Abstract:
It is an object of the invention to provide an antenna with small side-lobe while narrowing the beam width to a desired one without the antenna increased in weight. Besides, it is also an object of the invention to decrease the manufacturing process less than ever before and reduce the cost. Furthermore, it is also an object of the invention to improve stability when the antenna is turned. 
     In order to solve the above-mentioned problems, the antenna of the invention is characterized by comprising a radiator which is configured to radiate electromagnetic wave inside of an antenna housing, at least one dielectric which is configured to be contributory to directivity angle of the electromagnetic wave in the vertical direction, and characterized in that the dielectric is attached to the antenna housing ahead of the radiator.

Description:
[0001]    The antenna  30  is provided with slots  32  in front of the radiator  31 . It is possible to narrow the beam width in the horizontal direction by way of the slots  32 . Narrowing the beam width in the horizontal direction can improve the azimuth resolution of the antenna  30 . The dielectrics  34   a - c  are provided through the transition portion  33  ahead of the radiator  31 . The height of the transition portion  33  is practically equal to the height of the antenna housing  35   a - b  made up of the dielectric material, and the transition portion  33  electromagnetically-couples efficiently between the radiator  31  and the dielectrics  34   a - c.    
         [0002]    By providing the dielectrics  34   a - c  ahead of the radiator  31 , it is possible to keep the beam from spreading in the vertical direction and to keep the beam narrow in width. That is to say, it is possible to determine desired beam width by changing permittivity and length of the dielectrics  34   a - c.    
         [0003]    The dielectrics  34   a - c  are provided with their longitudinal sides disposed in the direction of the radial axis showing the center of radiation. The dielectrics  34   a - c  and the antenna housing  35   a - b  are provided in the form of a layer nearly perpendicular to the radial axis and uniformly spaced in the order of upper surface  35   a  of the antenna housing, a dielectric  34   a,  a dielectric  34   b,  a dielectric  34   c,  and lower surface  35   b  of the antenna housing. Furthermore, the dielectrics  34   a - c  are supported by the support member  36  disposed in the antenna housing  35   a - b.  In this manner, since the dielectrics  34   a - c  and the antenna housing  35   a - b  are provided in the form of a layer, interference occurs (that is, interference among the layers of the dielectrics  34   a - c ) which is attributable to interference between the upper surface  35   a  of the antenna housing and the lower surface  35   b  of the antenna housing, which causes the side-lobe. It is possible to reduce the side-lobe of the beam radiated outside of the antenna housing  35   a - b  due to interaction of each individual interference. 
         [0004]    In addition, although the support member  36  is made of the dielectric material similar to the dielectrics  34   a - c,  the support member does not contribute greatly to different permittivity, narrowing of the beam width, and reduction of the side-lobe. Therefore, the dielectrics and the support member shall be classified into different factors in view of object and effect. 
         [0005]    The antenna  30 , however, is heavier due to the presence of dielectrics  34   a - c  and the support member  36 . In general, with regard to the antenna capable of radiating desired beams, its weight is enumerated as one of the important subjects to be improved. The antenna is rotated by a motor for 360° degrees of detection around the antenna. Therefore, as antenna weight increases, the power of the motor must be increased and the strength of a base supporting the antenna must also be increased. 
         [0006]    For the antenna  30  described in Japanese Patent No. 3,634,372, the support member  36  is required to dispose and support the dielectrics  34   a - c.  In addition, it is necessary to form insertion slots for disposing the dielectrics  34   a - c  in the support member  36 , which complicates the manufacturing process of the antenna  30 . 
         [0007]    The present invention is devised to address the above-mentioned problems. It is an object of the invention to provide an antenna with small side-lobe while narrowing the beam width to a desired width without increasing antenna weight. It is also an object of the invention to reduce the manufacturing requirements and the cost. Furthermore, it is also an object of the invention to improve stability during antenna rotation. 
       SUMMARY OF THE INVENTION 
       [0008]    In order to solve the above-mentioned problems, the invention, in one aspect, is directed to an antenna characterized by comprising a radiator which is configured to radiate electromagnetic waves inside of an antenna housing, at least one dielectric which is configured to be contributory to directivity angle of the electromagnetic wave in the vertical direction, and characterized in that the dielectric is attached to the antenna housing ahead of the radiator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is an end elevation showing one example of the configuration of the antenna in one embodiment according to the invention. 
           [0010]      FIG. 2  is a view showing the result of simulation for the directivity characteristics of the antenna according to the embodiment of the invention shown in  FIG. 1 . 
           [0011]      FIG. 3  is a view showing the result of simulation when dielectric and antenna housing are not considered. 
           [0012]      FIG. 4  is an end elevation showing one example of the configuration of the antenna in another embodiment according to the invention. 
           [0013]      FIG. 5  is an end elevation showing one example of the configuration of the antenna in the embodiment shown in  FIG. 4 . 
           [0014]      FIG. 6  is an end elevation showing one example of the configuration of the antenna in the embodiment shown in  FIG. 4 . 
           [0015]      FIG. 7  is an end elevation showing the configuration of the antenna according to related art. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The antenna  10  in one embodiment of the invention is described below by referring to  FIG. 1  showing an end elevation of one example of the configuration of the antenna. 
         [0017]    The antenna  10  comprises a radiator  11 , slots  12 , a transition portion  13 , a plurality of dielectrics  14   a - d,  and antenna housing  15   a - b.    
         [0018]    The antenna  10  is provided with slots  12  in front of the radiator  11 . It is possible to narrow the beam width of the electromagnetic wave radiated from the radiator in the horizontal direction by way of the slots  12 . Narrowing the beam width in the horizontal direction can improve the azimuth resolution of the antenna  10 . The dielectrics  14   a - d  are provided through the transition portion  13  ahead of the radiator  11 . The height of the transition portion  13  is practically equal to the height of the antenna housing  15   a - b,  and the transition portion  13  electromagnetically-couples efficiently between the radiator  11  and the dielectrics  14   a - d.    
         [0019]    The dielectrics  14   a - d  are provided with longitudinal sides in parallel to the direction of the radial axis. Since the beam width in the vertical direction depends on the permittivity and length of the dielectrics  14   a - d,  it is possible to obtain desired beam width by changing these factors. The length of the dielectrics  14   b  and  14   c  are shorter than that of the dielectrics  14   a  and  14   d  as described below, with the end positions of the dielectrics  14   a - d  on the radiator side aligned to each other. Therefore, since the weight is more concentrated on the inner side than on the outer side of the antenna  10 , it is possible to impart more stability to the antenna  10  when rotated. 
         [0020]    The dielectrics  14   a - d  and the antenna housing  15   a - b  are provided in the form of a layer nearly perpendicular to the radial axis in the order of upper surface  15   a  of the antenna housing, the dielectric  14   a,  the dielectric  14   b,  the dielectric  14   c,  the dielectric  14   d,  and the lower surface  15   b  of the antenna housing. In addition, the outermost dielectrics  14   a  and  14   d  are in close contact with the antenna housing  15   a  and  15   b,  respectively. Furthermore, one side of the dielectric  14   b  comes into contact with the dielectric  14   a,  and one side of the dielectric  14   c  comes into contact with the dielectric  14   d.  In this manner, at least one side of the dielectrics  14   a  and  14   d  comes in contact with the antenna housing  15   a - b  and other dielectrics  14   b  and  14   c.  Therefore, when the dielectrics  14   a - d  are provided inside of the antenna housing  15   a - b,  it is possible to reduce the number of components without requiring a separate support member and so forth. Furthermore, the weight of the antenna can be reduced. 
         [0021]    The dielectrics  14   a - d  are provided symmetrically and perpendicularly to the radial axis. The permittivity and length of the dielectrics  14   a  and  14   d  and/or the dielectrics  14   b  and  14   c  are equally set, respectively. This allows the directivity characteristics of the beam to be symmetric with respect to the radial axis. However, since asymmetric configuration can also tilt and radiate the beam, it is not always necessary for the dielectrics  14   a - d  to be symmetric. 
         [0022]    In another embodiment, a foam dielectric can be used for the dielectrics  14   a - d.  The use of a foam dielectric enables the permittivity to be simply adjusted depending on the degree of foaming. In addition, the lower the permittivity, the larger the foaming area increases. Accordingly, it is also possible to lighten its weight. 
         [0023]    The permittivity and length of the dielectrics  14   a - d  can be determined using simulation so that a beam obtains the desired directivity characteristics. Each value of dielectrics  14   a - d  is described below in conjunction with the result of a simulation for the directivity characteristics of the antenna  10 . 
         [0024]      FIG. 2  shows the result of a simulation for the directivity characteristics of the antenna  10 . The results of  FIG. 2  can be compared to  FIG. 3  showing the results of a simulation wherein dielectrics  14   a - d  and antenna housing  15   a - b  are not considered. In  FIGS. 2 and 3 , the axis of abscissas indicates an angle, and an axis of ordinate indicates a gain. The dielectrics  14   a - d  are disposed at a distance of 20 mm from the radiator  11 . The dielectrics  14   a  and  14   d  are 1.5 in permittivity, 80 mm in length, and 7.0 mm in height. The dielectrics  14   b  and  14   c  are 1.3 in permittivity, 62 mm in length, and 7.0 mm in height. In addition, the antenna housing is 4.0 in permittivity, 32 mm in height, and 1.0 mm in thickness. These values are experimentally calculated so that adequate gain is obtained at 25° of directivity angle of a beam, provided that one wavelength is 32 mm. The antenna, however, is not restricted to these values. Directivity angle generally represents a range of angles in which a difference of gain from a point (with highest gain) in the direction of radial axis is within 3 dB. 
         [0025]    With regard to the result of the simulation shown in  FIG. 2 , point A (θ=90°) indicates the direction of radial axis, and points B (θ=78°) and C (θ=103°) indicate a maximum directivity angle of a beam. In addition, point D (θ=30°) indicates a point where the side-lobe occurs significantly. It is found from  FIG. 2  that a beam with desired directivity angle can be formed in a configuration like the antenna  10 . Furthermore, it is also found in reference to the side-lobe that a gain can be reduced up to a difference of about 15 dB or more as compared with that at point A (θ=90°) in the direction of radial axis of a beam. Accordingly, it is found that suitable beams for detecting targets at sea can be obtained by providing the antenna  10  with dielectrics  14   a - d  configured at the above-mentioned values. 
         [0026]    With regard to the result of a simulation as shown in  FIG. 3 , point A′ (θ=90°) indicates the direction of the radial axis of a beam, and points B′ (θ=65°) and C (θ=115°) indicate a maximum directivity angle of a beam. In addition, point D′ (θ=45°) indicates a point where the side-lobe occurs significantly. In  FIG. 3 , it is found that directivity angle of a beam is about 60° when the dielectrics  14   a - d  and the antenna housing  15   a - b  are not taken into account. That is to say, it is confirmed that the directivity angle of a beam is appropriately narrowed in the configuration of the antenna  10 . A difference of gain between the point D′ (θ=45°) where the side-lobe occurs significantly and the point A (θ=90°) in the direction of radial axis of a beam is about 7 dB. Namely, it is confirmed that, in reference to the side-lobe, the gain can also be reduced greatly in comparison with the gain in the radial axis. 
         [0027]    In the above-mentioned configuration, the antenna can radiate a beam with small side-lobe in a desired beam width. Since it is not necessary to support the dielectrics with a support member and so forth, the weight of the antenna can be reduced. Furthermore, since the weight is more concentrated on the inner side than on the outer side of the antenna, is possible to impart more stability to the antenna  10  when rotated. Furthermore, since a support member is not required to provide support to an insertion slot and the antenna, it is possible to reduce the manufacturing requirements and the cost. 
         [0028]    It is also possible to increase antenna  10  stability by filling in the space between the dielectrics  14   b  and  14   c.  Although there is a clearance between the dielectrics  14   b  and  14   c  of the antenna  10 , it is possible to obtain a desired beam by adjusting the permittivity and size of the dielectrics  14   a - d  without creating clearance. 
         [0029]    In another embodiment, the antenna  20  is described below by referring to  FIG. 4  showing an end elevation of one example of the configuration of the antenna  20 . 
         [0030]    The dielectric  24  of an antenna  20  is characterized as being a concave form. 
         [0031]    This dielectric  24  is in close contact with both upper and lower portions of the antenna housing  15   a - b  disposed ahead of a radiator  11 . Furthermore, the dielectric  24  can have a length extending in the direction of the radial axis which can become longer the further away it is from the radial axis. 
         [0032]    Furthermore, a foam dielectric can be used for the dielectric  24  wherein the foaming rate of the dielectric becomes low from the radial axis to the antenna housing in the vertical direction. Since the permittivity of the foam dielectric depends on its foaming rate, it is possible to increase the permittivity by decreasing the foaming rate. That is to say, the permittivity of the dielectric  24  increases from the radial axis to the antenna housing in the vertical direction. 
         [0033]    In another embodiment, the dielectric  24  can have its length changed radially as shown in  FIG. 5 . In another embodiment, the dielectrics  24  can be in the form of a layers as shown in  FIG. 6 . For example, the ends of the dielectrics  24   a - d  can be disposed at unequal distances from the radiator side of the radiator  11  to obtain desired properties of the antenna  20 . 
         [0034]    The invention is not restricted to the embodiments described above. For example, the invention is also applicable to a radome type antenna which rotates a radiator portion within a fixed antenna housing. Furthermore, the invention is not limited to use on ships to detect targets at sea, but may also be mounted on other vehicles such as aircraft and so forth for carrying out other types of detection. 
         [0035]    According to the invention, it is possible to manufacture a light weight antenna capable of radiating the beams with desired beam width and small side-lobe. Furthermore, it is also possible to decrease the manufacturing process and reduce the cost. Furthermore, it is also possible to improve stability when the antenna is rotated.