Abstract:
[OBJECT] It is directed to solving a problem that designing for a conventional waveguide slot antenna capable of radiating a circularly polarized wave involves complicated calculation, and a resulting circularly polarized wave antenna device can obtain a satisfactory axial ratio only in a narrow band. 
     [SOLUTION] The present invention provides a waveguide slot antenna which utilizes a waveguide as a feeding line and has a linear-shaped slot provided in a wall of the waveguide. The waveguide slot antenna is characterized in that it comprises a pair of polarized wave conversion members surrounding an outer periphery of the slot and divided by a slit intersecting the slot. 
     [EFFECT] The present invention can provide a waveguide slot antenna capable of radiating a circularly polarized wave with a satisfactory axial ratio characteristic, over a wide band, only by adding a simple component to a conventional waveguide slot antenna.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on Japanese Patent Application Serial No. 2011-287343 filed in Japan Patent Office on Dec. 28, 2011, the contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a waveguide slot antenna, and particularly to a structure of a waveguide slot antenna capable of generating a circularly polarized wave. 
         [0004]    2. Description of the Related Art 
         [0005]    A waveguide slot antenna utilizing a waveguide as a feeding line has been used as an antenna element usable in a microwave band and a millimeter-wave band, in a base station for wireless communication terminals. 
         [0006]    In use for wireless communication terminals, there are some situations in which, rather than a linearly polarized wave, a circularly polarized wave less susceptible to fading is desirable. Therefore, in connection with the need for a waveguide slot antenna adapted to radiate a circularly polarized wave, circularly polarized wave antenna devices using various waveguide slot antennas have been proposed. 
         [0007]    Basically, a waveguide slot antenna having a linear-shape slot radiates a linearly polarized wave therefrom. 
         [0008]    Thus, in a conventional waveguide slot antenna for radiating a circularly polarized wave, a linearly polarized wave is converted into a circularly polarized wave by combining a pair of linear slots to generate mutually orthogonal polarized waves, as disclosed in the following Non-Patent Documents 1 to 5. JP 2012-065229A discloses a waveguide slot antenna in which a linearly polarized wave is converted into a circularly polarized wave by coupling a parasitic element to a linear slot to generate orthogonal polarized wave components. 
         [0009]      FIG. 11  is a perspective view illustrating an example of a conventional waveguide slot antenna for radiating a circularly polarized wave. As illustrated in  FIG. 11 , one wall section of a hollow waveguide  89  has a cross slot  59  composed of a combination of two linear slots  79 ,  79  and provided offset from a center line CL thereof parallel to an axis of the hollow waveguide  89 . 
       LIST OF PRIOR ART DOCUMENTS 
     Patent Documents 
       [0010]    Patent Document 1: U.S. Pat. No. 6,028,562 A 
         [0011]    Patent Document 2: JP 2003-037432 A 
         [0012]    Patent Document 3: JP 2000-341030 A 
         [0013]    Patent Document 4: JP Application 2011-202765 
       Non-Patent Documents 
       [0014]    Non-Patent Document 1: A. J. Simmons, “Circularly polarized slot radiators,” IRE Trans. Antennas Propag., vol. 5, pp. 31-36, Jan. 1957. 
         [0015]    Non-Patent Document 2: W. J. Getsinger, “Elliptically polarized leaky-wave array,” IRE Trans. Antennas Propag., vol. 10, pp. 165-171, Mar. 1957. 
         [0016]    Non-Patent Document 3: T. Hirano, J. Hirokawa and M. Ando, “A design of a leaky waveguide crossed-slot linear array with a matching element by the method of moments with numerical-eigenmode basis functions,” IEICE Transactions on Communication, vol. E88-B, No. 3, pp. 1219-1226, Sep. 2004. 
         [0017]    Non-Patent Document 4: G. Montisci, M. Musa and G. Mazzarella “Waveguide slot antennas for circularly polarized radiated field”, IEEE Trans. Antennas Propag., vol. 52, pp. 619-623, 2004. 
         [0018]    Non-Patent Document 5: K. Min, J. Hirokawa, K. Sakurai, M. Ando, N. Goto and Y. Hara, “A Circularly Polarized Waveguide Narrow-wall Slot Array using a Single Layer Polarization Converter,” IEEE AP-S International Symposium 1996, pp. 1004-1007. 
       BRIEF SUMMARY OF THE INVENTION 
     Technical Problem 
       [0019]    Designing for each of the conventional waveguide slot antennas disclosed in the Non-Patent Documents 1 to 5 and the JP 2012-065229A involves complicated calculation, and a resulting circularly polarized wave antenna device can obtain a satisfactory axial ratio only in a narrow band. 
       Solution to the Technical Problem 
       [0020]    In order to solve the above problems, according to one aspect of the present invention, there is provided a waveguide slot antenna which utilizes a waveguide as a feeding line and has a linear slot provided in a wall of the waveguide. The waveguide slot antenna is characterized in that it comprises a pair of polarized wave conversion members surrounding an outer periphery of the slot and divided by a slit intersecting the slot. 
         [0021]    According to another aspect of the present invention, there is provided a waveguide slot antenna which utilizes a waveguide as a feeding line and has a linear slot provided in a wall of the waveguide. The waveguide slot antenna is characterized in that it comprises a flat-shaped conductor plate which has a first through-hole formed in a shape approximately identical to that of the slot and provided at a position opposed to the slot, and a second through-hole provided at a position intersecting the first through-hole. 
       Effect of the Invention 
       [0022]    The present invention can provide a waveguide slot antenna capable of radiating a circularly polarized wave with a satisfactory axial ratio characteristic, over a wide band, only by adding a simple component to a conventional waveguide slot antenna. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a perspective view of a waveguide slot antenna according to a first embodiment of the present invention. 
           [0024]      FIG. 2  is an exploded perspective view of the waveguide slot antenna in  FIG. 1 . 
           [0025]      FIGS. 3(   a ) and  3 ( b ) are explanatory diagrams illustrating details of the waveguide slot antenna in  FIG. 1 . 
           [0026]      FIGS. 4(   a ) and  4 ( b ) are perspective views of a waveguide slot antenna according to a second embodiment of the present invention. 
           [0027]      FIG. 5  is a perspective view of a waveguide slot antenna according to a third embodiment of the present invention. 
           [0028]      FIG. 6  is a perspective view of a waveguide slot antenna according to a fourth embodiment of the present invention. 
           [0029]      FIG. 7  is a graph illustrating respective gains in the first embodiment and the fourth embodiment. 
           [0030]      FIG. 8  is an exploded perspective view of a waveguide slot antenna according to a fifth embodiment of the present invention, wherein a technique of the fourth embodiment is applied to a dielectric waveguide. 
           [0031]      FIG. 9  is a graph illustrating a return loss in the dielectric waveguide slot antenna in  FIG. 8 . 
           [0032]      FIG. 10  is a graph illustrating an axial ratio characteristic of the dielectric waveguide slot antenna in  FIG. 8 . 
           [0033]      FIG. 11  is a perspective view of a conventional waveguide slot antenna. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
       [0034]    The present invention will now be described based on a first embodiment thereof with reference to  FIGS. 1 to 3(   b ). 
         [0035]    As illustrated in  FIGS. 1 to 2 , a waveguide slot antenna  11  according to the first embodiment of the present invention comprises: a waveguide member  81  composed of a cross-sectionally angular U-shaped conductor having an opening on one side thereof; a flat-shaped slot plate  61  composed of a conductor having a linear slot  71  and disposed to cover the opening while allowing the slot  71  to be located inside the opening, so as to form a waveguide; and a flat-shaped polarized wave conversion/radiation plate  21  disposed in superimposed relation to the slot plate  61 , wherein the polarized wave conversion/radiation plate  21  is composed of a conductor having a cross slot  51  which consists of a first through-hole  31  formed in a shape approximately identical to that of the slot  71  and provided at a position opposed to the slot  71 , and a second through-hole  41  formed in a linear shape and provided at a position intersecting the first through-hole  31 . 
         [0036]      FIGS. 3(   a ) and  3 ( b ) are plan views for explaining the polarized wave conversion/radiation plate  21  and the slot plate  61 , wherein  FIG. 3(   a ) is a plan view of the polarized wave conversion/radiation plate  21 , and  FIG. 3(   b ) is a plan view of the slot plate  61 . 
         [0037]    As illustrated in  FIG. 3(   a ), in the polarized wave conversion/radiation plate  21 , the first through-hole  31  is provided to have a longitudinal length L 1 , and the second through-hole  41  is provided to have a longitudinal length L 2  and intersect the first through-hole  31  at a predetermined intersection angle θ. As illustrated in  FIG. 3(   b ), in the slot plate  61 , the slot  71  is provided to have the longitudinal length L 1 . 
         [0038]    The slot  71  has a shape approximately identical to that the first through-hole  31 , and each of the slot  71  and the first through-hole  31  is formed at a position rotated by θ 0  with respect to a plane perpendicular to an axis of the waveguide. 
         [0039]    The longitudinal length L 2  of the second through-hole  41  is greater than the longitudinal length L 1  of the first through-hole  31 . 
         [0040]    The intersection angle θ between the first through-hole  31  and the second through-hole  41  is set in the following range: −90 degrees&lt;θ&lt;90 degrees (where θ≠0). 
         [0041]    The polarized wave conversion/radiation plate  21  has a thickness t which is 0.2 to 0.3 times a length of wavelength in the waveguide. 
         [0042]    Through the cross slot  51  consisting of the mutually intersecting first and second through-holes  31 ,  41 , an electric field B orthogonal to an electric field A from the slot  71  is generated. When the electric field A and the electric field B are orthogonal to each other, and a phase difference therebetween is 90 degrees, a synthetic wave of the electric field A and the electric field B will become a circularly polarized wave. 
         [0043]    A rotation direction of the circularly polarized wave is determined by the intersecting angle θ. 
         [0044]    When the second through-hole  41  is disposed at a position rotated in a counterclockwise direction (θ&gt;0) with respect to the first through-hole  31 , the antenna will generates a right-handed circularly polarized wave. 
         [0045]    The intersection angle θ is selected to provide a satisfactory axial ratio characteristic. For example, it may be set in the following range: 30 degrees≦θ&lt;90 degrees. 
         [0046]    The angle θ 0  of the slot  71  with respect to the plane perpendicular to the axis of the waveguide is an arbitrary value, and the slot  71  may be disposed at an arbitrary angle so as to facilitate impedance matching. 
       Second Embodiment 
       [0047]    Due to the polarized wave conversion/radiation plate having a relatively large thickness, the waveguide slot antenna for a circularly polarized wave (circularly polarized waveguide slot antenna) illustrated in  FIG. 1  significantly increases in weight, as compared to a waveguide slot antenna for a linearly polarized wave (linearly polarized waveguide slot antenna). 
         [0048]    As a result of experimental tests, the inventors have found that, even in a structure where a large portion of the polarized wave conversion/radiation plate is removed while leaving only a portion around the slot, so as to facilitate a reduction in weight, the resulting waveguide slot antenna can operate as an antenna capable of radiating a circularly polarized wave with a satisfactory axial ratio characteristic over a wide band. 
         [0049]      FIGS. 4(   a ) and  4 ( b ) are explanatory diagrams of a waveguide slot antenna according to a second embodiment of the present invention, wherein  FIG. 4(   a ) is a perspective view for explaining the second embodiment, compared to the first embodiment, and  FIG. 4(   b ) is a perspective view for explaining the waveguide slot antenna according to the second embodiment in detail. 
         [0050]    As illustrated in  FIG. 4(   a ), when the polarized wave conversion/radiation plate of the waveguide slot antenna  11  illustrated in  FIG. 1  is mostly cut away while leaving only a region surrounded by the dotted line in  FIG. 1 , a waveguide slot antenna  12  according to the second embodiment of the present invention is obtained as illustrated in  FIG. 4(   b ). 
         [0051]    As illustrated in  FIG. 4(   b ), the waveguide slot antenna  12  according to the second embodiment comprises: a waveguide member  82  composed of a cross-sectionally angular U-shaped conductor having an opening on one side thereof; a flat-shaped slot plate  62  composed of a conductor having a linear slot  72  and disposed to cover the opening while allowing the slot  72  to be located inside the opening, so as to form a waveguide; and a pair of polarized wave conversion members  22 ,  22  surrounding an outer periphery of the slot  72  and divided by a linear slit  42  intersecting the slot  72 . The polarized wave conversion members  22 ,  22  are arranged point-symmetrically with respect to a center of the slot  72 . 
         [0052]    Preferably, each of the polarized wave conversion members  22 ,  22  has a height dimension h which is 0.2 to 0.3 times a length of a wavelength in the waveguide, and the polarized wave conversion members  22 ,  22  are preferably arranged within one-half of the wavelength, with respect to the center of the slot. 
         [0053]    In the waveguide slot antenna  12 , a combination of the slot  72  and the slit  42  can be considered as a pseudo cross slot. Thus, the waveguide slot antenna  12  can radiate a circularly polarized wave, as with the waveguide slot antenna  11  according to the first embodiment. 
       Third to Fourth Embodiments 
       [0054]      FIG. 5  is a perspective view of a waveguide slot antenna  13  according to a third embodiment of the present invention. As illustrated in  FIG. 5 , the waveguide slot antenna  13  according to the third embodiment has the same structure as the waveguide slot antenna  12  according to the second embodiment, except that a slit  43  has a non-linear shape. Specifically, the slit  43  is radially expanded in a direction away from a center of the linear slot  72  so as to facilitate impedance matching. The change in shape of the slit makes it possible to reduce a return loss. 
         [0055]      FIG. 6  is a perspective view of a waveguide slot antenna  14  according to a fourth embodiment of the present invention. As illustrated in  FIG. 6 , the waveguide slot antenna  14  according to the fourth embodiment has the same structure as the waveguide slot antenna  12  according to the second embodiment, except that corners of each of a pair of polarized wave conversion members  24 ,  24  are chamfered so as to facilitate impedance matching. The chamfering of corners of each of the polarized wave conversion members  24 ,  24  makes it possible to reduce a return loss. 
         [0056]      FIG. 7  is a graph illustrating a comparison between respective gains of the waveguide slot antennas according to the first and fourth embodiments, calculated using an electromagnetic field simulator. 
         [0057]    In  FIG. 7 , the vertical axis represents gain [dBi], and the horizontal axis represents frequency [GHz], wherein the dotted line indicates a gain of the waveguide slot antenna according to the first embodiment, and the solid line indicates a gain of the waveguide slot antenna according to the fourth embodiment. In this test, each of the waveguide slot antennas according to the first and fourth embodiments was formed as a 60 GHz band single element antenna. 
         [0058]    As seen in  FIG. 7 , at a frequency of 60 GHz, the waveguide slot antenna according to the first embodiment had a gain of 6.1 dBi, whereas the waveguide slot antenna according to the fourth embodiment had a gain of 9.4 dBi. This shows that the structure of the waveguide slot antenna according to the fourth embodiment has an advantage of being able to not only reduce an antenna weight but also provide a significantly enhanced gain. 
       Fifth Embodiment 
       [0059]    The waveguide slot antenna of the present invention can be applied to not only a hollow waveguide but also a dielectric waveguide. 
         [0060]      FIG. 8  is a perspective view illustrating a waveguide slot antenna  15  according to fifth embodiment of the present invention. 
         [0061]    As illustrated in  FIG. 8 , the waveguide slot antenna  15  comprises: a dielectric waveguide member  85  comprised of a rectangular parallelepiped-shaped dielectric body and a conductor film covering a surface of the dielectric body and having a linear dielectric body-exposing area  95  provided in a part of a top region thereof to allow the dielectric body to be exposed therethrough; a flat-shaped slot plate  65  composed of a conductor having a linear slot  75  with a shape approximately identical to that of the dielectric body-exposing area  95  and disposed to allow the slot  75  to be located in opposed relation to the dielectric body-exposing area  95 ; and a pair of polarized wave conversion members  25 ,  25  surrounding an outer periphery of the slot  75  and divided by a slit  45  intersecting the slot  75 . 
         [0062]      FIG. 9  is a graph illustrating a return loss characteristic of the waveguide slot antennas according to the fifth embodiment, calculated using an electromagnetic field simulator. In  FIG. 9 , the vertical axis represents return loss [dB], and the horizontal axis represents frequency [GHz]. 
         [0063]    As seen in  FIG. 9 , in a frequency range of 55 GHz to 70 GHz, a fractional bandwidth having a return loss of 20 dB or more is about 18%. 
         [0064]      FIG. 10  is a graph illustrating an axial ratio characteristic of the dielectric waveguide slot antenna according to the fifth embodiment. In  FIG. 10 , the vertical axis represents axial ratio [dB], and the horizontal axis represents frequency [GHz]. 
         [0065]    As seen in  FIG. 10 , in a frequency range of 55 GHz to 70 GHz, a fractional bandwidth having an axial ratio characteristic of 2 dB or less is about 17%. 
         [0066]    As seen in the results in  FIGS. 9 and 10 , in a frequency range having a return loss of 20 dB or less, the axial ratio is 2 dB or less, which shows that a significantly wide band characteristic is obtained. 
         [0067]    As described in the above embodiments, a waveguide slot antenna capable of radiating a circularly polarized wave can be obtained, simply by: adding, to a waveguide slot antenna comprising a waveguide and a linear-shaped slot provided in a wall of the waveguide, a polarized wave conversion/radiation plate provided with a cross slot consisting of mutually intersecting first and second through-holes; or providing, to a waveguide slot antenna comprising a waveguide and a linear-shaped slot provided in a wall of the waveguide, a pair of polarized wave conversion members around an outer periphery of the slot. 
         [0068]    The waveguide slot antenna of the present invention can be applied to not only a single element antenna but also an array antenna. 
       EXPLANATION OF CODES 
       [0000]    
       
           11 ,  12 ,  13 ,  14 ,  15 ,  19 : waveguide slot antenna 
           21 : polarized wave conversion/radiation plate 
           22 ,  23 ,  24 ,  25 : polarized wave conversion member 
           31 ,  41 ,  71 ,  75 : through-hole 
           42 ,  43 ,  44 ,  45 : slit 
           51 ,  59 : cross slot 
           61 ,  62 ,  63 ,  64 ,  65 : slot plate 
           71 ,  75 ,  79 : slot 
           81 ,  82 ,  83 ,  84 ,  89 : hollow waveguide 
           85 : dielectric waveguide 
           95 : dielectric body-exposing area