Patent Publication Number: US-9843091-B2

Title: Electronic device with configurable symmetric antennas

Description:
BACKGROUND 
     This relates generally to electronic devices and, more particularly, to electronic devices with wireless communications circuitry. 
     Electronic devices often include wireless circuitry with antennas. For example, cellular telephones, computers, and other devices often contain antennas for supporting wireless communications. 
     It can be challenging to form electronic device antenna structures with desired attributes. In some wireless devices, the presence of conductive structures such as conductive housing structures can influence antenna performance. Antenna performance may not be satisfactory if the housing structures are not configured properly and interfere with antenna operation. Device size can also affect performance. It can be difficult to achieve desired performance levels in a compact device, particularly when the compact device has conductive housing structures and is used in a variety of operating environments. 
     It would therefore be desirable to be able to provide improved wireless circuitry for electronic devices such as electronic devices that include conductive housing structures. 
     SUMMARY 
     An electronic device may have wireless circuitry with antennas. An antenna resonating element arm for an antenna may be formed from peripheral conductive structures running along the edges of a device housing that are separated from a ground by an elongated opening. The resonating element arm may be an inverted-F antenna resonating element arm. 
     The electronic device may have a central longitudinal axis that divides the antenna resonating element arm and other antenna structures into symmetrical halves that exhibit mirror symmetry with respect to the central longitudinal axis. The antenna structures may include symmetrical slot antenna resonating elements on opposing sides of the central longitudinal axis. 
     Electrical components such as switches and antenna tuning inductors may be coupled to the antenna structures in a configuration that is symmetrical with respect to the central longitudinal axis. The electrical components may be used to place the antenna structures in an unflipped configuration or in a symmetrical flipped configuration. In the unflipped configuration, the antenna structures form a hybrid antenna with an antenna feed on one side a the central longitudinal axis. In the flipped configuration, the antenna structures form a symmetrical hybrid antenna with an antenna feed on an opposing side of the central longitudinal axis. 
     Control circuitry in the electronic device may be used to configure the antenna structures to optimize antenna performance in real time. The control circuitry may gather data to use in determining when to change the antenna structures between the flipped and unflipped states from sensors, impedance measurement circuitry, wireless circuitry that monitors signal strengths, or other suitable circuitry in the electronic device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative electronic device in accordance with an embodiment. 
         FIG. 2  is a schematic diagram of illustrative circuitry in an electronic device in accordance with an embodiment. 
         FIG. 3  is a schematic diagram of illustrative wireless circuitry in accordance with an embodiment. 
         FIG. 4  is a graph in which antenna performance (standing-wave ratio) has been plotted as a function of operating frequency in accordance with an embodiment. 
         FIG. 5  is a schematic diagram of an illustrative dual branch inverted-F antenna in accordance with an embodiment. 
         FIG. 6  is a schematic diagram of an illustrative slot antenna with two closed ends in accordance with an embodiment of the present invention. 
         FIG. 7  is a diagram of illustrative slot antenna with an open end and a closed end in accordance with an embodiment. 
         FIG. 8  is a diagram of illustrative antenna structures in accordance with an embodiment. 
         FIG. 9  is a flow chart of illustrative steps involved in operating an electronic device having antennas of the type shown in  FIG. 8  in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices such as electronic device  10  of  FIG. 1  may be provided with wireless communications circuitry. The wireless communications circuitry may be used to support wireless communications in multiple wireless communications bands. 
     The wireless communications circuitry may include one more antennas. The antennas of the wireless communications circuitry can include loop antennas, inverted-F antennas, strip antennas, planar inverted-F antennas, slot antennas, hybrid antennas that include antenna structures of more than one type, or other suitable antennas. Conductive structures for the antennas may, if desired, be formed from conductive electronic device structures. 
     The conductive electronic device structures may include conductive housing structures. The housing structures may include peripheral structures such as peripheral conductive structures that run around the periphery of an electronic device. The peripheral conductive structure may serve as a bezel for a planar structure such as a display, may serve as sidewall structures for a device housing, may have portions that extend upwards from an integral planar rear housing (e.g., to form vertical planar sidewalk or curved sidewalls), and/or may form other housing structures. 
     Gaps may be formed in the peripheral conductive structures that divide the peripheral conductive structures into peripheral segments. One or more of the segments may be used in forming one or more antennas for electronic device  10 . Antennas may also be formed using an antenna ground plane formed from conductive housing structures such as metal housing midplate structures and other internal device structures. Rear housing wall structures may be used in forming antenna structures such as an antenna ground. 
     Electronic device  10  may be a portable electronic device or other suitable electronic device. For example, electronic device  10  may be a laptop computer, a tablet computer, a somewhat smaller device such as a wrist-watch device, pendant device, headphone device, earpiece device, or other wearable or miniature device, a handheld device such as a cellular telephone, a media player, or other small portable device. Device  10  may also be a television, a set-top box, a desktop computer, a computer monitor into which a computer has been integrated, or other suitable electronic equipment. 
     Device  10  may include a housing such as housing  12 . Housing  12 . which may sometimes be referred to as a case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc,), other suitable materials, or a combination of these materials. In some situations, parts of housing  12  may be formed from dielectric or other low-conductivity material. In other situations, housing  12  or at least some of the structures that make up housing  12  may be formed from metal elements. 
     Device  10  may, if desired, have a display such as display  14 . Display  14  may be mounted on the front face of device  10 . Display  14  may be a touch screen that incorporates capacitive touch electrodes or may be insensitive to touch. 
     Display  14  may include pixels formed from light-emitting diodes (LEDs), organic LEDs (OLEDs), plasma cells, electrowetting pixels, electrophoretic pixels liquid crystal display (LCD) components, or other suitable pixel structures. A display cover layer such as a layer of clear glass or plastic may cover the surface of display  14  or the outermost layer of display  14  may be formed from a color filter layer, thin-film, transistor layer, or other display layer. Buttons such as button  24  may pass through openings in the cover layer. The cover layer may also have other openings such as an opening for speaker port  26 . 
     Housing  12  may include peripheral housing structures such as structures  16 . Structures  16  may run around the periphery of device  10  and display  14 . In configurations in which device  10  and display  14  have a rectangular shape with four edges, structures  16  max be implemented using peripheral housing structures that have a rectangular ring shape with four corresponding edges (as an example). Peripheral structures  16  or part of peripheral structures  16  may serve as a bezel for display  14  (e.g., a cosmetic trim that surrounds all four sides of display  14  and/or that helps hold display  14  to device  10 ). Peripheral structures  16  may also, if desired, form sidewall structures for device  10  (e.g., by forming a metal band with vertical sidewalls, curved sidewalls, etc.). 
     Peripheral housing structures  16  may be formed of a conductive material such as metal and may therefore sometimes be referred to as peripheral conductive housing structures, conductive housing structures, peripheral metal structures, or a peripheral conductive housing member (as examples). Peripheral housing structures  16  may be formed from a metal such as stainless steel, aluminum, or other suitable materials. One, two, or more than two separate structures may be used in forming peripheral housing structures  16 . 
     It is it necessary for peripheral housing structures  16  to have a uniform cross-section. For example, the top portion of peripheral housing structures  16  may, if desired, have an inwardly protruding lip that helps hold display  14  in place. The bottom portion of peripheral housing structures  16  may also have an enlarged lip (e.g., in the plane of the rear surface of device  10 ). Peripheral housing structures  16  may have substantially straight vertical sidewalls, may have sidewalk that are curved, or may have other suitable shapes. In some configurations (e.g., when peripheral housing structures  16  serve as a bezel for display  14 ), peripheral housing structures  16  may run around the lip of housing  12  (i.e., peripheral housing structures  16  may cover only the edge of housing  12  that surrounds display  14  and not the rest of the sidewalls of housing  12 ). 
     If desired, housing  12  may have a conductive rear surface. For example, housing  12  may be formed from a metal such as stainless steel or aluminum. The rear surface of housing  12  may lie in a plane that is parallel to display  14 . In configurations for device  10  in which the rear surface of housing  12  is formed from metal, it may be desirable to form parts of peripheral conductive housing structures  16  as integral portions of the housing structures forming the rear surface of housing  12 . For example, a rear housing wall of device  10  may be formed from a planar metal structure and portions of peripheral housing structures  16  on the sides of housing  12  may be formed as vertically extending integral metal portions of the planar metal structure. Housing structures such as these may, if desired, be machined from a block of metal and/or may include multiple metal pieces that are assembled together to form housing  12 . The planar rear wall of  12  may have one or more, two or more, or three or more portions. 
     Display  14  may have an array of pixels that form an active area AA that displays images for a user of device  10 . An inactive border region such as inactive area IA may run along one or more of the peripheral edges of active area AA. 
     Display  14  may include conductive structures such as an array of capacitive electrodes for a touch sensor, conductive lines for addressing pixels, driver circuits, etc. Housing  12  may include internal conductive structures such as metal frame members and a planar conductive housing member (sometimes referred to as a midplate) that spans the walls of housing  12  (i.e., a substantially rectangular sheet formed from one or more parts that is welded or otherwise connected between opposing sides of member  16  or other sheet metal parts that provide housing  12  with structural support). Device  10  may also include conductive structures such as printed circuit boards, components mounted on printed circuit boards, and other internal conductive structures. These conductive structures, which may be used in forming a ground plane in device  10 , may be located in the center of housing  12  and may extend under active area AA of display  14 . 
     In regions  22  and  20 , openings may be formed within the conductive structures of device  10  (e.g., between peripheral conductive housing structures  16  and opposing conductive ground structures such as conductive housing midplate or rear housing wall structures, a primed circuit board, and conductive electrical components in display  14  and device  10 ). These openings, which may sometimes be referred to as gaps, may be filled with air, plastic, and other dielectrics and may be used in forming slot antenna resonating elements for one or more antennas in device  10 . 
     Conductive housing structures and other conductive structures in device  10  such as a midplate, traces on a printed circuit board, display  14 , and conductive electronic components may serve as a ground plane for the antennas in device  10 . The openings in regions  20  and  22  may serve as slots in open or closed slot antennas, may serve as a central dielectric region that is surrounded by a conductive path of materials in a loop antenna, may serve as a space that separates an antenna resonating element such as a strip antenna resonating element or an inverted-F antenna resonating element from the ground plane, may contribute to the performance of a parasitic antenna resonating element, or may otherwise serve as part of antenna structures formed in regions  20  and  22 . If desired, the ground plane that is under active area AA of display  14  and/or other metal structures in device  10  may have portions that extend into parts of the ends of device  10  (e.g., the ground may extend towards the dielectric-filled openings in regions  20  and  22 ), thereby narrowing the slots in regions  20  and  22 . In configurations for device  10  with narrow U-shaped openings or other openings that run along the edges of device  10 , the ground plane of device  10  can be enlarged to accommodate additional electrical components (integrated circuits, sensors, etc.) 
     In general, device  10  may include any suitable number of antennas (e.g., one or more, two or more, three or more, four or more, etc.). The antennas in device  10  may be located at opposing first and second ends of an elongated device housing (e.g., at ends  20  and  22  of device  10  of  FIG. 1 ), along one or more edges of a device housing, in the center of a device housing, in other suitable locations, or in one or more of these locations. The arrangement of  FIG. 1  is merely illustrative. 
     Portions of peripheral housing structures  16  may be provided with peripheral gap structures. For example, peripheral conductive housing structures  16  may be provided with one or more gaps such as gaps  18 , as shown in  FIG. 1 . The gaps in peripheral housing structures  16  may be filled with dielectric such as polymer, ceramic, glass, air, other dielectric materials, or combinations of these materials. Gaps  18  may divide peripheral housing structures  16  into one or more peripheral conductive segments. There may be for example, two peripheral conductive segments in peripheral housing structures  16  (e.g., in an arrangement with two of gaps  18 ), three peripheral conductive segments (e.g., in an arrangement with three of gaps  18 ), four peripheral conductive segments (e.g., in an arrangement with four gaps  18 , etc.). The segments of peripheral conductive housing structures  16  that are formed in this way may form parts of antennas in device  10 . 
     If desired, openings in housing  12  such as grooves that extend partway or completely through housing  12  may extend across the width of the rear wall of housing  12  and may penetrate through the rear wall of housing  12  to divide the rear wall into different portions. These grooves may also extend into peripheral housing structures  16  and may form antenna slots, gaps  18 , and other structures in device  10 . Polymer or other dielectric may fill these grooves and other housing openings. In some situations, housing openings that form antenna slots and other structure may be filled with a dielectric such as air. 
     In a typical scenario, device  10  may have upper and lower antennas (as an example). An upper antenna may, for example, be formed at the upper end of device  10  in region  22 . A lower antenna may, for example, be formed at the lower end of device  10  in region  20 . The antennas may be used separately to cover identical communications bands, overlapping communications bands, or separate communications bands. The antennas may be used to implement an antenna diversity scheme or a multiple-input-multiple-output (MIMO) antenna scheme. 
     Antennas in device  10  may be used to support any communications bands of interest. For example, device  10  may include antenna structures for supporting local area network communications, voice and data cellular telephone communications, global positioning system (GPS) communications or other satellite navigation system communications, Bluetooth® communications, etc. 
     A schematic diagram showing illustrative components that may be used in device  10  of  FIG. 1  is shown in  FIG. 2 . As shown in  FIG. 2 , device  10  may include control circuitry such as storage and processing circuitry  28 . Storage and processing circuitry  28  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  28  may be used to control the operation of device  10 . This processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, application specific integrated circuits, etc. 
     Storage and processing circuitry  28  may be used to run software on device  10 , such as internet browsing applications, voice-over-internet-protocol (VOIP) telephone call applications, email applications, media playback applications, operating system functions, etc. To support interactions with external equipment, storage and processing circuitry  28  may be used in implementing communications protocols. Communications protocols that may be implemented using storage and processing circuitry  28  include internet protocols, wireless local area network protocols (e.g., IEEE 802.11 protocols—sometimes referred to as WiFi®), protocols for other short-range wireless communications links such as the Bluetooth® protocol, cellular telephone protocols, multiple-input and multiple-output (MIMO) protocols, antenna diversity protocols, etc. 
     Input-output circuitry  30  may include input-output devices  32 . Input-output devices  32  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. Input-output devices  32  may include user interface devices, data port devices, and other input-output components. For example, input-output devices  32  may include touch screens, displays without touch sensor capabilities, buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, cameras, buttons, speakers, status indicators, light sources, audio jacks and other audio port components, digital data port devices, light sensors, motion sensors (accelerometers), capacitance sensors, proximity sensors, fingerprint sensors e.g., a fingerprint sensor integrated with a button such as button  24  of  FIG. 1  or a fingerprint sensor that takes the place of button  24 ), etc. 
     Input-output circuitry  30  may include wireless communications circuitry  34  for communicating wirelessly with external equipment. Wireless communications circuitry  34  may include radio-frequency (RF) transceiver circuitry formed from one or more integrated circuits, power amplifier circuitry, low-noise input amplifiers, passive RF components, one or more antennas, transmission lines, and other circuitry for handling RF wireless signals. Wireless signals can also be sent using light (e.g., using infrared communications). 
     Wireless communications circuitry  34  may include radio-frequency transceiver circuitry  90  for handling various radio-frequency communications bands. For example, circuitry  34  may include transceiver circuitry  36 ,  38 , and  42 . Transceiver circuitry  36  may handle 2.4 GHz and 5 GHz bands for WiFi® (IEEE 802.11) communications and may handle the 2.4 GHz Bluetooth® communications band. Circuitry  34  may use cellular telephone transceiver circuitry  38  for handling wireless communications in frequency ranges such as a low communications band from 700 to 960 MHz, a low midband from 1400-1520 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  38  may handle voice data and non-voice data. Wireless communications circuitry  34  can include circuitry for other short-range and long-range wireless links if desired. For example, wireless communications circuitry  34  may include 60 GHz transceiver circuitry, circuitry for receiving television and radio signals, paging system transceivers, near field communications (NFC) circuitry, etc. Wireless communications circuitry  34  may include global positioning system (GPS) receiver equipment such as GPS receiver circuitry  42  for receiving GPS signals at 1575 MHz or for handling other satellite positioning data. 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. In cellular telephone links and other long-range links, wireless signals are typically used to convey data over thousands of feet or miles. 
     Wireless communications circuitry  34  may include antennas  40 . Antennas  40  may be formed using any suitable antenna types. For example, antennas  40  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. Different types of antennas may be used for different bands and combinations of bands. For example, one type of antenna may be used in forming a local wireless link antenna and another type of antenna may be used in forming a remote wireless link antenna. 
     As shown in  FIG. 3 , transceiver circuitry  90  in wireless circuitry  34  may be coupled to antenna structures  40  using paths such as: path  92 . Wireless circuitry  34  may be coupled to control circuitry  28 . Control circuitry  28  may be coupled to input-output devices  32 . Input-output devices  32  may supply output from device  10  and may receive input from sources that are external to device  10 . 
     To provide antenna structures such as antenna(s)  40  with the ability to cover communications frequencies of interest, antenna(s)  40  may be provided with circuitry such as filter circuitry (e.g., one or more passive filters and/or one or more tunable filter circuits). Discrete components such as capacitors, inductors, and resistors may be incorporated into the filter circuitry. Capacitive structures, inductive structures, and resistive structures may also be formed from patterned metal structures (e.g., part of an antenna). If desired, antenna(s)  40  may be provided with adjustable circuits such as tunable components  102  to tune antennas over communications bands of interest. Tunable components  102  may be part of a tunable filter or tunable impedance matching network, may be part of an antenna resonating element, may span a gap between an antenna resonating element and antenna ground, etc. Tunable components  102  may include tunable inductors, tunable capacitors, or other tunable components. Tunable components such as these may be based on switches and networks of fixed components, distributed metal structures that produce associated distributed capacitances and inductances, variable solid state devices for producing variable capacitance and inductance values, tunable filters, or other suitable tunable structures. During operation of device  10 , control circuitry  28  may issue control signals on one or more paths such as path  120  that adjust inductance values, capacitance values, or other parameters associated with tunable components  102 , thereby tuning antenna structures  40  to cover desired communications bands. 
     Path  92  may include one or more transmission lines. As an example, signal path  92  of  FIG. 3  may be a transmission line having a positive signal conductor such as line  94  and a ground signal conductor such as line  96 . Lines  94  and  96  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(s)  40  to the impedance of transmission line  92 . Matching network components may be provided as discrete components (e.g., surface mount technology components) or may be formed from housing structures, printed circuit hoard structures, traces on plastic supports, etc. Components such as these may also be used in forming filter circuitry in antenna(s)  40  and may be tunable and/or fixed components. 
     Transmission line  92  may be coupled to antenna feed structures associated with antenna structures  40 . As an example, antenna structures  40  may form an inverted-F antenna, a slot antenna, a hybrid inverted-F slot antenna or other antenna having an antenna feed with a positive antenna feed terminal such as terminal  98  and a ground antenna feed terminal such as ground antenna feed terminal  100 . Positive transmission line conductor  94  may be coupled to positive antenna feed terminal  98  and ground transmission line conductor  96  may be coupled to ground antenna feed terminal  92 . Other types of antenna feed arrangements may be used if desired. For example, antenna structures  40  may be fed using multiple feeds. The illustrative feeding configuration of  FIG. 3  is merely illustrative. 
     Antenna structures  40  may include components and antenna resonating element structures that are configured to implement redundant antennas. This allows device  10  to switch an optimum antenna into use during the operation of device  10 . Antenna performance can be affected by the presence of external objects along certain portions of housing  12  or other environmental effects. By using redundant antenna structures, the location of the transmitting and receiving antennas in device  10  can be altered in real time to avoid wireless performance degradation. 
     As an example, antenna structures  40  may include symmetrical structures on both the left and right sides of device  10  that serve as redundant antennas. These structures may be used in forming an antenna that operates on either the left or right side of device  10 , as needed. In one configuration, for example, antenna structures  40  may be used to form an antenna that operates primarily on the left of device  10  in a communications band of interest. In another configuration, adjustable circuitry in antenna structures  40  can be configured to flip the antenna so that the antenna operates primarily on the right side of device  10  in the communications band of interest. Switching circuitry can also be used to select between antennas on the upper and lower ends of device  10  and to adjust which antenna feeds are used by transceiver circuitry  90 . 
     Any suitable information from sensors or other data sources can be used by device  10  in determining how to configure the antenna structures of device  10 . With one suitable arrangement, control circuitry  28  may use an impedance measurement circuit to gather antenna impedance information in real time. Control circuitry  28  may also gather proximity information from a proximity sensor (see e.g., sensor  32  of  FIG. 2 ), received signal strength information (e.g., signal strength information or other link performance metrics from a baseband processor or other wireless circuit), information from an orientation sensor, and other information for determining when antenna structures  40  are being affected by the presence of nearby external objects or are otherwise being affected. In response, control circuitry  28  may reconfigure antenna structures  40  to ensure that antenna performance is optimized (e.g., by implementing a reconfigurable antenna with a feed on the left or right of device  10  and/or by selecting between upper and lower antennas). If desired, control circuitry  28  may also adjust an adjustable inductor or other tunable component  102  to counteract antenna detailing due to the presence of external objects and/or to extend the coverage of antenna structures  40  (e.g., to cover desired communications bands that extend over a range of frequencies larger than antenna structures  40  would cover without tuning). Device  10  may be provided with redundant tuning components so that both the left and right antennas may be tuned. 
     Antenna structures  40  may include resonating element structures and components (e.g., components  102 ) that are arranged symmetrically with respect to the center axis of device  10 . This allows antennas to be formed in either an unflipped or flipped (mirror) configuration as desired to optimize antenna performance. Antenna structures  40  may be configured to form any suitable types of antenna. With one suitable arrangement, which is sometimes described herein as an example, antenna structures  40  are used to implement a hybrid inverted-F-slot antenna that includes both inverted-F and slot antenna resonating elements. A graph of antenna performance (standing wave ratio SWR) as a function of operating frequency for an illustrative hybrid antenna is shown in  FIG. 4 . As shown in  FIG. 4 , the hybrid antenna may exhibit resonances in multiple communications bands such as a low band LB from 700-960 MHz, a low-midband LMB from 1400-1520 MHz, a midband MB from 1700-2200 MHz, and a high band HB from 2300-2700 MHz. Other frequencies (e.g., local area network frequencies in a 5 GHz band) may also be supported (e.g., using a separate monopole, etc.). The hybrid antenna may use the inverted-F antenna resonating element to support coverage in the low band LB and midband MB, and may use slot resonances associated with one or more slot antenna resonating elements to support coverage in low-midband LMB and high band HB (as an example). Other configurations may be used for forming a hybrid antenna for device  10 , if desired. 
     An illustrative inverted-F antenna is shown in  FIG. 5 . As shown in  FIG. 5 , inverted-F antenna  40 - 1  may have inverted-F antenna resonating element  106  and antenna ground  104 . Antenna ground  104  may be formed from conductive housing structures, metal traces on a printed circuit or other substrate, midplate structures, conductive components in device  10 , or other ground plane structures in device  10 . Antenna resonating element  106  may have a main arm such as arm  108 . Arm  108  may be formed from conductive housing structures such as peripheral conductive housing structures  16  (e.g., a segment of peripheral conductive housing structures  16  that extends along the periphery of device  10  between respective gaps  18 ) or may be formed from other conductive structures. A return path such as return path  110  may be coupled between arm  108  and ground  104 . If desired, return path  110  may be formed by a configurable switch to support antenna flipping operations. Antenna  40 - 1  may have an antenna feed that is coupled between arm  108  and ground  104  in parallel with return path  110 . For example, antenna  40 - 1  may have an antenna feed such as antenna feed  112  at the tip of one of the ends of arm  108  (i.e., a feed that includes positive antenna feed terminal  98  and ground antenna feed terminal  100 ) or may have an antenna feed located elsewhere in antenna  40 - 1  (see, e.g., feed  112 ′ with positive feed terminal  98 ′ and ground feed terminal  100 ′). Indirect feeding arrangements may also be used, if desired. 
     Arm  108  of antenna  40 - 1  of  FIG. 5  may have a first branch of length L 1  that supports an antenna resonance in midband MB and a second branch of length L 2  (longer than L 1 ) that supports an antenna resonance in low band LB. In a hybrid antenna, inverted-F antenna resonating element  106  may be combined with one or more slot antenna resonating elements to extent the frequency coverage of the antenna. 
     An illustrative slot antenna resonating element is shown in  FIG. 6 . Slot antenna resonating element  40 - 2  has been formed from slot  130  in ground plane  104 . Slot  130  may be filled with air, plastic, or other dielectric. Illustrative slot resonating element  40 - 2  forms a slot antenna that is directly feed at feed  112  using positive antenna feed terminal  98  and ground antenna feed terminal  100 . Other types of feeding arrangements may be used if desired (e.g., indirect feeding arrangement in which the slot resonating element is fed through near-field coupling from an indirect feed structure). 
     The slot resonating element of  FIG. 6  has first closed end  132  and second closed end  134  at the opposing end of slot  130 . Slots such as slot  130  that have two closed ends may sometimes be referred to as closed slots. 
     An illustrative open slot is shown in the example of  FIG. 7 . As shown in  FIG. 7 , slot  140  in ground  104  has closed end  136  and opposing open end  138 . Open end  138  is surrounded by dielectric (e.g., air, plastic, etc.), whereas closed end  136  is surrounded by portions of ground  104 . Slot  140  may form a slot antenna resonating element for slot antenna  40 - 3 . Slot antenna  40 - 3  of  FIG. 7  is directly feed at feed  112  using positive antenna feed terminal  98  and ground antenna feed terminal  100 . Other types of feeding arrangements may be used (e.g., indirect feeding). The arrangement of  FIG. 7  is merely illustrative. 
       FIG. 8  is a top interior view of a portion of electronic device  10  in which antenna structures  40  have been formed. Antenna structures  40  may include symmetric structures that that exhibit mirror symmetry along central axis  142 . Device  10  may have an elongated rectangular shape and axis  142  may form a central longitudinal axis for device  10  that extends along the elongated dimension of device  10 . Axis  142  may bisect device  10 , antenna structures  40 , and housing  12  into left and right portions (left-hand side structures LHS and right-hand side structures RHS of  FIG. 8 ). Left-hand structures LHS may be mirror images of right hand structures RHS (i.e., if device  10  were to be turned over by rotating device  10  180° about axis  142 , the LHS and RHS would swap places). Components such as switches SW 1  and SW 3  may be located at equal distances from axis  142 . Components such as switches SW 2  and SW 4  may likewise be located at equal distances from axis  142 . Tuning components such as inductors  102 LB and  102 LA may be placed on opposing sides of device  10  at equal distances from axis  142 . 
     The symmetrical design of antenna structures  40  allows antenna structures  40  to be configured to operate in a normal (unflipped) configuration in some situations and to be configured to operate in a flipped (mirror reversed) configuration in other situations. This may allow antenna operation to be optimized in real time (e.g., to avoid antenna degradation due to blocking from external objects, etc.). 
     Antenna structures  40  may form first and second hybrid antennas for unflipped and flipped operation, respectively. The hybrid antennas may be inverted-F-slot antennas. Peripheral conductive structures  16  extend between gaps  18  (e.g., plastic filled housing gaps) and can be used to form an inverted-F antenna resonating element that is shared between the first and second hybrid antennas. Slots may be formed in the structures of antenna structures  40 . The slots form slot antenna resonating elements. The slot antenna resonating elements and the inverted-F antenna resonating element formed from structures  16  contribute to the overall response of the hybrid antennas. 
     As shown in  FIG. 8 , ground  104  may have an extended portion such as U-shaped portion  104 ′ that forms slots for slot antenna resonating elements. Slot  148 L is formed on the left-hand side of device  10  from the opening between elongated ground portion  104 ′ on the left-hand side of device  10  and ground  104 . Inner slot  148 R is formed on the right-hand side of device  10  from the opening between elongated ground portion  104  on the right-hand side of device  10  and the ground  104 . Switches SW 2  and SW 4  may be used to adjust the lengths of slots  148 L and  148 R and thereby adjust the frequency response of the slot antenna resonating elements formed from slots  148 L and  148 R. 
     Switches SW 1  and SW 3  and tunable components such as tunable inductors  102 LA and  102 LB bridge opening  144  between peripheral conductive structures  16  and ground  104 . Switches SW 1  and SW 3  may be used in configuring the inverted-F antenna resonating element formed from peripheral conductive structures  16  to operate in either an unflipped or flipped configuration. When closed, switch SW 1  (or switch SW 3 ) may form a return path such as return path  110  of  FIG. 5 . Tunable inductors  102 LA and  102 LB may be used in tuning the inverted-F antenna resonating element. Other tuning components may be added to antenna structures  40  if desired. 
     Antenna structures  40  may be fed using feeds such as feeds  112 A and  112 B. The first hybrid antenna may be fed by positive antenna feed terminal  98 A and ground antenna teed terminal  100 A in feed  112 A. The second hybrid antenna may be fed by positive antenna feed terminal  98 B and ground antenna feed terminal  100 B in feed  112 B. Transmission lines may be used to couple feeds  112 A and  112 B to transceiver circuitry  90 . In the example of  FIG. 8 , the first and second hybrid antennas are formed at upper end  20  of device  10 . If desired, device  10  may be provided with a similar or identical set of hybrid antennas at lower end  22  (as an example). 
     Control circuitry  2 $ can use impedance information, proximity sensor information, signal strength information, and/or other information to configure the antennas of device  10  in real time to optimize antenna performance. For example, control circuitry  28  can switch the upper or lower antenna structures into use and can also configure the selected antenna structures (upper or lower) to operate in either an unflipped or flipped configuration. The shapes and layouts of the conductive structures (e.g., peripheral conductive structures  16 , ground portions  104 , ground  104 , switches SW 1 , SW 2 , SW 3 , SW 4 , inductors  102 LA and  102 LB, and feeds  112 A and  112 B) are symmetric with respect to central axis  142  (i.e., switch SW 3  and SW 1  are both located an equal distance from axis  142 , etc.). The use of symmetric antenna structures  40  at the top and bottom ends of device  10  effectively provides device  10  with four different selectable antenna configurations (effectively antennas at each of the four corners of device  10 ), thereby enhancing the ability of device  10  to avoid undesired antenna blocking scenarios and other situations in which wireless performance might be degraded. If desired, multiplexing circuitry can be used to allow portions of the upper and lower antenna structures in device  10  to be used simultaneously (e.g., to handle respective communications bands). 
     When it is desired in use structures  40  in an unflipped configuration, switches SW 1  and SW 2  may be placed in an open (open circuit) configuration and switches SW 3  and SW 4  may be placed in a closed (short circuit) configuration. In this scenario, structures  40  form an unflipped hybrid antenna. Feed  112 A serves as a feed for the hybrid antenna. Low band coverage in low band LB may be provided by portion LB(A) of peripheral conductive structures  16  (i.e., portion LB(A) of the inverted-F resonating element). Portion LB(A) of the inverted-F antenna resonating element terminates at the short circuit formed by closed switch SW 3  across slot  144 . Low band LB in the unflipped configuration may be tuned by adjusting tunable inductor  102 LA. Inductor  102 LB and switch SW 1  are open in the unflipped configuration and therefore do not influence tuning. Low-midband coverage in band LMB may be provided by slot  148 L, which forms low-midland slot resonating element LMB(A). Switch SW 2  is open and therefore allows the full length of slot  148 L to be used. Midband coverage in band MB may be provided by portion MB(A) of the inverted-F antenna resonating element formed by peripheral conductive structures  16  (extending from gap  18  to closed switch SW 3 ). High band coverage in band HB may be provided by the slot resonating element formed from portion HB(A) of slot  148 R, which has a closed end formed by closed switch SW 4  and which extends to open end  150 R. 
     When it is desired to use structures  40  in a flipped configuration, switches SW 1  and SW 2  may be closed and switches SW 3  and SW 4  may be opened. In this configuration, structures  40  form a flipped hybrid antenna that is identical as the unflipped antenna, but that is flipped with respect to central axis  142 . In the flipped hybrid antenna configuration, feed  112 B serves as a feed for the hybrid antenna. Low band coverage in low band LB may be provided by portion LB(B) of peripheral conductive structures  16  (i.e., portion LB(B) of the inverted-F resonating element). Portion LB(B) terminates at the short circuit formed by closed switch SW 1  across slot  144 ). Low band LB in the flipped configuration may be tuned using tunable inductor  102 LB. Inductor  102 LA and switch SW 3  may be opened. Low-midband coverage in band LMB may be provided by slot  148 R, which forms low-midband slot resonating element LMB(B). Switch SW 4  is open and therefore allows the full length of slot  148 R to be used. Midband coverage in band MB may be provided by portion MB(B) of the inverted-F antenna resonating element formed by peripheral conductive structures  16  (extending from gap  18  to closed switch SW 1 ). High band coverage in band HB may be provided by the slot resonating element formed from portion HB(B) of slot  148 L, which has a closed end formed by closed switch SW 4  and which extends to open end  150 L. 
     Device  10  may be provided with an upper set of symmetric structures  40  in region  22  and a lower set of symmetric structures  40  in region  20 . During operation, the upper structures may be configured to use the left or right feed and the lower structures may be configured to use the left or right feed to optimize antenna performance. If desired, the currently selected upper hybrid antenna may be used at the same time as the currently selected lower hybrid antenna (e.g., to implement a multiple-input-multiple-output scheme). Upper and tower antennas may be used to handle communications in different communications bands and/or in the same communications band. 
     Illustrative steps involved in operating an electronic device such as device  10  in a configuration in which device  10  has symmetric antenna structures  40  are shown in  FIG. 9 . 
     At step  160 , control circuitry  28  may use antenna impedance measurement circuitry, sensors, and wireless circuitry to gather information on antenna loading, the proximity of external objects, signal strength, and other information on the operation of antennas in device  10 . 
     At step  162 , control circuitry  28  may use information on antenna operation to switch one or more optimum antennas into use to transmit and/or receive wireless traffic. If, for example, it is desired to use a set of symmetric antenna structures at one of the ends of device  10 , control circuitry  28  can switch either the left-hand feed or right-hand feed at that end into use depending on which of these two feeds results in better data throughput or otherwise satisfies predetermined operating criteria. When the left-hand feed is used, structures  40  are placed in an unflipped configuration. When the right-hand feed is used, structures  40  are placed in a flipped configuration (in which switches and other components are reversed with respect to central axis  142 ). Both upper and lower symmetric antenna structures (or more such structures) may be configured in this way. 
     During the operations of step  164 , the selected antenna(s) may be used to transmit and receive wireless data. This process may be performed continuously, as indicated by line  166 . 
     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.