Abstract:
An active differential antenna is described that provides for improved performance for wireless communication systems across a wide set of use cases and environments. A balanced antenna structure along with switch assembly provides the differential mode radiation which results in minimal coupling to the components and items in the near field of the antenna. This results in an efficient antenna that is well isolated from the local environment of the antenna. The switch assembly is configured to switch the feed and ground connections of the differential design when needed to provide similar antenna performance for both “against head left” and “against head right” use cases for a cellular handset application for example. An active component or circuit can be integrated or coupled to the antenna design to provide the capability to dynamically balance the antenna to maintain pattern symmetry and efficiency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of priority with U.S. Provisional Ser. No. 61/636,553, filed Apr. 20, 2012, titled “LOOP ANTENNA WITH SWITCHABLE FEEDING AND GROUNDING POINTS”; the contents 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 communication. In particular, this invention relates to an active differential mode loop antenna configured to maintain efficient operation across a wide set of use cases for use in wireless communications. 
     2. Description of the Related Art 
     The availability of wireless services, such as Global System for Mobile Communications (GSM), Radio Frequency Identification (RFID), Distributed Control System (DCS), Personal Communications Service (PCS), UW, Digital Video Broadcasting-Terrestrial/Handheld (DVB-T/H), Wireless Fidelity (Wifi), Bt, Worldwide Interoperability for Microwave Access (Wimax), Long Term Evolution (LTE), Global Positioning System (GPS), and others, supported by modern handsets, such as MP3 player, mobile phone, laptop, video gaming devices, tablets, and the like have increased significantly during the last decade. 
     The Numbers of antennas in each device is increasing as well as the number of available wireless services and therefore, the embedded antennas need to be small and require high performance. Modern communication devices such as cellphones typically contain four or five antennas, with each antenna serving a specific function and frequency band. These antennas are closely spaced and are volume constrained, and good isolation between the antennas is needed for efficient operation. 
     With cellular communication systems becoming more loaded and capacity constrained, the antenna systems on the mobile side of the communication link are expected to become more efficient to assist in maintaining a level of acceptable network performance. Under-performing mobile devices in regard to the radiated performance of the device will degrade the cellular network, with these under-performing devices requiring more system resources compared to more efficient mobile devices. 
     Several solutions have been proposed over the years to improve the Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS) performance of the cellular antenna or to fulfill Specific Absorption Rate (SAR) and Hearing Aid Compatibility (HAC) requirements. Though various antenna techniques and topologies have been proposed and developed to improve antenna efficiency for internal applications, they all suffer from the limitation of being optimized for a single use case such as device in user&#39;s hand, device against the user&#39;s head, or device in free space environment. To improve on this situation, an antenna can be designed to provide a compromise solution, where the performance of the antenna is considered for a multitude of use cases and is not optimized for a preferred use case. 
     One antenna structure, called a folded loop antenna, has demonstrated several advantages for handset applications. It can be designed to have several resonances, with one resonance to cover low band cellular frequencies (&lt;1 GHz) and one or multiple resonances to cover high band cellular frequencies (1.5 GHz to 10 GHz bands) when applied to cellular applications. One important benefit of this antenna structure is that one of the different resonances of the folded loop antenna located in the high band (1710 MHZ to 2170 MHZ) is generated from a differential mode (also referred as a balanced mode). The advantages of this differential mode, are lower losses from the head when the phone is in “beside head” position, lower HAC and SAR values. 
     The differential mode existence is however tightly related to the symmetry of the way the antenna&#39;s E and H field are coupling with the mechanics of the host device. A symmetrical radiator design is required to generate the symmetrical coupling, which can be achieved during the antenna design process, but the non-symmetrical mechanical features of the host device will degrade the differential mode. Typically the non-symmetry of the mechanics of the host device is compensated for by introducing non-symmetry in the folded loop antenna radiator pattern. 
     When a folded loop antenna is designed and integrated into a wireless device for use in Free space conditions, the antenna can be tuned in a such way that the E and H are creating the desired differential mode. However, when the same antenna is used in other use cases such as against the user&#39;s head, in the user&#39;s hand, surrounded by external objects such as tables, the E and H fields will be disturbed. For example, the antenna performance will be different when the device is against the user&#39;s left side of the head as compared to the right side of the head, due to the local environment of the antenna changing between these two use cases when the host device is mobile phone. 
     Additionally, with the advent of 4G technologies such as LTE (Long Term Evolution) entering service in the mobile wireless industry, there is a need for MIMO (Multiple Input Multiple Output) antenna systems in small mobile devices such as smart phones. For optimal MIMO performance the antenna efficiencies for the two antennas in a MIMO system should be equal. High isolation and low ECC (Envelope Correlation Coefficient) is also required for optimal MIMO antenna system operation, and isolation and ECC can be difficult to achieve in these small form factors. It is difficult to keep the efficiencies of two antennas in a small mobile device equal across the several use cases previously mentioned. The antennas can be designed to provide equivalent performance for a preferred use case, but the efficiencies of the two antennas will diverge as the local environment changes. 
     SUMMARY OF THE INVENTION 
     A passive folded loop antenna is disclosed. The passive folded loop antenna, when the device is positioned beside the head (BH) or in the hand (FH), the relative position of the signal feeding point and grounding point of the antenna radiator, compared to the head or hand is not identical whether you are using it as right handed or as left handed person. This difference of position leads to a different E and H field distribution around the antenna which creates a difference in performance between beside head Left (BHL) and beside head right (BHR) positions, which can be several dB. 
     For the same reason, performances differences can also be of several dB if the device is held in the right hand (FHR) or in the left hand. Leveraging on the almost symmetrical shape of the antenna radiator, the antennas herein provide an improved solution to limit the performance drop between the right or left side usage of the device. 
     In certain embodiments, an antenna structure comprises at least one folded loop antenna element, a radiator, which has at least two signal connection points, one at the first end of the antenna radiator and one at the other end of the antenna radiator, and one active component which can swap the connections between the antenna&#39;s radiator&#39;s two connection points and the feeding and grounding pads on the device&#39;s Printed Circuit Board (PCB). 
     Other features and advantages are described in the appended detailed description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 (A-B) illustrate a loop antenna with swappable feed connection and ground connection. 
         FIG. 2  illustrates a loop antenna integrated into a communication device. 
         FIGS. 3 (A-B) illustrate two typical use positions of a cell phone against a user&#39;s head, phone in head right position and phone in head left position. 
         FIG. 4  illustrates a loop antenna connected to a switch assembly to provide the capability to alter feed connection and ground connection between the loop antenna and an external circuit. 
         FIG. 5  illustrates a loop antenna connected to a switch assembly to provide the capability to alter feed connection and ground connection between the loop antenna and an external circuit. 
         FIG. 6  illustrates a communication device  95  which contains two loop antennas according to one embodiment. 
         FIG. 7  illustrates a communication device  95  which contains two loop antennas according to another embodiment. 
         FIG. 8  illustrates a two antenna system that provides the capability to alter Envelope Correlation Coefficient (ECC) and/or isolation dynamically in accordance with one embodiment. 
         FIG. 9  illustrates a two antenna system that provides the capability to alter Envelope Correlation Coefficient (ECC) and/or isolation dynamically in accordance with another embodiment. 
         FIGS. 10 (A-B) illustrate a two antenna system where the loop antennas are co-located or nested together. 
         FIG. 11  illustrates a technique of coupling two loop antennas to a third, larger loop antenna. 
         FIGS. 12 (A-B) illustrate a technique of using a common switch assembly to feed two loop antennas. 
         FIGS. 13 (A-B) illustrate a swappable feed technique applied to an Isolated Magnetic Dipole (IMD) antenna. 
         FIG. 14  illustrates a loop antenna with swappable feed connection and ground connection. 
         FIG. 15  illustrates a folded loop antenna structure wherein the loop antenna is formed on a circuit board of the device. 
         FIG. 16  illustrates two opposing loop antenna structures formed at each opposing end of a device circuit board. 
         FIG. 17  illustrates the folded loop antenna formed about a device circuit board and comprising at least one parasitic element coupled to an active component for actively configuring the loop antenna. 
     
    
    
     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. 
     According to an example embodiment, the active swapping circuit can comprise transistors, diodes or Micro Electrical Mechanical System (MEMS) devices. 
     In another embodiment, the swapping circuit can have more than two inputs and two outputs and can offer a larger matrix of output connection for the radiator&#39;s connection points. 
     In another embodiment of the invention, a parasitic element can be coupled to a portion of the folded loop antenna. An active component can be connected to or coupled to the parasitic element, with this active component being used to alter the impedance loading on the parasitic element. By adjusting the impedance loading on the parasitic element the folded loop antenna can be tuned or compensated for to counteract the effects of loading on the loop antenna or the wireless device that the loop antenna is integrated in to. The swapping circuit can be used to determine which connection of the folded loop antenna is best for feeding the loop antenna; the parasitic element and active component can then be used to alter or fine tune the antenna element to compensate for loading effects. The active component can comprise an RF switch, tunable capacitor, MEMS switch or tunable capacitor, PIN diode, varactor diode, or tunable inductor. 
     In another embodiment of the invention, an active component can be connected to a portion of the folded loop radiator. This active component can be used to compensate for the effects of loading on the loop antenna or the wireless device the loop antenna is integrated in to. The active component can comprise an RF switch, tunable capacitor, MEMS switch or tunable capacitor, PIN diode, varactor diode, or tunable inductor. 
     In another embodiment of the invention, a pair of folded loop antennas can be used to comprise a MIMO antenna system. The pair of swappable feed circuits can be used to generate four combinations of feed configurations for the pair of antennas. An algorithm can be implemented in a processor on the host device, such as the baseband processor for example, wherein the four feed combinations can be sampled to determine which feed configuration provides the configuration for optimal isolation and/or ECC. As the loading on the host device changes, the antenna feed configuration can change to keep the pair of antennas optimized for MIMO system performance. 
     In another embodiment of the invention, two or more folded loop antennas can be connected to the same swapping circuit. Diplexers can be used to separate signals as a function of frequency and route the signals to the appropriate folded loop antenna. By adding additional diplexers, additional folded loops can be coupled to the same swapping circuit. The folded loop antennas can be nested or co-located to minimize volume required in the host device. 
     In yet another embodiment of the invention, a folded loop antenna with swapping circuit can be integrated into a host device such as a cell phone. A second larger loop antenna can be positioned in proximity to the first folded loop antenna with swapping circuit. The first folded loop antenna can act as a feed circuit for the larger loop antenna. 
     Now turning to the examples depicted in the drawings,  FIG. 1  illustrates an example of a loop antenna  1  with swappable feed connection  3  and ground connection  4 . A switch assembly  2  is used to change the feed and ground connections of the loop antenna  1  to an external transceiver or circuit. A control signal or signals  5  are provided to the switch assembly  2  to alter the feed and ground connections. “Antenna State  1 ” is shown in  FIG. 1A , where ground connection  4  is connected to the right portion of the loop antenna  1 , while feed connection  3  is connected to the left portion of loop antenna  1 . “Antenna State  2 ”, as illustrated in  FIG. 1B , illustrates a loop antenna  11  where the ground connection  14  is connected to the left side of loop antenna  11  and the feed connection  13  is connected to the right portion of loop antenna  11 . A control signal or signals  15  are provided to the switch assembly  12  to alter the feed and ground connections. 
       FIG. 2  illustrates an example of a loop antenna  31  integrated into a communication device  30 . 
       FIGS. 3 (A-B) illustrate two typical use positions of a cell phone against a user&#39;s head, phone beside head right (BHR) position  51  and phone beside head left (BHL) position  53 . Two primary hand positions for a phone are also illustrated, phone in right hand position  52  and phone in left hand position  54 . 
       FIG. 4  illustrates a loop antenna  60  connected to a switch assembly  62  to provide the capability to alter feed connection  63  and ground connection  64  between the loop antenna and an external circuit. A parasitic element  65  is positioned near the radiator and thereby coupled to a portion of loop antenna  60 . The parasitic element is in turn coupled to an active component  66 . Control signals  67  are provided to the active component  66  and the switch assembly  62 . 
       FIG. 5  illustrates a loop antenna  70  connected to a switch assembly  71  to provide the capability to alter feed connection  72  and ground connection  73  between the loop antenna and an external circuit. An active component  74  is connected to a portion of the loop antenna. Control signal  75  is provided to the active component to alter the characteristics of the loop antenna. Control signal  76  is provided to the switch assembly to alter the feed and ground connections. 
       FIG. 6  illustrates a communication device  95  which contains two loop antennas  90  and  97 . Loop antenna  90  is connected to switch assembly  91 . Transmission line  102  connects transceiver  100  to feed connection  92 . Control line  94  connects Baseband  101  to switch assembly  91 . Loop antenna  97  is connected to switch assembly  96 . Transmission line  103  connects transceiver  100  to feed connection  104 . Control line  99  connects Baseband  101  to switch assembly  96 . Control signals can be provided to both loop antennas simultaneously or serially to alter performance of the two antenna system. 
       FIG. 7  illustrates a communication device  95  which contains two loop antennas  110  and  117 . Each loop antenna contains a parasitic element and active component to adjust the antenna dynamically. Loop antenna  110  is connected to switch assembly  111 . Transmission line  126  connects transceiver  124  to feed connection  112 . Control line  116  connects Baseband  125  to switch assembly  111 . Parasitic element  114  is coupled to loop antenna  110 , and an active component  115  is connected to the parasitic element. A control line  116  from Baseband  125  is connected to the active component  115  to provide control signals to adjust the active component. A second loop antenna  117  is connected to switch assembly  118 . Transmission line  127  connects transceiver  124  to feed connection  119 . Control line  123  connects Baseband  125  to switch assembly  118 . Parasitic element  121  is coupled to loop antenna  117 , and an active component  122  is connected to the parasitic element. A control line  123  from Baseband  125  is connected to the active component  122  to provide control signals to adjust the active component. 
       FIG. 8  illustrates a two antenna system that provides the capability to alter Envelope Correlation Coefficient (ECC) and/or isolation dynamically. Loop antenna assembly  130  which contains a loop antenna and switch assembly is positioned in a communication device  136 . Loop antenna assembly  131  which contains a loop antenna and switch assembly is positioned at another location within the communication device  136 . An algorithm is resident in Baseband processor  133  which selects between  4  tuning states which are represented in Table  132 . Control lines  134  and  135  provide control signals to the loop antenna assemblies. 
       FIG. 9  illustrates a two antenna system that provides the capability to alter Envelope Correlation Coefficient (ECC) and/or isolation dynamically. Loop antenna assembly  140  which contains a loop antenna and switch assembly is positioned in a communication device  147 . A parasitic element  142  is coupled to the loop antenna and an active component  143  is connected to the parasitic to alter the impedance loading on the parasitic. Control line  150  provides control signals from Baseband  148  to active component  143 . Loop antenna assembly  141  which contains a loop antenna and switch assembly is positioned at another location within the communication device  147 . A parasitic element  144  is coupled to the loop antenna and an active component  145  is connected to the parasitic to alter the impedance loading on the parasitic. Control line  151  provides control signals from Baseband  148  to active component  145 . An algorithm is resident in Baseband processor  148  which selects between a plurality of tuning states which are represented in Table  146 . Control lines  149  and  152  provide control signals to the loop antenna assemblies. 
       FIGS. 10 (A-B) illustrate a two antenna system where the loop antennas are co-located or nested together. Loop antenna  160  is positioned on a ground plane  162 . A second loop antenna  161  is positioned beneath loop antenna  160 . 
       FIG. 11  illustrates a technique of coupling two loop antennas to a third, larger loop antenna. Loop antenna  181  is positioned on one side of a communication device  185  and connected to switch assembly  182 . A transceiver  190  is connected to port  183  of the switch assembly using a transmission line  191 , with port  184  being the ground connection. Loop antenna  186  is positioned on the opposing side of a communication device  185  and connected to switch assembly  187 . Transceiver  190  is connected to port  188  of the switch assembly using a transmission line  192 , with port  189  being the ground connection. A third loop  180  is positioned in the vicinity of both loop antennas  181  and  186 . One or both loop antennas  181  and  186  can couple a signal to loop  180  for use in transmitting a signal. Conversely, a received signal from loop  180  can be coupled to one or both loop antennas  181  and  186 , with the received signal coupled into the transceiver  190 . 
       FIG. 12 a    illustrates a technique of using a common switch assembly to feed two loop antennas. Switch assembly  204  is connected to diplexers  202  and  203 . The two output ports of diplexer  202  are connected to one end of loop antenna  200  and loop antenna  201 . The two output ports of diplexer  203  are connected to the opposing end of loop antenna  200  and loop antenna  201 . A signal applied to port  205  or port  206  will transgress through switch assembly  204  and will be coupled to loop antenna  200  or loop antenna  201 . The frequency characteristics of diplexer  202  and diplexer  203  will determine which frequencies are coupled to the two loop antennas. 
       FIG. 12 b    illustrates a technique of using a common switch assembly to feed three loop antennas. Switch assembly  214  is connected to diplexers  207 ,  208 , and  209 . One output port of diplexer  210  is connected to one end of loop antenna  209  and the second output port of diplexer  210  is connected to diplexer  211 . The two output ports of diplexer  211  are connected to one end of loop antennas  207  and  208 . One output port of diplexer  213  is connected to the second end of loop antenna  209  and the second output port of diplexer  213  is connected to diplexer  212 . The two output ports of diplexer  212  are connected to the second end of loop antennas  207  and  208 . The frequency characteristics of diplexers  210 ,  211 ,  212 , and  213  will determine which frequencies are coupled to the three loop antennas. 
       FIG. 13 a    illustrates the swappable feed technique applied to an IMD (Isolated Magnetic Dipole) antenna. An IMD antenna  220  is connected to a switching assembly  221 . A control line  222  is shown. 
       FIG. 13 b    illustrates another type of IMD antenna that can be used with a swappable feed assembly. IMD antenna  223  is positioned in proximity to a conductor  224 . A switching assembly  225  is attached to the feed point of the IMD antenna  223  and the conductor  224 . As control line  226  is shown. 
       FIG. 14  illustrates an example of a loop antenna  227  with a swapping circuit  228  used to change the feed and ground connections of the loop antenna  228  to a selection of connection point, chosen among the possible output  229 ,  230 ,  231 ,  232 ,  233 . 
       FIG. 15  illustrates a folded loop antenna structure wherein the loop antenna structure is formed by including part of the device (Cell Phone, Mp3 Player, Tablets) as part of the radiating structure. To control the differential mode generated by the loop, we can utilize the symmetrical nature of the device to form a symmetric loop with desired E and H Filed patterns. Using the swapping circuit to switch the feed and ground connections it will be possible to result in an efficient operating mode for different use cases (for example for switching between left hand to right hand in case of a cell phone). 
       FIG. 16  illustrates Two such folded loop antenna structures can be used in a MIMO configuration. The pair of swappable feeds can be used to generate  4  combinations of feeds for the two pair of antennas. 
       FIG. 17  illustrates an embodiment where two symmetric parasitic elements (can be traces on PCB) that can be connected to active components (RF Switches, tunable capacitors, MEMS switches, PIN diode). The swapping circuit will help to generate equal efficiencies by utilizing a balanced (differential) mode generated by the loop antenna for different use cases (for ex. left hand and right hand). The differential mode is generated due to the symmetry of the folded loop structure. However with changes in the local environment in the proximity of the antenna, there is an impact on the E and H Field distribution in addition to the detuning of the antenna due to the different loading effects. The parasitic elements can be used to retune the antenna element for either combination of feeding structures. This will enable to control the match and also help with maintaining the required field distribution to generate the balanced mode.