Patent Publication Number: US-11024973-B2

Title: Antenna structure

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 107141917, filed on Nov. 23, 2018. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND 
     Technology Field 
     The invention is related to an antenna structure, and particularly related to an antenna structure having a broadband and good return loss. 
     Description of Related Art 
     There are three metal layers in conventional coupled microstrip slot patch antennas. The middle metal layer is a ground plane. The upper metal layer is a patch antenna. The lower metal layer is a feed microstrip line. A dielectric plate is applied to separate the metal layers. A slot is configured on the middle metal layer, such that the microstrip line located below feeds input signals through the slot to feed the electric field to the patch antenna. 
     It is not easy to adjust the impedance matching for the conventional coupled microstrip slot patch antennas, the bandwidth of which is also limited by the modes excited by the size of the upper patch antenna. Therefore, the conventional patch antenna design has the disadvantage of narrow bandwidth. For example, the design of bandwidth of RFID frequency bands under American standard (0.902 GHz-0.928 GHz) fails to achieve the requirement of high return loss of 20 dB. 
     SUMMARY 
     The invention provides an antenna structure having a broadband and good return loss. 
     An antenna structure of the invention includes an antenna pattern, a ground layer, and two microstrip lines. The antenna pattern includes a first portion and a second portion. The first portion is a rectangular, which includes a first side, a second side, a third side and a fourth side connected in sequence. The second portion extends and protrudes outwardly from a corner formed by the first side and the second side. The ground layer is disposed below the antenna pattern and has two slots. Respective projections of the two slots projected onto the antenna pattern are close to the third side and the fourth side. The two microstrip lines are disposed below the ground layer. Respective projections of the two microstrip lines onto the antenna pattern are perpendicular to the third side and the fourth side, and Respective projections of the two microstrip lines onto the ground layer traverse the two slots. Each of the two microstrip lines has a first section and a second section in the extending direction. A projection of the second section onto the antenna pattern is closer to a center of the first portion than a projection of the first section to the antenna pattern, and the width of the first section is greater than the width of the second section. 
     In an embodiment of the invention, the antenna structure further includes a first circuit board and a second circuit board. The antenna pattern is disposed on a top surface of the first circuit board. The second circuit board is disposed below the first circuit board. The ground layer is disposed on a top surface of the second circuit board. The two microstrip lines are disposed on a bottom surface of the second circuit board. 
     In an embodiment of the invention, the antenna structure further includes a spacer, disposed between the first circuit board and the second circuit board. 
     In an embodiment of the invention, the antenna structure is suitable for resonating at a frequency band. A gap between the first circuit board and the second circuit board is 0.1 times a wavelength of the frequency band. 
     In an embodiment of the invention, the second portion is arranged in an L-shape. 
     In an embodiment of the invention, the length of the second portion protruding outwardly from the first side is between 0.05 times and 0.1 times the length of the fourth side, and the length of the second portion protruding outwardly from the second side is between 0.05 times and 0.1 times the length of the third side. 
     In an embodiment of the invention, the antenna structure is suitable for resonating at a frequency band. The length of each of the two microstrip lines is between 0.2 times and 0.3 times the frequency band. 
     In an embodiment of the invention, the width of the first section of each of the two microstrip lines is between 1.1 times and 2 times the width of the second section thereof. 
     In an embodiment of the invention, an extending direction of each of the slots is perpendicular to an extending direction of the corresponding microstrip line. 
     In an embodiment of the invention, the extending direction of one of the microstrip lines is perpendicular to the extending direction of the other one of the microstrip lines. 
     In view of the above, with the design that the width of the first section is greater than the width of the second section thereof, the antenna structure of the invention is able to adjust impedance matching. Matched with the antenna pattern of the antenna structure of the invention, the second portion extends and protrudes outwardly from the corner formed by the first side and the second side, such that the antenna structure of the invention is an antenna having a broadband and good return loss. 
     To make the aforementioned and other features and advantages of the invention more comprehensible, several embodiments accompanied with figures are described in detail as follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic top view of an antenna structure according to an embodiment of the invention. 
         FIG. 2  is a schematic cross-sectional view of an antenna structure of  FIG. 1 . 
         FIG. 3  is a schematic top view of a first circuit board of the antenna structure of  FIG. 1 . 
         FIG. 4  is a schematic top view of a second circuit board of the antenna structure of  FIG. 1 . 
         FIG. 5  is a schematic bottom view of a second circuit board of the antenna structure of  FIG. 1 . 
         FIG. 6  is a plot of frequency-return loss of the antenna structure of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic top view of an antenna structure according to an embodiment of the invention.  FIG. 2  is a schematic cross-sectional view of an antenna structure of  FIG. 1 . It should be noted that in  FIG. 1 , a ground layer  120  and a microstrip line  130  are both located below an antenna pattern  110 , and are thus illustrated in broken lines. Furthermore, the cross section of  FIG. 2  is a schematic cross-sectional view along the curved broken lines in  FIG. 1 . 
     Please refer to  FIG. 1  and  FIG. 2 . The antenna structure  100  is an example of a coupled microstrip slot dual-feeding patch antenna according to an embodiment of the present invention. However, the types of the antenna structure  100  shall not be limited thereto. In this embodiment, the antenna structure  100  has a broadband and high return loss, suitable for the applications of different kinds of RFID readers. The application frequency band of the antenna structure  100  is, for example, from 0.902 GHz to 0.928 GHz. Indeed, the application and the frequency band of the antenna structure  100  shall not be limited thereto. 
     It can be seen from  FIG. 2  that the antenna structure  100  of the embodiment includes, from top to the bottom, an antenna pattern  110 , a ground layer  120 , and two microstrip lines  130 .  FIG. 3  is a schematic top view of a first circuit board of the antenna structure  100  of  FIG. 1 . Please refer to  FIG. 1  and  FIG. 3  together. In this embodiment, the antenna pattern  110  is, for example, a patch antenna. As illustrated in  FIG. 1 , the antenna pattern  110  includes a first portion  112  and a second portion  118 . The first portion  112  has a rectangle shape, for example, a rectangle or a square. The first portion  112  has a rectangle shape, which includes a first side  113 , a second side  114 , a third side  115  and a fourth side  116  connected in sequence. The second portion  118  extends and protrudes outwardly from a corner formed by the first side  113  and the second side  114 . 
     The second portion  118  of the antenna pattern  110  is configured to allow the frequency band at which the first portion  112  resonates to slightly shift toward low frequency, such that the overall frequency band is widened. In this embodiment, the second portion  118  is arranged in an L-shape. Indeed, the shape of the second portion  118  shall not be limited thereto. In other embodiments, the second portion  118  may be in a ¾ circular shape, a serrated shape, or other irregular shapes. As illustrated in  FIG. 3 , in this embodiment, the length L 2  of the second portion  118  protruding outwardly from the first side  113  is between 0.05 times and 0.1 times the length L 1  of the fourth side  116 , and the length L 4  of the second portion  118  protruding outwardly from the second side  114  is between 0.05 times and 0.1 times the length L 3  of the third side  115 . Upon testing, the length relationship mentioned above allows the antenna pattern  110  to have better impedance matching. Indeed, the length relationship of the lengths L 1  and L 2  shall not be limited thereto. 
       FIG. 4  is a schematic top view of a second circuit board of the antenna structure of  FIG. 1 . Please refer to  FIG. 1  and  FIG. 4 . The ground layer  120  is disposed below the antenna pattern  110 . The ground layer  120  is a metal layer and has two slots  122 . It can be seen from  FIG. 1 , respective projections of the two slots  122  projected onto the antenna pattern  110  are close to the third side  115  and the fourth side  116 . 
     As illustrated in  FIG. 2 , the two microstrip lines  130  are disposed below the ground layer  120 .  FIG. 5  is a schematic bottom view of a second circuit board of the antenna structure of  FIG. 1 . Please refer to  FIG. 1  and  FIG. 5 . Respective projections of the two microstrip lines  130  projected onto the antenna pattern  110  are perpendicular to the third side  115  and the fourth side  116 , and respective projections of the two microstrip lines  130  onto the ground layer  120  traverse the two slots  122 . In this embodiment, each of the two microstrip lines  130  has a first section  132  and a second section  134  in the extending direction. A projection of the second section  134  onto the antenna pattern  110  is closer to a center of the first portion  112  than a projection of the first section  132  onto the antenna pattern  110 , and the width of the first section  132  is greater than the width of the second section  134 . 
     With the design that projections of the microstrip lines  130  onto the antenna pattern  110  traverse the positions of the third side  115  and the fourth side  116  and that the width of the first section  132  is greater than the width of the second section  134 , the antenna structure  100  of the invention is able to adjust impedance matching. The aforementioned design cooperates with the antenna pattern  110  to provide the antenna structure  110  with a broadband and high return loss though the second portion  118  extending and protruding outwardly from a corner formed by the first side  113  and the second side  114 . 
     In this embodiment, the extending direction of one of the microstrip lines  130  is perpendicular to the extending direction of the other microstrip line  130 , and the extending direction of each of the slots  122  is perpendicular to the extending direction of the corresponding microstrip line  130 . Indeed, in other embodiments, the extending directions of the two microstrip line  130  shall not be limited thereto. In addition, the relationship between the extending direction of each of the slots  122  and the extending direction of the corresponding microstrip line  130  shall not be limited thereto. 
     With reference back to  FIG. 2 , in this embodiment, the antenna structure  100  further includes a first circuit board  140 , a second circuit board  150 , and a spacer  160 . The antenna pattern  110  is disposed on a top surface  142  of the first circuit board  140 . The second circuit board  150  is disposed below the first circuit board  140 . The ground layer  120  is disposed on a top surface  152  of the second circuit board  150 . The two microstrip lines  130  are disposed on a bottom surface  154  of the second circuit board  150 . The spacer  160  is disposed between the first circuit board  140  and the second circuit board  150  to separate the first circuit board  140  and the second circuit board  150 , and to keep a certain distance between the antenna pattern  110  and the ground layer  120 . In this embodiment, the spacer  160  is, for example, a plastic post. However, the type of the spacer  160  shall not be limited thereto. 
     In this embodiment, the antenna structure  100  is suitable for resonating at a frequency (e.g., from 0.902 GHz to 0.928 GHz). A gap between the first circuit board  140  and the second circuit board  150  is 0.1 times a wavelength of the frequency band, which is about 5 mm to 10 mm. 
     It should be noted that, in other embodiments, the antenna structure  100  may be a single circuit board design. That is, the antenna pattern  110 , the ground layer  120 , and the two microstrip lines  130  are separately in different layers of the same circuit board. The antenna pattern  110  and the ground layer  120  are separated by two dielectric layers so are the ground layer  120  and the two microstrip lines  130 . The thickness of the dielectric layer between the antenna pattern  110  and the ground layer  120  may be about 0.1 times the wavelength of the frequency band at which the antenna structure  100  resonates. 
     Furthermore, it can be seen from  FIG. 5  that in this embodiment, the length of each of the two microstrip lines  130  is between 0.2 times and 0.3 times the wavelength of the frequency band, for example, 0.25 times the wavelength. In addition, the width of the first section  132  of each of the two microstrip lines  130  is between 1.1 times and 2 times the width of the second section  134 . Upon testing, when the microstrip lines  130  are in the aforementioned scopes, the antenna structure  100  has higher return loss. 
       FIG. 6  is a plot of frequency-return loss of the antenna structure of  FIG. 1 . Please refer to  FIG. 6 . In this embodiment, the antenna structure  100  is fed by the microstrip line  130  at the edge of the second circuit board  150 . Since there are two microstrip lines  130 , the antenna structure  100  has two feeding ports. The resonant mode obtained at the lower feeding port in  FIG. 1  (that is, from the lower microstrip line  130  at the edge of the second circuit board  150 ) is indicated by bold lines. The resonant mode obtained at the feeding port on the left in  FIG. 1  (i.e., from the left microstrip line  130  at the edge of the second circuit board  150 ) is indicated by a thin line. It can be seen from  FIG. 6  that the return loss of the resonant modes obtained at the two feeding ports in the frequency band from 0.902 GHz to 0.928 GHz is greater than or equal to 20 dB, and has good performance. 
     In summary of the above, with the design that the width of the first section is greater than the width of the second section, the antenna structure of the invention is able to adjust impedance matching. In the cooperation with the antenna pattern of the antenna structure of the invention the antenna structure of the invention achieves a broadband and high return loss through the second portion extending and protruding outwardly from the corner formed by the first side and the second side. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the invention covers modifications and variations of this disclosure provided that they fall within the scope of the following claims and their equivalents.