Patent Publication Number: US-2011063158-A1

Title: Array antenna apparatus and radar apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims the benefit of priority from earlier Japanese Patent Application No. 2009-215751 filed Sep. 17, 2009, the description of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates to an array antenna apparatus which can suppress grating lobes and to a radar apparatus that detects a target located within a scanning range of a main lobe by using the array antenna apparatus. 
     2. Related Art 
     The technique for providing such array antenna apparatus and radar apparatus is known as disclosed, for example, in the patent document JP-B-4147447. In the technique disclosed in the patent document JP-B-4147447, a radar apparatus includes an array antenna apparatus which is configured by a transmission array antenna having a plurality of antenna elements, a transmission processing unit connected to the transmission array antenna, a reception array antenna having a plurality of antenna elements, and a reception processing unit connected to the reception array antenna. 
     This radar apparatus detects a target located within a scanning range of a main lobe, with a composite transmission/reception directivity in which the direction of a grating lobe of the transmission array antenna, i.e. a grating suppressed side, agrees with the direction of null points of the reception array antenna, i.e. a grating suppressing side. The term “composite transmission/reception directivity” refers to a directivity resulting from the composition of the directivity on a grating suppressed side and the directivity on a grating suppressing side. 
     Thus, the composite transmission/reception directivity can permit the direction of the grating lobe on a grating suppressed side to agree with the direction of the null points on a grating suppressing side. Accordingly, the sensitivity in the direction of the grating lobe on a grating suppressed side can be suppressed, while the target located within the scanning range of the main lobe can be detected. 
     A grating suppressed side does not necessarily have a single grating lobe but has a plurality of grating lobes. Similarly, a grating suppressing side not only has a main lobe but also has a plurality of grating lobes. Further, the lobes on a grating suppressed side as well as the lobes on a grating suppressing side have the same directional intervals. These respective grating lobes may be referred to a primary grating lobe, a secondary grating lobe, . . . an Nth grating lobe, or may be referred to a first grating lobe, a second grating lobe, . . . an Nth grating lobe, in order of increasing distance from the main lobe. 
     According to the technique described in the above patent document JP-B-4147447, the direction of the first grating lobe on a grating suppressed side agrees with the direction of the null points between the main lobe and the first grating lobe on a grating suppressing side. Accordingly, the sensitivity in the direction of the first grating lobe on the grating suppressed side can be suppressed with the composite transmission/reception directivity. 
     However, although not disclosed in the above patent document JP-B-4147447, the directional interval between the lobes on the grating suppressing side is larger “by a factor of two” than the directional interval between the lobes on the grating suppressed side (refer to FIG. 2 of JP-B-4147447). Thus, the direction of the second grating lobe on the grating suppressed side will agree with the direction of the first grating lobe on the grating suppressing side. For this reason, with the technique described in JP-B-4147447, the sensitivity in the direction of the second grating lobe on the grating suppressed side cannot be suppressed with the composite transmission/reception directivity. Therefore, there is a concern that a target detection error may be caused if the direction of the second grating lobe on the grating suppressed side is included in the visible region. 
     In order to remove such a concern, the interval between the antenna elements may be reduced so that the direction of the second grating lobe on the grating suppressed side will not agree, in the visible region, with the direction of the first grating lobe on the grating suppressing side. Specifically, let us take an example in which a visible region extends to within “±90°”. In order that the directions of the grating lobes will not agree with each other in this visible region, the interval between the antenna elements on a grating suppressing side may be set to “one half” or less of a wavelength λ of the radio waves in use. 
     However, it is physically difficult to set the interval between the antenna elements on a grating suppressing side to such a small interval. Because of this difficulty, the visible region, per se, is necessitated to be made smaller so that no grating is caused. Reducing a visible region, per se, may lead to reducing the detection range of a target. Thus, the technique disclosed in the patent document JP-B-4147447 still leaves room for improvement. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in light of the circumstances described above, and has as its object to provide an array antenna apparatus which is able to broaden a visible region, while increasing the interval between antenna elements and to provide a radar apparatus which is able to detect a target using the array antenna apparatus. 
     In order to achieve the object, the present invention provides, as one aspect, an array antenna apparatus, including: a transmission array antenna which includes a plurality of transmission antenna elements; a transmission-side directivity control unit which controls a directivity of the transmission array antenna by controlling phases of at least part of the plurality of transmission antenna elements; a reception array antenna which includes a plurality of reception antenna elements; and a reception-side directivity control unit which controls a directivity of the reception array antenna by controlling phases of at least part of the plurality of reception antenna elements, wherein the array antenna apparatus has a composite transmission/reception directivity, in which the direction of a grating lobe of one of the transmission array antenna and the reception array antenna agrees with the direction of a null point or part of side lobes of the other of the transmission array antenna and the reception array antenna, and the direction of a grating lobe of the other of the transmission array antenna and the reception array antenna agrees with the direction of a null point or part of side lobes of the one of the transmission array antenna and the reception array antenna, and scans in a scanning range with the composite transmission/reception directivity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a block diagram illustrating a general configuration of a radar apparatus which includes an array antenna apparatus according to an embodiment; 
         FIG. 2  is a diagram illustrating an example of an array factor AF(u); 
         FIG. 3  is a diagram illustrating an example of an array factor AF(θ) in the case where the direction of a main lobe is “45°”; 
         FIG. 4  is a diagram illustrating an example of the directivity of a transmission array antenna and an example of the directivity of a reception array antenna, for explaining the principle of occurrence of a grating lobe; 
         FIG. 5  is a diagram illustrating an example of composite transmission/reception directivity resulting from the composition of the directivity of a transmission array antenna and the directivity of a reception array antenna, for explaining a method of suppressing grating lobes; 
         FIG. 6  is a diagram illustrating an example of the directivity of a transmission array antenna and an example of the directivity of a reception array antenna with a broken line and a solid line, respectively, according to the embodiment; 
         FIG. 7  is a diagram illustrating composite transmission/reception directivity resulting from the composition of the directivity of the transmission array antenna and the directivity of the reception array antenna, according to the embodiment; 
         FIG. 8  is a block diagram illustrating a general configuration of a radar apparatus, according to a modification of the embodiment; 
         FIG. 9  is a block diagram illustrating a general configuration of a radar apparatus, according to another modification of the embodiment; and 
         FIG. 10  is a block diagram illustrating a general configuration of a radar apparatus, according to still another modification of the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIGS. 1 to 7 , hereinafter will be described an embodiment of a radar apparatus that includes an array antenna apparatus.  FIG. 1  is a block diagram illustrating a general configuration of a radar apparatus  1  that includes an array antenna apparatus according to the present embodiment. Referring to  FIG. 1 , the configuration and functions of the radar apparatus  1  are described first. 
     As shown in  FIG. 1 , the radar apparatus  1  includes a microcomputer  10 , oscillator  11 , power splitter  12 , transmission-side phase shifter  13 , transmission array antenna  14 , reception array antenna  15 , reception-side phase shifter  16 , mixer  17  and A/D converter  18 . 
     The microcomputer  10  is connected to the oscillator  11  so as to be located upstream of the oscillator  11 , while the power splitter  12  is connected to the oscillator  11  so as to be located downstream of the oscillator  11 . The oscillator  11  gives oscillation in a frequency band (e.g., millimeter-wave band) allocated by the microcomputer  10 . At the same time, the oscillator  11  produces a high-frequency signal that changes into a triangular wave and outputs the produced high-frequency signal to the power splitter  12 . 
     The transmission array antenna  14  is connected to the power splitter  12  via the transmission-side phase shifter  13  so as to be located downstream of the power splitter  12 . The mixer  17  is connected to the power splitter  12 . The power splitter  12  splits the high-frequency signal inputted from the oscillator  11  into a transmission signal S and a local signal L. Then, the power splitter  12  outputs the transmission signal S to the transmission array antenna  14  via the transmission-side phase shifter  13  and outputs the local signal L to the mixer  17 . 
     The transmission-side phase shifter  13  is disposed between the transmission array antenna  14  and the power splitter  12 , while being connected to the microcomputer  10 . The transmission array antenna  14  is configured by a plurality of (e.g., five) transmission antenna elements  14   a  to  14   e  which are arrayed at regular intervals on a plane (on an antenna surface), not shown. The transmission-side phase shifter  13  sets an amount of phase shift of each of the transmission signals S inputted to the transmission antenna elements  14   a  to  14   e  from the power splitter  12 , according to the instructions of the microcomputer  10 . 
     Thus, the transmission signals (electric signals) S are inputted from the power splitter  12  via the transmission-side phase shifter  13 . The transmission array antenna  14  radiates these transmission signals S to the outside through the transmission antenna elements  14   a  to  14   e  in the form of transmission beams (radio waves). The direction of each transmission beam depends on the amount of phase shift of the transmission signal S inputted to each of the transmission antenna elements  14   a  to  14   e.    
     The direction of each transmission beam is determined based on the antenna surface of the transmission array antenna  14 . Specifically, the direction perpendicular to the antenna surface is “0°” and the direction parallel to the antenna surface is “±90°”. The phase of the transmission signal is controlled for each of the transmission antenna elements  14   a  to  14   e . Thus, the transmission array antenna  14  can radiate the transmission beams in a desired directional range. In other words, the directivity of the transmission array antenna  14  is variable. 
     On the other hand, the reception array antenna  15  is configured by a plurality of (e.g., three) reception antenna elements  15   a  to  15   c  which are arrayed at regular intervals on a plane, not shown. The reception antenna elements  15   a  to  15   c  are connected to the mixer  17  via the reception-side phase shifter  16 . When the transmission beams (radio waves) radiated from the transmission array antenna  14  are reflected by a target, the reception antenna elements  15   a  to  15   c  of the reception array antenna  15  receive the reflected beams (radio waves). Then, the reception antenna elements  15   a  to  15   c  output the received reflected beams to the reception-side phase shifter  16  in the form of reception signals (electric signals) R. The reception-side phase shifter  16  sets an amount of phase shift of each of the reception signals R inputted from the reception antenna elements  15   a  to  15   c , according to the instructions of the microcomputer  10 , and then outputs the reception signals R to the mixer  17 . 
     Thus, the reception signals R are inputted to the mixer  17  from the reception-side phase shifter  16  which is connected upstream of the mixer  17 . The microcomputer  10  is connected to the mixer  17  via the A/D converter  18  so as to be located downstream of the mixer  17 . The mixer  17  mixes the reception signals R inputted from the reception-side phase shifter  16  with the local signals L inputted from the power splitter  12  (performs synchronous detection) to thereby take out base band signals. The taken out base band signals are converted to digital signals by the A/D converter  18 , and then the converted digital signals are outputted to the microcomputer  10 . 
     The direction of each reflected beam incident (hereinafter referred to as “incident direction” of the reflected beam) on the reception array antenna  15  depends on the amount of phase shift (phase difference) of each of the reception signals R outputted from the reception antenna elements  15   a  to  15   c . The incident direction of the reflected beam is determined based on the antenna surface of the reception array antenna  15 . Specifically, the incident direction perpendicular to the antenna surface is “0°”, while the incident direction parallel to the antenna surface is “±90°”. The phase of the reception signal R is controlled for each of the reception antenna elements  15   a  to  15   c . Thus, the reception array antenna  15  can receive the reflected beams from within a desired directional range. 
     The microcomputer  10  is mainly configured by a microcomputer that includes a CPU, ROM, RAM, backup RAM and I/O (which are all not shown). The microcomputer  10  is provided with a processing unit (e.g., DSP (digital signal processor)) that performs a fast Fourier transform (FFT) process for the base band signals acquired from the mixer  17  via the A/D converter  18 . The microcomputer  10  performs various processes by executing various control programs stored in the ROM. 
     The microcomputer  10  scans, first, the radiation angle of a main lobe, in a detection range from “−90°” to “+90°” for a target, while permitting the transmission array antenna  14  to radiate transmission waves. (Thus, the detection range for a target corresponds to the scanning range.) When a target is located in the detection range, the transmission waves radiated from the transmission array antenna  14  are reflected by the target. The reflected waves are then received by the reception array antenna  15 . The microcomputer  10  determines whether or not the intensity of each reception signal R received by the reception array antenna  15  is equal to or more than a threshold. The microcomputer  10 , when it determines that the intensity of the signal is equal to or more than the threshold, detects the direction of the target using the phase difference (phase shift) of the reception signal R. 
     In this way, the radar apparatus  1  uses a so-called electronic scanning method. The microcomputer  10  corresponds to an object detection unit. In the present embodiment, the antenna surface of the transmission array antenna  14  is made parallel to that of the reception array antenna  15 , and the detection range for a target ranges from “−90°” to “+90°”. 
     Hereinafter is described the principle of occurrence of a grating lobe. An array factor AF that is a transfer function of an array antenna is expressed by the following Expression (1). In the expression, An is the amplitude of each wave source, d is the element interval of antenna elements, θ 0  is the direction of a main lobe and N is the number of antenna elements. 
     
       
         
           
             
               
                 
                   
                     AF 
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                   Expression 
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     An array antenna has a main lobe, a grating lobe, a side lobe, a null point and the like in a periodic fashion. Among them, the grating lobe refers to a lobe having a peak substantially the same as that of the main lobe. The side lobe refers to a lobe having a peak which is low compared to those of the main lobe and the grating lobe. Each of these lobes has a certain width which depends on the element interval of the antenna elements. In the present embodiment, the main lobe corresponds to a directional range that includes a peak and extends lowering from the peak by “10 dB”, for example. The grating lobe corresponds to a directional range that includes a peak and extends lowering from the peak by “10 dB”, for example. The side lobe corresponds to a directional range that includes a peak and extends lowering from the peak by “5 dB”, for example. However, the lowering extents are not limited to “10 dB” and “5 dB”. These lobes may have ranges that are not overlapped with each other and are identifiable from each other. Further, the null point corresponds to a direction that causes an abrupt drop of gain. Null points occur between lobes. 
     Here, a new variable u is defined as expressed by the following Expression (2): Also, for the sake of simplification of explanation, the above Expression (1) is expressed as the following Expression (3) when An is “1”. 
     
       
         
           
             
               
                 
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                     AF 
                      
                     
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       FIG. 2  is a diagram illustrating an array factor AF (u) in the case where, for example, the element interval d is set to be the same as the wavelength λ and the number of elements N of the antenna elements is set to “5”. 
     As shown in  FIG. 2 , the array factor AF(u) has a main lobe ML, a first grating lobe GL 1  and a second grating lobe GL 2  at a cycle “2π”. Meanwhile, an exponential function is used in the above Expression (3). Thus, it will be understood from  FIG. 2  and Expression (3) that the array factor AF(u) is a function having a cycle “2π” for the variable u. 
     Further, let us discuss the case where the direction θ 0  of the main lobe ML is set to “π/4 (=45°)”. In this case, the potential range of the variable u can be expressed by the following Expression (4). Such a range of the variable u is referred to a “visible range”. The visible range is indicated by an arrow in  FIG. 2 . 
       (√{square root over (2)}−2)π≦ u ≦(√{square root over (2)}+2)π  Expression (4)
 
     It will be understood from Expression (2) that the potential range of the variable u (i.e. visible range) becomes larger as the element interval d becomes larger. On the other hand, a direction θGL of the grating lobe corresponds to the direction defined by Expression (2) with its right side being “±2πm (m=1, 2, 3, . . . )”, that is, corresponds to the direction that constitutes the following Expression (5). Accordingly, it will be understood that, as the element interval d becomes larger, the grating lobe is likely to enter the visible region. It should be noted that the following Expression (6) is an expression that solves Expression (5) for the direction θ. For comparison, a direction θML of the main lobe corresponds to the direction defined by Expression (5) when “m=0”. 
     
       
         
           
             
               
                 
                   
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                     ( 
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                       θ 
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                     = 
                     
                       
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       FIG. 3  is a diagram illustrating an array factor AF(θ) with the horizontal axis of  FIG. 2  being replaced by on indicating the direction θ. In  FIG. 3 , the direction θ 0  of the main lobe is set to “π/4 (=45°)”. 
     As shown in  FIG. 3 , in the array factor AF(θ), the first grating lobe has a direction θGL 1  (=“−17°”) when the direction θML of the main lobe ML is “45°”. 
     Hereinafter is explained a method of suppressing the occurrence of grating in a visible region. In the explanation, AT represents the transmission array antenna and AR represents the reception array antenna. 
     First, an explanation is given for the technique corresponding to the technique disclosed in the patent document JP-B-4147447, a conventional art, mentioned above. The directional interval, in the conventional art, between the lobes of the transmission array antenna AT is larger by a factor of two than the directional interval between the lobes of the reception array antenna AR. 
     Specifically, the wavelength of the radio waves used for the transmission array antenna AT and the reception array antenna AR is represented by λ. In this case, for example, an element interval Dt of the transmission antenna elements that constitute the transmission array antenna AT is set to be “equal to twice of the wavelength λ”. Also, for example, an element interval Dr of the reception antenna elements that constitute the reception array antenna AR is set to be “equal to the wavelength λ”. In other words, let us assume that a relationship “Dt=Dr×2.0” is established between the element interval Dt of the transmission antenna elements and the element interval Dr of the reception antenna elements. 
     With this relationship, when a main lobe T:ML of the transmission array antenna AT has a direction expressed by “direction θ 0 =45° (=ASIN (1/√2))”, a first grating lobe T:GL 1  will have a direction expressed by “direction θTGL 1 =12° (=ASIN (−¼+1/√2))” as derived from Expression (6). Also, in this case, a second grating lobe T:GL 2  will have a direction expressed by “direction θTGL 2 =−17° (=ASIN (−1+1/√2))” as derived from Expression (6). 
     Further, when a main lobe R:ML of the reception array antenna AR has a direction expressed by “direction θ 0 =45° (=ASIN (1/√2))”, a first grating lobe R:GL 1  will have a direction expressed by “direction θRGL 1 =−17° (=ASIN (−1+1/√2))” as derived from Expression (6). Accordingly, by setting the relationship “Dt=Dr×2.0” between the element interval Dt of the transmission antenna elements and the element interval Dr of the reception antenna elements, it is understood that the directional interval between the lobes of the reception array antenna AR reliably becomes “twice” of the directional interval between the lobes of the transmission array antenna AT. It should be noted that “ASIN” indicates an inverse function of a sine function. 
     In the technique of this conventional art, the direction θTGL 2  of the second grating lobe of the transmission array antenna AT agrees, at “−17°”, with the direction θRGL 1  of the first grating lobe of the reception array antenna AR. Thus, grating occurs in the visible range (−90° to)+90°. In other words, sensitivity in the direction θTGL 2  of the second grating lobe of the transmission array antenna AT can no longer be suppressed with the composite transmission/reception directivity, leading to detection error of a target. 
     In this regard, in the present suppressing method, the directional interval between the lobes of the transmission array antenna AT is set so as not to be twice of the directional interval between the lobes of the reception array antenna AR. 
     Specifically, the wavelength of the radio waves used for the transmission array antenna AT and the reception array antenna AR is represented by λ. In this case, for example, the element interval Dt of the transmission antenna elements that constitute the transmission array antenna AT is set to be “equal to the wavelength λ”. Also, for example, the element interval Dr of the reception antenna elements that constitute the reception array antenna AR is set to be “equal to 1.5 times of the wavelength λ”. In other words, let us assume that a relationship “Dr=Dt×1.5” is established between the element interval Dt of the transmission antenna elements and the element interval Dr of the reception antenna elements. 
     With this relationship, when the main lobe T:ML of the transmission array antenna AT has a direction expressed by “direction θ 0 =45° (=ASIN (1/√2))”, the first grating lobe T:GL 1  will have a direction expressed by “direction θTGL 1 =−17° (=ASIN (−1+1/√2))” as derived from Expression (6). 
     Also, when the main lobe R:ML of the reception array antenna AR has a direction expressed by “direction θ 0 =45° (=ASIN (1/√2))”, the first grating lobe R:GL 1  will have a direction expressed by “direction θRGL 1 =2° (=ASIN (−⅔+1/√2))” as derived from Expression (6). Further, in this case, a second grating lobe R:GL 2  will have a direction expressed by “direction θRGL 2 =−39° (=ASIN(−4/3+1/√2))”. For comparison, reference is made to  FIG. 4 .  FIG. 4  is a diagram illustrating the directivity of the transmission array antenna AT and that of the reception array antenna AR by a broken line and a solid line, respectively. 
     Accordingly, by setting the relationship of “Dr=Dt×1.5” between the element interval Dt of the transmission antenna elements and the element interval Dr of the reception antenna elements, the directional interval between the lobes of the transmission array antenna AT reliably becomes “1.5 times” of the directional interval between the lobes of the reception array antenna AR. Further, it will be understood that the direction θTGL 1  of the first grating lobe T:GL 1  of the transmission array antenna AT agrees with the central direction between the directions θRGL 1  and θRGL 2  of the first and second grating lobes R:GL 1  and R:GL 2  of the reception array antenna AR. 
     As indicated by the solid line in  FIG. 4 , the reception array antenna AR has “three” side lobes and “four” null points between the directions of the first and second grating lobes R:GL 1  and R:GL 2 . Of the “three” side lobes, the one having the lowest peak (hereinafter referred to as “lowest-peak side lobe”) has a direction corresponding to the central direction mentioned above. Accordingly, the first grating lobe T:GL 1  of the transmission array antenna AT will be suppressed by the lowest-peak side lobe of the reception array antenna AR. 
     In this way, by setting the relationship “Dr=Dt×1.5” between the element interval Dt of the transmission antenna elements and the element interval Dr of the reception antenna elements, the amount of suppression of the first grating lobe of the transmission array antenna AT can be maximized. 
     Further, as indicated by the broken line in  FIG. 4 , the transmission array antenna AT has “three” side lobes and “four” null points between the directions of the main lobe T:ML and the first grating lobe T:GL 1 . Accordingly, the first grating lobe R:GL 1  of the reception array antenna AR will be suppressed by the side lobes and the null points of the transmission array antenna AT. 
     Similarly, the transmission array antenna AT also has “three” side lobes and “three” null points distanced from the main lobe T:ML with reference to the direction of the first grating lobe T:GL 1 . Accordingly, the second grating lobe R:GL 2  of the reception array antenna AR will be suppressed by the side lobes and the null points of the transmission array antenna AT. 
     Thus, by setting the relationship “Dr=Dt×1.5” between the element interval Dt of the transmission antenna elements and the element interval Dr of the reception antenna elements, the amount of suppression of the first grating lobe T:GL 1  of the transmission array antenna AT can be maximized. 
     Further, the first grating lobe R:GL 1  of the reception array antenna AR will be suppressed by the side lobes and the null points between the main lobe T:ML and the first grating lobe T:GL 1  of the transmission array antenna AT. Furthermore, the second grating lobe R:GL 2  of the reception array antenna AR will be suppressed by the side lobes and the null points distanced from the main lobe T:ML with reference to the first grating lobe T:GL 1  of the transmission array antenna AT. 
       FIG. 5  is a diagram illustrating a composite transmission/reception directivity in which the directivity of the transmission array antenna AT and that of the reception array antenna AR are composed. As shown in  FIG. 5 , in the composite transmission/reception directivity, the direction θTGL 1  of the first grating lobe T:GL 1  of the transmission array antenna AT agrees with the direction of the null points or the direction of the side lobes of the reception array antenna AR. In addition, the directions θRGL 1  and θRGL 2  of the first and second grating lobes R:GL 1  and R:GL 2  of the reception array antenna AR agree with the direction of the null points or the direction of the side lobes of the transmission array antenna AT. 
     In other words, the first grating lobe T:GL 1  of the transmission array antenna AT is suppressed, by about “10 dB”, by the null points or the side lobes of the reception array antenna AR. Similarly, the first and second grating lobes R:GL 1  and R:GL 2  of the reception array antenna AR are suppressed, by about “10 dB”, by the null points or the side lobes of the transmission array antenna AT. 
     However, when the element interval Dt of the transmission array antenna AT and the element interval Dr of the reception array antenna AR are set to have the relationship “Dr=Dt×1.5” as described above, the direction of a third grating lobe R:GL 3 , not shown, of the reception array antenna AR agrees with the direction of a second grating lobe T:GL 2 , not shown, of the transmission array antenna AT. Therefore, grating lobes occur in the composite transmission/reception directivlity. 
     When the direction in which the grating lobes have occurred enters the visible region, it is likely that a target may be erroneously detected. In order to avoid the occurrence of grating lobes in a visible region, the element interval Dt of the transmission array antenna AT is required to be an interval which will not permit the second grating lobe T:GL 2  of the transmission array antenna AT to enter the visible region. 
     Specifically, the element interval Dt of the transmission array antenna AT is required to be set so that the following Expression (7) is satisfied. Expression (7) is obtained by allowing the direction “θ 0 ” of the main lobe to be “+90°” and by substituting “2” and “−90°” for the factor “m” and the direction “θ”, respectively, in Expressions (2) and (5) provided above. Here, the direction of the main lobe, i.e. “θ 0 =+90°”, corresponds to the direction of one end of the visible region. Also, the conditions, i.e. “m=2” and “θ=−90°”, correspond to the conditions where the second grating lobe T:GL 2  of the transmission array antenna AT occurs in the direction of the other end of the visible region. 
     
       
         
           
             
               
                 
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     From the above Expression (7), it will be understood that, by setting the element interval Dt of the transmission array antenna AT to “Dt&lt;λ”, the occurrence of grating lobes in the visible region can be avoided. 
     As mentioned above, there is the relationship “Dr=Dt×1.5” between the element interval Dt of the transmission antenna elements and the element interval Dr of the reception antenna elements. Therefore, the amount of suppression is maximized, which is caused by the side lobes of the reception array antenna AR to the first grating lobe T:GL 1  of the transmission array antenna AT. 
     However, the relationship is not limited to “Dr=Dt×1.5”, but may be “Dt&lt;Dr&lt;Dt×2”. With this relationship as well, the first grating lobe T:GL 1  of the transmission array antenna AT can be suppressed by the null points or the side lobes of the reception array antenna AR. This is because the direction of the first grating lobe T:GL 1  of the transmission array antenna AT having the element interval Dt falls between the directions of the first and second grating lobes R:GL 1  and R:GL 2  of the reception array antenna AR having the element interval Dr. 
     Referring now to  FIGS. 6 and 7 , the present embodiment will again be described further based on the method of suppressing grating lobes described above.  FIG. 6  is a diagram illustrating the directivity of the transmission array antenna  14  and the directivity of the reception array antenna  15  with a solid line and a broken line, respectively.  FIG. 7  is a diagram illustrating composite transmission/reception directivity in which the directivity of the transmission array antenna  14  and that of the reception array antenna  15  are composed. 
     In the present embodiment, the element interval Dt between the transmission antenna elements  14   a  to  14   e  and the element interval Dr of the reception antenna elements  15   a  to  15   c  have the relationship expressed by “Dr=Dt×1.5”. 
     Thus, as shown in  FIG. 6 , the direction θTGL 1  of the first grating lobe T:GL 1  of the transmission array antenna  14  falls between the directions θRGL 1  and θRGL 2  of the first and second grating lobes R:GL 1  and R:GL 2  of the reception array antenna  15 . 
     Specifically, the direction θTGL 1  of the first grating lobe T:GL 1  of the transmission array antenna  14  agrees with the direction of the null points or the direction of the side lobes of the reception array antenna  15 . In addition, the directions θRGL 1  and θRGL 2  of the first and second grating lobes R:GL 1  and R:GL 2  of the reception array antenna  15  agree with the direction of the null points or the direction of the side lobes of the transmission array antenna  14 . 
     As shown in  FIG. 7 , in the composite transmission/reception directivity, the first grating lobe T:GL 1  of the transmission array antenna  14  will be suppressed, by about “10 dB”, by the null points or the side lobes of the reception array antenna  15 . Similarly, the first and second grating lobes R:GL 1  and R:GL 2  of the reception array antenna  15  will be suppressed, by about “10 dB”, by the null points or the side lobes of the transmission array antenna  14 . 
     Also, in the present embodiment, with the wavelength λ being used for the radar apparatus  1 , the element interval Dt of the transmission antenna elements  14   a  to  14   e  has been set to “0.9 times” of the wavelength λ, and the element interval Dr of the reception antenna elements  15   a  to  15   c  has been set to “1.35 times” of the wavelength λ. Thus, since the relationship “Dt&lt;λ” mentioned above can be satisfied, grating can be prevented from occurring in the visible region (±90°). 
     According to the technique disclosed in the patent document JP-B-4147447 provided as conventional art, the element interval Dt had to be set to “one half” or less of the wavelength λ in order that no grating is caused in the visible region (±90°). In this regard, according to the present embodiment, the element interval Dt can be set to be larger than one half of the wavelength λ if the element interval Dt is smaller than the wavelength λ. Therefore, the transmission array antenna  15  can be more easily configured. 
     It should be noted that the transmission-side phase shifter  13  and the reception-side phase shifter  16  of the radar apparatus  1  correspond to a transmission-side directivity control unit and a reception-side directivity control unit, respectively. 
     The radar apparatus  1  related to the present invention is not limited to the configuration exemplified in the above embodiment, but may be variously modified within a scope not departing from the spirit of the invention. For example, the above embodiment may be appropriately modified and implemented as set forth below. 
     In the above embodiment, the transmission array antenna  14  has been configured to have “five” transmission antenna elements  14   a  to  14   e . However, the number of the transmission antenna elements is not limited to this number. The number of the transmission antenna elements may be increased or decreased as required. Similarly, in the above embodiment, the reception array antenna  15  has been configured to have “three” reception antenna elements  15   a  to  15   c . However, the number of the reception antenna elements is not limited to this number. The number of the reception antenna elements may be increased or decreased as required. 
     In the above embodiment, the transmission-side phase shifter  13  has been allowed to set the directivity of the transmission array antenna  14  by setting the amount of phase shift for each of the plurality of transmission signals S inputted to the transmission antenna elements  14   a  to  14   e . However, the amount of phase shift may not necessarily be set for all of the plurality of transmission antenna elements  14   a  to  14   e  configuring the transmission array antenna  14 . 
     For example, a radar apparatus  1   a  shown in  FIG. 8 , which corresponds to  FIG. 1 , may be configured. In this radar apparatus, the amount of phase shift of the transmission antenna element  14   a  among the transmission antenna elements  14   a  to  14   e  may be used as a reference to set the amounts of phase shift of the remaining transmission antenna elements  14   b  to  14   e . In this case, since the phase of the transmission antenna element  14   a  is not required to be controlled, a transmission-side phase shifter  13   a  has no phase shifter for the transmission antenna element  14   a . Thus, the directivity of the transmission array antenna  14  can also be controlled by controlling the phases of at least part of the plurality of transmission antenna elements  14   a  to  14   e.    
     In the above embodiment, the reception-side phase shifter  16  has been allowed to set the directivity of the reception array antenna  15  by setting the amount of phase shift of each of the plurality of reception signals R inputted to the reception antenna elements  15   a  to  15   c . However, the amount of phase shift may not necessarily be set for all of the plurality of reception antenna elements  15   a  to  15   c  configuring the reception array antenna  15 . 
     For example, as in the radar apparatus  1   a  shown in  FIG. 8 , which corresponds to  FIG. 1 , the amount of phase shift of the reception antenna element  15   a  among the reception antenna elements  15   a  to  15   c  may be used as a reference to thereby set the amounts of phase shift of the remaining reception antenna elements  15   b  and  15   c . In this case, since the phase of the reception antenna element  15   a  is not required to be controlled, a reception-side phase shifter  16   a  is not provided with a phase shifter for the reception antenna element  15   a . Thus, the directivity of the reception array antenna  15  can also be controlled by controlling the phases of at least part of the plurality of reception antenna elements  15   a  to  15   c.    
     The reception-side phase shifter  16   a  may not necessarily be used in controlling the directivity of the reception array antenna  15 . Alternative to the reception-side phase shifter  16   a , a switch  16   b  may be used, for example, as shown in  FIG. 9  that corresponds to  FIG. 1  or  8 , in controlling the directivity of the reception array antenna  15  to thereby perform digital processing, such as the well-known DBF (digital beam forming). With this configuration as well, the directivity of the reception array antenna  15  can be controlled. 
     Further, the transmission-side phase shifter  13   a  may not necessarily be used in controlling the directivity of the transmission array antenna  14 . Alternative to the transmission-side phase shifter  13   a , a phase network  131 , such as the well-known Butler matrix, and a switch  132  may be used, as in the case of the radar apparatus  1   c  shown in  FIG. 10  that corresponds to  FIG. 1 ,  8  or  9 , in controlling the directivity of the transmission array antenna  14 . Specifically, the phase network  131  may be configured such that the transmission array antenna  14  would have five types of directivities, and the five types of directivities may be switched using the switch  132 . Further, the switch  16   b  may be configured such that the reception array antenna  15  would have directivities corresponding to the respective five types of directivities of the transmission array antenna  14 . With this configuration as well, the directivity of the transmission array antenna  14  can be controlled. 
     In the above embodiment, the transmission array antenna  14  has been configured as a planar antenna in which a plurality of transmission antenna elements are arrayed on a plane. However, the configuration is not limited to this. The transmission array antenna  14  may only have to be an array antenna, and thus a plurality of transmission antenna elements may not necessarily be arrayed on a plane. 
     Similarly, in the above embodiment, the reception array antenna  15  has been configured as a planar antenna in which a plurality of reception antenna elements are arrayed on a plane. However, the configuration is not limited to this. The reception array antenna  15  may only have to be an array antenna, and thus a plurality of reception antenna elements may not necessarily be arrayed on a plane. 
     In the above embodiment, the element interval Dt of the transmission antenna elements  14   a  to  14   e  and the element interval Dr of the reception antenna elements  15   a  to  15   c  have had the relationship “Dr=Dt×1.5”. Alternatively, a relationship “Dt&lt;Dr&lt;Dt×2” may be established. 
     In the above embodiment, when the wavelength of the radio waves used for the radar apparatus  1  is represented by λ, the element interval Dt of the transmission antenna elements  14   a  to  14   e  has been set to “0.9 times” of the wavelength λ. Alternative to this, the element interval Dt may be set to “0.99 times” or “0.5 times” of the wavelength λ. What matters is that the relationship “Dt&lt;λ” may only have to be established. 
     In the above embodiment, since the scanning ranges of the transmission and reception array antennas  14  and  15  have each been set ranging “±90°”, the relationship “Dt&lt;λ” has been set. However, the scanning ranges may not necessarily range “±90°”. When the scanning ranges of the transmission and reception array antennas  14  and  15  are set to range “±0”, a relationship “Dt&lt;λ/sin θ” may only have to be established. 
     For convenience, the above embodiment has been described fixing the direction of the main lobe radiated from the transmission array antenna  14  to be “45°”. However, the main lobe of the transmission array antenna  14  can be scanned ranging from “−90°” to “+90°”. Thus, the directions of the null points and the side lobes of the reception array antenna  15  can be permitted to follow the direction of the main lobe of the transmission array antenna  14 . 
     Hereinafter, aspects of the above-described embodiments will be summarized. 
     In order to achieve the above object, an array antenna apparatus of the embodiment has a composite transmission/reception directivity, in which the direction of a grating lobe of one of the transmission array antenna and the reception array antenna agrees with the direction of a null point or part of side lobes of the other of the transmission array antenna and the reception array antenna, and the direction of a grating lobe of the other of the transmission array antenna and the reception array antenna agrees with the direction of a null point or part of side lobes of the one of the transmission array antenna and the reception array antenna. 
     According to the configuration of the antenna apparatus, the grating lobe of the transmission array antenna is suppressed by the null points or the side lobes of the reception array antenna. Similarly, the grating lobe of the reception array antenna is suppressed by the null points or the side lobes of the transmission array antenna. 
     This means that, of the transmission array antenna and the reception array antenna, the direction of a second grating lobe of one array antenna (hereinafter referred to as “first array antenna”) does not agree with the direction of a first grating lobe of the other array antenna (hereinafter referred to as “second array antenna”). This is different from the above conventional art in which the direction of a second grating lobe on the grating suppressed side agrees with the direction of a first grating lobe on the grating suppressing side. 
     Compared to the above conventional art, the difference is that, in the composite transmission/reception directivity in the above configuration, the direction in which grating occurs will be more distanced from the direction of the main lobe, making it difficult for grating to occur in a visible region. Thus, with the above configuration, the visible region can be broadened without decreasing the element interval between the antenna elements. 
     In the array antenna apparatus, a relationship “Dt&lt;Dr&lt;Dt×2” is formed. This relationship allows the direction of a first grating lobe of the first array antenna to fall between the directions of a first grating lobe and a second grating lobe of the second array antenna. In this case, the second array antenna has a plurality of side lobes and a plurality of null points between the directions of the first and second grating lobes. Therefore, the first grating lobe of the first array antenna will be suppressed by the null points or the side lobes of the second array antenna. 
     Similarly, the first array antenna has a plurality of side lobes and a plurality of null points between the directions of the main lobe and the first grating lobe. Therefore, the first grating lobe of the second array antenna will be suppressed by the null points or the side lobes of the first array antenna. 
     Similarly, the first array antenna also has a plurality of side lobes and a plurality of null points distanced from the main lobe with reference to the first grating lobe. Therefore, the second grating lobe of the second array antenna will be suppressed by the null points or the side lobes of the first array antenna. 
     In the array antenna apparatus, a relationship “Dt&lt;λ/sin θ” is formed. This relationship can prevent the occurrence of grating as well in the visible region when a scanning range is “±θ”. 
     In the array antenna apparatus, an element interval Dt and an element interval Dr are set to establish a relationship “Dr=Dt×1.5” and a relationship “Dt&lt;λ” when the scanning range is “±90°”. This means that the direction of the first grating lobe of the first array antenna agrees with a central direction between the directions of the first and second grating lobes of the second array antenna. In this case, the second array antenna has a plurality of side lobes between the directions of the first and second grating lobes. Of the plurality of side lobes, the side lobe having the lowest peak is located at the central direction mentioned above. Accordingly, the first grating lobe of the first array antenna will be suppressed by the side lobe having the lowest peak of the second array antenna. Thus, according to the array antenna apparatus, the amount of suppression can be maximized. 
     Note that the array antenna apparatus may further include a phase shifter which controls phases of at least part of the reception antenna elements. The reception-side directivity control unit may control the directivity of the reception array antenna using the phase shifter. 
     Alternatively, the array antenna apparatus may further include a switch which controls phases of at least part of the reception antenna elements. The reception-side directivity control unit may control the directivity of the reception array antenna using the switch. 
     In order to achieve the above object, a radar apparatus includes the array antenna apparatus and an object detection unit which detects an object based on a reception signal, which is radio waves transmitted from the transmission array antenna and reflected by the object and is received by the reception array antenna. Thus, the radar apparatus can detect the object (target) using the array antenna apparatus. 
     It will be appreciated that the present invention is not limited to the configurations described above, but any and all modifications, variations or equivalents, which may occur to those who are skilled in the art, should be considered to fall within the scope of the present invention.