Patent Publication Number: US-9837724-B2

Title: Antenna system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority of Taiwan Patent Application No. 105106087 filed on Mar. 1, 2016, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The disclosure generally relates to an antenna system, and more particularly to a high-gain, multiband, and dual-polarized antenna system. 
     Description of the Related Art 
     With advancement in mobile communication technology, mobile devices such as portable computers, mobile phones, multimedia players, and other hybrid functional portable electronic devices have become more common. To satisfy consumer demand, mobile devices can usually perform wireless communication functions. Some devices cover a large wireless communication area; these include mobile phones using 2G, 3G, and LTE (Long Term Evolution) systems and using frequency bands of 700 MHz, 850 MHz, 900 MHz, 1800 MHz, 1900 MHz, 2100 MHz, 2300 MHz, and 2500 MHz. Some devices cover a small wireless communication area; these include mobile phones using Wi-Fi and Bluetooth systems and using frequency bands of 2.4 GHz, 5.2 GHz, and 5.8 GHz. 
     Wireless access points are indispensable elements for mobile devices in the room to connect to the Internet at a high speed. However, since indoor environments have serious signal reflection and multipath fading, wireless access points should process signals in a variety of polarization directions and from a variety of transmission directions simultaneously. Accordingly, it has become a critical challenge for antenna designers to design a high-gain, multiband, and dual-polarized antenna in the limited space of wireless access points. 
     BRIEF SUMMARY OF THE INVENTION 
     In an embodiment, the disclosure is directed to an antenna system including a dual-polarized antenna, a main reflector, and an auxiliary reflector. The dual-polarized antenna includes a first antenna element and a second antenna element. The first antenna element and the second antenna element operate in a low-frequency band and a high-frequency band. The first antenna element and the second antenna element have different polarization directions. The main reflector is configured to reflect the electromagnetic waves in the low-frequency band. The auxiliary reflector is positioned between the dual-polarized antenna and the main reflector, and is configured to reflect the electromagnetic waves in the high-frequency band. 
     In some embodiments, the first antenna element has a first polarization direction, and the second antenna element has a second polarization direction. The second polarization direction is perpendicular to the first polarization direction. 
     In some embodiments, the first antenna element is disposed on a first dielectric substrate, and the second antenna element is disposed on a second dielectric substrate. The second dielectric substrate is perpendicular to the first dielectric substrate. 
     In some embodiments, the main reflector is a box without a lid, and a top opening of the box faces the dual-polarized antenna. 
     In some embodiments, the auxiliary reflector is a plane. 
     In some embodiments, the electromagnetic waves in the low-frequency band are capable of penetrating the auxiliary reflector. 
     In some embodiments, the first antenna element and the second antenna element are dipole antenna elements or bowtie antenna elements. 
     In some embodiments, each of the first antenna element and the second antenna element includes a pair of first radiation elements, a pair of second radiation elements, and a pair of third radiation elements. The second radiation elements are disposed between the first radiation elements and the third radiation elements. 
     In some embodiments, the first radiation elements and the second radiation elements are excited to generate electromagnetic wave in the low-frequency band, and the third radiation elements are excited to generate electromagnetic wave in the high-frequency band. 
     In some embodiments, each of the first antenna element and the second antenna element further includes a pair of reflector elements for reflecting the electromagnetic waves in the high-frequency band. The reflector elements are disposed between the third radiation elements and the auxiliary reflector. 
     In some embodiments, each of the first antenna element and the second antenna element further includes a pair of director elements for directing the electromagnetic waves in the high-frequency band to transmit outwardly. The first radiation elements are disposed between the director elements and the second radiation elements. 
     In some embodiments, each of the first antenna element and the second antenna element further includes a signal source and a coaxial cable. 
     In some embodiments, the coaxial cable includes a conductive housing, and the conductive housing is soldered to the main reflector. 
     In some embodiments, the auxiliary reflector has an opening. The coaxial cable extends through the opening and does not directly touch the auxiliary reflector. 
     In some embodiments, each of the first antenna element and the second antenna element further includes a choke element. The choke element is applied to the coaxial cable. 
     In some embodiments, the choke element is a low-pass filter. 
     In some embodiments, the choke element is a hollow cylindrical tube which surrounds the coaxial cable. 
     In some embodiments, the hollow cylindrical tube has an open end and a closed end. The open end of the hollow cylindrical tube does not directly touch the coaxial cable. The closed end of the hollow cylindrical tube is soldered to the conductive housing of the coaxial cable. 
     In some embodiments, a length of the hollow cylindrical tube is shorter than 0.25 wavelength of the high-frequency band. 
     In some embodiments, the choke element is an L-shaped element. The L-shaped element has a connection end and an open end. The connection end of the L-shaped element is soldered to the conductive housing of the coaxial cable. The open end of the L-shaped element does not directly touch the coaxial cable. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1A  is a perspective view of an antenna system according to an embodiment of the invention; 
         FIG. 1B  is a side view of an antenna system according to an embodiment of the invention; 
         FIG. 1C  is a perspective view of an antenna system according to an embodiment of the invention; 
         FIG. 2A  is a partial perspective view below an auxiliary reflector of an antenna system according to an embodiment of the invention; 
         FIG. 2B  is a combined view of a choke element according to an embodiment of the invention; 
         FIG. 2C  is an exploded view of a choke element according to an embodiment of the invention; 
         FIG. 3  is a combined view of a choke element according to another embodiment of the invention; 
         FIG. 4A  is an S-parameter diagram of an antenna system operating in a low-frequency band, according to an embodiment of the invention; 
         FIG. 4B  is an S-parameter diagram of an antenna system operating in a high-frequency band, according to an embodiment of the invention; and 
         FIG. 5  is a perspective view of an antenna system according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to illustrate the purposes, features and advantages of the invention, the embodiments and figures of the invention are shown in detail as follows. 
       FIG. 1A  is a perspective view of an antenna system  100  according to an embodiment of the invention.  FIG. 1B  is a side view of the antenna system  100  according to an embodiment of the invention.  FIG. 1C  is a perspective view of the antenna system  100  according to an embodiment of the invention. Please refer to  FIG. 1A ,  FIG. 1B , and  FIG. 1C  together. The antenna system  100  can be applied in a wireless access point, and it can generate a dual-polarized radiation pattern. As shown in  FIG. 1A ,  FIG. 1B , and  FIG. 1C , the antenna system  100  at least includes a dual-polarized antenna  110 , a main reflector  140 , and an auxiliary reflector  150 . The aforementioned dual-polarized antenna  110 , main reflector  140 , and auxiliary reflector  150  are made of conductive materials, such as copper, silver, aluminum, iron, or their alloys. 
     The dual-polarized antenna  110  includes a first antenna element  120  and a second antenna element  130 . The first antenna element  120  is disposed on a first dielectric substrate  160 . The second antenna element  130  is disposed on a second dielectric substrate  165 . The second dielectric substrate  165  is perpendicular to the first dielectric substrate  160 . Each of the first dielectric substrate  160  and the second dielectric substrate  165  may be an FR4 (Flame Retardant 4) substrate. In some embodiments, each of the first dielectric substrate  160  and the second dielectric substrate  165  substantially has an inverted T-shape, and the two inverted T-shapes are combined with each other. The first antenna element  120  and the second antenna element  130  are multiband, and they operate in at least a low-frequency band and a high-frequency band. For example, the aforementioned low-frequency band may include LTE (Long Term Evolution) Band 5/13 from 746 MHz to 894 MHz, and the aforementioned high-frequency band may include LTE Band 2/4 from 1710 MHz to 2155 MHz. The first antenna element  120  and the second antenna element  130  have different polarization directions. In some embodiments, the first antenna element  120  has a first polarization direction (e.g., the +45-degree direction), and the second antenna element  130  has a second polarization direction (e.g., the +135-degree direction). The second polarization direction is perpendicular to the first polarization direction. The dual-polarized antenna  110  is configured to transmit and receive the signals in different polarization directions. 
     The main reflector  140  may be a box without a lid, and a top opening of the box may face the dual-polarized antenna  110 . Specifically, each side wall of the main reflector  140  may have a triangular concave notch, and the main reflector  140  may have an inverted pyramid structure. For example, the total area of the top opening of the main reflector  140  may be larger than the total area of the bottom plate of the main reflector  140 . The main reflector  140  is configured to reflect the electromagnetic waves in the low-frequency band. The auxiliary reflector  150  is a plane, which may be completely inside the top opening of the main reflector  140 . The auxiliary reflector  150  is disposed between the dual-polarized antenna  110  and the main reflector  140 , and is configured to reflect the electromagnetic waves in the high-frequency band. Ideally, the electromagnetic waves in the low-frequency band can penetrate the auxiliary reflector  150 , but they are completely reflected by the main reflector  140 ; on the other hand, the electromagnetic waves in the high-frequency band cannot penetrate the auxiliary reflector  150 , and they are completely reflected by the auxiliary reflector  150 . Both the main reflector  140  and the auxiliary reflector  150  are configured to enhance the antenna gain of the dual-polarized antenna  110 . Since the dual-polarized antenna  110  has a relatively wide operation bandwidth, the invention proposes the main reflector  140  and the auxiliary reflector  150  which correspond to the low-frequency band and the high-frequency band of the dual-polarized antenna  110 , respectively. As a result, the electromagnetic waves over the whole wide operation bandwidth of the dual-polarized antenna  110  can be completely reflected. 
     In some embodiments, the first antenna element  120  and the second antenna element  130  are dipole antenna elements or bowtie antenna elements. The first antenna element  120  and the second antenna element  130  have identical structures. The only difference is that the second antenna element  130  is considered as a duplicate of the first antenna element  120 , which is rotated by 90 degrees with respect to its central axis. Thus, the following embodiments and figures are merely arranged to describe the structure of the first antenna element  120 . 
     The first antenna element  120  includes a pair of first radiation elements  121 , a pair of second radiation elements  122 , and a pair of third radiation elements  123 . The second radiation elements  122  are disposed between the first radiation elements  121  and the third radiation elements  123 . Each of the first radiation elements  121 , the second radiation elements  122 , and the third radiation elements  123  may have a straight-line shape or a triangular shape. In some embodiments, the length of each first radiation element  121  is slightly longer than the length of each second radiation element  122 . In some embodiments, the length of each first radiation element  121  is at least two times the length of each third radiation element  123 . The first radiation elements  121  and the second radiation elements  122  can be excited to generate electromagnetic wave in the aforementioned low-frequency band. The third radiation elements  123  can be excited to generate electromagnetic wave in the aforementioned high-frequency band. The first antenna element  120  may further include a pair of reflector elements  124  for reflecting the electromagnetic waves in the high-frequency band. The reflector elements  124  are disposed between the third radiation elements  123  and the auxiliary reflector  150 . Each of the reflector elements  124  may substantially have a straight-line shape. In some embodiments, the length of each reflector element  124  may be slightly longer than the length of each third radiation element  123 , and the two reflector elements  124  are floating and not connected to each other. The first antenna element  120  may further include a pair of director elements  125  for directing the electromagnetic waves in the high-frequency band to transmit outwardly. The director elements  125  are positioned at one side of the first radiation elements  121 , such that the first radiation elements  121  are disposed between the director elements  125  and the second radiation elements  122 . Each of the director elements  125  may substantially have a straight-line shape. In some embodiments, the length of each director element  125  may be slightly shorter than the length of each third radiation element  123 , and the two director elements  125  are floating and connected to each other. The reflector elements  124  and the director elements  125  are optional, and they are configured to enhance the high-frequency antenna gain of the dual-polarized antenna  110 . 
       FIG. 2A  is a partial perspective view below the auxiliary reflector  150  of the antenna system  100  according to an embodiment of the invention. In the embodiment of  FIG. 2A , the first antenna element  120  further includes a signal source  128 , a coaxial cable  127 , and a choke element  170 . The signal source  128  may be an RF (Radio Frequency) module, and it can generate an RF signal or process the received RF signal. The signal source  128  is coupled through the coaxial cable  127  to the first antenna element  120 . The coaxial cable  127  includes a central conductive line (signal line) and a conductive housing (ground line). The conductive housing of the coaxial cable  127  is soldered to the main reflector  140 . The auxiliary reflector  150  has an opening  155 . The opening  155  may have a circular shape, a rectangular shape, or a square shape. The central conductive line and the conductive housing of the coaxial cable  127  extend through the opening  155  and do not directly touch the auxiliary reflector  150 . Ideally, the electromagnetic waves in high-frequency band cannot penetrate the auxiliary reflector  150 ; however, according to the simulation result of the electromagnetic simulation software, there are still partial electromagnetic waves penetrating the auxiliary reflector  150  in the frequency interval from 1710 MHz to 1755 MHz, and it degrades the radiation performance of the antenna system  100 . To solve this problem, the choke element  170  is newly added and applied to the coaxial cable  127 . The choke element  170  is considered as a low-pass filter for preventing the electromagnetic waves in the high-frequency band from penetrating the auxiliary reflector  150 . In some embodiments, the choke element  170  is positioned between the main reflector  140  and the auxiliary reflector  150 . In alternative embodiments, the position of the choke element  170  is moved slightly forward or backward, without affecting its performance. 
       FIG. 2B  is a combined view of the choke element  170  according to an embodiment of the invention.  FIG. 2C  is an exploded view of the choke element  170  according to an embodiment of the invention. Please refer to  FIG. 2B  and  FIG. 2C  together. The choke element  170  is a hollow cylindrical tube which surrounds the coaxial cable  127 . Specifically, the hollow cylindrical tube has an open end  171  and a closed end  172 . The open end  171  of the hollow cylindrical tube does not directly touch the coaxial cable  127 . The closed end  172  of the hollow cylindrical tube is soldered to the conductive housing of the coaxial cable  127 . The length L 1  of the hollow cylindrical tube is shorter than 0.25 wavelength of the aforementioned high-frequency band, so as to form a low-pass filter with inductive characteristics. A gap G 1  between the hollow cylindrical tube and the conductive housing of the coaxial cable  127  is used to adjust the impedance value of the choke element  170 . For example, if the gap G 1  between the hollow cylindrical tube and the conductive housing of the coaxial cable  127  becomes wider, the impedance value of the choke element  170  will decrease, and conversely, if the gap G 1  between the hollow cylindrical tube and the conductive housing of the coaxial cable  127  becomes narrower, the impedance value of the choke element  170  will increase. 
       FIG. 3  is a combined view of a choke element  180  according to another embodiment of the invention. The choke element  180  is an L-shaped element, and it can be applied to the coaxial cable  127 . Specifically, the L-shaped element has a connection end  181  and an open end  182 . The connection end  181  of the L-shaped element is soldered to the conductive housing of the coaxial cable  127 . The open end  182  of the L-shaped element extends parallel to the coaxial cable  127 , and does not directly touch the coaxial cable  127 . The length L 2  of the L-shaped element is shorter than 0.25 wavelength of the aforementioned high-frequency band, so as to form a low-pass filter with inductive characteristics. According to the simulation result of electromagnetic simulation software, the function of the L-shaped choke element  180  is similar to the function of the aforementioned choke element  170 . 
     In some embodiments, the element sizes of the antenna system  100  are as follows. The length of each first radiation element  121  is approximately equal to 0.25 wavelength of the aforementioned low-frequency band (e.g., from 50 mm to 60 mm, and can be 57.2 mm). The length of each second radiation element  122  is approximately equal to 0.25 wavelength of the aforementioned low-frequency band (e.g., from 50 mm to 60 mm, and can be 52.5 mm). The length of each third radiation element  123  is approximately equal to 0.25 wavelength of the aforementioned high-frequency band (e.g., from 20 mm to 40 mm, and can be 24 mm). The distance D 1  between the auxiliary reflector  150  and the main reflector  140  is from 50 mm to 60 mm, and can be 59 mm. The distance D 2  between the reflector elements  124  and the auxiliary reflector  150  is from 20 mm to 30 mm, and can be 24.5 mm. The distance D 3  between the director elements  125  and the first radiation elements  121  is from 10 mm to 20 mm, and can be 16 mm. The distance D 4  between the third radiation elements  123  and the auxiliary reflector  150  is approximately equal to 0.25 wavelength of the aforementioned high-frequency band (e.g., from 20 mm to 40 mm, and can be 34.5 mm). The distance D 5  between the first radiation elements  121  and the main reflector  140  (its bottom plate) is approximately equal to or longer than 0.5 wavelength of the aforementioned low-frequency band (e.g., from 100 mm to 120 mm, and can be 112.5 mm). The diameter of the conductive housing of the coaxial cable  127  is about 1.2 mm. The inner diameter of the choke element  170  (hollow cylindrical tube) is about 1.8 mm, and the outer diameter of the choke element  170  is about 2.4 mm. The above element sizes are calculated according to many simulation results, and they are arranged for optimizing the antenna gain and isolation of the antenna system  100 . 
     It should be noted that all of the components related to the first antenna element  120  can be applied to the second antenna element  130  correspondingly, and they will not be described again. 
       FIG. 4A  is an S-parameter diagram of the antenna system  100  operating in the low-frequency band, according to an embodiment of the invention. The horizontal axis represents the operation frequency (MHz), and the vertical axis represents the S-parameters (dB). The first antenna element  120  of the dual-polarized antenna  110  is considered as a first port (Port  1 ), and the second antenna element  130  of the dual-polarized antenna  110  is considered as a second port (Port  2 ). The curve S 11  represents the return loss of the first antenna element  120 . The curve S 22  represents the return loss of the second antenna element  130 . The curve S 21  represents the isolation between the first antenna element  120  and the second antenna element  130 . According to the result of the electromagnetic simulation software shown in  FIG. 4A , both the first antenna element  120  and the second antenna element  130  can cover the low-frequency band of LTE Band 5/13, and the S 21  parameter between the first antenna element  120  and the second antenna element  130  is below −25 dB over the low-frequency band. 
       FIG. 4B  is an S-parameter diagram of the antenna system  100  operating in the high-frequency band, according to an embodiment of the invention. According to the result of the electromagnetic simulation software shown in  FIG. 4B , both the first antenna element  120  and the second antenna element  130  can cover the high-frequency band of LTE Band 2/4, and the S 21  parameter between the first antenna element  120  and the second antenna element  130  is below −25 dB over the high-frequency band. 
     In addition, according to the simulation results of the electromagnetic simulation software, each of the first antenna element  120  and the second antenna element  130  has cross-polarization isolation which is equal to or higher than 17.3 dB. The incorporation of the choke element  170  can increase the cross-polarization isolation to at least 25.4 dB in the frequency interval from 1710 MHz to 1755 MHz. The above electromagnetic simulation data show that the antenna system  100  can meet the requirement of application in mobile communication devices. 
       FIG. 5  is a perspective view of an antenna system  500  according to another embodiment of the invention.  FIG. 5  is similar to  FIG. 1A . In the embodiment of  FIG. 5 , the antenna system  500  further includes an antenna cover  510 , a rotary motor  520 , and a metal bottom plate  530 . The antenna cover  510  is made of a nonconductive material, such as a plastic material. The antenna cover  510  may have a pyramid structure and a hollow cylindrical shape. The aforementioned dual-polarized antenna  110 , main reflector  140 , and auxiliary reflector  150  are all disposed inside the antenna cover  510 . The rotary motor  520  is connected to the dual-polarized antenna  110 , the main reflector  140 , and the auxiliary reflector  150 . In some embodiments, a processor generates a control signal, and the rotary motor  520  rotates the dual-polarized antenna  110 , the main reflector  140 , and the auxiliary reflector  150  according to the control signal, so as to fine-tune the maximum gain direction of the antenna system  500 . The metal bottom plate  530  may have a circular shape, a rectangular shape, or a square shape, and it can support the antenna cover  510  and the rotary motor  520 . The antenna cover  510  and the rotary motor  520  have vertical projections which are completely inside the metal bottom plate  530 . With such a design, the main beam of the antenna system  500  is adjustable in response to a variety of requirements, and it can be set toward different desired directions. Therefore, the antenna system  500  is considered as a product of smart antenna. 
     The invention proposes a dual-polarized antenna system which includes a main reflector and an auxiliary reflector. The main reflector and the auxiliary reflector correspond to a low-frequency band and a high-frequency band, respectively, such that the antenna gain over the wide operation frequency band is uniformly improved. In addition, a choke element is arranged for a solution of high-frequency suppression. If a rotary motor is added, the proposed antenna system can have a tunable main beam direction, and it can be used as a high-gain smart antenna. The invention is suitable for application in a variety of indoor environments, so as to solve the problem of poor communication quality due to signal reflection and multipath fading in conventional designs. 
     Note that the above element sizes, element shapes, and frequency ranges are not limitations of the invention. An antenna designer can fine-tune these settings or values according to different requirements. It should be understood that the antenna system of the invention is not limited to the configurations of  FIGS. 1-5 . The invention may merely include any one or more features of any one or more embodiments of  FIGS. 1-5 . In other words, not all of the features displayed in the figures should be implemented in the antenna system of the invention. 
     Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having the same name (but for use of the ordinal term) to distinguish the claim elements. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.