Patent Publication Number: US-2021184344-A1

Title: Antenna module, communication device, and array antenna

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of International Application No. PCT/JP2019/035606 filed on Sep. 11, 2019 which claims priority from Japanese Patent Application No. 2018-182098 filed on Sep. 27, 2018. The contents of these applications are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     Field of the Disclosure 
     The present disclosure relates to an antenna module, a communication device, and an array antenna and more specifically to a technique for broadening the antenna module. 
     Description of the Related Art 
     International Publication No. 2016/063759 (Patent Document 1) discloses a patch antenna in which a plurality of radiation electrodes (feed elements, parasitic elements) having a planar shape are stacked. 
     Patent Document 1: International Publication No. 2016/063759 
     BRIEF SUMMARY OF THE DISCLOSURE 
     For the above antenna, permittivity of a dielectric substrate on which antenna elements (radiation electrodes) are implemented has an effect on its antenna characteristics, such as a frequency band width, a peak gain, and a loss of a transmittable radio-frequency signal. Among them, the frequency band width typically increases with the increase in the thickness of the dielectric substrate (that is, the distance between a radiation electrode and a ground electrode and the distance between radiation electrodes). 
     In particular, mobile terminals, such as smartphones, have been increasingly required to be thinner in recent years, and thus an antenna module itself has been needed to be more compact and thinner. If a dielectric substrate becomes thinner, however, an issue arises in that the frequency band width of the antenna becomes narrower. 
     The present disclosure is made to solve that problem, and an object thereof is to achieve a broad band without increasing the size of an antenna module. 
     An antenna module includes a dielectric substrate having a multilayer structure, a first radiation electrode, a second radiation electrode, and a ground electrode. The second radiation electrode is arranged between the first radiation electrode and the ground electrode in a lamination direction of the dielectric substrate. In the dielectric substrate, a hollow portion is disposed in at least a portion between the first radiation electrode and the second radiation electrode. 
     In the antenna module according to the present disclosure, the hollow portion is disposed in at least the portion between the stacked two radiation electrodes. In that configuration, in comparison with an antenna module in which the dielectric substrate has no hollow portion, the effective permittivity between the two radiation electrodes is reduced. Accordingly, the broad band can be achieved without increasing the size of the antenna module. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a block diagram of a communication device on which an antenna module is mounted according to a first embodiment. 
         FIG. 2  includes a plan view and a cross-sectional view of the antenna module in  FIG. 1 . 
         FIG. 3  is an illustration for explaining the comparison between antenna characteristics of the antenna module according to the first embodiment and those according to a comparative example. 
         FIG. 4  includes a plan view and a cross-sectional view of an antenna module according to Variation 1. 
         FIG. 5  includes a plan view and a cross-sectional view of an antenna module according to Variation 2. 
         FIG. 6  includes a plan view and a cross-sectional view of an antenna module according to Variation 3. 
         FIG. 7  includes a plan view and a cross-sectional view of an antenna module according to Variation 4. 
         FIG. 8  includes a plan view and a cross-sectional view of an antenna module according to Variation 5. 
         FIG. 9  includes a plan view and a cross-sectional view of an antenna module according to Variation 6. 
         FIG. 10  includes a plan view and a cross-sectional view of an antenna module according to Variation 7. 
         FIG. 11  is a first illustration for explaining the relation of the position of a hollow portion in a Y-axis direction and the frequency band width. 
         FIG. 12  is a second illustration for explaining the relation of the position of the hollow portion in the Y-axis direction and the frequency band width. 
         FIG. 13  is a first illustration for explaining the relation of the position of the hollow portion in an X-axis direction and the frequency band width. 
         FIG. 14  is a second illustration for explaining the relation of the position of the hollow portion in the X-axis direction and the frequency band width. 
         FIG. 15  is a cross-sectional view of an antenna module according to Variation 8. 
         FIG. 16  is a cross-sectional view of an antenna module according to Variation 9. 
         FIG. 17  is a cross-sectional view of an antenna module according to Variation 10. 
         FIG. 18  includes a plan view and a cross-sectional view of an antenna module according to a second embodiment. 
         FIG. 19  includes a plan view and a cross-sectional view of an antenna module according to Variation 11. 
         FIG. 20  includes a plan view and a cross-sectional view of an antenna module according to Variation 12. 
         FIG. 21  includes a plan view and a cross-sectional view of an antenna module according to Variation 13. 
         FIG. 22  includes a plan view and a cross-sectional view of an antenna module according to a third embodiment. 
         FIG. 23  includes a plan view and a cross-sectional view of an antenna module according to Variation 14. 
         FIG. 24  includes a plan view and a cross-sectional view of an antenna module according to Variation 15. 
         FIG. 25  includes a plan view and a cross-sectional view of an antenna module according to a fourth embodiment. 
         FIG. 26  is a plan view of an antenna array according to a fifth embodiment. 
         FIG. 27  is a plan view of an antenna array according to Variation 16. 
         FIG. 28  includes a plan view and a cross-sectional view of an antenna module according to a sixth embodiment. 
         FIG. 29  is a cross-sectional view of an antenna module according to a reference example. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     Embodiments of the present disclosure are described in detail below with reference to the drawings. The same reference numerals are used in the same or corresponding sections in the drawings, and the description about them is not repeated. 
     First Embodiment 
     (Basic Configuration of Communication Device) 
       FIG. 1  is a block diagram of an example of a communication device  10  in which an antenna module  100  according to the present embodiment is used. Examples of the communication device  10  may include a mobile terminal, such as a cellular phone, a smartphone, or a tablet, and a personal computer having the communication function. 
     Referring to  FIG. 1 , the communication device  10  includes the antenna module  100  and a base band integrated circuit (BBIC)  200  constituting a baseband signal processing circuit. The antenna module  100  includes a radio frequency integrated circuit (RFIC)  110  being one example of a feeder circuit and an antenna array  120 . The communication device  10  is configured to upconvert signals conveyed from the BBIC  200  to the antenna module  100  into radio-frequency signals and radiate them from the antenna array  120 , and configured to downconvert radio-frequency signals received at the antenna array  120  and perform signal-processing on the resultant signals in the BBIC  200 . 
     In  FIG. 1 , for facilitating explanation, among a plurality of radiation electrodes (antenna elements)  121  constituting the antenna array  120 , only a configuration corresponding to four radiation electrodes  121  is illustrated, and a similar configuration corresponding to the other radiation electrodes  121  is omitted. 
     The RFIC  110  includes switches  111 A to  111 D,  113 A to  113 D, and  117 , power amplifiers  112 AT to  112 DT, low-noise amplifiers  112 AR to  112 DR, attenuators  114 A to  114 D, phase shifters  115 A to  115 D, a signal combiner/splitter  116 , a mixer  118 , and an amplifier circuit  119 . 
     In transmission of radio-frequency signals, the switches  111 A to  111 D and  113 A to  113 D are switched to the side corresponding to the power amplifiers  112 AT to  112 DT, and the switch  117  becomes connected to a transmission-side amplifier in the amplifier circuit  119 . In reception of radio-frequency signals, the switches  111 A to  111 D and  113 A to  113 D are switched to the side corresponding to the low-noise amplifiers  112 AR to  112 DR, and the switch  117  becomes connected to a reception-side amplifier in the amplifier circuit  119 . 
     A signal conveyed from the BBIC  200  is amplified in the amplifier circuit  119  and is upconverted in the mixer  118 . The transmission signal being the upconverted radio-frequency signal is split into four signals in the signal combiner/splitter  116 , and they pass through four signal paths and are fed to mutually different radiation electrodes  121 . At that time, the directivity of the antenna array  120  can be adjusted by individually adjusting the phase-shift degrees of the phase shifters  115 A to  115 D arranged in the signal paths. 
     Reception signals being radio-frequency signals received at the radiation electrodes  121  pass through mutually different signal paths and are combined in the signal combiner/splitter  116 . The combined reception signal is downconverted in the mixer  118 , is amplified in the amplifier circuit  119 , and is conveyed to the BBIC  200 . 
     One example of the RFIC  110  may be formed as a one-chip integrated circuit component having the above-described circuitry. Alternatively, equipment (switches, power amplifiers, low-noise amplifier, attenuator, phase shifter) corresponding to each of the radiation electrodes  121  in the RFIC  110  may be formed as a one-chip integrated circuit component for each corresponding radiation electrode  121 . 
     (Structure of Antenna Module) 
       FIG. 2  includes a plan view (upper row) and a cross-sectional view (lower row) of the antenna module  100  according to the first embodiment. Referring to  FIG. 2 , the antenna module  100  includes the radiation electrode  121 , a radiation electrode  122 , a dielectric substrate  160 , a ground electrode GND, and the RFIC  110 . The cross-sectional view in the lower row is taken at a plane II-II extending through a feed point SP 1  for the radiation electrode  121  being a feed element in the plan view. In the following description, the positive direction and the negative direction of the Z axis in  FIG. 2  may be referred to as an upper-surface side and a lower-surface side, respectively. 
     In the following description, an example in which the radiation electrode  121  is a feed element and the radiation electrode  122  is a parasitic element is described. Both the radiation electrode  121  and the radiation electrode  122  may be feed elements. Conversely, the radiation electrode  121  may be a parasitic element, and the radiation electrode  122  may be a feed element. 
     The dielectric substrate  160  has a substantially rectangular shape when the antenna module  100  is seen in plan view from the direction of the normal to the dielectric substrate  160  (Z-axis direction in the drawing) and has a first side  161  to a fourth side  164 . In the example of the dielectric substrate  160  in  FIG. 2 , the short sides are the first side  161  and the third side  163 , and the long sides are the second side  162  and the fourth side  164 . The second side  162  and the fourth side  164  are adjacent to the first side  161 . The third side  163  is opposite to the first side  161 . 
     The dielectric substrate  160  has a multilayer structure in which a plurality of dielectric layers are laminated. The dielectric layers in the dielectric substrate  160  may be made of a resin, such as epoxy or polyimide. The dielectric layers may also be made by using a liquid crystal polymer (LCP) having lower permittivity, a fluorine-based resin, low temperature co-fired ceramics (LTCC), or the like. The RFIC  110  is implemented on one principal surface (lower surface) of the dielectric substrate  160  with solder bumps  130  disposed therebetween. 
     A plurality of columnar conductors  145  are arranged at predetermined intervals along the sides of the dielectric substrate  160  in its outer region. The plurality of columnar conductors  145  are connected to the ground electrode GND inside the dielectric substrate  160 . The plurality of columnar conductors  145  function as a shield on the side-surface side of the dielectric substrate  160 . In antenna modules described below with reference to  FIG. 3  and the subsequent drawings, the description of the columnar conductors  145  is omitted. 
     The ground electrode GND is arranged on a layer near the lower surface of the dielectric substrate  160 . The rectangular radiation electrode  122  (first radiation electrode) is arranged on a layer near the other principal surface (upper surface) of the dielectric substrate  160 . The rectangular radiation electrode  121  (second radiation electrode) is arranged on a layer between the radiation electrode  122  and the ground electrode GND. The radiation electrode  121  and the radiation electrode  122  overlap each other such that the points of intersection of their respective diagonal lines (that is, centers) coincide when the antenna module  100  is seen in plan view. In the example illustrated in  FIG. 2 , the radiation electrode  122  is larger than the radiation electrode  121 . However, both of the radiation electrodes may have the same size, or the radiation electrode  121  may be larger. 
     The radiation electrode  121  is electrically connected to the RFIC  110  with a feed line  140  disposed therebetween. The feed line  140  extends through the ground electrode GND and is connected to the feed point SP 1  for the radiation electrode  121 . The feed point SP 1  is arranged in a position displaced from the center of the radiation electrode  121  toward the second side  162 , which extends along the X axis, on the radiation electrode  121 . Thus, the radiation electrode  121  radiates a radio wave whose polarization direction is the Y-axis direction. 
     When the radiation electrode  122  is the feed element, one example of the feed line  140  may extend through the radiation electrode  121  and be connected to a feed point for the radiation electrode  122  by a via extending through a hollow portion  150 . Alternatively, the feed line  140  may be diverted around the hollow portion  150 , extend inside the dielectric substrate  160 , and be connected to the radiation electrode  122 . 
     In the dielectric substrate  160 , the hollow portion  150  is disposed in a layer between the radiation electrodes  121  and  122 . The dielectric substrate  160  includes a layer  165  supported by the first side  161  (hereinafter also referred to as “beam portion”) on the upper-surface side of the hollow portion  150 , and the radiation electrode  122  is arranged in the beam portion  165 . A cavity portion  152  is disposed along the second side  162  to the fourth side  164  around the beam portion  165 , and the cavity portion  152  extends through the dielectric substrate  160  to the hollow portion  150 . 
     It is known that in the above-described stack-type antenna module including the plurality of radiation electrodes stacked, the frequency band width of radio waves that can be radiated by the radiation electrodes is determined by the strength of electromagnetic-field coupling between the radiation electrode and the ground electrode and the strength of electromagnetic-field coupling between the radiation electrodes. As the strength of electromagnetic-field coupling increases, the frequency band width decreases, and as the strength of electromagnetic-field coupling decreases, the frequency band width increases. 
     Typically, an increase in the thickness of the dielectric substrate is needed for expanding the frequency band width of a radio wave radiated by a radiation electrode. The increased thickness of the dielectric substrate, however, may be a hindrance to a reduction in size and thickness of a communication device, such as a smartphone, that uses an antenna module and that is required to be smaller and thinner. 
     Here, the effective permittivity between the two electrodes also has an effect on the strength of electromagnetic-field coupling. More specifically, as the effective permittivity increases, the electromagnetic-field coupling becomes stronger, and as the effective permittivity decreases, the electromagnetic-field coupling becomes weaker. That is, the frequency band width can be expanded by a reduction in the effective permittivity between the two electrodes. 
     In the antenna module  100  according to the first embodiment, as described above, the hollow portion  150  is disposed between the radiation electrodes  121  and  122 . Typically, the permittivity of air is lower than that of the dielectric forming the dielectric substrate  160 . Thus, the effective permittivity between the radiation electrodes  121  and  122  can be reduced by the presence of the hollow portion  150 . That can result in weakened electromagnetic-field coupling between the radiation electrodes  121  and  122 . Accordingly, in the antenna module  100  according to the first embodiment, the frequency band width can be expanded without increasing the overall size of the module. 
     Because the loss of electric energy inside the dielectric can be reduced by the presence of the hollow portion  150 , the efficiency of the antenna module can be improved. 
     (Simulation Results) 
       FIG. 3  illustrates the simulation results of the comparison between the antenna characteristics of the antenna module  100  according to the first embodiment and those of an antenna module in which the dielectric substrate does not include the hollow portion  150  (comparative example).  FIG. 3  illustrates the reflection characteristic (upper row), gain (middle row), and efficiency (lower row) at a specific frequency (60.48 GHz). 
     In the simulation described below, an example in which the used frequency range is a millimeter-wave frequency range (gigahertz range) is described. The configuration of the present disclosure is also applicable to frequency ranges other than the millimeter wave. 
     Referring to  FIG. 3 , in the return loss in the comparative example (line LN 1 A in  FIG. 3 ), the frequency range where the return loss is below 10 dB is the range of 55.4 to 69.7 GHz (RNG 1 A), and the frequency band width is 14.3 GHz. On the other hand, in the return loss in the first embodiment (line LN 1  in  FIG. 3 ), the frequency range where the return loss is below 10 dB is the range of 55.2 to 77.1 GHz (RNG 1 ), and the frequency band width is 21.9 GHz. Hence, the frequency band width of the antenna module  100  according to the first embodiment is wider than that according to the comparative example. 
     In the graph of the gain in the middle row, the lines LN 2  and LN 2 A indicate the gain directivity, and the lines LN 3  and LN 3 A indicate the performance gain. The difference between the gain directivity and the performance gain is the loss in the antenna module. In the graph of the gain, the range where the gain directivity and the performance gain are close is also the above-described range RNG 1 A in the comparative example and the range RNG 1  in the first embodiment, and it is revealed that the range where the loss is low in the antenna module  100  according to the first embodiment is wider. The efficiency at 60.48 GHz (ratio of the radiated power to the input power), which is 91.4% in the comparative example, is improved to 94.0% in the first embodiment. 
     Hence, in the stack-type antenna module, the frequency band width can be expanded and the efficiency can be improved by disposing the hollow portion between the two radiation electrodes. 
     (Variations) 
     Next, antenna modules  100 A to  100 G according to variations are described with reference to  FIGS. 4 to 10 . 
       FIG. 4  includes a plan view and a cross-sectional view of the antenna module  100 A according to Variation 1. The antenna module  100 A is an example that differs from the antenna module  100  in the feed point to which the feed line  140  from the RFIC  110  is connected. Specifically, a feed point SP 1 A for the radiation electrode  121  in the antenna module  100 A is in a position displaced from the center of the radiation electrode  121  toward the first side  161 . In the antenna module  100 A, the polarization direction of a radio wave radiated by the radiation electrode  121  is the X-axis direction in  FIG. 4 . 
     Variations 2 to 5 in  FIGS. 5 to 8  are examples that differ from the antenna module  100  in the cavity portion  152  in the upper surface of the dielectric substrate  160 . Specifically, in the antenna module  100 B according to Variation 2 in  FIG. 5 , the cavity portion  152  is disposed in only the portion along the third side  163 , and the beam portion  165  is supported by the first side  161 , the second side  162 , and the fourth side  164 . 
     In the antenna module  100 C according to Variation 3 in  FIG. 6 , the cavity portion  152  is disposed in the portion along the second side  162  and the portion along the fourth side  164 , and the beam portion  165  is supported by the first side  161  and the third side  163 . In the antenna module  100 D according to Variation 4 in  FIG. 7 , the cavity portion  152  is disposed in the portion along the neighboring sides (second side  162  and third side  163 ), and the beam portion  165  is supported by the first side  161  and the fourth side  164 . 
       FIG. 8  includes a plan view and a cross-sectional view of the antenna module  100 E according to Variation 5. In  FIG. 8 , the cross-sectional view in the lower row is taken along a plane VIII-VIII extending through the feed point SP 1  and the cavity portion  152 . The cavity portion  152  in the antenna module  100 E does not have a slit shape illustrated in  FIGS. 5 to 7 , has a relatively small circular shape, and is near the third side  163 . The number of cavity portions  152  in  FIG. 8  may be two or more. The cavity portion  152  in  FIG. 8  may be disposed in a different position. 
     The antenna module  100 F according to Variation 6 in  FIG. 9  and the antenna module  100 G according to Variation 7 in  FIG. 10  are examples in which the dielectric substrate  160  has no cavity portion in its upper surface and the hollow portion  150  is a closed space. 
     In the antenna module  100 F in  FIG. 9 , the hollow portion  150  is disposed inside the dielectric substrate  160  such that when the antenna module  100 F is seen in plan view, the hollow portion  150  overlaps the entire area of the radiation electrodes  121  and  122 . In the antenna module  100 G in  FIG. 10 , the hollow portion  150  is disposed such that it overlaps only the portion of the radiation electrodes  121  and  122  along the second side  162  and the fourth side  164  of the dielectric substrate  160 . 
     Here, the relation between the position of the hollow portion  150  and the frequency band width in the cases where the hollow portion  150  partially overlaps a portion of the radiation electrodes, as in the antenna module  100 G in  FIG. 10 , is described with reference to  FIGS. 11 to 14 . 
     First, the relation between the position of the hollow portion in the Y-axis direction and the frequency band width is described with reference to  FIGS. 11 and 12 . As illustrated in  FIG. 11 , in an antenna module in which the length of one side of each of the two radiation electrodes (corresponding to the radiation electrodes  121  and  122 ) is 0.9 mm and the feed point is in a position displaced from the center of the radiation electrode toward the negative direction of the Y axis, the position of the rectangular hollow portion having the dimension in the Y-axis direction of 0.3 mm and being long in the X-axis direction is moved in the Y-axis direction. The frequency band width in that case is simulated, and its results are illustrated in  FIG. 12 . 
     In  FIG. 12 , the horizontal axis indicates the displacement amount Yoff of the position of the center of the hollow portion in the Y-axis direction from the position of the center of the radiation electrode in the Y-axis direction (X axis in  FIG. 11 ), and the vertical axis indicates the frequency band width of a radiated radio wave. The line LN 10  in  FIG. 12  indicates the simulation result for the frequency band width in a comparative example that has no hollow portion, and its frequency band width is 6.98 GHz. 
     The line LN 11  in  FIG. 12  indicates the simulation result for the frequency band width when the hollow portion in  FIG. 11  is moved. It is revealed that in the range of −0.6≤Yoff≤0.6, where the hollow portion overlaps the radiation electrodes, the frequency band width wider than that in the comparative example, which has no hollow portion, is achieved. In particular, the frequency band width is large in the vicinities where Yoff is ±0.3. 
     In the case where a radio wave whose polarization direction is the Y-axis direction is radiated, as in  FIG. 11 , it is known that the intensity of an electric field occurring between two radiation electrodes is typically the largest in the vicinity of end portions of the radiation electrodes in the Y-axis direction. Accordingly, when the hollow portion is disposed in the portions where the intensity of the electric field is large, the advantage of reducing the effective permittivity is large, and that results in the increased amount of improvement in the frequency band width. On the contrary, in the vicinity of the centers of the radiation electrodes in the Y-axis direction (Yoff=0), the intensity of the electric field is lower than that in the end portions in the Y-axis direction, and thus the advantage of improving the frequency band width obtained from the presence of the hollow portion is slightly small. 
     Next, the relation between the position of the hollow portion in the X-axis direction and the frequency band width is described with reference to  FIGS. 13 and 14 . In the antenna module in which the length of one side of each of the two radiation electrodes is 0.9 mm and the polarization direction is the Y-axis direction, as in the case of  FIG. 11 , the frequency band width obtained when the rectangular hollow portion having the dimension in the X-axis direction of 0.3 mm and being long in the Y-axis direction is moved in the X-axis direction is simulated, and its results are illustrated in  FIG. 14 . 
     In  FIG. 14 , the horizontal axis indicates the displacement amount Xoff of the position of the center of the hollow portion in the X-axis direction from the position of the center of the radiation electrode in the X-axis direction (Y axis in  FIG. 13 ), as illustrated in  FIG. 13 , and the vertical axis indicates the frequency band width of a radiated radio wave. The line LN 15  in  FIG. 14  indicates the simulation result for the frequency band width in the comparative example, which has no hollow portion. 
     The line LN 16  in  FIG. 14  indicates the simulation result for the frequency band width when the hollow portion  150  in  FIG. 13  is moved. It is revealed that in the range of −0.6≤Xoff≤0.6, where the hollow portion overlaps the radiation electrodes, the frequency band width wider than that in the comparative example, which has no hollow portion, is also achieved. Unlike the case where the position in the Y-axis direction is moved illustrated in  FIGS. 11 and 12 , the advantage of improving the frequency band width in the vicinity of the centers of the radiation electrodes in the X-axis direction (Xoff=0) is large, and the advantage of the improvement in the end portions in the X-axis direction is slightly smaller than that in the vicinity of the centers. That is because the feed point for the radiation electrode is on the Y axis, as illustrated in  FIG. 13 , and thus the electric field occurring between the two radiation electrodes is the largest in the vicinity of the centers of the radiation electrodes in the X-axis direction. 
     The above-described simulation results reveal that, as in the antenna module  100 G in  FIG. 10 , in the case where the hollow portion is partially disposed between the two radiation electrodes, the hollow portion may preferably be in a position that overlaps the end portions of the radiation electrodes with respect to the polarization direction (Y-axis direction) and may preferably be in the vicinity of the centers, which are near the feed point, of the radiation electrodes with respect to a direction perpendicular to the polarization direction (X-axis direction). 
     As described above, in the stack-type antenna modules including the two radiation electrodes, the expanded frequency band width of a radiated radio wave can be achieved by disposing the hollow portion in at least a portion between the two radiation electrodes. 
     The size and position of the hollow portion  150  and the arrangement of the cavity portion  152  can be determined in accordance with a desired frequency band width and stiffness (durability) of the antenna module. 
     The hollow portion  150  disposed inside the dielectric substrate  160  may consist of a plurality of sections separated by a dielectric wall portion  167 , as in an antenna module  100 X according to Variation 8 in  FIG. 15 . The hollow portion  150  may extend to a portion close to the ground electrode GND in a zone around the radiation electrode  121  being the feed element, as in an antenna module  100 Y according to Variation 9 in  FIG. 16 . Furthermore, the hollow portion  150  may consist of sections separated in the lamination direction (thickness direction) of the dielectric substrate  160 , as in an antenna module  100 Z according to Variation 10 in  FIG. 17 . 
     Second Embodiment 
     In the first embodiment, the hollow portion  150  disposed inside the dielectric substrate  160  is basically an air layer. 
     In a second embodiment, an example in which the hollow portion  150  disposed between the two the radiation electrodes  121  and  122  is at least partially filled with another dielectric having permittivity lower than that of the dielectric substrate  160  is described. 
       FIG. 18  includes a plan view and a cross-sectional view of an antenna module  100 H according to the second embodiment. The antenna module  100 H is the one in which the hollow portion  150  and the cavity portion  152  in the antenna module  100  according to the first embodiment are filled with a dielectric material  170  having permittivity lower than that of the dielectric forming the dielectric substrate  160 . 
     Because the hollow portion  150  is filled with the different dielectric material having the lower permittivity, the effective permittivity can be more reduced than that in the case where the substrate is entirely made of the same dielectric material, and the frequency band width can be expanded. In that configuration, although the amount of expansion of the frequency band width is smaller than that in the case where the hollow portion  150  is the air layer, the stiffness of the antenna module can be enhanced. In the antenna module  100 H, the hollow portion  150  is entirely filled with another dielectric material. The hollow portion  150  may be only partially filled with another dielectric material. 
     As in an antenna module  100 I according to Variation 11 in  FIG. 19 , the cavity portion  152  may be filled with a dielectric material  171  different from the dielectric material  170  with which the hollow portion  150  is filled. 
     Similarly, the hollow portion  150  in each of the variations of the first embodiment may be filled with a dielectric material having low permittivity. For example, an antenna module  100 J according to Variation 12 in  FIG. 20  is the one in which the hollow portion  150  in the antenna module  100 F according to Variation 6 of the first embodiment is filled with the different dielectric material  170 . An antenna module  100 K according to Variation 13 in  FIG. 21  is the one in which the hollow portion  150  in the antenna module  100 E according to Variation 5 of the first embodiment is filled with the different dielectric material  170 . 
     Third Embodiment 
     The antenna module in the first embodiment has the configuration in which the two radiation electrodes are stacked. The number of radiation electrodes stacked may be three or more. 
     In a third embodiment and its variations, examples in which the same configuration as that of the first embodiment is applied to an antenna module including three stacked radiation electrodes are described. 
       FIG. 22  includes a plan view and a cross-sectional view of an antenna module  100 L according to the third embodiment. The antenna module  100 L in  FIG. 22  further includes a radiation electrode  123  (third radiation electrode) being a parasitic element, in addition to the radiation electrode  121 , which is a feed element, and the radiation electrode  122 , which is a parasitic element. 
     The radiation electrode  123  is disposed on a layer between the radiation electrodes  121  and  122 . In the example of the antenna module  100 L, the radiation electrodes  122  and  123  have the same dimensions and the same shape, and when the antenna module  100 L is seen in plan view, the radiation electrodes  122  and  123  overlap each other. 
     The hollow portion  150  is disposed between the radiation electrodes  121  and  123 , and the cavity portion  152  extends from the upper surface of the dielectric substrate  160  through the dielectric substrate  160  to the hollow portion  150 . The cavity portion  152  in the antenna module  100 L is disposed along the second side  162 , the third side  163 , and the fourth side  164  of the antenna module  100 L having a rectangular shape as seen in plan view, as in the case of the antenna module  100  according to the first embodiment. The radiation electrodes  122  and  123 , which are parasitic elements, are arranged in the beam portion  165  supported by the first side  161 . 
     The layer where the hollow portion  150  is disposed is not limited to the layer between the radiation electrodes  121  and  123 . The hollow portion  150  may be disposed between the radiation electrodes  122  and  123 , as in an antenna module  100 M according to Variation 14 in  FIG. 23 . 
     As in an antenna module  100 N according to Variation 15 in  FIG. 24 , the hollow portion  150  may be disposed both between the radiation electrodes  122  and  123  and between the radiation electrodes  121  and  123 . In the antenna module  100 N, the radiation electrode  123  is arranged inside a beam portion  166  disposed in a middle area in the lamination direction of the dielectric substrate  160 . 
     Although not illustrated, the hollow portion  150  in the third embodiment may be at least partially filled with the dielectric material having lower permittivity than that of the dielectric material forming the dielectric substrate  160 , as in the case of the second embodiment. 
     In the above-described antenna modules including the three or more stacked radiation electrodes, the expanded frequency band width of a radiated radio wave can be achieved by disposing the hollow portion between any radiation electrodes. 
     Fourth Embodiment 
     In each of the antenna modules described in the first to third embodiments, the beam portion  165  where the radiation electrode  122 , which is a parasitic element, is arranged includes the upper surface of the dielectric substrate  160 . 
     However, in the configuration in which the hollow portion is an air layer and the cavity portion is open in the upper surface of the dielectric substrate, because the portion supporting the beam portion is limited, the portion supporting the beam portion may be broken, depending on the force acting thereon during handling of the antenna module. 
     In a fourth embodiment, the beam portion where the radiation electrode is arranged is disposed so as to be supported in a position displaced from the uppermost surface of the dielectric substrate in the lamination direction. In that configuration, the occurrence of incidents in which an external force directly acts on the beam portion during handling is reduced, and the possibility of breakage of the beam portion is decreased. 
       FIG. 25  includes a plan view and a cross-sectional view of an antenna module  100 P according to the fourth embodiment. In the antenna module  100 P, as illustrated in the cross-sectional view, a beam portion  165 A is disposed in a position displaced from the upper surface of the dielectric substrate  160  toward the negative direction of the Z axis (that is, toward the hollow portion  150 ). In other words, the level of the outer region of the dielectric substrate  160  is higher than the level of the upper surface of the beam portion  165 A. In an example case where antenna modules are stacked, although another antenna module is likely to come into contact with the outer region of the dielectric substrate  160 , the occurrence of incidents in which an external force directly acts on the beam portion  165 A is reduced in the above-described configuration. Thus, the possibility of breakage of the beam portion  165 A can be decreased. 
     In the example of the antenna module  100 P in  FIG. 25 , the configuration in which the level of the overall outer region of the dielectric substrate  160  is higher than the level of the upper surface of the beam portion  165 A is described. The level of the outer region of the dielectric substrate  160  may not entirely be higher, that is, the outer region may not have a wall-like shape. For example, columnar dielectrics may be arranged in part in the outer region of the dielectric substrate  160  such that the uppermost surface of the dielectric substrate  160  is higher than the level of the upper surface of the beam portion  165 A. 
     Fifth Embodiment 
     In the first to fourth embodiments, the antenna modules including the single unit of the antenna element and the RFIC are described. In a fifth embodiment, an array antenna, in which antenna elements are arranged in an array, is described. 
       FIG. 26  is a plan view of an array antenna  300  according to the fifth embodiment. The array antenna  300  is a two-by-two array in which four antenna modules  100 - 1  to  100 - 4  having the same configuration as the antenna module  100  described in the first embodiment are arranged. The number of antenna modules constituting the array is not limited to four, and it may be two, three, five or more. 
     In that array antenna  300 , the expanded frequency band width of a radiated radio wave can be achieved by disposing the hollow portion between the radiation electrodes in each of the antenna modules. Although not illustrated, in the case of the array antenna, the plurality of antenna modules may include their respective RFICs or may share a single RFIC. 
     In the case of the array antenna, the dielectric wall between the neighboring antenna modules may be omitted such that the hollow portions communicate with each other. 
       FIG. 27  is a plan view of an array antenna  300 A according to Variation 16. In the array antenna  300 A, the wall between neighboring antenna modules  100 - 1 A and  100 - 3 A is removed, and the hollow portions in the two antenna modules communicate with each other. The hollow portions in neighboring antenna modules  100 - 2 A and  100 - 4 A also communicate with each other. In the example illustrated in  FIG. 27 , the walls in the end portions in the Y-axis direction in the antenna modules are also removed. 
     As described above, because the hollow portions in the neighboring antenna modules in the array antenna communicate with each other, the dielectric section is decreased, and the effective permittivity can be further reduced, and the frequency band width can be still further expanded. 
     Sixth Embodiment 
     In a sixth embodiment, a configuration where in a so-called dual-band type antenna module, which can radiate radio waves in two frequency ranges, the expanded frequency band widths of radiated radio waves can be achieved by disposing a hollow in a dielectric substrate is described. 
       FIG. 28  includes a plan view and a cross-sectional view of an antenna module  100 Q according to the sixth embodiment. Referring to  FIG. 28 , the antenna module  100 Q includes the radiation electrode  121  being a feed element and a radiation electrode  124  being a parasitic element. The radiation electrode  121  is arranged on an inner layer near the upper surface of the dielectric substrate  160 . The radiation electrode  124  is arranged on a layer on the lower-surface side with respect to the radiation electrode  121 , that is, on a layer between the radiation electrode  121  and the ground electrode GND and is opposite to the radiation electrode  121 . 
     Two feed points SP 1  and SP 2  are arranged on the radiation electrode  121 . The feed point SP 1  is arranged in a position displaced from the center of the radiation electrode  121  toward the negative direction of the Y axis when the antenna module  100 Q is seen in plan view. A radio-frequency signal is conveyed from the RFIC  110  to the feed point SP 1  through a feed line  141 . When the radio-frequency signal is supplied to the feed point SP 1 , a radio wave whose polarization direction is the Y-axis direction is radiated. 
     The feed point SP 2  is arranged in a position displaced from the center of the radiation electrode  121  toward the positive direction of the X axis when the antenna module  100 Q is seen in plan view. A radio-frequency signal is conveyed from the RFIC  110  to the feed point SP 2  through a feed line  142 . When the radio-frequency signal is supplied to the feed point SP 2 , a radio wave whose polarization direction is the X-axis direction is radiated. That is, the antenna module  100 Q is also a dual-polarization type antenna module capable of radiating radio waves in two different polarization directions. 
     The feed lines  141  and  142  extend from the RFIC  110  through the radiation electrode  124  to the radiation electrode  121 . Thus, when radio-frequency signals corresponding to the resonant frequency of the radiation electrode  124  being the parasitic element are supplied to the feed lines  141  and  142 , the radiation electrode  124  radiates radio waves. 
     The size of the radiation electrode  124  is larger than that of the radiation electrode  121 . The resonant frequency of the radiation electrode  124  is lower than that of the radiation electrode  121 . Thus, the radiation electrode  124  radiates a radio wave in a frequency range lower than that for the radiation electrode  121 . 
     In the antenna module  100 Q, a hollow portion  155  is disposed in a layer between the radiation electrodes  121  and  124 . When the antenna module  100 Q is seen in plan view, the hollow portion  155  has substantially the same shape as that of the radiation electrode  121  and is disposed in a position overlapping the radiation electrode  121 . 
     The radiation electrode  121  functions as an antenna when an electric line of force occurs between the radiation electrodes  121  and  124 . Thus, the effective permittivity between the radiation electrodes  121  and  124  has an effect on the antenna characteristics. In the antenna module  100 Q, because the hollow portion  155  is disposed in the layer between the radiation electrodes  121  and  124 , as described above, the effective permittivity is lower than that when the hollow portion  155  is filled with the dielectric. Therefore, the electromagnetic-field coupling between the radiation electrodes  121  and  124  can be weakened, and the expanded frequency band width of a radio wave radiated by the radiation electrode  121  can be achieved. 
     The effective permittivity between the radiation electrode  124  and the ground electrode GND has an effect on the frequency band width of the radio wave radiated by the radiation electrode  124 . Thus, when the hollow portion  155  is disposed between the radiation electrodes  121  and  124 , the frequency band width of the radio wave radiated by the radiation electrode  124  basically remains unchanged. That is, when the hollow portion  155  is disposed between the radiation electrodes  121  and  124 , the expanded frequency band width of the radio wave radiated by the radiation electrode  121  can be achieved while at the same time the frequency band width of the radio wave radiated by the radiation electrode  124  is maintained. 
     The expanded frequency band width of the radio wave radiated by the radiation electrode  124  can be achieved by disposing a hollow portion  156  in a layer between the radiation electrode  124  and the ground electrode GND, as in an antenna module  100 R in a reference example illustrated in  FIG. 29 . 
     Furthermore, although not illustrated, the expanded frequency band widths of both the radio wave radiated by the radiation electrode  121  and that by the radiation electrode  124  can be achieved by disposing the hollow portion in each of a layer between the radiation electrodes  121  and  124  and a layer between the radiation electrode  124  and the ground electrode GND. 
     In the antenna modules illustrated in  FIGS. 28 and 29 , a portion of each of the feed lines  141  and  142  vertically extends through the hollow portion. One example of the feed line inside the hollow portion may be formed by connecting a columnar conductor to a via or a feed element disposed on a dielectric layer by the use of silver paste. Alternatively, the feed line inside the hollow portion may be formed by laminating small flat-shaped electrodes in the thickness direction. When the technique of connecting a feed element previously formed on a dielectric layer to a feed line is used in connecting the feed line inside the hollow portion and the feed element, the degree of flatness of the feed element can be more ensured, in comparison with the case where a feed element alone is connected to another feed element. 
     In the antenna module  100 Q in  FIG. 28  and the antenna module  100 R in  FIG. 29 , the hollow portions  155  and  156  may consist of sections separated in the lamination direction of the dielectric substrate  160 , as in the configuration in  FIG. 17 . 
     As described above, in dual-band type antenna modules including the two stacked radiation electrodes and capable of radiating radio waves in different frequency ranges, the frequency band width of each of the radio waves can be individually adjusted by disposing the hollow portion in the layer between the two radiation electrodes and/or the layer between the radiation electrode on the low-frequency side and the ground electrode. 
     It is to be understood that the embodiments disclosed here are illustrative and not restrictive in all respects. The scope of the present disclosure is indicated by not the above description of the embodiments but the claims, and it is intended to include all changes in the meaning and scope equivalent to the claims. 
       10  communication device,  100 ,  100 A to  100 N,  100 P to  100 R,  100 X to  100 Z antenna module,  110  RFIC,  111 A to  111 D,  113 A to  113 D,  117  switch,  112 AR to  112 DR low-noise amplifier,  112 AT to  112 DT power amplifier,  114 A to  114 D attenuator,  115 A to  115 D, phase shifter,  116  signal combiner/splitter,  118  mixer,  119  amplifier circuit,  120  antenna array,  121  to  124  radiation electrode,  130  solder bump,  140  to  142  feed line,  145  columnar conductor,  150 ,  155 ,  156  hollow portion,  152  cavity portion,  160  dielectric substrate,  161  to  164  side,  165 ,  165 A,  166  beam portion,  167  wall portion,  170 ,  171  dielectric material,  300 ,  300 A array antenna, GND ground electrode, SP 1 , SP 1 A, SP 2  feed point