Patent Publication Number: US-2023147065-A1

Title: Antenna array

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of Taiwan Patent Application No. 110141789 filed on Nov. 10, 2021, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The disclosure generally relates to an antenna array, and more particularly, to an antenna array for increasing radiation gain. 
     Description of the Related Art 
     With the advancements being made 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 user 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. 
     Antenna arrays are widely used in the fields of military technology, radar detection, life detection, and health monitoring. Therefore, it has become a critical challenge for a current designer to design an antenna array with high radiation gain and thereby improve communication performance. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment, the invention is directed to an antenna array that includes a first antenna unit, a second antenna unit, a third antenna unit, a fourth antenna unit, a first auxiliary metal element, a second auxiliary metal element, a third auxiliary metal element, and a fourth auxiliary metal element. The first auxiliary metal element is adjacent to the first antenna unit. The second auxiliary metal element is adjacent to the second antenna unit. The third auxiliary metal element is adjacent to the third antenna unit. The fourth auxiliary metal element is adjacent to the fourth antenna unit. The first auxiliary metal element, the second auxiliary metal element, the third auxiliary metal element, and the fourth auxiliary metal element are configured to increase the radiation gain of the antenna array. 
     In some embodiments, the first antenna unit, the second antenna unit, the third antenna unit, and the fourth antenna unit cover a first frequency band and a second frequency band of millimeter-wave operations. 
     In some embodiments, each of the first auxiliary metal element, the second auxiliary metal element, the third auxiliary metal element, and the fourth auxiliary metal element substantially has a square shape. 
     In some embodiments, each of the first auxiliary metal element, the second auxiliary metal element, the third auxiliary metal element, and the fourth auxiliary metal element substantially has a square-ring shape. 
     In some embodiments, each of the first auxiliary metal element, the second auxiliary metal element, the third auxiliary metal element, and the fourth auxiliary metal element substantially has a circular-ring shape. 
     In some embodiments, the antenna array further includes a dielectric substrate and a ground metal plane. The dielectric substrate has a first surface and a second surface which are opposite to each other. The ground metal plane is disposed on the second surface of the dielectric substrate. 
     In some embodiments, the first antenna unit includes a first metal loop and a first feeding metal element. The first feeding metal element is coupled to a first signal source and is adjacent to the first metal loop. The second antenna unit includes a second metal loop and a second feeding metal element. The second feeding metal element is coupled to a second signal source and is adjacent to the second metal loop. The third antenna unit includes a third metal loop and a third feeding metal element. The third feeding metal element is coupled to a third signal source and is adjacent to the third metal loop. The fourth antenna unit includes a fourth metal loop and a fourth feeding metal element. The fourth feeding metal element is coupled to a fourth signal source and is adjacent to the fourth metal loop. The first metal loop, the second metal loop, the third metal loop, and the fourth metal loop are disposed on the first surface of the dielectric substrate. 
     In some embodiments, the first auxiliary metal element has a first vertical projection on the first surface of the dielectric substrate, and the first vertical projection at least partially overlaps the first metal loop. The second auxiliary metal element has a second vertical projection on the first surface of the dielectric substrate, and the second vertical projection at least partially overlaps the second metal loop. The third auxiliary metal element has a third vertical projection on the first surface of the dielectric substrate, and the third vertical projection at least partially overlaps the third metal loop. The fourth auxiliary metal element has a fourth vertical projection on the first surface of the dielectric substrate, and the fourth vertical projection at least partially overlaps the fourth metal loop. 
     In some embodiments, the first auxiliary metal element, the second auxiliary metal element, the third auxiliary metal element, the fourth auxiliary metal element, the first metal loop, the second metal loop, the third metal loop, and the fourth metal loop substantially have the same perimeters. 
     In some embodiments, a first distance is defined between the first auxiliary metal element and the first metal loop, a second distance is defined between the second auxiliary metal element and the second metal loop, a third distance is defined between the third auxiliary metal element and the third metal loop, and a fourth distance is defined between the fourth auxiliary metal element and the fourth metal loop. Each of the first distance, the second distance, the third distance, and the fourth distance is from 0.125 to 0.5 wavelength of the first frequency band. 
     In some embodiments, each of the first metal loop, the second metal loop, the third metal loop, and the fourth metal loop substantially has a relatively large square shape. 
     In some embodiments, the first metal loop has a first hollow portion, the second metal loop has a second hollow portion, the third metal loop has a third hollow portion, and the fourth metal loop has a fourth hollow portion. Each of the first hollow portion, the second hollow portion, the third hollow portion, and the fourth hollow portion substantially has a relatively small square shape. 
     In some embodiments, the length of each of the first hollow portion, the second hollow portion, the third hollow portion, and the fourth hollow portion is substantially equal to 0.25 wavelength of the first frequency band. 
     In some embodiments, the center-to-center distance between any adjacent two of the first metal loop, the second metal loop, the third metal loop, and the fourth metal loop is from 0.4 to 1 wavelength of the first frequency band. 
     In some embodiments, the first feeding metal element, the second feeding metal element, the third feeding metal element, and the fourth feeding metal element are embedded in the dielectric substrate and between the first surface and the second surface. 
     In some embodiments, each of the first feeding metal element, the second feeding metal element, the third feeding metal element, and the fourth feeding metal element substantially has an L-shape. 
     In some embodiments, each of the first feeding metal element, the second feeding metal element, the third feeding metal element, and the fourth feeding metal element is at least partially perpendicular to and at least partially parallel to the corresponding one of the first metal loop, the second metal loop, the third metal loop, and the fourth metal loop. 
     In some embodiments, the length of each of the first feeding metal element, the second feeding metal element, the third feeding metal element, and the fourth feeding metal element is substantially equal to 0.25 wavelength of the second frequency band. 
     In some embodiments, a first feeding point and a second feeding point are respectively positioned at two ends of the first feeding metal element, a third feeding point and a fourth feeding point are respectively positioned at two ends of the second feeding metal element, a fifth feeding point and a sixth feeding point are respectively positioned at two ends of the third feeding metal element, and a seventh feeding point and an eighth feeding point are respectively positioned at two ends of the fourth feeding metal element. 
     In some embodiments, the first signal source is coupled to the first feeding point or the second feeding point so as to excite the first antenna unit, the second signal source is coupled to the third feeding point or the fourth feeding point so as to excite the second antenna unit, the third signal source is coupled to the fifth feeding point or the sixth feeding point so as to excite the third antenna unit, and the fourth signal source is coupled to the seventh feeding point or the eighth feeding point so as to excite the fourth antenna unit. 
    
    
     
       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.  1    is a diagram of an antenna array according to an embodiment of the invention; 
         FIG.  2    is a diagram of return loss of an antenna array according to an embodiment of the invention; 
         FIG.  3 A  is a top view of an antenna array according to an embodiment of the invention; 
         FIG.  3 B  is a side view of an antenna array according to an embodiment of the invention; 
         FIG.  4 A  is a perspective view of an antenna array according to an embodiment of the invention; 
         FIG.  4 B  is a diagram of radiation gain of an antenna array operating in a first frequency band according to an embodiment of the invention; 
         FIG.  5 A  is a perspective view of an antenna array according to an embodiment of the invention; 
         FIG.  5 B  is a diagram of radiation gain of an antenna array operating in a first frequency band according to an embodiment of the invention; 
         FIG.  6 A  is a perspective view of an antenna array according to an embodiment of the invention; and 
         FIG.  6 B  is a diagram of radiation gain of an antenna array operating in a first frequency band according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to illustrate the foregoing and other purposes, features and advantages of the invention, the embodiments and figures of the invention will be described in detail as follows. 
     Certain terms are used throughout the description and following claims to refer to particular components. As one skilled in the art will appreciate, manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following description and in the claims, the terms “include” and “comprise” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . . ”. The term “substantially” means the value is within an acceptable error range. One skilled in the art can solve the technical problem within a predetermined error range and achieve the proposed technical performance. Also, the term “couple” is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is coupled to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Furthermore, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
       FIG.  1    is a diagram of an antenna array  100  according to an embodiment of the invention. The antenna array  100  may be applied to a mobile device, such as a smartphone, a tablet computer, or a notebook computer. As shown in  FIG.  1   , the antenna array  100  includes a first antenna unit  101 , a second antenna unit  102 , a third antenna unit  103 , a fourth antenna unit  104 , a first auxiliary metal element  105 , a second auxiliary metal element  106 , a third auxiliary metal element  107 , and a fourth auxiliary metal element  108 . The shapes and types of aforementioned antenna units and auxiliary metal elements are not limited in the invention. It should be understood that the antenna array  100  may further include other elements, such as an RF (Radio Frequency) module including a plurality of signal sources, and a plurality of power amplifiers, although they are not displayed in  FIG.  1   . 
     The first auxiliary metal element  105  is disposed adjacent to the first antenna unit  101 , and they may be substantially aligned with each other. The second auxiliary metal element  106  is disposed adjacent to the second antenna unit  102 , and they may be substantially aligned with each other. The third auxiliary metal element  107  is disposed adjacent to the third antenna unit  103 , and they may be substantially aligned with each other. The fourth auxiliary metal element  108  is disposed adjacent to the fourth antenna unit  104 , and they may be substantially aligned with each other. It should be noted that the term “adjacent” or “close” over the disclosure means that the distance (spacing) between two corresponding elements is smaller than a predetermined distance (e.g., 10 mm or shorter), but usually does not mean that the two corresponding elements directly touch each other (i.e., the aforementioned distance/spacing therebetween is reduced to 0). For example, a first distance DA may be defined between the first auxiliary metal element  105  and the first antenna unit  101 . A second distance DB may be defined between the second auxiliary metal element  106  and the second antenna unit  102 . A third distance DC may be defined between the third auxiliary metal element  107  and the third antenna unit  103 . A fourth distance DD may be defined between the fourth auxiliary metal element  108  and the fourth antenna unit  104 . 
       FIG.  2    is a diagram of return loss of the antenna array  100  according to an embodiment of the invention. The horizontal axis represents the operation frequency (GHz), and the vertical axis represents the return loss (dB). According to the measurement of  FIG.  2   , the first antenna unit  101 , the second antenna unit  102 , the third antenna unit  103 , and the fourth antenna unit  104  of the antenna array  100  can cover a first frequency band FB 1  and a second frequency band FB 2  of millimeter-wave operations. For example, the first frequency band FB 1  may be at about 28 GHz, and the second frequency band FB 2  may be at about 39 GHz. Accordingly, the antenna array  100  can support the wideband operations of next-generation 5G communication. 
     It should be noted that the first auxiliary metal element  105 , the second auxiliary metal element  106 , the third auxiliary metal element  107 , and the fourth auxiliary metal element  108  resonate with the first antenna unit  101 , the second antenna unit  102 , the third antenna unit  103 , and the fourth antenna unit  104 , respectively, so as to increase the radiation gain of the antenna array  100  operating in the first frequency band FB 1  and the second frequency band FB 2 . According to practical measurements, when each of the first distance DA, the second distance DB, the third distance DC, and the fourth distance DD is from 0.125 to 0.5 wavelength of the first frequency band FB 1  (i.e., λ/8˜λ/2), the radiation gain of the antenna array  100  can be maximized. With such a design, the whole radiation performance of the antenna array  100  will not be negatively affected even if the antenna array  100  is covered by a nonconductive housing of a mobile device or is blocked by an antenna window. 
     The following embodiments will introduce different configurations and detailed structural features of the antenna array  100 . It should be understood that these figures and descriptions are merely exemplary, rather than limitations of the invention. 
       FIG.  3 A  is a top view of an antenna array  300  according to an embodiment of the invention.  FIG.  3 B  is a side view of the antenna array  300  according to an embodiment of the invention. In the embodiment of  FIG.  3 A  and  FIG.  3 B , the antenna array  300  at least includes a dielectric substrate  110 , a ground metal plane  120 , a first antenna unit  130 , a second antenna unit  140 , a third antenna unit  150 , and a fourth antenna unit  160 . The antenna array  300  can also cover the first frequency band FB 1  and the second frequency band FB 2  as mentioned above. In order to simplify the figures, the first auxiliary metal element, the second auxiliary metal element, the third auxiliary metal element, and the fourth auxiliary metal element are not displayed in  FIG.  3 A  and  FIG.  3 B , but they will be illustrated in detail in the following embodiments. 
     The dielectric substrate  110  has a first surface E 1  and a second surface E 2  which are opposite to each other. The ground metal plane  120  is disposed on the second surface E 2  of the dielectric substrate  110 , so as to provide a ground voltage. The dielectric substrate  110  may be a Rogers substrate made of, for example, an RO4350B material. However, the invention is not limited thereto. In alternative embodiments, adjustments to the design may be made to the effect that the dielectric substrate  110  may be an FR4 (Flame Retardant 4) substrate, a PCB (Printed Circuit Board), or an FPC (Flexible Printed Circuit). The ground metal plane  120  may substantially have a rectangular shape to cover the whole second surface E 2  of the dielectric substrate  110 . 
     The first antenna unit  130  includes a first metal loop  131  and a first feeding metal element  132 . For example, the first metal loop  131  may substantially have a relatively large square shape. The first metal loop  131  is disposed on the first surface E 1  of the dielectric substrate  110 . The first metal loop  131  has a first hollow portion  135 . The first hollow portion  135  may substantially have a relatively small square shape. The first feeding metal element  132  may substantially have an L-shape. The first feeding metal element  132  may be at least partially perpendicular to and at least partially parallel to the first metal loop  131 . The first feeding metal element  132  may be embedded in the dielectric substrate  110  and between the first surface E 1  and the second surface E 2 . The first feeding metal element  132  is coupled to a first signal source  191  and is adjacent to the first metal loop  131 . A first coupling gap GC 1  may be formed between the first metal loop  131  and the first feeding metal element  132 . Specifically, a first feeding point FP 1  and a second feeding point FP 2  are respectively positioned at two ends of the first feeding metal element  132 . The first signal source  191  is coupled to either the first feeding point FP 1  or the second feeding point FP 2 , so as to excite the first antenna unit  130 . 
     The second antenna unit  140  includes a second metal loop  141  and a second feeding metal element  142 . For example, the second metal loop  141  may substantially have a relatively large square shape. The second metal loop  141  is disposed on the first surface E 1  of the dielectric substrate  110 . The second metal loop  141  has a second hollow portion  145 . The second hollow portion  145  may substantially have a relatively small square shape. The second feeding metal element  142  may substantially have an L-shape. The second feeding metal element  142  may be at least partially perpendicular to and at least partially parallel to the second metal loop  141 . The second feeding metal element  142  may be embedded in the dielectric substrate  110  and between the first surface E 1  and the second surface E 2 . The second feeding metal element  142  is coupled to a second signal source  192  and is adjacent to the second metal loop  141 . A second coupling gap GC 2  may be formed between the second metal loop  141  and the second feeding metal element  142 . Specifically, a third feeding point FP 3  and a fourth feeding point FP 4  are respectively positioned at two ends of the second feeding metal element  142 . The second signal source  192  is coupled to either the third feeding point FP 3  or the fourth feeding point FP 4 , so as to excite the second antenna unit  140 . 
     The third antenna unit  150  includes a third metal loop  151  and a third feeding metal element  152 . For example, the third metal loop  151  may substantially have a relatively large square shape. The third metal loop  151  is disposed on the first surface E 1  of the dielectric substrate  110 . The third metal loop  151  has a third hollow portion  155 . The third hollow portion  155  may substantially have a relatively small square shape. The third feeding metal element  152  may substantially have an L-shape. The third feeding metal element  152  may be at least partially perpendicular to and at least partially parallel to the third metal loop  151 . The third feeding metal element  152  may be embedded in the dielectric substrate  110  and between the first surface E 1  and the second surface E 2 . The third feeding metal element  152  is coupled to a third signal source  193  and is adjacent to the third metal loop  151 . A third coupling gap GC 3  may be formed between the third metal loop  151  and the third feeding metal element  152 . Specifically, a fifth feeding point FP 5  and a sixth feeding point FP 6  are respectively positioned at two ends of the third feeding metal element  152 . The third signal source  193  is coupled to either the fifth feeding point FP 5  or the sixth feeding point FP 6 , so as to excite the third antenna unit  150 . 
     The fourth antenna unit  160  includes a fourth metal loop  161  and a fourth feeding metal element  162 . For example, the fourth metal loop  161  may substantially have a relatively large square shape. The fourth metal loop  161  is disposed on the first surface E 1  of the dielectric substrate  110 . The fourth metal loop  161  has a fourth hollow portion  165 . The fourth hollow portion  165  may substantially have a relatively small square shape. The fourth feeding metal element  162  may substantially have an L-shape. The fourth feeding metal element  162  may be at least partially perpendicular to and at least partially parallel to the fourth metal loop  161 . The fourth feeding metal element  162  may be embedded in the dielectric substrate  110  and between the first surface E 1  and the second surface E 2 . The fourth feeding metal element  162  is coupled to a fourth signal source  194  and is adjacent to the fourth metal loop  161 . A fourth coupling gap GC 4  may be formed between the fourth metal loop  161  and the fourth feeding metal element  162 . Specifically, a seventh feeding point FP 7  and an eighth feeding point FP 8  are respectively positioned at two ends of the fourth feeding metal element  162 . The fourth signal source  194  is coupled to either the seventh feeding point FP 7  or the eighth feeding point FP 8 , so as to excite the fourth antenna unit  160 . 
     As a whole, the first metal loop  131 , the second metal loop  141 , the third metal loop  151 , and the fourth metal loop  161  may have the same structures, and they may be arranged in the same straight-line. In some embodiments, the first metal loop  131 , the second metal loop  141 , the third metal loop  151 , and the fourth metal loop  161  have vertical projections on the second surface E 2  of the dielectric substrate  110 , and the entirety of each vertical projection is inside the ground metal plane  120 . The shapes of the first metal loop  131 , the second metal loop  141 , the third metal loop  151 , and the fourth metal loop  161  are not limited in the invention. In alternative embodiments, each of the first metal loop  131 , the second metal loop  141 , the third metal loop  151 , and the fourth metal loop  161  substantially has a circular shape, a rectangular shape, an elliptical shape, a regular triangular shape, or a regular hexagonal shape. 
     In some embodiments, the operation principles of the antenna array  300  are described as follows. The radiation pattern of the antenna array  300  will provide a first polarization direction if the first signal source  191  is coupled to the first feeding point FP 1 , the second signal source  192  is coupled to the third feeding point FP 3 , the third signal source  193  is coupled to the fifth feeding point FP 5 , and the fourth signal source  194  is coupled to the seventh feeding point FP 7 . Conversely, the radiation pattern of the antenna array  300  will provide a second polarization direction which is substantially perpendicular to the first polarization direction if the first signal source  191  is coupled to the second feeding point FP 2 , the second signal source  192  is coupled to the fourth feeding point FP 4 , the third signal source  193  is coupled to the sixth feeding point FP 6 , and the fourth signal source  194  is coupled to the eighth feeding point FP 8 . For example, the first polarization direction may be horizontally-polarized (parallel to the XY-plane), and the second polarization direction may be vertically-polarized (parallel to the Z-axis), but they are not limited thereto. Thus, the antenna array  300  can transmit or receive signals with different polarization directions by selecting appropriate feeding points. Furthermore, the main beam direction of the antenna array  300  is adjustable by changing the phase differences between the first signal source  191 , the second signal source  192 , the third signal source  193 , and the fourth signal source  194 . 
     In some embodiments, the element sizes and element parameters of the antenna array  300  are described as follows. The thickness H 1  of the dielectric substrate  110  may be from 0.6 mm to 1 mm, such as about 0.8 mm. The dielectric constant of the dielectric substrate  110  may be from 3 to 5, such as about 3.48. The length L 1  of the first hollow portion  135  of the first metal loop  131 , the length L 2  of the second hollow portion  145  of the second metal loop  141 , the length L 3  of the third hollow portion  155  of the third metal loop  151 , and the length L 4  of the fourth hollow portion  165  of the fourth metal loop  161  may all be substantially equal to 0.25 wavelength (λ/4) of the first frequency band FB 1  of the antenna array  300 . The width W 1  of the first metal loop  131 , the width W 2  of the second metal loop  141 , the width W 3  of the third metal loop  151 , and the width W 4  of the fourth metal loop  161  may all be from 0.1 mm to 0.5 mm, such as 0.3 mm. The length L 5  of the first feeding metal element  132 , the length L 6  of the second feeding metal element  142 , the length L 7  of the third feeding metal element  152 , and the length L 8  of the fourth feeding metal element  162  may all be substantially equal to 0.25 wavelength (λ/4) of the second frequency band FB 2  of the antenna array  300 . The center-to-center distance D 1  between the first metal loop  131  and the second metal loop  141 , the center-to-center distance D 2  between the second metal loop  141  and the third metal loop  151 , and the center-to-center distance D 3  between the third metal loop  151  and the fourth metal loop  161  may all be from 0.4 to 1 wavelength (0.4λ˜1λ) of the first frequency band FB 1  of the antenna array  300 . The width of the first coupling gap GC 1 , the width of the second coupling gap GC 2 , the width of the third coupling gap GC 3 , and the width of the fourth coupling gap GC 4  may all be from 0.1 mm to 0.3 mm, such as 0.2 mm. The above ranges of element sizes and element parameters are calculated and obtained according to many experiment results, and they help to optimize the total beam width, the operational bandwidth, and the impedance matching of the antenna array  300 . Other features of the antenna array  300  of  FIG.  3 A  and  FIG.  3 B  are similar to those of the antenna array  100  of  FIG.  1   . Accordingly, the two embodiments can achieve similar levels of performance. 
       FIG.  4 A  is a perspective view of an antenna array  400  according to an embodiment of the invention.  FIG.  4 A  is similar to  FIG.  3 A  and  FIG.  3 B . In the embodiment of  FIG.  4 A , the antenna array  400  further includes a first auxiliary metal element  405 , a second auxiliary metal element  406 , a third auxiliary metal element  407 , and a fourth auxiliary metal element  408 , each of which may substantially have a square shape (solid). The antenna array  400  can also cover the first frequency band FB 1  and the second frequency band FB 2  as mentioned above. 
     The first auxiliary metal element  405  has a first vertical projection on the first surface E 1  of the dielectric substrate  110 , and the first vertical projection at least partially overlaps the first metal loop  131 . For example, the central point of the first auxiliary metal element  405  may be exactly aligned with the central point of the first metal loop  131 . The second auxiliary metal element  406  has a second vertical projection on the first surface E 1  of the dielectric substrate  110 , and the second vertical projection at least partially overlaps the second metal loop  141 . For example, the central point of the second auxiliary metal element  406  may be exactly aligned with the central point of the second metal loop  141 . The third auxiliary metal element  407  has a third vertical projection on the first surface E 1  of the dielectric substrate  110 , and the third vertical projection at least partially overlaps the third metal loop  151 . For example, the central point of the third auxiliary metal element  407  may be exactly aligned with the central point of the third metal loop  151 . The fourth auxiliary metal element  408  has a fourth vertical projection on the first surface E 1  of the dielectric substrate  110 , and the fourth vertical projection at least partially overlaps the fourth metal loop  161 . For example, the central point of the fourth auxiliary metal element  408  may be exactly aligned with the central point of the fourth metal loop  161 . 
     In some embodiments, a first distance DA is defined between the first auxiliary metal element  405  and the first metal loop  131 , a second distance DB is defined between the second auxiliary metal element  406  and the second metal loop  141 , a third distance DC is defined between the third auxiliary metal element  407  and the third metal loop  151 , and a fourth distance DD is defined between the fourth auxiliary metal element  408  and the fourth metal loop  161 . Each of the first distance DA, the second distance DB, the third distance DC, and the fourth distance DD may be from 0.125 to 0.5 wavelength of the first frequency band FB 1  (i.e., λ/8˜λ/2). In some embodiments, the first auxiliary metal element  405 , the second auxiliary metal element  406 , the third auxiliary metal element  407 , the fourth auxiliary metal element  408 , the first metal loop  131 , the second metal loop  141 , the third metal loop  151 , and the fourth metal loop  161  substantially have the same perimeters LE (i.e., the outer perimeters). According to practical measurements, the above ranges of element sizes can help to maximize the radiation gain of the antenna array  400 . 
     It should be understood that the distances between the first auxiliary metal element  405 , the second auxiliary metal element  406 , the third auxiliary metal element  407 , and the fourth auxiliary metal element  408  substantially correspond to the distances between the first metal loop  131 , the second metal loop  141 , the third metal loop  151 , and the fourth metal loop  161 . In alternative embodiments, the shift angle of the main beam of the antenna array  400  is fine-tuned by changing the distances between the first auxiliary metal element  405 , the second auxiliary metal element  406 , the third auxiliary metal element  407 , and the fourth auxiliary metal element  408 . 
       FIG.  4 B  is a diagram of radiation gain of the antenna array  400  operating in the first frequency band FB 1  according to an embodiment of the invention (it may be measured on the XZ-plane). The horizontal axis represents the zenith angle (Theta) (degrees), and the vertical axis represents the radiation gain (dBi). As shown in  FIG.  4 B , a first curve CC 1  represents the radiation pattern of the antenna array  400  when the aforementioned feeding phase difference is equal to −120 degrees, a second curve CC 2  represents the radiation pattern of the antenna array  400  when the aforementioned feeding phase difference is equal to −60 degrees, a third curve CC 3  represents the radiation pattern of the antenna array  400  when the aforementioned feeding phase difference is equal to 0 degrees, a fourth curve CC 4  represents the radiation pattern of the antenna array  400  when the aforementioned feeding phase difference is equal to 60 degrees, and a fifth curve CC 5  represents the radiation pattern of the antenna array  400  when the aforementioned feeding phase difference is equal to 120 degrees. Therefore, the antenna array  400  can provide an almost omnidirectional radiation pattern by controlling its feeding phase difference. It should be noted that the maximum radiation gain of the antenna array  400  can be enhanced by about 2.7 dBi after the first auxiliary metal element  405 , the second auxiliary metal element  406 , the third auxiliary metal element  407 , and the fourth auxiliary metal element  408  are used. Other features of the antenna array  400  of  FIG.  4 A  are similar to those of the antenna array  300  of  FIG.  3 A  and  FIG.  3 B . Accordingly, the two embodiments can achieve similar levels of performance. 
     In some embodiments, the first auxiliary metal element  405  is moved outwardly by a first shift distance DM 1 , and the fourth auxiliary metal element  408  is moved outwardly by a second shift distance DM 2  (the first auxiliary metal element  405  and the fourth auxiliary metal element  408  may be both moved parallel to the dielectric substrate  110 ). That is, according to the normal direction of the dielectric substrate  110 , a first shift angle θ 1  can be provided to the first auxiliary metal element  405 , and a second shift angle θ 2  can be provided to the fourth auxiliary metal element  408 . Their relationship may be described according to the following equations (1) and (2). 
         DM 1= DA ·tan(θ1)  (1)
 
         DM 2= DD ·tan(θ2)  (2)
 
     where “DM 1 ” represents the first shift distance DM 1 , “DM 2 ” represents the second shift distance DM 2 , “DA” represents the first distance DA, “DD” represents the fourth distance DD, “01” represents the first shift angle θ 1 , and “02” represents the second shift angle θ 2 . 
     According to practical measurements, a designer can fine-tune and rotate the main beam direction of the antenna array  400  by changing the first shift angle θ 1  and the second shift angle θ 2 . In some embodiments, if the first shift angle θ 1  and the second shift angle θ 2  are between 0 and 30 degrees, the main beam direction of the antenna array  400  will be rotated by 0 to 30 degrees, so as to meet different requirements of designs. 
       FIG.  5 A  is a perspective view of an antenna array  500  according to an embodiment of the invention.  FIG.  5 A  is similar to  FIG.  4 A . In the embodiment of  FIG.  5 A , the antenna array  500  further includes a first auxiliary metal element  505 , a second auxiliary metal element  506 , a third auxiliary metal element  507 , and a fourth auxiliary metal element  508 , each of which may substantially have a square-ring shape (hollow). The antenna array  500  can also cover the first frequency band FB 1  and the second frequency band FB 2  as mentioned above. 
     The first auxiliary metal element  505  has a first vertical projection on the first surface E 1  of the dielectric substrate  110 , and the first vertical projection at least partially (or completely) overlaps the first metal loop  131 . For example, the central point of the first auxiliary metal element  505  may be exactly aligned with the central point of the first metal loop  131 . The second auxiliary metal element  506  has a second vertical projection on the first surface E 1  of the dielectric substrate  110 , and the second vertical projection at least partially (or completely) overlaps the second metal loop  141 . For example, the central point of the second auxiliary metal element  506  may be exactly aligned with the central point of the second metal loop  141 . The third auxiliary metal element  507  has a third vertical projection on the first surface E 1  of the dielectric substrate  110 , and the third vertical projection at least partially (or completely) overlaps the third metal loop  151 . For example, the central point of the third auxiliary metal element  507  may be exactly aligned with the central point of the third metal loop  151 . The fourth auxiliary metal element  508  has a fourth vertical projection on the first surface E 1  of the dielectric substrate  110 , and the fourth vertical projection at least partially (or completely) overlaps the fourth metal loop  161 . For example, the central point of the fourth auxiliary metal element  508  may be exactly aligned with the central point of the fourth metal loop  161 . In some embodiments, the first auxiliary metal element  505 , the second auxiliary metal element  506 , the third auxiliary metal element  507 , the fourth auxiliary metal element  508 , the first metal loop  131 , the second metal loop  141 , the third metal loop  151 , and the fourth metal loop  161  substantially have the same perimeters LE. 
       FIG.  5 B  is a diagram of radiation gain of the antenna array  500  operating in the first frequency band FB 1  according to an embodiment of the invention. The horizontal axis represents the zenith angle (Theta) (degrees), and the vertical axis represents the radiation gain (dBi). As shown in  FIG.  5 B , a sixth curve CC 6  represents the radiation pattern of the antenna array  500  when the aforementioned feeding phase difference is equal to −120 degrees, a seventh curve CC 7  represents the radiation pattern of the antenna array  500  when the aforementioned feeding phase difference is equal to −60 degrees, an eighth curve CC 8  represents the radiation pattern of the antenna array  500  when the aforementioned feeding phase difference is equal to 0 degrees, a ninth curve CC 9  represents the radiation pattern of the antenna array  500  when the aforementioned feeding phase difference is equal to 60 degrees, and a tenth curve CC 10  represents the radiation pattern of the antenna array  500  when the aforementioned feeding phase difference is equal to 120 degrees. It should be noted that the maximum radiation gain of the antenna array  500  can be enhanced by about 2.9 dBi after the first auxiliary metal element  505 , the second auxiliary metal element  506 , the third auxiliary metal element  507 , and the fourth auxiliary metal element  508  are used. Other features of the antenna array  500  of  FIG.  5 A  are similar to those of the antenna array  400  of  FIG.  4 A . Accordingly, the two embodiments can achieve similar levels of performance. 
       FIG.  6 A  is a perspective view of an antenna array  600  according to an embodiment of the invention.  FIG.  6 A  is similar to  FIG.  4 A . In the embodiment of  FIG.  6 A , the antenna array  600  further includes a first auxiliary metal element  605 , a second auxiliary metal element  606 , a third auxiliary metal element  607 , and a fourth auxiliary metal element  608 , each of which may substantially have a circular-ring shape (hollow). The antenna array  600  can also cover the first frequency band FB 1  and the second frequency band FB 2  as mentioned above. 
     The first auxiliary metal element  605  has a first vertical projection on the first surface E 1  of the dielectric substrate  110 , and the first vertical projection at least partially overlaps the first metal loop  131 . For example, the central point of the first auxiliary metal element  605  may be exactly aligned with the central point of the first metal loop  131 . The second auxiliary metal element  606  has a second vertical projection on the first surface E 1  of the dielectric substrate  110 , and the second vertical projection at least partially overlaps the second metal loop  141 . For example, the central point of the second auxiliary metal element  606  may be exactly aligned with the central point of the second metal loop  141 . The third auxiliary metal element  607  has a third vertical projection on the first surface E 1  of the dielectric substrate  110 , and the third vertical projection at least partially overlaps the third metal loop  151 . For example, the central point of the third auxiliary metal element  607  may be exactly aligned with the central point of the third metal loop  151 . The fourth auxiliary metal element  608  has a fourth vertical projection on the first surface E 1  of the dielectric substrate  110 , and the fourth vertical projection at least partially overlaps the fourth metal loop  161 . For example, the central point of the fourth auxiliary metal element  608  may be exactly aligned with the central point of the fourth metal loop  161 . In some embodiments, the first auxiliary metal element  605 , the second auxiliary metal element  606 , the third auxiliary metal element  607 , the fourth auxiliary metal element  608 , the first metal loop  131 , the second metal loop  141 , the third metal loop  151 , and the fourth metal loop  161  substantially have the same perimeters LE. 
       FIG.  6 B  is a diagram of radiation gain of the antenna array  600  operating in the first frequency band FB 1  according to an embodiment of the invention. The horizontal axis represents the zenith angle (Theta) (degrees), and the vertical axis represents the radiation gain (dBi). As shown in  FIG.  6 B , an eleventh curve CC 11  represents the radiation pattern of the antenna array  600  when the aforementioned feeding phase difference is equal to −120 degrees, a twelfth curve CC 12  represents the radiation pattern of the antenna array  600  when the aforementioned feeding phase difference is equal to −60 degrees, a thirteenth curve CC 13  represents the radiation pattern of the antenna array  600  when the aforementioned feeding phase difference is equal to 0 degrees, a fourteenth curve CC 14  represents the radiation pattern of the antenna array  600  when the aforementioned feeding phase difference is equal to 60 degrees, and a fifteenth curve CC 15  represents the radiation pattern of the antenna array  600  when the aforementioned feeding phase difference is equal to 120 degrees. It should be noted that the maximum radiation gain of the antenna array  600  can be enhanced by about 2.9 dBi after the first auxiliary metal element  605 , the second auxiliary metal element  606 , the third auxiliary metal element  607 , and the fourth auxiliary metal element  608  are used. Other features of the antenna array  600  of  FIG.  6 A  are similar to those of the antenna array  400  of  FIG.  4 A . Accordingly, the two embodiments can achieve similar levels of performance. 
     The invention proposes a novel antenna array. In comparison to the conventional design, the invention has at least the advantages of high radiation gain, multiple polarization directions, small size, wide bandwidth, and low manufacturing cost, and therefore it is suitable for application in a variety of mobile communication devices. 
     Note that the above element sizes, element shapes, element parameters, 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 array of the invention is not limited to the configurations of  FIGS.  1 - 6   . The invention may include any one or more features of any one or more embodiments of  FIGS.  1 - 6   . In other words, not all of the features displayed in the figures should be implemented in the antenna array 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. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the invention. It is intended that the standard and examples be considered as exemplary only, with the true scope of the disclosed embodiments being indicated by the following claims and their equivalents.