Patent Publication Number: US-11043749-B2

Title: Antenna structure

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 108116011, filed on May 9, 2019. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to an antenna structure, and in particular, to a wideband antenna structure. 
     Related Art 
     Currently, a millimeter-wave radar applied to the automotive market has good signal penetration and high distance detection accuracy due to high operating frequencies (77 GHz and 79 GHz), and is applicable to a long distance detection system, such as an automatic emergency braking (AEB) system, an adaptive cruise (ACC) system, a forward collision prevention (FCW) system, etc. However, currently, most millimeter-wave radar antennas are designed in a general series-fed antenna form, and therefore a bandwidth thereof is limited by about 2%. 
     SUMMARY 
     The present disclosure provides an antenna structure that may have a wideband characteristic. 
     The antenna structure of the present disclosure includes a ground plane and at least one series-fed antenna. Each series-fed antenna includes a first patch, a plurality of second patches, a first microstrip line, a first grounding structure group, a plurality of second microstrip lines, and a plurality of second grounding structure groups. The first patch is disposed beside the ground plane. The first patch is arranged between the ground plane and the second patches, and the first patch and the second patches are arranged along a straight line. The first microstrip line extends from the first patch in a direction away from the second patches and has a first end and a second end opposite to each other. The first end is a feeding point, and the second end is connected to the first patch. The first grounding structure group includes two first grounding traces. The two first grounding traces extend symmetrically from opposite sides of the first microstrip line to the ground plane. The second microstrip lines are respectively connected between the first patch and the second patch adjacent to the first patch and connected between the second patches. The second grounding structure groups are respectively disposed on both sides of the second microstrip lines, and are coupled to ground plane. 
     Based on the above, in an embodiment of the present disclosure, in the antenna structure, the two grounding traces are symmetrically disposed on the two opposite sides of the first microstrip line and extend to the ground plane, and the second grounding structure groups are respectively disposed on both sides of the second microstrip lines and are coupled to the ground plane. According to a simulation result in the embodiment, through the above design, a range of a frequency band coupled out by the antenna structure and an impedance bandwidth can be increased, so that the antenna structure has a good antenna characteristic. 
     To make the features and advantages of the present disclosure clear and easy to understand, the following gives a detailed description of embodiments with reference to accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic diagram of an antenna structure according to an embodiment of the present disclosure. 
         FIG. 1B  and  FIG. 1C  are respectively partial schematic enlarged views of an antenna structure in  FIG. 1A . 
         FIG. 2  is a schematic diagram of an antenna structure according to another embodiment of the present disclosure. 
         FIG. 3A  to  FIG. 3C  are radiation pattern diagrams corresponding to an antenna structure in  FIG. 2  at three frequency points of 77 GHz, 79 GHz, and 81 GHz. 
         FIG. 4  is a diagram of frequency-return loss relationships of an antenna structure in  FIG. 1A  and an antenna structure in  FIG. 2 . 
         FIG. 5  is a schematic diagram of an antenna structure according to another embodiment of the present disclosure. 
         FIG. 6  is a diagram of a frequency-return loss relationship of an antenna structure in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  is a schematic diagram of an antenna structure according to an embodiment of the present disclosure.  FIG. 1B  and  FIG. 1C  are respectively partial schematic enlarged views of an antenna structure in  FIG. 1A . Referring to  FIG. 1A  to  FIG. 1C , an antenna structure  10  in this embodiment includes a ground plane  130  and at least one series-fed antenna  100 . In this embodiment, that the antenna structure  10  has one series-fed antenna  100  is used as an example, but a number of the series-fed antennas  100  is not limited thereto. In this embodiment, the series-fed antenna  100  includes a first patch  114 , a plurality of second patches  115  and  116 , a first microstrip line  111 , a first grounding structure group (two first grounding traces  113 ), a plurality of second microstrip lines  112 , and a plurality of second grounding structure groups (two second grounding traces  122  and  124 ). 
     As shown in  FIG. 1A , in this embodiment, the first patch  114  is disposed beside the ground plane  130 . The first patch  114  is arranged between the ground plane  130  and the second patches  115  and  116 , and in particular, the first patch  114  is arranged between the ground plane  130  and the second patch adjacent to the first patch  114  (i.e., the second patch  115 ). The first patch  114  and the second patches  115  and  116  are arranged along one straight line. In this embodiment, there are two second patches  115  and  116 , but a number of the second patches  115  and  116  is not limited thereto. 
     In this embodiment, an area of the first patch  114  and areas of the second patches  115  and  116  increase and then decrease along a direction (a direction A 1 ) in which the straight line extends. The area of the first patch  114  is the same as an area of the second patch  116  far away from the first patch  114  and less than an area of the second patch  115  adjacent to the first patch  114 . In other words, the series-fed antenna  100  is a patch antenna assembled in a tapered manner. Definitely, in other embodiments, the area of the first patch  114  may be the same as the area of each of the second patches  115  and  116 . An area relationship between the first patch  114  and the second patches  115  and  116  is not limited thereto. 
     In addition, in this embodiment, the first patch  114  and each of the second patches  115  and  116  are rectangular. One side length (for example, a side length along the direction A 1 ) of any of the first patch  114  and the second patches  115  and  116  is between 0.9 millimeters and 1.05 millimeters, and another side length (for example, a side length along a direction A 2 ) is between 0.7 millimeters and 1.6 millimeters. Definitely, a relationship between dimensions of the first patch  114  and the second patches  115  and  116  is not limited thereto. 
     The first microstrip line  111  extends from the first patch  114  in a direction away from the second patches  115  and  116 . More specifically, as shown in  FIG. 1B , the first microstrip line  111  has a first end A and a second end C opposite to each other. The first end A is a feeding point, and the second end C is connected to the first patch  114 . There is a distance between the first end A of the first microstrip line  111  and the ground plane  130  without contacting the ground plane  130 . In this embodiment, the antenna structure  10  is adapted to couple out a frequency band ranging from about 77 GHz to 81 GHz, but the range of the frequency band is not limited thereto. A length of the first microstrip line  111  (that is, a distance between the first end A and the second end C) is between 0.39 times and 0.42 times a wavelength of the frequency band. 
     As shown in  FIG. 1B , in this embodiment, the first grounding structure group includes two first grounding traces  113  that extend symmetrically from two opposite sides of the first microstrip line  111  to the ground plane  130 . In each series-fed antenna  100 , a length of the first grounding trace  113  is between 0.22 times and 0.28 times the wavelength of the frequency band, for example, 0.25 times the wavelength. 
     In this embodiment, the first grounding trace  113  includes a first segment (that is, a line segment B 1 B 2 ) and a second segment (that is, a line segment B 2 B 3 ) connected in a bent manner. The first segment (the line segment B 1 B 2 ) extends vertically from the first microstrip line  111 , and the second segment (the line segment B 2 B 3 ) is parallel to the first microstrip line  111  and connected to the ground plane  130 . A distance L 1  between the first segment (the line segment B 1 B 2 ) and the ground plane  130  is between 0.2 millimeters and 0.4 millimeters. It is worth mentioning that after simulation, when the distance L 1  between the first segment (the line segment B 1 B 2 ) and the ground plane  130  is gradually changed from 0.2 millimeters to 0.3 millimeters and 0.4 millimeters, a Smith chart of the antenna structure  10  has a clockwise rotation characteristic. When the distance L 1  between the first segment (the line segment B 1 B 2 ) and the ground plane  130  is 0.3 millimeters, a frequency band of the first grounding trace  113  may range from 77 GHz to 81 GHz, and therefore has good performance. 
     In addition, when the first segment (the line segment B 1 B 2 ) or the second segment (the line segment B 2 B 3 ) of the first grounding trace  113  widens outward, for example, the line segment B 1 B 2  of the first grounding trace  113  is thickened rightward by 0.1 millimeters, 0.2 millimeters, and 0.3 millimeters, the upper line segment B 2 B 3  is thickened upward by 0.1 millimeters, 0.2 millimeters, and 0.3 millimeters, and the lower line segment B 2 B 3  is thickened downward by 0.1 millimeters, 0.2 millimeters, and 0.3 millimeters, the Smith chart of the antenna structure  10  has a clockwise rotation characteristic. When the second segment (the line segment B 2 B 3 ) of the first grounding trace  113  is widened inward, for example, the upper line segment B 2 B 3  of the first grounding trace  113  is thickened downward by 0.1 millimeters, 0.15 millimeters, and 0.2 millimeters, and the lower line segment B 2 B 3  is thickened upward by 0.1 millimeters, 0.15 millimeters, and 0.2 millimeters, the Smith chart of the antenna structure  10  has a counterclockwise rotation characteristic. A designer may adjust a dimension of the first grounding trace  113  according to the above characteristics to obtain good antenna performance. 
     In addition, in this embodiment, a distance W 1  between the second segment (the line segment B 2 B 3 ) and the first microstrip line  111  is between 0.2 millimeters and 0.25 millimeters. It is worth mentioning that after simulation, the distance W 1  between the second segment (the line segment B 2 B 3 ) and the first microstrip line  111  is gradually changed from 0.2 millimeters to 0.23 millimeters, and 0.25 millimeters. Therefore, the Smith chart of the first grounding trace  113  has a clockwise rotation characteristic. When the distance W 1  between the second segment (the line segment B 2 B 3 ) and the first microstrip line  111  is 0.2 millimeters, an impedance matching effect at 77 GHz to 79 GHz is better. When the distance W 1  between the second segment (the line segment B 2 B 3 ) and the first microstrip line  111  is 0.25 millimeters, an impedance matching effect at 79 GHz to 81 GHz is better. When the distance W 1  between the second segment (the line segment B 2 B 3 ) and the first microstrip line  111  is 0.23 millimeters, the first grounding trace  113  may have a frequency ranging from 77 GHz to 81 GHz, and therefore has wideband performance. Definitely, the distances L 1  and W 1  are not limited thereto. 
     Returning back to  FIG. 1A , in this embodiment, there are two second microstrip lines  112  corresponding to the two second patches  115  and  116 . However, a number of the second microstrip lines  112  is not limited thereto. The second microstrip lines  112  are respectively connected between the first patch  114  and the second patch  115  adjacent to the first patch  114  and connected between the second patches  115  and  116 . In addition, in this embodiment, the second microstrip lines  112  have a same length. However, in other embodiments, the second microstrip lines  112  may have different lengths. 
     In addition, in this embodiment, there are two second grounding structure groups corresponding to the two second microstrip lines  112 , but a number of the second grounding structure groups is not limited thereto. The two second grounding structure groups are respectively disposed on both sides of the two second microstrip lines  112 . Each of the second grounding structure groups includes two second grounding traces  122  and  124  symmetrically arranged on two opposite sides of the corresponding second microstrip line  112  and are respectively connected to the ground plane  130 . The second grounding traces  122  and  124  are, for example, connected to a ground terminal located on a back surface of a substrate through a through hole, and are coupled to the ground plane  130 . 
     As shown in  FIG. 1C , in this embodiment, in each of the grounding structure groups, each of the second grounding traces  122  and  124  includes a first end  123  and  125  and a second end  126  and  127  respectively. In each of the grounding structure groups, the first end  123  and the second end  126  of the second grounding trace  122  respectively correspond to the second end  127  and the first end  125  of the second grounding trace  124 , and the two first ends  123  and  125  are coupled to the ground plane to serve as two grounding terminals. In other words, the first end  123  of the second grounding trace  122  and the first end  125  of the second grounding trace  124  are respectively close to two opposite ends of the corresponding second microstrip line  112 . In the design of grounding on the opposite sides, the Smith chart may be slightly smaller and an impedance bandwidth may be increased. Definitely, in other embodiments, relative positions of the first end  123  of the second grounding trace  122  and the first end  125  of the second grounding trace  124  are not limited thereto. 
     In addition, in this embodiment, a length of the second grounding traces  122  and  124  (that is, a distances between positions D 1  and D 2  in  FIG. 1C ) is between 0.2 times and 0.3 times the wavelength of the frequency band. For example, lengths of the second grounding traces  122  and  124  are between 0.65 millimeters and 0.85 millimeters, and widths of the second grounding traces  122  and  124  are between 0.08 millimeters and 0.12 millimeters. Definitely, the lengths and the widths of the second grounding traces  122  and  124  are not limited thereto. When the length (a line segment D 1 D 2 ) of the second grounding traces  122  and  124  is gradually changed from 0.577 millimeters to 0.677 millimeters and 0.777 millimeters, it may be learned from the Smith chart that an impedance circle becomes larger and a frequency tends to be low. In this embodiment, when the lengths (the line segment D 1 D 2 ) of the second grounding traces  122  and  124  are 0.777 millimeters, the second grounding traces  122  and  124  may have a frequency band ranging from 77 GHz to 81 GHz, and therefore have a relatively large impedance bandwidth. 
     In addition, in this embodiment, a distance G 1  between the second microstrip line  112  and the second grounding traces  122 , which is the same as the distance between the second microstrip line  112  and the second grounding traces  124 , is between 0.08 millimeters and 0.12 millimeters, for example, is 0.1 millimeters, but the distance G 1  is not limited thereto. 
     In this embodiment, in the antenna structure  100 , the two first grounding traces  113  are symmetrically disposed on the two opposite sides of the first microstrip line  111  and extend to the ground plane  130 , and the two second grounding traces  122  and  124  are symmetrically disposed on two opposite sides of the second microstrip line  112  and grounded in different directions respectively. According to a simulation result in the embodiment, through the above design, a range of a frequency band coupled out by the antenna structure  10  and an impedance bandwidth can be increased, so that the antenna structure  10  has a good antenna characteristic. 
       FIG. 2  is a schematic diagram of an antenna structure according to another embodiment of the present disclosure. Referring to  FIG. 2 , a main difference between an antenna structure  10   a  in  FIG. 2  and the antenna structure  10  in  FIG. 1A  is that in this embodiment, a series-fed antenna  100   a  includes second patches  115 ,  116 ,  117 , and  118 . In other words, there are four second patches  115 ,  116 ,  117 , and  118 . There are four second microstrip lines  112 , and there are four second grounding structure groups. 
     In this embodiment, an area of the first patch  114  and areas of the second patches  115 ,  116 ,  117 , and  118  increase and then decrease along a direction (a direction A 1 ) in which the straight line extends. More specifically, the second patch  116  at a central position has a largest area, the second patch  115  and the second patch  117  have second largest areas, and the first patch  114  and the second patch  118  have smallest areas. In this embodiment, the area of the first patch  114  is the same as the area of the second patch  118  farthest away from the first patch  114 , the area of the second patch  115  is the same as the area of the second patch  117 , and the area of first patch  114  is a half of the area of the second patch  116  at the central position. 
     In particular, in this embodiment, a dimension of the antenna structure  10   a  is 9.65 millimeters×1.57 millimeters×0.102 millimeters (which is a thickness of a substrate). A side length of the first patch  114  along the direction A 1  is, for example, 0.96 millimeters, which is 0.416 times the wavelength of the frequency band coupled out by the antenna structure  10   a . The side length of the first patch  114  along the direction A 2  is, for example, 0.785 millimeters. A length of the first microstrip line  111  is 0.955 millimeters, which is 0.41 times the wavelength of the frequency band (77 GHz to 81 GHz) coupled out by the antenna structure  10   a . A width of the first microstrip line  111  is 0.1 millimeters. 
     Side lengths of the second patches  115 ,  116 ,  117 , and  118  along the direction A 1  are, for example, 0.96 millimeters, which is 0.416 times the wavelength of the frequency band coupled out by the antenna structure  10   a . The side lengths of the second patches  115 ,  116 ,  117 , and  118  along the direction A 2  are, for example, 1.24 millimeters, 1.57 millimeters, 1.24 millimeters, and 0.785 millimeters. A length of the second microstrip line  112  is 0.95 millimeters, which is 0.39 times the wavelength of the frequency band coupled out by the antenna structure  10   a . A width of the second microstrip line  112  is 0.1 millimeters. Lengths of the second grounding traces  122  and  124  are about 0.777 millimeters and widths of the second grounding traces  122  and  124  are about 0.1 millimeters. 
     In this embodiment, through the first grounding structure group, a bandwidth of a frequency band coupled out by the antenna structure  10   a  can be increased to 4.82%. In this embodiment, through the second grounding structure group, the bandwidth of the frequency band coupled out by the antenna structure  10   a  can be increased to 5.06%. The antenna structure  10   a  can have a maximum gain from 11.09 dBi to 12.4 dBi at the frequency band of 77 GHz to 81 GHz. 
       FIG. 3A  to  FIG. 3C  are radiation pattern diagrams corresponding to an antenna structure in  FIG. 2  at different frequency points of 77 GHz, 79 GHz, and 81 GHz. Referring to  FIG. 3A  to  FIG. 3C , in this embodiment, maximum values of the antenna structure  10   a  in  FIG. 2  in a field pattern in which ψ is 0° and in a field pattern in which ψ is 90° are both at a position of zero degrees on a Z axis, so that a mainlobe is more likely to aim at the zero degrees on the Z axis. In such a design, a sidelobe is about 10 dB lower than the mainlobe, so that a characteristic of the sidelobe is suppressed. Therefore, performance is good. 
       FIG. 4  is a diagram of frequency-return loss relationships of an antenna structure in  FIG. 1A  and an antenna structure in  FIG. 2 . Referring to  FIG. 4 , the antenna structure  10  in  FIG. 1A  and the antenna structure  10   a  in  FIG. 2  both have a resonance frequency band at 77 GHz to 79 GHz, and a return loss at the frequency band from 77 GHz to 81 GHz can be less than −10 dB. Therefore, performance is good. The antenna structure  10   a  in  FIG. 2  has two valleys in the resonance frequency band at 77 GHz to 79 GHz, and a junction of the two valleys is 79 GHz. A current return loss can be increased to 11.6 dB, and the bandwidth can be synchronously increased to 5.06%. 
       FIG. 5  is a schematic diagram of an antenna structure according to another embodiment of the present disclosure. In particular, a multi-antenna arrangement structure is shown. Referring to  FIG. 5 , in this embodiment, an antenna structure  10   b  includes a plurality of series-fed antennas  100   a  disposed beside the ground plane  130  side by side. The series-fed antenna  100   a  is the series-fed antenna  100   a  in  FIG. 2  as an example. The series-fed antenna  100   a  has four second patches  115 ,  116 ,  117 , and  118 . However, in other embodiments, a number of the second patches of the series-fed antenna  100   a  is not limited thereto. In addition, in this embodiment, for example, there are three series-fed antennas  100   a , but a number of the series-fed antennas  100   a  is not limited thereto. 
     As shown in  FIG. 5 , in this embodiment, a distance G 2  between two feeding points of two adjacent ones of the series-fed antennas  100   a  is between 1.7 millimeters and 2.1 millimeters, for example, 1.9 millimeters. In addition, a minimum distance G 3  between two adjacent ones of the series-fed antennas  100   a  is between 0.29 millimeters and 0.37 millimeters, for example, 0.33 millimeters. In this embodiment, when the series-fed antennas  100   a  are disposed at a transmitter end or a receiver end, the minimum distance G 3  in the range can meet all antenna characteristics of each of the series-fed antennas  100   a.    
       FIG. 6  is a diagram of a frequency-return loss relationship of an antenna structure in  FIG. 5 . Referring to  FIG. 6 , in this embodiment, if an uppermost series-fed antenna  100   a  in  FIG. 5  is used as a first series-fed antenna  100   a , a central series-fed antenna  100   a  is used as a second series-fed antenna  100   a , and a lowermost series-fed antenna  100   a  is used as a third series-fed antenna  100   a , it may be learned from  FIG. 6  that return losses S 11 , S 22 , and S 33  of the three series-fed antennas  100   a  at the frequency band from 77 GHz to 81 GHz are all less than −10 dB. Therefore, performance is good. In addition, isolations S 21 , S 32 , and S 31  between two adjacent series-fed antennas  100   a  can be below −17.9 dB, and therefore the isolation is good. 
     In summary, in an embodiment of the present disclosure, in the antenna structure, the two first grounding traces are symmetrically disposed on the two opposite sides of the first microstrip line and extend to the ground plane, and the two second grounding traces are symmetrically disposed on two opposite sides of the second microstrip line and grounded in different directions respectively. After test, through the above design, a range of a frequency band coupled out by the antenna structure and an impedance bandwidth can be increased, so that the antenna structure has a good antenna characteristic. 
     Although the present disclosure is described with reference to the above embodiments, the embodiments are not intended to limit the present disclosure. A person of ordinary skill in the art may make variations and modifications without departing from the spirit and scope of the present disclosure. Therefore, the protection scope of the present disclosure should be subject to the appended claims.