Patent Publication Number: US-10312571-B2

Title: Electronic device having isolated antenna structures

Description:
BACKGROUND 
     This relates generally to electronic devices and, more particularly, to electronic devices with wireless communications circuitry. 
     Electronic devices often include wireless communications circuitry. For example, cellular telephones, computers, and other devices often contain antennas and wireless transceivers for supporting wireless communications. 
     It can be challenging to form electronic device antenna structures with desired attributes. In some wireless devices, antennas are bulky. In other devices, antennas are compact, but are sensitive to the position of the antennas relative to external objects. If care is not taken, antennas may become detuned, may emit wireless signals with a power that is more or less than desired, or may otherwise not perform as expected. 
     It would therefore be desirable to be able to provide improved wireless circuitry for electronic devices. 
     SUMMARY 
     An electronic device may be provided with wireless circuitry and control circuitry. The wireless circuitry may include multiple antennas and transceiver circuitry. The antennas may include antenna structures at opposing first and second ends of the electronic device. The antenna structures at a given end of the device may include multiple antennas and adjustable components that are adjusted by the control circuitry to place the antenna structures and the electronic device in one of a number of different operating modes or states. 
     The antenna structures at a first end of the electronic device may include an inverted-F antenna resonating element for a first antenna formed from portions of a peripheral conductive electronic device housing structure and an antenna ground that is separated from the antenna resonating element by a gap. A short circuit path may bridge the gap. An antenna feed may be coupled across the gap in parallel with the short circuit path. The inverted-F antenna resonating element arm may have a first end adjacent a first dielectric-filled gap and an opposing second end adjacent a second dielectric-filled gap. 
     The antenna structures at the first end of the electronic device may include an additional antenna resonating element for a second antenna formed from traces on a dielectric substrate. The additional antenna resonating element arm may have a first end coupled to a positive antenna feed terminal and a second end that opposes the first end. The second end of the additional antenna resonating element arm may be interposed between the first dielectric-filled gap and the first end of the additional antenna resonating element arm. 
     When configured in this way, the second end of the additional antenna resonating element arm may be interposed between the positive antenna feed terminal of the second antenna and relatively high magnitude electric fields generated by the first antenna around the first dielectric-filled gap. The second end of the additional antenna resonating element arm may shield other portions of the second antenna from the high magnitude electric field to improve isolation. 
    
    
     
       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 communications circuitry in accordance with an embodiment. 
         FIG. 4  is a schematic diagram of an illustrative inverted-F antenna in accordance with an embodiment. 
         FIG. 5  is a top view of illustrative antenna structures in an electronic device in accordance with an embodiment. 
         FIG. 6  is a top view of an illustrative antenna having relatively strong coupling to an adjacent antenna in accordance with an embodiment. 
         FIG. 7  is a top view of an illustrative antenna having relatively strong isolation from an adjacent antenna in accordance with an embodiment. 
         FIG. 8  is a cross-sectional side view of illustrative antenna structures of the type shown in  FIGS. 5 and 7  in accordance with an embodiment. 
         FIG. 9  is a schematic diagram showing how illustrative portions of an electronic device may be grounded in accordance with an embodiment. 
         FIG. 10  is a graph of antenna performance (antenna isolation) between illustrative antennas of the type shown in  FIGS. 5-9  as a function of frequency 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 structures 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 sidewalls 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 and/or an antenna resonating element formed from conductive housing structures (e.g., internal and/or external structures, support plate structures, etc.). 
     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 set-top box, a desktop computer, a display into which a computer or other processing circuitry has been integrated, a display without an integrated computer, 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 (e.g., glass, ceramic, plastic, sapphire, etc.). 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. The rear face of housing  12  (i.e., the face of device  10  opposing the front face of device  10 ) may have a planar housing wall. The rear housing wall may have slots that pass entirely through the rear housing wall and that therefore separate housing wall portions (and/or sidewall portions) of housing  12  from each other. The rear housing wall may include conductive portions and/or dielectric portions. If desired, the rear housing wall may include a planar metal layer covered by a thin layer or coating of dielectric such as glass, plastic, sapphire, or ceramic. Housing  12  (e.g., the rear housing wall, sidewalls, etc.) may also have shallow grooves that do not pass entirely through housing  12 . The slots and grooves may be filled with plastic or other dielectric. If desired, portions of housing  12  that have been separated from each other (e.g., by a through slot) may be joined by internal conductive structures (e.g., sheet metal or other metal members that bridge the slot). 
     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 if desired. 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  may 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, 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 not 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 sidewalls 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 or wall. 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 flat or curved 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 housing  12  may have one or more, two or more, or three or more portions. Peripheral conductive housing structures  16  and/or the conductive rear wall of housing  12  may form one or more exterior surfaces of device  10  (e.g., surfaces that are visible to a user of device  10 ) and/or may be implemented using internal structures that do not form exterior surfaces of device  10  (e.g., conductive housing structures that are not visible to a user of device  10  such as conductive structures that are covered with layers such as thin cosmetic layers, protective coatings, and/or other coating layers that may include dielectric materials such as glass, ceramic, plastic, or other structures that form the exterior surfaces of device  10  and/or serve to hide structures  16  from view of the user). 
     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 backplate) that spans the walls of housing  12  (i.e., a substantially rectangular sheet formed from one or more metal parts that is welded or otherwise connected between opposing sides of member  16 ). The backplate may form an exterior rear surface of device  10  or may be covered by layers such as thin cosmetic layers, protective coatings, and/or other coatings that may include dielectric materials such as glass, ceramic, plastic, or other structures that form the exterior surfaces of device  10  and/or serve to hide the backplate from view of the user. 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 extend under active area AA of display  14 , for example. 
     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 portions of housing  12 , conductive traces on a printed circuit board, conductive electrical components in display  14 , etc.). These openings, which may sometimes be referred to as gaps, may be filled with air, plastic, and/or other dielectrics and may be used in forming slot antenna resonating elements for one or more antennas in device  10 , if desired. 
     Conductive housing structures and other conductive structures in device  10  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 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 peripheral 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 of 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 one or more upper antennas and one or more 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, position and orientation sensors (e.g., sensors such as accelerometers, gyroscopes, and compasses), capacitance sensors, proximity sensors (e.g., capacitive proximity sensors, light-based proximity sensors, etc.), 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 960 to 1710 MHz, a midband from 1710 to 2170 MHz, a high band from 2300 to 2700 MHz, an ultra-high band from 3400 to 3700 MHz or other communications bands between 600 MHz and 4000 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, dipole antenna structures, monopole 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  103  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, a stripline transmission line, or a microstrip transmission line (as examples). A matching network (e.g., an adjustable matching network formed using tunable components  102 ) may include components such as inductors, resistors, and capacitors 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 board 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  112  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  100 . 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. 
     Control circuitry  28  may use information from a proximity sensor (see, e.g., sensors  32  of  FIG. 2 ), wireless performance metric data such as received signal strength information, device orientation information from an orientation sensor, device motion data from an accelerometer or other motion detecting sensor, information about a usage scenario of device  10 , information about whether audio is being played through speaker  26 , information from one or more antenna impedance sensors, and/or other information in determining when antenna(s)  40  is being affected by the presence of nearby external objects or is otherwise in need of tuning. In response, control circuitry  28  may adjust an adjustable inductor, adjustable capacitor, switch, or other tunable component  102  to ensure that antenna structures  40  operate as desired. Adjustments to component  102  may also be made 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). 
     The presence or absence of external objects such as a user&#39;s hand may affect antenna loading and therefore antenna performance. Antenna loading may differ depending on the way in which device  10  is being held. For example, antenna loading and therefore antenna performance may be affected in one way when a user is holding device  10  in the user&#39;s right hand and may be affected in another way when a user is holding device  10  in the user&#39;s left hand. In addition, antenna loading and performance may be affected in one way when a user is holding device  10  to the user&#39;s head and in another way when the user is holding device  10  away from the user&#39;s head. To accommodate various loading scenarios, device  10  may use sensor data, antenna measurements, information about the usage scenario or operating state of device  10 , and/or other data from input-output circuitry  32  to monitor for the presence of antenna loading (e.g., the presence of a user&#39;s hand, the user&#39;s head, or another external object). Device  10  (e.g., control circuitry  28 ) may then adjust adjustable components  102  in antenna  40  to compensate for the loading. 
     Antennas  40  may include slot antenna structures, inverted-F antenna structures (e.g., planar and non-planar inverted-F antenna structures), loop antenna structures, combinations of these, or other antenna structures. 
     An illustrative inverted-F antenna structure is shown in  FIG. 4 . As shown in  FIG. 4 , inverted-F antenna structure  40  (sometimes referred to herein as antenna  40  or inverted-F antenna  40 ) may include an inverted-F antenna resonating element such as antenna resonating element  106  and an antenna ground (ground plane) such as antenna ground  104 . Antenna resonating element  106  may have a main resonating element arm such as arm  108 . The length of arm  108  may be selected so that antenna structure  40  resonates at desired operating frequencies. For example, the length of arm  108  (or a branch of arm  108 ) may be a quarter of a wavelength at a desired operating frequency for antenna  40 . Antenna structure  40  may also exhibit resonances at harmonic frequencies. If desired, slot antenna structures or other antenna structures may be incorporated into an inverted-F antenna such as antenna  40  of  FIG. 4  (e.g., to enhance antenna response in one or more communications bands). As an example, a slot antenna structure may be formed between arm  108  or other portions of resonating element  106  and ground  104 . In these scenarios, antenna  40  may include both slot antenna and inverted-F antenna structures and may sometimes be referred to as a hybrid inverted-F and slot antenna. 
     Arm  108  may be separated from ground  104  by a dielectric-filled opening such as dielectric gap  101 . Antenna ground  104  may be formed from housing structures such as a conductive support plate, printed circuit traces, metal portions of electronic components, conductive portions of display  14 , and/or other conductive ground structures. Gap  101  may be formed by air, plastic, and/or other dielectric materials. 
     Main resonating element arm  108  may be coupled to ground  104  by return path  110 . Antenna feed  112  may include positive antenna feed terminal  98  and ground antenna feed terminal  100  and may run parallel to return path  110  between arm  108  and ground  104 . If desired, inverted-F antenna structures such as illustrative antenna structure  40  of  FIG. 4  may have more than one resonating arm branch (e.g., to create multiple frequency resonances to support operations in multiple communications bands) or may have other antenna structures (e.g., parasitic antenna resonating elements, tunable components to support antenna tuning, etc.). Arm  108  may have other shapes and may follow any desired path if desired (e.g., paths having curved and/or straight segments). 
     If desired, antenna  40  may include one or more adjustable circuits (e.g., tunable components  102  of  FIG. 3 ) that are coupled to antenna resonating element structures  106  such as arm  108 . As shown in  FIG. 4 , for example, tunable components  102  such as adjustable inductor  114  may be coupled between antenna resonating element arm structures in antenna  40  such as arm  108  and antenna ground  104  (i.e., adjustable inductor  114  may bridge gap  101 ). Adjustable inductor  114  may exhibit an inductance value that is adjusted in response to control signals  116  provided to adjustable inductor  114  from control circuitry  28 . 
     A top interior view of an illustrative portion of device  10  that contains antennas is shown in  FIG. 5 . As shown in  FIG. 5 , device  10  may have peripheral conductive housing structures such as peripheral conductive housing structures  16 . Peripheral conductive housing structures  16  may be divided by dielectric-filled peripheral gaps (e.g., plastic gaps)  18  such as gaps  18 - 1  and  18 - 2 . Antenna structures  40  may include a first antenna  40 F and a second antenna  40 W. Antenna  40 F (sometimes referred to as a cellular telephone antenna or a cellular and satellite navigation antenna) may include an inverted-F antenna resonating element arm  108  formed from the segment of peripheral conductive housing structures  16  extending between gaps  18 - 1  and  18 - 2 . Air and/or other dielectrics may fill slot  101  between arm  108  and ground structures  104 . If desired, opening  101  may be configured to form a slot antenna resonating element structure that contributes to the overall performance of the antenna. Antenna ground  104  may be formed from conductive housing structures, from electrical device components in device  10 , from printed circuit board traces, from strips of conductor such as strips of wire and metal foil, conductive portions of display  14 , and/or other conductive structures. In one suitable arrangement, ground  104  includes both conductive portions of housing  12  (e.g., portions of a rear wall of housing  12  such as a conductive backplate and portions of peripheral conductive housing structures  16  that are separated from arm  108  by peripheral gaps  18 ) as well as conductive portions of display  14 . 
     Antenna  40 F may support resonances in one or more desired frequency bands. The length of arm  108  may be selected to resonate in one or more desired frequency bands. For example, arm  108  may support a resonance in a cellular low band LB, midband MB, high band HB, and/or satellite navigation bands. In order to handle wireless communications at other frequencies (e.g., frequencies in 2.4 GHz and 5 GHz wireless local area network bands and Bluetooth bands or other bands), an additional antenna such as antenna  40 W may be formed within region  206 . 
     As shown in  FIG. 5 , ground  104  may have portions that are separated from the segment of peripheral conductive housing structures  16  between gaps  18 - 2  and  18 - 1  by a distance  140 . Slot  101  may have a width  140  in these regions. Other portions of ground plane  104  may be separated from peripheral conductive housing structures  16  by a shorter distance  142 . Slot  101  may have a width  142  in these regions. 
     Ground  104  may serve as antenna ground for one or more antennas. For example, inverted-F antenna  40 F may include an antenna ground formed from ground  104 . Antenna  40 W (sometimes referred to as wireless local area network antenna  40 W) may include an antenna resonating element within region  230  and ground  104 . 
     Positive transmission line conductor  94  and ground transmission line conductor  96  of transmission line  92  may be coupled between transceiver circuitry  90  and antenna feed  112 . Positive antenna feed terminal  98  of feed  112  may be coupled to arm  108  of antenna  40 F. Ground antenna feed terminal  100  of feed  112  may be coupled to ground  104 . Antenna feed  112  may be coupled across slot  101  at a location along ground plane  104  that is separated from peripheral conductive structures  16  by distance  142 . Distance  142  may, for example, be selected so that a desired distributed capacitance is formed between ground  104  and peripheral conductive housing structures  16 . The distributed capacitance may be selected to ensure that antenna  40  is impedance matched to transmission line  92 , for example. The portion of ground plane  104  that is separated from peripheral conductive housing structures  16  by distance  142  may be interposed between two regions where ground plane  104  is separated from peripheral conductive housing structures  16  by distance  140 , if desired. Transceiver circuitry  90  (e.g., remote wireless transceiver circuitry  38 , local wireless transceiver circuitry  36 , and/or GPS receiver circuitry  42  in  FIG. 2 ) may convey radio-frequency signals in frequency ranges such as a low communications band from 700 to 960 MHz, a low-midband from 960 to 1710 MHz, a midband from 1710 to 2170 MHz, a high band from 2300 to 2700 MHz, an ultra-high band from 3400 to 3700 MHz, 2.4 GHz and 5 GHz bands for WiFi® (IEEE 802.11) communications, and/or a 1575 MHz GPS band using antenna  40  and feed  112 . 
     Wireless local area network antenna  40 W in region  230  may include an inverted-F antenna resonating element or other suitable antenna resonating element. Wireless local area network antenna  40 W may be fed using a corresponding antenna feed  220  having a positive antenna feed terminal  222  coupled to the antenna resonating element of antenna  40 W and ground antenna feed terminal  224  coupled to ground  104 . Feed  220  of the wireless local area network antenna may convey radio-frequency over positive signal conductor  226  and ground signal conductor  228  of signal path  232  (e.g., a radio-frequency transmission line). Lines  226  and  228  may form parts of a coaxial cable, a stripline transmission line, or a microstrip transmission line (as examples). 
     Wireless local area network antenna  40 W may resonate in multiple frequency bands. For example, antenna  40 W may cover both 2.4 GHz and 5 GHz bands for wireless local area network (WLAN) communications (e.g., WiFi® communications) and/or Bluetooth communications or other wireless personal area network (WPAN) communications. Transmission line  232  may be coupled between wireless local area network transceiver circuitry  36  and feed  220  of antenna  40 W. Wireless local area network transceiver circuitry  36  may handle wireless local area network communications and/or wireless personal area network communications using transmission line  232 , feed  220 , and antenna  40 W. 
     Ground plane  104  may have any desired shape within device  10 . For example, the lower edge of ground plane  104  may be aligned with gap  18 - 1  in peripheral conductive hosing structures  16  (e.g., the upper or lower edge of gap  18 - 1  may be aligned with the edge of ground plane  104  defining slot  101  adjacent to gap  18 - 1 ). This example is merely illustrative. If desired, as shown in  FIG. 5 , ground  104  may include a vertical slot such as slot  162  adjacent to gap  18 - 1  that extends above the edges of gap  18 - 1  (e.g., along the Y-axis of  FIG. 5 ). Similarly, the lower edge of ground plane  104  may be aligned with the gap  18 - 2  (e.g., the upper or lower edge of gap  18 - 2  may be aligned with the edge of ground plane  104  defining slot  101  adjacent to gap  18 - 2 ) or may extend above the edges of gap  18 - 2 . 
     As shown in  FIG. 5 , vertical slot  162  adjacent to gap  18 - 1  may extend beyond the upper edge (e.g., upper edge  174 ) of gap  18 - 1  (e.g., in the direction of the Y-axis of  FIG. 5 ). Slot  162  may, for example, have two edges that are defined by ground  104  and one edge that is defined by peripheral conductive structures  16 . Slot  162  may have an open end defined by an open end of slot  101  at gap  18 - 1 . Slot  162  may have a width  176  that separates ground  104  from the portion of peripheral conductive structures  16  above gap  18 - 1  (e.g., in the direction of the X-axis of  FIG. 5 ). Because the portion of peripheral conductive structures  16  above gap  18 - 1  is shorted to ground  104  (and thus forms part of the antenna ground for antenna structures  40 ), slot  162  may effectively form an open slot having three sides defined by the antenna ground for antenna structures  40 . Slot  162  may have any desired width (e.g., about 2 mm, less than 4 mm, less than 3 mm, less than 2 mm, less than 1 mm, more than 0.5 mm, more than 1.5 mm, more than 2.5 mm, 1-3 mm, etc.). Slot  162  may have an elongated length  178  (e.g., perpendicular to width  176 ). Slot  162  may have any desired length (e.g., 10-15 mm, more than 5 mm, more than 10 mm, more than 15 mm, more than 30 mm, less than 30 mm, less than 20 mm, less than 15 mm, less than 10 mm, between 5 and 20 mm, etc.). 
     Electronic device  10  may be characterized by longitudinal axis  282 . Length  178  may extend parallel to longitudinal axis  282  (e.g., the Y-axis of  FIG. 5 ). Portions of slot  162  may contribute slot antenna resonances to antenna  40  in one or more frequency bands if desired. For example, the length and width of slot  162  (e.g., the perimeter of slot  162 ) may be selected so that antenna  40  resonates at desired operating frequencies. If desired, the overall length of slots  101  and  162  may be selected so that antenna  40  resonates at desired operating frequencies. 
     If desired, ground plane  104  may include an additional vertical slot  182  adjacent to gap  18 - 2  that extends beyond the upper edge (e.g., upper edge  184 ) of gap  18 - 2  (e.g., in the direction of the Y-axis of  FIG. 5 ). Slot  182  may, for example, have two edges that are defined by ground  104  and one edge that is defined by peripheral conductive structures  16 . Slot  182  may have an open end defined by an open end of slot  101  at gap  18 - 2 . Slot  182  may have a width  186  that separates ground  104  from the portion of peripheral conductive structures  16  above gap  18 - 1  (e.g., in the direction of the X-axis of  FIG. 5 ). Because the portion of peripheral conductive structures  16  above gap  18 - 2  is shorted to ground  104  (and thus forms part of the antenna ground for antenna structures  40 ), slot  182  may effectively form an open slot having three sides defined by the antenna ground for antenna structures  40 . Slot  182  may have any desired width (e.g., about 2 mm, less than 4 mm, less than 3 mm, less than 2 mm, less than 1 mm, more than 0.5 mm, more than 1.5 mm, more than 2.5 mm, 1-3 mm, etc.). Slot  182  may have an elongated length  188  (e.g., perpendicular to width  186 ). Slot  182  may have any desired length (e.g., 10-15 mm, more than 5 mm, more than 10 mm, more than 15 mm, more than 30 mm, less than 30 mm, less than 20 mm, less than 15 mm, less than 10 mm, between 5 and 20 mm, etc.). 
     Length  188  may extend parallel to longitudinal axis  282  (e.g., the Y-axis of  FIG. 5 ). Portions of slot  182  may contribute slot antenna resonances to antenna  40  in one or more frequency bands if desired. For example, the length and width of slot  182  may be selected so that antenna  40  resonates at desired operating frequencies. If desired, the overall length of slots  101  and  182  may be selected so that antenna  40  resonates at desired operating frequencies. If desired, the overall length of slots  101 ,  162 , and  182  may be selected so that antenna  40  resonates at desired operating frequencies. 
     A return path such as path  110  of  FIG. 4  may be formed by a fixed conductive path bridging slot  101  and/or one or more adjustable components such as adjustable components  202  and/or  208  as shown in  FIG. 5  (e.g., adjustable components such as tuning components  102  of  FIG. 3 ). Adjustable components  202  and  208  may sometimes be referred to herein as tuning components, tunable components, tuning circuits, tunable circuits, adjustable components, or adjustable tuning components. 
     Adjustable component  202  may bridge slot  101  at a first location along slot  101  (e.g., component  202  may be coupled between terminal  206  on ground plane  104  and terminal  204  on peripheral conductive structures  16 ). Adjustable component  208  may bridge slot  101  at a second location along slot  101  (e.g., component  208  may be coupled between terminal  212  on ground plane  104  and terminal  210  on peripheral conductive structures  16 ). Ground antenna feed terminal  100  may be interposed between terminal  206  and terminal  212  on ground plane  104 . Positive antenna feed terminal  98  may be interposed between terminal  204  and terminal  210  on peripheral conductive structures  16 . Terminal  212  may be closer to ground antenna feed terminal  100  than terminal  206 . Terminal  210  may be closer to positive antenna feed terminal  98  than terminal  204 . Terminals  206  and  212  may be formed on portions of ground plane  104  that are separated from peripheral conductive housing structures  16  by distance  140 . 
     Components  202  and  208  may include switches coupled to fixed components such as inductors for providing adjustable amounts of inductance or an open circuit between ground  104  and peripheral conductive structures  16 . Components  202  and  208  may also include fixed components that are not coupled to switches or a combination of components that are coupled to switches and components that are not coupled to switches. These examples are merely illustrative and, in general, components  202  and  208  may include other components such as adjustable return path switches, switches coupled to capacitors, or any other desired components (e.g., resistors, capacitors, inductors, and/or inductors arranged in any desired manner). 
     Components  202  and  208  may be adjusted based on the operating environment of the electronic device. For example, a tuning mode for antenna  40 F may be selected based on the presence or absence of external objects such as a user&#39;s hand or other body part in the vicinity of antenna  40  and/or based on required communication bands. Components  202  and  208  provide antenna  40  with flexibility to accommodate different loading conditions (e.g., different loading conditions that may arise due to the presence of a user&#39;s hand or other external object on various different portions of device  10  adjacent to various different corresponding portions of antenna  40 ). 
     Components  202  and  208  may be formed between peripheral conductive housing structures  16  and ground plane  104  using any desired structures. For example, components  202  and  208  may each be formed on a respective printed circuit such as a flexible printed circuit board that is coupled between peripheral conductive housing structures  16  and ground plane  104 . 
     The frequency response of antenna  40 F may be dependent upon the tuning mode of adjustable components  202  and  208 . For example, in a first tuning mode, adjustable component  202  may form an open circuit between antenna resonating element arm  108  and antenna ground  104 , whereas adjustable component  208  may selectively couple one or more inductors between antenna resonating element arm  108  and antenna ground  104  to tune antenna  40 F. In the first tuning mode, the resonance of antenna  40  in low band LB (e.g., from 700 MHz to 960 MHz or another suitable frequency range) may be associated with the distance along peripheral conductive structures  16  between feed  112  of  FIG. 5  and gap  18 - 1 , for example.  FIG. 5  is a view from the front of device  10 , so gap  18 - 1  of  FIG. 5  lies on the left edge of device  10  when device  10  is viewed from the front (e.g., the side of device  10  on which display  14  is formed) and lies on the right edge of device  10  when device  10  is viewed from behind. The resonance of antenna  40  at midband MB (e.g., from 1710 MHz to 2170 MHz) may be associated with the distance along peripheral conductive structures  16  between feed  112  and gap  18 - 2 , for example. Antenna performance in midband MB may also be supported by slot  182  in ground plane  104 . Antenna performance in high band HB (e.g., 2300 MHz to 2700 MHz) may be supported by slot  162  in ground plane  104  and/or by a harmonic mode of a resonance supported by antenna arm  108 . 
     In a second tuning mode, adjustable component  208  may form an open circuit between antenna resonating element arm  108  and antenna ground  104  to tune the antenna, whereas adjustable component  202  may selectively couple one or more inductors between antenna resonating element arm  108  and antenna ground  104  to tune antenna  40 F. In the second tuning mode, the resonance of antenna  40 F in low band LB may be associated with the distance along peripheral conductive structures  16  between the position of component  202  (i.e., terminal  204 ) of  FIG. 5  and gap  18 - 2 , for example. The resonance of antenna  40  in midband MB may be associated with the distance along peripheral conductive structures  16  between the position of component  202  (i.e., terminal  204 ) and gap  18 - 1 , for example. Antenna performance in high band HB may also be supported by slot  162  in ground plane  104 . 
     In a third tuning mode, adjustable components  202  and  208  may both selectively couple one or more inductors between antenna resonating element arm  108  and antenna ground  104  to tune antenna  40 F. In the third tuning mode, the resonance of antenna  40  at midband MB and high band HB may be associated with a loop including portions of peripheral conductive structures  16  (e.g., the portion of peripheral conductive structures  16  between terminal  204  of component  202  and terminal  210  of component  208 ) component  202 , ground plane  104 , and component  208 . 
     Antennas  40  may be configured to handle different frequency bands in each tuning mode. For example, in the first tuning mode, antenna  40 F may be configured to perform communications in a low band, midband, and high band. In the second tuning mode of antenna  40 F may also be configured to perform communications in the low band, midband, and high band. However, the first and second tuning modes may compensate for antenna loading by an external device such as a user&#39;s hand in different ways. For example, in the first tuning mode, antenna  40  may be configured to operate with a relatively high antenna efficiency if device  10  is being held by a user&#39;s right hand and a relatively low antenna efficiency if device  10  is being held by a user&#39;s left hand, whereas in the second tuning mode antenna  40  may be configured to operate with a relatively high antenna efficiency if device  10  is being held by a user&#39;s left hand and a relatively low antenna efficiency if device  10  is being held by a user&#39;s right hand. In other words, in the first and second tuning modes, antenna  40  may perform wireless communications in the low band, midband, and high band, but may be sensitive to certain operating conditions such as which hand a user is using to hold device  10 . 
     In general, antenna  40  may be more susceptible to changing loading conditions and detuning when operating in the low band than when operating in the midband or high band. In the third tuning mode, antenna  40  may be configured to operate with a relatively high efficiency regardless of which hand a user is using to hold device  10  (e.g., antenna  40  may be resilient or reversible to the handedness of the user). However, when placed in the third tuning mode, antenna  40  may only cover a subset of the frequency bands that antenna  40  is capable of covering in the first and second tuning modes. For example, in the third tuning mode antenna  40  may cover the midband and high band without covering the low band. 
     When operated in the first tuning mode, adjustable component  202  may form an open circuit between terminals  204  and  206 . However, when operated in the second or third tuning modes, one or more inductors of adjustable component  202  may be coupled between terminals  204  and  206 . In the second and third tuning modes when at least one inductor is connected between terminals  204  and  206 , a relatively strong (e.g., high magnitude) electric field may be present around gap  18 - 1 . If care is not taken, the relatively high magnitude electric field may interfere with adjacent antenna structures such as the resonating element of antenna  40 W within region  230 . 
       FIG. 6  is a top view of antenna  40 W adjacent to gap  18 - 1  in one particular scenario. As shown in  FIG. 6 , antenna  40 W may include an antenna resonating element such as antenna resonating element  242  (e.g., an inverted-F antenna resonating element). Antenna resonating element  242  may, for example, be formed from metal traces on a dielectric substrate. Positive antenna feed terminal  222  of feed  220  may be coupled to antenna resonating element  242  whereas ground antenna feed terminal  224  is coupled to ground  104 . A return path  244  may be coupled between the antenna resonating element  242  and ground  104 . Antenna resonating element  242  may exhibit a relatively high current density within region  246  (e.g., a region of resonating element  242  closest to feed terminal  222 ). The relatively high current density in region  246  may electromagnetically couple to the relatively high magnitude electric field generated by antenna resonating element  108  of antenna  40 F within region  248 . This electromagnetic coupling may, for example, serve to limit the electromagnetic isolation between antenna  40 F and the antenna  40 W and may subsequently generate electromagnetic interference on the antenna signals handled by antenna  40 W and/or antenna  40 F. Such interference may introduce errors in the data conveyed by antennas  40 W and/or  40 F, may lead to a reduction in corresponding wireless link quality, and/or may cause the corresponding wireless link to be dropped. 
     In  FIG. 6 , positive antenna feed terminal  222  is separated from gap  18 - 1  by distance  250 . Electromagnetic coupling between antenna  40 F and antenna  40 W may be mitigated by increasing this distance, for example. 
     An arrangement for antenna  40 W with greater electromagnetic isolation between antennas  40 W and  40 F relative to the arrangement of  FIG. 6  is shown in  FIG. 7 . As shown in  FIG. 7 , antenna  40 W may have an antenna resonating element  242 . Antenna resonating element  242  may, for example, be formed from metal traces on a dielectric substrate. Antenna resonating element  242  of antenna  40 W may include a first segment  256  that is coupled to positive antenna feed terminal  222 . Segment  256  may extend along a longitudinal axis that is approximately parallel to the left edge of the device and approximately perpendicular to the lower edge of the device (e.g., segment  256  may extend parallel to the Y-axis of  FIGS. 5 and 7 ). 
     Antenna resonating element  242  in  FIG. 7  includes a first branch (arm)  258  that extends from segment  256  and resonates in a first wireless local area network antenna band (e.g., a 5 GHz WiFi® band between 5150 MHz and 5850 MHz). Branch  258  may include a first segment  257  that extends away from segment  256  towards gap  18 - 1  (e.g., parallel to the X-axis) and a second segment  259  that extends away from the end of segment  257  opposing segment  256  and perpendicular to segment  257  (e.g., parallel to the Y-axis). Extending the tip of arm  258  in a direction perpendicular to the horizontal portion of antenna resonating element  108  may, for example, serve to maximize isolation between arm  258  and antenna  40 W at frequencies in the first wireless local area network band. 
     The antenna resonating element may also include a second branch (arm)  260  that extends from segment  256  and resonates in a second wireless area network band (e.g., a 2.4 GHz WiFi® band between 2400 MHz and 2500 MHz and/or in a Bluetooth band). Branch  260  may include a first antenna resonating element segment  261  that extends from segment  256  in a direction away from gap  18 - 1  (e.g., parallel to the X-axis). Branch  260  may include a second segment  263  that extends from the end of segment  261  opposite segment  256  in a direction away from positive antenna feed terminal  222  and perpendicular to segment  261  (e.g., parallel to the Y-axis). Branch  260  may also include a third antenna resonating element segment  265  that extends from the end of segment  263  opposite segment  261  in a direction perpendicular to segment  263  and parallel to segment  261  (e.g., parallel to the X-axis). If desired, branch  260  may further include a fourth antenna resonating element segment  267  that extends from the end of segment  265  opposite segment  263  and in a direction perpendicular to segments  261  and  265  and parallel to segment  263  and segment  256  (e.g., parallel to the Y-axis). When configured in this way, segment  267  may extend parallel to the portion of resonating element arm  108  adjacent to gap  18 - 1  and may terminate at a gap that is interposed between the tip of segment  267  and ground  104 . Segment  267  (e.g., a first end of branch  260 ) may be interposed between the second end of branch  260  (coupled to positive antenna feed terminal  222 ) and the end of antenna resonating element arm  108 , may be interposed between the second end of branch  260  (coupled to positive antenna feed terminal  222 ) and gap  18 - 1 , or may extend beyond gap  18 - 1  such that a portion of segment  267  is interposed between the second end of branch  260  (coupled to positive antenna feed terminal  222 ) and the end of antenna resonating element arm  108 , gap  18 - 1 , and/or portions of peripheral conductive housing structures  16 . Segment  265  may extend parallel to the horizontal portion of resonating element arm  108  on which feed  112  of antenna  40 F is formed. In this way, antenna resonating element arm  260  may follow or mirror the shape of the adjacent antenna resonating element arm  108  of antenna  40 F to help to minimize the amount of electromagnetic coupling between the antennas. 
     In addition, when configured in this way, segment  267  may be interposed between feed  220  (segment  256 ) and the relatively high magnitude electric fields generated by antenna  40 F within region  248  when operated in the second and third tuning modes. Segment  267  may shield branch  258  and/or antenna feed  220  from the high magnitude electric field to improve isolation. Also, isolation between antenna  40 F and antenna  40 W may be improved by increasing the distance between the positive antenna feed terminal  222  and gap  18 - 1 . For example, positive antenna feed terminal  222  is separated from gap  18 - 1  by distance  252  in  FIG. 7  and distance  250  in  FIG. 6 . Distance  252  may be greater than distance  250 . Since electromagnetic coupling is inversely proportional to the distance between positive antenna feed terminal  222  and gap  18 - 1 , the increased distance in  FIG. 7  will reduce electromagnetic coupling, enhance antenna performance (antenna efficiency), increase corresponding wireless link quality, and/or may reduce the likelihood of the corresponding wireless link being dropped relative to the arrangement of  FIG. 6 , for example. 
     As shown in  FIG. 7 , the wireless local area network antenna may also include a return path  244  that couples antenna resonating element  242  to ground  104  (e.g., antenna currents conveyed over resonating element  242  may be shorted to ground  104  over return path  244 ). If desired, an optional capacitive circuit such as capacitor  262  may be interposed on return path  244  between segment  261  and terminal  264  on ground plane  104 . Capacitor  262  may, for example, serve as a high-pass filter that blocks currents at frequencies in the cellular midband from passing to ground terminal  264 . This may, for example, further improve isolation between wireless local area network antenna  40 W and cellular antenna  40 F at corresponding frequencies of operation. Capacitor  262  may be omitted if desired. 
     Ground terminal  264  may include a screw and/or screw boss that is electrically connected to a conductive support plate that forms a portion of ground  104 . Ground terminal  264  may be shared with other components if desired. For example, inductor  202  may be coupled to ground terminal  264  (e.g., without contacting the conductive traces of resonating element  242 ). 
     In some of the aforementioned arrangements, fasteners are described as being used to short conductive components to the antenna ground. In general, any desired fastener such as a bracket, clip, spring, pin, screw, solder, weld, conductive adhesive, or a combination of these may be used. Fasteners may be used to electrically connect and/or mechanically secure components within electronic device  10 . Fasteners may be used at any desired terminals within electronic device  10  (e.g., terminals  224 ,  204 ,  206 ,  264 ,  98 ,  100 ,  210 , and/or  212 ). 
     Additionally, at each ground terminal within the device (e.g., terminals  224 ,  206 ,  264 ,  100 , and/or  212 ), different components of the device ground (e.g., ground  104  in  FIG. 5 ) may be electrically connected so that the conductive structures that are located the closest to resonating element arm  108  are held at a ground potential and form a part of antenna ground  104 . In one suitable arrangement, ground  104  includes both conductive portions of housing  12  (e.g., portions of a rear wall of housing  12  such as a conductive backplate and portions of peripheral conductive housing structures  16  that are separated from arm  108  by peripheral gaps  18 ) as well as conductive portions of display  14  (e.g., conductive portions of a display panel, a conductive plate for supporting the display panel, and/or a conductive frame for supporting the conductive plate and/or the display panel). Vertical conductive structures (e.g., a bracket, clip, spring, pin, screw, solder, weld, conductive adhesive, wire, metal strip, or a combination of these) may couple conductive portions of housing  12  to conductive portions of display  14  at terminals  224 ,  206 ,  264 ,  100 , and/or  212 . Ensuring that the conductive structures closest to resonating element arm  108  such as conductive portions of display  14  are held at a ground potential may, for example, serve to optimize the antenna efficiency of antenna structures  40 . 
     A cross-sectional side view of electronic device  10  showing how antenna  40 W and antenna  40 F may be grounded to antenna ground  104  within device  10  is shown in  FIG. 8  (e.g., as taken in the direction of arrow  283  in  FIG. 7 ). As shown in  FIG. 8 , display  14  for electronic device  10  may include a display cover layer such as display cover layer  302  that covers display panel  304 . Display panel  304  (sometimes referred to as a display module) may be any desired type of display panel and 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. The lateral area of display panel  304  may, for example, determine the size of active area AA of display  14  ( FIG. 1 ). Display panel  304  may include active light emitting components, touch sensor components (e.g., touch sensor electrodes), force sensor components, and/or other active components. Display cover layer  302  may be a layer of clear glass, plastic, or other dielectric that covers the light-emitting surface of the underlying display panel. In another suitable arrangement, display cover layer  302  may be the outermost layer of display panel  304  (e.g., layer  302  may be a color filter layer, thin-film transistor layer, or other display layer). Buttons may pass through openings in cover layer  302  (see button  24  in  FIG. 1 ). The cover layer may also have other openings such as an opening for a speaker port (see speaker port  26  in  FIG. 1 ). 
     Display panel  304  may be supported within electronic device  10  by a conductive display support plate (sometimes referred to as a midplate or display plate) such as display plate  306 . Conductive display frame  308  may hold display plate  306  and/or display panel  304  in place on housing  12 . For example, display frame  308  may be ring-shaped and may include a portion that runs around the periphery of the display panel  304  and surrounds a central opening. Display plate  306  and display frame  308  may both be formed from conductive material (e.g., metal). Display plate  306  and display frame  308  may be in direct contact such that the display plate  306  and the display frame  308  are electrically connected. If desired, display plate  306  and display frame  308  may be formed integrally (e.g., from the same piece of metal). 
     Conductive display frame  308  may be electrically connected to a radio-frequency shield  312  by conductive spring  310 . The conductive spring may directly contact both the display frame  308  and the radio-frequency shield  312 . The example of a conductive spring electrically connecting frame  308  and shield  312  is merely illustrative, and any other desired structure (e.g., a bracket, clip, spring, pin, screw, solder, weld, conductive adhesive, wire, metal strip, or a combination of these) may electrically connect frame  308  and shield  312 . Alternatively, display frame  308  may directly contact radio-frequency shield  312  without an intervening structure. 
     Radio-frequency shield  312  may shield the cellular antenna and the wireless local area network antenna in electronic device  10  from interference. The cellular antenna may be formed from conductive structures such as peripheral conductive housing structures  16  and other desired structures. The wireless local area network antenna may be formed at least partially from traces on a circuit board. As shown in  FIG. 8 , antenna resonating element  242  may be formed on printed circuit  322 . Other antenna traces and components such as return path  244  and capacitor  262  may also be formed on printed circuit  322  if desired. Printed circuit  322  may be a rigid printed circuit board (e.g., a printed circuit board formed from fiberglass-filled epoxy or other rigid printed circuit board material) or may be a flexible printed circuit (e.g., a flexible printed circuit formed from a sheet of polyimide or other flexible polymer layer). Because printed circuit  322  with antenna resonating element  242  is formed underneath radio-frequency shield  312 , the wireless local area network antenna may be shielded from radio-frequency signals generated by other components within electronic device  10  (e.g., radio-frequency signals originating on the other side of the radio-frequency shield). 
     As shown in  FIG. 8 , housing  12  may include a conductive portion such as conductive housing layer  320  (e.g., a conductive backplate for device  10  that extends between the left and right edges of device  10  and that forms a portion of antenna ground  104 ). Printed circuit  322  may be formed in a cutout region of conductive housing layer  320 . Additional electronic components may be formed above printed circuit  322  if desired. 
     Housing  12  may include dielectric housing portions such as dielectric layer  324  and conductive housing portions such as conductive layer  320  (sometimes referred to herein as conductive housing wall  320 ). If desired, dielectric layer  324  may by formed under layer  320  such that layer  324  forms an exterior surface of device  10  (e.g., thereby protecting layer  320  from wear and/or hiding layer  320  from view of a user). Conductive housing portion  320  may form a portion of ground  104 . As examples, conductive housing portion  320  may be a conductive support plate or wall (e.g., a conductive back plate or rear housing wall) for device  10 . Conductive housing portion  320  may, if desired, extend across the width of device  10  (e.g., between two opposing sidewalls formed by peripheral housing structures  16 ). If desired, conductive housing portion  320  and the opposing sidewalls of device  10  may be formed from a single integral piece of metal or portion  320  may otherwise be shorted to the opposing sidewalls of device  10 . Dielectric layer  324  may be a thin glass, sapphire, ceramic, or sapphire layer or other dielectric coating, as examples. In another suitable arrangement, layer  324  may be omitted if desired. 
     Printed circuit  322  may be secured to and electrically connected to conductive housing layer  320  using one or more conductive structures. Each conductive structure may serve to electrically connect two or more components, attach two or more components, or both. Conductive structure  326 , which may be a clip, may help secure flexible printed circuit  322  to conductive support plate  320  and/or electrically connect flexible printed circuit  322  to conductive support plate  320 . Fasteners  328  and  330  may attach radio-frequency shield  312 , conductive support plate  320 , and printed circuit  322  together. Fasteners  328  and  330  may be conductive so that they also electrically connect components. For example, fasteners  328  and/or  330  may electrically connect radio-frequency shield  312  to conductive housing layer  320 . Fastener  330  may be a screw and fastener  328  may be a screw-boss that receives screw  330 . Conductive structure  326  and fasteners  328  and  330  may collectively form ground terminal  264  for the return path of the wireless local area network antenna (shown in  FIG. 7 ). 
     Conductive support plate  320 , radio-frequency shield  312 , display frame  308 , display plate  306 , and portions of peripheral conductive housing structures  16 , may collectively form ground  104  for electronic device  10 . As shown in  FIG. 8 , adjustable component  202  may be coupled to ground at radio-frequency shield  312  (e.g., terminal  206  may be located on shield  312 ). Adjustable component  202  may include an inductor  316  coupled to a switch  318 . In a first state (e.g., a closed state), switch  318  may connect inductor  316  between terminal  204  on peripheral conductive hosing structure  16  and terminal  206  on radio-frequency shield  312 . In a second state (i.e., an open state), switch  318  may disconnect the inductor between terminal  204  and terminal  206 . In the first state when inductor  316  is connected between terminals  204  and  206 , a high strength electric field may be present around gap  18 - 1  ( FIG. 7 ). Inductor  316  may be connected between terminals  204  and  206  in the second and third tuning states (as discussed in connection with  FIG. 5 ). Inductor  316  and switch  318  may be formed on a printed circuit such as flexible printed circuit  314  if desired. 
     The arrangement of  FIG. 8  is merely illustrative. If desired, conductive structure  310  may be shorted directly to conductive housing layer  320 . Ground terminal  206  may be formed on conductive housing layer  320  instead of radio-frequency shield  312 . A return path may couple antenna resonating element  242  to any desired portion of ground  104  (e.g., the radio-frequency shield  312 , the conductive housing layer  320 , the display frame  308 , the display plate  306 , etc.). 
       FIG. 9  is a schematic diagram showing the relationship between various components in electronic device  10  and antenna ground  104 . As shown in  FIG. 9 , display plate  306 , display frame  308 , radio-frequency shield  312 , and conductive support plate  320  may collectively form portions of antenna ground  104 . It should be noted that this example is merely illustrative and, in general, ground  104  may include additional or alternate components and conductive structures if desired. 
     As shown in  FIG. 9 , flexible printed circuit  314  for adjustable inductor  202  may be coupled to radio-frequency shield  312 , whereas flexible printed circuit  322  for the wireless local area network antenna traces may be coupled to conductive support plate  320 . Each connection in  FIG. 9  may be formed directly (i.e., from direct contact between the components) or using any desired intervening conductive structures (e.g., a bracket, clip, spring, pin, screw, solder, weld, conductive adhesive, wire, metal strip, or a combination of these). For example, display plate  306  and display frame  308  may be directly connected. Display frame  308  and radio-frequency shield  312  may be electrically connected with a conductive component (e.g., spring  310  in  FIG. 8 ). Radio-frequency shield  312  may be electrically connected to conductive support plate  320  using fasteners such as screw and/or screw-boss (e.g., fasteners  328  and  330  in  FIG. 8 ). Radio-frequency shield  312  may be electrically connected to the flexible printed circuit  314 . Conductive support plate  320  may be directly connected to flexible printed circuit  322  or may be electrically connected to flexible printed circuit  322  using a conductive structure such a clip (e.g., clip  326  in  FIG. 8 ). The arrangement shown in  FIGS. 8 and 9  is merely illustrative, and other arrangements may be used for the components of electronic device  10  if desired. 
       FIG. 10  is a graph of the electromagnetic isolation (e.g., S21 scattering parameter measurements) between antenna  40 F and antenna  40 W as a function of frequency. As shown in  FIG. 10 , antenna  40 F may exhibit resonances in a cellular midband MB (e.g., 1710 to 2170 MHz) and a cellular high band HB (e.g., 2300 to 2700 MHz). Antenna  40 W may exhibit a resonance in a 2.4 GHz wireless local area network band that overlaps with some of the cellular high band HB. This is merely illustrative and, if desired, antennas  40 W and  40 F may exhibit resonances in additional bands not shown in the graph of  FIG. 10  (e.g., a cellular low band from 700 to 960 MHz, a 5 GHz WiFi® band, etc.). 
     Midband MB may extend from 1710 MHz to 2170 MHz or other suitable frequency range. High band HB may extend from 2300 MHz to 2700 MHz. Threshold  408  may illustrate the minimum isolation threshold (e.g., −10 dB) between antenna  40 F and antenna  40 W. As shown in  FIG. 10 , when antennas  40 W and  40 F are implemented using the arrangement shown in  FIG. 6  (e.g., with high current density region  246  in close proximity to high strength electric field region  248 ), antennas  40 W and  40 F may exhibit an isolation characterized by curve  402 . Curve  402  exceeds threshold  408  because the high current in region  246  is strongly coupled to the nearby high magnitude electric field in region  248 , thereby minimizing isolation between the two. When antennas  40 W and  40 F are implemented using the arrangement shown in  FIG. 7  and in the absence of capacitor  262 , antennas  40 W and  40 F may exhibit an isolation characterized by curve  404 . As shown by curve  404 , there may be sufficient isolation between antenna  40 F and antenna  40 W to meet threshold  408 , even in the absence of capacitor  262  (e.g., due to the increased distance between the positive antenna feed terminal  222  dielectric-filled gap  18 - 1 , segment  267  shielding branch  258  and/or antenna feed  220  from the high magnitude electric field, etc.). The presence of capacitor  262  may further improve isolation between the cellular antenna and the wireless local area network antenna. As shown in  FIG. 7 , curve  406  characterizes the isolation of antennas  40 F and  40 W when capacitor  262  is formed on return path  244 . Capacitor  262  may serve to further improve isolation (particularly within midband MB and the 2.4 GHz wireless local area network band) relative to scenarios where capacitor  262  is not present (curve  404 ). This example is merely illustrative and, if desired, the curves may have any shapes in any bands. Antenna structures  40  may exhibit resonances in a subset of these bands and/or in additional bands. 
     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.