PATENT DOCUMENT

Publication Number: US-10651670-B1
Application Number: US-201715491899-A
Country: US
Kind Code: B1

Title: Electronic devices with wireless charging antenna arrays

Abstract:
An electronic device may transfer power wirelessly to an external device. The electronic device may include a housing having a cavity. A cover layer may be formed over the cavity. An array of antennas may be formed within the cavity. Antennas in the array may transfer wireless charging signals to the external device through the cover layer to charge the external device while the external device is in contact with the cover layer. Impedance detection circuitry may gather impedance matching information from each antenna in the array. Control circuitry may select an antenna having a best impedance match with the external device for transmitting the wireless charging signals. One or more antennas in the array may form a block filter for the selected antenna. Multiple near-field coupled antennas in the array may be selected for transmitting the charging signals to focus the charging signals on the external device.

Claims:
What is claimed is: 
     
       1. A wireless charging mat for transmitting wireless power to an electronic device while the electronic device is placed on the wireless charging mat, the wireless charging mat comprising:
 a plurality of antennas that includes a selected transmit antenna configured to transmit wireless charging signals to the electronic device and that includes a set of antennas that surrounds the selected transmit antenna; 
 wireless charging circuitry operatively coupled to the plurality of antennas; and 
 control circuitry coupled to the wireless charging circuitry, wherein the control circuitry is configured to control the wireless charging circuitry to transmit the wireless charging signals over the selected transmit antenna and wherein the control circuitry is configured to control the set of antennas to form a guard ring structure for the selected transmit antenna. 
 
     
     
       2. The wireless charging mat defined in  claim 1 , wherein the control circuitry is configured to disable the set of antennas to prevent the set of antennas from transmitting any wireless charging signals while the selected transmit antenna transmits the wireless charging signals. 
     
     
       3. The wireless charging mat defined in  claim 2 , wherein a given antenna in the set of antennas comprises first and second antenna feed terminals, the wireless charging mat further comprising:
 a transmission line having a positive signal line coupled between the wireless charging circuitry and the first antenna feed terminal and a ground signal line coupled between the wireless charging circuitry and the second antenna feed terminal; and 
 a switch interposed on the ground signal line, wherein the control circuitry is configured to control the given antenna to form a portion of the guard ring structure by closing the switch. 
 
     
     
       4. The wireless charging mat defined in  claim 3 , further comprising:
 an additional switch interposed on the positive signal line, wherein the control circuitry is configured to control the given antenna to transmit additional wireless charging signals by opening the switch and closing the additional switch. 
 
     
     
       5. The wireless charging mat defined in  claim 1 , wherein the set of antennas is configured to reflect at least some of the wireless charging signals transmitted by the selected transmit antenna back towards the selected transmit antenna. 
     
     
       6. The wireless charging mat defined in  claim 1 , wherein the set of antennas surrounds at least four sides of the selected transmit antenna. 
     
     
       7. The wireless charging mat defined in  claim 6 , wherein the plurality of antennas are formed in an array having consecutive first, second, and third rows, the selected transmit antenna is formed in the second row, and the guard ring structure includes antennas in the first, second, and third rows. 
     
     
       8. The wireless charging mat defined in  claim 1 , wherein the plurality of antennas comprises an additional selected transmit antenna that is configured to transmit the wireless charging signals, the additional selected transmit antenna is near field coupled to the selected transmit antenna, the control circuitry is configured to control the wireless charging circuitry to transmit the wireless charging signals over the additional selected transmit antenna, and the set of antennas surrounds both the selected transmit antenna and the additional selected transmit antenna. 
     
     
       9. The wireless charging mat defined in  claim 1 , wherein the selected transmit antenna and each antenna in the set of antennas comprise a respective patch antenna resonating element located in a common plane. 
     
     
       10. The wireless charging mat defined in  claim 1 , further comprising:
 a housing, wherein the plurality of antennas is formed within a cavity in the housing; and 
 a cover layer positioned over the cavity and the plurality of antennas, wherein each antenna in the plurality of antennas comprises a respective antenna resonating element that is formed at a first distance with respect to a rear wall of the cavity and at a second distance with respect to the cover layer, the first distance being different from the second distance. 
 
     
     
       11. The wireless charging mat defined in  claim 1 , wherein the selected transmit antenna comprises a patch antenna, the wireless charging mat further comprising:
 a first transmission line coupled to the patch antenna at a first signal feed terminal; and 
 a second transmission line coupled to the patch antenna at a second signal feed terminal that is different from the first signal feed terminal, wherein the wireless charging circuitry is configured to transmit the wireless charging signals over the patch antenna via the first and second signal feed terminals, and the wireless charging signals transmitted by the patch antenna exhibit first and second different radio-frequency polarizations. 
 
     
     
       12. An electronic device configured to wirelessly transfer power to an external device, the electronic device comprising:
 a housing; 
 wireless power transfer circuitry in the housing; 
 a cavity formed in the housing; 
 a cover layer positioned over the cavity; 
 a plurality of antennas within the cavity, wherein at least one antenna in the plurality of antennas is configured to transmit wireless charging signals at a transmit frequency from the wireless power transfer circuitry to the external device through the cover layer while the external device is on the cover layer, the transmit frequency being greater than 1 GHz, wherein the plurality of antennas in the cavity comprises an array of patch antennas; and 
 control circuitry coupled to the wireless power transfer circuitry, wherein the control circuitry is configured to:
 select a patch antenna in the array to transmit the wireless charging signals to the external device, and 
 disable a ring of patch antennas around the selected patch antenna in the array to form a guard ring structure for the selected patch antenna, wherein the guard ring structure is configured to block at least some of the wireless charging signals at the transmit frequency. 
 
 
     
     
       13. The electronic device defined in  claim 12 , further comprising:
 a dielectric support structure formed on a rear surface of the cavity that opposes the cover layer, wherein each of the patch antennas in the array comprises a respective patch antenna resonating element formed on a surface of the dielectric support structure, the surface of the dielectric support structure is located at a first distance that is between 0.5 and 10 mm from the rear surface of the cavity, and the surface of the dielectric support structure is located at a second distance that is between 2 and 20 mm from the cover layer. 
 
     
     
       14. The electronic device defined in  claim 12 , wherein the control circuitry is configured to gather impedance measurements using the plurality of antennas and to select the patch antenna in the array to transmit the wireless charging signals to the external device based on the gathered impedance measurements. 
     
     
       15. An electronic device configured to provide wireless power to an external device that is placed on the electronic device, the electronic device comprising:
 wireless power transfer circuitry that is configured to generate wireless charging signals; 
 a plurality of antennas operatively coupled to the wireless power transfer circuitry; and 
 control circuitry coupled to the wireless power transfer circuitry, wherein the control circuitry is configured to control the wireless power transfer circuitry to transmit the wireless charging signals over a selected antenna of the plurality of antennas, wherein the control circuitry is configured to control a set of antennas in the plurality of antennas that is adjacent to the selected antenna to form a filter, and wherein the filter is configured to block at least some of the transmitted wireless charging signals from being transmitted past the set of antennas. 
 
     
     
       16. The electronic device defined in  claim 15 , wherein each antenna in the set of antennas is coupled to the wireless power transfer circuitry by a respective transmission line having a corresponding positive signal line and a corresponding ground signal line, wherein a respective switch is interposed on each of the ground signal lines, and wherein the control circuitry is configured to short the wireless charging signals transmitted by the selected antenna to ground at the location of each antenna in the set of antennas by closing each of the respective switches. 
     
     
       17. The electronic device defined in  claim 16 , wherein the wireless power transfer circuitry is configured to generate the wireless charging signals at a transmit frequency that is greater than 1 GHz. 
     
     
       18. The electronic device defined in  claim 15 , wherein the set of antennas that forms the filter comprises a ring of antennas that surrounds the selected antenna. 
     
     
       19. The electronic device defined in  claim 15 , wherein the control circuitry is configured to control the wireless power transfer circuitry to transmit the wireless charging signals over an additional selected antenna in the plurality of antennas, the additional selected antenna being near field coupled to the selected antenna, and the set of antennas that forms the filter comprising a ring of antennas that surrounds the selected antenna and the additional selected antenna.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of patent application Ser. No. 15/426,954, filed on Feb. 7, 2017, which claims the benefit of provisional patent application No. 62/297,538, filed Feb. 19, 2016, both of which are hereby incorporated by reference herein in their entireties. This application claims the benefit of and claims priority to patent application Ser. No. 15/426,954, filed on Feb. 7, 2017 and provisional patent application No. 62/297,538 filed on Feb. 19, 2016. 
    
    
     BACKGROUND 
     This relates generally to electronic devices and, more particularly, to electronic devices with wireless circuitry. 
     Electronic devices often include circuitry for performing wireless communications. For example, cellular telephones, computers, and other devices often contain antennas and wireless transceivers for supporting wireless communications. Some devices include circuitry to support wireless charging operations. 
     SUMMARY 
     Challenges can arise in implementing wireless charging and communications systems. If care is not taken, sensitivity to antenna misalignment and other issues can make it difficult or impossible to achieve desired levels of performance when integrating antennas and other structures into devices of interest. 
     It would therefore be desirable to be able to provide systems with improved wireless circuitry. 
     An electronic device may be provided with wireless circuitry. The electronic device may use the wireless circuitry to transfer power wirelessly to an external device or to communicate wirelessly with external device. The electronic device may include housing structures. A cavity may be formed in the housing structures. A dielectric cover layer may be formed over the cavity. An array of antennas may be formed within the cavity. One or more of the antennas in the array may transfer wireless charging signals to the external device through the dielectric cover layer to charge the external device. The antennas may wirelessly charge the external device while the external device is in contact with the dielectric cover layer. 
     The electronic device may include impedance detection circuitry coupled to the antennas. The impedance detection circuitry may detect when the cover layer is in contact with the external device. For example, the impedance detection circuitry may gather impedance matching information from each antenna in the array. The impedance detection circuitry may include power measurement circuitry that measures a power level of the transmitted wireless charging signals. The power measurement circuitry may measure a power level of a reflected version of the transmitted wireless charging signals. Control circuitry in the device may identify the impedance matching information by comparing the measured power levels. The impedance matching information may identify which antennas are to be used for transmitting the wireless charging signals. For example, the control circuitry may select an antenna in the array having a best impedance match with the external device for transmitting the wireless charging signals. If desired, the impedance matching information may identify an optimal frequency with which the selected antenna is to transmit the wireless charging signals. 
     The array of antennas may include antenna structures such as one or more patch antennas. The patch antenna may have first and second feeding points for transmitting the wireless charging signals using first and second different polarities. Antennas in the array that are located around the antenna that transmits the wireless charging signals may be disabled from transmitting the wireless charging signals. The disabled antennas may form a block filter for the selected antenna. If desired, multiple adjacent antennas in the array may be selected for transmitting the wireless charging signals. Each of the adjacent antennas may be electromagnetically near-field coupled to focus the wireless charging signals on the external device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an electronic device and wireless charging equipment in accordance with an embodiment. 
         FIG. 2  is a schematic view of an illustrative electronic device having antennas for receiving wireless charging signals in accordance with an embodiment. 
         FIG. 3  is a perspective view showing how an electronic device may be placed on top of illustrative wireless charging equipment for wirelessly charging the device in accordance with an embodiment. 
         FIG. 4  is a cross-sectional side view of illustrative wireless charging equipment having multiple antennas for wirelessly charging an electronic device placed on top of the wireless charging equipment in accordance with an embodiment. 
         FIG. 5  is a top-down view of illustrative wireless charging equipment having an array of wireless charging antennas in accordance with an embodiment. 
         FIG. 6  is a schematic diagram of illustrative wireless circuitry that communicates using an antenna in accordance with an embodiment. 
         FIG. 7  is a perspective view of an illustrative patch antenna for transmitting wireless charging signals in accordance with an embodiment. 
         FIG. 8  is a perspective view of an illustrative patch antenna with dual ports for transmitting wireless charging signals in accordance with an embodiment. 
         FIG. 9  is a flow chart of illustrative steps that may be performed by wireless charging equipment having an array of antennas for wirelessly charging an electronic device in accordance with an embodiment. 
         FIG. 10  is a top-down view of illustrative wireless charging equipment having antennas that are configured to form a guard ring structure in accordance with an embodiment. 
         FIG. 11  is a cross-sectional side view of illustrative wireless charging equipment having antennas that are configured to form a guard ring structure in accordance with an embodiment. 
         FIG. 12  is a top-down view of illustrative wireless charging equipment that concurrently transmits wireless charging signals using multiple adjacent antennas in accordance with an embodiment. 
         FIGS. 13A and 13B  are side views showing how transmitting wireless charging signals using multiple antennas may concentrate the wireless charging signals on a charged electronic device in accordance with an embodiment. 
         FIG. 14  is a top-down view of illustrative wireless charging equipment having multiple antennas that transmit wireless charging signals and that are surrounded by an antenna guard ring in accordance with an embodiment. 
         FIG. 15  is a diagram showing how switching circuitry may be interposed between wireless power transmitter circuitry and a corresponding antenna on wireless charging equipment for configuring the antenna to form a selected transmit antenna or part of a guard ring structure in accordance with an embodiment. 
         FIG. 16  is a schematic diagram showing how illustrative wireless charging circuitry may include circuitry for detecting impedance matching information associated with a charged device in accordance with an embodiment. 
         FIG. 17  is a plot showing an example of how scattering parameter values may be used to identify a selected antenna for wireless power transmission in accordance with an embodiment. 
         FIG. 18  is a flow chart of illustrative steps that may be performed by wireless charging equipment to select antennas for transmitting wireless charging signals in accordance with an embodiment. 
         FIG. 19  is a flow chart of illustrative steps that may be performed by wireless charging equipment to select a frequency for transmitting wireless charging signals in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A system of the type that may support wireless charging and wireless communications is shown in exemplary  FIG. 1 . As shown in  FIG. 1 , the system may include electronic devices such as electronic devices  10  and  12 . Devices such as  10  and  12  may communicate wirelessly over a wireless communications link. The wireless communications link may be a cellular telephone link (e.g., a wireless link at frequencies of 700 MHz to 2700 MHz or other suitable cellular telephone frequencies), may be a wireless local area network link operating at 2.4 GHz, 5 GHz, or other suitable wireless local area network frequencies, may involve millimeter wave communications (e.g., communications of the type sometimes referred to as extremely high frequency (EHF) communications that involve signals at 60 GHz or other frequencies between about 10 GHz and 400 GHz), may involve WiGig communications (millimeter wave IEEE 802.11 ad communications in a communications band at 60 GHz), or may involve communications at any other wireless communications frequencies in any other desired communications band (e.g., at frequencies above 700 MHz, frequencies below 700 MHz, frequencies above 400 GHz, frequencies below 400 GHz, frequencies from 1-1000 MHz, frequencies above 100 MHz, frequencies above 500 MHz, frequencies above 1 GHz, frequencies from 1-400 GHz, frequencies below 100 GHz, in a 243 GHz frequency band, a 2.45 GHz frequency band, a 5.75 GHz frequency band, other frequency bands such as Industrial Scientific and Medical (ISM) bands, unlicensed frequency bands, or at any other frequencies of interest). 
     Power may also be transferred wirelessly between devices  12  and  10  at these frequencies or any other suitable frequencies. For example, device  12  may transfer power wirelessly to device  10  (e.g., to power device  10  and/or to charge a battery in device  10 ). Wireless communications and wireless power transfer operations may be supported using wireless paths such as wireless path  14  of  FIG. 1 . Device  12  may sometimes be referred to as wireless charging equipment  12 , wireless charging device  12 , or charging device  12 , because device  12  performs wireless charging for device  10 . 
     Device  10  may be, for example, a mobile telephone device, tablet computer device, laptop computer device, or any other desired electronic device. Wireless charging device  12  may be a peripheral or docking device. In one suitable arrangement, wireless charging device  12  is a mat-shaped device or other relatively flat structure (sometimes referred to herein as a wireless charging mat). If desired, charging device  12  may be, for example, a mobile telephone device, tablet computer device, laptop computer device, or any other desired electronic device. Device  10  may be docked or otherwise placed onto or into contact with device  12  for performing wireless charging. The example of  FIG. 1  is illustrative. In general, devices  10  and  12  may be any desired electronic devices. For example, it is possible for device  12  to be a desktop computer that wireless charges a watch. 
     As shown in  FIG. 1 , device  10  may include control circuitry such as storage and processing circuitry  16 . Device  12  may include control circuitry such as storage and processing circuitry  18 . Storage and processing circuitry  16  and  18  may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in storage and processing circuitry  16  may be used to control the operation of device  10  whereas processing circuitry in storage and processing circuitry  18  is used to control the operation of device  12 . This processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processor integrated circuits, application specific integrated circuits, etc. 
     Storage and processing circuitry  16  may be used to run software on device  10  and/or storage and processing circuitry  18  may be used to run software on device  12  such as internet browsing applications, voice-over-internet-protocol (VOIP) telephone call applications, email applications, media playback applications, operating system functions, functions related to supporting wireless charging operations, etc. To support interactions with external equipment, storage and processing circuitry  16  and  18  may be used in implementing communications protocols. Communications protocols that may be implemented using storage and processing circuitry  16  and  18  include internet protocols, wireless local area network protocols (e.g., IEEE 802.11 protocols—sometimes referred to as WiFi® and WiGig), protocols for other short-range wireless communications links such as the Bluetooth® protocol, cellular telephone protocols, MIMO protocols, antenna diversity protocols, satellite navigation system protocols, etc. 
     Device  10  may include input-output circuitry  20 . Input-output circuitry  20  may include input-output devices  22 . Input-output devices  22  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices such as device  12 . Input-output devices  22  may include user interface devices, data port devices, and other input-output components. For example, input-output devices may include touch screens (i.e., displays with touch sensors), displays without touch sensor capabilities, buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, cameras, speakers, status indicators, light sources, audio jacks and other audio port components, digital data port devices, light sensors, accelerometers or other components that can detect motion and device orientation relative to the Earth, capacitance sensors, proximity sensors (e.g., a capacitive proximity sensor and/or an infrared proximity sensor), magnetic sensors, a connector port sensor or other sensor that determines whether a device is mounted in a dock, and other sensors and input-output components. 
     Input-output circuitry  20  may include wireless circuitry  24 . Wireless circuitry  24  may include wireless circuitry  26  (sometimes referred to as transmitter circuitry, receiver circuitry, transceiver circuitry, etc.) for supporting wireless charging (e.g., using wireless power circuitry  28 ) and/or wireless communications (e.g., using wireless communications circuitry  30 ). Wireless circuitry  26  may be formed from one or more integrated circuits, may include power amplifier circuitry, low-noise input amplifiers, passive RF components, and/or other circuitry. Circuitry  26  may transmit and/or receive wireless signals over path  14  using one or more antennas  32 . 
     Device  12  may include wireless circuitry  34 . Wireless circuitry  34  may include wireless circuitry  36  (sometimes referred to as transmitter circuitry, receiver circuitry, transceiver circuitry etc.) for supporting wireless charging (e.g., using wireless power circuitry  40 ) and/or wireless communications (e.g., using wireless communications circuitry  38 ). Wireless circuitry  34  may be formed from one or more integrated circuits, may include power amplifier circuitry, low-noise input amplifiers, passive RF components, and/or other circuitry. Circuitry  36  may transmit and/or receive wireless signals over path  14  using one or more antennas  42 . If desired, device  12  may include input-output devices such as input-output devices similar to devices  22  on electronic device  10 . 
     Wireless communications circuitry  30  on device  10  and/or wireless communications circuitry  38  on device  12  may include wireless local area network transceiver circuitry that may handle 2.4 GHz and 5 GHz bands for WiFi® (IEEE 802.11) communications and that may handle the 2.4 GHz Bluetooth® communications band. Circuitry  30  and/or circuitry  38  may also include cellular telephone transceiver circuitry for handling wireless communications in frequency ranges such as a low communications band from 700 to 960 MHz, a midband from 1710 to 2170 MHz, and a high band from 2300 to 2700 MHz or other communications bands between 700 MHz and 2700 MHz or other suitable frequencies (as examples). Circuitry  30  and/or circuitry  38  may handle voice data and non-voice data. Circuitry  30  and/or circuitry  38  may include millimeter wave transceiver circuitry that may support communications at extremely high frequencies (e.g., millimeter wave frequencies from 10 GHz to 400 GHz or other millimeter wave frequencies). Circuitry  30  and/or circuitry  38  may handle IEEE 802.11 ad (WiGig) communications at 60 GHz (millimeter wave frequencies). If desired, circuitry  30  and/or  38  may include satellite navigation system circuitry such as Global Positioning System (GPS) receiver circuitry for receiving GPS signals at 1575 MHz or for handling other satellite positioning data (e.g., GLONASS signals at 1609 MHz). Satellite navigation system signals may be received from a constellation of satellites orbiting the earth. 
     In satellite navigation system links, cellular telephone links, and other long-range links, wireless signals are typically used to convey data over thousands of feet or miles. In WiFi® and Bluetooth® links and other short-range wireless links, wireless signals are typically used to convey data over tens or hundreds of feet. Extremely high frequency (EHF) wireless transceiver circuitry (e.g., WiGig circuitry) may convey signals over these over these short distances that travel between transmitter and receiver over a line-of-sight path. If desired, antenna diversity schemes may be used to ensure that the antennas that have become blocked or that are otherwise degraded due to the operating environment of devices  10  and  12  can be switched out of use and higher-performing antennas used in their place. 
     Wireless circuitry  24  and  34  can include circuitry for other wireless operations if desired. For example, wireless communications circuitry  30  and circuitry  38  may include circuitry for receiving television and radio signals, paging system transceivers, near field communications (NFC) circuitry, etc. 
     Transmission line paths may be used to route antenna signals within devices  10  and  12 . For example, transmission line paths may be used to couple antenna structures  32  to circuitry  26 . Similarly, transmission line paths may be used to couple antenna structures  42  to circuitry  36 . Transmission lines in devices  10  and  12  may include coaxial cable paths, microstrip transmission lines, stripline transmission lines, edge-coupled microstrip transmission lines, edge-coupled stripline transmission lines, transmission lines formed from combinations of transmission lines of these types, etc. Filter circuitry, switching circuitry, impedance matching circuitry, and other circuitry may be interposed within the transmission lines, if desired. 
     Device  10  may contain multiple antennas  32  and device  12  may include multiple antennas  42 . The antennas may be used together or one of the antennas may be switched into use while other antenna(s) are switched out of use. If desired, control circuitry  16  may be used to select an optimum antenna to use in device  10  in real time and/or to select optimum settings for wireless circuitry  24 . Antenna adjustments may be made to tune antennas  32  to perform in desired frequency ranges and to otherwise optimize antenna performance. Sensors may be incorporated into antennas  32  to gather sensor data in real time that is used in adjusting antennas  32 . If desired, control circuitry  18  may be used to select an optimum antenna to use in device  12  in real time and/or to select optimum settings for wireless circuitry  34 . Antenna adjustments may be made to tune antennas  42  to perform in desired frequency ranges and to otherwise optimize antenna performance. Sensors may be incorporated into antennas  42  to gather sensor data in real time that is used in adjusting antennas  42 . 
     Antennas  32  and  42  may be formed using any suitable antenna type. For example, antennas  32  and  42  may include antennas with resonating elements that are formed from loop antenna structures, patch antenna structures, inverted-F antenna structures, slot antenna structures, planar inverted-F antenna structures, helical antenna structures, hybrids of these designs, etc. Multiple different antenna types may be used in implementing antennas  32  on device  10  and multiple different antenna types may be used in implementing antennas  42  on device  12 , if desired. 
     Wireless charging equipment  12  may receive power from a corresponding power source. Power sources that provide power to wireless charging equipment include other wireless charging devices, wired inputs such as an alternating-current (AC) powering input or a direct current (DC) powering input, solar power inputs, or any other desired power source. In the example of  FIG. 1 , wireless charging equipment  12  receives power over power plug  44 . Plug  44  may mate with a wall socket or other power supply equipment for powering device  12 . Device  12  may include AC/DC conversion circuitry or any other desired circuitry to convert power received over plug  44  into wireless power for transmission to device  10 . If desired, wireless communications circuitry  12  may include battery circuitry (not shown). 
     During wireless power transfer operations, wireless power transfer circuitry in circuit  40  of charging device  12  and circuit  28  of charged device  10  may be used to transfer power between the devices. Circuitry  40  and  28  may sometimes be referred to herein as wireless charging circuitry because circuitry  40  and  28  handles transmission and reception of wireless power associated with charging battery  46  on device  10 . For example, charging device  12  may use circuitry  40  and antennas  42  to transfer power wirelessly over path  14 . Device  10  may use antennas  32  and circuitry  28  to receive the transmitted wireless power. Circuitry  28  in charged device  10  may include power receiver circuitry that converts received wireless power into a form that is suitable for powering the components of device  10 . For example, circuitry  28  may include power rectifier circuitry or other conversion circuitry (e.g., AC/DC converter circuitry, DC/DC converter circuitry, etc.) that converts received wireless charging signals  14  into voltages that can be used to power the components of device  10  and/or to charge battery  46  (e.g., circuitry  28  may convert AC wireless charging signals received over antennas  32  into DC power to supply power rails to the components of device  10  and/or to charge battery  46 ). 
     Any desirable frequencies may be used for wireless charging (e.g., frequencies in the kHz range, the MHz range, or in the GHz range, frequencies of 1 kHz to 1 MHz, frequencies of 1 kHz to 100 MHz, frequencies in the 100 MHz-400 GHz range, etc.). Wireless power may be transmitted over path  14  according to any desired wireless powering protocol. Wireless power may be transmitted over path  14  in the form of radio-frequency signals, near field coupling (NFC) signals, magnetic resonance signals, or any other desired wireless signals that are capable of conveying power from charger device  12  to receiving device  10  over the air (e.g., without a direct electrical connection between devices  10  and  12 ). 
     Antenna diversity schemes may be implemented in which multiple redundant antennas are used in handling communications for a particular band or bands. In an antenna diversity scheme, storage and processing circuitry  16  on device  10  may select which antenna to use in real time based on signal strength measurements or other data. In multiple-input-multiple-output (MIMO) schemes, multiple antennas may be used to transmit and receive multiple data streams, thereby enhancing data throughput. 
     Illustrative locations in which antennas  32  may be formed in device  10  are shown in  FIG. 2 . As shown in  FIG. 2 , electronic device  10  may have a housing such as housing  50 . Housing  50  may include plastic walls, metal housing structures, structures formed from carbon-fiber materials or other composites, glass, ceramics, or other suitable materials. Housing  50  may be formed using a single piece of material (e.g., using a unibody configuration) or may be formed from a frame, housing walls, and other individual parts that are assembled to form a completed housing structure. The components of device  10  that are shown in  FIG. 1  may be mounted within housing  50 . Antenna structures  32  may be mounted within housing  50  and may, if desired, be formed using parts of housing  50 . For example, housing  50  may include metal housing sidewalls, peripheral conductive members such as band-shaped members (with or without dielectric gaps), conductive bezels, and other conductive structures that may be used in forming antenna structures  50 . Portions of housing  50  such as metal housing sidewalls may form all or some of the ground plane structures and/or resonating element structures for one or more antennas  32 . Housing  50  may include a planar rear metal surface (e.g., a surface forming a rear face of the device). The metal housing side walls may, for example, form external surfaces for device  10  that extend from the planar rear metal surface to a front face of device  10 . A display may be formed at the front face of device  10  if desired. 
     As shown in  FIG. 2 , antenna structures  32  may be coupled to transceiver circuitry  26  by paths such as paths  52 . Paths  52  may include transmission line structures such as coaxial cables, microstrip transmission lines, stripline transmission lines, etc. Paths  52  may also include impedance matching circuitry, filter circuitry, and switching circuitry. Impedance matching circuitry may be used to ensure that antennas  32  are efficiently coupled to transceiver circuitry  26  in communications bands of interest. Filter circuitry may be used to implement frequency-based multiplexing circuits such as diplexers, duplexers, or other multiplexing circuits. Switching circuitry may be used to selectively couple antennas  32  to desired ports of transceiver circuitry  26 . For example, in one operating mode a switch may be configured to route one of paths  52  to a given antenna and in another operating mode the switch may be configured to route a different one of paths  52  to the given antenna. The use of switching and filtering circuitry between transceiver circuitry  26  and antennas  32  allows device  10  to support multiple communications bands of interest with a limited number of antennas. 
     In a device such as a cellular telephone that has an elongated rectangular outline, it may be desirable to place antennas  32  at one or both ends of the device. As shown in  FIG. 2 , for example, some of antennas  32  may be placed in upper end region  54  of housing  50  and some of antennas  32  may be placed in lower end region  56  of housing  50 . The antenna structures in device  10  may include a single antenna in region  54 , a single antenna in region  56 , multiple antennas in region  54 , multiple antennas in region  56 , or may include one or more antennas located elsewhere in housing  50 . 
     Antenna structures  32  may be formed within some or all of regions such as regions  54  and  56 . For example, an antenna such as antenna  32 T- 1  may be located within region  54 - 1  or an antenna such as antenna  32 T- 2  may be formed that fills some or all of region  54 - 2 . An antenna such as antenna  32 B- 1  may fill some or all of region  56 - 2  or an antenna such as antenna  32 B- 2  may be formed in region  56 - 1 . These types of arrangements need not be mutually exclusive. For example, region  56  may contain a first antenna such as antenna  32 B- 1  and a second antenna such as antenna  32 B- 2 . 
     Transceiver circuitry  26  may contain transmitters such as transmitters  58  and receivers such as receivers  60 . Transmitters  58  and receivers  60  may be implemented using one or more integrated circuits (e.g., cellular telephone communications circuits, wireless local area network communications circuits, circuits for Bluetooth® communications, circuits for receiving satellite navigation system signals, power amplifier circuits for increasing transmitted signal power, low noise amplifier circuits for increasing signal power in received signals, other suitable wireless communications circuits, and combinations of these circuits). One or more receivers  60  may be used to receive wireless power from wireless charging device  12  (e.g., one or more receivers  60  may form part of wireless power circuitry  28  of  FIG. 1 ). If desired, wireless power receivers  60  may receive wireless charging signals over the same antennas  32  that are used by radio-frequency transceivers  30  or may receive wireless charging signals over dedicated charging antennas that are not used by radio-frequency transceivers  30 . 
     In one suitable arrangement, wireless charging device  12  may be a mat-shaped device or other relatively flat device. When it is desired to charge device  10 , a user may place device  10  on top of wireless charging device  12 . For example, a user may place device  10  onto a top surface of charging device  12  such that device  10  is in contact with the top surface of charging device  12 . Wireless power may be conveyed from charging device  12  to electronic device  10  over wireless path  14  while device  10  is in mechanical (physical) contact with charging device  12 . Even though device  10  is in physical contact with charging device  12  in this scenario, antennas  32  on device  10  may be separated from antennas  42  on charging device  12  such that wireless charging signals may be conveyed between the antennas. 
       FIG. 3  is a perspective view showing how device  10  may be placed on a surface of device  12  for wirelessly charging device  10 . Wireless charging device  12  may, for example, be a mat-shaped device. Charging device  12  may therefore sometimes be referred to herein as wireless charging mat  12  or wireless charging receptacle  12 . 
     As shown in  FIG. 3 , wireless charging device  12  may have a housing  68 . Housing  68  may include plastic walls, metal housing structures, structures formed from carbon-fiber materials or other composites, glass, ceramics, or other suitable materials. Housing  68  may be formed using a single piece of material (e.g., using a unibody configuration) or may be formed from a frame, housing walls, and other individual parts that are assembled to form a completed housing structure. Top surface  70  of housing  68  may extend across a length and width of charging device  12 . Housing  68  may have a bottom surface  72  that opposes top surface  70 . Sidewalls of housing  68  may extend from top surface  70  to bottom surface  72 . The side walls of housing  68  may be shorter than the length and width of surface  70 , for example. The example of  FIG. 3  in which top surface  70  has a rectangular outline is merely illustrative. In general, surface  70  may have any desired outline (e.g., a polygonal outline, a square outline, a circular outline, an elliptical outline, etc.). 
     When a user desires to wirelessly charge device  10  using charging device  12 , the user may place device  10  onto top surface  70  of device  12 . In the example of  FIG. 3 , device  12  is placed at location  74  on top surface  70 . Wireless charging device  12  may transmit wireless charging signals to device  10  while device  10  is placed on surface  70 . For example, one or more antennas  42  within housing  68  may transmit wireless charging signals to device  10  through top surface  70 . If desired, device  10  may be placed at other locations and at other orientations on surface  70 . For example, device  10  may be placed at locations such as location  76  or location  78 . Wireless charging device  12  may be configured to transmit wireless charging signals to device  10  regardless of the location and orientation at which device  10  is placed on surface  70 . 
       FIG. 4  is a cross-sectional side view of wireless charging device  12  (e.g., as taken across line AA′ and in the Y-Z plane of  FIG. 3 ). As shown in  FIG. 4 , housing  68  of wireless charging device  12  may include base housing portion  68 B and cover layer  68 C. Base housing portion  68 B may include dielectric materials, conductive materials, or combinations of conductive and dielectric materials. Cavity  82  may be formed extending from the top surface of base housing portion  68 B. Antennas  42  may be formed within cavity  82 . The portion of housing base  68 B behind cavity  82  may be continuous with the side walls of housing base  68 B or may be formed from separate structures than the side walls of housing base  68 B. 
     Cover layer  68 C may be placed over a top surface of housing base  68 B. Antennas  42  may transmit wireless charging signals  90  for charging device  10  while device  10  is in contact with top surface  70  of cover layer  68 C. Cover layer  70  may include dielectric materials to allow wireless charging signals  90  to pass through cover layer  70  for charging device  10 . If desired, cover layer  70  may include pigmented material and/or one or more ink layers so that cover layer  70  is opaque to visible light (e.g., to hide cavity  82  from view of a user and to enhance the aesthetic appearance of device  12  while still allowing charging signals  90  to pass through the cover layer). Wireless charging signals  90  may be transmitted at any desired frequencies (e.g., at frequencies between 700 MHz and 2700 MHz, in a 2.4 GHz frequency band, a 5 GHz frequency band, a 60 GHz frequency band, a 243 GHz frequency band, a 2.45 GHz frequency band, a 5.75 GHz frequency band, in an ISM band, in an unlicensed frequency band, at frequencies above 700 MHz, frequencies below 700 MHz, frequencies above 400 GHz, frequencies below 400 GHz, frequencies from 1-1000 MHz, frequencies above 100 MHz, frequencies above 500 MHz, frequencies above 1 GHz, frequencies from 1-400 GHz, frequencies below 100 GHz, or any other frequencies of interest). 
     Antennas  42  may include antenna resonating elements  92 . Resonating elements  92  may be formed at a distance  88  from the rear wall of cavity  82 . Distance  88  may be selected to adjust the radio-frequency performance of antennas  42 . As an example, distance  88  may be between 0.5 mm and 1 cm (e.g., in scenarios where a 5.8 GHz frequency is used), may be less than 10 mm, or may be any other desired distance. If desired, a dielectric support structure  84  may be formed in cavity  82  to support antennas  42 . Antenna resonating elements  92  may be formed on a top surface of support structure  84 . Support structure  84  may be formed from any desired dielectric materials (e.g., plastic, polymer, flexible printed circuit board substrate, foam, ceramics, glass, etc.). Support structure  84  may have openings or via structures that allow antenna resonating elements  92  to be coupled to transceiver circuitry  36  and/or ground plane structures in charging device  12 . Support structure  84  may include a single structure that supports each of antennas  42  in cavity  82  or may include separate structures that each support a corresponding antenna  42 . 
     The portion of cavity  82  that is interposed between antenna resonating elements  92  and cover layer  70  may be filled with air or any other desired dielectric. For example, cavity  82  may be filled with a dielectric support structure such as non-conductive foam or other materials that protect antennas  42  and support cover layer  68 C. Antenna resonating elements  92  may be formed within cavity  82  at a distance  86  from cover layer  70 . Distance  86  may be selected to provide a desired amount of directionality for wireless charging signals  90 . For example, if distance  86  is too short, wireless charging signals  90  may unfocused with respect to the device  10  that is placed on top surface  70 . If distance  86  is too long, wireless power transfer may be excessively inefficient. As an example, distance  86  may be between 2 mm and 20 mm (e.g., in scenarios where a 5.8 GHz frequency is used), may be less than 20 mm, or may be any other desired distance to optimize efficiency and directionality of wireless charging signals  90 . 
     Charging device  12  may include multiple antennas  42  in cavity  82  to ensure that at least one antenna  42  is within wireless charging range of device  10  regardless of where device  10  is placed on top surface  70 . The antennas within cavity  82  may be arranged in any desired manner. For example, antennas  42  may be evenly or unevenly distributed across surface  70  of charging device  12 , may be concentrated within a portion of charging device  12 , may be arranged in a predetermined shape across surface  70 , may be concentrated within multiple areas across surface  70 , etc. If desired, antennas  42  may be arranged in a pattern such as an array, grid, or matrix. As an example, antennas  42  may be arranged in an array having rows and columns of antennas. Distributing multiple antennas  42  across charging device  12  may, for example, allow charging device  12  to cover wireless charging for situations in which device  10  is placed at any desired location across surface  70 . 
       FIG. 5  is a top-down view of wireless charging device  12  having an array of charging antennas  42  (e.g., as taken in direction B and in the X-Y plane of  FIG. 3 ). As shown in  FIG. 5 , antennas  42  may be laterally distributed across surface  70  in an array having rows and columns. Antennas  42  may be evenly distributed in the array (e.g., the distance of the gap between each antenna in the array may be equal) or may be unevenly distributed in the array (e.g., the distance of the gaps between each antenna in the array may be unequal). As an example, the gap between each antenna  42  in the array may be between 20 and 30 mm. When device  10  is placed on top surface  70  for charging, a corresponding antenna  32 C may overlap with one or more antennas  42  on charging device  42 . Antenna  32 C may be an antenna on device  10  that is dedicated to receiving wireless charging signals or may be an antenna that handles wireless charging signals and other radio-frequency signals. If desired, multiple antennas  32 C may be used for wireless charging. For example, device  10  may include multiple antennas  32 C having different polarizations for receiving the wireless charging signals. One or more antennas  42  may transmit wireless charging signals  90  for reception by antenna  32 C. For example, the antenna  42  that overlaps antenna  32 C on device  10 , the antenna  42  that is closest to antenna  32 C, or multiple antennas  42  in the vicinity of antenna  32 C may transmit wireless charging signals  90  for device  10 . 
     As shown in  FIG. 6 , wireless circuitry  36  may be coupled to each antenna  42  on charging device  12  using paths such as path  100  (e.g., a transmission line path). Each antenna  42  in charging device  12  may be coupled to wireless circuitry  36  via a respective transmission line path  100 . If desired, switching circuitry may be interposed on path  100  to allow wireless circuitry  36  to convey wireless charging signals or radio-frequency signals over a selected set of antennas  42  (e.g., over a single antenna  42 , over two antennas  42 , over four antennas  42 , over every antenna  42 , etc.). 
     Wireless circuitry  36  may be coupled to control circuitry  18  so that circuitry  36  can be controlled by circuitry  18  during wireless power transfer operations and/or wireless communications operations. Path  100  may include one or more transmission lines. As an example, signal path  100  of  FIG. 6  may be a transmission line having a first conductor such as line  102  (e.g., a positive signal conductor) and a second conductor such as line  104  (e.g., a negative signal conductor or ground signal conductor). Lines  102  and  104  may form parts of a coaxial cable or a microstrip transmission line (as examples). A matching network formed from components such as inductors, resistors, and capacitors may be used in matching the impedance of antenna  42  to the impedance of transmission line  100 . Matching network components may be provided as discrete components (e.g., surface mount technology components) or may be formed from electronic device housing structures, printed circuit board structures, traces on plastic supports, etc. Components such as these may also be used in forming filter circuitry in antenna  42  if desired. 
     Transmission line  100  may be coupled to antenna feed structures associated with antenna  42 . As an example, antenna  42  may form a patch antenna, a dipole antenna, or other antenna having an antenna feed with a first antenna feed terminal such as terminal  106  (e.g., a positive feed terminal) and a second antenna feed terminal such as terminal  108  (e.g., a negative or ground feed terminal). First transmission line conductor  102  may be coupled to first antenna feed terminal  106  and second transmission line conductor  104  may be coupled to second antenna feed terminal  108 . Other types of antenna feed arrangements may be used if desired. The illustrative feeding configuration of  FIG. 6  is merely illustrative. 
     If desired, patch antenna structures may be used for implementing antenna  42 . An illustrative patch antenna is shown in  FIG. 7 . As shown in  FIG. 7 , patch antenna  42  may have a patch antenna resonating element such as patch  92  that is separated from a ground plane structure such as ground  110 . Antenna patch resonating element  92  and ground  110  may be formed from metal foil, machined metal structures, metal traces on a printed circuit or a molded plastic carrier, electronic device housing structures, or other conductive structures in charging device  12 . Patch resonating element  92  may be separated from ground  110  by dielectric support structure  84  and/or portions of base housing structure  68 B (as shown in  FIG. 4 ). Patch resonating element  92  may have any desired size. For example, patch resonating element  92  may have a length and a width between 10 mm and 20 mm, or any other desired size. If desired, patch resonating element  92  may have any desired shape (e.g., a circular shape, oval shape, square shape, rectangular shape, polygonal shape, etc.). 
     Antenna patch resonating element  92  may lie within a plane such as the X-Y plane of  FIGS. 3 and 5 . Ground  110  may line within a plane that is substantially parallel to the plane of antenna patch resonating element (patch)  92 . Patch  92  and ground  110  may therefore lie in separate parallel planes that are separated by a distance H. Distance H may be equal to distance  88  of  FIG. 4  (e.g., in scenarios where ground plane  110  is formed on the rear surface of cavity  82 ) or may be greater than distance  88  (e.g., in scenarios where ground plane  110  is embedded within base housing portion  68 B or otherwise formed below the rear surface of cavity  82 ). 
     Conductive path  112  may be used to couple terminal  106 ′ to terminal  106 . Antenna  42  may be fed using transmission line  100  having positive conductor  102  coupled to terminal  106 ′ and thus terminal  106  and with a ground conductor  104  coupled to terminal  108 . Other feeding arrangements may be used if desired. Moreover, patch  92  and ground  110  may have different shapes and orientations (e.g., planar shapes, curved patch shapes, patch element shapes with non-rectangular outlines, shapes with straight edges such as squares, shapes with curved edges such as ovals and circles, shapes with combinations of curved and straight edges, etc.). If desired, a via, through-hole, or other opening may be formed in ground plane  110  to allow feed line  102  to couple to path  112  without contacting ground plane  110 . 
     The arrangement of  FIG. 7  in which antenna  42  is provided with a single feed may result in transmitted wireless charging signals  90  having a single polarization. Transmitting wireless charging signals  90  with a single polarization may reduce wireless charging efficiency for device  10  at certain orientations of device  10  with respect to charging device  12 . To enhance the frequency coverage and polarizations handled by patch antenna  42 , antenna  42  may be provided with multiple feeds. 
     An illustrative patch antenna with multiple feeds is shown in  FIG. 8 . As shown in  FIG. 8 , antenna  42  may have a first feed at antenna port P 1  that is coupled to transmission line  100 - 1  and a second feed at antenna port P 2  that is coupled to transmission line  100 - 2 . The first antenna feed may have a first ground feed terminal coupled to ground  110  and a first positive feed terminal  106 -P 1  coupled to patch antenna resonating element  92 . The second antenna feed may have a second ground feed terminal coupled to ground  110  and a second positive feed terminal  106 -P 2 . 
     Patch  92  may have a rectangular shape with a pair of longer edges running parallel to dimension X and a pair of perpendicular shorter edges running parallel to dimension Y. The dimension of patch  92  in dimension X is L 1  and the dimension of patch  92  in dimension Y is L 2 . With this configuration, antenna  42  may be characterized by orthogonal polarizations and multiple frequencies of operation. 
     When using the first antenna feed associated with port P 1 , antenna  42  may transmit and/or receive antenna signals in a first communications band at a first frequency (e.g., a frequency at which a half of a wavelength is equal to dimension L 1 ). These signals may have a first polarization (e.g., the electric field E 1  of antenna signals  90  associated with port P 1  may be oriented parallel to dimension X). When using the antenna feed associated with port P 2 , antenna  42  may transmit and/or receive antenna signals in a second communications band at a second frequency (e.g., a frequency at which a half of a wavelength is equal to dimension L 2 ). These signals may have a second polarization (e.g., the electric field E 2  of antenna signals  90  associated with port P 2  may be oriented parallel to dimension Y so that the polarizations associated with ports P 1  and P 2  are orthogonal to each other). During wireless power transfer operations and/or wireless communications using system  10 , device  12  may use one or more antennas such dual-polarization patch antenna  42  of  FIG. 8  and may use port P 1 , port P 2 , or both port P 1  and P 2  of each of these antennas. By transmitting wireless charging signals  90  with dual polarizations, wireless charging efficiency of device  10  may be optimized regardless of the orientation of device  10  relative to charging device  12 . 
     If desired, one, multiple, or all of antennas  42  on charging device  12  may be provided with the dual port arrangement of  FIG. 8 . In another suitable arrangement, antennas  42  may be provided with a single feed and may be oriented along different directions so that the sum of the wireless charging signals  90  transmitted by antennas  42  has different polarities. For example, each adjacent pair of antennas  42  on charging device  12  may include a first single feed antenna  42  that transmits charging signals  90  that are horizontally polarized and a second single feed antenna  42  that transmits charging signals  90  that are vertically polarized. If desired, charging antenna  32 C on device  10  may include a dual port arrangement such as that shown in  FIG. 8  for receiving charging signals at different polarities. Device  10  may, if desired, include multiple adjacent antennas having different polarities (e.g., a first antenna  32 C that receives vertically polarized charging signals and a second antenna  32 C that receives horizontally polarized charging signals). This example is merely illustrative and, in general, any desired combination of single and dual feed antennas  42  may be used for charging device  12 . In general, antennas  42  may include any desired type of antenna that transmits wireless charging signals  90  with any desired polarization (e.g., dipole antennas, monopole antennas, loop antennas, inverted-F antennas, etc.). If desired, antennas  42  may transmit circularly polarized or elliptically polarized wireless charging signals  90 . 
     Transmitting wireless power signals  90  using every antenna  42  in charging device  12  may be inefficient and consume excessive power. For example, when wireless device  10  is placed at location  74  ( FIG. 3 ), antennas  42  that are not within wireless charging range of location  74  (e.g., antennas  42  at locations  76  or  78 ) need not transmit wireless charging signals  90 , since signals transmitted by those antennas are unlikely to be received by device  74 . In order to conserve power consumption in the system, charging device  12  may actively select one or more antennas  42  for transmitting wireless charging signals  90 . 
       FIG. 9  is a flow chart of illustrative steps that may be performed by wireless charging device  12  for transmitting wireless charging signals to device  10 . The steps of  FIG. 9  may, for example, be performed continuously (e.g., even when no device  10  has been placed on top surface  70 ), or may be performed once device  10  has been placed on top surface  70  for charging. 
     At step  120 , charging device  12  may select one or more antennas  42  in the array of antennas for wireless power transmission. For example, device  12  may select one or more antennas  42  in the vicinity of charging device  10  for transmitting wireless charging signals  90 . If desired, charging device  12  may select the antennas for wireless power transmission based on antenna impedance information gathered from each of the antennas  42 . For example, the gathered impedance information may identify when device  10  has been placed onto surface  70  and may identify which antennas  42  are within wireless charging range of device  10 . 
     At step  122 , charging device  12  may select an optimal frequency for transmitting wireless charging signals  90 . If desired, charging device  12  may select the optimal frequency for transmission based on impedance matching information gathered from the selected antennas  42  (e.g., as selected at step  120 ). If desired, charging device  12  may select multiple frequencies for transmission (e.g., in scenarios where dual port antennas such as those shown in  FIG. 8  are used). 
     At step  124 , charging device  12  may transmit wireless charging signals  90  using the selected antennas  42  (e.g., as selected at step  120 ) and at the selected frequency (e.g., as selected at step  122 ). The charging signals may be transmitted to device  10  through cover layer  68 C as shown in  FIG. 4 , for example. 
     At step  126 , charging device  12  may determine whether updates to the wireless power transmission configuration are needed. For example, device  12  may identify whether a different transmit frequency is needed or whether other antennas should be selected for transmission. If desired, device  12  may determine whether updates are needed based on impedance matching information gathered from antennas  42 . For example, the gathered impedance matching information may identify that device  10  is no longer placed on surface  70  or may identify that device  10  has moved to a different location on surface  70 . If charging device  12  determines that an update to the wireless power transmit configuration is needed, processing may loop back to step  120  as shown by path  130  to select different antennas  42  and/or different transmission frequencies for wireless charging. If charging device  12  determines that no updates are needed, processing may loop back to step  124  as shown by path  128  to continue wirelessly charging device  10  using the selected antennas and frequency. 
     If desired, antennas  42  that are adjacent to a selected antenna may be configured to form a filter such as a block filter for the selected transmit antenna.  FIG. 10  is a top-down view (e.g., in the X-Y plane of  FIGS. 3, 5, 7, and 8 ) showing how antennas  42  that are adjacent to a selected antenna may form a block filter for the selected antenna. 
     As shown in  FIG. 10 , antenna  42 - 1  on charging device  12  may be selected for wireless power transmission (TX). Antenna  42 - 1  may, for example, be within wireless charging range of charging antenna  32 C on charged device  10 . Antenna  42 - 1  may be selected, for example, based on impedance information gathered by each of antennas  42 . 
     Selected antenna  42 - 1  may transmit wireless charging signals  90 . A set of antennas  42  that surrounds transmit antenna  42 - 1  may not transmit any wireless charging signals (e.g., the set of antennas may be switched out of use or transceiver circuitry coupled to those antennas may be disabled). A portion  140  of the transmitted charging signals  90  may be transmitted laterally outwards and towards the set of antennas  42  surrounding transmit antenna  42 - 1 . 
     The set of antennas  42  surrounding transmit antenna  42 - 1  may have a frequency dependent impedance and may exhibit a frequency response (e.g., based on the geometry of the corresponding antenna resonating element, a distance between the resonating element and ground, etc.). At the frequency of operation of transmit antenna  42 - 1 , the presence of the set of surrounding antennas  42  may cause an impedance discontinuity at the location of the surrounding antennas. Such an impedance discontinuity may tend to block signals  140  from leaking laterally outwards in the X-Y plane and out of charging device  12 . As an example, the set of surrounding antennas may exhibit an impedance that is less than the impedance of the space between the set of surrounding antennas and the transmit antenna. This may allow the set of surrounding antennas to form a low impedance path to ground for wireless signals  140 , causing an effective short of transmitted wireless signals  140  to ground at the location of the surrounding antennas that blocks wireless signals  140  from travelling outwards past the surrounding antennas. As another example, the set of surrounding antennas may exhibit an impedance that is much greater than the impedance of the space between the surrounding antennas and the transmit antenna (e.g., an infinite impedance). This may also serve to block wireless signals  140  from travelling outwards past the surrounding antennas. Forming an impedance discontinuity for laterally radiated signals  140  using the set of antennas surrounding the transmit antenna may prevent undesirable signal loss out of the sides of charging device  12  (e.g., preventing absorption of the charging signals by people or objects in the vicinity of charging device  12 ) and may increase the charging efficiency of charging device  12 . In this way, the set of antennas surrounding the transmit antenna may form a block filter or guard ring structure for the transmit antenna. 
     In the example of  FIG. 10 , the set of surrounding antennas that forms the guard ring for selected antenna  42 - 1  is abutting selected antenna  42 - 1  (e.g., the antennas in the guard ring are located in immediately preceding and succeeding rows and columns as selected antenna  42 - 1 ). If desired, the antennas that form the block filter or guard ring may be any desired antennas adjacent to selected antenna  42 - 1  (e.g., the antennas in the guard ring may be the set of antennas that are closest to selected antenna  42 - 1  in scenarios where the antennas are not arranged in a grid). The guard ring may include one, two, three, four, or more than four antennas  42 . This is merely illustrative and, in general, the antennas that form the guard ring may include any of the antennas in the array. If desired, one or more other antennas may be interposed between the selected antenna and the antennas in the guard ring. The guard ring may have any desired shape (e.g., the antennas in the guard ring may be arranged in a square, rectangle, triangle, circle, oval, or any other desired shape around or adjacent to the selected antenna or may be arranged in a linear pattern, curved pattern, or any other desired pattern). 
       FIG. 11  is a cross-sectional side view of charging device  12  when placed into contact with device  10  (e.g., as taken across line CC′ of  FIG. 10  and in the Y-Z plane of  FIG. 3 ). As shown in  FIG. 11 , device  10  may be placed on surface  70  of charging device  12 . Antenna  32 C may be within charging distance of selected transmit antenna  42 - 1 . Antenna  42 - 1  may be fed wireless charging signals over feed terminal  106 . A portion  144  of charging signals  90  transmitted by selected antenna  42 - 1  may be transmitted towards charging antenna  32 C. Device  10  may receive charging signals  144  and use the charging signals to power battery  46 , for example. Portion  140  of charging signals  90  may be transmitted towards adjacent antennas  42 . Adjacent antennas  42  may not be fed any wireless charging signals from transceiver circuitry  36 . Adjacent antennas  42  may form an impedance discontinuity at the frequency of operation of transmit antenna  42 - 1 . This impedance discontinuity may, for example, form a low impedance path to ground for signals  140  at the location of the adjacent antennas. This low impedance path to ground may serve to short charging signals  140  to ground  110  at the location of the adjacent antennas  42 , effectively serving to trap or reflect signals  140  back towards selected antenna  42 - 1  as shown by paths  142 . In this way, antennas  42  adjacent to transmit antenna  42 - 1  may form a part of a guard ring that blocks charging signal portions  140  from escaping out the side of device  12 , while allowing charging signal portion  144  to be received at antenna  32 C for charging device  10 . 
     If desired, multiple adjacent antennas  42  may concurrently transmit wireless charging signals.  FIG. 12  is a top-down view of charging device  12  (e.g., in the X-Y plane of  FIG. 3 ) having multiple antennas selected for wireless power transmission. 
     As shown in  FIG. 12 , charging device  12  may select four adjacent antennas  42  (e.g., a first antenna  42 - 1 , a second antenna  42 - 2 , a third antenna  42 - 3 , and a fourth antenna  42 - 4 ) for wireless power transmission (TX). Antennas  42 - 1 ,  42 - 2 ,  42 - 3 , and  42 - 4  may, for example, be selected for transmission while processing step  120  of  FIG. 9 . Antennas  42 - 1 ,  42 - 2 ,  42 - 3 , and  42 - 4  may be fed wireless charging signals  90  by wireless circuitry  36  and may transmit wireless charging signals  90  towards device  10 . Antennas  42 - 1 ,  42 - 2 ,  42 - 3 , and  42 - 4  may, for example, be switched into use using switching circuitry interposed between the antennas and wireless circuitry  36 . Antennas  42 - 1 ,  42 - 2 ,  42 - 3 , and  42 - 4  may wirelessly interact (e.g., via near field electromagnetic coupling) to redirect or focus wireless charging signals  90  towards antenna  32 C on charged device  10 . 
       FIGS. 13A and 13B  are side views (e.g., in the Z-Y plane of  FIG. 3 ) showing how transmitting wireless charging signals  90  using multiple adjacent antennas  42  may serve to focus charging signals  90  towards receiver antenna  32 C. In the example of  FIG. 13A , a single antenna  42  is used to transmit wireless charging signals  90  (e.g., a single antenna  42  receives wireless charging signals  90  via feed terminal  106 ). Antenna  42  in  FIG. 13A  transmits wireless signals  90  along paths  160 . Paths  160  may have excessive lateral spread along the Y-axis such that the wireless signals may leak out of charging device  12  or may otherwise be directed away from charged device  10  (e.g., thereby reducing the charging efficiency of the system). 
     In the example of  FIG. 13B , antennas  42 - 1  and  42 - 2  are concurrently fed wireless charging signals  90  over feed points  106 . Antenna  42 - 1  may electromagnetically interact with antenna  42 - 2  (e.g., via near-field coupling) such that charging signals  90  are collectively transmitted by both antennas  42 - 1  and  42 - 2  along paths  162 . Paths  162  may exhibit less lateral spread along the Y-axis than paths  160  in  FIG. 13A . In other words, charging signals  90  may be more focused towards receiver device  10  and less leakage out of device  12  may occur relative to the example of  FIG. 13A  (e.g., the arrangement of  FIG. 13B  may result in improved charging efficiency relative to the arrangement of  FIG. 13A ). 
     If desired, antennas  42  that are adjacent to selected antennas  42 - 1 ,  42 - 2 ,  42 - 3 , and  42 - 4  may form a block filter for the selected antennas.  FIG. 14  is a top-down view of charging device  12  having concurrently selected antennas and an antenna-based block filter. 
     As shown in  FIG. 14 , antennas  42 - 1 ,  42 - 2 ,  42 - 3 , and  42 - 4  may be selected for transmission (e.g., as described in connection with  FIG. 12 ). The antennas  42  that surround the selected antennas (e.g., a ring of antennas  42  around antennas  42 - 1 ,  42 - 2 ,  42 - 3 , and  42 - 4 ) may be decoupled from wireless power transmitters  40  and may form a block filter for the selected antennas. For example, antennas  42 - 1 ,  42 - 2 ,  42 - 3 , and  42 - 4  may transmit wireless charging signals  90  for charging device  10 . A portion  140  of charging signals  90  may be transmitted outwards towards the surrounding ring of non-fed antennas. Surrounding antennas  42  may form an impedance discontinuity at the frequency of charging signals  90  that serves to block signals  140  from being transmitted past the surrounding antennas. This may serve to effectively trap signals  140  within the ring of surrounding antennas, as shown by paths  142 . 
     In this way, antennas  42  that surround selected antennas  42 - 1  through  42 - 4  may block wireless signals  140  from leaking laterally in the X-Y plane out of charging device  12 . This may prevent undesirable signal loss out of the sides of charging device  12  (e.g., preventing absorption of the charging signals by people or objects in the vicinity of charging device  12 ) and may increase the charging efficiency of charging device  12 . By transmitting charging signals  90  using multiple antennas, wireless charging signals  90  may be focused on receiving device  10 , thereby improving charging efficiency relative to scenarios where only one transmit antenna is selected (e.g., as described in connection with  FIGS. 13A and 13B ). 
     The example of  FIG. 14  is merely illustrative. If desired, two, three, or more than four antennas  42  may be selected for wireless power transmission. The selected antennas may be arranged in any desired manner (e.g., across two adjacent rows and two adjacent columns, across two adjacent rows and in one column, across two adjacent columns and in one row, across one row, across one column, etc.). 
     If desired, control circuitry  18  may control switching circuitry to select one or more antennas for transmission and to disable one or more antennas so that the disabled antennas are configured to form the guard ring structure. The switching circuitry may, for example, be interposed between wireless power transmitter circuitry  40  and antennas  42  (e.g., on transmission lines  100 ). 
       FIG. 15  is a diagram showing how switching circuitry may be interposed between wireless power transmitter circuitry  40  and a corresponding antenna  42 . As shown in  FIG. 15 , a given antenna  42  may be coupled to wireless power transmitter circuitry  40  via a corresponding transmission line  100 . Transmission line  100  may include a positive signal line  102  coupled between wireless power transmitter circuitry  40  and positive antenna feed terminal  106  on antenna  42 . Transmission line  100  may include a ground signal line  104  coupled between wireless power transmitter circuitry  40  and ground antenna feed terminal  108  on antenna  42 . A first switch such as switch SW 1  may be interposed on positive signal line  102  between wireless power transmitter circuitry  40  and positive antenna feed terminal  106 . A second switch such as switch SW 2  may be interposed on ground signal line  104  between wireless power transmitter circuitry  40  and ground antenna feed terminal  106 . 
     Storage and processing circuitry  16  ( FIG. 1 ) may provide control signals  171  to control the state of switch SW 1  and may provide control signals  173  to control the state of switch SW 2 . Storage and processing circuitry  16  may provide control signals  171  and  173  to configure antenna  42  to perform wireless power transmission or to form a portion of a block filter (e.g., guard ring structure). For example, storage and processing circuitry  16  may place switch SW 1  in a closed (ON) state and switch SW 2  in an open (OFF) state when the corresponding antenna  42  is selected for signal transmission (e.g., when antenna  42  is enabled for wireless power transmission). This may allow wireless charging signals to be transmitted by antenna  42  towards device  10 . Storage and processing circuitry  16  may disable antenna  42  to configure antenna  42  to form a portion of the guard ring structure. Storage and processing circuitry  16  may disable antenna  42  by placing switch SW 2  in a closed (ON) state. When switch SW 2  is closed, laterally transmitted antenna signals  140  ( FIGS. 10 and 14 ) may be shorted to ground at the location of antenna  42 , thereby serving to block the laterally transmitted antenna signals from passing beyond the location of antenna  42 . Switch SW 1  may be placed in either the open or closed state when antenna  42  is configured to form a portion of the guard ring structure. In this way, storage and processing circuitry  18  may configure antenna  42  to serve as a portion of a guard ring structure or to serve as a selected antenna for wireless power transmission. 
     The example of  FIG. 15  is merely illustrative. In general, any desired switching circuitry may be used to configure antenna  42  to transmit wireless charging signals or form a part of the guard ring structure. Similar switches may be coupled between wireless power transmitter circuitry  40  and each antenna  42  in wireless charging equipment  12  to allow equipment  12  to actively adjust which antenna  42  is used for signal transmission or used to form part of a guard ring structure in real time (e.g., as the position of device  10  relative to wireless charging equipment  12  is changed over time). 
     Storage and processing circuitry  16  may use any desired process for selecting which antennas  42  are configured to form the guard ring structure after the transmit antennas have been selected (e.g., after processing step  120  and before or concurrent with processing step  124  of  FIG. 9 ). As one example, storage and processing circuitry  16  may configure one or more antennas  42  that are adjacent to the selected transmit antennas (e.g., each antenna  42  that is adjacent to the selected transmit antenna) to form the guard ring structure. As another example, storage and processing circuitry  16  may use predetermined information identifying which antennas  42  to configure to form the guard ring structure for a given corresponding transmit antenna. In this scenario, calibration data may be stored on storage and processing circuitry  16  that identifies which antennas  42  to configure to form the guard ring structure for each possible transmit antenna. The calibration data may be generated during factory testing or manufacture of device  10 . For example, the radio-frequency transmit performance (e.g., wireless power transfer efficiency, reflection coefficients, etc.) of each antenna  42  may be tested in the factory while different combinations of other antennas  42  are configured to form part of the guard ring structure. The particular guard ring antennas that resulted in the optimal radio-frequency transmit performance for each antenna  42  may be stored in the calibration data and used during normal operation of charging equipment  12 . 
     As another example, storage and processing circuitry  16  may toggle switch SW 2  for each antenna  42  other than the selected transmit antennas to identify which antennas to configure to form the guard ring structure. In this scenario, storage and processing circuitry  16  may gather radio-frequency transmit performance information for the selected transmit antennas as the switches SW 2  in different combinations of antennas  42  are selectively turned on. Storage and processing circuitry  16  may identify which combination of antennas  42  resulted in the optimal radio-frequency transmit performance of the selected transmit antennas and may configure that combination of antennas  42  to form the guard ring structure for subsequent wireless charging using the selected transmit antennas. In this way, storage and processing circuitry  16  may actively identify optimal antennas to configure to form the guard ring structure for a given transmit antenna even if some antennas  42  reflect wireless charging signals back towards the transmit antenna that are otherwise out of phase with the transmitted wireless charging signals. These examples are merely illustrative and, in general, any desired process may be used to identify which antennas  42  to configure to form the guard ring structure. 
     In general, effective wireless charging of device  10  occurs when there is a close impedance match between transmitting antenna  42  on charging device  12  and receiving antenna  32  on device  10 . Impedance discontinuities in the vicinity of transmitting antenna  42  (e.g., between transmitting antenna  42  and the environment of antenna  42  above the antenna) cause signal reflections that prevent wireless power from being transferred efficiently. If desired, charging device  12  may include monitoring circuitry that is used to detect whether or not receive antenna  32  is efficiently coupled to transmitting antenna  42  for wireless charging (e.g., to gather data indicative of whether or not impedance discontinuities are present). 
     The monitoring circuitry may include impedance measurement circuitry that is used to characterize how well transmit antenna  42  is coupled to a receiving antenna  32 . If the receiving antenna is not present or if a piece of metal or object other than receiving antenna  32  is present, an impedance discontinuity will be present at transmit antenna  42 . In this case, the impedance measurement circuitry will gather impedance data or impedance information that identifies the presence of the discontinuity. The impedance measurement circuitry may gather impedance data by determining whether or not antenna  42  is coupled to antenna  32 , by determining whether or not there is an impedance discontinuity at antenna  42 , etc. When there is an impedance discontinuity present at antenna  42 , there will be a spike in reflected transmit power at antenna  42 . The impedance data can be gathered by measuring and analyzing the reflected power if desired. Because measurements of impedance mismatch are indicative of whether or not there is satisfactory coupling between transmit antenna  42  and receive antenna  32 , gathering impedance data in charging device  12  may help determine which if any of antennas  42  in device  12  are well coupled to receiving antenna  32 . 
     In order to gather the impedance data, the impedance measurement circuitry may make complex impedance measurements (e.g., measurements of complex impedance values having phases and magnitudes), measurements of reflected power (e.g., reflected power indicative of an impedance discontinuity), measurements of scattering parameters (sometimes referred to as S-parameters), or any other desired measurements for monitoring impedance discontinuities associated with transmitting antenna  42 . Impedance data or other information about impedance discontinuities associated with transmit antenna  42  may be used to select one or more antennas for transmitting wireless charging signals  90  (e.g., while processing step  120  of  FIG. 9 ). 
     In one suitable arrangement, charging device  12  may include circuitry for monitoring reflected signal power to detect impedance discontinuities associated with transmit antenna  42 . As shown in  FIG. 16 , wireless circuitry  34  on charging device  12  may include radio-frequency coupler circuitry  170 . Radio-frequency coupler  170  may be interposed on transmission path  100  between a corresponding antenna  42  and wireless power circuitry  40 . Wireless circuitry  34  may include power measurement circuitry  174  coupled to radio-frequency coupler  170  over transmission path  172 . Power measurement circuitry  174  and coupler  170  may sometimes be referred to collectively herein as impedance measurement circuitry (e.g., because coupler  170  and circuitry  174  are used to gather impedance data for a corresponding antenna  42 ). The data gathered by the impedance measurement circuitry may, for example, identify whether or not there is an impedance discontinuity associated with antenna  42 . If desired, portions of control circuitry  18  ( FIG. 1 ) may form part of the impedance measurement circuitry. Power measurement circuitry  174  may be an integrated circuit such as an application specific integrated circuit, field-programmable gate array, microprocessor, or other discrete power measurement circuit. If desired, power measurement circuitry  174 , RF coupler  170 , and/or wireless power circuitry  40  may be formed on a common integrated circuit, application specific integrated circuit, field-programmable gate array, microprocessor, etc. 
     Wireless power circuitry  40  may transmit charging signals  90  or other signals over transmission line  100  as shown by arrows  180 . Coupler  170  may route transmit signals  90  to antenna  42  so that these signals are transmitted over the air. Coupler  170  may also serve as a tap that routes a fraction of the transmitted signals from path  100  to power measurement circuitry  174  over path  172 , as shown by arrow  184 . If an impedance discontinuity exists at antenna  42 , the impedance discontinuity will cause some or all of the signals transmitted to antenna  42  to be reflected back towards coupler  170 . Coupler  170  may route the received signal reflections to power measurement circuitry  174  over path  172  as shown by arrow  182 . 
     Power measurement circuitry  174  may receive the tapped transmit signals and the reflected power over path  172 . Power measurement circuitry  174  may measure power levels of the tapped transmit signals and the reflected signals. Coupler  170  may, for example, include switching or other circuitry that routes reflected signals and transmit signals to power measurement circuitry  174  during respective time periods (e.g., so that circuitry  174  may distinguish between the reflected power and the transmit signals). Power measurement circuitry  174  may compare the measured power level of the transmitted signal to the magnitude of the reflected power to gather impedance data associated with antenna  42 . For example, relatively high levels of reflected power relative to the transmitted power may indicate that there is an impedance discontinuity associated with antenna  42  whereas relatively low levels of reflected power may indicate that there is no impedance discontinuity associated with antenna  42 . The presence or absence of an impedance discontinuity may identify how well matched antenna  42  is to a corresponding receive antenna  32  on device  10  for wireless charging. 
     As an example, when no objects are located over transmit antenna  42  (or when objects other than antenna  32  such as a metal device housing case are located over transmit antenna  42 ), there may be a large impedance discontinuity associated with transmit antenna  42 . This impedance discontinuity may cause a relatively large amount (e.g., a maximum amount) of the signals transmitted to antenna  42  to be reflected back towards measurement circuitry  174  via coupler  170 . Power measurement circuitry  174  may thereby measure a relatively large amount (e.g., a maximum amount) of reflected power in this scenario. For example, measurement circuitry  174  may compare the magnitude of the reflected power to the magnitude of the transmit power to identify that there is a relatively large (e.g., a maximum amount) of reflected power. The corresponding impedance data gathered by the impedance measurement circuitry may identify (e.g., based on the magnitude of the reflected power relative to the magnitude of the transmit power as measured at circuitry  174 ) that there is a relatively large impedance discontinuity at antenna  42 , and that antenna  42  is not efficiently coupled to a corresponding receive antenna  32  for wireless charging. If desired, measurement circuitry  174  may transmit the measured power levels to control circuitry  18  and control circuitry  18  may compare the power levels to detect the impedance discontinuities. 
     As receive antenna  32  on device  10  is moved over transmit antenna  42 , the impedance discontinuity between transmit antenna  42  and its surroundings (e.g., the space above antenna  42 ) will reduce. This reduction in the impedance discontinuity associated with antenna  42  will reduce the amount of power reflected back towards power measurement circuitry  174  via coupler  170 . When receive antenna  32  is aligned with transmit antenna  42  (e.g., when the efficiency of wireless power transmission from antenna  42  to antenna  32  is at a maximum), the amount of power reflected back towards power measurement circuitry  174  via coupler  170  is at a minimum. Power measurement circuitry  174  may thereby measure a relatively small amount (e.g., a minimum amount) of reflected power in this scenario. For example, measurement circuitry  174  may compare the magnitude of the reflected power to the magnitude of the transmitted power to identify that there is a relatively small (e.g., a minimum amount) of reflected power. The corresponding impedance data gathered by the impedance measurement circuitry may identify (e.g., based on the magnitude of the reflected power relative to the magnitude of the transmit power) that there is a relatively small or no impedance discontinuity at antenna  42 , and that antenna  42  is efficiently coupled to a corresponding receive antenna  32  for wireless charging. 
     This example in which the impedance measurement circuitry includes power measurement circuitry for detecting reflected power is merely illustrative. If desired, the impedance measurement circuitry may include phase and magnitude detection circuitry for identifying complex impedance values associated with antenna  42 , or any other circuitry for gathering impedance data that identifies how well matched antenna  42  is with a corresponding receive antenna  32  (e.g., that identifies impedance discontinuities associated with antenna  42 ). If desired, the impedance measurement circuitry may gather the impedance data based on scattering parameter values (sometimes referred to as S-parameters or S-values). As an example, the circuitry may gather S 11  parameter values. The S 11  parameter values may be indicative of the amount of reflected power received by coupler  170  and may be calculated based on measured power levels of the reflected signal, for example. The S 11  values may also be calculated using complex impedance values in scenarios where the impedance measurement circuitry gathers complex impedance data. In general, lower magnitudes of measured S 11  values may be indicative of lower levels of impedance discontinuity at antenna  42  (e.g., higher levels of impedance matching) whereas higher magnitudes of measured S 11  values may be indicative of higher levels of impedance discontinuity at antenna  42  (e.g., lower levels of impedance matching). 
     The circuitry of  FIG. 16  may be used to monitor, one, two, more than two, or all of the antennas  42  in device  12 . The impedance data (e.g., the information identifying impedance discontinuities or impedance matching between antennas  42  and  43 ) gathered by the impedance measurement circuitry may be gathered in real time. For example, real time power level measurements and/or measured complex impedance values gathered by the impedance measurement circuitry may be processed by control circuitry  18  to determine which antennas  42  to select for transmission. Control circuitry  18  may, for example, identify reflected power levels or S 11  values for each of the antennas  42  and may identify an antenna having the minimum measured reflected power level or minimum S 11  value as the selected antenna for wireless power transmission. In another example, control circuitry  18  may identify any antennas having a measured S 11  value or a measured reflected power level that is less than or equal to a threshold value as the selected antennas. Control circuitry  18  may provide control signals to wireless power circuitry  40  and/or to switching circuitry interposed on transmit path  100  to transmit wireless charging signals  90  over the selected antennas. 
     The impedance measurement circuitry (e.g., circuitry  174 ,  170 , and portions of control circuitry  18 ) may continue to gather impedance data for each antenna  42  in real time (e.g., as device  10  is wirelessly charged using the selected antennas). In this way, control circuitry  18  may update or actively change which antennas  42  are used for wireless power transmission as the gathered impedance data changes over time (e.g., while processing step  126  of  FIG. 9 ). For example, control circuitry  18  may identify, based on the gathered reflected power level, that device  10  has been removed from surface  70 . Control circuitry  18  may subsequently end wireless power transmission using antennas  42 . In another example, control circuitry  18  may identify, based on gathered S 11  values, that device  10  has changed locations on surface  70  (e.g., when device  10  has moved from a first location  74  to a second location  78 ). Control circuitry  18  may subsequently change which antennas  42  are used for transmitting wireless charging signals  90 . 
     This example shown in  FIG. 16  is merely illustrative. If desired, wireless signals other than wireless charging signals  90  may be used to gather the antenna impedance information. For example, wireless probe signals at any desired frequency may be transmitted and measured by measurement circuitry  174  for determining the antenna impedance information. The probe signals may, for example, include short pulse waves and may, if desired, be transmitted at power levels less than that used by wireless charging signals  90  for charging the battery on device  10 . Charging signals  90  that are used to charge the device using the selected antenna(s) may, for example, be at a maximum power level of device  12  or at any power level greater than the probe signals. Use of wireless probe signals to characterize the impedance of antennas  42  may, for example, allow for device  12  to conserve power while searching for a receive antenna  32  within wireless charging range of antennas  42 . Once device  12  has detected that an antenna  32  is in wireless charging range using the wireless probe signals, one or more antennas  42  that are best matched with that antenna  32  may be used to transmit wireless charging signals at a greater power level than the probe signals. Those transmitted wireless charging signals may be used to determine whether the transmitting antenna needs to be changed and/or additional wireless probe signals may be transmitted to determine whether the transmitting antenna needs to be changed. If desired, other sensor circuitry on charging device  12  may detect which antennas  42  are in wireless charging range of receiver antenna  32 . If desired, antennas  42  at different locations across surface  70  may concurrently transmit signals for charging multiple devices at once (e.g., in scenarios where multiple devices  10  are placed on surface  70  for charging). 
       FIG. 17  is a plot showing how measured scattering parameter values S 11  may be used to identify a selected antenna  42  for wireless power transmission. The S 11  values may be gathered using S 11  detection circuitry on charging device  12  or may be computed based on the amount of reflected power received by power measurement circuitry  174  (e.g., scattering parameter S 11  may be directly proportional to the magnitude of the reflected power). In scenarios where the impedance measurement circuitry includes circuitry that measured complex impedance values, scattering parameter S 11  may be computed based on the measured complex impedance values using known relations.  FIG. 17  plots measured S 11  values for two different antennas  42  as a function of horizontal distance between each antenna and a given receive antenna  32  (e.g., when device  10  having receive antenna  32  is placed on top of charging device  12 ). Curve  185  plots measured S 11  values for a first antenna  42  whereas curve  187  plots measured S 11  values for a second antenna  42 . 
     When receive antenna  32  is aligned with the second antenna (e.g., when there is zero horizontal distance between the second antenna  42  and receive antenna  32 ) as shown at point  189 , a minimum S 11  value may be measured from the second antenna (e.g., because there may be a minimum amount of reflected power due to the minimal impedance discontinuity between receive antenna  32  and the second antenna  42  at this location). This may indicate that the second antenna is well-matched with receive antenna  32 . As receive antenna  32  is moved horizontally towards the first antenna (e.g., as device  10  is laterally moved across the surface of device  12 ), the magnitude of S 11  measured for the second antenna may increase. When receive antenna  32  is located at a horizontal distance of D 2  with respect to the second antenna, a maximum S 11  value may be measured for the second antenna, as shown at point  191 . However, when receive antenna  32  is located at distance D 2  from the second antenna, receive antenna  32  may be well-aligned with respect to the first antenna (e.g., in scenarios where the first antenna  42  is located at a distance D 2  from the second antenna  42 ). A minimum S 11  value may thereby be measured for the first antenna when receive antenna  32  is at this position, as shown by point  193 . 
     As receive antenna  32  is moved laterally back towards the second antenna, the value of S 11  measured for the first antenna may increase. When receive antenna  32  is located at horizontal distance D 2  with respect to the first antenna (e.g., at a distance of zero with respect to the second antenna), a maximum S 11  value may be gathered for the first antenna as shown by point  181 . Conversely, receive antenna  32  may be well-aligned with respect to the second antenna at this point. At point  183  located between the first and second antennas (e.g., at a distance D 1  from the second antenna or distance D 2 -D 1  from the first antenna), S 11  values gathered for both the first and second antennas are equal. When antenna  32  is located to the left of point  183  (e.g., when antenna  32  is closer to the second antenna associated with curve  187 ), control circuitry  18  may select the second antenna for wireless charging (e.g., because the second antenna associated with curve  187  measures a smaller S 11  value than the first antenna associated with curve  185  for antenna positions to the left of D 1 ). When antenna  32  is located to the right of point  183  (e.g., when receive antenna  32  is closer to the first antenna associated with curve  185 ), control circuitry  18  may select the first antenna for wireless charging (e.g., because the first antenna associated with curve  185  measures a smaller S 11  value than the second antenna associated with curve  187  for antenna positions to the right of D 1 ). When receive antenna  32  is located at point  183 , either or both antennas  42  may be selected for wireless charging. The example of  FIG. 17  is merely illustrative. If desired, control circuitry  18  may select any antennas  42  that measure a value of S 11  that is below a threshold value to perform wireless charging. 
       FIG. 18  is a flow chart of illustrative steps that may be performed by wireless charging device  12  to select one or more antennas  42  for wireless power transmission. The steps of  FIG. 18  may, for example, be performed while processing step  120  of  FIG. 9 . 
     At step  190 , control circuitry  18  on charging device  12  may select a first antenna  42  for transmission. For example, control circuitry  18  may switch the selected antenna into use and/or activate transceiver circuitry coupled to the selected antenna. 
     At step  192 , charging device  12  may transmit wireless charging signals  90  using the selected antenna. A portion of the transmitted signals may be received at power measurement circuitry  174  via path  172  and coupler  170  ( FIG. 16 ). Power measurement circuitry  174  may receive reflected versions of the transmitted signals over the selected antenna, coupler  170 , and path  172 . This example is merely illustrative. If desired, device  12  may transmit radio-frequency signals other than charging signals  90  (e.g., wireless signals at any desired frequency). For example, device  12  may transmit wireless probe signals or other signals formatted for characterizing the impedance of antenna  42  (e.g., signals such as short pulse waves at power levels lower than those used to charge device  10 ). 
     At step  194 , power measurement circuitry  174  may measure the power level of the transmitted signals received through coupler  170 . Power measurement circuitry  174  may measure the power level of the reflected signals received through coupler  170 . If desired, circuitry  174  or other circuitry may generate S 11  values based on the reflected signals. In another suitable arrangement, complex impedance measurement circuitry may measure complex impedance values for antenna  42 . 
     At step  196 , power measurement circuitry  174  and/or control circuitry  18  may identify impedance matching information (sometimes referred to herein as impedance data) associated with the selected antenna based on the measured power levels (e.g., the power levels measured while processing step  194 ). For example, measurement circuitry  174  may compare the measured power level of the transmitted signal to the measured power level of the reflected signal to identify the impedance matching information (e.g., impedance discontinuities) associated with the selected antenna  42 . The identified impedance matching information may be indicative of how well aligned the selected antenna is to a corresponding receiver antenna  32 C on device  10 . 
     At step  198 , control circuitry  18  may determine whether additional antennas  42  remain on charging device  12  for processing. If antennas remain, processing may loop back to step  190  as shown by path  200  to identify impedance information associated with additional antennas  42 . If no antennas remain, processing may proceed to step  204  as shown by path  202 . In this way, charging device  12  may cycle through each antenna  42  in the array for identifying the corresponding impedance information. This example is merely illustrative. If desired, charging device  12  may perform power measurements and identify impedance matching information for two or more (e.g., all) of antennas  42  in parallel. 
     At step  204 , processing circuitry  18  may identify one or more antennas  42  as the selected antennas for charging device  10  based on the identified impedance matching information. For example, processing circuitry  18  may identify the one or more antennas  42  that are best impedance matched with receiver antenna  32 A on device  10  (e.g., one or more antennas having a minimum measured S 11  value or antennas having a measured S 11  values that are less than or equal to a threshold value, one or more antennas having a minimum reflected power level or having a reflected power level that is less than a threshold value, etc.). In some scenarios, processing circuitry  18  may identify a single best impedance matched antenna  42  for wireless charging. If desired, processing circuitry  18  may select one or more antennas  42  adjacent to that antenna for concurrently charging device  10  (e.g., such as in the examples of  FIGS. 12 and 14 ). The antennas  42  that are adjacent to the selected antenna(s) may, if desired, serve as a block filter or guard ring for the selected antennas (e.g., as shown in  FIGS. 10 and 14 ). The selected antennas  42  may be fed using a single port or using two ports for concurrently transmitting wireless charging signals  90  at different polarizations (e.g., as shown in  FIG. 8 ). 
     In practice, the impedance discontinuity associated with antenna  42  may be a function of the transmit frequency.  FIG. 19  is a flow chart of illustrative steps that may be performed by charging device  12  to select a desired frequency for wireless charging signals  90 . The steps of  FIG. 19  may, for example, be performed while processing step  122  of  FIG. 9 . 
     At step  210 , processing circuitry  18  may select a first frequency for transmission. 
     At step  212 , charging device  12  may transmit wireless charging signals over the selected antenna(s) (e.g., as selected while processing the step  120  of  FIG. 9 ) and at the selected frequency. If desired, device  12  may transmit radio-frequency signals other than charging signals  90  (e.g., wireless signals at any desired frequency). For example, device  12  may transmit wireless probe signals or other signals formatted for characterizing the impedance of antenna  42 . 
     At step  214 , power measurement circuitry  174  ( FIG. 16 ) may measure power levels of the transmitted signals and reflected versions of the transmitted signals at the selected frequency. In another suitable arrangement, circuitry  174  may measure S 11  values based on the reflected power. In yet another suitable arrangement, complex impedance measurement circuitry may measure complex impedance values for antenna  42  at the selected frequency. 
     At step  216 , power measurement circuitry  174  and/or control circuitry  18  may identify impedance matching information for the selected frequency based on the measured power levels (e.g., circuitry  174  and/or  18  may compare the reflected and transmit power levels to identify how well matched the selected antenna is to the receive antenna). At step  218 , control and processing circuitry  18  may identify whether additional frequencies remain for processing. If additional frequencies remain, processing may loop back to step  210  as shown by path  220  to gather impedance matching information for the selected antennas at different frequencies. If no additional frequencies remain, processing may proceed to step  224  as shown by path  222 . 
     At step  224 , processing circuitry  18  may identify one or more frequencies for charging device  10  based on the identified impedance matching information. For example, processing circuitry  18  may identify the frequency that generated a best impedance match over the selected antennas for charging device  10  (e.g., one or more frequencies that generated a minimum amount of reflected power for the selected antennas, that generated a minimum S 11  value for the selected antennas, etc.). The selected antenna(s) may subsequently transmit wireless charging signals  90  at the selected frequency to charge device  10  (e.g., while processing step  124  of  FIG. 9 ). 
     The example of  FIG. 19  is merely illustrative. In one suitable arrangement, charging device  12  may change the selected frequency in coarse increments upon looping back over path  220 . The coarse increments may be, for example, 50-100 MHz increments. After adjusting through coarse frequency increments, processing circuitry  18  may identify a coarse frequency having a best impedance match over the selected antenna(s). Once an optimal coarse frequency has been identified, charging device  12  may loop through fine frequency increments (e.g., 0.1 MHz, 1 MHz, 5 MHz, 10 MHz, or other increments that are finer than the coarse frequency increments) around the identified optimal coarse frequency. As an example, device  12  may loop through coarse frequencies in 50 MHz increments. Device  12  may identify an optimal coarse frequency of 150 MHz. Device  12  may subsequently loop through frequencies from 100 MHz to 200 MHz in 10 MHz increments. Once an optimal fine frequency has been found (e.g., a frequency having a best impedance match with the selected antenna), that frequency may be used for wireless charging. 
     By selecting best impedance matched antennas  42  and best impedance matched frequencies for wireless charging, charging device  12  may ensure that the efficiency with which device  10  is wirelessly charged is maximized, regardless of whether device  10  is moved to another location on the surface of charging device  12 . These selection operations may also minimize signal leakage by preventing the transmission of wireless power using antennas that are not in the vicinity of device  10 . Minimizing signal leakage may, for example, enhance the capability of device  12  to comply with regulatory limits or standards on emitted radiofrequency signals. Charging device  12  may further increase the efficiency with which device  10  is charged while minimizing signal leakage by forming a blocking filter from antennas  42  that are located around the selected antennas, and/or by concurrently transmitting wireless charging signals using multiple adjacent antennas  42 . The examples described above in connection with wireless charging signals  90  are merely illustrative. If desired, antennas  42  may transmit any desired radio-frequency signals (e.g., for conveying data with device  10 ). The processes of  FIGS. 9, 16 , and  17  may, if desired, be performed for selecting optimal antennas and frequencies with which to convey the radio-frequency data with device  10 . 
     The operations of devices  12  and  10  (e.g., the operations of  FIGS. 9, 18, and 19 ) may be performed by control circuitry  16  and/or  18 . During operation, this control circuitry (which may sometimes be referred to as processing circuitry, processing and storage, computing equipment, a computer, etc.) may be configured to perform the methods of  FIGS. 9, 18, and 19  and/or other operations (e.g., using dedicated hardware and/or using software code running on hardware such as control circuitry  16  and/or  18 ). Software code for performing these operations may be stored on non-transitory (tangible) computer readable storage media. The software code may sometimes be referred to as software, data, program instructions, instructions, or code. The non-transitory computer readable storage media may include non-volatile memory such as non-volatile random-access memory (NVRAM), one or more hard drives (e.g., magnetic drives or solid state drives), one or more removable flash drives or other removable media, other computer readable media, or combinations of these computer readable media. Software stored on the non-transitory computer readable storage media may be executed on the processing circuitry of control circuitry  16  and/or  18 . The processing circuitry may include application-specific integrated circuits with processing circuitry, one or more microprocessors, or other processing circuitry. 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20170419
Publication Date: 20200512
Grant Date: 20200512
Priority Date: 20160219
Inventors: JIANG, BING
SEN, INDRANIL S.
NARANG, MOHIT
BAE, MUN SOO
Assignee: APPLE INC
CPC Classifications: [{"code": "H02J50/80", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J50/23", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J50/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B5/0075", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02J50/23", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J50/80", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J7/025", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02J50/40", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J50/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B5/0037", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02J50/005", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02J50/402", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J50/402", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J50/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02J50/005", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02J50/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B5/24", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04B5/79", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04B5/24", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04B5/79", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 70612958