Patent Publication Number: US-2023139670-A1

Title: Antenna module and communication device incorporating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of International Patent Application No. PCT/JP2021/016805, filed Apr. 27, 2021, which claims priority to Japanese Patent Application No. 2020-114111, filed Jul. 1, 2020, the entire contents of each of which being incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an antenna module and a communication device incorporating the antenna module, and more specifically relates to an antenna layout of an antenna module in which a board dimension is constrained. 
     BACKGROUND ART 
     In recent years, a communication terminal device typified by a mobile phone or a smartphone is configured in such a manner as to be able to transmit and receive a plurality of radio waves of different frequency bands. In such a multiband communication device, antenna elements each corresponding to a radio wave of each frequency band are installed. 
     Japanese Unexamined Patent Application Publication No. 2003-152431 (Patent Document 1) discloses a multifrequency planar antenna that can be used in a plurality of frequency bands, at least one of which uses a circularly polarized wave. In the antenna of Japanese Unexamined Patent Application Publication No. 2003-152431 (Patent Document 1), a plurality of radiating electrodes are formed in a concentric fashion on the same plane, and further a 90 degree hybrid for supplying a radio frequency signal to each radiating electrode is formed in such a manner as to be concentric with the radiating electrodes. With such a configuration, a small-size planar-type composite antenna having good circularly polarized wave characteristics can be realized. 
     CITATION LIST 
     Patent Document 
     
         
         Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-152431 
       
    
     SUMMARY 
     Technical Problems 
     In mobile terminals such as mobile phones or smartphones, there are needs for further downsizing and thinning. Because of this, internal devices such as an antenna module also need to be downsized, and profile heights of these internal devices need to be reduced. Particularly, in the case where a display screen is formed on the whole principal surface of the device such as a smartphone, there is a limitation on the area where an antenna device, in which antenna elements (radiating elements) are formed in the inside of a communication device, can be arranged. Further, the dimension of the antenna device itself may also be constrained. 
     In the case where a patch antenna having a flat plate shape is used as a radiating element, the patch antenna functions as an antenna by using electromagnetic coupling established between the radiating element and a ground electrode arranged opposite the radiating element. However, in the case where the dimension constraint of the antenna device makes it difficult to secure a sufficient area for the ground electrode, there is a possibility that desired antenna characteristics cannot be realized because the electromagnetic coupling between the radiating element and the ground electrode becomes insufficient or the electromagnetic coupling is disturbed. 
     Issues such as the ones described above caused by the dimension constraint are not considered in Japanese Unexamined Patent Application Publication No. 2003-152431 (Patent Document 1). 
     The present disclosure is made to resolve such issues, as well as other issues, and thus an aspect thereof is to suppress degradation of antenna characteristics in an antenna module capable of radiating radio waves of two different frequency bands. 
     Solutions to Problems 
     An antenna module in accordance with a certain aspect of the present disclosure includes a first radiating element and a second radiating element each having a flat plate shape and a ground electrode arranged opposite the first radiating element and the second radiating element. The first radiating element radiates a radio wave in a first frequency band. The second radiating element radiates a radio wave in a second frequency band that is higher than the first frequency band. In a plan view, as seen from a normal direction of the first radiating element, a distance from a center of the first radiating element to an end portion of the ground electrode in a first direction is shorter than ½ of a free space wavelength of a radio wave radiated from the first radiating element. The first radiating element radiates a radio wave of a single polarization direction. A feed point of the first radiating element is arranged at a location shifted in a second direction from the center of the first radiating element, and the second direction is different from the first direction. A feed point of the second radiating element is arranged at a location shifted in a third direction from a center of the second radiating element. 
     An antenna module in accordance with another aspect of the present disclosure includes a first radiating element and a second radiating element each having a flat plate shape and a ground electrode arranged opposite the first radiating element and the second radiating element. The first radiating element radiates a radio wave in a first frequency band. The second radiating element radiates a radio wave in a second frequency band that is higher than the first frequency band. In a plan view seen from a normal direction of the first radiating element, a dimension of the ground electrode in a first direction is shorter than a dimension of the ground electrode in a second direction that is different from the first direction. The first radiating element radiates a radio wave in a single polarization direction. A feed point of the first radiating element is arranged at a location shifted in the second direction from a center of the first radiating element, and the second direction is different from the first direction. Feed points of the second radiating element are arranged at a location shifted in a third direction from a center of the second radiating element and at a location shifted in a fourth direction from the center of the second radiating element, and the fourth direction is different from the third direction. 
     Advantageous Effects of Disclosure 
     According to the antenna modules in accordance with the present disclosure, under an environment where there is a constraint on the dimension of the ground electrode, with regard to the first radiating element on the low frequency side, for which a sufficient distance from the ground electrode cannot be secured in the plan view, a feed point is provided in a direction that is different from a dimension constraint direction (first direction), and with regard to the second radiating element on the high frequency side, which suffers less impact of the dimension constraint, feed points are provided in two directions. With such configuration, in the radiating element on the low frequency side, a polarized wave for which the antenna characteristics become insufficient is suppressed, and in the radiating element on the high frequency side, radio waves of two polarized waves can be radiated. Therefore, in a dual-band-type antenna module, degradation of antenna characteristics can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a block diagram of a communication device in which an antenna module according to an embodiment is employed. 
         FIG.  2    is a plan transparent view of the antenna module according to the embodiment. 
         FIG.  3    is a side transparent view of the antenna module of  FIG.  2   . 
         FIG.  4    is a plan transparent view of an antenna module of a modified example 1. 
         FIG.  5    is a side transparent view of an antenna module of a modified example 2. 
         FIG.  6    is a plan transparent view of an antenna module of a modified example 3. 
         FIG.  7    is a plan transparent view of an antenna module of a modified example 4. 
         FIG.  8    is a plan transparent view of an antenna module of a modified example 5. 
         FIG.  9    is a perspective view of an antenna module of a modified example 6. 
         FIG.  10    is a side transparent view of an antenna module of a modified example 7. 
         FIG.  11    is a side transparent view of an antenna module of a modified example 8. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the same reference symbols are assigned to the same or corresponding parts in the drawings, and descriptions thereof will not be repeated. 
     Embodiment 
     (Basic Configuration of Communication Device) 
       FIG.  1    is a block diagram of one example of a communication device  10  in which an antenna module  100  according to the present embodiment is employed. The communication device  10  is, for example, a mobile terminal such as a mobile phone, a smartphone, a tablet, or the like, a personal computer with communication capability, or the like. 
     Referring to  FIG.  1   , the communication device  10  includes the antenna module  100  and a BBIC  200 , which makes up a baseband signal processing circuit. The antenna module  100  includes a RFIC  110 , which is one example of a feed circuit, and an antenna device  120 . The communication device  10  up-converts a signal, which is sent from the BBIC  200  to the antenna module  100 , into a radio frequency signal and radiates the radio frequency signal from the antenna device  120 , and further down-converts a radio frequency signal received by the antenna device  120  and performs processing on a down-converted signal in the BBIC  200 . 
     The antenna device  120  of  FIG.  1    has a configuration in which radiating elements  125  are arranged in a two-dimensional array shape. Each of the radiating elements  125  includes two feed elements  121  and  122 . The feed elements  121  and  122  are arranged in such a manner as to overlap each other in the normal direction of the feed elements, as will be described below with reference to  FIG.  2   . The antenna device  120  is configured to be able to radiate radio waves of different frequency bands from the feed element  121  and the feed element  122  of the radiating element  125 . That is to say, the antenna device  120  is a stack-type dual-band-type antenna device. Different radio frequency signals are supplied to the feed elements  121  and  122  from the RFIC  110 . 
     In  FIG.  1   , for ease of description, of a plurality of radiating elements  125  that makes up the antenna device  120 , only a configuration corresponding to four radiating elements  125  is illustrated, and configurations corresponding to other radiating elements  125  having a similar configuration are omitted. Note that the antenna device  120  is not necessarily a two-dimensional array. Alternatively, the antenna device  120  may be made up of a single radiating element  125 . Further, the antenna device  120  may alternatively be a one-dimensional array in which a plurality of radiating elements  125  are arranged in a row. In the present embodiment, the feed elements  121  and  122  included in the radiating element  125  are each a patch antenna having a flat plate shape. 
     The RFIC  110  includes switches  111 A to  111 H,  113 A to  113 H,  117 A, and  117 B, power amplifiers  112 AT to  112 HT, low noise amplifiers  112 AR to  112 HR, attenuators  114 A to  114 H, phase shifters  115 A to  115 H, signal combiner/splitters  116 A and  116 B, mixers  118 A and  118 B, and amplifier circuits  119 A and  119 B. Of these, the configuration including the switches  111 A to  111 D,  113 A to  113 D, and  117 A, the power amplifiers  112 AT to  112 DT, the low noise amplifiers  112 AR to  112 DR, the attenuators  114 A to  114 D, the phase shifters  115 A to  115 D, the signal combiner/splitter  116 A, the mixer  118 A, and the amplifier circuit  119 A is a circuit for a radio frequency signal of a first frequency band on the low frequency side, which is radiated from the feed element  121 . Further, the configuration including the switches  111 E to  111 H,  113 E to  113 H, and  117 B, the power amplifiers  112 ET to  112 HT, the low noise amplifiers  112 ER to  112 HR, the attenuators  114 E to  114 H, the phase shifters  115 E to  115 H, the signal combiner/splitter  116 B, the mixer  118 B, and the amplifier circuit  119 B is a circuit for a radio frequency signal of a second frequency band on the high frequency side, which is radiated from the feed element  122 . 
     When a radio frequency signal is transmitted, the switches  111 A to  111 H and  113 A to  113 H are switched to the sides of the power amplifiers  112 AT to  112 HT, and the switches  117 A and  117 B are connected to transmitting side amplifiers of the amplifier circuits  119 A and  119 B. When a radio frequency signal is received, the switches  111 A to  111 H and  113 A to  113 H are switched to the sides of the low noise amplifiers  112 AR to  112 HR, and the switches  117 A and  117 B are connected to receiving side amplifiers of the amplifier circuits  119 A and  119 B. 
     Signals sent from the BBIC  200  are amplified in the amplifier circuits  119 A and  119 B and up-converted in the mixers  118 A and  118 B. Transmitting signals, which are up-converted radio frequency signals, are split into four signals in the signal combiner/splitters  116 A and  116 B, and these four signals are each fed to different feed elements  121  and  122  after traveling through corresponding signal paths. The directivity of the antenna device  120  can be adjusted by separately adjusting the degree of phase shift in the phase shifters  115 A to  115 H that are arranged in the respective signal paths. 
     Received signals, which are radio frequency signals received by the respective feed elements  121  and  122 , are sent to the RFIC  110  and multiplexed in the signal combiner/splitters  116 A and  116 B after traveling through the four different signal paths. Multiplexed received signals are down-converted in the mixers  118 A and  118 B, amplified in the amplifier circuits  119 A and  119 B, and sent to the BBIC  200 . 
     The RFIC  110  is formed as, for example, a one-chip integrated circuit component including the foregoing circuit configuration. Alternatively, devices (switch, power amplifier, low noise amplifier, attenuator, and phase shifter) corresponding to each radiating element  125  in the RFIC  110  may be formed as a one-chip integrated circuit component for each radiating element  125 . 
     (Configuration of Antenna Module) 
     Next, referring to  FIG.  2    and  FIG.  3   , the configuration of the antenna module  100  according to the present embodiment is described in detail. 
       FIG.  2    illustrates a plan transparent view of the antenna module  100 , and  FIG.  3    is a side transparent view of the antenna module  100 . In the following description, for ease of description, the antenna module is described using an example in which a single radiating element  125  is formed. Note that as illustrated in  FIG.  2    and  FIG.  3   , the thickness direction of the antenna module  100  is the Z-axis direction, and a plane vertical to the Z-axis direction is defined by the X-axis and the Y-axis. Further, in some cases, the positive direction of the Z-axis is referred to as a top surface side and the negative direction of the Z-axis is referred to as a bottom surface side in the respective drawings. 
     Referring to  FIG.  2    and  FIG.  3   , the antenna module  100  includes, in addition to the RFIC  110  and the radiating element  125  (feed elements  121  and  122 ), a dielectric substrate  130 , feed lines  141 A,  141 B,  142 A, and  142 B, and a ground electrode GND. In the plan transparent view, the RFIC  110 , the dielectric substrate  130 , and the respective feed lines are omitted. 
     The dielectric substrate  130  is, for example, a low temperature co-fired ceramics (LTCC) multilayer substrate, a multilayer resin substrate formed by laminating a plurality of resin layers, each of which is composed of a resin such as epoxy, polyimide, or the like, a multilayer resin substrate formed by laminating a plurality of resin layers, each of which is composed of a liquid crystal polymer (LCP) having a lower dielectric constant, a multilayer resin substrate formed by laminating a plurality of resin layers, each of which is composed of a fluorine-based resin, or a ceramics multilayer substrate other than LTCC. Note that the dielectric substrate  130  is not necessarily a multilayer substrate and may alternatively be a single layer substrate. 
     In the plan view seen from the normal direction (Z-axis direction), the dielectric substrate  130  is formed in a rectangular shape or a substantially rectangular shape, which has long sides parallel to the X-axis and short sides parallel to the Y-axis. On a bottom surface  132  (surface in the negative direction of the Z-axis) side of the dielectric substrate  130 , the ground electrode GND that has substantially the same rectangular shape as the dielectric substrate  130  is arranged. On a top surface  131  (surface in the positive direction of the Z-axis) side of the dielectric substrate  130 , the feed element  122  is arranged opposite the ground electrode GND. The feed element  122  may be configured in such a manner as to be exposed on the top surface  131  of the dielectric substrate  130  or may be arranged in an internal layer of the dielectric substrate  130  as illustrated in the example of  FIG.  3   . The feed element  121  is arranged opposite the ground electrode GND in a layer that is located closer to the ground electrode GND than the feed element  122 . In other words, the feed element  121  is arranged in a layer located between the layer in which the feed element  122  is formed and the layer in which the ground electrode GND is formed. 
     The feed elements  121  and  122  each have a flat plate shape and are each formed of an electric conductor such as copper, aluminum, or the like. In the example of  FIG.  2   , in the plan view seen from the normal direction of the dielectric substrate  130 , the feed elements  121  and  122  each have a square shape or a substantially square shape and are each arranged in such a way that each side of the feed elements  121  and  122  is parallel to a side of the dielectric substrate  130  (and the ground electrode GND) having a rectangular shape. Note that the shapes of the feed elements  121  and  122  are not limited to a square shape and may alternatively be a polygonal shape, a circular shape, an elliptical shape, or a cross-like shape. 
     Further, in the plan view seen from the normal direction of the dielectric substrate  130 , the feed element  121  and the feed element  122  are arranged in such a manner as to overlap each other. The size of the feed element  122  is smaller than the size of the feed element  121 , and a resonant frequency of the feed element  122  is higher than a resonant frequency of the feed element  121 . That is to say, a frequency band (second frequency band) of a radio wave radiated from the feed element  122  is higher than a frequency band (first frequency band) of a radio wave radiated from the feed element  121 . For example, the frequency band of a radio wave radiated from the feed element  122  is a 39 GHz band, and the frequency band of a radio wave radiated from the feed element  121  is a 28 GHz band. 
     The RFIC  110  is mounted on the bottom surface  132  of the dielectric substrate  130  with solder bumps  150  interposed therebetween. Note that instead of using the solder connection, the RFIC  110  may be connected to the dielectric substrate  130  using a multipole connector. 
     Radio frequency signals are sent to the feed element  121  from the RFIC  110  via the feed lines  141 A and  141 B. The feed lines  141 A and  141 B extend from the RFIC  110 , penetrate the ground electrode GND, and are connected to feed points SP 1 A and SP 1 B from the bottom surface side of the feed element  121 , respectively. That is to say, the feed lines  141 A and  141 B send radio frequency signals to the feed points SP 1 A and SP 1 B of the feed element  121 , respectively. Note that in the plan view seen from the normal direction of the dielectric substrate  130 , the locations of the feed points SP 1 A and SP 1 B of the feed element  121  on the low frequency side may overlap the feed element  122  on the high frequency side. However, from the viewpoint of isolation of two radio waves, it is an option that the feed points SP 1 A and SP 1 B are arranged at locations that do not overlap the feed element  122 . 
     The feed point SP 1 A is arranged at a location shifted in the negative direction of the X-axis from the center of the feed element  121 . Further, the feed point SP 1 B is arranged at a location shifted in the positive direction of the X-axis from the center of the feed element  121 . Therefore, when radio frequency signals are sent to the feed points SP 1 A and SP 1 B, a radio wave whose polarization direction is the X-axis direction is radiated from the feed element  121 . A signal whose phase is opposite to the phase of a radio frequency signal supplied to the feed point SP 1 A is supplied to the feed point SP 1 B. In other words, the phase difference between a radio frequency signal supplied to the feed point SP 1 A and a radio frequency signal supplied to the feed point SP 1 B is 180 degrees. As described above, by separately supplying radio frequency signals whose phases are opposite to each other to the feed points SP 1 A and SP 1 B, it becomes possible to double the power of a radio wave radiated from the feed element  121 . 
     Radio frequency signals are sent to the feed element  122  from the RFIC  110  via the feed lines  142 A and  142 B. The feed lines  142 A and  142 B extend from the RFIC  110 , penetrate the ground electrode GND and the feed element  121 , and are connected to feed points SP 2 A and SP 2 B from the bottom surface side of the feed element  122 . That is to say, the feed lines  142 A and  142 B send radio frequency signals to the feed points SP 2 A and SP 2 B of the feed element  122 . 
     The feed point SP 2 A is arranged at a location shifted in the negative direction of the X-axis from the center of the feed element  122 . Further, the feed point SP 2 B is arranged at a location shifted in the negative direction of the Y-axis from the center of the feed element  122 . Therefore, when a radio frequency signal is sent to the feed point SP 2 A, a radio wave whose polarization direction is the X-axis direction is radiated from the feed element  122 . Further, when a radio frequency signal is sent to the feed point SP 2 B, a radio wave whose polarization direction is the Y-axis direction is radiated from the feed element  122 . 
     Note that as illustrated in  FIG.  3   , for the antenna module  100 , the configuration has been described in which each of the feed elements  121  and  122  is fed by being directly connected to the feed line. However, the configuration may alternatively be such that one or both of the feed elements  121  and  122  are fed by using capacitive coupling between the one or both of the feed elements  121  and  122  and the corresponding feed line or lines. 
     As illustrated in  FIG.  2   , in the plan view seen from the Z-axis direction, the antenna module  100  has a rectangular shape whose long side is in the X-axis direction and whose short side is in the Y-axis direction. In other words, the dimension in the Y-axis direction (first direction) is shorter than the dimension in the X-axis direction (second direction). In the antenna module  100 , the distance from an end portion of the feed element  121  to an end portion of the dielectric substrate  130  (that is to say, the ground electrode GND) in the Y-axis direction is made shorter than the corresponding distance in the X-axis direction. For example, in the case where an antenna module is arranged on a side surface of a thin communication device such as a smartphone, the dimension of the communication device in the thickness direction (for example, the dimension in the Y-axis direction in  FIG.  2   ) is limited. 
     As in the antenna module  100 , in the case where a patch antenna having a flat plate shape is used as a radiating element, the patch antenna functions as an antenna by using electromagnetic coupling established between the radiating element and a ground electrode arranged opposite the radiating element. In the case of the antenna module  100 , the feed element  121  functions as an antenna by using electromagnetic coupling formed between the feed element  121  and the ground electrode GND. On the other hand, the feed element  122  functions as an antenna because the feed element  121  plays a role of the ground electrode and electromagnetic coupling is formed between the feed element  122  and the feed element  121 . 
     In the case where a sufficient area of the ground electrode cannot be secured due to the dimension constraint of the dielectric substrate and the like, there is a possibility that desired antenna characteristics cannot be realized because the electromagnetic coupling between the radiating element and the ground electrode becomes insufficient or the electromagnetic coupling is disturbed. Accordingly, as in the antenna module  100  illustrated in  FIG.  2   , in the case where the dimension of the dielectric substrate  130  in the Y-axis direction is constrained, there is a possibility of having an impact on antenna characteristics relating to a radio wave whose polarization direction is the Y-axis direction in the feed element  121  on the low frequency side. More specifically, in the case where the shortest distance L 2  from the center of the feed element  121  to an end portion of the ground electrode GND in the Y-axis direction is shorter than ½ of a free space wavelength λ LO  of a radio wave radiated from the feed element  121 , degradation of the antenna characteristics may become prominent. 
     Therefore, in the foregoing case where the shortest distance L 2  is shorter than ½ of the free space wavelength λ LO , with regard to the feed element  121 , the feed points SP 1 A and SP 1 B are arranged at locations shifted in the X-axis direction from the center of the feed element  121  in such a way that only a radio wave whose polarization direction is the X-axis direction is radiated. Because of this, a degrading impact on the antenna characteristics associated with the dimension constraint in the Y-axis direction can be removed. 
     On the other hand, the feed element  122  on the high frequency side functions as an antenna by using the electromagnetic coupling formed between the feed element  122  on the high frequency side and the feed element  121  on the low frequency side as described above, and thus, an impact on the antenna characteristics caused by the dimension constraint of the ground electrode GND is small. Accordingly, in the feed element  122 , the feed points SP 2 A and SP 2 B are arranged at a location shifted in the X-axis direction and a location shifted in the Y-axis direction from the center of the feed element  122 , respectively. Because of this, two radio waves having polarization directions different from each other can be radiated. 
     Note that the “feed element  121 ” and the “feed element  122 ” in the embodiment correspond to a “first radiating element” and a “second radiating element” in the present disclosure, respectively. Further, in the embodiment, the “Y-axis direction” corresponds to the “first direction” and a “third direction” in the present disclosure, and the “X-axis direction” corresponds to the “second direction” and a “fourth direction” in the present disclosure. 
     Note that in the foregoing description, the example is described in which the direction (first direction) along which the distance between the feed element  121  and the ground electrode GND becomes the shortest and the polarization direction (second direction) of a radio wave radiated from the feed element  121  are orthogonal to each other. However, the first direction and the second direction are not necessarily orthogonal to each other. Further, two polarization directions (third direction and fourth direction) of the feed element  122  are not necessarily orthogonal to each other. 
     Modified Example 1 
       FIG.  4    is a plan transparent view of an antenna module  100 X of a modified example 1. The antenna module  100 X of the modified example 1 is different from the antenna module  100  in that a radio wave of a single polarization direction is radiated from the feed element  122  on the high frequency side. More specifically, in the feed element  122 , a radio frequency signal is supplied only to the feed point SP 2 A. 
     As described above, compared with the feed element  121 , in the feed element  122  on the high frequency side, the dimension constraint of the ground electrode GND causes a less impact on the antenna characteristics. However, it is not always required to radiate radio waves of two polarization directions, and the configuration may be such that only a radio wave of a single polarization direction is radiated as illustrated in  FIG.  4   . 
     Modified Example 2 
       FIG.  5    is a side transparent view of an antenna module  100 A of a modified example 2. Compared with the antenna module  100 , the antenna module  100 A of the modified example 2 uses a different method of supplying a radio frequency signal to the feed point SP 1 B of the feed element  121  on the low frequency side. More specifically, a separate radio frequency signal is not supplied to the feed point SP 1 B from the RFIC  110 . Instead, for the feed point SP 1 B, a radio frequency signal is supplied by a feed line  141 C that branches from the feed line  141 A, which supplies a radio frequency signal to the feed point SP 1 A. At that time, the path length of the feed line  141 C is set to a length that gives the phase opposite to that of a signal transmitted to the feed point SP 1 A (for example, ½ of the wavelength of the signal being transmitted). 
     Note that in the case of the antenna module  100 A, a radio frequency signal is supplied to the feed element  121  from the RFIC  110  using a single path. Thus, the power of a radio wave being radiated becomes ½ of the power of the antenna module  100 . 
     Modified Example 3 
       FIG.  6    is a plan transparent view of an antenna module  100 B of a modified example 3. In the antenna module  100 B of the modified example 3, the feed element  122  on the high frequency side is arranged in such a manner as to be tilted against the feed element  121 . Specifically, the feed element  122  is arranged in such a way that an angle formed by each side of the feed element  122  and the X-axis and an angle formed by each side of the feed element  122  and the Y-axis are 45 degrees. Because of this, the feed element  122  radiates radio waves whose polarization directions are directions of 45 degrees and −45 degrees from the X-axis. 
     As described above, the feed element  122  on the high frequency side functions as an antenna by using electromagnetic coupling formed between the feed element  122  and the feed element  121 . Accordingly, in the case where the distance from the center of feed element  122  to the end portion of the feed element  121  in the polarization direction is limited, degradation of the antenna characteristics of the feed element  122  can be suppressed by increasing the foregoing distance by arranging the feed element  122  in such a manner as to be tilted against the feed element  121 , as in the antenna module  100 B. 
     Note that in the modified example 3, the “Y-axis direction” and the “X-axis direction” respectively correspond to the “first direction” and the “second direction” of the present disclosure, and the direction of 45 degrees and the direction of −45 degrees from the X-axis respectively correspond to the “third direction” and the “fourth direction” of the present disclosure. 
     Modified Example 4 
     In a modified example 4, a configuration is described in which two feed elements are arranged side by side.  FIG.  7    is a plan transparent view of an antenna module  100 C of the modified example 4. The antenna module  100 C is not a stack-type antenna module such as the one illustrated in  FIG.  3   , and in the antenna module  100 C, two feed elements  121  and  122 A are arranged side by side with a gap in between. More specifically, in the example of  FIG.  7   , the feed element  121  and the feed element  122 A are arranged side by side in the X-axis direction. 
     In the antenna module  100 C, the shortest distance L 2  from the centers of the feed elements  121  and  122 A to the end portion of the ground electrode GND in the Y-axis direction is shorter than ½ of the free space wavelength λ LO  of a radio wave radiated from the feed element  121  and is longer than ½ of a free space wavelength λ HO  of a radio wave radiated from the feed element  122 A. On the other hand, a shortest distance L 4  from the center of the feed element  121  to an end portion of the ground electrode GND in the X-axis direction is longer than ½ of the free space wavelength λ LOf  and a shortest distance L 5  from the center of the feed element  122 A to an end portion of the ground electrode GND in the X-axis direction is longer than ½ of the free space wavelength λ HO . 
     Accordingly, the feed points of the feed element  121  are arranged at locations shifted in the X-axis direction from the center of the feed element  121 , and the feed points of the feed element  122 A are arranged at a location shifted in the X-axis direction and a location shifted in the Y-axis direction from the center of the feed element  122 A. 
     As described above, also in the antenna module in which two feed elements having frequency bands different from each other are arranged side by side, degradation of the antenna characteristics can be suppressed by causing the feed element to radiate a radio wave of a single polarization direction in the case where there is a constraint on the distance from the center of the radiating element to the end portion of the ground electrode GND and by causing the feed element to radiate radio waves of two polarization directions in the case where there is no constraint on that distance. 
     Modified Example 5 
     In a modified example 5, a case of an array antenna is described in which a plurality of stack-type radiating elements is arranged. 
       FIG.  8    is a plan transparent view of an antenna module  100 D of the modified example 5. In the antenna module  100 D, four radiating elements  125 - 1  to  125 - 4  are arranged in a row along the X-axis direction with gaps in between. The radiating element  125 - 1  includes a feed element  121 - 1  on the low frequency side and a feed element  122 - 1  on the high frequency side. The radiating element  125 - 2  includes a feed element  121 - 2  on the low frequency side and a feed element  122 - 2  on the high frequency side. The radiating element  125 - 3  includes a feed element  121 - 3  on the low frequency side and a feed element  122 - 3  on the high frequency side. The radiating element  125 - 4  includes a feed element  121 - 4  on the low frequency side and a feed element  122 - 4  on the high frequency side. 
     The shortest distance L 2  from the center of each feed element to the end portion of the ground electrode GND in the Y-axis direction is longer than ½ of the free space wavelength λ HO  of radio waves radiated from the feed elements  122 - 1  to  122 - 4  on the high frequency side, and is shorter than ½ of the free space wavelength λ LO  of radio waves radiated from the feed elements  121 - 1  to  121 - 4  on the low frequency side. Further, with respect to the radiating elements  125 - 1  and  125 - 4  arranged on end portions, the shortest distance L 4  from the center of the radiating element to the end of the ground electrode GND in the X-axis direction is longer than ½ of the free space wavelength λ LO . 
     Accordingly, with respect to the feed elements  121 - 1  to  121 - 4  on the low frequency side, the feed points are arranged at locations shifted in the X-axis direction from the center of each feed element. On the other hand, with respect to the feed elements  122 - 1  to  122 - 4  on the high frequency side, the feed points are arranged at a location shifted in the X-axis direction and at a location shifted in the Y-axis direction from the center of each feed element. 
     As described above, also in the case where the antenna module is an array antenna, degradation of the antenna characteristics can be suppressed by causing the feed element to radiate a radio wave of a single polarization direction in the case where there is a constraint on the distance from the center of the feed element to the end portion of the ground electrode GND and by causing the feed element to radiate radio waves of two polarization directions in the case where there is no constraint on that distance. 
     Note that in the modified example 5, for example, the “feed element  121 - 1 ” and the “feed element  122 - 1 ” of the radiating element  125 - 1  respectively correspond to the “first radiating element” and the “second radiating element” of the present disclosure, and the “feed element  121 - 2 ” and the “feed element  122 - 2 ” of the radiating element  125 - 2  respectively correspond to a “third radiating element” and a “fourth radiating element” of the present disclosure. Further, in the modified example 5, the “Y-axis direction” corresponds to the “first direction” and the “third direction” in the present disclosure, and the “X-axis direction” corresponds to the “second direction” and a “fourth direction” in the present disclosure. 
     Modified Example 6 
     In a modified example 6, a case of an antenna module having two array antennas is described. 
       FIG.  9    is a perspective view of an antenna module  100 Y of the modified example 6. The antenna module  100 Y includes two different dielectric substrates  130 B and  130 C extending in the Y-axis direction. Each of the dielectric substrates  130 B and  130 C has a substantially rectangular shape whose long sides are in the Y-axis direction and has a plurality of stack-type radiating elements arranged along the Y-axis direction. Further, the RFIC  110  is arranged on a bottom surface of the dielectric substrate  130 B. 
     The normal direction of the dielectric substrate  130 B is the Z-axis direction, and the normal direction of the dielectric substrate  130 C is the X-axis direction. The dielectric substrate  130 B and the dielectric substrate  130 C are connected to each other by a bent connection member  123 . That is to say, the antenna module  100 Y has a substantially L shape in the plan view seen from the Y-axis direction. With such configuration, the antenna module  100 Y can radiate a radio wave in two different directions, which are the X-axis direction and the Z-axis direction. 
     In the dielectric substrate  130 B, four radiating elements are arranged in a row along the Y-axis direction with gaps in between. Each radiating element of the dielectric substrate  130 B includes a feed element  121 B on the low frequency side and a feed element  122 B on the high frequency side. Further, also in the dielectric substrate  130 C, four radiating elements are arranged in a row along the Y-axis direction with gaps in between. Each radiating element of the dielectric substrate  130 C includes a feed element  121 C on the low frequency side and a feed element  122 C on the high frequency side. 
     Here, a dimension L 20  in the short side direction (Z-axis direction) of the dielectric substrate  130 C is shorter than a dimension L 10  in the short side direction (X-axis direction) of the dielectric substrate  130 B (L 10 &gt;L 20 ). Accordingly, in the dielectric substrate  130 C, the dimension of the ground electrode in the Z-axis direction is limited. Therefore, as is the case with the modified example 5, in the feed elements  121 C on the low frequency side of the dielectric substrate  130 C, the feed points are arranged at locations shifted in the Y-axis direction from the center of each feed element while in the feed elements  122 C on the high frequency side, the feed points are arranged at a location shifted in the Y-axis direction and a location shifted in the Z-axis direction from the center of each feed element. 
     In the dielectric substrate  130 B having a less limitation on the dimension of the ground electrode, in both the feed elements  121 B and  122 B, the feed points are arranged at a location shifted in the X-axis direction and at a location shifted in the Y-axis direction from the center of each feed element. Note that also in the dielectric substrate  130 B, as is the case with the dielectric substrate  130 C, in the case where the dimension L 10  in the short side direction is made shorter, the feed element  121 B on the low frequency side may be configured in such a way that the polarization direction of the feed element  121 B is the Y-axis direction only. 
     As described above, also in the case where the antenna module has two array antennas capable of radiating radio waves in directions different from each other, degradation of the antenna characteristics can be suppressed by causing the feed element to radiate a radio wave of a single polarization direction in the case where there is a constraint on the distance from the center of the feed element to the end portion of the ground electrode GND and by causing the feed element to radiate radio waves of two polarization directions in the case where there is no constraint on that distance (modified examples 7 and 8). In the antenna module  100  of the embodiment illustrated in  FIG.  3   , the configuration in which the feed elements  121  and  122  are arranged in the same dielectric substrate  130  is illustrated. However, the configuration may alternatively be such that one or both of the feed elements  121  and  122  are arranged in separated different dielectric substrates. 
       FIG.  10    is a side transparent view of an antenna module  100 E of a modified example 7. 
     The antenna module  100 E has the configuration in which the feed elements  121  and  122  are formed in a dielectric substrate  170  and the ground electrode GND is formed in a dielectric substrate  160 . The dielectric substrate  170  corresponds to, for example, a housing of the communication device  10 , and radio frequency signals are supplied from the RFIC  110  arranged in the dielectric substrate  160  to the radiating elements buried in the housing in advance. 
     In the dielectric substrate  170 , the feed element  122  is formed on a top surface  171  side, and the feed element  121  is formed on a bottom surface  172  side in such a manner as to face the feed element  122 . The dielectric substrates  160  and  170  are arranged in such a way that the bottom surface  172  of the dielectric substrate  170  and a top surface  161  of the dielectric substrate  160  face each other. The RFIC  110  is mounted on the bottom surface  162  of the dielectric substrate  160  with solder bumps  150  interposed therebetween. 
     Connection terminals  180  such as solder bumps or the like are formed between the dielectric substrate  160  and the dielectric substrate  170  and electrically connect the dielectric substrate  160  and the dielectric substrate  170 . Specifically, feed lines  141 A,  141 B,  142 A, and  142 B are connected to corresponding feed points of the feed elements via the connection terminals  180 . 
     Further,  FIG.  11    is a side transparent view of an antenna module  100 F of a modified example 8. The antenna module  100 F has the configuration in which the feed element  122  on the high frequency side is arranged in a dielectric substrate  170 A, and the feed element  121  and the ground electrode GND are formed in a dielectric substrate  160 A. 
     The connection terminals  180  are formed between the dielectric substrate  160 A and the dielectric substrate  170 A and electrically connect the dielectric substrate  160 A and the dielectric substrate  170 A. Specifically, feed lines  142 A and  142 B are connected to the corresponding feed points of the feed element  122  via the connection terminals  180 . 
     As described above, also in the antenna module having the configuration in which one or both of the feed elements are arranged in the separated different dielectric substrates, as is the case in  FIG.  2   , in the case where it is difficult to secure a sufficient distance between the center of the feed element  121  on the low frequency side and the end portion of the ground electrode GND, degradation of the antenna characteristics can be suppressed by not radiating a radio wave whose polarization direction is a direction of an insufficient distance and arranging the feed point in the direction (for example, an orthogonal direction) in which a sufficient distance to the ground electrode GND can be secured. 
     It is to be understood that the embodiments disclosed herein are exemplary in all aspects and are not restrictive. It is intended that the scope of the present invention is defined by the claims, not by the description of the foregoing embodiments, and includes all variations which come within the meaning and range of equivalency of the claims. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10  Communication device 
               100 ,  100 A to  100 F,  100 X,  100 Y Antenna module 
               110  RFIC 
               111 A to  111 H,  113 A to  113 H,  117 A,  117 B Switch 
               112 AR to  112 HR Low noise amplifier 
               112 AT to  112 HT Power amplifier 
               114 A to  114 H Attenuator 
               115 A to  115 H Phase shifter 
               116 A,  116 B Signal combiner/splitter 
               118 A,  118 B Mixer 
               119 A,  119 B Amplifier circuit 
               120  Antenna device 
               121 ,  121 - 1  to  121 - 4 ,  121 B,  121 C,  122 ,  122 - 1  to  122 - 4 ,  122 A to  122 C Feed element 
               125 ,  125 - 1  to  125 - 4  Radiating element 
               130 ,  130 B,  130 C,  160 ,  160 A,  170 ,  170 A dielectric substrate 
               141 A to  141 C,  142 A,  142 B Feed line 
               150  Solder bump 
               180  Connection terminal 
               200  BBIC 
             GND Ground electrode 
             SP 1 A, SP 1 B, SP 2 A, SP 2 B Feed point