Patent Publication Number: US-2010123619-A1

Title: Antenna device and radar apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the Japanese Patent Application No. 2008-292492, filed on Nov. 14, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an antenna device and a radar apparatus. 
     2. Description of the Related Art 
     In monopulse radar systems, an array antenna forms a beam to transmit a signal. Then, the array antenna receives an echo signal which is corresponded to the signal in order to measure a target angle. 
     The array antenna includes several subarray antennas as disclosed in “Antenna Engineering Handbook”, Ohmsha, pp. 339-pp. 445. One side of each subarray antenna is connected to a feeding interface such as a waveguide or a line such as a triplate line and a microstrip line in order to feed a signal. These feeding methods are disclosed by H. Iizuka, K. Sakakibara, T. Watanabe, K. Sato, and K. Nishikawa, “Antennas for Automotive Millimeter-wave Rader Systems”, IEICE, SB-1-7, pp. 743-pp. 744, 2001, and in JP-A 2000-124727 (KOKAI). 
     A waveguide feeding method is popular for the antenna in automotive radar systems using the millimeter wave. In the case that the width of the feeding interface which is the waveguide is larger than interval of the subarray antenna an extra space is required between adjacent subarray antennas when all feeding interfaces are formed at the same side of all subarray antennas. As a result, an aperture area of the array antenna gets large. 
     On the other hand, the space between the adjacent subarray antennas should be narrow in order to achieve a wide coverage angle in the automotive radar systems. 
     One of the waveguide feeding methods is disclosed by Y. Okajima, S. Park, J. Hirokawa, and M. Ando, “A Slotted Post-wall Waveguide Array with Inter-digital Structure for 45-deg Linear and Dual Polarization”, IEICE Technical Report, AP2003-149, RCS2003-155, pp. 21-26, 2003. In this reference, the subarray antennas in the array antennas are arranged in an interdigital structure. 
     In the array antenna with the inter-digital structure, the feeding interfaces are formed at a different side of the subarray antennas alternately. Therefore, since the adjacent subarray antennas are arranged with no space, it can achieve a small aperture area of the array antenna. 
     However, the array antenna with the asymmetrical inter-digital structure for a scan plane causes an asymmetrical phase difference of a signal beam of each subarray antenna because of manufacturing tolerance. As a result, measurement accuracy of the target angle degrades in the monopulse radar systems using the array antenna with the inter-digital structure. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, an antenna device includes: 
     subarray antennas arranged parallel to each other with an interval on a plane, each subarray antenna including an antenna element; and 
     feeding interfaces, each being connected to each of the subarray antennas, wherein 
     the interval of the subarray antennas is less or equal than a free-space wavelength, 
     the subarray antennas are symmetrically arranged about a central axis on the plane, 
     the central axis being along with the center of two adjacent subarray antennas arranged at middle of the subarray antennas when the number of the subarray antennas is even, and being along with one subarray antenna arranged at the middle of the subarray antennas when the number of the subarray antennas is odd. 
     According to other aspect of the invention, a radar apparatus includes: 
     the antenna device of claim  1 , which receives an RF signal; 
     an RF chip amplifying the RF signal and down-converting a frequency of the first signal to a lower frequency to obtain a baseband signal; 
     an A/D converter converting the baseband signal to a digital signal; 
     a DBF circuit measuring a target angle based on the digital signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of an antenna device; 
         FIG. 2  is a top view of an antenna device; 
         FIG. 3  is a block diagram showing a radar apparatus; 
         FIG. 4  is a top view of a prototype of the radar apparatus; 
         FIG. 5  is a top view of an antenna device; 
         FIG. 6  is a top view of a prototype of the antenna device; 
         FIG. 7  is a top view of a subarray antenna with an alignment of the antenna elements; 
         FIG. 8  is a top view of a subarray antenna with another alignment of the antenna elements; 
         FIG. 9  is a top view of a subarray antenna with another alignment of the antenna elements; and 
         FIG. 10  is a top view of an antenna device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiment will be explained with reference to the accompanying drawings. 
     As shown in  FIG. 1 , an antenna device  100  includes subarray antennas  101  and feeding interfaces  104 . The subarray antennas  101  are set parallel to each other on a same plane. The subarray antennas  101  provide an array antenna. One side of each subarray antenna  101  is connected to the feeding interface  104  in order to feed a signal. Each subarray antenna  101  includes antenna elements  102  and feeding lines  103 . The antenna element  102  may be any one of a slot, horn, and patch antenna elements. The feeding line  103  feeds the signal to the antenna element  102 . The feeding line  103  may be a waveguide, a triplate line, a microstrip line, and a post-wall waveguide. 
     The distance of the between adjacent subarray antennas  101  (hereinafter, “subarray interval”) is shown as “d” in the  FIG. 1 . The subarray interval “d” is following the expression (1) in order to reduce a grating lobe level. In the expression (1), a free-space wavelength of operating frequency is “λ” and a maximum coverage angle is “θm”. 
     
       
         
           
             
               
                 
                   
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     According to the expression (1), the subarray interval “d” is smaller than the free-space wavelength of operating frequency. For example, the subarray interval “d” should be smaller than 0.6λ to achieve the coverage angle of 40 degrees. 
     The number of the subarray antennas  101  is “8” in  FIG. 1 . However, it is not limited. For example, it may be “15” in other case. 
     Also, the subarray antennas  101  are arranged symmetrically with a central axis which is a center line of the antenna device  100 . In  FIG. 1 , since the number of the subarray antennas  101  is even (shown as “2n”), the central axis is located in the middle of two adjacent subarray antennas  101  which are n th and (n+1) th. The subarray antennas  101  are arranged in the inter-digital structure. Therefore, the feeding interfaces  104  are located at different side of the subarray antennas  101  alternately, except for the n th and (n+1) th feeding interfaces  104 . The n th and (n+1) th feeding interfaces  104 , which are the closest to the central axis, are located at the same side of the n th and (n+1) th subarray antennas  101 . The n th and (n+1) th feeding interfaces  104  are shifted away from each other to avoid giving interference. The distance of the shift should be more than a value which is following as the expression (2). “w” is a width of the feeding interfaces  104 . 
     
       
         
           
             
               
                 
                   
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     In  FIG. 1 , the n th feeding interface  104  is shifted to leftward to be away from the central axis. Also, the (n+1) th feeding interface  104  is shifted to rightward. The n th and (n+1) th connection points “A” between the feeding interfaces  104  and the subarray antennas  101  are not in the middle of the width of the feeding interfaces  104  compared with the other connection points “B”. 
       FIG. 2  shows the antenna device  100  which the number of the subarray antennas is odd (shown as “2n+1”). The central axis is located at the (n+1) th subarray antenna  101 . The subarray antennas  101  are arranged in the inter-digital structure. Therefore, the n th feeding interface  104  is located at one side of the n th subarray antenna  101 . The (n+1) th feeding interface  104  is located at the opposite side of the (n+1) th subarray antenna  101 . 
     Hereinafter, we will explain a monopulse radar system. As shown in  FIG. 3 , the monopulse radar system  300  includes the antenna device  100 , an RF chip  302 , an A/D (Analog/Digital) converter  303 , and a DBF (Digital Beam Forming) circuit  304 . The antenna device  100  includes the subarray antennas  101   a ,  101   b ,  101   c ,  101   d . The number of the subarray antennas  101  is not limited to four. 
     Each subarray antenna  101  receives an analog signal. The antenna device  100  outputs the analog signals from the subarray antennas  101   a ,  101   b ,  101   c ,  101   d  to the RF chip  302 . The RF chip  302  amplifies the analog signals. Also, the RF chip  302  down-converts a frequency of each analog signal to a lower frequency. Then, the RF chip  302  outputs the analog signals to the A/D converter  303 . The A/D converter  303  converts the analog signals to digital signals. Then, the A/D converter  303  outputs the digital signals to the DBF circuit  304 . 
     The DBF circuit  304  measures the target angle by using the digital signals. First, the DBF circuit  304  combines all digital signals in same phase to obtain a sum signal. Next, the DBF circuit  304  combines two digital signals due to the subarray antennas  101   a  and  101   b  in same phase to obtain a first combine signal. Similarly, the DBF circuit  304  combines two digital signals due to the subarray antennas  101   c  and  101   d  in same phase to obtain a second combine signal. Then, the DBF circuit  304  combines the first and second combine signals in inverse phase to obtain a differential signal. At last, the DBF circuit  304  measures the target angle by the sum signal and the differential signal. Explain of the detail to measure the target angle is skipped because it is same as conventional methods. 
       FIG. 4  shows a prototype  400  of the antenna device  100 . The prototype  400  has four subarray antennas  101   a - 101   d , four feeding lines  401   a - 401   d , and a package  402 . The package  402  includes the RF chip  302 , the A/D converter  303 , and the DBF circuit  304 . The prototype  400  adopts post-wall waveguide slotted subarray antennas as the subarray antennas  101   a - 101   d . The detail of the post-wall waveguide slotted subarray antenna will be explained later. The subarray antennas  101   a - 101   d  are connected to the package  402  through the feeding lines  401   a - 401   d , respectively. Each subarray antenna  101   a - 101   d  receives a signal and inputs the signal into the package  402  through the feeding line  401   a - 401   d.    
     Even if a phase of the RF signal in the feeding line  401   a ,  401   b  is shifted by manufacturing tolerance, the phase shift for each feeding line appears symmetry because the prototype  400  has the symmetrical structure with the central axis. Therefore, the phase shifts of each feeding line are canceled out each other, when these four signals through the feeding line  401   a - 401   d  are combined in the package  402 . As a result, the prototype  400  keeps forming a beam (or a null) without tilt. 
     As described above, since the antenna device  100  has the inter-digital structure, it can achieve a small aperture area without giving interferences each other among the subarray antennas  101 . Moreover, since the antenna device  100  also has the symmetrical structure, the phase shifts of the signals due to manufacturing tolerance are canceled out each other among the subarray antennas  101 . Therefore, the measurement accuracy of the target angle does not degrade in the antenna device  100 . 
     Modified Example 1 
     Hereinafter, a modified example of an antenna device  100 ′ will be described.  FIG. 5  shows the antenna device  100 ′ which the number of the subarray antennas is even. 
     The antenna device  100 ′ includes the subarray antennas  101  and the feeding interfaces  104  as same as the antenna device  100 . While the n th and (n+1) th feeding interfaces  104 , which are the closest to the central axis, are shifted away from each other to avoid giving interference in the antenna device  100  of  FIG. 1 , they are located at both outside of the 1st and 2n th subarray antennas in the antenna device  100 ′ of  FIG. 5 . The n th and (n+1) th feeding lines  103  are extended longer than other feeding lines  103 . In the antenna device  101 ′, the n th and (n+1) th feeding lines  103  have bend structures to connect to the n th and (n+1) th feeding interfaces  104 , respectively. 
       FIG. 6  shows a prototype  600  of the antenna device  100 ′. The prototype  600  is same as the prototype  400 , except that the feeding lines  401   a - 401   d  and the package  402  are not shown. The prototype  600  includes a dielectric substrate  605  and four subarray antennas  101   a - 101   d . The dielectric substrate  605  has a layer which is made of a material such as liquid crystal polymer or Polytetrafluoroethylene (PTFE). Both top and under surfaces of the layer are covered by membranes of conductive metal. The prototype  600  adopts the post-wall waveguide slotted subarray antennas as the subarray antennas  101   a - 101   d . The subarray antenna  101   a - 101   d  includes through holes  601 , antenna elements  602 , feeding interfaces  603   a - 603   d , and matching pins  604 . The through hole  601  is a hole through the dielectric substrate  605 . The hole is filled with metal to connect electrically between the top and under surfaces. Many through holes  601  align in order to form a post-wall waveguide. The post-walls corresponds to a waveguide wall. The antenna element  602  is a slot which is formed by etching the top surface. In  FIG. 6 , the antenna element  602  is formed transverse to the aligned through holes  601 . The antenna element  602  may be formed longitudinal or 45-degree to the aligned through holes  601 . Moreover, the antenna elements  602  align at regular or unequally intervals in this embodiment. 
     The feeding interface  603   a - 603   d  is an aperture which is formed by etching the top surface. Each feeding interface  603   a - 603   d  is surrounded by many through holes  601 . The matching pin  604  provides matching impedance between subarray antennas  600   a - 600   d  and the feeding lines  101   a - 101   d  (not shown). The matching pin  604  may be the through hole  601 . The subarray antennas  101   b ,  101   c  are bent to be connected to the feeding interfaces  603   b ,  603   c , respectively. In  FIG. 6 , the subarray antennas  101   b ,  101   c  are bent with L-shaped. The subarray antennas  101   b ,  101   c  may be bent with U-shaped. Since the subarray antennas  101   b ,  101   c  are bent to outside of the subarray antennas  101   a ,  101   d , respectively, the feeding interfaces  603   b ,  603   c  do not give interferences each other. 
     According to the modified example 1, the antenna device  100 ′ keeps the symmetrical structure without giving interference each other among the feeding interfaces  104 . 
     Modified Example 2 
     Hereinafter, another modified example will be described. In the modified example 2, the subarray antenna  101  is any one of a waveguide slotted subarray antenna, a conductive waveguide slotted subarray antenna, a patch antenna with the triplate line, a patch antenna with the microstrip line, and a horn array antenna. In the modified example 2, we will describe variation of alignments of the antenna elements  102 . 
       FIGS. 7-9  show subarray antennas  701 - 901  which have different alignments of the antenna elements  102 . As shown in  FIG. 7 , each antenna element  102  may be located at an end of a sub feeding line  705  which is branched to one side from the feeding line  103 . As shown in  FIG. 8 , each antenna element  102  may be located at the end of a sub feeding line  805  which is branched to both sides from the feeding line  103 . Moreover, as shown in  FIG. 9 , the antenna elements  102  may be located at the end of a sub feeding line  905  branching T-shaped three times from the feeding lines  103 . One branch from the feeding lines  103  has eight antenna elements  102 . 
       FIG. 10  shows an antenna device  1000  using the subarray antennas  701 . Each subarray antenna  701   a - 701   d  does not have the symmetrical structure. However, the antenna device  1000  has the symmetrical structure by arranging the subarray antennas  701   a ,  701   b  pointing to the right and the subarray antennas  701   c ,  701   d  pointing to the left. Similarly, the subarray antennas  801  and  901  can realize the antenna device which has the symmetrical structure. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.