Patent Publication Number: US-2022216618-A1

Title: Reflector antenna device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of PCT International Application No. PCT/JP2019/046266, filed on Nov. 27, 2019, which is hereby expressly incorporated by reference into the present application. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a reflector antenna device including a primary radiator and a reflector. 
     BACKGROUND ART 
     There is a reflector antenna device that includes a primary radiator that radiates radio waves in a plurality of frequency bands and a reflector that reflects the radio waves in the plurality of frequency bands radiated by the primary radiator to output the radio waves in the plurality of frequency bands. In a case where the primary radiator radiates radio waves in a plurality of frequency bands, the beam widths of main lobes of the radio waves in the plurality of frequency bands radiated by the primary radiator are greatly different. 
     In the above-described reflector antenna device, a part of radio waves in a high frequency band that is a higher frequency band among radio waves in a plurality of frequency bands radiated by the primary radiator may be incident on the reflector as side lobes. Since the side lobe closest to the main lobe has a phase inverted with respect to the main lobe, in a case where the side lobe incident on the reflector is reflected by the reflector, a gain of a secondary radiation pattern, which is a radiation pattern of the radio wave reflected by the reflector, decreases. 
     Patent Literature 1 discloses an antenna device in which in a dual reflector antenna including a sub-reflector that shares at least two frequency bands, a reflecting mirror face of the sub-reflector is concentrically divided into two regions of a first center region and a second outer peripheral region, the first center region is formed of a metal reflection face, and the second outer peripheral region is formed of a frequency-selective reflection face having transmission characteristic in a high frequency band and reflection characteristic in a low frequency band. The antenna device (hereinafter, referred to as a “conventional reflector antenna device”) disclosed in Patent Literature 1 has the above-described configuration to suppress a decrease in gain of the secondary radiation pattern. 
     CITATION LIST 
     Patent Literatures 
     
         
         Patent Literature: Japanese Patent Laid-open Publication No. 55-092002 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the conventional reflector antenna device, the side lobe of the radio wave in the high frequency band radiated by the primary radiator passes through the second outer peripheral region. Therefore, the conventional reflector antenna device can suppress a decrease in gain of a secondary radiation pattern of the radio wave in the high frequency band radiated by the primary radiator, but spillover of a side lobe occurs, and a secondary radiation pattern with high gain cannot be obtained in the radio wave in the high frequency band. 
     The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a reflector antenna device capable of suppressing spillover of a side lobe of a radio wave in a high frequency band while suppressing a decrease in gain of a secondary radiation pattern of the radio wave in the high frequency band. 
     Solution to Problem 
     A reflector antenna device according to the present invention includes: a primary radiator to radiate a first radio wave that is a radio wave in a first frequency band and radiate a second radio wave that is a radio wave in a second frequency band lower in frequency than the first frequency band; and a reflector having a reflection face that receives the first radio wave and the second radio wave radiated by the primary radiator and reflects the first radio wave and the second radio wave, in which the reflection face included in the reflector has a first region including a center point of the reflection face and a second region that is an outer peripheral region of the first region and is a region provided with a plurality of recesses, and each of the plurality of recesses provided in the second region of the reflection face included in the reflector is configured to allow the first radio wave to enter the recess, restrict the second radio wave from entering the recess, and reflect the first radio wave that has entered the recess on a bottom face of the recess. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to suppress spillover of a side lobe of a radio wave in a high frequency band while suppressing a decrease in gain of a secondary radiation pattern of the radio wave in a high frequency band. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a configuration diagram illustrating an example of a configuration of a main part of a reflector antenna device according to a first embodiment.  FIG. 1B  is a configuration diagram illustrating an example of the configuration of the main part of the first reflector  120  included in the reflector antenna device according to the first embodiment.  FIG. 1C  is a configuration diagram illustrating an example of the configuration of the main part of the first reflector included in the reflector antenna device according to the first embodiment.  FIG. 1D  is a configuration diagram illustrating an example of the configuration of the main part of the first reflector included in the reflector antenna device according to the first embodiment. 
         FIG. 2  is a configuration diagram illustrating an example of a shape of each of a plurality of recesses according to the first embodiment. 
         FIG. 3  is a diagram illustrating an example of behavior of a first radio wave and a second radio wave incident on a certain recess provided on a reflection face in a second region according to the first embodiment. 
         FIG. 4  is a configuration diagram illustrating a configuration of the reflector antenna device according to the first embodiment, a reflector antenna device according to a first example. 
         FIG. 5  is a diagram illustrating radiation patterns of a first radio wave and a second radio wave radiated by a primary radiator included in the reflector antenna device according to the first example. 
         FIG. 6  is a secondary radiation pattern of the first radio wave output from the reflector antenna device according to the first example. 
         FIG. 7A  is a configuration diagram illustrating an example of a configuration of a main part of a reflector antenna device according to another modification of the first embodiment.  FIG. 7B  is a configuration diagram illustrating an example of a configuration of a main part of a first reflector included in the reflector antenna device according to another modification of the first embodiment.  FIG. 7C  is a configuration diagram illustrating the example of the configuration of the main part of the first reflector included in the reflector antenna device according to another modification of the first embodiment.  FIG. 7D  is a configuration diagram illustrating the example of the configuration of the main part of the first reflector included in the reflector antenna device according to another modification of the first embodiment. 
         FIG. 8A  is a diagram illustrating an example of a configuration of a main part of a reflector antenna device according to a second embodiment.  FIG. 8B  is a configuration diagram illustrating an example of a configuration of a main part of a first reflector included in the reflector antenna device according to the second embodiment.  FIG. 8C  is a configuration diagram illustrating the example of the configuration of the main part of the first reflector included in the reflector antenna device according to the second embodiment.  FIG. 8D  is a configuration diagram illustrating the example of the configuration of the main part of the first reflector included in the reflector antenna device according to the second embodiment. 
         FIG. 9A  is a configuration diagram illustrating an example of a configuration of a main part of a reflector antenna device according to a third embodiment.  FIG. 9B  is a configuration diagram illustrating an example of a configuration of a main part of a first reflector included in the reflector antenna device according to the third embodiment.  FIG. 9C  is a configuration diagram illustrating an example of a configuration of the main part of the first reflector included in the reflector antenna device according to the third embodiment. 
         FIG. 10A  is a diagram illustrating an example of behaviors of a first radio wave and a second radio wave incident on a second region in a case where the second region according to the third embodiment does not include a dielectric.  FIG. 10B  is a diagram illustrating an example of behaviors of the first radio wave and the second radio wave incident on the dielectric constituting a reflection face in the second region according to the third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In order to explain the present invention in more detail, a mode for carrying out the present invention will be described below with reference to the accompanying drawings. 
     First Embodiment 
     A configuration of a main part of a reflector antenna device  100  according to a first embodiment will be described with reference to  FIG. 1 . 
       FIG. 1  is a configuration diagram illustrating an example of a configuration of a main part of the reflector antenna device  100  according to the first embodiment. 
     The reflector antenna device  100  includes a primary radiator  110 , a first reflector  120 , and a second reflector  130 . 
     The reflector antenna device  100  is, for example, a reflector antenna including a plurality of reflectors such as a Gregorian antenna or a Cassegrain antenna. In the first embodiment, the reflector antenna device  100  will be described as a Gregorian antenna as illustrated in  FIG. 1  as an example. 
       FIG. 1A  is a configuration diagram illustrating an example of a configuration of a main part of the reflector antenna device  100  according to the first embodiment, and is a cross-sectional view of the reflector antenna device  100  on a plane including a radiation axis of a primary radiator  110  included in the reflector antenna device  100 . 
       FIG. 1B  is a configuration diagram illustrating an example of a configuration of a main part of the first reflector  120  included in the reflector antenna device  100  according to the first embodiment, and is a configuration diagram of the first reflector  120  viewed from the primary radiator  110  included in the reflector antenna device  100  according to the first embodiment. 
       FIG. 1C  is a configuration diagram illustrating an example of a configuration of the main part of the first reflector  120  included in the reflector antenna device  100  according to the first embodiment, and is an enlarged view of the first reflector  120  in a region surrounded by a rectangle indicated by a broken line in  FIG. 1A . 
       FIG. 1D  is a configuration diagram illustrating an example of a configuration of the main part of the first reflector  120  included in the reflector antenna device  100  according to the first embodiment, and is an enlarged view of the first reflector  120  in a region surrounded by a rectangle indicated by a broken line in  FIG. 1B . 
     The primary radiator  110  is a radiator that radiates a first radio wave that is a radio wave in a first frequency band and radiates a second radio wave that is a radio wave in a second frequency band lower in frequency than the first frequency band. 
     In the first embodiment, the primary radiator  110  is described as one radiator that radiates the first radio wave and the second radio wave, but the primary radiator  110  may be a radiator in which two radiators are combined, such as a radiator in which a radiator that radiates the first radio wave and another radiator that radiates the second radio wave are combined. 
     The first reflector  120  is a reflector having a reflection face that receives the first radio wave and the second radio wave radiated from the primary radiator  110  and reflects the first radio wave and the second radio wave. 
     In the reflector antenna device  100  according to the first embodiment, the first reflector  120  is a sub-mirror. 
     The reflection face of the first reflector  120  as a reflector is, for example, a curved face such as a quadratic face or a parabolic face. 
     The reflection face of the first reflector  120  as a reflector includes a first region  121  including a center point of the reflection face, and a second region  122  that is an outer peripheral region of the first region  121  and is a region provided with a plurality of recesses  123 . 
     Note that the plurality of recesses  123  (hereinafter, simply referred to as a “plurality of recesses  123 ”) provided on the reflection face in the second region  122  may be periodically arranged or may be arranged at any positions in the second region  122 . 
     The reflection face in the first region  121  (hereinafter, simply referred to as a “first region  121 ”) included in the first reflector  120  is made of, for example, a conductor such as metal, and the reflection face in the first region  121  is processed into a smooth shape without unevenness. 
     The reflection face in the first region  121  receives a main lobe of the first radio wave radiated by the primary radiator  110  and a main lobe of the second radio wave radiated by the primary radiator  110 . The reflection face in the first region  121  reflects the main lobe of the first radio wave and the main lobe of the second radio wave toward the second reflector  130 . 
     The reflection face in the second region  122  (hereinafter, simply referred to as a “second region  122 ”) included in the first reflector  120  is made of, for example, a conductor such as metal, and the plurality of recesses  123  are formed by processing such as casting, cutting, or tapping. 
     The reflection face in the second region  122  receives a side lobe of the first radio wave radiated by the primary radiator  110  and the main lobe of the second radio wave radiated by the primary radiator  110 . 
     Each of the plurality of recesses  123  allows the first radio wave to enter the recess  123 , restricts the second radio wave from entering the recess  123 , and reflects the first radio wave having entered the recess  123  on a bottom face  125  of the recess  123 . 
     Specifically, each of the plurality of recesses  123  allows the side lobe of the first radio wave radiated by the primary radiator  110  to enter the recess  123 , and reflects the side lobe of the first radio wave having entered the recess  123  on the bottom face  125  of the recess  123 . More specifically, each of the plurality of recesses  123  reflects the side lobe of the first radio wave having entered the recess  123  toward the second reflector  130 . Further, each of the plurality of recesses  123  restricts the main lobe of the second radio wave radiated by the primary radiator  110  from entering the recess  123 , and reflects the main lobe of the second radio wave not entering the recess  123  toward the second reflector  130 . 
     With such a configuration, the reflector antenna device  100  can suppress the spillover of the side lobe of the radio wave in the high frequency band while suppressing the decrease in the gain of the secondary radiation pattern of the radio wave in the high frequency band. 
     Each of the plurality of recesses  123  has, for example, a circular shape in a cross section in a plane parallel to the reflection face. That is, each of the plurality of recesses  123  is a cylindrical recess provided on the reflection face in the second region  122 . 
     The shape of the cross section in the plane parallel to the reflection face of each of the plurality of recesses  123  is not limited to a circular shape. 
       FIG. 2  is a configuration diagram illustrating an example of a shape of each of the plurality of recesses  123  according to the first embodiment, and is a configuration diagram illustrating an example of the shape of the cross section in a plane parallel to the reflection face of each of the plurality of recesses  123 . 
     As illustrated in  FIG. 2 , the shape of the cross section in the plane parallel to the reflection face of each of the plurality of recesses  123  may be an elliptical shape, a rectangular shape, a doughnut shape, a cross shape, or the like. The plurality of recesses  123  may be a combination of recesses having different shapes of the cross section in a plane parallel to the reflection face. 
     The second reflector  130  is a reflector having a reflection face that receives the first radio wave and the second radio wave reflected by the first reflector  120  and reflects the first radio wave and the second radio wave. 
     In the reflector antenna device  100  according to the first embodiment, the second reflector  130  is a main mirror. 
     For example, the second reflector  130  reflects the first radio wave and the second radio wave reflected by the first reflector  120  in a predetermined direction in which the reflector antenna device  100  outputs the first radio wave and the second radio wave. 
     The reflector antenna device  100  outputs the first radio wave and the second radio wave reflected by the second reflector  130  in a predetermined direction. 
     The maximum value “L” of the length in the plane parallel to the reflection face of each of the plurality of recesses  123  falls, for example, within a range determined by the following formula (1). 
     
       
         
           
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       
                         
                           C 
                           z 
                         
                         
                           
                             ? 
                           
                           ⁢ 
                           F 
                           ⁢ 
                           
                             ? 
                           
                         
                       
                       &lt; 
                       L 
                       &lt; 
                       
                         
                           C 
                           x 
                         
                         
                           
                             ? 
                           
                           ⁢ 
                           
                             F 
                             L 
                           
                         
                       
                     
                     ⁢ 
                     
                       
 
                     
                     ⁢ 
                     
                       
                         ? 
                       
                       ⁢ 
                       
                         indicates text missing or illegible when filed 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Here, “C” is the speed of light, “χ” is the positive minimum root in the first derivative of the Bessel function of the first type, “π” is the circular constant, “F H ” is the first frequency band, and “F L ” is the second frequency band. 
     Note that the value of χ, which is the positive minimum root in the first derivative of the Bessel function of the first type, is 1.841. 
     With reference to  FIG. 3 , behaviors of the first radio wave and the second radio wave incident on a certain recess  123  provided on the reflection face in the second region  122  according to the first embodiment will be described. 
       FIG. 3  is a diagram illustrating an example of behaviors of the first radio wave and the second radio wave incident on a certain recess  123  provided on the reflection face in the second region  122  according to the first embodiment. 
     For example, in a case where the maximum value of the length in the plane parallel to the reflection face of each of the plurality of recesses  123  satisfies the condition shown in the formula (1), the second radio wave in the second frequency band having a frequency lower than that of the first frequency band which is a high frequency band is reflected at an opening  124  of each recess  123  since the maximum value of the length is shorter than the wavelength of the second radio wave. 
     On the other hand, in this case, since the maximum value of the length is longer than the wavelength of the first radio wave, the first radio wave in the first frequency band that is a high frequency band enters each recess  123  and is reflected on the bottom face  125  of each recess  123  facing the opening  124  of each recess  123 . 
     For example, each of the plurality of recesses  123  is processed so that the depth is an odd multiple of ¼ wavelength of the first radio wave. 
     The depth of each of the plurality of recesses  123  does not need to be strictly ¼ wavelength of the first radio wave, and the ¼ wavelength of the first radio wave herein includes approximately ¼ wavelength. 
     Further, as for the depths of the plurality of recesses  123 , all the depths of the plurality of recesses  123  do not need to be ¼ wavelength of the first radio wave, and may be, for example, any depth depending on the distances from the center point of the reflection face or the like. 
     In a case where the depth of each of the plurality of recesses  123  is an odd multiple of ¼ wavelength of the first radio wave, the phase of the first radio wave reflected on the bottom face  125  of the recess  123  is inverted with respect to the phase of the first radio wave incident on the recess  123  at the opening  124  of the recess  123 . 
     Note that the depth of the recess  123  is a distance from the opening  124  of the recess  123  to the bottom face  125  of the recess  123 . 
     The side lobe closest to the main lobe has a phase inverted with respect to the main lobe. 
     As described above, the reflection face in the first region  121  receives the main lobe of the first radio wave radiated by the primary radiator  110  and the main lobe of the second radio wave radiated by the primary radiator  110 . As described above, the reflection face in the second region  122  receives the side lobe of the first radio wave radiated by the primary radiator  110  and the main lobe of the second radio wave radiated by the primary radiator  110 . 
     Therefore, in a case where the depth of each of the plurality of recesses  123  is an odd multiple of the ¼ wavelength of the first radio wave, the side lobe of the first radio wave reflected on the bottom face  125  of the recess  123  has the same phase as the main lobe of the first radio wave reflected by the reflection face in the first region  121  at the opening  124  of the recess  123 . Further, the main lobe of the second radio wave reflected at the opening  124  of the recess  123  has the same phase as the main lobe of the second radio wave reflected by the reflection face in the first region  121 . 
     Note that the same phase referred to herein does not need to be strictly the same phase, and includes substantially the same phase. 
     Although the case where the depth of each of the plurality of recesses  123  is an odd multiple of the ¼ wavelength of the first radio wave has been described, the depth may not be an odd multiple of the ¼ wavelength of the first radio wave. In each of the plurality of recesses  123 , the phase of the first radio wave having entered the recess  123  and reflected on the bottom face  125  of the recess  123  may be the same phase as the phase of the first radio wave reflected by the first region  121  of the reflection face of the reflector at the opening  124  of the recess  123 . For example, in a case where the plurality of recesses  123  are filled with a dielectric, the depth may be set so that the side lobe of the first radio wave reflected on the bottom face  125  of the recess  123  and the main lobe of the first radio wave reflected by the reflection face in the first region  121  have the same phase at the opening  124  of the recess  123  in consideration of the relative permittivity of the dielectric. 
     First Example 
     An example of the reflector antenna device  100  according to the first embodiment will be described with reference to  FIGS. 4 to 6 . 
       FIG. 4  is a configuration diagram illustrating a configuration of the reflector antenna device  100  according to the first embodiment and the reflector antenna device  100  according to a first example. 
     The reflector antenna device  100  illustrated in  FIG. 4  includes a primary radiator  110 , a first reflector  120 , and a second reflector  130 . 
     As illustrated in  FIG. 4 , the reflector antenna device  100  according to the first example is a ring-focus type Gregorian antenna. 
     The primary radiator  110  is an ideal horn antenna that excites the radio wave in the HE 11  mode. The primary radiator  110  radiates a first radio wave in a 30 GHz (gigahertz) band that is a first frequency band and a second radio wave in a 20 GHz band that is a second frequency band lower in frequency than the first frequency band. 
       FIG. 5  is a diagram illustrating radiation patterns of the first radio wave and the second radio wave radiated by the primary radiator  110  included in the reflector antenna device  100  according to the first example. 
     In  FIG. 5 , the horizontal axis represents an angle (hereinafter, referred to as “prospective half angle”) formed between a direction in which the primary radiator  110  radiates the first radio wave and the second radio wave and the radiation axis with a predetermined point on the radiation axis at which the primary radiator  110  radiates the first radio wave and the second radio wave as an origin. In  FIG. 5 , the vertical axis represents the intensity of each of the first radio wave and the second radio wave radiated by the primary radiator  110 . 
     As illustrated in  FIG. 5 , the primary radiator  110  radiates the main lobe of the first radio wave in the prospective half angle of less than 15 degrees, and radiates the side lobe of the first radio wave in the prospective half angle of more than or equal to 15 degrees and less than or equal to 22.5 degrees. In addition, the primary radiator  110  radiates the main lobe of the second radio wave in the prospective half angle of less than or equal to 22.5 degrees. 
     The first reflector  120  is a ring focus mirror having a mirror diameter of 0.14 m (meters). The reflection face of the first reflector  120  reflects, among the first radio wave and the second radio wave radiated by the primary radiator  110 , the first radio wave and the second radio wave having the prospective half angle of more than or equal to 0 degrees and less than or equal to 22.5 degrees toward the second reflector  130 . Specifically, the reflection face in the first region  121  reflects, among the first radio wave and the second radio wave radiated by the primary radiator  110 , the first radio wave and the second radio wave having the prospective half angle of more than or equal to 0 degrees and less than 15 degrees toward the second reflector  130 . That is, the reflection face in the first region  121  reflects the main lobe of the first radio wave and the main lobe of the second radio wave toward the second reflector  130 . Further, the reflection face in the first region  121  reflects, among the first radio wave and the second radio wave radiated by the primary radiator  110 , the first radio wave and the second radio wave having the prospective half angle of more than or equal to 15 degrees and less than 22.5 degrees toward the second reflector  130 . That is, the reflection face in the first region  121  reflects the side lobe of the first radio wave and the main lobe of the second radio wave toward the second reflector  130 . 
     The second reflector  130  is a ring focus mirror having a mirror diameter of 1 m. The second reflector  130  receives the first radio wave and the second radio wave reflected by the first reflector  120 , and reflects the first radio wave and the second radio wave in a predetermined direction. 
     The reflector antenna device  100  outputs the first radio wave and the second radio wave reflected by the second reflector  130  to the outside of the reflector antenna device  100 . 
       FIG. 6  is a diagram illustrating a secondary radiation pattern of the first radio wave output from the reflector antenna device  100  according to the first example, the secondary radiation pattern of the first radio wave after the first radio wave radiated by the primary radiator  110  included in the reflector antenna device  100  according to the first example is reflected by the first reflector  120  and the second reflector  130 .  FIG. 6  also illustrates a secondary radiation pattern of the first radio wave output from the conventional reflector antenna device for comparison with the secondary radiation pattern of the first radio wave output from the reflector antenna device  100  according to the first example. 
     The horizontal axis in  FIG. 6  represents an angle formed with the radiation axis of the first radio wave output from the reflector antenna device  100 . The vertical axis in  FIG. 6  represents a gain of the first radio wave output from the reflector antenna device  100 . 
     As illustrated in  FIG. 6 , the gain of the first radio wave output from the reflector antenna device  100  according to the first example is improved by about 1 dB in the radiation axis direction as compared with a gain of the first radio wave output from the conventional reflector antenna device. 
     As described above, the reflector antenna device  100  includes the primary radiator  110  to radiate the first radio wave that is the radio wave in the first frequency band and radiate the second radio wave that is the radio wave in the second frequency band lower in frequency than the first frequency band, and the first reflector  120  that is a reflector having the reflection face that receives the first radio wave and the second radio wave radiated by the primary radiator  110  and reflects the first radio wave and the second radio wave. The reflection face included in the first reflector  120  that is the reflector has the first region  121  including the center point of the reflection face and the second region  122  that is the outer peripheral region of the first region  121  and is the region provided with the plurality of recesses  123 . Each of the plurality of recesses  123  provided in the second region  122  of the reflection face included in the first reflector  120  that is the reflector allows the first radio wave to enter the recess  123 , restricts the second radio wave from entering the recess  123 , and reflects the first radio wave that has entered the recess  123  on the bottom face  125  of the recess  123 . 
     With such a configuration, the reflector antenna device  100  can suppress the spillover of the side lobe of the radio wave in the high frequency band while suppressing the decrease in the gain of the secondary radiation pattern of the radio wave in the high frequency band. 
     Furthermore, with such a configuration, the reflector antenna device  100  can improve the gain of the secondary radiation pattern of the radio wave in the high frequency band output from the reflector antenna device  100  by suppressing the spillover of the side lobe of the radio wave in the high frequency band. 
     Furthermore, as described above, in the above-described configuration, the reflector antenna device  100  is configured so that the maximum value “L” of the length in the plane parallel to the reflection face of each of the plurality of recesses  123  provided in the second region  122  of the reflection face included in the first reflector  120  that is a reflector falls within the range defined by the above-described formula (1). 
     With this configuration, each of the plurality of recesses  123  provided in the second region  122  of the reflection face included in the first reflector  120  that is a reflector can allow the first radio wave to enter the recess  123 , restrict the second radio wave from entering the recess  123 , and reflect the first radio wave that has entered the recess  123  on the bottom face  125  of the recess  123 . 
     Furthermore, as described above, in the above-described configuration, the reflector antenna device  100  is configured so that each of the plurality of recesses  123  provided in the second region  122  of the reflection face included in the first reflector  120  that is a reflector enters the recess  123 , and the phase of the first radio wave reflected on the bottom face  125  of the recess  123  is the same phase as the phase of the first radio wave reflected by the first region  121  of the reflection face included in the first reflector  120  that is a reflector at the opening  124  of the recess  123 . 
     With such a configuration, the reflector antenna device  100  can suppress the spillover of the side lobe of the radio wave in the high frequency band while suppressing the decrease in the gain of the secondary radiation pattern of the radio wave in the high frequency band. 
     Furthermore, with such a configuration, the reflector antenna device  100  can improve the gain of the secondary radiation pattern of the radio wave in the high frequency band output from the reflector antenna device  100  by suppressing the spillover of the side lobe of the radio wave in the high frequency band. 
     Furthermore, as described above, in the above-described configuration, the reflector antenna device  100  is configured so that the depth of each of the plurality of recesses  123  provided in the second region  122  of the reflection face included in the first reflector  120  that is a reflector is an odd multiple of the ¼ wavelength of the first radio wave. 
     With such a configuration, the reflector antenna device  100  can suppress the spillover of the side lobe of the radio wave in the high frequency band while suppressing the decrease in the gain of the secondary radiation pattern of the radio wave in the high frequency band. 
     Furthermore, with such a configuration, the reflector antenna device  100  can improve the gain of the secondary radiation pattern of the radio wave in the high frequency band output from the reflector antenna device  100  by suppressing the spillover of the side lobe of the radio wave in the high frequency band. 
     Furthermore, as described above, in the above-described configuration, the reflector antenna device  100  is configured so that the reflection face included in the first reflector  120  that is a reflector is a quadratic face or a parabolic face. 
     With such a configuration, the reflector antenna device  100  can suppress the spillover of the side lobe of the radio wave in the high frequency band while suppressing the decrease in the gain of the secondary radiation pattern of the radio wave in the high frequency band. 
     Furthermore, with such a configuration, the reflector antenna device  100  can improve the gain of the secondary radiation pattern of the radio wave in the high frequency band output from the reflector antenna device  100  by suppressing the spillover of the side lobe of the radio wave in the high frequency band. 
     Furthermore, as described above, in the above-described configuration, the reflector antenna device  100  is configured so that the second region  122  of the reflection face included in the first reflector  120  that is a reflector is a region that receives the side lobe of the first radio wave radiated by the primary radiator  110  and the main lobe of the second radio wave radiated by the primary radiator  110 . 
     With such a configuration, the reflector antenna device  100  can suppress the spillover of the side lobe of the radio wave in the high frequency band while suppressing the decrease in the gain of the secondary radiation pattern of the radio wave in the high frequency band. 
     Furthermore, with such a configuration, the reflector antenna device  100  can improve the gain of the secondary radiation pattern of the radio wave in the high frequency band output from the reflector antenna device  100  by suppressing the spillover of the side lobe of the radio wave in the high frequency band. 
     Modification of First Embodiment 
     The reflector antenna device  100  according to the first embodiment includes the primary radiator  110 , the first reflector  120 , and the second reflector  130  as illustrated in  FIG. 1 , but the reflector antenna device  100  may include one or more reflectors different from the first reflector  120  and the second reflector  130  in addition to the first reflector  120  and the second reflector  130 . 
     More specifically, for example, in the reflector antenna device  100  according to a modification of the first embodiment, the first reflector  120  reflects the first radio wave and the second radio wave radiated by the primary radiator  110  toward a reflector different from the first reflector  120  and the second reflector  130 . Furthermore, in the reflector antenna device  100  according to the modification of the first embodiment, the second reflector  130  receives the first radio wave and the second radio wave reflected by the reflector different from the first reflector  120  and the second reflector  130 , and reflects the first radio wave and the second radio wave in a predetermined direction. 
     As described above, the reflector antenna device  100  according to the modification of the first embodiment includes the primary radiator  110  to radiate the first radio wave that is the radio wave in the first frequency band and radiate the second radio wave that is the radio wave in the second frequency band lower in frequency than the first frequency band, and the first reflector  120  that is the reflector having the reflection face that receives the first radio wave and the second radio wave radiated by the primary radiator  110  and reflects the first radio wave and the second radio wave. The reflection face included in the first reflector  120  that is the reflector has the first region  121  including the center point of the reflection face and the second region  122  that is the outer peripheral region of the first region  121  and is the region provided with the plurality of recesses  123 . Each of the plurality of recesses  123  provided in the second region  122  of the reflection face included in the first reflector  120  that is a reflector is configured to allow the first radio wave to enter the recess  123 , restrict the second radio wave from entering the recess  123 , and reflect the first radio wave that has entered the recess  123  on the bottom face  125  of the recess  123 . 
     With such a configuration, the reflector antenna device  100  according to the modification of the first embodiment can suppress the spillover of the side lobe of the radio wave in the high frequency band while suppressing the decrease in the gain of the secondary radiation pattern of the radio wave in the high frequency band. 
     Furthermore, with such a configuration, the reflector antenna device  100  according to the modification of the first embodiment can improve the gain of the secondary radiation pattern of the radio wave in the high frequency band output from the reflector antenna device  100  by suppressing the spillover of the side lobe of the radio wave in the high frequency band. 
     Another Modification of First Embodiment 
     The reflector antenna device  100  according to the first embodiment includes the primary radiator  110 , the first reflector  120 , and the second reflector  130  as illustrated in  FIG. 1 , but a reflector antenna device  100   a  may include only a first reflector  120   a  without including the second reflector  130 . 
     That is, while the reflector antenna device  100  according to the first embodiment is a reflector antenna including a plurality of reflectors such as a Cassegrain antenna or a Gregorian antenna, the reflector antenna device  100   a  is a reflector antenna including one reflector such as a parabola antenna, an offset parabola antenna, or a horn reflector antenna. 
     A configuration of the reflector antenna device  100   a  according to another modification of the first embodiment will be described with reference to  FIG. 7 . 
       FIG. 7  is a configuration diagram illustrating an example of a configuration of a main part of the reflector antenna device  100   a  according to another modification of the first embodiment. 
     The reflector antenna device  100   a  includes a primary radiator  110  and a first reflector  120   a.    
       FIG. 7A  is a configuration diagram illustrating an example of a configuration of a main part of the reflector antenna device  100   a  according to another modification of the first embodiment, and is a cross-sectional view of the reflector antenna device  100   a  on a plane including a radiation axis of the primary radiator  110  included in the reflector antenna device  100   a.    
       FIG. 7B  is a configuration diagram illustrating an example of the configuration of the main part of the first reflector  120   a  included in the reflector antenna device  100   a  according to another modification of the first embodiment, and is a configuration diagram of the first reflector  120   a  viewed from the primary radiator  110  included in the reflector antenna device  100   a  according to another modification of the first embodiment. 
       FIG. 7C  is a configuration diagram illustrating an example of a configuration of the main part of the first reflector  120   a  included in the reflector antenna device  100   a  according to another modification of the first embodiment, and is an enlarged view of the first reflector  120   a  in a region surrounded by a rectangle indicated by a broken line in  FIG. 7A . 
       FIG. 7D  is a configuration diagram illustrating an example of a configuration of the main part of the first reflector  120   a  included in the reflector antenna device  100   a  according to another modification of the first embodiment, and is an enlarged view of the first reflector  120   a  in a region surrounded by a rectangle indicated by a broken line in  FIG. 7B . 
     In  FIG. 7 , the same reference numerals are given to the same blocks as those illustrated in  FIG. 1 , and the description thereof will be omitted. 
     The first reflector  120   a  is a reflector having a reflection face that receives the first radio wave and the second radio wave radiated from the primary radiator  110  and reflects the first radio wave and the second radio wave. 
     The reflection face included in the first reflector  120   a  that is a reflector is, for example, a curved face such as a quadratic face or a parabolic face. 
     For example, the first reflector  120   a  reflects the first radio wave and the second radio wave reflected by the first reflector  120   a  in a predetermined direction in which the reflector antenna device  100   a  outputs the first radio wave and the second radio wave. 
     The reflector antenna device  100   a  outputs the first radio wave and the second radio wave reflected by the first reflector  120   a  in a predetermined direction. 
     The reflection face included in the first reflector  120   a  that is a reflector includes a first region  121  including a center point of the reflection face, and a second region  122  that is an outer peripheral region of the first region  121  and is a region provided with a plurality of recesses  123 . 
     The reflection face included in the first reflector  120   a  in the first region  121  corresponds to the reflection face in the first region  121  according to the first embodiment, and thus the description thereof is omitted. 
     In addition, the reflection face included in the first reflector  120   a  in the second region  122  corresponds to the reflection face in the second region  122  according to the first embodiment, and thus description thereof is omitted. 
     In addition, the plurality of recesses  123  provided on the reflection face included in the first reflector  120   a  in the second region  122  correspond to the plurality of recesses  123  according to the first embodiment, and thus description thereof is omitted. 
     As described above, the reflector antenna device  100   a  includes the primary radiator  110  to radiate the first radio wave that is the radio wave in the first frequency band and radiate the second radio wave that is the radio wave in the second frequency band lower in frequency than the first frequency band, and the first reflector  120   a  that is the reflector having the reflection face that receives the first radio wave and the second radio wave radiated by the primary radiator  110  and reflects the first radio wave and the second radio wave. The reflection face included in the first reflector  120   a  that is the reflector has the first region  121  including the center point of the reflection face and the second region  122  that is the outer peripheral region of the first region  121  and is the region provided with the plurality of recesses  123 . Each of the plurality of recesses  123  provided in the second region  122  of the reflection face included in the first reflector  120   a  that is the reflector is configured to allow the first radio wave to enter the recess  123 , restrict the second radio wave from entering the recess  123 , and reflect the first radio wave that has entered the recess  123  on the bottom face  125  of the recess  123 . 
     With this configuration, the reflector antenna device  100   a  can suppress the spillover of the side lobe of the radio wave in the high frequency band while suppressing the decrease in the gain of the secondary radiation pattern of the radio wave in the high frequency band. 
     Furthermore, with such a configuration, the reflector antenna device  100   a  can improve the gain of the secondary radiation pattern of the radio wave in the high frequency band output from the reflector antenna device  100   a  by suppressing the spillover of the side lobe of the radio wave in the high frequency band. 
     Second Embodiment 
     The primary radiator  110  included in the reflector antenna device  100  according to the first embodiment is a radiator that radiates the first radio wave that is the radio wave in the first frequency band and radiates the second radio wave that is the radio wave in the second frequency band lower in frequency than the first frequency band. However, the primary radiator  110  may be a radiator that radiates the first radio wave and the second radio wave and radiates a third radio wave that is a radio wave in a third frequency band lower in frequency than the first frequency band and higher in frequency than the second frequency band. 
     A configuration of a reflector antenna device  100   b  according to a second embodiment will be described with reference to  FIG. 8 . 
       FIG. 8  is a configuration diagram illustrating an example of a configuration of a main part of the reflector antenna device  100   b  according to the second embodiment. 
     The reflector antenna device  100   b  includes a primary radiator  110   b , a first reflector  120   b , and a second reflector  130 . 
     The reflector antenna device  100   b  is, for example, a reflector antenna including a plurality of reflectors such as a Gregorian antenna or a Cassegrain antenna. In the second embodiment, the reflector antenna device  100   b  will be described as a Gregorian antenna as illustrated in  FIG. 8  as an example. The reflector antenna device  100   b  may be a reflector antenna having one reflector such as a parabolic antenna, an offset parabolic antenna, or a horn reflector antenna. In a case where the reflector antenna device  100   b  is a reflector antenna including one reflector, the second reflector  130  is not an essential configuration in the reflector antenna device  100   b.    
       FIG. 8A  is a configuration diagram illustrating an example of a configuration of a main part of the reflector antenna device  100   b  according to the second embodiment, and is a cross-sectional view of the reflector antenna device  100   b  on a plane including a radiation axis of the primary radiator  110   b  included in the reflector antenna device  100   b.    
       FIG. 8B  is a configuration diagram illustrating an example of the configuration of a main part of the first reflector  120   b  included in the reflector antenna device  100   b  according to the second embodiment, and is a configuration diagram of the first reflector  120   b  viewed from the primary radiator  110   b  included in the reflector antenna device  100   b  according to the second embodiment. 
       FIG. 8C  is a configuration diagram illustrating the example of the configuration of the main part of the first reflector  120   b  included in the reflector antenna device  100   b  according to the second embodiment, and is an enlarged view of the first reflector  120   b  in a region surrounded by a rectangle indicated by a broken line in  FIG. 8A . 
       FIG. 8D  is a configuration diagram illustrating the example of the configuration of the main part of the first reflector  120   b  included in the reflector antenna device  100   b  according to the second embodiment, and is an enlarged view of the first reflector  120   b  in a region surrounded by a rectangle indicated by a broken line in  FIG. 8B . 
     In  FIG. 8 , the same reference numerals are given to the same blocks as those illustrated in  FIG. 1 , and the description thereof will be omitted. 
     The primary radiator  110   b  is a radiator that radiates a first radio wave that is a radio wave in a first frequency band, a second radio wave that is a radio wave in a second frequency band lower in frequency than the first frequency band, and a third radio wave that is a radio wave in the third frequency band lower in frequency than the first frequency band and higher in frequency than the second frequency band. 
     In the second embodiment, the primary radiator  110   b  is described as one radiator that radiates the first radio wave, the second radio wave, and the third radio wave, but the primary radiator  110   b  may be a radiator in which three radiators are combined, such as a radiator in which a radiator that radiates the first radio wave, another radiator that radiates the second radio wave, and another radiator that radiates the third radio wave are combined. 
     The first reflector  120   b  is a reflector having a reflection face that receives the first radio wave, the second radio wave, and the third radio wave radiated by the primary radiator  110   b  and reflects the first radio wave, the second radio wave, and the third radio wave. 
     In the reflector antenna device  100   b  according to the second embodiment, the first reflector  120   b  is a sub-mirror. 
     The reflection face included in the first reflector  120   b  that is a reflector is, for example, a curved face such as a quadratic face or a parabolic face. 
     The reflection face included in the first reflector  120   b  that is a reflector includes a first region  121  including a center point of the reflection face, a second region  122   b   1  that is an outer peripheral region of the first region  121  and is a region provided with a plurality of recesses  123   b   1 , and a third region  122   b   2  that is an outer peripheral region of the second region  122   b   1  and is a region provided with a plurality of recesses  123   b   2 . 
     Note that the plurality of recesses  123   b   1  provided on the reflection face in the second region  122   b   1  may be periodically arranged or may be arranged at any positions in the second region  122   b   1 . In addition, the plurality of recesses  123   b   2  provided on the reflection face in the third region  122   b   2  may be periodically arranged, or may be arranged at any positions in the third region  122   b   2 . 
     The reflection face included in the first reflector  120   b  in the first region  121  is made of, for example, a conductor such as metal, and the reflection face in the first region  121  is processed into a smooth shape without unevenness. 
     The reflection face in the first region  121  receives a main lobe of the first radio wave radiated by the primary radiator  110   b , a main lobe of the second radio wave radiated by the primary radiator  110   b , and a main lobe of the third radio wave radiated by the primary radiator  110   b . The reflection face in the first region  121  reflects the main lobe of the first radio wave, the main lobe of the second radio wave, and the main lobe of the third radio wave toward the second reflector  130 . 
     The reflection face included in the first reflector  120   b  in the second region  122   b   1  is made of, for example, a conductor such as metal, and the plurality of recesses  123   b   1  (hereinafter, simply referred to as a “plurality of recesses  123   b   1 ”) provided in the reflection face in the second region  122   b   1  is formed by casting, shaving, or tapping. 
     The reflection face in the second region  122   b   1  receives a side lobe of the first radio wave radiated by the primary radiator  110   b , the main lobe of the second radio wave radiated by the primary radiator  110   b , and the main lobe of the third radio wave radiated by the primary radiator  110   b.    
     Each of the plurality of recesses  123   b   1  allows the first radio wave to enter the recess  123   b   1 , restricts the second radio wave and the third radio wave from entering the recess  123   b   1 , and reflects the first radio wave having entered the recess  123   b   1  on a bottom face  125   b   1  of the recess  123   bl.    
     Specifically, each of the plurality of recesses  123   b   1  allows the side lobe of the first radio wave radiated by the primary radiator  110   b  to enter the recess  123   b   1 , and reflects the side lobe of the first radio wave having entered the recess  123   b   1  on the bottom face  125   b   1  of the recess  123   b   1 . More specifically, each of the plurality of recesses  123   b   1  reflects the side lobe of the first radio wave having entered the recess  123   b   1  toward the second reflector  130 . In addition, each of the plurality of recesses  123   b   1  restricts the main lobe of the second radio wave and the main lobe of the third radio wave radiated by the primary radiator  110   b  from entering the recess  123   b   1 , and reflects the main lobe of the second radio wave and the main lobe of the third radio wave not entering the recess  123   b   1  toward the second reflector  130 . 
     The reflection face included in the first reflector  120   b  in the third region  122   b   2  is made of, for example, a conductor such as metal, and the plurality of recesses  123   b   2  (hereinafter, simply referred to as a “plurality of recesses  123   b   2 ”) provided in the reflection face in the third region  122   b   2  is formed by casting, shaving, or tapping. 
     The reflection face in the third region  122   b   2  receives the side lobe of the first radio wave radiated by the primary radiator  110   b , the main lobe of the second radio wave radiated by the primary radiator  110   b , and a side lobe of the third radio wave radiated by the primary radiator  110   b.    
     Each of the plurality of recesses  123   b   2  allows the first radio wave and the third radio wave to enter the recess  123   b   2 , restricts the second radio wave from entering the recess  123   b   2 , and reflects the first radio wave and the third radio wave having entered the recess  123   b   2  on a bottom face  125   b   2  of the recess  123   b   2 . 
     Specifically, each of the plurality of recesses  123   b   2  allows the side lobe of the first radio wave radiated by the primary radiator  110   b  and the side lobe of the third radio wave radiated by the primary radiator  110   b  to enter the recess  123   b   2 , and reflects the side lobe of the first radio wave and the side lobe of the third radio wave having entered the recess  123   b   2  on the bottom face  125   b   2  of the recess  123   b   2 . More specifically, each of the plurality of recesses  123   b   2  reflects the side lobe of the first radio wave and the side lobe of the third radio wave having entered the recess  123   b   2  toward the second reflector  130 . Each of the plurality of recesses  123   b   2  restricts the main lobe of the second radio wave radiated by the primary radiator  110   b  from entering the recess  123   b   2 , and reflects the main lobe of the second radio wave not entering the recess  123   b   2  toward the second reflector  130 . 
     With this configuration, the reflector antenna device  100   b  can suppress the spillover of the side lobe of the radio wave in the high frequency band while suppressing the decrease in the gain of the secondary radiation pattern of the radio wave in the high frequency band. 
     Each of the plurality of recesses  123   b   1  and each of the plurality of recesses  123   b   2  have, for example, a circular shape in a cross section in a plane parallel to the reflection face. That is, each of the plurality of recesses  123   b   1  and each of the plurality of recesses  123   b   2  are cylindrical recesses provided on the reflection face in the second region  122   b   1  or the third region  122   b   2  included in the first reflector  120   b.    
     The shape of the cross section in the plane parallel to the reflection face of each of the plurality of recesses  123   b   1  and each of the plurality of recesses  123   b   2  is not limited to a circular shape. 
     As illustrated in  FIG. 2 , the shape of the cross section in the plane parallel to the reflection face of each of the plurality of recesses  123   b   1  and each of the plurality of recesses  123   b   2  may be an elliptical shape, a rectangular shape, a doughnut shape, a cross shape, or the like. The plurality of recesses  123   b   1  and the plurality of recesses  123   b   2  may be a combination of recesses having different cross-sectional shapes in a plane parallel to the reflection face. 
     The second reflector  130  is a reflector having a reflection face that receives the first radio wave, the second radio wave, and the third radio wave reflected by the first reflector  120   b  and reflects the first radio wave and the second radio wave. 
     In the reflector antenna device  100   b  according to the second embodiment, the second reflector  130  is a main mirror. 
     For example, the second reflector  130  reflects the first radio wave, the second radio wave, and the third radio wave reflected by the first reflector  120   b  in a predetermined direction in which the reflector antenna device  100   b  outputs the first radio wave, the second radio wave, and the third radio wave. 
     The reflector antenna device  100   b  outputs the first radio wave, the second radio wave, and the third radio wave reflected by the second reflector  130  in a predetermined direction. 
     The maximum value “La” of the length in the plane parallel to the reflection face of each of the plurality of recesses  123   b   1  falls, for example, within a range defined by the following formula (2). 
     
       
         
           
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       
                         
                           C 
                           z 
                         
                         
                           
                             ? 
                           
                           ⁢ 
                           F 
                           ⁢ 
                           
                             ? 
                           
                         
                       
                       &lt; 
                       L 
                       &lt; 
                       
                         
                           C 
                           z 
                         
                         
                           
                             ? 
                           
                           ⁢ 
                           
                             F 
                             M 
                           
                         
                       
                     
                     ⁢ 
                     
                       
 
                     
                     ⁢ 
                     
                       
                         ? 
                       
                       ⁢ 
                       
                         indicates text missing or illegible when filed 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In addition, the maximum value “Lb” of the length in the plane parallel to the reflection face of each of the plurality of recesses  123   b   2  falls, for example, within a range defined by the following formula (3). 
     
       
         
           
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       
                         
                           C 
                           x 
                         
                         
                           
                             ? 
                           
                           ⁢ 
                           
                             F 
                             M 
                           
                         
                       
                       &lt; 
                       L 
                       &lt; 
                       
                         
                           C 
                           x 
                         
                         
                           
                             ? 
                           
                           ⁢ 
                           
                             F 
                             L 
                           
                         
                       
                     
                     ⁢ 
                     
                       
 
                     
                     ⁢ 
                     
                       
                         ? 
                       
                       ⁢ 
                       
                         indicates text missing or illegible when filed 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Here, “C” is the speed of light, “χ” is the positive minimum root in the first derivative of the Bessel function of the first type, “π” is the circular constant, “F H ” is the first frequency band, “F L ” is the second frequency band, and “F M ” is the third frequency band. 
     Note that the value of χ, which is the positive minimum root in the first derivative of the Bessel function of the first type, is 1.841. 
     For example, in a case where the maximum value of the length in the plane parallel to the reflection face of each of the plurality of recesses  123   b   1  satisfies the condition shown in formula (2), the second radio wave in the second frequency band and the third radio wave in the third frequency band having frequencies lower than that of the first frequency band which is the high frequency band are reflected at an opening  124   b   1  of each recess  123   b   1  since the maximum value of the length is shorter than the wavelengths of the second radio wave and the third radio wave. 
     On the other hand, in this case, since the maximum value of the length is longer than the wavelength of the first radio wave, the first radio wave in the first frequency band that is a high frequency band enters the inside of each recess  123   b   1  and is reflected on the bottom face  125   b   1  of each recess  123   b   1  facing the opening  124   b   1  of each recess  123   bl.    
     In addition, for example, in a case where the maximum value of the length in the plane parallel to the reflection face of each of the plurality of recesses  123   b   2  satisfies the condition shown in formula (3), the second radio wave in the second frequency band having a frequency lower than that of the third frequency band, which is a high frequency band, is reflected at an opening  124   b   2  of each recess  123   b   2  since the maximum value of the length is shorter than the wavelength of the third radio wave. 
     On the other hand, in this case, since the maximum value of the length is longer than the wavelengths of the first radio wave and the third radio wave, the first radio wave in the first frequency band and the third radio wave in the third frequency band, which are high frequency bands, enter the inside of each recess  123   b   2 , and are reflected on the bottom face  125   b   2  of each recess  123   b   2  facing the opening  124   b   2  of each recess  123   b   2 . 
     For example, the plurality of recesses  123   b   1  are processed so that the depth of each recess is an odd multiple of ¼ wavelength of the first radio wave. 
     Note that the depth of each of the plurality of recesses  123   b   1  does not need to be strictly ¼ wavelength of the first radio wave, and the ¼ wavelength of the first radio wave herein includes approximately ¼ wavelength. 
     Further, as for the depths of the plurality of recesses  123   b   1 , the depths of all of the plurality of recesses  123   b   1  do not need to be ¼ wavelength of the first radio wave, and may be, for example, any depth depending on the distance from the center point of the reflection face or the like. 
     In a case where the depth of each of the plurality of recesses  123   b   1  is an odd multiple of ¼ wavelength of the first radio wave, the phase of the first radio wave reflected on the bottom face  125   b   1  of the recess  123   b   1  is inverted with respect to the phase of the first radio wave incident on the recess  123   b   1  at the opening  124   b   1  of the recess  123   bl.    
     The depth of the recess  123   b   1  is a distance from the opening  124   b   1  of the recess  123   b   1  to the bottom face  125   b   1  of the recess  123   b   1 . 
     For example, the plurality of recesses  123   b   2  are processed so that the depth of each recess is an odd multiple of ¼ wavelength of the first radio wave or an odd multiple of ¼ wavelength of the third radio wave. 
     Note that the depth of each of the plurality of recesses  123   b   2  does not need to be strictly ¼ wavelength of the first radio wave or the third radio wave, and the ¼ wavelength of the first radio wave or the third radio wave here includes approximately ¼ wavelength. 
     For example, the plurality of recesses  123   b   2  may be processed so that the depth of each recess is an odd multiple of the ¼ wavelength of the first radio wave and an odd multiple of the ¼ wavelength of the third radio wave. 
     For example, the plurality of recesses  123   b   2  may be processed so that the depth of each recess is substantially odd multiple of ¼ wavelength of the first radio wave and substantially odd multiple of ¼ wavelength of the third radio wave. 
     Further, as for the depths of the plurality of recesses  123   b   2 , the depths of all of the plurality of recesses  123   b   2  do not need to be ¼ wavelength of the first radio wave or the third radio wave, and may be, for example, any depth depending on the distance from the center point of the reflection face or the like. 
     In a case where the depth of each of the plurality of recesses  123   b   2  is an odd multiple of ¼ wavelength of the first radio wave, the phase of the first radio wave reflected on the bottom face  125   b   2  of the recess  123   b   2  is inverted with respect to the phase of the first radio wave incident on the recess  123   b   2  at the opening  124   b   2  of the recess  123   b   2 . 
     In a case where the depth of each of the plurality of recesses  123   b   2  is an odd multiple of ¼ wavelength of the third radio wave, the phase of the third radio wave reflected on the bottom face  125   b   2  of the recess  123   b   2  is inverted with respect to the phase of the third radio wave incident on the recess  123   b   2  at the opening  124   b   2  of the recess  123   b   2 . 
     In a case where the depth of each of the plurality of recesses  123   b   2  is approximately an odd multiple of the ¼ wavelength of the first radio wave and approximately an odd multiple of the ¼ wavelength of the third radio wave, the phases of the first radio wave and the third radio wave reflected on the bottom face  125   b   2  of the recess  123   b   2  are substantially inverted with respect to the phases of the first radio wave and the third radio wave incident on the recess  123   b   2  at the opening  124   b   2  of the recess  123   b   2 . 
     Note that the depth of the recess  123   b   2  is a distance from the opening  124   b   2  of the recess  123   b   2  to the bottom face  125   b   2  of the recess  123   b   2 . 
     The detailed behavior of the recess  123   b   1  and the recess  123   b   2  is similar to that of the recess  123  according to the first embodiment, and thus the detailed description thereof is omitted. 
     As described above, the reflector antenna device  100   b  includes the primary radiator  110   b  to radiate the first radio wave that is the radio wave in the first frequency band and radiate the second radio wave that is the radio wave in the second frequency band lower in frequency than the first frequency band and the third radio wave that is the radio wave in the third frequency band lower in frequency than the first frequency band and higher in frequency than the second frequency band, and the first reflector  120   b  that is the reflector having the reflection face that receives the first radio wave, the second radio wave, and the third radio wave radiated by the primary radiator  110   b  and reflects the first radio wave, the second radio wave, and the third radio wave. The reflection face included in the first reflector  120   b  that is the reflector has the first region  121  including the center point of the reflection face, the second region  122   b   1  that is the outer peripheral region of the first region  121  and is a region provided with the plurality of recesses  123   b   1 , and the third region  122   b   2  that is the outer peripheral region of the second region  122   b   1  and is a region provided with the plurality of recesses  123   b   2 . Each of the plurality of recesses  123   b   1  provided in the second region  122   b   1  of the reflection face included in the first reflector  120   b  that is a reflector is configured to allow the first radio wave to enter the recess  123   b   1 , restrict the second radio wave and the third radio wave from entering the recess  123   b   1 , and reflect the first radio wave that has entered the recess  123   b   1  on the bottom face  125   b   1  of the recess  123   b   1 . Each of the plurality of recesses  123   b   2  provided in the third region  122   b   2  of the reflection face included in the first reflector  120   b  that is a reflector is configured to allow the first radio wave and the third radio wave to enter the recess  123   b   2 , restrict the second radio wave from entering the recess  123   b   2 , and reflect the first radio wave and the third radio wave that have entered the recess  123   b   2  on the bottom face  125   b   2  of the recess  123   b   2 . 
     With this configuration, the reflector antenna device  100   b  can suppress the spillover of the side lobe of the radio wave in the high frequency band while suppressing the decrease in the gain of the secondary radiation pattern of the radio wave in the high frequency band. 
     Furthermore, with such a configuration, the reflector antenna device  100   b  can improve the gain of the secondary radiation pattern of the radio wave in the high frequency band output from the reflector antenna device  100   b  by suppressing the spillover of the side lobe of the radio wave in the high frequency band. 
     Third Embodiment 
     A configuration of a main part of a reflector antenna device  100   c  according to a third embodiment will be described with reference to  FIG. 9 . 
       FIG. 9  is a configuration diagram illustrating an example of the configuration of the main part of the reflector antenna device  100   c  according to the third embodiment. 
     The reflector antenna device  100   c  includes a primary radiator  110 , a first reflector  120   c , and a second reflector  130 . 
     The reflector antenna device  100   c  is, for example, a reflector antenna including a plurality of reflectors such as a Gregorian antenna or a Cassegrain antenna. In the third embodiment, the reflector antenna device  100   c  will be described as a Gregorian antenna as illustrated in  FIG. 9  as an example. Note that the reflector antenna device  100   c  may be a reflector antenna having one reflector such as a parabolic antenna, an offset parabolic antenna, or a horn reflector antenna. In a case where the reflector antenna device  100   c  is a reflector antenna including one reflector, the second reflector  130  is not an essential configuration in the reflector antenna device  100   c.    
       FIG. 9A  is a configuration diagram illustrating an example of the configuration of the main part of the reflector antenna device  100   c  according to the third embodiment, and is a cross-sectional view of the reflector antenna device  100   c  on a plane including the radiation axis of the primary radiator  110  included in the reflector antenna device  100   c.    
       FIG. 9B  is a configuration diagram illustrating an example of the configuration of the main part of the first reflector  120   c  included in the reflector antenna device  100   c  according to the third embodiment, and is a configuration diagram of the first reflector  120   c  viewed from the primary radiator  110  included in the reflector antenna device  100   c  according to the third embodiment. 
       FIG. 9C  is a configuration diagram illustrating an example of a configuration of a main part of the first reflector  120   c  included in the reflector antenna device  100   c  according to the third embodiment, and is an enlarged view of the first reflector  120   c  in a region surrounded by a rectangle indicated by a broken line in  FIG. 9A . 
     In  FIG. 9 , the same reference numerals are given to the same blocks as those illustrated in  FIG. 1 , and the description thereof will be omitted. 
     The primary radiator  110  is a radiator that radiates a first radio wave that is a radio wave in a first frequency band and radiates a second radio wave that is a radio wave in a second frequency band lower in frequency than the first frequency band. 
     The first reflector  120   c  is a reflector having a reflection face that receives the first radio wave and the second radio wave radiated by the primary radiator  110  and reflects the first radio wave and the second radio wave. 
     In the reflector antenna device  100   c  according to the third embodiment, the first reflector  120   c  is a sub-mirror. 
     The reflection face included in the first reflector  120   c  that is a reflector is, for example, a curved face such as a quadratic face or a parabolic face. 
     The reflection face included in the first reflector  120   c  that is a reflector includes a first region  121  including a center point of the reflection face, and a second region  122   c  that is an outer peripheral region of the first region  121  and is a region including a conductor  126  and a dielectric  127  provided on the conductor  126 . 
     The reflection face in the first region  121  (hereinafter, simply referred to as a “first region  121 ”) included in the first reflector  120   c  is made of, for example, a conductor such as metal, and the reflection face in the first region  121  is processed into a smooth shape without unevenness. 
     The reflection face in the first region  121  receives a main lobe of the first radio wave radiated by the primary radiator  110  and a main lobe of the second radio wave radiated by the primary radiator  110 . The reflection face in the first region  121  reflects the main lobe of the first radio wave and the main lobe of the second radio wave toward the second reflector  130 . 
     In the conductor  126  (hereinafter, simply referred to as a “conductor  126 ”) constituting the reflection face in the second region  122   c  (hereinafter, simply referred to as a “second region  122   c ”) included in the first reflector  120   c , the face of the conductor  126  in contact with the dielectric  127  is processed into a smooth shape without unevenness, and is disposed on the same curved face as the curved face formed by the reflection face in the first region  121 . 
     The conductor  126  may be the same member as the conductor constituting the reflection face in the first region  121 , or may be a member different from the conductor constituting the reflection face in the first region  121 . 
     A face in contact with the conductor  126  of the dielectric  127  (hereinafter, simply referred to as a “dielectric  127 ”) constituting the reflection face in the second region  122   c  and a face facing the face and receiving the first radio wave and the second radio wave radiated by the primary radiator  110  are both processed into a smooth shape without unevenness. 
     The dielectric  127  receives the first radio wave and the second radio wave radiated by the primary radiator  110  and transmits the first radio wave and the second radio wave. 
     The conductor  126  reflects the first radio wave and the second radio wave transmitted through the dielectric  127 . 
     The second region  122   c  reflects the first radio wave and the second radio wave radiated by the primary radiator  110  by transmitting the first radio wave and the second radio wave reflected by the conductor  126  through the dielectric  127  again and radiating the first radio wave and the second radio wave. 
     The dielectric  127  increases the phase of the first radio wave reflected by the second region  122   c  by an odd multiple of 180 degrees with respect to the phase of the first radio wave reflected by the second region  122   c  in a case where the second region  122   c  does not have the dielectric  127 , and increases the phase of the second radio wave reflected by the second region  122   c  by an even multiple of 180 degrees with respect to the phase of the second radio wave reflected by the second region  122   c  in a case where the second region  122   c  does not have the dielectric  127 . 
     It should be noted that 180 degrees referred to herein need not be strictly 180 degrees and include approximately 180 degrees. 
     The dielectric  127  has a thickness calculated based on the following formula (4). 
     
       
         
           
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       ϕ 
                       = 
                       
                         2 
                         × 
                         
                           
                             360 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             D 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               ( 
                               
                                 
                                   
                                     ? 
                                   
                                 
                                 - 
                                 1 
                               
                               ) 
                             
                           
                           λ 
                         
                       
                     
                     ⁢ 
                     
                       
 
                     
                     ⁢ 
                     
                       
                         ? 
                       
                       ⁢ 
                       
                         indicates text missing or illegible when filed 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Here, “D” is the thickness of the dielectric  127 , “ε r ” is the relative permittivity of the dielectric  127 , “λ” is the wavelength of the radio wave, and “φ” is the amount of increase in the phase of the radio wave reflected by the second region  122   c  with respect to the phase of the radio wave reflected by the second region  122   c  in a case where the second region  122   c  does not have the dielectric  127 . 
     The behaviors of the first radio wave and the second radio wave incident on the second region  122   c  according to the third embodiment will be described with reference to  FIG. 10 . 
       FIG. 10A  is a diagram illustrating an example of behaviors of the first radio wave and the second radio wave incident on the second region  122   c  in a case where the second region  122   c  according to the third embodiment does not have the dielectric  127 . 
       FIG. 10B  is a diagram illustrating an example of behaviors of the first radio wave and the second radio wave incident on the dielectric  127  constituting the reflection face in the second region  122   c  according to the third embodiment. 
     As an example, the dielectric  127  illustrated in  FIG. 10B  has a relative permittivity of 2.25 and a thickness of 15 mm (millimeters). 
     As an example, the frequency band of the first radio wave illustrated in  FIGS. 10A and 10B  is 30 GHz, and the frequency band of the second radio wave is 20 GHz. 
     Assuming that the light speed is 3.0×10 8  m per second, the wavelength of the first radio wave is 1.0×10 −2  m, and the wavelength of the first radio wave is 1.5×10 −2  m. 
     Therefore, as illustrated in  FIG. 10B , the phase of the first radio wave advances by 1620 degrees while the first radio wave advances by 30 mm through the dielectric  127  having a relative permittivity of 2.25, and the phase of the second radio wave advances by 1080 degrees while the second radio wave advances by 30 mm through the dielectric  127 . As illustrated in  FIG. 10A , the phase of the first radio wave advances by 1080 degrees while the first radio wave advances by 30 mm in vacuum or air, and the phase of the second radio wave advances by 720 degrees while the second radio wave advances by 30 mm in vacuum or air. 
     That is, the dielectric  127  illustrated in  FIG. 10B  increases the phase of the first radio wave reflected by the second region  122   c  by 540 degrees, which is an odd multiple of 180 degrees with respect to the phase of the first radio wave reflected by the second region  122   c  in a case where the second region  122   c  does not have the dielectric  127 , and increases the phase of the second radio wave reflected by the second region  122   c  by 360 degrees, which is an even multiple of 180 degrees with respect to the phase of the second radio wave reflected by the second region  122   c  in a case where the second region  122   c  does not have the dielectric  127 . 
     The side lobe closest to the main lobe has a phase inverted with respect to the main lobe. 
     As described above, the reflection face in the first region  121  receives the main lobe of the first radio wave radiated by the primary radiator  110  and the main lobe of the second radio wave radiated by the primary radiator  110 . As described above, the reflection face in the second region  122   c  receives the side lobe of the first radio wave radiated by the primary radiator  110  and the main lobe of the second radio wave radiated by the primary radiator  110 . 
     Therefore, in a case where the dielectric  127  increases the phase of the first radio wave reflected by the second region  122   c  by an odd multiple of 180 degrees with respect to the phase of the first radio wave reflected by the second region  122   c  in a case where the second region  122   c  does not have the dielectric  127 , and increases the phase of the second radio wave reflected by the second region  122   c  by an even multiple of 180 degrees with respect to the phase of the second radio wave reflected by the second region  122   c  in a case where the second region  122   c  does not have the dielectric  127 , the side lobe of the first radio wave reflected by the second region  122   c  has the same phase as the main lobe of the first radio wave reflected by the reflection face in the first region  121 . In this case, the main lobe of the second radio wave reflected by the second region  122   c  has the same phase as the main lobe of the first radio wave reflected by the reflection face in the first region  121 . 
     Note that the same phase referred to herein does not need to be strictly the same phase, and includes substantially the same phase. 
     In addition, the reflector antenna device  100   c  according to the third embodiment has been described as including the primary radiator  110 , the first reflector  120   c , and the second reflector  130  as an example, but it is not limited thereto. 
     For example, the reflector antenna device  100   c  according to the third embodiment may include, as the reflectors, one or more reflectors different from the first reflector  120   c  and the second reflector  130 , in addition to the first reflector  120   c  and the second reflector  130 . 
     Furthermore, for example, the reflector antenna device  100   c  according to the third embodiment may not include the second reflector  130 , and may include only the first reflector  120   c  as a reflector with the first reflector  120   c  as a main mirror. 
     Furthermore, for example, the primary radiator  110  included in the reflector antenna device  100   c  according to the third embodiment is a radiator that radiates the first radio wave that is a radio wave in the first frequency band and radiates the second radio wave that is a radio wave in the second frequency band lower in frequency than the first frequency band. However, the primary radiator  110  may be a radiator that radiates the first radio wave and the second radio wave and radiates the third radio wave that is a radio wave in the third frequency band lower in frequency than the first frequency band and higher in frequency than the second frequency band. 
     In a case where the primary radiator  110  included in the reflector antenna device  100   c  according to the third embodiment radiates the first radio wave, the second radio wave, and the third radio wave, the reflection face included in the first reflector  120   c  according to the third embodiment may include a third region that is an outer peripheral region of the second region  122   c  or a third region that is an outer peripheral region of the first region  121  and an inner peripheral region of the second region  122   c  in addition to the first region  121  and the second region  122   c . Further, the third region of the reflection face included in the first reflector  120   c  (hereinafter, simply referred to as a “third region”) includes a dielectric having a different thickness or a different relative permittivity from the dielectric  127  constituting the second region  122   c.    
     In this case, for example, the second region  122   c  receives the side lobe of the first radio wave, the main lobe of the second radio wave, and the main lobe of the third radio wave, and the dielectric  127  constituting the second region  122   c  increases the phase of the first radio wave by an odd multiple of 180 degrees with respect to the phase of the first radio wave reflected by the second region  122   c  in a case where the second region  122   c  does not have the dielectric  127 , and increases the phases of the second radio wave and the third radio wave by an even multiple of 180 degrees with respect to the phases of the second radio wave and the third radio wave reflected by the second region  122   c  in a case where the second region  122   c  does not have the dielectric  127 . In addition, the third region receives the side lobe of the first radio wave, the main lobe of the second radio wave, and the side lobe of the third radio wave, and the dielectric included in the third region increases the phases of the first radio wave and the third radio wave by an odd multiple of 180 degrees with respect to the phases of the first radio wave and the third radio wave reflected by the second region  122   c  in a case where the second region  122   c  does not have the dielectric  127 , and increases the phase of the second radio wave by an even multiple of 180 degrees with respect to the phase of the second radio wave reflected by the second region  122   c  in a case where the second region  122   c  does not have the dielectric  127 . 
     As described above, the reflector antenna device  122   c  includes the primary radiator  110  to radiate the first radio wave that is the radio wave in the first frequency band and radiate the second radio wave that is the radio wave in the second frequency band lower in frequency than the first frequency band, and the reflector having the reflection face that receives the first radio wave and the second radio wave radiated by the primary radiator  110  and reflects the first radio wave and the second radio wave, and is configured so that the reflection face included in the reflector includes the first region  121  including the center point of the reflection face and the second region  122   c  that is the outer peripheral region of the first region  121  and is the region including the conductor  126  and the dielectric  127  provided on the conductor  126 , the dielectric  127  constituting the second region  122   c  of the reflection face included in the reflector receives the first radio wave and the second radio wave radiated by the primary radiator  110  and transmits the first radio wave and the second radio wave, the conductor  126  constituting the second region  122   c  of the reflection face included in the reflector reflects the first radio wave and the second radio wave transmitted through the dielectric  127 , the second region  122   c  of the reflection face included in the reflector reflects the first radio wave and the second radio wave reflected by the conductor  126  by transmitting the first radio wave and the second radio wave reflected by the conductor  126  through the dielectric  127  again and radiating the first radio wave and the second radio wave, and the dielectric  127  constituting the second region  122   c  of the reflection face included in the reflector increases the phase of the first radio wave reflected by the second region  122   c  by an odd multiple of 180 degrees with respect to the phase of the first radio wave reflected by the second region  122   c  in a case where the second region  122   c  does not have the dielectric  127 , and increases the phase of the second radio wave reflected by the second region  122   c  by an even multiple of 180 degrees with respect to the phase of the second radio wave reflected by the second region  122   c  in a case where the second region  122   c  does not have the dielectric  127 . 
     With this configuration, the reflector antenna device  100   c  can suppress the spillover of the side lobe of the radio wave in the high frequency band while suppressing the decrease in the gain of the secondary radiation pattern of the radio wave in the high frequency band. 
     Furthermore, with such a configuration, the reflector antenna device  100   c  can improve the gain of the secondary radiation pattern of the radio wave in the high frequency band output from the reflector antenna device  100   c  by suppressing the spillover of the side lobe of the radio wave in the high frequency band. 
     Furthermore, as described above, in the above-described configuration, the reflector antenna device  100   c  is configured so that the reflection face included in the first reflector  120   c  that is a reflector is a quadratic face or a parabolic face. 
     With this configuration, the reflector antenna device  100   c  can suppress the spillover of the side lobe of the radio wave in the high frequency band while suppressing the decrease in the gain of the secondary radiation pattern of the radio wave in the high frequency band. 
     Furthermore, with such a configuration, the reflector antenna device  100   c  can improve the gain of the secondary radiation pattern of the radio wave in the high frequency band output from the reflector antenna device  100   c  by suppressing the spillover of the side lobe of the radio wave in the high frequency band. 
     Furthermore, as described above, in the above-described configuration, the reflector antenna device  100   c  is configured so that the second region  122   c  of the reflection face included in the first reflector  120   c  that is a reflector is a region that receives the side lobe of the first radio wave radiated by the primary radiator  110  and the main lobe of the second radio wave radiated by the primary radiator  110 . 
     With this configuration, the reflector antenna device  100   c  can suppress the spillover of the side lobe of the radio wave in the high frequency band while suppressing the decrease in the gain of the secondary radiation pattern of the radio wave in the high frequency band. 
     Furthermore, with such a configuration, the reflector antenna device  100   c  can improve the gain of the secondary radiation pattern of the radio wave in the high frequency band output from the reflector antenna device  100   c  by suppressing the spillover of the side lobe of the radio wave in the high frequency band. 
     It should be noted that the invention of the present application can freely combine the embodiments, modify any constituent element of each embodiment, or omit any constituent element in each embodiment within the scope of the invention. 
     INDUSTRIAL APPLICABILITY 
     The present invention is suitable for a reflector antenna device including a primary radiator and a reflector. 
     REFERENCE SIGNS LIST 
     
         
         
           
               100 ,  100   a ,  100   b ,  100   c : Reflector antenna device,  110 ,  110   b : Primary radiator,  120 ,  120   a ,  120   b ,  120   c : First reflector,  121 : First region.  122 ,  122   b   1 ,  122   c : Second region,  122   b   2 : Third region,  123 ,  123   b   1 ,  123   b   2 : Recess,  124 ,  124   b   1 ,  124   b   2 : Opening,  125 ,  125   b   1 ,  125   b   2 : Bottom face,  126 : Conductor,  127 : Dielectric,  130 : Second reflector