Patent Publication Number: US-6340953-B1

Title: Antenna device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an antenna device, and more specifically, to an antenna device used for a base station in a radio communication system with a submillimeter wave band or a millimeter wave band. 
     2. Description of the Related Art 
     Installation of optical fibers is being tried in order to achieve effective and high-speed transmission of a large amount of data in the current information-oriented society; however, high installment cost results in boosting the ratio of the total communication fees of ordinary households to access line fees (fixed fees portion). 
     Under these circumstances, establishing a radio communication system with a millimeter wave band (local radio network WLL) is now drawing attentions. This system makes it unnecessary to install lines up to users&#39; houses and further provides a local area with services unique to the area at a low cost. 
     The important questions in establishing a radio communication system are how many terminal users each base station can accommodate and how fast the system can transmit large amounts of information accurately. As one solution of the questions, developing an antenna device is an urgent necessity. 
     As a base station antenna in the radio communication system with the millimeter wave band, using an antenna device with a horn antenna has been considered. The antenna device must be provided with wide radiation characteristics in a horizontal plane in order to cover a large service area. 
     FIG. 19 show s a prior art antenna device employing a horn antenna. The horn antenna  102  has the geometry of a pyramid in which the width A 1  of an antenna opening  102   a  is larger than the width of the opening portion  101   a  of a waveguide  101 . 
     However, the horn antenna often employed in a radio communication with a millimeter wave band is generally used as a high-gain antenna with a narrowed beam, so that the half-power beamwidth is generally narrow. 
     SUMMARY OF THE INVENTION 
     In view of the aforementioned problem, the main object of the present t invention is to secure wide radiation characteristics In a horizontal plane, in spite of the antenna device having a simple structure. 
     In order to achieve the object, the antenna device of the present invention comprises a plurality of directional antennas which are aligned on the same axis in one direction and which are also arranged in a manner that the main radiation directions of the plurality of directional antennas deviate from each other in a direction orthogonal to the direction. In this structure the radio waves from the directional antennas are radiated at least with the point coinciding with the same axis in the one direction as its center, so that the occurrence of a drop in directivity due to the array factor is prevented. Since the directivity in the antenna device is the combination of the directivities of all the directional antennas, the main radiation directions of the directional antennas can be deviated from each other in the one direction to expand the radiation characteristics in the plane orthogonal to the one direction. 
     The above-mentioned one direction is preferably a vertical direction, which could expand the radiation characteristics in the horizontal plane. 
     The antenna device preferably comprises a distribution circuit for supplying the plurality of directional antennas with electric powers. 
     In this case, various directivities can be obtained according to the following variations. 
     The distribution circuit supplies the plurality of directional antennas with electric powers of the same amplitude and the same phase. 
     The distribution circuit supplies the plurality of directional antennas with electric powers of the same amplitude and different phases. 
     The distribution circuit supplies the plurality of directional antennas with electric powers of the same phase and different amplitudes. 
     The distribution circuit supplies the plurality of directional antennas with electric powers of different amplitudes and different phases. 
     It is preferred that the antenna device further comprises a power supply part waveguide, and that the plurality of directional antennas are horn antennas connected to the power supply part waveguide. In this case, the half-power beamwidth can be further increased by making at least one of the plurality of directional antennas have an antenna opening shorter in width than the opening portion of the power supply part waveguide. 
     It is preferred that the antenna device further comprises a fixing plate which is arranged in a direction orthogonal to the one direction, wherein the plurality of directional antennas are provided with respective placing surfaces which are arranged parallel to the direction orthogonal to the one direction and which are in contact with the front and back sides of the fixing plate so as to fix the antenna device thereonto. In this case, the angles of the directional antennas made of horn antennas can be freely set by providing an installment angle setting unit for freely adjusting the main radiation directions of the plurality of directional antennas fixed on the fixing plate. 
     In an antenna device comprising a horn antenna connected to a power supply part waveguide, the half-power beamwidth can be increased by making the antenna opening of the horn antenna shorter in width than the opening portion of the power supply part waveguide. 
     In this case, the side surfaces of the horn antenna are tapered from the opening portion of the power supply part waveguide to the antenna opening. This makes the antenna opening of the horn antenna shorter in width than the opening portion of the power supply part waveguide, without causing a large drop in gain or increasing the internal volume of the antenna. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The other objects of the invention will be clarified by the following description and specified in the attached claims. Executing the present invention will remind those skilled in the art of various advantages not mentioned in the present specification. 
     FIG. 1 is a side view showing the structure of an antenna device in accordance with a first preferred embodiment of the present invention. 
     FIG. 2 is a plan view showing the structure of the antenna device in accordance with the first preferred embodiment. 
     FIG. 3 shows the horizontal directivity of the antenna device in accordance with the first preferred embodiment. 
     FIG. 4 shows the horizontal directivity of a modified antenna device in accordance with the first preferred embodiment. 
     FIG. 5 is a plan view showing the structure of another modified antenna device in accordance with the first preferred embodiment. 
     FIG. 6 is a plan view showing the structure of an antenna device in accordance with a second preferred embodiment. 
     FIG. 7 is a perspective view showing the structure of the waveguide in the second preferred embodiment. 
     FIG. 8 is a plan view showing the structure of a modified antenna device in accordance with the second preferred embodiment. 
     FIG. 9 is a perspective view showing the structure of a modified waveguide in the second preferred embodiment. 
     FIG. 10 is a perspective view showing the structure of another modified waveguide in the second preferred embodiment. 
     FIG. 11 is a side view showing the structure of an antenna device in accordance with a third preferred embodiment of the present invention. 
     FIG. 12 is a plan view showing the structure of the antenna device in accordance with the third preferred embodiment. 
     FIG. 13 is a side view showing the structure of a modified antenna device in accordance with the third preferred embodiment. 
     FIG. 14 shows the horizontal directivity of the antenna device in accordance with the third preferred embodiment. 
     FIG. 15 is a plan view showing the structure of another modified antenna device in accordance with the third preferred embodiment. 
     FIG. 16 is a perspective view showing the structure of an antenna device in accordance with a fourth preferred embodiment. 
     FIG. 17A is a plan view showing the structure of the antenna device in accordance with the fourth preferred embodiment. 
     FIG. 17B is a side view showing the structure of the antenna device in accordance with the fourth preferred embodiment. 
     FIG. 18 is a graph showing the measurement results of the half-power beamwidth of the antenna device in accordance with the fourth preferred embodiment. 
     FIG. 19 is a perspective view showing the structure of a prior art antenna device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred embodiments of the present invention will be described as follows with reference to the drawings. 
     (First Preferred Embodiment) 
     As shown in FIGS. 1 and 2 an antenna device of the present embodiment comprises a first horn antenna  1   a  and a second horn antenna  1   b  each having a directivity in the horizontal direction, and a distribution circuit  2  which supplies an electric power to each of the antennas  1   a  and  1   b.    
     In the present embodiment, the vertical direction is selected as a reference direction for the antenna device, and the horizontal direction is selected as the direction orthogonal to the reference direction; however, it goes without saying that any other direction could be selected as the reference direction. 
     The horn antennas  1   a  and  1   b  have their respective radiation centers aligned on the same axis P in the vertical direction, and are also arranged vertically in a manner that their respective main radiation directions X and Y deviate from each other in the horizontal direction, that is, the directions X and Y cross each other at the same axis P. 
     Although the radiation centers of the horn antennas  1   a  and  1   b  are illustrated on the antenna openings as shown in FIGS.  1  and  2 , there are cases where the centers are inside the openings, so that in reality the horn antennas  1   a  and  1   b  are superimposed each other after their radiation centers are found through analysis or experiments. 
     The horn antennas  1   a  and  1   b  are connected, via power supply paths  3   a  and  3   b , respectively, to the distribution circuit  2  which has an input/output terminal  4  connected to a radio (not shown). Through the input/output terminal  4 , the electric power is entered into the distribution circuit  2  which distributes it to the hone antennas  1   a  and  1   b  via the power supply paths  3   a  and  3   b , respectively. 
     The horn antennas  1   a  and  1   b  are thus arranged in the present embodiment because of the following reason. In the prior art structure, a plurality of horn antennas are too large in size to be arranged on the same horizontal plane, making it impossible to align their radiation centers on the same axis P. This forces one of these horn antennas to be arranged out of the axis P, which inevitably causes a drop in directivity due to the array factor. 
     In contrast, in the present embodiment, the horn antennas  1   a  and  1   b  are deviated from each other in the vertical direction while being aligned on the same axis P. Furthermore, the setting angle of each of the antennas  1   a  and  1   b  in the horizontal direction is determined so that the main radiation directions X and Y can turn laterally. 
     In this structure of the present embodiment, the radio waves in the horizontal plane coming from the horn antennas  1   a  and  1   b  are radiated with the point coinciding with the same axis P in the vertical direction as the radiation center, which effectively holds down dropping directivity due to the array factor. In addition, the directivities of the horn antennas  1   a  and  1   b  whose main radiation directions X and Y deviate from each other in the horizontal direction are composed. This expands the radiation of the entire antenna device in the horizontal plane. 
     When it is necessary to keep the radiation characteristics in the horizontal plane within a range of 120 degrees in the antenna device of the present embodiment, it can be done as follows. The horn antennas  1   a  and  1   b  having a half-power beamwidth of 60 degrees in the horizontal direction are prepared. These antennas  1   a  and  1   b  are aligned on the same axis P in the vertical direction in such a manner that their radiation centers are arranged on the same axis P. Furthermore, the setting angle of these antennas  1   a  and  1   b  in the horizontal direction are determined so as to deviate their radiation directions X and Y from each other by 60 degrees in the horizontal direction. 
     With the horn antennas  1   a  and  1   b  arranged as described above, the distribution circuit  2  will be structured as follows. A one-to-one Wilkinson power distribution circuit may be used as the distribution circuit  2 , which supplies the horn antennas  1   a  and  1   b  with electric powers of the same amplitude and phase. The power supply paths  3   a  and  3   b  are made of coaxial cables of the same length. Thus the horn antennas  1   a  and  1   b  are supplied with powers of the same amplitude and phase via a waveguide coaxial converter (not shown). 
     The antenna device with the above-mentioned structure forms a composed directivity  5 A in the horizontal direction as shown in FIG.  3 . The composed directivity  5 A is formed by composing the respective horizontal directivities  6 A and  7 A of the horn antennas  1   a  and  1   b  whose main radiation directions X and Y deviate from each other in the horizontal direction. The composed directivity  5 A offers radiation characteristics which can cover a 120-degree range in the horizontal plane. The symbol P in FIG. 3 indicates a single axis, which coincides with the radiation centers of the overlapped horn antennas la and  1   b.    
     In this case, supplying the horn antennas  1   a  and  1   b  with powers of the same phase achieves strong radiation characteristics in the horizontal direction. Alternatively, the horn antennas  1   a  and  1   b  can be supplied with powers which only differ in phase by making the power supply paths  3   a  and  3   b  differ in length. When the horn antennas  1   a  and  1   b  are supplied with powers of the same amplitude and different phases, radiation characteristics become wider in the vertical direction than in the horizontal direction. 
     It is another alternation to use as the distribution circuit  2  a one-to-n (n≠1) Wilkinson power distribution circuit which supplies the horn antennas  1   a  and  1   b  with electric powers of the same phase and different amplitudes, in place of the one-to-one Wilkinson power distribution circuit. Using this distribution circuit  2  and making the power supply paths  3   a  and  3   b  equal in length makes it possible to supply the horn antennas  1   a  and  1   b  with powers of different amplitudes, thereby obtaining a composed directivity  5 B in the horizontal direction as shown in FIG.  4 . 
     The composed directivity  5 B shown in FIG. 4 is a combination of the directivity  6 B having weaker radiation in the main radiation direction X of the horn antenna  1   a  supplied with a power of a smaller amplitude and the directivity  7 B having stronger radiation in the main radiation direction Y of the horn antenna  1   b  supplied with a power of a larger amplitude. Thus, changing the amplitudes of supplying powers allows the composed directivity  5 B to be formed in a desired shape. 
     It is also possible that the horn antennas  1   a  and  1   b  are supplied with powers of different phases by making the power supply paths  3   a  and  3   b  differ in length. When the supplying powers differ both in phase and amplitude, radiation characteristics become wider in the vertical direction than in the horizontal direction. 
     It is possible to structure an antenna device by combining horn antennas  1   a ′ and  1   b ′ whose antenna openings are different in size as shown in FIG. 5, which has an effect of setting the directivity as desired. To be more specific, increasing the antenna opening in width sharpens the horizontal directivity, thereby narrowing the half-power beamwidth. On the other hand, decreasing the antenna opening in width makes the horizontal directivity dull, thereby widening the half-power beamwidth. This feature can be used to combine the horn antennas  1   a ′ and  1   b ′ having antenna openings different in size, so as to allow horizontal radiation characteristics to be formed in a desired shape. 
     (Second Preferred Embodiment) 
     While the antenna device of the first preferred embodiment uses the distribution circuit and power supply paths to connect the horn antennas and the radio, the antenna device of the present embodiment employs waveguides in place of the distribution circuit and power supply paths. The waveguides must be able to supply a power to each horn antenna with a minor power loss. 
     As shown in FIG. 6, the antenna device of the present embodiment comprises a first horn antenna  1   a  and a second horn antenna  1   b  each having a directivity in the horizontal direction and a waveguide  11 A which supplies an electric power to each of the antennas  1   a  and  1   b . The arrangement relation between the horn antennas  1   a  and  1   b , the single axis P, and the main radiation directions X and Y of the horn antennas  1   a  and  1   b , respectively, is not described here because it is equal to that of the first preferred embodiment. 
     The waveguide  11 A consists of a first and second waveguide paths  11   a  and  11   b , which are connected to the horn antennas  1   a  and  1   b , respectively, and further connected to a waveguide flange  12  via an E-plane T-branch waveguide  13 . The waveguide flange  12  is an input/output terminal provided between the E-plane T-branch waveguide  13  and a radio (not shown). 
     FIG. 7 illustrates waveguide flanges  14  connected to the horn antennas  1   a  and  1   b , respectively, E-bend waveguides E  15 , and H-bend waveguides  16  having respective curves corresponding to the curves of the horn antennas  1   a  and  1   b.    
     In the present embodiment, the horn antennas  1   a  and  1   b  are supplied with powers of the same amplitude via the waveguide  11 A, and the phases of the powers are adjusted by making the waveguide paths  11   a  and  11   b  differ in length. The lengths correspond to the lengths extending between the E-plane T-branch waveguide  13  and each of the waveguide flanges  14 . 
     This structure allows the horn antennas  1   a  and  1   b  to be supplied with the powers of the same phase, and an antenna device having such horn antennas to form the same composed directivity  5 A as shown in FIG.  3 . Moreover, the powers to be supplied can be adjusted to have different phases from each other, and an antenna device having the adjusted power phases can obtain wider radiation characteristics in the vertical direction than in the horizontal direction. 
     The structure of the antenna device of the present embodiment is not restricted to the one shown in FIGS. 6 and 7, and can be the one shown in FIGS. 8 and 9. The antenna device shown in FIGS. 8 and 9 is characterized by the structure of a waveguide  11 B. To be more specific, a waveguide flange  12 ′, which is a component of the waveguide  11 B differs in structure from the one shown in FIGS. 6 and 7, and also slightly differs from the one shown in FIGS. 6 and 7 in the attached positions of the E-plane T-branch waveguide  13 , E-bend waveguides  15 , H-bend waveguides  16 , and other components. 
     The waveguide flange  12  in the structure shown in FIGS. 6 and 7 is provided along the plane approximately parallel to the direction toward which the horn antennas  1   a  and  1   b  radiate radio waves. In contrast, the waveguide flange  12 ′ in the structure shown in FIGS. 8 and 9 is provided along the plane approximately diagonal to the direction toward which the horn antennas  1   a  and  1   b  radiate radio waves. However, the antenna device shown in FIGS. 8 and 9 does not differ in basic structure from the one shown in FIGS. 6 and 7, so that like components in FIGS. 8 and 9 are labeled with like reference numerals with respect to FIGS. 6 and 7, and the description of these components is not repeated. 
     The waveguides  11 A and  11 B could be replaced by a waveguide  11 C shown in FIG.  10 . The waveguide  11 C has a bond hole  17  for varying the amplitude of an electric power supplied, depending on the size of the antenna opening area. An antenna device provided with the waveguide  11 C has the same effects as the one shown in FIGS. 6 and 7. To be more specific, with the waveguide  11 C, the horn antennas  1   a  and  1   b  can be supplied with electric powers of the same phase and different amplitudes or with electric powers which differ both in phase and amplitude. Consequently, the directivity in the horizontal direction can be formed in a desired shape. 
     The two horn antennas used as directional antennas in the antenna devices of the first and second preferred embodiments could be replaced by patch antennas, or by three or more directional antennas. 
     (Third Preferred Embodiment) 
     The antenna device of the present embodiment comprises a first horn antenna  23  and a second horn antenna  26  as shown in FIG.  11 . The horn antennas  23  and  26  are arranged in the vertical direction and connected to waveguides  21  and  24 , respectively. 
     The horn antennas  23  and  26  are structured as follows. A cabinet  22  including the horn antenna  23  has a bottom surface  22   a  parallel to a line L 1  which links the center  21   a  of the opening portion of the waveguide  21  and the center  23   b  of the antenna opening  23   a  of the horn antenna  23 . Similarly, a cabinet  25  including the horn antenna  26  has a top surface  25   a  parallel to a line L 2  which links the center  24   a  of the opening portion of the waveguide  24  and the center  26   b  of the antenna opening  26   a  of the horn antenna  26 . These horn antennas  23  and  26  are arranged vertically in a manner as to position the antenna  23  above the antenna  26 , and a fixing plate  27  is disposed between these antennas. Then, the bottom surface  22   a  and top surface  25   a  are touched with the front and back sides of the fixing plate  27  so as to fix the horn antennas  23  and  26  on the fixing plate  27 . In the present embodiment, the bottom surface  22   a  and top surface  25   a  serve as bases to install the antennas  23  and  26  onto the fixing plate  27 . 
     Hence, the main radiation directions X and Y in the horizontal planes of the horn antennas  23  and  26  are horizontal and parallel to each other. The waveguides  21  and  24  of the horn antennas  23  and  26 , respectively, are connected to a distribution circuit  31  via power supply paths  30   a  and  30   b , respectively. The symbol P in FIG. 11 indicates a single axis which coincides with the radiation centers of the horn antennas  23  and  26 . 
     Attachment of the horn antennas  23  and  26  to the fixing plate  27  will be described as follows with reference to FIG.  12 . The fixing plate  27  is provided with a guide groove  28 , which is included in an attachment angle setting unit. The guide groove  28  is arc-shaped, and the horn antennas  23  and  26  are fixed to the groove  28  by a screw  29 . Loosing the screw  29  makes it possible to turn the horn antennas  23  and  26  as desired under the conditions that the same axis P, which is their radiation centers, is used as the turning axis. As a result, the radiation angle formed by the main radiation directions X and Y of the horn antenna  23  and  26 , respectively, can be set arbitrary while the axis P, which is the center of radiation, is fixed. 
     The specific attachment of the horn antennas  23  and  26  to the guide groove  28  may be conducted as follows. The screw  29  is composed of a male and female threads, and the male thread is so attached as to prevent the horn antennas  23  and  26  from turning. In addition, a square prism portion is formed at some midpoint in the longitudinal direction of the male thread so as to have the approximate same size as the width of the guide groove  28 . On the other hand, the curvature of the arc-shaped guide groove  28  is set so that the center of the arc coincides with the axis P, which is the centers of the radiations of the horn antennas  23  and  26 . Then, the square prism portion is inserted to the guide groove  28 , and the horn antennas  23  and  26  are arranged in the direction of the diameter of the arc formed by the guide groove  28 . Under these conditions, the female thread is screwed against the tip of the male thread. Thus, the horn antennas  23  and  26  sandwich the fixing plate  27  so as to be fixed to the fixing plate  27  by the female thread. Since the square prism portion is in contact with the guide groove  28 , the male thread does not turn, which further prevents the horn antennas  23  and  26  from turning. This structure allows the radiation angle formed by the main radiation directions X and Y of the antennas  23  and  26  to be set freely within the range between both ends of the guide groove  28  under the conditions that the axis P as the centers of the radiations is firmly fixed. 
     The fixing plate  27  is preferably made of an insulating material of an organic system such as acrylic or polypropylene, or an inorganic system in order to obtain better radiation characteristics. 
     FIG. 13 shows another example of attaching horn antennas. The horn antennas  33  and  36  in this example consist of a pair of antennas formed by dividing one horn antenna vertically along the line linking the center of the opening portion of a waveguide and the center of the antenna opening of the horn antenna. The horn antennas  33  and  36  are connected to a first and second waveguides  31  and  34 , respectively. The bottom surface of the horn antenna  33  and the top surface of the horn antenna  36  sandwich the fixing plate  37 . Since these antennas  33  and  36  are installed to the fixing plate  37  in the same manner as in FIGS. 11 and 12, a detailed description will be omitted. 
     In this modified example, the radiation directions of the horn antennas  33  and  36  slightly expand in the vertical direction, failing to be parallel; however, the radiation angle formed by the main radiation directions of the antennas  33  and  36  can be set arbitrary under the conditions that the axis P as the radiation centers is fixed. 
     FIG. 14 shows the directional characteristics formed by the antenna device of the present embodiment. The horn antenna  23  ( 33 ) has the main radiation direction X and the directivity  6 C, and the horn antenna  26  ( 36 ) has the main radiation direction Y and the directivity  7 C. The horn antennas  23  ( 33 ) and  26  ( 36 ) are arranged so as to make the main radiation directions X and Y apart from each other laterally at an angle of θ. With this arrangement of the antennas  23  ( 33 ) and  26  ( 36 ), the directivity  5 C having a large half-power beamwidth can be obtained by composing the directivities  6 C and  7 C. 
     In the present embodiment, the opening portions of the power supply part waveguides  21  ( 31 ) and  24  ( 34 ) have the same width as the antenna openings  23   a  ( 33   a ) and  26   a  ( 36   a ). Alternatively, at least one of the antenna openings  23   a  ( 33   a ) and  26   a  ( 36   a ) can be made shorter in width than the opening portions of the power supply part waveguides  21  ( 31 ) and  24  ( 34 ) so as to have a larger half-power beamwidth of the horizontal plane. FIG. 15 shows an example where the antenna openings  23   a  and  26   a  of the antennas  23 ′ and  26 ′ are shorter in width than the opening portions of the power supply part waveguides  21  and  24 . 
     As described hereinbefore, the present embodiment offers an antenna device provided with horn antennas whose radiation characteristics have an efficiently expanded half-power beamwidth. 
     (Fourth Preferred Embodiment) 
     As shown in FIGS. 16 and 17, the antenna device of the present embodiment comprises a horn antenna  42  connected to an end of a waveguide  41 , and is characterized by the structure of the horn antenna  42 . 
     As shown in FIG. 19 a prior art horn antenna  102  has the geometry of a pyramid in which the width A 1  of an antenna opening  102   a  is larger than the width of the opening portion  101   a  of a waveguide  101 . 
     In contrast, in the present embodiment, as shown in FIG. 17A, the width A 2  of an antenna opening  42   a  of the horn antenna  42  is smaller than the width w of the opening portion  41   a  of the waveguide  41 . To be more specific, the left and right side surfaces  42   b  of the cabinet including the horn antenna  42  is tapered from the opening portion  41   a  of the waveguide  41  to the antenna opening  42   a  so as to make the width A 2  of the antenna opening  42   a  smaller than the width w of the opening portion  41   a  of the waveguide  41 . 
     As shown in FIG. 17B, the height B of the antenna opening  42   a  is made larger than the height h of the opening portion  41   a  of the waveguide  41  and can be determined as indicated in the formula below to offer a gain suitable for a required system. To the be more specific, when the horn antenna  42  has a length of R p , the optimum gain can be obtained by setting the height B as found in the following formula: 
     B≈(2R p λ) ½ (λ: spatial wavelength of usable frequency) 
     According to the present embodiment, the half-power beamwidth of the horizontal plane is made larger by making the width A 2  of the antenna opening  42   a  smaller than the width w of the opening portion  41   a  of the waveguide  41  so as to taper the side surfaces. 
     The following is a description of the measurement results on the effects of increasing the half-power beamwidth of the antenna device. A half-power beamwidth θ (deg) and a gain (dBi) were measured while the width A 2  of the antenna opening  42   a  is changed under the conditions that the opening portion  41   a  of the waveguide  41  has a width w of 8.6 mm and a height h of 4.3 mm, and the antenna opening  42   a  has a height B of 50 mm and a length of 100 mm. The measurement results are shown in FIG. 18, where the horizontal axis indicates the width A 2  of the opening  42   a  and the vertical axis indicates the half-power beamwidth θ (deg) and the gain (dBi). 
     As apparent from FIG. 18, when the width A 2  of the antenna opening  42   a  is equal to the width w (8.6 mm) of the opening portion  42   a , the half-power beamwidth θ is only about 64 degrees; however, when the width w of the antenna opening  42   a  is reduced down to around 6.6 mm, the half-power beamwidth θ can be expanded as large as 80 degrees or so. The width w of the antenna opening  42   a  being smaller than 6.6 mm could not make the half-power beamwidth θ larger than 80 degrees, and on the contrary, the gain would be rapidly reduced. This is considered to result from the influence of the cutoff frequency of the waveguide  41 . The measurement results indicate that when the ratio of the width A 2  of the antenna opening  42   a  to the width w of the waveguide  41  (A 2 /w) is set at about 6.6/8.6, a large half-power beamwidth can be obtained while preventing the gain from decreasing due to the cutoff frequency of the waveguide  41 . 
     Alternatively, the width of the antenna opening  42   a  can be reduced as follows. A shielding wall for shielding the antenna opening  42   a  is provided at the left and right ends of the antenna opening  42   a , and a radio wave absorber is applied onto the wall to reduce the width of the antenna opening  42   a . However, that case has a drawback that a loss caused by the radio wave absorber decreases the gain, and growing internal volume of the antenna prevents the miniaturization of the antenna device. In contrast, the present embodiment does not need a radio wave absorber, which reduces the gain drop. Moreover, the internal volume of the antenna does not grow, and, on the contrary, can be reduced, which facilitates the miniaturization of the antenna device. 
     The above-mentioned embodiments can provide horn antennas with excellent radiation characteristics having a large half-power beamwidth, thereby largely contributing to the establishment of information communication network provided with an inexpensive and high information density mainly for general household telephones. In addition, the occurrence of a drop in directivity due to the array factor can be effectively prevented. Hence, wide radiation characteristics in the horizontal plane can be secured easily, regardless of the simple structure. 
     The preferred embodiments of the present invention are described hereinbefore in detail. Combination and arrangement of the components contained in these embodiments can be changed variously without departing from the spirit or scope of the present invention claimed below.