showing respective transmission characteristics of the mode converters in accordance with Example Group 10 of the present invention. 
         FIG. 31  is a plan view illustrating a sixth variation of the mode converter illustrated in  FIG. 1 . 
         FIG. 32  is a graph showing respective reflection characteristics of the mode converters in accordance with Example Group 11 of the present invention. 
         FIG. 33  is a graph showing respective transmission characteristics of the mode converters in accordance with Example Group 11 of the present invention. 
         FIG. 34  is a graph showing respective reflection characteristics of the mode converters in accordance with Example Group 12 of the present invention. 
         FIG. 35  is a graph showing respective transmission characteristics of the mode converters in accordance with Example Group 12 of the present invention. 
         FIG. 36  is a plan view illustrating an RF module in accordance with Embodiment 3 of the present invention. 
         FIG. 37  is a cross-sectional view of a seventh variation of the mode converter illustrated in  FIG. 1 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
     The following description will discuss, with reference to  FIGS. 1 and 2 , a mode converter  10  in accordance with Embodiment 1 of the present invention.  FIG. 1  is a plan view illustrating the mode converter  10 .  FIG. 2  is a cross-sectional view illustrating the mode converter  10  and is a cross-sectional view taken along the line A-A′ illustrated in  FIG. 1 . Note that all of mode converters and RF modules in accordance with embodiments of the present invention assume a 28 GHz band (for example, a band of not less than 27 GHz and not more than 29.5 GHz) as an operating band. 
     &lt;Configuration of Mode Converter  10 &gt; 
     As illustrated in  FIGS. 1 and 2 , the mode converter  10  includes a post-wall waveguide PW, a microstrip line MS, and a solder  18 . The post-wall waveguide PW and the microstrip line MS will be discussed later. The solder  18  short-circuits a conductor layer  12  of the post-wall waveguide PW and a conductor layer  17  of the microstrip line MS and is an example of a joining member for joining the conductor layer  12  of the post-wall waveguide PW and the conductor layer  17  of the microstrip line MS. The solder  18  joins the conductor layer  12  and the conductor layer  17  in a state in which the conductor layer  12  and the conductor layer  17  are parallel or substantially parallel to each other. As discussed later, the conductor layer  17  is formed on a main surface of the substrate  15  which main surface is on a side closer to the conductor layer  12  out of a pair of opposed main surfaces of the substrate  15 . Thus, it can be said that the solder  18  indirectly joins the conductor layer  12  and the substrate  15  via the conductor layer  17 . Hereinafter, out of the pair of opposed main surfaces of the substrate  15 , a main surface on a side farther from the conductor layer  12  is also referred to as a first main surface, and a main surface on a side closer to the conductor layer  12  is also referred to as a second main surface. Hereinafter, the shortest distance between central axes of through-vias  14   i  constituting a short wall  14   c  (discussed later) and the solder  18  is referred to as a distance D3. The distance D3 can be determined as appropriate, but in the present embodiment, D3=850 μm. 
     (Post-Wall Waveguide PW) 
     As illustrated in  FIGS. 1 and 2 , the post-wall waveguide PW includes a substrate  11 , conductor layers  12  and  13 , and a post wall  14 . 
     The substrate  11  is a plate-like member made of a dielectric and is made of quartz in the present embodiment. The dielectric constituting the substrate  11  is not limited to quartz, and can be selected as appropriate from among dielectrics capable of suppressing transmission loss when employed as a substrate of a post-wall waveguide, according to, for example, the center frequency or the like of the mode converter  10 . Note that the thickness T11 of the substrate  11  can be selected as appropriate. 
     The conductor layers  12  and  13  are layer members that are formed respectively on a pair of opposed main surfaces of the substrate  11 . The conductor layers  12  and  13  are layer members made of a conductive material, and are made of copper in the present embodiment. The conductive material of which the conductor layers  12  and  13  are made is not limited to copper and can be selected as appropriate. Further, the thicknesses of the conductor layers  12  and  13  can be selected as appropriate, and each of the conductor layers  12  and  13  may be a relatively thin layer member called a conductor film, or may be a relatively thick layer member called a conductor plate. 
     The post wall  14  is constituted by a plurality of through-vias  141  to  14   n  provided in a fence-like manner inside the substrate  11 . Here, n is an arbitrary integer greater than or equal to 2. Further, hereinafter, the through vias  141  to  14   n  are generalized and are also each referred to as through-vias  14   i . Here, i is an integer not less than 1 and not more than n. The post wall  14  is composed of a pair of opposed narrow walls  14   a  and  14   b , a short wall  14   c , and another short wall (not illustrated in  FIGS. 1 and 2 ) facing the short wall  14   c . Each of the through-vias  14   i  is constituted by a cylindrical or columnar conductor (cylindrical conductor in the present embodiment). 
     Each of the through-vias  14   i  extends from the first main surface of the substrate  11  to the second main surface of the substrate  11 , and short-circuits the conductor layer  12  and the conductor layer  13 . Further, the diameter DT (see  FIG. 1 ) of each of the through-vias  14   i  can be determined as appropriate according to the width W1 of the post-wall waveguide PW, the complexity of the shape of the post-wall waveguide PW, and the like, but in the present embodiment, DT=100 μm. 
     In the mode converter  10 , the conductor layers  12  and are provided so as to sandwich the substrate  11  therebetween from two directions (for example, up and down directions), the narrow walls  14   a  and  14   b  are provided so as to sandwich a partial region of the substrate  11  therebetween from two directions (for example, left and right directions), and the short wall  14   c  and the another short wall are provided so as to sandwich the partial region of the substrate  11  therebetween from two directions (for example, front and rear directions). The partial region of the substrate  11  sandwiched by the conductor layers  12  and  13 , the narrow walls  14   a  and  14   b , the short wall  14   c , and the another short wall from six directions functions as a waveguide region of the mode converter  10 . This waveguide region is illustrated as a region surrounded on three sides by a chain double-dashed line in  FIG. 1 . In  FIG. 2 , the waveguide region is illustrated as a region which is on the right side of the through-vias  14   i  and which is sandwiched between the conductor layer  12  and the conductor layer  13 . Note that the chain double-dashed line illustrated in  FIG. 1  is a straight line passing through the respective centers of the through-vias  14   i . As illustrated in  FIG. 1 , the narrow wall  14   a  and the narrow wall  14   b  are parallel to each other, and the short wall  14   c  is orthogonal to the narrow walls  14   a  and  14   b . Hereinafter, spacing between the narrow wall  14   a  and the narrow wall  14   b  is referred to as a width W1 of the post-wall waveguide PW. The width W1 can be determined as appropriate according to the operating band or the like, but in the present embodiment, W1=4 mm. 
     As illustrated in  FIGS. 1 and 2 , an opening  121  is provided in the conductor layer  12  constituting a first wide wall of the post-wall waveguide PW. The opening  121  is rectangular in shape and is provided in the vicinity of the short wall  14   c  such that a long side of the opening  121  extends along the short wall  14   c  (the long side of the opening  121  is parallel to the short wall  14   c  in the present embodiment) and a short side of the opening  121  extends along the narrow walls  14   a  and  14   b  (the short side of the opening  121  is parallel to the narrow walls  14   a  and  14   b  in the present embodiment). Hereinafter, the shortest distance between the opening  121  and the short wall  14   c  is referred to as a distance D1, the width of the opening  121  (length along the short side) is referred to as a width W2, and the length (length along the long side) is referred to as a length L2. The distance D1, the width W2, and the length L2 can be determined as appropriate according to the operating band or the like, but in the present embodiment, D1=100 μm, W2=400 μm, and L2=3.2 mm. The post-wall waveguide PW configured as above functions as a TE line, and guides high-frequency waves coupled to the waveguide region via the opening  121 , along the long-axis direction of the waveguide region. 
     (Microstrip Line MS) 
     As illustrated in  FIGS. 1 and 2 , the microstrip line MS includes the substrate  15 , a strip-shaped conductor  16 , and the conductor layer  17 . 
     The substrate  15  is a plate-like member made of a dielectric. The dielectric constituting the substrate  15  can be selected as appropriate from among dielectrics capable of suppressing transmission loss when employed as a substrate of a microstrip line, according to, for example, the center frequency or the like of the mode converter  10 . Note that the thickness T15 of the substrate  15  can be selected as appropriate. 
     Further, a substrate portion of a commercially available mounting substrate (for example, Megtron6 (registered trademark), Rogers RT/duroid (registered trademark) 5880, etc.) can also be used as the substrate  15 . In this case, the strip-shaped conductor  16  (discussed later) can be formed by patterning a conductor layer which is a first main surface out of a pair of opposed main surfaces of the mounting substrate, and the conductor layer  17  (discussed later) can be formed by patterning a conductor layer which is a second main surface of the mounting substrate. 
     The strip-shaped conductor  16  is a rectangular conductor pattern formed on a first main surface of the substrate  15  (in  FIG. 2 , a main surface on a side farther from the conductor layer  12  of the post-wall waveguide PW) and functions as a signal line of the microstrip line MS. Hereinafter, the width of the strip-shaped conductor  16  is referred to as a width W3, and the length of a tip part of the strip-shaped conductor  16  which tip part protrudes from the opening  121  in a plan view is referred to as a length L3. The width W3 and the length L3 can be determined as appropriate according to the operating band or the like, but in the present embodiment, W3=600 μm and L3=600 μm. 
     The length L3 affects impedance matching between the microstrip line MS and the post-wall waveguide PW. Thus, suitably designing the length L3 enables enhancement of the impedance matching, and hence enables suppression of reflection loss in the mode converter  10 . 
     The conductor layer  17  is a conductor pattern formed on a second main surface of the substrate  15  (in  FIG. 2 , a main surface on a side closer to the conductor layer  12  of the post-wall waveguide PW), and functions as a ground layer of the microstrip line MS. In the present embodiment, as illustrated in  FIG. 1 , the conductor layer  17  is formed outside the waveguide region of the post-wall waveguide PW (that is, the region surrounded by the post wall  14 ) in a plan view. Hereinafter, the shortest distance between the central axes of the through-vias  14   i  constituting the short wall  14   c  and the conductor layer  17  is referred to as a distance D2. The distance D2 can be determined as appropriate, but in the present embodiment, D2=850 μm. That is, in the present embodiment, D2=D3 (see  FIGS. 1 and 2 ). Note that the region where the conductor layer  17  is formed and the distance D2 can be designed as appropriate according to the position where the opening  121  is formed or the like, and may extend from outside of the waveguide region to the vicinity of the opening  121 . Furthermore, in a plan view, the conductor layer  17  may not be formed in a region facing at least the opening  121  in the second main surface of the substrate  15 , and the conductor layer  17  may be formed in a part of a region other than the region facing the opening  121  or may be formed in an entire region other than the region facing the opening  121 . 
     The strip-shaped conductor  16  and the conductor layer  17  are made of a conductive material, and are made of copper in the present embodiment. The conductive material of which the strip-shaped conductor  16  and the conductor layer  17  are made is not limited to copper and can be selected as appropriate. Further, the thickness and width of the strip-shaped conductor  16  and the thickness of the conductor layer  17  can be selected as appropriate, and each of the strip-shaped conductor  16  and the conductor layer  17  may be a relatively thin layer member called a conductor film, or may be a relatively thick layer member called a conductor plate. 
     In the region illustrated in  FIG. 1 , the strip-shaped conductor  16  is rectangular in shape. However, the strip-shaped conductor  16  only needs to be arranged such that at least a vicinity of a region which overlaps the opening  121  in a plan view is in the shape of a strip, and an end portion of the strip-shaped conductor  16  on a side farther from the opening  121  may be patterned in any shape. At the end portion on the side farther from the opening  121 , a conductor pad can be formed for connecting, for example, a terminal of a radio frequency integrated circuit (RFIC). Further, in the present embodiment, an end portion of the strip-shaped conductor  16  on the side closer to the opening  121  is patterned so as to have two corners. However, the shape of the end portion on the side closer to the opening  121  is not limited. 
     In a plan view of the post-wall waveguide PW (see  FIG. 1 ), the strip-shaped conductor  16  is arranged such that the end portion of the strip-shaped conductor  16  on the side farther from the opening  121  is arranged outside the waveguide region of the post-wall waveguide PW, a strip-shaped part of the strip-shaped conductor  16  crosses the short wall  14   c  and the opening  121 , and the end portion of the strip-shaped conductor  16  on the side closer to the opening  121  is arranged in the waveguide region in the vicinity of the opening  121 . Thus, in the mode converter  10 , a portion of the opening  121  in the vicinity of a center of the opening  121  and a portion of the strip-shaped conductor  16  overlap each other, in a plan view of the post-wall waveguide PW. 
     The microstrip line MS configured as above is an aspect of a line called a quasi-TEM line or a two-conductor line. 
     In the present embodiment, the strip-shaped conductor  16  is formed only on the first main surface of the substrate  15 . For example, a strip-shaped conductor, as with the strip-shaped conductor  16 C illustrated in  FIG. 5 , may be formed on both of the main surfaces of the substrate  15 . 
     Further, in the present embodiment, the mode converter  10  has been discussed in which the microstrip line MS is used as a line provided by forming a strip-shaped conductor on at least one of the first main surface of the substrate  15  and the second main surface thereof. However, in an aspect of the present invention, the line provided by forming the strip-shaped conductor on at least one of the first main surface of the substrate  15  and the second main surface thereof is not limited to the microstrip line MS. Such a line may be a strip line, may be a coplanar line, may be a grounded coplanar line, or may be a parallel two-wire line. 
     (Mode Conversion) 
     In the mode converter  10 , the mode of the microstrip line MS and the mode of the post-wall waveguide PW are coupled via a region in which a portion of the strip-shaped conductor  16  and a portion of the opening  121  overlap each other in a plan view. That is, the mode converter  10  can convert between these modes via the opening  121  which does not directly contact the strip-shaped conductor  16 , without using an excitation pin. 
     (Gap Between the Conductor Layer  12  and the Substrate  15 ) 
     As illustrated in  FIG. 2 , a void, including a portion of the opening  121 , is formed between the conductor layer  12  and the substrate  15 . This void may remain in the state of a gap (i.e., the void may remain filled with air) or may be filled with a dielectric such as a resin material. In a case where the resin material is filled into the void, most part of the void can be considered to be filled with the resin material, for example, if the resin material which is in a liquid form before curing is injected through any of the through-vias  14   i , and it is observed that the resin material is poured out from another through-via  14   i  which differs from the through-via  14   i  through which the resin material is injected. Thereafter, the resin material may be cured. 
     In a case where the void is filled with a dielectric, it is possible to lessen discontinuity of a relative dielectric constant which can occur between the substrate  11  and the substrate  15  by appropriately selecting the dielectric with which the void is to be filled. 
     (Function of the Post-Wall Waveguide PW) 
     In the mode converter  10 , the post-wall waveguide PW may be such that the waveguide region is configured so that the post-wall waveguide PW functions as any of a filter, a directional coupler, a diplexer, and an antenna. In a case where the post-wall waveguide PW is used to realize the function of any of the filter, the directional coupler, the diplexer, and the antenna, a component suitable for an application may be selected as appropriate from an existing filter, an existing directional coupler, an existing diplexer, and an existing antenna. Note that the antenna is preferably an array antenna. 
     &lt;First Variation&gt; 
     The following description will discuss, with reference to  FIG. 3 , a mode converter  10 A in accordance with a first variation of the present invention, wherein the mode converter  10 A is a variation of the mode converter  10  illustrated in  FIGS. 1 and 2 .  FIG. 3  is a cross-sectional view illustrating the mode converter  10 A. 
     The mode converter  10 A is based on the mode converter  10  and is obtained by omitting the conductor layer  17  from the mode converter  10  and replacing the solder  18  with an adhesive  18 A. The adhesive  18 A is made of a non-conductive resin material. The adhesive  18 A directly joins the conductor layer  12  and the substrate  15 . Hereinafter, the thickness of the adhesive  18 A is referred to as a thickness TA. By designing the thickness TA as appropriate, it is possible to adjust the degree of joining between the strip-shaped conductor  16  and the conductor layer  12  in the microstrip line MS. 
     Even when the conductor layer  17  is omitted, the conductor layer  12  of the post-wall waveguide PW functions as a ground layer of the microstrip line MS. Thus, it can be said that the microstrip line MS of the mode converter  10 A includes the substrate  15 , the strip-shaped conductor  16 , and the conductor layer  12 . 
     The mode converter  10 A configured as above brings about the same effect as the effect brought about by the mode converter  10 . 
     Further, in  FIG. 3 , a void is formed between the conductor layer  12  and the substrate  15 . However, as discussed earlier, this void may remain in the state of a gap (i.e., the void may remain filled with air) or may be filled with a dielectric such as a resin material. In the mode converter  10 A, the conductor layer  12  and the substrate  15  can be joined, without forming the void, by, for example, applying the adhesive  18 A onto the main surface of the substrate  15  and then putting the substrate  15  on the conductor layer  12 . 
     The adhesive  18 A has a higher modulus of elasticity than the solder  18 , and thus can reduce the stress that can occur with a change in environmental temperature. 
     &lt;Second Variation&gt; 
     The following description will discuss, with reference to  FIG. 4 , a mode converter  10 B in accordance with a second variation of the present invention, wherein the mode converter  10 B is a variation of the mode converter  10  illustrated in  FIGS. 1 and 2 .  FIG. 4  is a plan view illustrating the mode converter  10 B. 
     The mode converter  10 B is based on the mode converter  10  and is obtained by replacing the strip-shaped conductor  16  included in the mode converter  10  with a strip-shaped conductor  16 B. In the present variation, the strip-shaped conductor  16 B is such that the length L3 of the strip-shaped conductor  16 B is designed so as to be shorter than the length L3 of the strip-shaped conductor  16 , and L3=200 μm. 
     The strip-shaped conductor  16 B, as illustrated in  FIG. 4 , includes a strip-shaped main part  16 B 1  and two stubs  16 B 1  and  16 B 2  each of which is rectangular in shape. The main part  16 B 1  is a conductor pattern corresponding to the strip-shaped conductor  16 , and each of the stubs  16 B 1  and  16 B 2  is a conductor pattern which is rectangular in shape. The main part  16 B 1  and the stubs  16 B 1  and  16 B 2  form the strip-shaped conductor  16 B, which is a single conductor pattern. 
     The stubs  16 B 1  and  16 B 2  are provided, in a plan view, at any respective positions in a section outside the waveguide region of the post-wall waveguide PW in the main part  16 B 1 , wherein the positions are linearly symmetrical about the central axis of the strip-shaped main part  16 B 1  as a symmetrical axis. 
     In the present variation, the stubs  16 B 1  and  16 B 2  are rectangular in shape, and are arranged such that long sides of the stubs  16 B 1  and  16 B 2  cross the symmetrical axis (in the present embodiment, they are orthogonal to the symmetrical axis) and short sides of the stubs  16 B 1  and  16 B 2  extend along the symmetrical axis (in the present embodiment, they are parallel to the symmetrical axis). However, the shape of the stubs  16 B 1  and  16 B 2  and the orientation of the stubs  16 B 1  and  16 B 2  being arranged are not limited to those in accordance with an aspect of the present variation. The stubs  16 B 1  and  16 B 2  may be open stubs having open ends, as illustrated in  FIG. 4 , or may be stubs called short stubs which are short-circuited. 
     As discussed earlier, in the present variation, the length L3 is designed so as to be shorter than the length L3 of the strip-shaped conductor  16 . Thus, in a case where a strip-shaped conductor with no stubs, as with the strip-shaped conductor  16 , is employed in the mode converter  10 B, impedance matching becomes poorer than that in the mode converter  10 . In a case where impedance matching is poor as discussed above, designing the stubs  16 B 1  and  16 B 2  as appropriate achieves impedance matching between the microstrip line MS and the post-wall waveguide PW, and hence enables suppression of reflection loss in the mode converter  10 B. 
     &lt;Third Variation&gt; 
     The following description will discuss, with reference to  FIG. 5 , a mode converter  10 C in accordance with a third variation of the present invention, wherein the mode converter  10 C is a variation of the mode converter  10  illustrated in  FIGS. 1 and 2 .  FIG. 5  is a cross-sectional view illustrating the mode converter  10 C. 
     The mode converter  10 C is based on the mode converter  10  and is obtained by replacing the strip-shaped conductor  16  included in the mode converter  10  with the strip-shaped conductor  16 C. 
     The strip-shaped conductor  16 C, as illustrated in  FIG. 5 , includes a first conductor pattern  16 C 1  and a second conductor pattern  16 C 2 . The first conductor pattern  16 C 1  and the second conductor pattern  16 C 2  are each strip-shaped conductor patterns. As discussed later, the first conductor pattern  16 C 1  is formed on a first main surface of the substrate  15  (on a main surface on a side farther from the conductor layer  12 ), and the second conductor pattern  16 C 2  is formed on a second main surface of the substrate  15  (on a main surface on a side closer to the conductor layer  12 ). The first conductor pattern  16 C 1  and the second conductor pattern  16 C 2  are electrically connected using a through-via  16 C 3 . 
     The first conductor pattern  16 C 1  is a strip-shaped conductor pattern formed on the first main surface of the substrate  15 , as in the strip-shaped conductor  16 , and a tip part of the first conductor pattern  16 C 1  is arranged outside the guidewave region of the post-wall waveguide PW. 
     The second conductor pattern  16 C 2  is a strip-shaped conductor pattern formed on the second main surface of the substrate  15 . A first tip part of the second conductor pattern  16 C 2  is arranged outside the waveguide region of the post-wall waveguide PW and is arranged so as to overlap a first tip part of the first conductor pattern  16 C 1  in a plan view. A strip-shaped part of the second conductor pattern  16 C 2  crosses the short wall  14   c  and the opening  121  in a plan view, and a second tip part of the second conductor pattern  16 C 2  is arranged in the vicinity of the opening  121  in a plan view and is arranged in the waveguide region. Thus, in the mode converter  10 C, a portion of the opening  121  in the vicinity of the center of the opening  121  and a portion of the strip conductor  16 C overlap each other in a plan view of the post-wall waveguide PW. In the strip-shaped conductor  16 C as well, the length of a portion of the second tip part of the second conductor pattern  16 C 2  which portion protrudes from the opening  121  is referred to as a length L3. 
     The through-via  16 C 3  is constituted by a cylindrical or columnar conductor (cylindrical conductor in the present embodiment) formed in a region which is outside the waveguide region in a plan view and in which a first tip part of the first conductor pattern  16 C 1  overlaps a first tip part of the second conductor pattern  16 C 2 . In the present variation, the diameter of the through-via  16 C 3  is 300 μm. 
     In the mode converter  10 , the strip-shaped conductor  16  is realized by using a single conductor pattern formed only on the first main surface of the substrate  15 . However, as in the mode converter  10 C, the strip-shaped conductor  16 C may be such that the first conductor pattern  16 C 1  and the second conductor pattern  16 C 2  are formed on the first main surface of the substrate  15  and on the second main surface thereof, respectively, and are electrically connected to each other by using the through-via  16 C 3 . In this case, the first conductor pattern  16 C 1  and the conductor layer  17  form a microstrip line, and the second conductor pattern  16 C 2  and the conductor layer  12  form a microstrip line. In the present variation, by appropriately designing the thickness TS of the solder  18 C, it is possible to adjust the degree of coupling between the second conductor pattern  16 C 2  and the conductor layer  12  in the microstrip line MS. 
     Further, in an aspect of the present invention, a strip-shaped conductor corresponding to the strip-shaped conductor  16  may be formed on the second main surface of the substrate  15 , not on the first main surface of the substrate  15 . In this case, by joining at least a region in which the strip-shaped conductor and the conductor layer  12  face each other with use of the adhesive  18 A having no conductivity, it is possible to prevent the strip-shaped conductor and the conductor layer  12  from being short-circuited. In this case, the strip-shaped conductor and the conductor layer  12  form a microstrip line. 
     Embodiment 2 
     The following description will discuss an RF module  1  in accordance with Embodiment 2 of the present invention with reference to  FIGS. 6 and 7 .  FIG. 6  is an exploded perspective view of the RF module  1 . Note that in  FIG. 6 , the conductor layer  17  and the solder  18  are not illustrated.  FIG. 7  is a cross-sectional view illustrating the RF module  1  in a cross section passing through a straight line that coincides with central axes of strip-shaped conductors  16   a  and  16   b  (described later). For convenience of description, members having functions identical to those described in Embodiment are assigned identical referential numerals, and their descriptions are not repeated. 
     As illustrated in  FIGS. 6 and 7 , a mode converter  10   a , a mode converter  10   b , an RFIC  21 , and an antenna  22  are provided. Each of the mode converters  10   a  and  10   b  is a specific example of the mode converter  10  illustrated in  FIGS. 1 and 2  and is configured in the same manner as the mode converter  10 . Members constituting the mode converter  10   a  are given reference numerals that are made up of the reference numerals of the members constituting the mode converter  10  and the suffix “a”, and members constituting the mode converter  10   b  are given reference numerals that are made up of the reference numerals of the members constituting the mode converter  10  and the suffix “b”. Note that a strip-shaped conductor  16   a  is an aspect of a first strip-shaped conductor, and a strip-shaped conductor  16   b  is an aspect of a second strip-shaped conductor. 
     The RF module  1  is obtained by (1) forming the mode converters  10   a  and  10   b , which function as an input port and an output port, respectively, in the vicinity of the pair of short walls of the post-wall waveguide PW, (2) mounting the RFIC  21  on the strip-shaped conductor  16   a  of the mode converter  10   a , and (3) mounting the antenna  22  on the strip-shaped conductor  16   b  of the mode converter  10   b . The RF module  1  is an RF module in which the RFIC  21  and the antenna  22  are mounted on a first main surface of the substrate  15  (on a main surface on a side farther from the conductor layer  12 ), and a second main surface of the substrate  15  is mounted on a first main surface of the post-wall waveguide PW (on a surface of the conductor layer  12 ). 
     In the present embodiment, the following description will discuss only a configuration which has not been discussed above about the mode converter  10 , of the configurations of the mode converters  10   a  and  10   b . Specifically, the following description will discuss only, out of the tip parts of the strip-shaped conductor  16   a , a second tip part (a tip part on a side farther from the opening  121   a ) on a side opposite the first tip part that protrudes from the opening  121   a , and out of the tip parts of the strip-shaped conductor  16   b , a second tip part (a tip part on a side farther from the opening  121   b ) on a side opposite the first tip part that protrudes from the opening  121   b.    
     In the second tip part of the strip-shaped conductor  16   a , a strip-shaped part of the strip-shaped conductor  16   a  is connected to a signal conductor pad, which is wider than the strip-shaped part, for connecting to a signal terminal of the RFIC  21 . Further, on both sides of the signal conductor pad, two ground conductor pads for connecting to a ground terminal of the RFIC  21  are formed so as to sandwich the signal conductor pad therebetween. Each of the ground conductor pads is short-circuited to the conductor layer  12 . As described above, in the vicinity of the second tip part of the strip-shaped conductor  16   a , is formed a terminal which is for mounting the RFIC  21  and which has a GSG arrangement in which the conductor pads are arranged in the order of a ground conductor pad, a signal conductor pad, and a ground conductor pad. The RFIC  21  is mounted on the terminal of this GSG arrangement using bumps. 
     The antenna  22  is mounted on (connected to) the second tip part of the strip-shaped conductor  16   b . Note that in the present embodiment, an aspect of the antenna  22  is not limited, and can be appropriately selected from among antennas capable of constituting a radiating element using a conductor pattern. Thus, in  FIGS. 6 and 7 , any specific shape of the antenna  22  is not illustrated. As the antenna  22 , a patch antenna having a plurality of radiating elements each of which is constituted by a conductor pattern is suitable. In the present embodiment, the strip-shaped conductor  16   b  and the antenna  22  are realized as a single conductor pattern. The strip-shaped conductor  16   b  functions as a feed line. 
     As discussed above, the RF module  1  includes: the post-wall waveguide PW in which the opening  121   a , which is a first opening, is provided in the vicinity of a first short wall, which is one of the short walls of the conductor layer  12 , and the opening  121   b , which is a second opening, is provided in the vicinity of a second short wall, which is the other short wall of the conductor layer  12 ; the dielectric substrate  15  having the first main surface on which the strip-shaped conductor  16   a  and the antenna, which uses the strip-shaped conductor  16   a  as a feed line, are formed; the solder  18 , which is a joining member, joining the conductor layer  12  and the conductor layer  17  formed on the second main surface of the substrate  15  in a state in which the conductor layer  12  and the second main surface of the substrate  15  are parallel to or substantially parallel to each other; and the RFIC  21  being mounted on the first main surface of the substrate  15  and having terminals any of which is connected to a signal conductor pad which is connected to the second tip part of the strip-shaped conductor  16   a . In the RF module  1 , in a plan view of the post-wall waveguide PW, a portion of the opening  121   a  and a portion of the strip-shaped conductor  16   a  overlap each other, and a portion of the opening  121   b  and a portion of the strip-shaped conductor  16   b  overlap each other. 
     The RF module  1  functions as any of a transmission module, a reception module, and a transmission/reception module, depending on the functions of the RFIC  21  and the antenna  22 . 
     Further, an aspect of the present invention includes a mobile terminal that is provided with the RF module  1 . The RF module  1 , which includes the microstrip lines MSa and MSb, the mode converters  10   a  and  10   b , and the post-wall waveguide PW can suppress a transmission loss in a 28 GHz band as an operating band, and thus can be suitably used as an RF module for 5G. 
     Note that in the present embodiment, the configuration of the mode converter  10  is employed as the two mode converters provided in the RF module  1 . However, the configuration of the two mode converters provided in the RF module  1  is not limited to the configuration of the mode converter  10 , and may be any of the configurations discussed in the variations, or may be a configuration obtained by appropriately combining the configurations discussed in Embodiment 1 and/or the configurations discussed in the variations. 
     EXAMPLES 
     Example 1 
     The following description will discuss Example of the mode converter  10 B illustrated in  FIG. 4 , which is Example 1 of the present invention, with reference to  FIGS. 8 to 11 .  FIGS. 8 and 9  are a cross-sectional view and a plan view, respectively, illustrating the mode converter  10 B showing results of simulation of a conversion characteristic of the mode converter  10 B of Example 1.  FIG. 10  is a graph showing respective reflection characteristics and respective transmission characteristics of the mode converter  10 B of Example 1 and a mode converter of Reference Example.  FIG. 11  is an enlarged graph showing respective transmission characteristics of the mode converter  10 B of Example 1 and the mode converter of Reference Example. Note that the reflection characteristic means a frequency dependency of S-parameter S (1, 1), and the transmission characteristic means a frequency dependency of S-parameter S (2, 1). The same applies to Examples discussed later. 
     The mode converter  10 B of Example 1 employed the following design parameters, with the aim of realizing an operating band including a 28 GHz band in the mode converter  10 B illustrated in  FIG. 4 . In the present specification, a band of not less than 27 GHz and not more than 29.5 GHz is an operating band, and a mode converter is deemed to have excellent performance if the S-parameter S (1, 1) is below −20 dB and the S-parameter S (2, 1) is above −0.5 dB. The same applies to Examples discussed later, wherein the band of not less than 27 GHz and not more than 29.5 GHz is an operating band. Note that, of dot-and-dash lines shown in  FIGS. 10 and 11 , dot-and-dash lines parallel to a y-axis indicate 27 GHz and 29.5 GHz. Further, of the dot-and-dash lines shown in  FIG. 10 , a dot-and-dash line parallel to an x-axis indicates −20 dB, and, of the dot-and-dash lines shown in  FIG. 11 , a dot-and-dash line parallel to an x-axis indicates −0.5 dB. 
     (Design Parameters) 
     Substrate  11 : Made of quartz (T11=0.86 mm) 
     Post-wall waveguide PW: W1=4 mm 
     Substrate  15 : Megtron6 (registered trademark) (T15=300 μm) 
     Conductor layers  12 ,  13 , and  17 : made of copper (thickness of 18 μm) 
     Opening  121 : L2=3.2 mm, W2=400 μm, D1=100 μm, D2=D3=850 μm 
     Strip-shaped conductor  16 B: made of copper (thickness of 18 μm), L3=200 μm, W3=600 μm 
     Stubs  16 B 1  and  16 B 2 : shape of a rectangle having a long side of 600 μm in length and a short side of 300 μm in length 
     Solder  18 : TS=30 μm 
     Note that the mode converter of Reference Example is obtained by omitting the stubs  16 B 1  and  16 B 2  from the mode converter  10 B of Example 1. 
     Referring to  FIGS. 8 and 9 , it was found that the mode converter  10 B excellently converts between the quasi-TEM mode of the microstrip line MS and the TE mode of the post-wall waveguide PW through the opening  121 . 
     Referring to  FIGS. 10 and 11  for Reference Example, it was found that the reflection characteristic is above −20 dB in the entire band of not less than 24 GHz and not more than 32 GHz, and the transmission characteristic is below −0.5 GHz in the entire band of not less than 24 GHz and not more than 32 GHz. That is, it was found that the mode converter of Reference Example does not have excellent performance in the operating band of not less than 27 GHz and not more than 29.5 GHz. 
     On the other hand, in the mode converter  10 B of Example 1 in which the stubs  16 B 1  and  16 B 2  are added to the strip-shaped conductor  16 B, it was found that the S-parameter S (1, 1) is below −20 dB in the band of not less than 25.2 GHz and not more than 30.4 GHz as shown in  FIG. 10 , and the S-parameter S (2, 1) is above −0.5 dB in the band of not less than 26.3 GHz and not more than 32 GHz as shown in  FIG. 11 . That is, it was found that the mode converter  10 B of Example 1 has excellent performance in the operating band of not less than 27 GHz and not more than 29.5 GHz. 
     Examples 2 and 3 
     The following description will discuss Examples of the mode converters  10  illustrated in  FIGS. 1 and 2 , which are Examples 2 and 3 of the present invention, with reference to  FIGS. 12 and 13 .  FIG. 12  is a graph showing respective reflection characteristics and respective transmission characteristics of the mode converter  10  of Example 2 and the mode converter  10  of Example 3.  FIG. 13  is an enlarged graph showing a transmission characteristic of the mode converter  10  of Example 2 and a transmission characteristic of the mode converter  10  of Example 3. 
     The mode converter  10  of Example 2 employed the following design parameters. 
     (Design Parameters) 
     Substrate  11 : Made of quartz (T11=0.86 mm) 
     Post-wall waveguide PW: W1=4 mm 
     Substrate  15 : Megtron6 (registered trademark) (T15=300 μm) 
     Conductor layers  12 ,  13 , and  17 : made of copper (thickness of 18 μm) 
     Opening  121 : L2=3.2 mm, W2=400 μm, D1=100 μm, D2=D3=850 μm 
     Strip-shaped conductor  16 B: made of copper (thickness of 18 μm), L3=600 μm, W3=600 μm 
     Solder  18 : TS=30 μm 
     Further, the mode converter  10  of Example 3 was based on the mode converter  10  of Example 2 and was obtained by changing the thickness T15 of the substrate  15  from 300 μm to 100 μm and changing the thickness TS of the solder  18  to TS=150 μm. 
     Referring to  FIGS. 12 and 13  for Example 2, it was found that the reflection characteristic is below −20 dB in the entire band of not less than 24 GHz and not more than 32 GHz, and the transmission characteristic is above −0.5 GHz in the entire band of not less than 24 GHz and not more than 32 GHz. That is, it was found that Example 2 has excellent performance in the operating band of not less than 27 GHz and not more than 29.5 GHz. 
     Referring to  FIGS. 12 and 13  for Example 3, it was found that the reflection characteristic is below −20 dB in the entire band of not less than 24 GHz and not more than 32 GHz, and the transmission characteristic is above −0.5 GHz in the entire band of not less than 24 GHz and not more than 32 GHz. That is, it was found that Example 3 has excellent performance in the operating band of not less than 27 GHz and not more than 29.5 GHz. 
     Example Group 4 
     The following description will discuss Example Group 4 of the present invention, which is an example group of the mode converters  10  illustrated in  FIGS. 1 and 2 , with reference to  FIGS. 14 and 15 .  FIG. 14  is a graph showing respective reflection characteristics of the mode converters  10  of Example Group 4.  FIG. 15  is a graph showing respective transmission characteristics of the mode converters  10  of Example Group 4. 
     The mode converters  10  in Example Group 4 are based on the mode converter  10  of Example 3 as discussed above and are obtained by changing the thickness TS in the range of not less than 100 μm and not more than 300 μm. Thus, of the plots shown in  FIGS. 14 and 15 , the plots of TS=150 μm are the same as the plots of Example 3 shown in  FIGS. 12 and 13 . 
     For Example Group 4, referring to  FIG. 14 , the mode converter  10  such that the S-parameter S (1, 1) is below −20 dB in the band of not less than 27 GHz and not more than 29.5 GHz was the mode converter  10  having the thickness TS of not less than 150 μm and not more than 225 μm. 
     Further, for Example Group 4, referring to  FIG. 15 , the mode converter  10  such that the S-parameter S (2, 1) is above −0.5 GHz in the band of not less than 27 GHz and not more than 29.5 GHz was the mode converter  10  having the thickness TS of not less than 100 μm and not more than 250 μm. 
     From the above results, it was found that in Example Group 4, the mode converter  10  having the thickness TS of not less than 150 μm and not more than 225 μm has excellent performance in the operating band of not less than 27 GHz and not more than 29.5 GHz. 
     Example 5 
     The following description will discuss Example of the mode converter  10  illustrated in  FIGS. 1 and 2 , which is Example 5 of the present invention, with reference to  FIGS. 19 and 20 .  FIG. 19  and  FIG. 20  are graphs respectively showing a reflection characteristic of a mode converter  10  of Example 5 and a transmission characteristic thereof. 
     The mode converter  10  of Example 5 employed the following design parameters, with the aim of setting a band called E-band of not less than 71 GHz and not more than 86 GHz as an operating band in the mode converter  10  illustrated in  FIGS. 1 and 2 . Specifically, in the mode converter  10  of Example 5, 59.62 GHz was employed as a cutoff frequency. Note that a guide wavelength at 77.5 GHz, which is 1.5 times the cutoff frequency, is 3.1 mm. 
     Note that, as discussed earlier, in the present specification, the E-band is an operating band, and a mode converter is deemed to have excellent performance if the S-parameter S (1, 1) is below −20 dB and the S-parameter S (2, 1) is above −0.5 dB. The same applies to Examples discussed later, wherein the E-band is an operating band. Note that, of dot-and-dash lines shown in  FIGS. 19 and 20 , dot-and-dash lines parallel to a y-axis indicate 71 GHz and 86 GHz. Further, of the dot-and-dash lines shown in  FIG. 19 , a dot-and-dash line parallel to an x-axis indicates −20 dB, and, of the dot-and-dash lines shown in  FIG. 20 , a dot-and-dash line parallel to an x-axis indicates −0.5 dB. 
     (Design Parameters) 
     Substrate  11 : Made of quartz (T11=0.45 mm) 
     Post-wall waveguide PW: W1=1.54 mm 
     Substrate  15 : Megtron6 (registered trademark) (T15=125 μm) 
     Conductor layers  12  and  13 : made of copper (thickness of 10 μm) 
     Conductor layer  17 : made of copper (thickness of 18 μm) 
     Opening  121 : L2=1.4 mm, W2=150 μm, D1=75 μm, D2=D3=375 μm 
     Strip-shaped conductor  16 B: made of copper (thickness of 20 μm), L3=275 μm, W3=250 μm 
     Solder  18 : TS=30 μm 
     In the mode converter  10  of Example 5, it was found that the S-parameter S (1, 1) is below −20 dB in the band of not less than 69.8 GHz and not more than 88.2 GHz as shown in  FIG. 19 , and the S-parameter S (2, 1) is above −0.5 dB in the band of not less than 68.4 GHz and not more than 90 GHz as shown in  FIG. 20 . That is, it was found that the mode converter  10 B of Example 5 has excellent performance in the E-band which is the operating band. 
     Example Group 6 
     The following description will discuss Example Group 6 of the present invention, which is an example group of the mode converters  10  illustrated in  FIGS. 1 and 2 , with reference to  FIGS. 21 and 22 .  FIG. 21  is a graph showing respective reflection characteristics of the mode converters  10  of Example Group 6.  FIG. 22  is a graph showing respective transmission characteristics of the mode converters  10  of Example Group 6. 
     The mode converter  10  of Example Group 6 employed the following design parameters, with the aim of realizing an operating band including a 28 GHz band in the mode converter  10  illustrated in  FIGS. 1 and 2 . Specifically, in the mode converter  10  of Example Group 6, 19.68 GHz was employed as a cutoff frequency. Note that a guide wavelength at 28 GHz, which is 1.42 times the cutoff frequency, is 7.7 mm. 
     Note that, of dot-and-dash lines shown in  FIGS. 21 and 22 , dot-and-dash lines parallel to a y-axis indicate 27 GHz and 29.5 GHz. Further, of the dot-and-dash lines shown in  FIG. 21 , a dot-and-dash line parallel to an x-axis indicates −20 dB. 
     The mode converters  10  of Example Group 6 employed the following design parameters. The mode converters  10  of Example Group 6 are each obtained by changing the thickness T15 of the substrate  15  in a range of not less than 50 μm and not more than 300 μm. Further, with the change of the thickness T15 in the above range, the width W3 of the strip-shaped conductor  16  is changed so as to be twice the thickness T15. That is, in the mode converters  10  of Example Group 6, the width W3 is changed in a range of not less than 100 μm and not more than 600 μm. Thus, by determining the width W3 so that the width W3 is twice the thickness T15, an input impedance of the microstrip line MS can be set to approximately 50Ω. 
     (Design Parameters) 
     Substrate  11 : Made of quartz (T11=0.86 mm) 
     Post-wall waveguide PW: W1=4 mm 
     Substrate  15 : Megtron6 (registered trademark) (T15=50 μm, 100 μm, 200 μm, and 300 μm) 
     Conductor layers  12  and  13 : made of copper (thickness of 10 μm) 
     Conductor layer  17 : made of copper (thickness of 18 μm) 
     Opening  121 : L2=3.2 mm, W2=400 μm, D1=100 μm, D2=D3=850 μm 
     Strip-shaped conductor  16 B: made of copper (thickness of 18 μm), L3=600 μm, W3=100 μm, 200 μm, 400 μm, and 600 μm 
     Solder  18 : TS=30 μm 
     In each of the mode converters  10  in Example Group 6, it was found that the S-parameter S (1, 1) is below −20 dB in the operating band of not less than 27 GHz and not more than 29.5 GHz as shown in  FIG. 21 , and the S-parameter S (2, 1) is above −0.5 dB in the band of not less than 27 GHz and not more than 29.5 GHz as shown in  FIG. 22 . That is, it was found that each of the mode converters  10  of Example Group 6 has excellent performance in the above operating band. 
     &lt;Fourth Variation&gt; 
     The following description will discuss, with reference to  FIG. 23 , a mode converter  10 D in accordance with Fourth Variation of the present invention, which is a variation of the mode converter  10  illustrated in  FIGS. 1 and 2 .  FIG. 23  is a cross-sectional view illustrating the mode converter  10 D. 
     The mode converter  10 D is based on the mode converter  10  and is obtained by arranging such that the position of an edge of the conductor layer  17  on a side closer to the opening  121  out of edges of the conductor layer  17  is different from the position of an edge of a solder  18 D, which corresponds to the solder  18 , on a side closer to the opening  121  out of edges of the solder  18 D. Note that in Fourth Variation as well, the shortest distance between the central axes of the through-vias  14   i  constituting the short wall  14   c  and the conductor layer  17  is referred to as a distance D2, and the shortest distance between the central axes of the through-vias  14   i  constituting the short wall  14   c  and the solder  18  is referred to as a distance D3. In the mode converter  10 D illustrated in  FIG. 23 , the distance D2 and the distance D3 are determined so as to satisfy D3=D2+ΔD, that is, D3&gt;D2, on the assumption that a difference ΔD is positive. 
     In an aspect of the present invention, the distance D2 and the distance D3 may differ from each other as in the mode converter  10 D. Further, the description in Fourth Variation has discussed a case where the distance D2 and the distance D3 satisfy D3&gt;D2. However, in an aspect of the mode converter  10 D, the distance D2 and the distance D3 may be arranged so as to satisfy D3&lt;D2. 
     Example Group 7 
     The following description will discuss, with reference to  FIG. 24 , mode converters  10 D in accordance with Example Group 7 of the present invention, which is an example group of Fourth Variation illustrated in  FIG. 23 .  FIG. 24  is a graph showing respective reflection characteristics of the mode converters  10 D of Example Group 7. Note that the mode converters  10 D of Example Group 7 are designed with the aim of using the E-band as the operating band. 
     The mode converters  10 D of Example Group 7 are based on the mode converter  10  of Example 5 and are obtained by employing, as the difference ΔD, ΔD=100 μm and 200 μm. Note that the plot of ΔD=0 μm shown in  FIG. 24  is the reflection characteristic of the mode converter  10  of Example 5. 
     Referring to  FIG. 24 , it was found that the larger the difference ΔD, the narrower the bandwidth of the operating band. In other words, it is preferable that the difference ΔD is small in order to realize an operating band having as a wide bandwidth as possible. 
     Example Group 8 
     The following description will discuss, with reference to  FIG. 25 , mode converters  10 D in accordance with Example Group 8 of the present invention, which is an example group of Fourth Variation illustrated in  FIG. 23 .  FIG. 25  is a graph showing a reflection characteristic of the mode converters  10 D of Example Group 8. Note that the mode converters  10 D of Example Group 8 are designed with the aim of realizing an operating band including a 28 GHz band. 
     The mode converters  10 D of Example Group 8 are based on the mode converter  10  in which the thickness T15 of the substrate  15  is 300 μm among the mode converters  10  of Example Group 6, and are obtained by employing, as the difference ΔD, ΔD=100 μm and 200 μm. 
     Referring to  FIG. 25 , it was found that the larger the difference ΔD, the narrower the bandwidth of the operating band. In other words, it is preferable that the difference ΔD is small in order to realize an operating band having as a wide bandwidth as possible. 
     Example 9 
     The following description will discuss Example of the mode converter  10  illustrated in  FIGS. 1 and 2 , which is Example 9 of the present invention, with reference to  FIGS. 26 and 27 .  FIG. 26  is a graph showing a reflection characteristic of a mode converter  10  of Example 9.  FIG. 27  is a graph showing a transmission characteristic of the mode converter  10  of Example 9. 
     The mode converter  10  of Example Group 9 employed the following design parameters, with the aim of realizing an operating band including a 32 GHz band and having as a wide bandwidth as possible in the mode converter  10  illustrated in  FIGS. 1 and 2 . Specifically, in the mode converter  10  of Example 9, 21.3 GHz was employed as a cutoff frequency. Note that a guide wavelength at 32 GHz, which is 1.5 times the cutoff frequency, is 9.38 mm. 
     (Design Parameters) 
     Substrate  11 : Made of quartz (T11=0.86 mm) 
     Post-wall waveguide PW: W1=4 mm 
     Substrate  15 : Megtron6 (registered trademark) (T15=100 μm) 
     Conductor layers  12  and  13 : made of copper (thickness of 10 μm) 
     Conductor layer  17 : made of copper (thickness of 20 μm) 
     Opening  121 : L2=3.2 mm, W2=400 μm, D1=100 μm, D2=D3=850 μm 
     Strip-shaped conductor  16 B: made of copper (thickness of 20 μm), L3=600 μm, W3=200 μm 
     Solder  18 : TS=30 μm 
     In the mode converters  10  of Example 9, it was found that the S-parameter S (1, 1) is below −20 dB in the band of not less than 21.5 GHz and not more than 44 GHz as shown in  FIG. 26 , and the S-parameter S (2, 1) is above −0.5 dB in the band of not less than 22 GHz and not more than 44.8 GHz as shown in  FIG. 27 . That is, it was found that the mode converters  10  of Example Group 9 have excellent performance in the band of not less than 22 GHz and not more than 44 GHz. 
     &lt;Fifth Variation&gt; 
     The following description will discuss, with reference to  FIG. 28 , a mode converter  10 E in accordance with Fifth Variation of the present invention, which is a variation of the mode converter  10  illustrated in  FIGS. 1 and 2 .  FIG. 28  is a plan view illustrating the mode converter  10 E. 
     The mode converter  10 E is based on the mode converter  10  and is obtained by replacing the strip-shaped conductor  16  with a strip-shaped conductor  16 E. 
     The strip-shaped conductor  16 E is a rectangular conductor pattern formed in the same manner as the strip-shaped conductor  16 . The strip-shaped conductor  16 E is different in position on the first main surface of the substrate  15 , as compared with the strip-shaped conductor  16 . Specifically, in a plan view of the first main surface of the substrate  15 , the strip-shaped conductor  16  is arranged such that the central axis of the strip-shaped conductor  16  coincides with a B-B′ line, which is a straight line that is parallel to the lengthwise direction of the post-wall waveguide PW and that is a set of points equally dividing the width W1 of the post-wall waveguide PW. On the other hand, in a plan view of the first main surface of the substrate  15 , the strip-shaped conductor  16 E is arranged so as to be displaced in parallel by a gap ΔG from the B-B′ line. In  FIG. 28 , a straight line that coincides with the central axis of the strip-shaped conductor  16 E is indicated by a C-C′ line. 
     In such a manner, the central axis of the strip-shaped conductor  16 E can be provided so as to be displaced from the B-B′ line, so that the mode converter  10 E can increase the degree of freedom of the arrangement of the strip-shaped conductor  16 E. 
     Example Group 10 
     The following description will discuss, with reference to  FIGS. 29 and 30 , mode converters  10 E in accordance with Example Group 10 of the present invention, which is an example group of the mode converter  10 E illustrated in  FIG. 28 .  FIG. 29  is a graph showing respective reflection characteristics of the mode converters  10 E of Example Group 10.  FIG. 30  is a graph showing respective transmission characteristics of the mode converters  10 E of Example Group 10. 
     The mode converters  10 E of Example Group 10 employed the following design parameters, with the aim of realizing an operating band including a 28 GHz band in the mode converter  10 E illustrated in  FIG. 28 . The mode converters  10 E of Example Group 10 are obtained by changing the gap ΔG in a range of not less than 0 μm and not more than 900 μm. 
     (Design Parameters) 
     Substrate  11 : Made of quartz (T11=0.86 mm) 
     Post-wall waveguide PW: W1=4 mm 
     Substrate  15 : Megtron6 (registered trademark) (T15=100 μm) 
     Conductor layers  12  and  13 : made of copper (thickness of 10 μm) 
     Conductor layer  17 : made of copper (thickness of 18 μm) 
     Opening  121 : L2=3.2 mm, W2=400 μm, D1=100 μm, D2=D3=850 μm 
     Strip-shaped conductor  16 B: made of copper (thickness of 18 μm), L3=600 μm, W3=200 μm, ΔG=0, 100, 200, 300, 400, 500, 600, 700, 800, 900 μm 
     Solder  18 : TS=30 μm 
     Referring to  FIG. 29 , in the mode converter  10 E of Example Group 10, it was found that (1) in a case where the gap ΔG is not less than 0 μm and not more than 500 μm, the S-parameter S (1, 1) is below −20 dB in the operating band of not less than 27 GHz and not more than 29.5 GHz, that (2) in a case where the gap ΔG is 600 μm, the S-parameter S (1, 1) is above −20 dB in a part of the above operating band, and that (3) in a case where the gap ΔG is not less than 700 μm and not more than 900 μm, the S-parameter S (1, 1) is above −20 dB in the above operating band. 
     Further, referring to  FIG. 30 , in the mode converter  10 E of Example Group 10, it was found that (4) in a case where the gap ΔG is not less than 0 μm and not more than 800 μm, the S-parameter S (2, 1) is above −0.5 dB in the above operating band, and that (5) in a case where the gap ΔG is 900 μm, the S-parameter S (2, 1) is below −0.5 dB in the above operating band. 
     From the above results, it was found that it is preferable that the mode converters  10 E of Example Group 10 employ, as the gap ΔG, a value included in the range of not less than 0 μm and not more than 500 μm, to exhibit excellent performance in the above operating band. 
     &lt;Sixth Variation&gt; 
     The following description will discuss, with reference to  FIG. 31 , a mode converter  10 F in accordance with Sixth Variation of the present invention, which is a variation of the mode converter  10 E illustrated in  FIG. 28 .  FIG. 31  is a plan view illustrating the mode converter  10 F. 
     The mode converter  10 F is based on the mode converter  10 E and is obtained by replacing the opening  121  with an opening  121 F. Note that the strip-shaped conductor  16 F included in the mode converter  10 F has the same configuration as the strip-shaped conductor  16 E included in the mode converter  10 E. 
     The opening  121 F is an opening provided in the conductor layer  12  that constitutes a first wide wall of the post-wall waveguide PW, as with the opening  121 , but the shape of the opening  121 F differs from that of the opening  121 . Specifically, in a plan view of the first main surface of the substrate  15 , the opening  121  is in the shape of a rectangle having a long side parallel to the short wall  14   c  and a short side parallel to the narrow walls  14   a  and  14   b . On the other hand, in the present variation, the opening  121 F is in the shape of an isosceles trapezoid in which respective portions of a pair of short sides of the opening  121  correspond to a pair of bases (upper base and lower base). Note that the shape of the opening  121 F only needs to be at least a trapezoidal shape and is preferably an isosceles trapezoidal shape. 
     Hereinafter, a minimum value (that is, a length of the upper base in the arrangement illustrated in  FIG. 31 ) of the width (a length along the short side) of the opening  121 F is referred to as a width W2a, and a maximum value (that is, a length of the lower base in the arrangement illustrated in  FIG. 31 ) of the width of the opening  121 F is referred to as a width W2b. 
     Among two regions of the mode converter  10 F into which the mode converter  10 F is divided by a B-B′ line, which serves as a boundary and is a set of points equally dividing the width W1 of the post-wall waveguide PW, (1) a region on a side where the central axis (which can be said to be a C-C′ line) of the strip-shaped conductor  16 F is provided (a lower region in the arrangement illustrated in  FIG. 31 ) is referred to as a first region, and (2) a region on a side where the central axis of the strip-shaped conductor  16 F is not provided (an upper region in the arrangement illustrated in  FIG. 31 ) is referred to as a second region. The opening  121 F is provided in the conductor layer  12  such that a longer base, out of a pair of bases of the opening  121 F, is located in the first region, while a shorter base is located in the second region. In other words, a distance from the central axis of the strip-shaped conductor  16 F to the upper base exceeds a distance from the central axis of the strip-shaped conductor  16 F to the lower base. Note that in the arrangement illustrated in  FIG. 31 , the upper base is a shorter base out of the pair of bases, and the lower base is a longer base out of the pair of bases. 
     The width W2a and the width W2b can be set as appropriate according to the operating band, the gap ΔG, and the like. However in the present embodiment, W2a=100 μm and W2b=400 μm are employed as the widths W2a and W2b, respectively. 
     The arrangement in which the central axis of the strip-shaped conductor  16 E is provided so as to be displaced from the B-B′ line as in the mode converter  10 E illustrated in  FIG. 28 , may deteriorate the reflection characteristic and the transmission characteristic, as compared with the arrangement in which the central axis coincides with the B-B′ line. In the mode converter  10 F, the strip-shaped conductor  16 F is provided such that the central axis of the strip-shaped conductor  16 F is displaced from the B-B′ line, and the opening  121 F shaped in an isosceles trapezoid is employed. Thus, it is possible to suppress the deterioration of the reflection characteristic and transmission characteristic that may be caused by the arrangement in which the central axis of the strip-shaped conductor  16 F is provided so as to be displaced from the B-B′ line. That is, the mode converter  10 F can suppress the deterioration of the reflection characteristic and transmission characteristic while increasing the degree of freedom of the arrangement of the strip-shaped conductor  16 F. 
     Example Group 11 
     The following description will discuss, with reference to  FIGS. 32 and 33 , mode converters  10 F in accordance with Example Group 11 of the present invention, which is an example group of the mode converter  10 F illustrated in  FIG. 31 .  FIG. 32  is a graph showing respective reflection characteristics of the mode converters  10 F of Example Group 11.  FIG. 33  is a graph showing respective transmission characteristics of the mode converters  10 F of Example Group 11. 
     The mode converter  10 F of Example Group 11 is based on the mode converter  10 E having ΔG=600 μm out of the mode converters  10 E in accordance with Example Group 10, with the aim of realizing an operating band including a 28 GHz band in the mode converter  10 F illustrated in  FIG. 31 . Then, the mode converter  10 F of Example Group 11 is obtained by fixing the width W2b to 400 μm and changing the width W2a in a range of not less than 50 μm and not more than 400 μm. Note that of the plots shown in  FIGS. 32 and 33 , the plot of W2a=400 μm corresponds to the mode converter  10 E having ΔG=600 μm. 
     Referring to  FIG. 32 , in the mode converter  10 F of Example Group 11, it was found that the reflection characteristics are improved by reducing the width W2a from 400 μm to 50 μm. More specifically, in the mode converter  10 F of Example Group 11, it was found that (1) in a case where the width W2a is not less than 50 μm and not more than 300 μm, the S-parameter S (1, 1) is below −20 dB in the operating band of not less than 27 GHz and not more than 29.5 GHz, and that (2) in a case where the width W2a is not less than 350 μm and not more than 400 μm, the S-parameter S (1, 1) is above −20 dB in a part of the above operating band. 
     Further, referring to  FIG. 33 , in the mode converters  10 F of Example Group 11, it was found that the transmission characteristics are improved by reducing the width W2a from 400 μm to 50 μm. Note that in the mode converters  10 F of Example Group 11, in a case where the width W2a is not less than 50 μm and not more than 400 μm, the S-parameter S (2, 1) is above −0.5 dB in the above operating band. 
     From the above results, it was found that it is preferable that the mode converters  10 F of Example Group 11 employ, as the width W2a, a value included in the range of not less than 50 μm and not more than 300 μm, to exhibit excellent performance in the above operating band. 
     Example Group 12 
     The following description will discuss, with reference to  FIGS. 34 and 35 , mode converters  10 F in accordance with Example Group 12 of the present invention, which is an example of the mode converter  10 F illustrated in  FIG. 31 .  FIG. 34  is a graph showing respective reflection characteristics of the mode converters  10 F of Example Group 12.  FIG. 35  is a graph showing respective transmission characteristics of the mode converters  10 F of Example Group 12. 
     The mode converter  10 F of Example 12 is based on the mode converter  10 E having ΔG=700 μm out of the mode converters  10 E in accordance with Example Group 10, with the aim of realizing an operating band including a 28 GHz band in the mode converter  10 F illustrated in  FIG. 31 . Then, the mode converter  10 F of Example Group 12 is obtained by fixing the width W2b to 400 μm and employing 50 μm and 400 μm as the width W2a. Note that of the plots shown in  FIGS. 34 and 35 , the plot of W2a=400 μm corresponds to the mode converter  10 E having ΔG=700 μm. 
     Referring to  FIG. 34 , in the mode converter  10 F of Example Group 12, it was found that the reflection characteristics are improved by employing W2a=50 μm, as compared with the mode converter  10 F employing W2a=400 μm. More specifically, in the mode converter  10 F of Example Group 12, it was found that (1) in a case where the width W2a is 50 μm, the S-parameter S (1, 1) is below −20 dB in the operating band of not less than 27 GHz and not more than 29.5 GHz, and that (2) in a case where the width W2a is 400 μm, the S-parameter S (1, 1) is above −20 dB in a part of the above operating band. 
     Further, referring to  FIG. 35 , in the mode converters  10 F of Example Group 12, it was found that the transmission characteristics are improved by employing W2a=50 μm, as compared with the mode converter  10 F employing W2a=400 μm. In the mode converters  10 F of Example Group 12, in a case where the width W2a is not less than 50 μm and not more than 400 μm, the S-parameter S (2, 1) is above −0.5 dB in the above operating band. 
     From the above results, it was found that it is preferable that the mode converters  10 F of Example Group 12 employ, as the width W2a, a value included in the range of not less than 50 μm and not more than 400 μm, to exhibit excellent performance in the above operating band. 
     Embodiment 3 
     The following description will discuss, with reference to  FIG. 36 , an RF module  1 F in accordance with Embodiment 3 of the present invention.  FIG. 36  is a plan view illustrating the RF module  1 F. 
     As illustrated in  FIG. 36 , the RF module  1 F includes a mode converter  10 Fa and a mode converter  10 Fb, which are a pair of mode converters, and an RFIC  21 F. The RFIC  21 F is mounted on a first main surface of a substrate  15 F, which includes a microstrip line MSFa of the mode converter  10 Fa and a microstrip line MSFb of the mode converter  10 Fb, wherein the first main surface is a main surface on a side farther from a post-wall waveguide PWFa of the mode converter  10 Fa and a post-wall waveguide PWFb of the mode converter  10 Fb. 
     The RFIC  21 F includes an output port and an input port. The output port is composed of a signal terminal E1a and ground terminals E2a and E3a which are arranged such that the signal terminal E1a is sandwiched between the ground terminals E2a and E3a. The input port is composed of a signal terminal E1b and ground terminals E2b and E3b which are arranged such that the signal terminal E1b is sandwiched between the ground terminals E2b and E3b. That is, each of the output port and the input port is a port of a GSG arrangement in which the terminals are arranged in the order of the ground terminal, the signal terminal, and the ground terminal. 
     &lt;Configuration of Mode Converters  10 Fa and  10 Fb&gt; 
     Each of the mode converters  10 Fa and  10 Fb is a specific example of the mode converter  10 F illustrated in  FIG. 31 , and is configured in the same manner as the mode converter  10 F. Thus, in the present embodiment, the description regarding the mode converters  10 Fa and  10 Fb will only provide a correspondence between members constituting the mode converters  10 Fa and  10 Fb and the members constituting the mode converter  10 F, and explanation of the members constituting the mode converters  10 Fa and  10 Fb will be omitted. 
     As illustrated in  FIG. 36 , the mode converters  10 Fa and  10 Fb include respective post-wall waveguides PWFa and PWFb, and respective microstrip lines MSFa and MSFb. 
     (Post-Wall Waveguides PWFa and PWFb) 
     The post-wall waveguide PWFa and the post-wall waveguide PWFb are arranged such that their respective extension directions are parallel to each other and the post-wall waveguide PWFa and the post-wall waveguide PWFb are close to each other. The post-wall waveguide PWFa and the post-wall waveguide PWFb are electromagnetically coupled in a region not illustrated in  FIG. 36  to form one post-wall waveguide PWF. Thus, the post-wall waveguide PWFa is one end region of the post-wall waveguide PWF, and the post-wall waveguide PWFb is the other end region of the post-wall waveguide PWF. The shape of the post-wall waveguide PWF in a plan view is not particularly limited and can be determined as appropriate. 
     The post-wall waveguide PWFa includes a short wall  14   ac , which is one short wall of a pair of short walls that constitute the post-wall waveguide PWF. The post-wall waveguide PWFb includes a short wall  14   bc , which is the other short wall of the pair of short walls that constitute the post-wall waveguide PWF. In the post-wall waveguide PWFa, an opening  121 Fa (an example of the first opening) is provided in the vicinity of the short wall  14   ac  in the conductor layer  12 F constituting a first wide wall of the post-wall waveguide PWFa. In the post-wall waveguide PWFb, an opening  121 Fb (an example of the second opening) is provided in the vicinity of the short wall  14   bc  in the conductor layer  12 F constituting a first wide wall of the post-wall waveguide PWFb. The openings  121 Fa and  121 Fb correspond to the opening  121 F in the mode converter  10 F. 
     The openings  121 Fa and  121 Fb are shaped in an isosceles trapezoid. Note that the openings  121 Fa and  121 Fb are shaped in a trapezoid. In a plan view of the post-wall waveguide PWF, the opening  121 Fa is arranged such that a pair of bases forming the shape of an isosceles trapezoid are parallel to a narrow wall  14 Faa and a narrow wall  14 Fabb constituting the post-wall waveguide PWFa, and such that a distance from a central axis of a strip-shaped conductor  16 Fa (discussed later) to a shorter base out of the pair of bases of the opening  121 Fa exceeds a distance from the central axis of the strip-shaped conductor  16 Fa to a longer base out of the pair of bases of the opening  121 Fa. Further, in a plan view of the post-wall waveguide PWF, the opening  121 Fb is arranged such that a pair of bases forming the shape of an isosceles trapezoid are parallel to a narrow wall  14 Fba and the narrow wall  14 Fabb constituting the post-wall waveguide PWFb, and such that a distance from a central axis of a strip-shaped conductor  16 Fb (discussed later) to a shorter base out of the pair of bases of the opening  121 Fb exceeds a distance from the central axis of the strip-shaped conductor  16 Fb to a longer base out of the pair of bases of the opening  121 Fb. Further, the longer base out of the pair of bases of the opening  121 Fa is on a side closer to the narrow wall  14 Fabb, and the longer base out of the pair of bases of the opening  121 Fb is on a side closer to the narrow wall  14 Fabb. 
     The post-wall waveguides PWFa and PWFb and the microstrip lines MSFa and MSFb are joined with use of a solder (not illustrated in  FIG. 36 ). This solder is an example of a joining member that short-circuits and joins the conductor layer  12 F and the conductor layer  17 F, and corresponds to the solder  18  provided in the mode converter  10 . 
     The post-wall waveguide PWF has a cutoff frequency that depends on the shape and size of the waveguide region. Thus, the post-wall waveguide PWF functions as a high-pass filter. Further, in a case where a plurality of resonators that are electromagnetically coupled to each other are provided inside the waveguide region, the post-wall waveguide PWF functions as a bandpass filter. Thus, the RF module  1 F can (1) subject, to a predetermined filtering process, a high frequency supplied from the output port of the RFIC  21 F to the post-wall waveguide PWF via the microstrip line MSFa, and then (2) supply the high frequency subjected to the filtering process from the post-wall waveguide PWF to the input port of the RFIC  21 F via the microstrip line MSFb. 
     The post-wall waveguide PWF including the post-wall waveguides PWFa and PWFb includes a substrate  11 F and the conductor layers  12 F and  13 F. The substrate  11 F and the conductor layers  12 F and  13 F are common members in the post-wall waveguide PWF including the post-wall waveguides PWFa and PWFb. Further, each of the post-wall waveguides PWFa and PWFb includes post walls  14 Fa and  14 Fb, respectively. The substrate  11 F corresponds to the substrate  11  of the mode converter  10 , the conductor layers  12 F and  13 F correspond to the conductor layers  12  and  13  of the mode converter  10 , and the post walls  14 Fa and  14 Fb correspond to the post wall  14  of the mode converter  10 . Note that the conductor layer  13 F is not illustrated in  FIG. 36 . 
     In the RF module  1 F, the post-wall waveguides PWFa and PWFb are arranged so that the directions in which the respective waveguide regions are extended are parallel to each other and an interval between the post-wall waveguides PWFa and PWFb are as narrow as possible. Thus, the post-wall waveguides PWFa and PWFb share the narrow wall  14 Fabb that separates the post-wall waveguides PWFa and PWFb out of the narrow walls constituting the post walls  14 Fa and  14 Fb. The narrow wall  14 Faa and the narrow wall  14 Fabb, which form the post wall  14 Fa together with the short wall  14 Fac, are parallel to each other. Further, the narrow wall  14 Fba and the narrow wall  14 Fabb, which form the post wall  14 Fb together with the short wall  14 Fbc, are parallel to each other. 
     (Microstrip Lines MSFa and MSFb) 
     The microstrip lines MSFa and MSFb include the substrate  15 F and the conductor layer  17 F. The substrate  15 F and the conductor layer  17 F are common members in the microstrip lines MSFa and MSFb. The microstrip lines MSFa and MSFb include the strip-shaped conductors  16 Fa and  16 Fb, respectively. The substrate  15 F corresponds to the substrate  15  of the mode converter  10 , the conductor layer  17 F corresponds to the conductor layer  17  of the mode converter  10 , and the strip-shaped conductors  16 Fa and  16 Fb correspond to the strip-shaped conductor  16  of the mode converter  10 . Further, the strip-shaped conductors  16 Fa and  16 Fb are an example of a third strip-shaped conductor and an example of a fourth strip-shaped conductor, respectively. 
     In the mode converter  10 Fa, the mode of the microstrip line MSFa and the mode of the post-wall waveguide PWFa are coupled via a region in which a portion of the strip-shaped conductor  16 Fa and a portion of the opening  121 Fa overlap each other in a plan view. Similarly, in the mode converter  10 Fb, the mode of the microstrip line MSFb and the mode of the post-wall waveguide PWFb are coupled via a region in which a portion of the strip-shaped conductor  16 Fb and a portion of the opening  121 Fb overlap each other in a plan view. That is, the mode converters  10 Fa and  10 Fb can each convert between these modes via the openings  121 Fa and  121 Fb which do not directly contact the strip-shaped conductor  16 Fa and  16 Fb, without using an excitation pin. 
     In the RF module  1 F, the mode converters  10 Fa and  10 Fb are configured so as to be mirror symmetrical about a plane, as a plane of symmetry, including the central axes of the through-vias constituting the narrow wall  14 Fabb. 
     As with the B-B′ line indicated in  FIG. 31 , a B-B′ line indicated in  FIG. 36  is a straight line that is a set of points equally dividing the width of the post-wall waveguide PWFa. As with the C-C′ line indicated in  FIG. 31 , a C-C′ line indicated in  FIG. 36  is a straight line that coincides with the central axis of the strip-shaped conductor  16 Fa. A D-D′ line indicated in  FIG. 36  is a straight line that is a set of points equally dividing the width of the post-wall waveguide PWFb. An E-E′ line indicated in  FIG. 36  is a straight line that coincides with the central axis of the strip-shaped conductor  16 Fb. 
     As illustrated in  FIG. 36 , the strip-shaped conductors  16 Fa and  16 Fb are arranged so as to be displaced in parallel by gaps ΔGa and ΔGb, respectively, in the direction in which the distance between the central axes of the strip-shaped conductors  16 Fa and  16 Fb approach each other, with reference to the position of the B-B′ line and the position of the D-D′ line, respectively. That is, in a plan view of the post-wall waveguide PWF, both the central axis of the strip-shaped conductor  16 Fa and the central axis of the strip-shaped conductor  16 Fb are located between the B-B′ line and the D-D′ line. Note that in the RF module  1 F, ΔGa=ΔGb. Thus, hereinafter, the gaps ΔGa and ΔGb are also simply referred to as a gap ΔG. 
     &lt;Configuration for Mounting RFIC  21 F&gt; 
     Out of a pair of tip parts of the strip-shaped conductor  16 Fa, a tip part which is close to the opening  121 Fa and protrudes from the opening  121 Fa is referred to as a first tip part, and a tip part on a side farther from the opening  121 Fa is referred to as a second tip part. The second tip part of the strip-shaped conductor  16 Fa functions as a signal conductor pad for connecting to a signal terminal E1a of the RFIC  21 F. On both sides of the second tip part of the strip-shaped conductor  16 Fa, ground conductor pads G2a and G3a for connecting to ground terminals E2a and E3a of the RFIC  21 F are formed so as to sandwich the second tip part therebetween. Each of the ground conductor pads G2a and G3a is short-circuited to the conductor layer  12 F. As described above, in the second tip part of the strip-shaped conductor  16 Fa and in the vicinity of the second tip part of the strip-shaped conductor  16 Fa, is formed a terminal which is for connecting to an output port of the RFIC  21 F and which has a GSG arrangement in which the conductor pads are arranged in the order of a ground conductor pad, a signal conductor pad, and a ground conductor pad. 
     The second tip part of the strip-shaped conductor  16 Fa is connected to the signal terminal E1a with use of a bump B1a, and the ground conductor pads G2a and G3a are connected to the respective ground terminals E2a and E3a with use of respective bumps B2a and B3a. 
     Similarly, out of a pair of tip parts of the strip-shaped conductor  16 Fb, a tip part which is close to the opening  121 Fb and protrudes from the opening  121 Fb is referred to as a first tip part, and a tip part on a side farther from the opening  121 Fb is referred to as a second tip part. In the second tip part of the strip-shaped conductor  16 Fb and in the vicinity of the second tip part of the strip-shaped conductor  16 Fb, is formed a terminal which is for connecting to an input port of the RFIC  21 F and which has a GSG arrangement. The terminal having the GSG arrangement is constituted by the second tip part of the strip-shaped conductor  16 Fb and ground conductor pads G2b and G3b which are arranged so as to sandwich the second tip part therebetween. The second tip part of the strip-shaped conductor  16 Fb is connected to the signal terminal E1b with use of a bump B1b, and the ground conductor pads G2b and G3b are connected to the respective ground terminals E2b and E3b with use of respective bumps B2b and B3b. 
     According to the RF module  1 F, the configuration of the RF module  1 F makes it possible to narrow, by 2 ΔG, an interval between the central axes of the strip-shaped conductors  16 Fa and  16 Fb, as compared with the configuration in which the strip-shaped conductor  16 Fa is provided such that the central axis of the strip-shaped conductor  16 Fa coincides with the B-B′ line, and the strip-shaped conductor  16 Fb is provided such that the central axis of the strip-shaped conductor  16 Fb coincides with the D-D′ line. Thus, even in a case where the RFIC  21 F, in which an interval between the signal terminal E1a constituting the output port and the signal terminal E1b constituting the input port is narrower than the interval between the B-B′ line and the D-D′ line (that is, the widths of the post-wall waveguides PWFa and PWFb), is mounted on the microstrip lines MSFa and MSFb, the RFIC  21 F can be easily mounted. 
     &lt;Seventh Variation&gt; 
     The following description will discuss, with reference to  FIG. 37 , a mode converter  10 G in accordance with Seventh Variation of the present invention, which is a variation of the mode converter  10  illustrated in  FIGS. 1 and 2 .  FIG. 37  is a cross-sectional view illustrating the mode converter  10 G. 
     The mode converter  10 G is based on the mode converter  10  and is obtained by adding a connector  19 G to the mode converter  10 . The connector  19 G may be any connector that can transmit high frequencies included in a desired operating band, and can be appropriately selected from, for example, commercially available connectors. Thus, in the mode converter  10 G, the internal structure of the connector  19 G is not limited. Therefore,  FIG. 37  omits the illustration of the internal structure of the connector  19 G. In the present variation, a jack-side connector is employed, as the connector  19 G, out of the jack-side connector and a plug-side connector both of which constitute a pair of coaxial connectors. 
     The connector  19 G is fixed to a substrate  15 G at an end portion of the microstrip line MS on a side opposite an end portion coupled to the post-wall waveguide PW via the opening  121 . 
     The connector  19 G, which is a coaxial connector, includes a central conductor and an outer conductor. The central conductor is electrically connected to the strip-shaped conductor  16  of the microstrip line MS. The outer conductor is also called a shell and is electrically connected to the conductor layer  17  of the microstrip line MS. 
     The mode converter  10 G having such a configuration can convert, via the microstrip line MS, between the mode coupled to the connector  19 G from outside and the mode of the post-wall waveguide PW. Moreover, the mode converter  10 G does not use the excitation pin provided in the conventional mode converter, and thus brings about the same effect as the effect brought about by the mode converter  10 . 
     Aspects of the present invention can also be expressed as follows: 
     In order to solve the above problem, a mode converter in accordance with a first aspect of the present invention includes: a post-wall waveguide in which an opening is provided in a first wide wall; a dielectric substrate having a first main surface and a second main surface on at least one of which a strip-shaped conductor is formed; and a joining member joining directly or indirectly the first wide wall and the substrate, wherein at least a portion of the opening and at least a portion of the strip-shaped conductor overlap each other in a plan view of the post-wall waveguide. 
     According to the above configuration, it is possible to couple, via the opening provided in the first wide wall of the post-wall waveguide, the mode of the post-wall waveguide and the mode of the line in which the strip-shaped conductor formed on another substrate different from the substrate of the post-walled waveguide serves as a signal line. That is, the present mode converter can convert between the mode of the post-wall waveguide and the mode of the line in which the strip-shaped conductor formed on another substrate different from the substrate of the post-walled waveguide serves as a signal line, without using an excitation pin provided in the conventional mode converter. Thus, the present mode converter is less likely to cause a transmission failure resulting from a change in environmental temperature than the conventional mode converter. 
     In a second aspect of the present invention, the mode converter is configured such that, in the first aspect of the present invention, the strip-shaped conductor constitutes a signal line of a TEM line or a quasi-TEM line. 
     Thus, the present mode converter is suitable as a mode converter that converts between the mode of the TEM line or the quasi-TEM line and the mode of the post-wall waveguide. 
     In a third aspect of the present invention, the mode converter is configured such that, in the second aspect of the present invention, the TEM line or the quasi-TEM line is any of a microstrip line, a strip line, a coplanar line, a grounded coplanar line, and a parallel two-wire line. 
     Thus, the present mode converter is suitable as a mode converter that converts between the mode of any of a microstrip line, a strip line, a coplanar line, a grounded coplanar line, and a parallel two-wire line and the mode of the post-wall waveguide. 
     In a fourth aspect of the present invention, the mode converter is configured such that, in any of the first to third aspects of the present invention, in a plan view of the post-wall waveguide, the strip-shaped conductor has a stub formed in a vicinity of a region where the strip-shaped conductor overlaps the opening. 
     According to the above configuration, designing the stub as appropriate achieves impedance matching and hence enables suppression of reflection loss in the present mode converter. 
     In a fifth aspect of the present invention, the mode converter is configured such that, in any of the first to fourth aspects of the present invention, the post-wall waveguide functions as any of a filter, a directional coupler, a diplexer, and an antenna. 
     The above configuration, when the line in which the strip-shaped conductor serves as a signal line is coupled to any of the filter, the directional coupler, the diplexer, and the antenna, can make it less likely to cause a transmission failure resulting from a change in environmental temperature than the conventional configuration. 
     In a sixth aspect of the present invention, the mode converter is configured such that, in any of the first to fifth aspects of the present invention, in a plan view of the post-wall waveguide, a pair of narrow walls constituting the post-wall waveguide are parallel to each other, and a straight line which is a set of points equally dividing a width of the post-wall waveguide and a central axis of the strip-shaped conductor are displaced from each other. 
     According to the above configuration, it is possible to provide the strip-shaped conductor such that the central axis of the strip-shaped conductor is displaced from the straight line which is a set of points equally dividing the width of the post-wall waveguide. This allows the present mode converter to increase the degree of freedom of the arrangement of the strip-shaped conductor. 
     In a seventh aspect of the present invention, the mode converter is configured such that, in the sixth aspect of the present invention, the opening is shaped in a trapezoid, and in a plan view of the post-wall waveguide, the opening is arranged such that a pair of bases forming the trapezoid are parallel to each of the pair of narrow walls, and a distance from the central axis to a shorter base out of the pair of bases exceeds a distance from the central axis to a longer base out of the pair of bases. 
     According to the above configuration, it is possible to suppress the deterioration of a reflection characteristic and transmission characteristic that may be caused by the arrangement in which the central axis of the strip-shaped conductor is provided so as to be displaced from the line which is a set of points equally dividing the width of the post-wall waveguide. That is, the present mode converter can suppress the deterioration of the reflection characteristic and transmission characteristic while increasing the degree of freedom of the arrangement of the strip-shaped conductor. 
     In an eighth aspect of the present invention, the mode converter is configured such that, in any of the first to seventh aspects of the present invention, the mode converter further includes a coaxial connector which includes a central conductor electrically connected to the strip-shaped conductor and is fixed to at least the substrate. 
     According to the above configuration, it is possible to convert, via the microstrip line, between the mode coupled to the coaxial connector from outside and the mode of the post-wall waveguide. Moreover, the present mode converter does not use the excitation pin provided in the conventional mode converter, and thus brings about the same effect as the effect brought about by the mode converter in accordance with the first aspect of the present invention. 
     In order to solve the above problem, an RF module in accordance with a ninth aspect of the present invention includes: a post-wall waveguide in which a first opening is provided in a vicinity of a first short wall of a first wide wall, and a second opening is provided in a vicinity of a second short wall of the first wide wall; a dielectric substrate having a first main surface and a second main surface, wherein a first strip-shaped conductor is formed on at least one of the first main surface and the second main surface, a second strip-shaped conductor is formed on at least one of the first main surface and the second main surface, and an antenna is formed on at least one of the first main surface and the second main surface; a joining member joining directly or indirectly the first wide wall and the substrate; and an RFIC being mounted on the first main surface of the substrate and having terminals any of which is electrically connected to the first strip-shaped conductor, wherein the antenna is electrically connected to the second strip-shaped conductor, and in a plan view of the post-wall waveguide, at least a portion of the first opening and at least a portion of the first strip-shaped conductor overlap each other, and at least a portion of the second opening and at least a portion of the second strip-shaped conductor overlap each other. 
     According to the above configuration, the present RF module, as with the mode converter in accordance with the first aspect of the present invention, can convert between the mode of the post-wall waveguide and the mode of the line in which the strip-shaped conductor connected to the RFIC serves as a signal line, and convert between the mode of the post-wall waveguide and the mode of the antenna, without using an excitation pin provided in the conventional mode converter. Thus, the present RF module is less likely to cause a transmission failure resulting from a change in environmental temperature than the conventional RF module. 
     In order to solve the above problem, the mobile terminal in accordance with a tenth aspect of the present invention includes the RF module in accordance with the ninth aspect as at least one of a transmission module, a reception module, and a transmission/reception module. 
     According to the above configuration, the present mobile terminal brings about the same effect as the effect brought about by the RF module in accordance with the sixth aspect of the present invention. 
     In order to solve the above problem, an RF module in accordance with an eleventh aspect of the present invention includes: a post-wall waveguide having a waveguide region formed by a pair of wide walls, narrow walls, and a pair of short walls, wherein (1) a first opening is provided in a first end region which is in a first wide wall of the pair of wide walls and includes a first short wall, (2) a second opening is provided in a second end region which is in the first wide wall of the pair of wide walls and includes a second short wall, and (3) the first end region and the second end region are arranged so as to be parallel to and close to each other; a dielectric substrate having a first main surface and a second main surface on at least one of which a third strip-shaped conductor and a fourth strip-shaped conductor are formed; a joining member joining directly or indirectly the first wide wall and the substrate; and an RFIC being mounted on the first main surface of the substrate and having an output terminal and an input terminal which are electrically connected to the third strip-shaped conductor and the fourth strip-shaped conductor, respectively, wherein in a plan view of the post-wall waveguide, at least a portion of the first opening and at least a portion of the third strip-shaped conductor overlap each other, and at least a portion of the second opening and at least a portion of the fourth strip-shaped conductor overlap each other. 
     According to the above configuration, it is possible to electromagnetically couple, via the third strip-shaped conductor, the output terminal of the RFIC and the first end region of the post-wall waveguide, and electromagnetically couple, via the fourth strip-shaped conductor, the second end region of the post-wall waveguide and the input terminal of the RFIC. This allows the present RF module to pass, through the post-wall waveguide, a mode supplied from the output terminal of the RFIC and then supply the mode to the input terminal of the RFIC. Moreover, the present RF module can couple the mode of the post-wall waveguide and the mode of the line in which the strip-shaped conductor formed on another substrate different from the substrate of the post-walled waveguide serves as a signal line, without using an excitation pin provided in the conventional mode converter. Thus, the present RF module brings about the same effect as the effect brought about by the mode converter in accordance with the first aspect of the present invention. 
     In a twelfth aspect of the present invention, the RF module is configured such that, in the eleventh aspects of the present invention, the post-wall waveguide functions as a filter. 
     According to the above configuration, it is possible to supply, to the input terminal of the RFIC, the mode obtained by subjecting the mode supplied from the output terminal of the RFIC to a filtering process. 
     In a thirteenth aspect of the present invention, the RF module is configured such that, in the eleventh or twelfth aspect of the present invention, in a plan view of the post-wall waveguide, the narrow walls constituting the first end region and the narrow walls constituting the second end region are parallel to each other, a straight line which is a set of points equally dividing a width of the first end region and a central axis of the third strip-shaped conductor are displaced from each other, and a straight line which is a set of points equally dividing a width of the second end region and a central axis of the fourth strip-shaped conductor are displaced from each other. 
     According to the above configuration, it is possible to provide the central axis of the third strip-shaped conductor so that the central axis of the third strip-shaped conductor is displaced from the straight line which is the set of points equally dividing the width of the first end region, and it is possible to provide the central axis of the fourth strip-shaped conductor so that the central axis of the fourth strip-shaped conductor is displaced from the straight line which is the set of points equally dividing the width of the second end region. Therefore, the present RF module can increase the degree of freedom of the arrangement of the third strip-shaped conductor and the fourth strip-shaped conductor according to a terminal-to-terminal distance between the output terminal and the input terminal of the RFIC. 
     In a fourteenth aspect of the present invention, the RF module is configured such that, in the thirteenth aspect of the present invention, the first opening and the second opening are each shaped in a trapezoid, and in a plan view of the post-wall waveguide, (1) the first opening is arranged such that a pair of bases forming the trapezoid are parallel to the narrow walls constituting the first end region, and a distance from the central axis of the third strip-shaped conductor to a shorter base out of the pair of bases of the first opening exceeds a distance from the central axis of the third strip-shaped conductor to a longer base out of the pair of bases of the first opening, and (2) the second opening is arranged such that a pair of bases forming the trapezoid are parallel to the narrow walls constituting the second end region, and a distance from the central axis of the fourth strip-shaped conductor to a shorter base out of the pair of bases of the second opening exceeds a distance from the central axis of the fourth strip-shaped conductor to a longer base out of the pair of bases of the second opening. 
     According to the above configuration, it is possible to suppress the deterioration of the reflection characteristic and transmission characteristic that may be caused by the arrangement in which the central axis of the third strip-shaped conductor is provided so as to be displaced from the straight line which is the set of points equally dividing the width of one waveguide region, and it is possible to suppress the deterioration of the reflection characteristic and transmission characteristic that may be caused by the arrangement in which the central axis of the fourth strip-shaped conductor is provided so as to be displaced from the straight line which is the set of points equally dividing the width of the other waveguide region. That is, the present RF module can suppress the deterioration of the reflection characteristic and transmission characteristic while increasing the degree of freedom of the arrangement of the third strip-shaped conductor and the fourth strip-shaped conductor according to the terminal-to-terminal distance between the output terminal and the input terminal of the RFIC. 
     In a fifteenth aspect of the present invention, the RF module is configured such that, in the thirteenth or fourteenth aspect of the present invention, in a plan view of the post-wall waveguide, the central axis of the third strip-shaped conductor and the central axis of the fourth strip-shaped conductor are located between the straight line which is the set of points equally dividing the width of the first end region and the straight line which is the set of points equally dividing the width of the second end region. 
     The terminal-to-terminal distance between the output terminal and the input terminal of the RFIC can be narrower than the distance between the straight line which is the set of points equally dividing the width of the one waveguide region and the straight line which is the set of points equally dividing the width of the other waveguide region. According to the above configuration, it is possible to make narrower an axis-to-axis distance between the central axis of the third strip-shaped conductor and the central axis of the fourth strip-shaped conductor by locating the central axis of the third strip-shaped conductor and the central axis of the fourth strip-shaped conductor between the straight line which is the set of points equally dividing the width of the one waveguide region and the straight line which is the set of points equally dividing the width of the other waveguide region. Therefore, the present RF module can further increase the degree of freedom of the arrangement of the third strip-shaped conductor and the fourth strip-shaped conductor according to the terminal-to-terminal distance between the output terminal and the input terminal of the RFIC. 
     [Supplementary Remarks] 
     The present invention is not limited to the embodiments, but can be altered by a skilled person in the art within the scope of the claims. The present invention also encompasses, in its technical scope, any embodiment derived by combining technical means disclosed in differing embodiments. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10 ,  10 A,  10 B,  10 C,  10 D,  10 E,  10 F,  10 Fa,  10 Fb,  10 G: mode converter 
             PW, PWFa, PWFb: post-wall waveguide 
               11 : substrate 
               12 ,  13 : conductor layer 
               121 ,  121 F: opening 
               121 Fa,  121 Fb: opening (first opening, second opening) 
               14 ,  14 Fa,  14 Fb: post wall 
               14   a ,  14   b ,  14 Faa,  14 Fba,  14 Fb: narrow wall 
               14   c ,  14 Fac,  14 Fbc: short wall 
               14   i : through-via 
               15 : substrate 
               16 ,  16 B,  16 C,  16 E,  16 F: strip-shaped conductor 
               16   a ,  16   b : strip-shaped conductor (first strip-shaped conductor, second strip-shaped conductor) 
               16 Fa,  16 Fb: strip-shaped conductor (third strip-shaped conductor, fourth strip-shaped conductor) 
               16 B 1 ,  16 B 2 : stub 
               16 C 1  first conductor pattern 
               16 C 2  second conductor pattern 
               16 C 3 : through-via 
               17 : conductor layer 
               18 ,  18 C,  18 D: solder (joining member) 
               18 A: adhesive (joining member) 
               19 G: connector 
             MS, MSFa, MSFb: microstrip line 
               1 ,  1 F: RF module 
               21 ,  21 F: RFIC 
               22 : antenna