Abstract:
A multi-band antenna system for MIMO applications is adapted to provide high isolation between antennas across a wide range of frequencies. Multiple Isolated Magnetic Dipole (IMD) antennas are co-located and connected with a feed network that can include switches that adjust phase length for transmission lines connecting the antennas. Filtering is integrated into the feed network to improve rejection of unwanted frequencies. Filtering can also be implemented on the antenna structure. Either one or multi-port antennas can be used.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 13/548,221, filed Jul. 13, 2012, and titled “MULTI-BAND MIMO ANTENNA”; 
     which is a CIP of U.S. patent application Ser. No. 13/548,211, filed Jul. 13, 2012, and titled “Multi-Feed Antenna for Path Optimization”; 
     which is a CIP of U.S. patent application Ser. No. 13/289,901, filed Nov. 4, 2011, and titled “Antenna With Active Elements”; 
     which is a CON of U.S. patent application Ser. No. 12/894,052, filed Sep. 29, 2010, and also titled “Antenna With Active Elements”; 
     which is a CON of U.S. patent application Ser. No. 11/841,207, filed Aug. 20, 2007, and also titled “Antenna With Active Elements”; 
     the contents of each of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of wireless communications and devices, and more particularly to the design of antennas configured for robust multiple band multi-input multi-output (MIMO) implementations for use in wireless communications. 
     2. Description of the Related Art 
     Commonly owned U.S. Pat. Nos. 7,339,531; 6,943,730; 6,919,857; 6,900,773; 6,859,175; 6,744,410; 6,323,810; and 6,515,634; describe an IMD antenna formed by coupling one element to another in a manner that forms a capacitively loaded inductive loop, setting up a magnetic dipole mode; the entire disclosures of which are hereby incorporated by reference. The magnetic dipole mode can also be generated by inducing a current mode onto a conductive element with specific slot geometry. This magnetic dipole mode provides a single or dual resonance and forms an antenna that is efficient and well isolated from the surrounding structure. This is, in effect, a self resonant structure that is de-coupled from the local environment. This antenna typically has a single feed for connection of the antenna to the transceiver. The IMD antenna is well isolated from the surrounding environment and two or more IMD antennas can be closely spaced and maintain high levels of isolation. This high isolation is a desired attribute for antennas directed toward multi-input multi-output (MIMO) implementations. 
     Current and future communication systems will require MIMO antenna systems capable of operation over multiple frequency bands. Isolation between adjacent elements as well as de-correlated radiation patterns will need to be maintained across multiple frequency bands, with antenna efficiency needing to be optimized for the antenna system. 
     SUMMARY OF THE INVENTION 
     Various embodiments of a multi-band antenna system are disclosed which provide high isolation between multiple antennas at two or more frequency bands. A transmission line network is described which optimizes isolation between antennas, and that incorporates filters, switches, and/or passive and active components to provide a fixed or dynamically tuned multi-antenna system. A beam steering feature is described capable of changing the radiation pattern of one or multiple antennas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and characteristics of the invention will become apparent from the examples illustrated below pertaining to a hand-operated tool and methods for use therewith for which reference will be made to the attached figures, where: 
         FIG. 1  illustrates a schematic of two antennas, the feed ports of the antennas being connected with two transmission lines, and a filter is located in the second transmission line. 
         FIG. 2  illustrates a graph of the frequency response from the antenna system provided in  FIG. 1 , the graph illustrating both return loss and isolation. 
         FIG. 3  illustrates the isolation provided between antenna  1  and antenna  2  of in  FIG. 1 , and the combination plot of the two transmission lines. 
         FIG. 4  illustrates a system having two antennas, the feed ports of the antennas being connected with two transmission lines, and a filter is located in the second transmission line. The location of the filter in the transmission line is used to optimize antenna system performance by improving isolation. 
         FIG. 5  illustrates a system having two antennas, the feed ports of the antennas connected with two transmission lines, and a filter is positioned in both transmission lines. The location of the filters in each of the transmission lines is configured to optimize antenna system performance by improving rejection at specific frequencies. 
         FIG. 6  illustrates an a system having two antennas, the feed ports of the antennas connected with two transmission lines, and a filter and switch are positioned in each of the transmission lines. 
         FIG. 7  illustrates a pair of antennas with the antenna feed ports connected by a single transmission line. The transmission line consists of a multi-port switch assembly comprising two four-port switches allowing the electrical length of the transmission line to be varied. 
         FIG. 8  illustrates a pair of antennas with the antenna feed ports connected by a single transmission line. The transmission line consists of a multi-port switch assembly comprising two four port switches in addition to a circuit for impedance matching in series with the with the four port switches. 
         FIG. 9  illustrates a pair of antennas with the antenna feed ports connected by a single transmission line. The transmission line consists of a multi-port switch assembly comprising two four-port switches in addition to a circuit for impedance matching in parallel with the with the four-port switches. 
         FIG. 10  illustrates an antenna system having two antennas, each with three feed ports and transmission lines connecting pairs of feed ports. Filters are incorporated into the antenna structures improve rejection of unwanted frequencies for the specific transmission lines. A combiner is used to combine the three feed ports into a single port for connection of the antenna to a transceiver or other component or subsystem. 
         FIGS. 11(   a - c ) illustrate the antenna system configuration described in  FIG. 10  with the exception that the feed ports of the antennas are capacitively coupled to the transmission lines. Two illustrations are shown of Isolated Magnetic Dipole (IMD) antennas with feed ports capacitively coupled to a region of the antenna by placing a second conductive element in close proximity to the main antenna element. 
         FIGS. 12  ( a - b ) illustrate an isolated magnetic dipole (IMD) antenna with two feed ports and with filters integrated into the antenna element. The feed ports are connected to separate transceivers. Several types of conductive elements with distributed reactance incorporated into the element are shown. 
         FIG. 13  illustrates an antenna system having two antennas with feed ports that are capacitively coupled to the transmission lines. Filters are incorporated into the second antenna to improve rejection of unwanted frequencies for the specific transmission lines. A combiner is used to combine some of the feed ports into a single port. 
         FIGS. 14(   a - b ) illustrate an antenna system having two antennas with the feed ports of the antennas connected with two transmission lines. The electrical length of each transmission line is chosen to provide optimal isolation between the pair of antennas at a specific frequency band. A filter is incorporated in the second transmission line to improve rejection of the frequencies that the second transmission line is optimized for. An additional element, a parasitic element, is connected to an active element and positioned in proximity to one or both antennas. The active tuning element can, for example, be any one or more of voltage controlled tunable capacitors, voltage controlled tunable phase shifters, FET&#39;s, switches, MEMs device, transistor, or circuit capable of exhibiting ON-OFF and/or actively controllable conductive/inductive characteristics. 
         FIGS. 15(   a - d ) illustrate an antenna system having two antennas with the feed ports of the antennas connected with two transmission lines. The electrical length of each transmission line is chosen to provide optimal isolation between the pair of antennas at a specific frequency band. A filter is incorporated in the second transmission line to improve rejection of the frequencies that the second transmission line is optimized for. One or multiple additional elements with one or multiple active elements are positioned in proximity to one or both antennas. The active tuning elements can, for example, be any one or more of voltage controlled tunable capacitors, voltage controlled tunable phase shifters, FET&#39;s, switches, MEMs device, transistor, or circuit capable of exhibiting ON-OFF and/or actively controllable conductive/inductive characteristics. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, for purposes of explanation and not limitation, details and descriptions are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments that depart from these details and descriptions. 
     In accordance with one embodiment,  FIG. 1  illustrates an antenna system having two antenna elements  1 ,  2  with the feed ports  3 ,  4  of the antennas connected with two transmission lines  5  and  6 . The two antenna elements can be referred to as a first antenna element  1  and a second antenna element  2 , respectively. The electrical length of each transmission line is chosen to provide optimal isolation between the pair of antennas  1  and  2  at a specific frequency band. A filter  7  is incorporated in the second transmission line  6  to improve rejection of one or more frequencies. 
       FIG. 2  illustrates an example of the frequency response from the antenna system shown in  FIG. 1 . The electrical characteristics of transmission line  5  in  FIG. 1  are optimized to provide good isolation between antennas  1  and  2  at the low frequency resonance  21 . The electrical characteristics of transmission line  6  in  FIG. 1  are optimized to provide good isolation between antennas  1  and  2  at the high frequency resonance  22 . The isolation between antenna  1  and antenna  2  in  FIG. 1  is shown by dotted line  23 . The isolation at both low and high frequency resonance is below the solid lines  24  labeled “Isolation Requirement”. 
       FIG. 3  shows a more detailed plot of the isolation between antenna  1  and antenna  2  as shown in  FIG. 1 . The plots of the return losses for antenna  1  and antenna  2  with low and high resonances are shown by lines  31  and  32 , respectively. A plot of the isolation for antenna  1  is shown by dotted line  33 . A plot of the isolation for antenna  2  is shown by dotted line  34 . A combination of the transmission lines  1  and  2  provides good isolation at both low and high frequency resonances as shown by plot line  35 . 
     In accordance with another embodiment,  FIG. 4  illustrates two antenna elements  41  and  42  with the feed ports  43  and  44  of the antennas connected with two transmission lines  45  and  46 . The electrical length of each transmission line is chosen to provide optimal isolation between the pair of antennas  41  and  42  at a specific frequency band. The location of the filter  47  in the second transmission line is chosen to optimize antenna isolation by increasing or decreasing the distance between the filter  47  and the feed points  43  and  44  of the antenna. This feature provides a method to use the coupling between the transmission lines and coupling between the antennas and the transmission lines to optimize antenna system performance by improving isolation. 
     In accordance with another embodiment,  FIG. 5  illustrates two antenna elements  51  and  52  with the feed ports  53  and  54  of the antennas connected with two transmission lines  55  and  56 . The electrical length of each transmission line is chosen to provide optimal isolation between the pair of antennas at a specific frequency band. Filters  57  and  58  are incorporated into each transmission line to improve rejection of the frequencies that each transmission line is optimized for. The location of each filter is chosen to optimize antenna isolation by increasing or decreasing the distance between the filters and the feed points of the antenna. This feature provides a method to use the coupling between the transmission lines and coupling between the antennas and the transmission lines to optimize antenna system performance by improving isolation 
     In accordance with another embodiment,  FIG. 6  illustrates two antenna elements  61  and  62  with the feed ports  63  and  64  of the antennas connected with two transmission lines  65  and  66 . The electrical length of each transmission line is chosen to provide optimal isolation between the pair of antennas at a specific frequency band. Filters  67   a  and  67   b  and switches  68  and  69  are incorporated into each respective transmission line. Filters  67   a  and  67   b  are used to improve rejection of the frequencies that each transmission line is optimized for. Switches  68  and  69  provide the ability to dynamically connect or disconnect the transmission line used to connect the antenna feed ports. 
     In accordance with another embodiment,  FIG. 7  illustrates a pair of antenna elements  71  and  72  with the antenna feed ports  73  and  74  connected by a single transmission line  75 . A multi-port switch assembly  76  comprising two four port switches with transmission lines connecting adjacent ports is incorporated into the transmission line. This provides the ability to switch in different selections of transmission line to vary the electrical length of the total feed network. The feed network includes the transmission line  75  connecting the two antennas  71  and  72  along with the multi-port switch assembly  76 . 
     In accordance with another embodiment,  FIG. 8  illustrates a pair of antenna elements  81  and  82  with the antenna feed ports  83  and  84  connected by a single transmission line  85 . A multi-port switch assembly  86  comprising two four port switches with transmission lines connecting adjacent ports is incorporated into the transmission line  85 . This provides the ability to switch in different selections of transmission line to vary the electrical length of the total feed network, the feed network including the transmission line connecting the two antennas along with the multi-port switch assembly. A passive or active circuit  87  is attached in a series configuration to the switch assembly  86  and provides a method of adjusting the impedance match of the transmission line connecting the pair of antennas either statically for a passive circuit, or dynamically for an active circuit. 
     In accordance with another embodiment,  FIG. 9  illustrates a pair of antenna elements  91  and  92  with the antenna feed ports  93  and  94  connected by a single transmission line  95 . A multi-port switch assembly  96  comprising two four port switches with transmission lines connecting adjacent ports is incorporated into the transmission line. This provides the ability to switch in different selections of transmission line to vary the electrical length of the total feed network, the feed network including the transmission connecting the two antennas along with the multi-port switch assembly. A passive or active circuit  97  is attached in a shunt configuration to the switch assembly  96  and provides a method of adjusting the impedance match of the transmission line connecting the pair of antennas either statically for a passive circuit, or dynamically for an active circuit. 
     In accordance with another embodiment,  FIG. 10  illustrates a first antenna  101  with a first feed port  101   a , a second feed port  101   b , and a third feed port  101   c , and a second antenna  102  with a fourth feed port  102   a , a fifth feed port  102   b , and a sixth feed port  102   c . Transmission lines  104   a ,  104   b  and  104   c  are used to connect pairs of respective feed ports as illustrated. Filters  103   a ,  103   b ,  104   a  and  104   b  are incorporated into the antenna structures  101  and  102  to improve rejection of unwanted frequencies for the specific transmission lines. The electrical length of the transmission lines connecting pairs of antenna feed ports is chosen to provide optimal isolation between the pair of antennas at a specific frequency band. A combiner  105  is used to combine the three feed ports into a single port for connection of the antenna to a transceiver or other component or subsystem. For example, the schematic in this figure shows the high band response optimized with the electrical delay line L 1  for frequency Fh. Filters  103   a  and  104   a  are low pass filters that pass frequencies below Fh. Filters  103   b  and  104   b  are low pass filters that pass frequencies below Fm. This schematic allows three separate frequency bands to be optimized simultaneously. 
     In accordance with another embodiment,  FIG. 11(   a ) illustrates the antenna system configuration described in  FIG. 10  with the exception that the feed ports of the antennas are capacitively coupled at points  110   a ,  110   b ,  110   c ,  111   a ,  111   b  and  111   c  to the transmission lines. 
       FIG. 11(   b ) illustrates an isolated magnetic dipole (IMD) antenna  114  with a feed port  112 . A second element  115  is located below the IMD element providing an additional feed port  113  as a result of the coupling between the IMD antenna  112  and the second element  115 . This structure creates a low band frequency resonance with two feed ports. 
       FIG. 11(   c ) illustrates an exemplary example of an isolated magnetic dipole (IMD) antenna  118  with a feed port  116 . A second element  119  is located below the IMD element providing an additional feed port  117  as a result of the coupling between the IMD antenna  118  and the second element  119 . This structure creates a high band frequency resonance with two feed ports. 
     In accordance with another embodiment,  FIG. 12  illustrates an isolated magnetic dipole (IMD) antenna  125  with two feed ports  121  and  122  and with filters  123  and  124  integrated into the antenna element  125 . The feed ports  121  and  122  are connected to separate transceivers. Several types of conductive elements with distributed reactance incorporated into the element are shown. The distributed reactance can be adjusted to alter the frequency response of the conductive element. A distributed LC section  126   a  is designed into a conductive element. Two distributed LC sections  126   b  and  126   c  are designed into a single conductive element. A series of capacitive sections are formed by coupling regions  126   d  designed into a conductive element. A method to reduce the frequency of operation is shown in the design  126   e  incorporated into a conductive element. Another method of applying a distributed LC circuit is shown in pattern  126   f.    
     In accordance with another embodiment,  FIG. 13  illustrates a pair of antennas, the first antenna  131  having a single feed port  131   a  and the second antenna  132  having three feed ports,  132   a ,  132   b , and  132   c . A transmission line  133  is used to connect the single feed port  131   a  of the first antenna to the three feed ports  132   a ,  132   b , and  132   c  of the second antenna  132  using capacitive coupling. Filters  134  and  135  are incorporated into the antenna structure of the second antenna  132  to improve rejection of unwanted frequencies for the specific transmission lines. A combiner  136  is used to combine the three feed ports into a single port for connection of the antenna to a transceiver or other component or subsystem. 
     In accordance with another embodiment,  FIG. 14  illustrates two antennas  141  and  142  with the feed ports  143  and  144  of the antennas connected with two transmission lines  145  and  146 . The electrical length of each transmission line is chosen to provide optimal isolation between the pair of antennas at a specific frequency band. A filter  147  is incorporated in the second transmission line  146  to improve rejection of the frequencies that the first transmission line is optimized for. An additional element, a parasitic element  148 , is connected to an active element  149  and positioned in proximity to one or both antennas. The active tuning element  149  can, for example, be any one or more of voltage controlled tunable capacitors, voltage controlled tunable phase shifters, FET&#39;s, switches, MEMs device, transistor, or circuit capable of exhibiting ON-OFF and/or actively controllable conductive/inductive characteristics. It should be further noted that coupling of the various active control elements to different antenna and/or parasitic elements may be accomplished in different ways. For example, active elements may be deposited generally within the feed area of the antenna and/or parasitic elements by electrically coupling one end of the active element to the feed line, and coupling the other end to the ground portion. This element is coupled to one or both antennas and will alter the radiation pattern of one or both antennas as the active element is transitioned from one reactance to a second, different reactance. The simplest method is to transition from an open to short condition to adjust the antenna beam position. 
     In yet another embodiment,  FIG. 15  illustrates two antennas  151  and  152  with the feed ports  153  and  154  of the antennas connected with two transmission lines  155  and  156 . The electrical length of each transmission line is chosen to provide optimal isolation between the pair of antennas at a specific frequency band. A filter  157  is incorporated in the second transmission line  156  to improve rejection of the frequencies that the second transmission line is optimized for. Two active elements  148  and  149  are attached to a parasitic element and positioned in proximity to one or both antennas. The active tuning elements  158  and  159  can, for example, be any one or more of voltage controlled tunable capacitors, voltage controlled tunable phase shifters, FET&#39;s, switches, MEMs device, transistor, or circuit capable of exhibiting ON-OFF and/or actively controllable conductive/inductive characteristics. This element is coupled to one or both antennas and will alter the radiation pattern of one or both antennas as the active element is transitioned from one reactance to a second, different reactance. The simplest method is to transition from an open to short condition to adjust the antenna beam position. The first top view illustrates multiple parasitic elements with active elements surrounding the two antennas. These parasitic elements provide the ability to alter the antenna beam position of one or both antennas. The second top view illustrates an alternate configuration for radiation pattern control. 
     The above examples are set forth for illustrative purposes and are not intended to limit the spirit and scope of the invention. One having skill in the art will recognize that deviations from the aforementioned examples can be created which substantially perform the same task and obtain similar results.