Patent Publication Number: US-9847571-B2

Title: Compact, multi-port, MIMO antenna with high port isolation and low pattern correlation and method of making same

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to a compact, multi-port, multiple-input and multiple-output (MIMO) antenna with high port isolation and low pattern correlation and to a method of making such an antenna. 
     BACKGROUND 
     As the use of smart phones, cellular telephones, and personal digital assistants, and like mobile devices in wireless communication systems continues to dramatically grow, a need exists to provide increased system performance. One technique for improving such system performance is to provide uncorrelated propagation paths using multiple-input and multiple-output (MIMO) smart antenna technology. MIMO uses multiple transmitting antennas, which are typically spatially arranged apart, at a transmitter for simultaneously transmitting spatially multiplexed signals along multiple propagation paths; and multiple receiving antennas, which are also typically spatially arranged apart, at a receiver to demultiplex the spatially multiplexed signals. MIMO technology offers significant increases in data throughput and system range without additional bandwidth or increased transceiver power by spreading the same total power over the multiple antennas. MIMO is an important part of modern wireless communication standards, such as at least one version of IEEE 802.11 (Wi-Fi), 4G, 3GPP Long Term Evolution (LTE), WiMax and HSPA+. 
     However, the use of multiple antennas results in an unfavorable trade-off between device size and system performance. Effective MIMO performance requires relatively high port isolation and low pattern correlation. This is typically accomplished by increasing the distance between the antennas, thereby resulting in larger devices, which are undesirable in many applications, such as handheld mobile devices or Wi-Fi access points. Although decreasing the distance between the antennas results in a desirably smaller device, it is typically obtained at the expense of higher pattern correlation, lower port isolation, and poorer performance caused by mutual coupling. Mutual coupling between the antennas typically results in wasted transmit power during transmission, and a lower received power from incoming signals during reception. 
     Accordingly, there is a need for a compact, multi-port, MIMO antenna with the characteristics of high port isolation and low pattern correlation for enhanced performance, as well as to a method of making such an antenna. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views, together with the detailed description below, are incorporated in and form part of the specification, and serve to further illustrate embodiments of concepts that include the claimed invention, and explain various principles and advantages of those embodiments. 
         FIG. 1  is a perspective view of one embodiment of a compact, multi-port, MIMO antenna with high port isolation and low pattern correlation in accordance with the present disclosure. 
         FIG. 2  is a top plan view of the embodiment of  FIG. 1 . 
         FIG. 3  is a close-up, perspective view of a detail of the embodiment of  FIG. 1 . 
         FIG. 4  is an enlarged, sectional view taken on line  4 - 4  of  FIG. 1 . 
         FIG. 5  is a perspective view of another embodiment of a compact, multi-port, MIMO antenna with high port isolation and low pattern correlation in accordance with the present disclosure. 
         FIG. 6  is a perspective view of still another embodiment of a compact, multi-port, MIMO antenna with high port isolation and low pattern correlation in accordance with the present disclosure. 
         FIG. 7  is a perspective view of yet another embodiment of a compact, multi-port, MIMO antenna with high port isolation and low pattern correlation in accordance with the present disclosure. 
         FIG. 8  is a perspective view of an additional embodiment of a compact, multi-port, MIMO antenna with high port isolation and low pattern correlation in accordance with the present disclosure. 
         FIG. 9  is a sectional view analogous to  FIG. 4  of a further embodiment of a compact, multi-port, MIMO antenna with high port isolation and low pattern correlation in accordance with the present disclosure. 
         FIG. 10  is a view analogous to  FIG. 9 , but showing a different physical embodiment providing a signal feed. 
     
    
    
     Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and locations of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. 
     The method and structural components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
     DETAILED DESCRIPTION 
     One aspect of this disclosure relates to an antenna that includes a ground support, e.g., a ground plane; an electrically conductive, endless element, e.g., a circular element, mounted at a distance relative to the ground support; and a trio of ports arranged, preferably circumferentially, along the endless element for conveying radio frequency signals in an operating band of frequencies. The ports are successively spaced apart, preferably at equal electrical distances, along the endless element by a spacing of one-half of a wavelength at a center frequency of the operating band. 
     The wavelength referenced herein is the guided wavelength relative to an open transmission line formed, between the ports, by the endless element and the ground support. More particularly, this guided wavelength is such that a signal applied at one port undergoes a phase inversion to arrive at another port through the shortest connecting path therebetween along the endless element. Preferably, the endless element has a symmetrical shape about each port. For instance, each port could be located at a respective corner of an equilateral triangularly-shaped element, or at every other corner of an equilateral hexagonally-shaped element. Correspondingly, the trio of ports is arranged preferably equiangularly. 
     Also, preferably, the above-mentioned open transmission line formed between the ground support and the endless element features constant characteristic impedance. When this condition is met, a radio frequency signal fed at any one port will split approximately equally in opposite directions along the endless element. This signal split is exactly equal if the input impedance seen on either side of each port is the same. One split signal will arrive at an adjacent port a half wavelength away (180 degrees phase shift) along the shorter connecting path, while the other split signal will arrive at the same adjacent port a full wavelength away (360 degrees phase shift) along the longer connecting path. The split signals are thus in opposite phase at the same adjacent port. Thus, there is a high (near ideal) port isolation between the ports, and a corresponding low pattern correlation between the respective radiated electromagnetic field patterns, since it is well known that, for lossless antennas, coupling between the ports corresponds to pattern correlation, and the same is approximately true for low-loss antennas. Antennas are typically designed to have low ohmic losses, and thus a high efficiency in order to maximize communication range and data throughput rate. 
     Low pattern correlation yields a high data throughput in MIMO communication systems. Other known means may be used that can concurrently achieve phase inversion and approximately equal amplitude when transmitting between any pair of ports of a three-port antenna structure, to thereby produce high port isolation and low pattern correlation. For instance, it may be possible to load sections of the endless element with distributed or lumped resistive and reactive components in order to obtain the so desired phase and amplitude relationships. In this case, the endless element may be mechanically discontinuous if series elements, e.g., capacitors, are placed along its contour in order to achieve said phase relationships. 
     In a preferred embodiment, the ground support has an outer contoured support surface, e.g., flat or curved, and the endless element has an outer antenna surface of complementary contour, i.e., also flat or curved, relative to the contoured support surface. At any given point along the endless element, the outer antenna surface has preferably a constant dimension, e.g., width, if the endless element is formed by a strip-like structure, in the direction orthogonal to the direction along which the endless element develops, as well as the direction crossing said point and orthogonal to the ground support, and is preferably maintained at a constant distance from the outer contoured support surface. 
     In this way, the characteristic impedance of the transmission line formed by the endless element and the ground support is maintained essentially constant, thus substantially facilitating the energy flow and the determination of the distance between the ports, because the guided wavelength is essentially constant. For instance, the distance between the endless element and the ground support can be selected and adjusted to yield a 50 ohm impedance match at each port, as it happens, for instance, if the input impedance seen on either side of each port along the endless element is 100 ohms Advantageously, the endless element radiates radio frequency waves in an operating band of frequencies, e.g., 2.40 GHz to 2.48 GHz, and also radiates radio frequency waves in an additional operating band of higher frequencies, e.g., 5 GHz to 6 GHz, thereby allowing a wireless device to operate across the most common Wi-Fi frequency bands world-wide. 
     A method of making an antenna, in accordance with another aspect of this disclosure, is performed by mounting an electrically conductive, endless element at a distance relative to a ground support; arranging a trio of ports along the endless element for conveying radio frequency signals in an operating band of frequencies; and successively spacing the ports apart along the endless element by a spacing of one-half of a guided wavelength at a center frequency of the operating band. 
     Turning now to  FIGS. 1-4  of the drawings, reference numeral  10  generally identifies a first embodiment of a compact, three-port, multiple-input and multiple-output (MIMO) antenna with high port isolation and low pattern correlation. Antenna  10  includes a ground support, which is configured as a ground plane  12 ; an electrically conductive, endless element, which is configured as a flat ring or circular element  14 , that is mounted at a constant distance relative to the ground plane  12 ; and a trio of ports  16 ,  18 ,  20  that are equiangularly arranged along the circumference of the circular element  14  for conveying radio frequency signals in an operating band of frequencies, e.g., 2.40 GHz to 2.48 GHz. Adjacent ports  16 ,  18 ,  20  are successively spaced circumferentially apart along the circular element  14  by a spacing of one-half of a guided wavelength (λ/2) at a center frequency, e.g., 2.44 GHz, of the operating band. The circumference of the circular element  14  is 3λ/2. This numerical operating band of frequencies is merely exemplary. It will be understood that different operating frequency bands and different operating frequency ranges, as described below, could also be used. 
     As shown in  FIGS. 3-4  for representative port  20 , in a preferred embodiment, each port includes an electrically insulating component or dielectric  22 , e. g., constituted of Teflon, for holding the circular element  14  at the distance; an electrical center conductor  24  extending through the dielectric  22  and galvanically connected, or electromagnetically coupled, to the circular element  14 ; and an electrically shielding component or outer electrically conductive shield  26  surrounding the dielectric  22  and shielding the electrical conductor  24 . Evidently, in this embodiment, the center conductor  24 , the dielectric  22 , and the conductive shield  26  form a coaxial cable. This cable, if sufficiently rigid, provides the mechanical function of suspending and supporting the circular element  14  above the ground plane  12 . In the preferred embodiment of  FIG. 4 , which is an enlarged, sectional view taken on line  4 - 4  of  FIG. 1 , an upper end of the conductor  24  extends through a hole that extends through the circular element  14  and is soldered at weld joint  28 . A lower end  48  of the conductive shield  26  is galvanically connected to the ground plane  12 . A lower end of the conductor  24  extends through a hole in the ground plane  12 , the hole having a diameter approximately equal to the inner diameter of the conductive shield  26 . The lower end of the conductor  24  extends through the ground plane  12  and, as illustrated in  FIG. 4 , is electrically connected to a microstrip feed line  30  on a dielectric substrate  32  provided at the underside of the ground plane  12 . It will be understood that a different feed arrangement, such as a coaxial cable and a pair of connectors for each port, could also be used instead of the microstrip arrangement to feed a signal to the conductor  24 . 
     In a preferred embodiment, the ground plane  12  has an outer contoured support surface, and the circular element  14  has an outer antenna surface of complementary contour to the contoured support surface. As shown in the embodiment of  FIGS. 1-3 , the circular element  14  is planar and its outer antenna surface is generally parallel to, and at approximately a constant distance relative to, the outer planar support surface of the ground plane  12 . The circular element  14  is maintained at the aforementioned constant distance from the ground plane  12  by the dielectric  22  of each port  16 ,  18 ,  20 . The constant distance between the circular element  14  and the ground plane  12  is selected and/or adjusted, as described below, to produce a desired impedance match, e.g., 50 ohms, at each port  16 ,  18 ,  20  to efficiently radiate/receive radio frequency power at any of the ports. 
     In an exemplary embodiment, the circular element  14  is constituted of a metal, such as steel, preferably with a gold or nickel plating. When operative at the operating band of frequencies, e.g., 2.40 GHz to 2.48 GHz, the circular element  14  has a width of about 1-5 mm, preferably about 2-3 mm, and is maintained at the distance of about 17 mm relative to the ground plane  12  to obtain approximately the desired 50 ohm impedance match. The aforementioned spacing of one-half of a guided wavelength between adjacent ports, along the circular element  14 , is about 57.5 mm. 
     In use as a transmitting antenna, a plurality of radio frequency sources together with antenna matching circuits (not illustrated), preferably one matching circuit for each port, are mounted at the opposite side of the ground plane  12 , preferably between the microstrip line  30  and the center conductor  24 . Each source generates a radio frequency signal that is conducted along the respective microstrip line  30  to the respective center conductor  24 , through a matching circuit, if needed, and to the circular element  14 . Thus, each radio frequency signal is fed to each port, preferably simultaneously, and is radiated from the entire circular element  14 . The three ports, so decoupled, serve as three independent channels. The radio frequency signal emitted at any one port, e.g., port  16 , will split equally in opposite circumferential directions along the circular element  14 . One split signal will arrive at an adjacent port, e.g., port  18 , a half wavelength away (180 degrees out of phase), while the other split signal will arrive at the same adjacent port  18  a full wavelength away (360 degrees; thus, in phase). The same analysis is valid for any other pair of neighboring ports. The split signals thus feature opposite phases, and cancel each other out, at the same adjacent port  18 . Due to symmetry, all three ports have the same properties. 
     Thus, there is a high (near ideal) port isolation between the ports  16 ,  18 , across the aforementioned narrow fractional operating band, and a corresponding low pattern correlation between the radiated electromagnetic patterns, provided that the ohmic losses of the antenna are moderate. This yields a high data throughput and an enhanced antenna performance in MIMO wireless communication systems, for instance, Wi-Fi devices operating under at least one version of the IEEE 802.11 standard. Advantageously, the circular element  14  is a dual-band antenna and radiates radio frequency waves not only in the aforementioned operating band of frequencies, e.g., 2.40 GHz to 2.48 GHz, but also efficiently radiates radio frequency waves in an additional operating band of higher frequencies, e.g., 5 GHz to 6 GHz, thereby making the antenna especially desirable for use in dual-band, wireless, Wi-Fi routers. 
       FIGS. 5-8  depict variations of the antenna. In the embodiment of  FIG. 5 , the ground support  12  is large enough to accommodate and support three circular elements  14 A;  14 B; and  14 C, each with its own set of respective ports  16 A,  18 A,  20 A;  16 B,  18 B,  20 B; and  16 C,  18 C,  20 C. As illustrated, the antennas are translated in position relative to one another, i.e., the same numbered ports have the same angular positions relative to the ground support  12 . As an example, the ports  18 A,  18 B,  18 C all face generally rightwardly and downwardly in  FIG. 5 . It will be understood that the antennas could also be rotated in position relative to one another, i.e., the same numbered ports have different relative positions relative to the ground support  12 . This rotation is about an axis that is perpendicular to the ground support  12  and is centrally located within the respective endless element  14 A,  14 B, and  14 C. As an example, the port  18 B could be located in either the illustrated position of port  20 B or port  16 B. It will be further understood that one or more of the antennas in  FIG. 5  could be translated and rotated. 
     In the embodiment of  FIG. 6 , the circular element  14  and its ports  16 ,  18 ,  20  are mounted at one side  12 A of the ground support  12 , and an additional circular element  14 D and its ports  16 D,  18 D,  20 D are mounted at an opposite side  12 B of the ground support  12 . The additional ports  16 D,  18 D,  20 D are arranged along the additional circular element  14 D for conveying radio frequency signals in one or multiple operating bands of frequencies. The additional ports  16 D,  18 D,  20 D are spaced apart along the additional circular element  14 D by a spacing of one-half of a guided wavelength at the center frequency of an operating band. Although the ports  16 ,  16 D; ports  18 ,  18 D; and ports  20 ,  20 D are illustrated as being aligned, i.e., collinear, it will be understood that one of the antennas could be rotated about an axis that is perpendicular to the ground support  12  and is centrally located within the respective endless element  14  and  14 D. The back-to-back configuration of the embodiment of  FIG. 6  provides six ports with high port isolation and can advantageously be positioned on corridor walls to provide independent Wi-Fi zones in opposite directions of the corridor. Furthermore, the double-faced ground support  12  of  FIG. 6  can be hollow and thick enough to contain Wi-Fi router circuitry, batteries, and the like, thereby forming a wholly functional device. 
     The embodiment of  FIG. 6  also depicts an annular adjustment element  34  fixedly mounted on the ground support  12  for adjusting the distance between the circular element  14  and the ground support  12  to achieve the aforementioned 50 ohm impedance match. The adjustment element  34  may be one of a set of such adjustment elements of different heights. A user selects an adjustment element  34  of the proper height (H), thereby setting the constant distance between the circular element  14  and the ground support  12  to an optimum value. In a preferred embodiment, the adjustment element  34  has a thin cross-section and is galvanically connected to the ground support  12  and to the conductive shield  26  of each port. This adjustment element  34  may be used in any of the other disclosed antenna embodiments. 
     Furthermore, other embodiments of the adjustment element  34  may include the case where the adjustment element  34  is suspended between the ground support  12  and the circular element  14 . For instance, the adjustment element  34  may be galvanically connected to the conductive shield  26  of each port and be supported mechanically by each conductive shield  26  at some distance from the ground support  12 , and at another distance from the circular element  14 . 
     The ground support  12  need not lie in a plane, but, as illustrated in the embodiments of  FIGS. 7-8 , may be curved. In  FIG. 7 , the ground support is a frustoconical support  36 . In  FIG. 7 , the ground support is a cylindrical support  38 . In these preferred embodiments, the outer antenna surface of the circular element is of complementary contour with, and maintained at a constant distance from, the outer contoured support surface. Hence, in  FIG. 7 , the circular element  14 E (associated with ports  16 E,  18 E,  20 E) is likewise conically shaped, and, in  FIG. 8 , the circular element  14 F (associated with ports  16 F (hidden),  18 F,  20 F) is likewise cylindrically shaped. 
       FIG. 9  is a view analogous to  FIG. 4 , but depicting another preferred embodiment, in which the endless element  14  is again suspended above a ground plane  12 . However, in contrast to the above-described coaxial cable configuration of the representative port  20  in  FIG. 4 , the representative port  40  in  FIG. 9  is configured as a solid metal post  42 . An upper metal disk  44  at or adjacent the top of the post  42  is spaced from the endless element  14  and serves as a series capacitor therewith. A dielectric (not illustrated so as to simplify the drawing) is located between the disk  44  and the endless element  14  to support the latter. A lower metal disk  46  at or adjacent the bottom of the post  42  is spaced from the ground plane  12  and serves as a shunt capacitor therewith. A dielectric (not illustrated so as to simplify the drawing) is located between the disk  46  and the ground plane  12 . The size and spacing of these disks  44 ,  46 , as well as the permittivity of the aforementioned dielectrics, control the value of their capacitances and are employed to optimize the aforementioned impedance match, and may replace the aforementioned adjustment element  34 . The post  42  in  FIG. 9  extends through the ground support  12 , and the bottom end of the post  42  is galvanically connected to the aforementioned microstrip feed line  30 . Again, a dielectric support between the feed line  30  and the ground support  12  has been omitted so as not to encumber the drawing. 
       FIG. 10  is a view analogous to  FIG. 9 , but depicting another preferred embodiment, in which a conductor  48  at the bottom of the post  42  extends through the ground plane  12 , and an RF connector  50  is used to feed a signal to port  40 . 
     In the foregoing specification, specific embodiments have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present teachings. 
     The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued. 
     Moreover in this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” “has,” “having,” “includes,” “including,” “contains,” “containing,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises, has, includes, contains a list of elements does not include only those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a,” “has . . . a,” “includes . . . a,” or “contains . . . a,” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises, has, includes, or contains the element. The terms “a” and “an” are defined as one or more unless explicitly stated otherwise herein. The terms “substantially,” “essentially,” “approximately,” “about,” or any other version thereof, are defined as being close to as understood by one of ordinary skill in the art, and in one non-limiting embodiment the term is defined to be within 10%, in another embodiment within 5%, in another embodiment within 1%, and in another embodiment within 0.5%. The term “coupled” as used herein is defined as connected, although not necessarily directly and not necessarily mechanically. A device or structure that is “configured” in a certain way is configured in at least that way, but may also be configured in ways that are not listed. 
     It will be appreciated that some embodiments may be comprised of one or more generic or specialized processors (or “processing devices”) such as microprocessors, digital signal processors, customized processors, and field programmable gate arrays (FPGAs), and unique stored program instructions (including both software and firmware) that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of the method and/or apparatus described herein. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. 
     Moreover, an embodiment can be implemented as a computer-readable storage medium having computer readable code stored thereon for programming a computer (e.g., comprising a processor) to perform a method as described and claimed herein. Examples of such computer-readable storage mediums include, but are not limited to, a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a ROM (Read Only Memory), a PROM (Programmable Read Only Memory), an EPROM (Erasable Programmable Read Only Memory), an EEPROM (Electrically Erasable Programmable Read Only Memory) and a Flash memory. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein, will be readily capable of generating such software instructions and programs and ICs with minimal experimentation. 
     The Abstract of the Disclosure is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in various embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter.