Patent Publication Number: US-11387561-B2

Title: Antenna

Description:
TECHNICAL FIELD 
     The present disclosure relates to an antenna. 
     BACKGROUND ART 
     Patent Literature 1 discloses a slot antenna of a triplate line feeding system. Specifically, a feed line is formed between two dielectric layers, a front conductive foil is formed on a front surface of one of the dielectric layers, a back conductive foil is formed on a back surface of the other dielectric layer, and a slot is formed in the front conductive foil. The feed line is wired from a transmitter and receiver circuit to the position facing the center of the slot. 
     Further, a slot-coupled patch antenna of a triplate line feeding system has also been generally known. In the slot-coupled patch antenna, a dielectric layer is further formed on the foregoing front conductive foil, a patch antenna element is formed on the dielectric layer such that the antenna element faces the foregoing slot. Thus, the antenna element is configured to be electromagnetically coupled to the feed line through the slot. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] Japanese Patent Application Publication No. 2017-46107 
     SUMMARY OF INVENTION 
     Technical Problem 
     In a conventional slot-coupled patch antenna of a triplate line feeding system, when a signal wave is transmitted between a feed line and an antenna element, a dielectric loss occurs in a dielectric layer between the antenna element and a conductive foil. Such a dielectric loss causes a decrease in gain. 
     Thus, the present disclosure has been achieved in view of the circumstances described above. An object of the present disclosure is to reduce a dielectric loss when a signal wave is transmitted between a feed line and an antenna element via a slot. 
     Solution to Problem 
     A primary aspect of the present disclosure to achieve the aforementioned object is an antenna comprising: a dielectric substrate including a recess; a conductive ground layer bonded to the dielectric substrate so as to cover the recess, the conductive ground layer including a slot that is arranged on an inner side with respect to the recess; a dielectric layer bonded to the conductive ground layer on a side opposite to the dielectric substrate with respect to the conductive ground layer; an antenna element formed on a bottom of the recess at position facing the slot; and a feed line formed on a side opposite to the conductive ground layer with respect to the dielectric layer, the feed line configured to be electromagnetically coupled to the antenna element via the slot. 
     Other features of the present disclosure are made apparent from the following description and the drawings. 
     Advantageous Effects of Invention 
     According to an embodiment of the present disclosure, it is possible to reduce a dielectric loss when a signal wave is transmitted between a feed line and an antenna element via a slot. This improves a gain of an antenna. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view of an antenna according to a first embodiment. 
         FIG. 2  is a graph illustrating a simulation result of a gain of an antenna according to a first embodiment and a gain of an antenna according to a comparative example. 
         FIG. 3  is a graph illustrating a simulation result of a reflection coefficient of an antenna according to a first embodiment and a reflection coefficient of an antenna according to a comparative example. 
         FIG. 4  is a graph illustrating a simulation result of a gain of an antenna according to a first embodiment and a gain of an antenna according to a comparative example. 
         FIG. 5  is a graph illustrating a simulation result of a reflection coefficient of an antenna according to a first embodiment and a reflection coefficient of an antenna according to a comparative example. 
         FIG. 6  is a graph illustrating a simulation result of a gain of an antenna according to a first embodiment and a gain of an antenna according to a comparative example. 
         FIG. 7  is a graph illustrating a simulation result of a reflection coefficient of an antenna according to a first embodiment and a reflection coefficient of an antenna according to a comparative example. 
         FIG. 8  is a graph illustrating a simulation result of a gain of an antenna according to a first embodiment and a gain of an antenna according to a comparative example. 
         FIG. 9  is a graph illustrating a simulation result of a reflection coefficient of an antenna according to a first embodiment and a reflection coefficient of an antenna according to a comparative example. 
         FIG. 10  is a plan view of an antenna according to a second embodiment. 
         FIG. 11  is a cross-sectional view illustrating taken along XI-XI in  FIG. 10 . 
         FIG. 12  is a plan view of a slot. 
         FIG. 13  is a plan view of a slot. 
         FIG. 14  is a graph illustrating a simulation result of a gain of an antenna according to a second embodiment and a gain of an antenna according to a comparative example. 
         FIG. 15  is a graph illustrating a simulation result of a reflection coefficient of an antenna according to a second embodiment and a reflection coefficient of an antenna according to a comparative example. 
         FIG. 16  is a graph illustrating a simulation result of a gain of an antenna according to a second embodiment and a gain of an antenna according to a comparative example. 
         FIG. 17  is a graph illustrating a simulation result of a reflection coefficient of an antenna according to a second embodiment and a reflection coefficient of an antenna according to a comparative example. 
         FIG. 18  is a graph illustrating a simulation result of a gain of an antenna according to a second embodiment and a gain of an antenna according to a comparative example. 
         FIG. 19  is a graph illustrating a simulation result of a reflection coefficient of an antenna according to a second embodiment and a reflection coefficient of an antenna according to a comparative example. 
         FIG. 20  is a graph illustrating a simulation result of a gain of an antenna according to a second embodiment and a gain of an antenna according to a comparative example. 
         FIG. 21  is a graph illustrating a simulation result of a reflection coefficient of an antenna according to a second embodiment and a reflection coefficient of an antenna according to a comparative example. 
         FIG. 22  is a graph illustrating a simulation result of a gain of an antenna according to a second embodiment and a gain of an antenna according to a comparative example. 
         FIG. 23  is a graph illustrating a simulation result of a reflection coefficient of an antenna according to a second embodiment and a reflection coefficient of an antenna according to a comparative example. 
         FIG. 24  is a cross-sectional view of an antenna according to a modified example of a second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     At least the following matters are made apparent from the following description and the drawings. 
     An antenna will be made apparent which comprises: a dielectric substrate including a recess; a conductive ground layer bonded to the dielectric substrate so as to cover the recess, the conductive ground layer including a slot that is arranged on an inner side with respect to the recess; a dielectric layer bonded to the conductive ground layer on a side opposite to the dielectric substrate with respect to the conductive ground layer; an antenna element formed on a bottom of the recess at position facing the slot; and a feed line formed on a side opposite to the conductive ground layer with respect to the dielectric layer, the feed line configured to be electromagnetically coupled to the antenna element via the slot. 
     As described above, the conductive ground layer is bonded to the dielectric substrate so as to cover the recess, resulting in the recess being a hollow, and the hollow is interposed between the antenna element and the slot. This can reduce a dielectric loss when a signal wave is transmitted between the feed line and the antenna element via the slot, thereby improving a gain of the antenna. 
     The dielectric substrate is rigid. 
     This can allow the dielectric layer to be thin. Thus, it is possible to reduce a dielectric loss of a signal wave transmitted with the feed line, and also improve a gain of the antenna. 
     When the dielectric substrate is rigid, the space between the antenna element and the feed line is also less likely to change. This stabilizes radiation characteristics of the antenna. 
     The antenna element comprises a plurality of antenna elements, the plurality of antenna elements being aligned at intervals, the slot comprises a plurality of slots, the plurality of slots being aligned at intervals, and the antenna elements face the slots, respectively. 
     This can achieve improvement in gain of the antenna. 
     The number of the antenna elements is an even number, the number of the slots is an even number, and the feed line branches at a point between the slots adjacent to each other that are positioned in the center of a row of the slots, the feed line having branch portions that extend from the point of branch until the branch portions cross the slots at both ends of the row of the slots in plan view, respectively. 
     This adjusts the impedance of a portion of the feed line from each end thereof to immediately below the slot. Thus, impedance matching can be achieved between the portion of the feed line from each end thereof to immediately below the slot, the slot, and the antenna element. 
     The recess comprises a plurality of recesses, the antenna elements are individually formed on bottoms of the recesses, respectively, and the slots are individually arranged on an inner side with respect to the recesses, respectively. 
     This improves strength of the dielectric substrate by virtue of a portion between the recesses adjacent to each other, so that the dielectric substrate is less likely to be deformed. Thus, radiation characteristics of the antenna  21  are stabilized. 
     The slot is formed in a shape obtained by cutting out, in a rectangular shape or square shape, from both end portions of both long sides of a rectangular hole portion, in a short-side direction. 
     This can achieve improvement in gain of the antenna. 
     EMBODIMENTS 
     Embodiments of the present disclosure will be described below with reference to the drawings. Note that, various limitations that are technically preferable for carrying out the present disclosure are imposed on embodiments which will be described below, however, the scope of the disclosure is not to be limited to the following embodiments or illustrated examples. 
     First Embodiment 
       FIG. 1  is a cross-sectional view of an antenna  1  according to a first embodiment. The antenna  1  is used for transmitting, receiving, or both transmitting and receiving a radio wave in a frequency band of a microwave or a millimeter wave. 
     A dielectric layer  3  and a dielectric layer  6  are bonded to each other, using a dielectric adhesive layer  5 , with a conductive pattern layer  4  sandwiched therebetween. The dielectric layer  3  and the dielectric layer  6  are made of a liquid crystal polymer. 
     The conductive pattern layer  4  is formed between the dielectric layer  3  and the adhesive layer  5 . Note that the conductive pattern layer  4  may be formed between the dielectric layer  6  and the adhesive layer  5 . 
     A conductive ground layer  2  is formed on a surface  3   a  of the dielectric layer  3  on a side opposite to the conductive pattern layer  4  with respect to the dielectric layer  3 . 
     The dielectric layer  6  and a dielectric substrate  8  are bonded to each other with a conductive ground layer  7  sandwiched therebetween. The dielectric layer  6  is bonded to the conductive ground layer  7  on a side opposite to the dielectric substrate  8  with respect to the conductive ground layer  7 . 
     The conductive ground layer  7  is formed between the dielectric layer  6  and the dielectric substrate  8 . 
     As described above, the conductive ground layer  2 , the dielectric layer  3 , the conductive pattern layer  4 , the adhesive layer  5 , the dielectric layer  6 , the conductive ground layer  7 , and the dielectric substrate  8  are laminated in this order. A laminated body from the conductive ground layer  2  to the conductive ground layer  7  is flexible, and the dielectric substrate  8  is rigid. Bending deformation of the antenna  1  is less likely to occur by virtue of the rigid dielectric substrate  8  bonded to the laminated body that is from the conductive ground layer  2  to the conductive ground layer  7 . 
     The thickness of the dielectric substrate  8  is greater than the thickness of each of the dielectric layers  3  and  6  and the adhesive layer  5 , and is also greater than the total thickness of the dielectric layers  3  and  6  and the adhesive layer  5 . 
     The conductive ground layer  2 , the conductive pattern layer  4 , and the conductive ground layer  7  are made of a conductive metal material such as copper. 
     The conductive ground layer  7  is processed and shaped by an additive method, a subtractive method, or the like, and thus a slot  7   a  is formed in the conductive ground layer  7 . The shape of the slot  7   a  may be an I shape, a rectangular shape, a round shape, or other shapes. 
     The conductive pattern layer  4  is processed and shaped by an additive method, a subtractive method, or the like, and thus the conductive pattern layer  4  includes a feed line  4   a . The feed line  4   a  is formed on a side opposite to the conductive ground layer  7  with respect to the dielectric layer  6 , and is formed on a side opposite to the conductive ground layer  2  with respect to the dielectric layer  3 . Since the feed line  4   a  is located between the conductive ground layer  2  and the conductive ground layer  7 , the feed line  4   a  constitutes a triplate or strip-line transmission line together with the conductive ground layer  2  and the conductive ground layer  7 . 
     The feed line  4   a  crosses the slot  7   a  in plan view, and the feed line  4   a  is open at one end  4   b  thereof. Herein, the plan view refers to viewing the antenna  1  from above the antenna  1 , in other words, viewing the antenna  1  in a direction of an arrow A illustrated in  FIG. 1 . 
     The impedance of a portion from the one end  4   b  to immediately below the slot  7   a  in the feed line  4   a  is adjusted according to a length from the position facing the center of the slot  7   a  to the one end  4   b  of the feed line  4   a.    
     The other end portion of the feed line  4   a  is connected to a terminal of a radio frequency integrated circuit (RFIC). 
     A recess  8   b  is formed in a bonding surface  8   a  to be bonded to the conductive ground layer  7  out of two surfaces of the dielectric substrate  8 . An opening  8   c  of the recess  8   b  faces the slot  7   a , and the bonding surface  8   a  of the dielectric substrate  8  is bonded to the conductive ground layer  7 . The opening  8   c  of the recess  8   b  is covered with the conductive ground layer  7 , resulting in the recess  8   b  being a hollow. The slot  7   a  is arranged on the inner side with respect to the edge of the opening  8   c  of the recess  8   b . A bottom  8   d  of the recess  8   b  faces the conductive ground layer  7 . The depth of the recess  8   b , in other words, the height of the hollow is greater than the thickness of each of the dielectric layers  3  and  6  and the adhesive layer  5 . 
     A patch antenna element  9  is formed on the bottom  8   d  of the recess  8   b . The antenna element  9  faces the slot  7   a . The antenna element  9  is configured to be electromagnetically coupled to the feed line  4   a  through the slot  7   a . Therefore, when the RFIC is a transmitter or a transceiver, a signal wave transmitted from the RFIC with the feed line  4   a  is transmitted to the antenna element  9  through the slot  7   a , and an electromagnetic wave generated with the signal wave is radiated from the antenna element  9 . When the RFIC is a receiver or a transceiver, a signal wave generated with an electromagnetic wave being incident on the antenna element  9  is transmitted to the feed line  4   a  through the slot  7   a , and the signal wave is transmitted to the RFIC with the feed line  4   a.    
     Herein, since the feed line  4   a  crosses the slot  7   a  in plan view, impedance matching is achieved among the portion from the one end  4   b  to immediately below the slot  7   a  in the feed line  4   a , the slot  7   a , and the antenna element  9 . 
     According to an embodiment according to the present disclosure as described above, the rigid dielectric substrate reduces bending of the laminated body that is from the conductive ground layer  2  to the conductive ground layer  7 . Thus, reduction in thickness of the dielectric layers  3  and  6  and the adhesive layer  5  can be achieved. The reduction in thickness of the dielectric layers  3  and  6  and the adhesive layer  5  contributes to reduction in dielectric loss and improvement in radiation efficiency. Accordingly, a gain of the antenna  1  is high, and an applicable frequency band of the antenna  1  is wide. 
     The hollow formed with the recess  8   b  is present between the antenna element  9  and the slot  7   a . A dielectric loss tangent in the hollow is substantially zero when the hollow is under an atmosphere of the air. Thus, a signal wave is not affected by a dielectric when the signal wave is transmitted between the antenna element  9  and the slot  7   a , thereby being able to reduce occurrence of a dielectric loss. Accordingly, a gain of the antenna  1  is high, and an applicable frequency band of the antenna  1  is wide. 
     Since the recess  8   b  is formed in the rigid dielectric substrate  8 , the depth of the recess  8   b  (i.e., the height of the hollow) is less likely to change. Furthermore, a space between the antenna element  9  and the feed line  4   a  is also less likely to change. Thus, radiation characteristics of the antenna  1  are stabilized. 
     Since the conductive ground layer  7  is located between the antenna element  9  and the feed line  4   a , radiation of an electromagnetic wave in the feed line  4   a  is less likely to affect radiation in the antenna element  9 . 
     A contribution of the hollow existing between the antenna element  9  and the slot  7   a  to improvement in radiation characteristics of the antenna  1  has been verified by simulations. A simulation result when the depth of the recess  8   b , in other words, the height of the hollow is 0.25 mm is illustrated in  FIGS. 2 and 3 . A simulation result when the height of the hollow is 0.3 mm is illustrated in  FIGS. 4 and 5 . A simulation result when the height of the hollow is 0.35 mm is illustrated in  FIGS. 6 and 7 . A simulation result when the height of the hollow is 0.4 mm is illustrated in  FIGS. 8 and 9 . The vertical axis represents a gain, and the horizontal axis represents a frequency in each graph in  FIGS. 2, 4, 6, and 8 . The vertical axis represents S 11  of S-parameters, and the horizontal axis represents a frequency in each graph in  FIGS. 3, 5, 7, and 9 . S 11  refers to a reflection coefficient in a connecting section between the feed line  4   a  and the terminal of the RFIC. In all of  FIGS. 2 to 9 , a solid line indicates a result using the antenna  1  as a simulation target. A broken line indicates a result using, as a simulation target, an antenna without a hollow obtained by filling the recess  8   b  with a liquid crystal polymer that is a dielectric. 
     As is apparent from  FIGS. 2, 4, 6, and 8 , it is found that when gains in a use band of 57 to 67 GHz are averaged, the average gain of the antenna  1  with the hollow is higher than the average gain of the antenna without the hollow. In particular, in the use band of 57 to 67 GHz, a band in which the gain of the antenna  1  with the hollow is higher than the gain of the antenna without the hollow is wider than a band in which the gain of the antenna  1  with the hollow is lower than the gain of the antenna without the hollow. 
     As is apparent from  FIGS. 3, 5, 7, and 9 , it is found that the reflection coefficient of the antenna  1  with the hollow is lower than the reflection coefficient of the antenna without the hollow in the use band of 57 to 67 GHz. 
     From the foregoing simulation results, it is found that the hollow existing between the antenna element  9  and the slot  7   a  contributes to improvement in radiation characteristics of the antenna  1 . 
     Second Embodiment 
       FIG. 10  is a schematic plan view of an antenna  21  according to a second embodiment.  FIG. 11  is a cross-sectional view taken along XI-XI in  FIG. 10 . 
     The antenna  21  is used for transmitting, receiving, or both transmitting and receiving a radio wave in a frequency band of a microwave or a millimeter wave. 
     A dielectric layer  23  and a dielectric layer  26  sandwich a conductive pattern layer  24  therebetween, and are bonded to each other using a dielectric adhesive layer  25 . The dielectric layer  23  and the dielectric layer  26  are made of a liquid crystal polymer. 
     The conductive pattern layer  24  is formed between the dielectric layer  23  and the adhesive layer  25 . Note that the conductive pattern layer  24  may be formed between the dielectric layer  26  and the adhesive layer  25 . 
     A conductive ground layer  22  is formed on a surface  23   a  of the dielectric layer  23  on a side opposite to the conductive pattern layer  24  with respect to the dielectric layer  23 . 
     The dielectric layer  26  and a dielectric substrate  28  sandwich a conductive ground layer  27  therebetween, and are bonded to each other. The dielectric layer  26  is bonded to the conductive ground layer  27  on a side opposite to the dielectric substrate  28  with respect to the conductive ground layer  27 . 
     The conductive ground layer  27  is formed between the dielectric layer  26  and the dielectric substrate  28 . 
     As described above, the conductive ground layer  22 , the dielectric layer  23 , the conductive pattern layer  24 , the adhesive layer  25 , the dielectric layer  26 , the conductive ground layer  27 , and the dielectric substrate  28  are laminated in this order. A laminated body from the conductive ground layer  22  to the conductive ground layer  27  is flexible, and the dielectric substrate  28  is rigid. Bending deformation of the antenna  21  is less likely to occur by virtue of the dielectric substrate  28  being bonded to the laminated body that is from the conductive ground layer  22  to the conductive ground layer  27 . 
     The thickness of the dielectric substrate  28  is greater than the thickness of each of the dielectric layers  23  and  26  and the adhesive layer  25 , and is also greater than the total thickness of the dielectric layers  23  and  26  and the adhesive layer  25 . 
     The conductive ground layer  22 , the conductive pattern layer  24 , and the conductive ground layer  27  are made of a conductive metal material such as copper. 
     The conductive ground layer  27  is processed and shaped by an additive method, a subtractive method, or the like, and thus a plurality of slots  27   a  to  27   d  are formed in the conductive ground layer  27 . The slots  27   a  to  27   d  are aligned at regular intervals in a short-side direction of the slots  27   a  to  27   d.    
     The slot  27   a  is formed in an I shape as illustrated in  FIG. 12 or 13 . 
     In a case of  FIG. 12 , the slot  27   a  is formed in a shape obtained by cutting out, in a rectangular shape or square shape, from both end portions of one of long sides of a rectangular hole portion  270   a , in the short-side direction (see reference signs  271   a  and  272   a ), and cutting out, in a rectangular shape or square shape, from both end portions of the other long side of the hole portion  270   a , in the short-side direction (see reference signs  273   a  and  274   a ). 
     In a case of  FIG. 13 , the slot  27   a  is formed in a shape obtained by cutting out, in a trapezoidal shape, from both end portions of one of long sides of a rectangular hole portion  275   a , in the short-side direction (see reference signs  276   a  and  277   a ), and cutting out, in a trapezoidal shape, from both end portions of the other long side of the hole portion  275   a , in the short-side direction (see reference signs  278   a  and  279   a ). The portions  276   a  and  277   a  obtained by being cut into the trapezoidal shape are tapered, and the widths of the portions  276   a  and  277   a  obtained by being cut into the trapezoidal shape gradually decreases as a distance from one of the long sides of the hole portion  275   a  increases. The portions  278   a  and  279   a  obtained by being cut into the trapezoidal shape are tapered, and the widths of the portions  278   a  and  279   a  obtained by being cut into the trapezoidal shape gradually decreases as a distance from the other long side of the hole portion  275   a  increases. 
     The shape and size of the slots  27   b  to  27   d  are the same as those of the slot  27   a.    
     Note that the shape of the slots  27   a  to  27   d  is not limited to the I shape, but may be a rectangular shape, a round shape, or other shapes. 
     The conductive pattern layer  24  is processed and shaped by an additive method, a subtractive method, or the like, and thus the conductive pattern layer  24  includes a feed line  24   a . The feed line  24   a  is formed on a side opposite to the conductive ground layer  27  with respect to the dielectric layer  26 , and is formed on a side opposite to the conductive ground layer  22  with respect to the dielectric layer  23 . Since the feed line  24   a  is located between the conductive ground layer  22  and the conductive ground layer  27 , the feed line  24   a  constitutes a triplate or strip-line transmission line together with the conductive ground layer  22  and the conductive ground layer  27 . 
     The feed line  24   a  is a T-shaped line having branches. The feed line  24   a  includes a main line portion  24   b  and branch line portions  24   f  and  24   h.    
     The main line portion  24   b  is formed in an L shape. 
     The branch line portions  24   f  and  24   h  are formed by branching from one end portion  24   c  of the main line portion  24   b  at the position between the slots  27   b  and  27   c  adjacent to each other in the center of a row of the slots  27   a  to  27   d . The branch line portions  24   f  and  24   h  extend linearly in directions opposite to each other from a branch point. A direction in which the branch line portions  24   f  and  24   h  extend is parallel to a direction in which the slots  27   a  to  27   d  are aligned. 
     The other end portion  24   d  of the main line portion  24   b  is connected to a terminal of an RFIC. 
     The width of the one end portion  24   c  and the other end portion  24   d  of the main line portion  24   b  are wider than the width of a portion  24   e  between the one end portion  24   c  and the other end portion  24   d . Thus, the impedance of the one end portion  24   c  and the other end portion  24   d  of the main line portion  24   b  is smaller than the impedance of the portion  24   e  between the one end portion  24   c  and the other end portion  24   d . For example, the impedance of the one end portion  24   c  and the other end portion  24   d  of the main line portion  24   b  is a half of the impedance of the portion  24   e  between the one end portion  24   c  and the other end portion  24   d.    
     The width of the branch line portions  24   f  and  24   h  is smaller than the width of the one end portion  24   c  and the other end portion  24   d  of the main line portion  24   b , and is equal to the width of the portion  24   e  between the one end portion  24   c  and the other end portion  24   d . Thus, the impedance of the branch line portions  24   f  and  24   h  is greater than the impedance of the one end portion  24   c  and the other end portion  24   d  of the main line portion  24   b . For example, the impedance of the branch line portions  24   f  and  24   h  is twice the impedance of the one end portion  24   c  and the other end portion  24   d  of the main line portion  24   b.    
     The branch line portion  24   f  extends from the branch point and crosses the slots  27   b  and  27   a  in plan view, and the branch line portion  24   f  is open at one end  24   g  thereof. The impedance of a portion from the one end  24   g  to immediately below the slot  27   a  in the branch line portion  24   f  is adjusted according to a length from the position facing the center of the slot  27   a  to the one end  24   g  of the branch line portion  24   f.    
     The branch line portion  24   h  extends from the branch point and crosses the slots  27   c  and  27   d  in plan view, and the branch line portion  24   h  is open at one end  24   i  thereof. The impedance of a portion from the one end  24   i  to immediately below the slot  27   d  in the branch line portion  24   h  is adjusted according to a length from the position facing the center of the slot  27   d  to the one end  24   i  of the branch line portion  24   h.    
     The electrical length of the portion from the branch point to immediately below the slot  27   b  is different from the electrical length of the portion from the branch point to immediately below the slot  27   c  in the feed line  24   a . Specifically, a difference between the electrical length of the portion from the branch point to immediately below the slot  27   b  and the electrical length of the portion from the branch point to immediately below the slot  27   c  in the feed line  24   a  is equal to a quarter of the effective wavelength in the center of a band to be used. This improves a gain of the antenna  1 . Note that a difference between the electrical length of the portion from the branch point to immediately below the slot  27   b  and the electrical length of the portion from the branch point to immediately below the slot  27   c  in the feed line  24   a  may be equal to a half of an effective wavelength in the center of a band to be used. The electrical length of the portion from the branch point to immediately below the slot  27   b  in the feed line  24   a  may be equal to the electrical length of the portion from the branch point to immediately below the slot  27   c  in the feed line  24   a.    
     A recess  28   b  is formed in a bonding surface  28   a  to be bonded to the conductive ground layer  27  out of two surfaces of the dielectric substrate  28 . An opening  28   c  of the recess  28   b  faces the slots  27   a  to  27   d , and the bonding surface  28   a  of the dielectric substrate  28  is bonded to the conductive ground layer  27 . The opening  28   c  of the recess  28   b  is covered with the conductive ground layer  27 , resulting in the recess  28   b  being a hollow. The slots  27   a  to  27   d  are arranged on the inner side with respect to the edge of the opening  28   c  of the recess  28   b . A bottom  28   d  of the recess  28   b  faces the conductive ground layer  27 . A bottom  28   d  of the recess  28   b  is flat, and is parallel to the conductive ground layer  27 . The depth of the recess  28   b , in other words, the height of the hollow is greater than the thickness of each of the dielectric layers  23  and  26  and the adhesive layer  25 . 
     Patch antenna elements  29   a  to  29   d  are formed on the bottom  28   d  of the recess  28   b . The antenna elements  29   a  to  29   d  are aligned at regular intervals in a direction parallel to a direction in which the slots  27   a  to  27   d  are aligned. The antenna element  29   a  faces the slot  27   a , the antenna element  29   b  faces the slot  27   b , the antenna element  29   c  faces the slot  27   c , and the antenna element  29   d  faces the slot  27   d . The antenna element  29   a  is configured to be electromagnetically coupled to the branch line portion  24   f  of the feed line  24   a  through the slot  27   a . The antenna element  29   b  is configured to be electromagnetically coupled to the branch line portion  24   f  of the feed line  24   a  through the slot  27   b . The antenna element  29   c  is configured to be electromagnetically coupled to the branch line portion  24   h  of the feed line  24   a  through the slot  27   c . The antenna element  29   d  is configured to be electromagnetically coupled to the branch line portion  24   h  of the feed line  24   a  through the slot  27   d . Accordingly, when the RFIC is a transmitter or a transceiver, a signal wave transmitted from the RFIC using the feed line  24   a  is transmitted to the antenna elements  29   a  to  29   d  through the slots  27   a  to  27   d , respectively, and electromagnetic waves generated with the signal waves are radiated from the antenna elements  29   a  to  29   d . When the RFIC is a receiver or a transceiver, signal waves generated with electromagnetic waves being incident on the antenna elements  29   a  to  29   d  are transmitted to the feed line  24   a  through the slots  27   a  to  27   d , and the signal waves are transmitted to the RFIC using the feed line  24   a.    
     Herein, since the branch line portion  24   f  of the feed line  24   a  crosses the slot  27   a  in plan view, impedance matching is achieved among the portion from the one end  24   g  to immediately below the slot  27   a  in the branch line portion  24   f , the slot  27   a , and the antenna element  29   a . Since the branch line portion  24   h  of the feed line  24   a  crosses the slot  27   d  in plan view, impedance matching is achieved among the portion from the one end  24   i  to immediately below the slot  27   d  in the branch line portion  24   h , the slot  27   d , and the antenna element  29   d.    
     According to an embodiment according to the present disclosure described above, the rigid dielectric substrate  28  reduces bending of the laminated body that is from the conductive ground layer  22  to the conductive ground layer  27 . This can achieve reduction in thickness of the dielectric layers  23  and  26  and the adhesive layer  25 . The reduction in thickness of the dielectric layers  23  and  26  and the adhesive layer  25  contributes to reduction in dielectric loss and improvement in radiation efficiency. Accordingly, a gain of the antenna  21  is high, and an applicable frequency band of the antenna  21  is wide. 
     The hollow formed with the recess  28   b  is present between the antenna elements  29   a  to  29   d  and the slots  27   a  to  27   d . A dielectric loss tangent in the hollow is substantially zero when the hollow is under an atmosphere of the air. Thus, a signal wave is not affected by a dielectric when the signal waves are transmitted between the antenna elements  29   a  to  29   d  and the slots  27   a  to  27   d , thereby being able to reduce occurrence of a dielectric loss. Accordingly, a gain of the antenna  21  is high, and an applicable frequency band of the antenna  21  is wide. 
     Since the recess  28   b  is formed in the rigid dielectric substrate  28 , the depth of the recess  28   b  (i.e., the height of the hollow) is less likely to change. Furthermore, a space between the antenna elements  29   a  to  29   d  and the feed line  24   a  is also less likely to change. Thus, radiation characteristics of the antenna  21  are stabilized. 
     Since the conductive ground layer  27  is located between the antenna elements  29   a  to  29   d  and the feed line  24   a , radiation of electromagnetic waves in the feed line  24   a  is less likely to affect radiation in the antenna elements  29   a  to  29   d.    
     When the slots  27   a  to  27   d  have the shape illustrated in  FIG. 12 , a contribution of the hollow existing between the antenna elements  29   a  to  29   d  and the slots  27   a  to  27   d  to improvement in gain of the antenna  21  has been verified by simulations. A simulation result when the depth of the recess  28   b , in other words, the height of the hollow is 0.25 mm is illustrated in  FIGS. 14 and 15 . A simulation result when the height of the hollow is 0.3 mm is illustrated in  FIGS. 16 and 17 . A simulation result when the height of the hollow is 0.35 mm is illustrated in  FIGS. 18 and 19 . A simulation result when the height of the hollow is 0.4 mm is illustrated in  FIGS. 20 and 21 . The vertical axis represents a gain, and the horizontal axis represents a frequency in each graph in  FIGS. 14, 16, 18, and 20 . The vertical axis represents S 11  of S-parameters, and the horizontal axis represents a frequency in each graph in  FIGS. 15, 17, 19, and 21 . S 11  refers to a reflection coefficient in a connecting section between the feed line  24   a  and the terminal of the RFIC. In all of  FIGS. 14 to 21 , a solid line indicates a result using the antenna  21  as a simulation target. A broken line indicates a result using, as a simulation target, an antenna without a hollow obtained by filling the recess  28   b  with a liquid crystal polymer that is a dielectric. 
     As is apparent from  FIGS. 14, 16, 18, and 20 , it is found that a gain of the antenna  21  with the hollow takes a local maximum value in a use band of 57 to 67 GHz, whereas a gain of the antenna without the hollow does not take a local maximum value in the use band of 57 to 67 GHz. It is also found that the gain of the antenna  21  with the hollow is higher than the gain of the antenna without the hollow. 
     As is apparent from  FIGS. 15, 17, 19, and 21 , it is found that the reflection coefficient of the antenna  21  with the hollow is lower than the reflection coefficient of the antenna without the hollow, in the use band of 57 to 67 GHz. 
     It is found from the foregoing simulation results that the hollow existing between the antenna elements  29   a  to  29   d  and the slots  27   a  to  27   d  contributes to improvement in gain of the antenna  21 . 
     When the slots  27   a  to  27   d  have the shape illustrated in  FIG. 12 or 13 , a contribution of the hollow existing between the antenna elements  29   a  to  29   d  and the slots  27   a  to  27   d  to improvement in gain of the antenna  21  has been verified by simulations. A simulation result is illustrated in  FIGS. 22 and 23 . The vertical axis represents a gain and the horizontal axis represents a frequency, in the graph of  FIG. 22 . The vertical axis represents S 11  of S-parameters and the horizontal axis represents a frequency, in the graph of  FIG. 23 . In both of  FIGS. 22 and 23 , a solid line indicates a result using the antenna  21  as a simulation target when the slots  27   a  to  27   d  have the shape illustrated in  FIG. 12 . A broken line indicates a result using the antenna  21  as a simulation target when the slots  27   a  to  27   d  have the shape illustrated in  FIG. 13 . A chain double-dashed line indicates a result using, as a simulation target, the antenna without the hollow obtained by filling the recess  28   b  with a liquid crystal polymer that is a dielectric, when the slots  27   a  to  27   d  have the shape illustrated in FIG.  12 . An alternate long and short dashed line indicates a result using, as a simulation target, the antenna without the hollow obtained by filling the recess  28   b  with a liquid crystal polymer that is a dielectric, when the slots  27   a  to  27   d  have the shape illustrated in  FIG. 13 . 
     As is apparent from  FIG. 22 , it is found that even when the slots  27   a  to  27   d  have either of the shapes of  FIGS. 12 and 13 , a gain of the antenna  21  with the hollow (see the solid line and the broken line) is higher than a gain of the antenna without the hollow (see the chain double-dashed line and the alternate long and short dashed line), in a band of 53 to 64 GHz. It is also found that a gain of the antenna  21  (see the solid line) in which the slots  27   a  to  27   d  have the shape illustrated in  FIG. 12  is higher than a gain of the antenna  21  (see the broken line) in which the slots  27   a  to  27   d  have the shape illustrated in  FIG. 13 , in the band of 53 to 63 GHz. 
     As is apparent from  FIG. 23 , it is found that even when the slots  27   a  to  27   d  have either of the shapes of  FIGS. 12 and 13 , the reflection coefficient of the antenna  21  with the hollow (see the solid line and the broken line) is lower than the reflection coefficient of the antenna without the hollow (see the chain double-dashed line and the alternate long and short dashed line) in bands of 52 to 60.5 and 63.5 to 68 GHz. 
     Modification Examples of Second Embodiment 
     Next, some modifications from the second embodiment will be described. The modifications which will be described below can be applied separately or in combination. 
     (1) In an embodiment described above, the antenna elements  29   a  to  29   d  are disposed in the single recess  28   b . In contrast, as illustrated in  FIG. 24 , the same number of recesses  28   e  to  28   h  as the number of the antenna elements  29   a  to  29   d  may be formed in the bonding surface  28   a  of the dielectric substrate  28 , and the antenna elements  29   a  to  29   d  may be individually disposed in the recesses  28   e  to  28   h , respectively. In this case, the antenna element  29   a  is formed on a bottom  28   i  of the recess  28   e , the antenna element  29   b  is formed on a bottom  28   j  of the recess  28   f , the antenna element  29   c  is formed on a bottom  28   k  of the recess  28   g , and the antenna element  29   d  is formed on a bottom  28   m  of the recess  28   h . The slot  27   a  is arranged on the inner side with respect to an opening  28   p  of the recess  28   e . The slot  27   b  is arranged on the inner side with respect to an opening  28   q  of the recess  28   f . The slot  27   c  is arranged on the inner side with respect to an opening  28   r  of the recess  28   g . The slot  27   d  is arranged on the inner side with respect to an opening  28   s  of the recess  28   h . The antenna elements  29   a  to  29   d  face the slots  27   a  to  27   d , respectively. This improves strength of the dielectric substrate  28  by virtue of portions each between adjacent two of the recesses  28   e  to  28   h , so that the dielectric substrate  28  is less likely to be deformed. Thus, radiation characteristics of the antenna  21  are stabilized. 
     (2) In an embodiment described above, there is one group of the antenna elements  29   a  to  29   d , the slots  27   a  to  27   d , and the feed line  24   a . In contrast, there may be a plurality of groups each including the antenna elements  29   a  to  29   d , the slots  27   a  to  27   d , and the feed line  24   a . In this case, the plurality of groups each including the antenna elements  29   a  to  29   d , the slots  27   a  to  27   d , and the feed line  24   a  are aligned in a direction orthogonal to a row direction of the antenna elements  29   a  to  29   d . The positions in the row direction of the antenna elements  29   a  in the groups are aligned, the positions in the row direction of the antenna elements  29   b  in the groups are aligned, the positions in the row direction of the antenna elements  29   c  in the groups are aligned, and the positions in the row direction of the antenna elements  29   d  in the groups are aligned. The antenna elements  29   a  to  29   d  in all of the groups may be arranged in the single recess  28   b . The antenna elements  29   a  to  29   d  in each of the groups may be arranged in each recess  28   b . The antenna elements  29   a  to  29   d  may be individually arranged in recesses, respectively. The directivity of an electromagnetic wave can be controlled by controlling the phase of a signal wave of each feed line  24   a.    
     (3) In an embodiment described above, the four antenna elements  29   a  to  29   d  are aligned, and the four slots  27   a  to  27   d  are aligned. In contrast, two, six, or more even-number of the antenna elements may be aligned, and the same number of slots as the number of antenna elements may be aligned. In this case, the feed line  24   a  branches into two at a point between the slots adjacent to each other, and the branch line portions  24   f  and  24   h  extend from the point of branch until the branch line portions cross the slots at both ends of the row of the slots in plan view, respectively. The feed line  24   a  preferably branches at a point between the slots adjacent to each other that are positioned in the center of the row of the slots. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  21  antenna 
           4   a ,  24   a  feed line 
           6 ,  26  dielectric layer 
           7 ,  27  conductive ground layer 
           7   a ,  27   a ,  27   b ,  27   c ,  27   d  slot 
           8 ,  28  dielectric substrate 
           8   b ,  28   b ,  28   e ,  28   f ,  28   g ,  28   h  recess 
           8   d ,  28   d ,  28   i ,  28   j ,  28   k ,  28   m  bottom of recess 
           9 ,  28   a - 29   d  antenna element 
           270   a ,  275   a  hole portion 
           271   a - 274   a ,  276   a - 279   a  cut-out portion