Patent Publication Number: US-2013241800-A1

Title: Electronic Device with Tunable and Fixed Antennas

Description:
BACKGROUND 
     This relates generally to electronic devices, and more particularly, to antennas for electronic devices. 
     Electronic devices such as portable computers and cellular telephones are often provided with wireless communications capabilities. For example, electronic devices may use long-range wireless communications circuitry such as cellular telephone circuitry to communicate using cellular telephone bands. Electronic devices may use short-range wireless communications circuitry such as wireless local area network communications circuitry to handle communications with nearby equipment. Electronic devices may also be provided with satellite navigation system receivers and other wireless circuitry. 
     To satisfy consumer demand for small form factor wireless devices, manufacturers are continually striving to implement wireless communications circuitry such as antenna components using compact structures. At the same time, it may be desirable to include conductive structures in an electronic device such as metal device housing components. Because conductive components can affect radio-frequency performance, care must be taken when incorporating antennas into an electronic device that includes conductive structures. For example, care must be taken to ensure that the antennas and wireless circuitry in a device are able to exhibit satisfactory performance over a range of operating frequencies without causing undesired interference. 
     It would therefore be desirable to be able to provide wireless electronic devices with improved antenna structures. 
     SUMMARY 
     Electronic devices may be provided that contain wireless communications circuitry. The wireless communications circuitry may include radio-frequency transceiver circuitry and antennas. The antennas may include a non-tunable antenna and a tunable antenna. 
     The non-tunable antenna may serve as the primary antenna in the electronic device and the tunable antenna may serve as a secondary antenna in the electronic device. The non-tunable antenna may be configured to operate in at least one communications band. The tunable antenna may contain adjustable circuitry. The adjustable circuitry may be used to tune the tunable antenna to cover the same communications band used by the non-tunable antenna, even in configurations in which the tunable antenna has been implemented in a smaller volume within the electronic device than the non-tunable antenna. 
     The tunable antenna may have a resonating element and an antenna ground. The adjustable circuit may be coupled between the resonating element and the antenna ground. The adjustable circuit may include electrical components such as inductors and capacitors and a radio-frequency switch for antenna tuning. 
     The electronic device may have a metal housing from which a common antenna ground is formed for the tunable and non-tunable antennas. A dielectric antenna window may be provided in the metal housing that overlaps the tunable and non-tunable antennas. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative electronic device with wireless communications circuitry in accordance with an embodiment of the present invention. 
         FIG. 2  is a schematic diagram of an illustrative electronic device with wireless communications circuitry in accordance with an embodiment of the present invention. 
         FIG. 3  is a diagram of an illustrative array of antennas that may be used in wireless electronic devices of the type shown in  FIGS. 1 and 2  in accordance with an embodiment of the present invention. 
         FIG. 4  is a diagram of an illustrative fixed (non-tunable) antenna that may be used in an antenna array in wireless communications circuitry in accordance with an embodiment of the present invention. 
         FIG. 5  is a diagram of an illustrative tunable antenna that may be used in an antenna array in wireless communications circuitry in accordance with an embodiment of the present invention. 
         FIG. 6  is a diagram of an illustrative switch-based tunable capacitor that may be used a tunable antenna in accordance with an embodiment of the present invention. 
         FIG. 7  is a diagram of an illustrative switch-based bypassable inductor that may be used in a tunable antenna in accordance with an embodiment of the present invention. 
         FIG. 8  is a diagram of an illustrative switch-based bypassable capacitor that may be used in a tunable antenna in accordance with an embodiment of the present invention. 
         FIG. 9  is a diagram of an illustrative tunable capacitor that may be used in a tunable antenna in accordance with an embodiment of the present invention. 
         FIG. 10  is an antenna performance graph showing how a non-tunable antenna may have a resonance peak that covers a communications band of interest and how a tunable antenna may be tuned so that its narrower resonance peak can cover the same communications band of interest in accordance with an embodiment of the present invention. 
         FIG. 11  is a top view of a portion of an electronic device in which first and second antennas have been implemented using antenna resonating elements of different sizes in accordance with an embodiment of the present invention. 
         FIG. 12  is a diagram showing how a switchless antenna may be used for transmitting and receiving wireless signals while a tunable antenna that contains an adjustable component such as a switch-based adjustable component may be used only in receiving wireless signals in accordance with an embodiment of the present invention. 
         FIG. 13  is a diagram showing how a switchless antenna may be used for transmitting wireless signals at a first maximum power while an antenna that contains an adjustable component such as a switch-based adjustable component may be used in transmitting wireless signals at a second power that is lower than the first maximum power in accordance with an embodiment of the present invention. 
     
    
    
     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 multiple antennas. 
     The antennas 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 a peripheral conductive member that runs around the periphery of an electronic device. The peripheral conductive member may serve as a bezel for a planar structure such as a display, may serve as sidewall structures for a device housing, and/or may form other housing structures. Gaps in the peripheral conductive member may be associated with the antennas. 
     The antennas may, if desired, be formed from patterned metal foil or other metal structures or may be formed from conductive traces such as metal traces on a substrate. The substrate may be a plastic structure or other dielectric structure, a rigid printed circuit board substrate such as a fiberglass-filled epoxy substrate (e.g., FR4), a flexible printed circuit (“flex circuit”) formed from a sheet of polyimide or other flexible polymer, or other substrate material. The housing for electronic device  10  may be formed from conductive structures (e.g., metal) or may be formed from dielectric structures (e.g., glass, plastic, ceramic, etc.). Antenna windows formed from plastic or other dielectric material may, if desired, be formed in conductive housing structures. Antennas for device  10  may be mounted so that the antenna window structures overlap the antennas. During operation, radio-frequency antenna signals may pass through the dielectric antenna windows and other dielectric structures in device  10 . 
     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 cellular telephone, or a media player. 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 have a display such as display  14  that is mounted in a housing such as housing  12 . Display  14  may, for example, be a touch screen that incorporates capacitive touch electrodes. Display  14  may include image pixels formed from light-emitting diodes (LEDs), organic LEDs (OLEDs), plasma cells, electrowetting pixels, electrophoretic pixels, liquid crystal display (LCD) components, or other suitable image pixel structures. A cover glass layer may cover the surface of display  14 . The cover glass may have one or more openings such as an opening to accommodate button  16 . 
     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, housing or 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. In configurations for device  10  in which housing  12  is formed from conductive materials such as metal, one or more dielectric antenna windows such as antenna window  18  of  FIG. 1  may be formed in housing  12 . 
     Antenna window  18  may be formed from a dielectric such as plastic (as an example). Antennas in device  10  may be mounted within housing  12  so that antenna window  18  overlaps the antennas. During operation, radio-frequency antenna signals can pass through antenna window  18  and other dielectric structures in device  10  (e.g., edge portions of the cover glass for display  14 ). 
     Device  10  may have two or more antennas. The antennas may be used to implement an antenna array in which signals for multiple identical data streams (e.g., Code Division Multiple Access data streams) are combined to improve signal quality or may be used to implement a multiple-input-multiple-output (MIMO) antenna scheme that enhances performance by handling multiple independent data streams (e.g., independent Long Term Evolution data streams). Multiple antennas may be used together in both transmit and receive modes of operation or may only be used together during signal reception operations. 
     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 of an illustrative configuration that may be used for electronic device  10  is shown in  FIG. 2 . As shown in  FIG. 2 , electronic 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 . The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio codec chips, 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, etc. 
     Circuitry  28  may be configured to implement control algorithms that control the use of antennas in device  10 . For example, circuitry  28  may perform signal quality monitoring operations, sensor monitoring operations, and other data gathering operations and may, in response to the gathered data and information on which communications bands are to be used in device  10 , control which antenna structures within device  10  are being used to receive and process data and/or may adjust one or more switches, tunable elements, or other adjustable circuits in device  10  to adjust antenna performance. As an example, circuitry  28  may control which of two or more antennas is being used to receive incoming radio-frequency signals, may control which of two or more antennas is being used to transmit radio-frequency signals, may control the process of routing incoming data streams over two or more antennas in device  10  in parallel, may tune an antenna to cover a desired communications band, etc. In performing these control operations, circuitry  28  may open and close switches, may turn on and off receivers and transmitters, may adjust impedance matching circuits, may configure switches in front-end-module (FEM) radio-frequency circuits that are interposed between radio-frequency transceiver circuitry and antenna structures (e.g., filtering and switching circuits used for impedance matching and signal routing), may adjust switches, tunable circuits, and other adjustable circuit elements that are formed as part of an antenna or that are coupled to an antenna or a signal path associated with an antenna, and may otherwise control and adjust the components of device  10 . 
     Input-output circuitry  30  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 circuitry  30  may include input-output devices  32 . Input-output devices  32  may include touch screens, buttons, joysticks, click wheels, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, tone generators, vibrators, cameras, sensors, light-emitting diodes and other status indicators, data ports, etc. A user can control the operation of device  10  by supplying commands through input-output devices  32  and may receive status information and other output from device  10  using the output resources of input-output devices  32 . 
     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, 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 satellite navigation system receiver circuitry such as Global Positioning System (GPS) receiver circuitry  35  (e.g., for receiving satellite positioning signals at 1575 MHz) or satellite navigation system receiver circuitry associated with other satellite navigation systems. 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 cellular telephone bands such as bands in frequency ranges of about 700 MHz to about 2200 MHz or bands at higher or lower frequencies. 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 wireless circuitry for receiving radio and television signals, paging circuits, etc. 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 types of antenna. For example, antennas  40  may include antennas with resonating elements that are formed from loop antenna structure, patch antenna structures, inverted-F antenna structures, closed and open slot antenna structures, planar inverted-F antenna structures, helical antenna structures, strip antennas, monopoles, dipoles, 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. 
     There is generally a tradeoff between antenna volume and antenna bandwidth. An antenna that is implemented in a constrained volume will tend to exhibit a smaller bandwidth than a comparable antenna that is implemented in a larger volume. One way to overcome the tendency of small-volume antennas to exhibit narrow bandwidths involves providing the antennas with adjustable components. An adjustable component can be used to place the antenna in different configurations to support different desired frequencies of operation. Using antenna tuning, the frequency coverage of a compact narrow-bandwidth antenna can be expanded to match that of a less compact wider-bandwidth antenna. 
     It is not, however, always acceptable to use adjustable components in antennas. For example, radio-frequency switches and other adjustable circuits may exhibit non-linear behavior that can lead to the creation of undesired intermodulation distortion (IMD). If care is not taken, out-of-band emissions may be created due to the presence of the adjustable circuit (e.g., due to harmonics resulting from non-linear behavior). Electrostatic discharge events can damage adjustable components such as switches, so the presence of adjustable components may lead to reliability issues in a device. The presence of digital control lines for routing control signals to an adjustable component may potentially disrupt antenna performance (e.g., by providing a pathway for interference). The potential for interference from other circuits operating in an electronic device may also be increased by the presence of an adjustable component. 
     These potential performance issues with the use of adjustable components in an antenna may be exacerbated by higher antenna signal powers. At the typically low powers associated with received over-the-air antenna signals, nonlinearities may be minimal. At some or all transmit powers of interest, however, issues with intermodulation distortion, out-of-band emissions requirements, and other types of interference may make antenna performance unacceptable. 
     Due to considerations such as these, there are tradeoffs associated with using switches and other adjustable devices in an antenna. The inclusion of the switches or other adjustable devices may make it possible for an antenna to be tuned across a desired range of frequencies while minimizing antenna volume. The inclusion of the switches or other adjustable devices may, however, limit the maximum power handling capability of a tunable antenna. In contrast, antennas without adjustable components (e.g., non-tunable antennas that are switchless) may be capable of handling antenna signals with larger powers. Fixed (non-tunable) antennas may, however, consume more space within an electronic device than tunable antennas that cover comparable operating frequencies. 
     To maximize overall device performance, antennas  40  may be provided with one or more tunable antennas and one or more fixed antennas. For example, in a two-antenna configuration, antennas  40  may include a fixed antenna and a tunable antenna. The fixed antenna and the tunable antenna may both be used to handle wireless signals in device  10 . For example, the fixed antenna and the tunable antenna may both be used for receiving data streams in a multiple antenna array (e.g., in a MIMO scheme or in a scheme in which identical antenna signals from each of the antennas are combined to improve signal quality). When it is desired to transmit antenna signals, the signals may be transmitted using the fixed antenna. Because higher-power (transmitted) signals are routed through the fixed antenna, the tunable antenna will not be subjected to higher-power signals and will not exhibit undesired nonlinearities. Because the tunable antenna is included in the electronic device, device size may be minimized (i.e., the size of the tunable antenna may be made smaller than a comparable fixed antenna covering the same frequency bands). 
       FIG. 3  is a diagram showing how antennas  40  may include multiple types of antenna. In the illustrative configuration of  FIG. 3 , antennas  40  include at least a first antenna of a first type such as antenna  40 N and at least one second antenna of a second type such as antenna  40 Y. Antenna  40 N and  40 Y may, for example, include different types and amounts of tunable circuit capabilities. With one suitable arrangement, which is sometimes described herein as an example, antenna  40 N may be a fixed (non-tunable) antenna that is devoid of any antenna switching components, whereas antenna  40 Y may be a tunable antenna that includes one or more adjustable components. In general, there may be any suitable number of antennas  40 N (each of which may be identical or some or all of which may be different from each other) and any suitable number of antennas  40 Y (each of which may be identical or some or all of which may be different from each other) among antennas  40  of device  10 . Illustrative configurations in which antennas  40  include first antenna  40 N and second antenna  40 Y are sometimes described herein as an example. 
     Because antenna  40 N does not contain any switch-based components or other potentially non-linear adjustable components (in this example), it may be desirable to use antenna  40 N whenever device  10  is transmitting radio-frequency signals. Antenna  40 Y contains one or more adjustable components (in this example) and may therefore most suitably be used for transmitting lower-power radio-frequency signals or be used exclusively for receiving radio-frequency signals. In this type of configuration, antenna  40 N may be used to transmit and receive signals and may therefore sometimes be referred to as a primary antenna for device  10 , whereas antenna  40 Y may be used only in receiving signals (or in receiving signals and transmitting only lower power signals) and may therefore sometimes be referred to as a secondary antenna for device  10 . 
     Antennas  40  may be coupled to radio-frequency transceiver circuitry  46  using signal paths  44  (e.g., transmission line paths) and front-end circuitry  42 . Front-end circuitry  42  may include switches, transmission lines, filters, impedance matching circuits, amplifiers, and other circuitry. Radio-frequency transceiver circuitry  46  may operate in wireless local area network bands, satellite navigation bands, cellular telephone bands, and/or other communications bands of interest. One or more integrated circuits may be used in implementing radio-frequency transceiver circuitry  46 . 
     Radio-frequency transceiver circuitry  46  may be supplied with data to be transmitted from a circuit such as a baseband processor using a path such as path  48 . Antenna signals that have been received by radio-frequency transceiver circuitry  46  may be supplied to circuitry such as baseband processor circuitry using a path such as path  48 . 
     Radio-frequency transceiver circuitry  46  may have multiple ports. For example, in a configuration in which antennas  40  include first antenna  40 N and second antenna  40 Y, radio-frequency transceiver circuitry  46  may include a first port (port A) and a second port (port B). Port A may include a receiver (RX) and a transmitter (TX). Port B may include a receiver (RX) and may or may not include a transmitter (TX). Front-end circuitry  42  may contain fixed pathways that couple antennas  40 N and  40 Y to ports A and B, respectively. If desired, front-end circuitry  42  may contain switching circuitry (e.g., a cross-bar switch) that allows antenna  40 N to be coupled to either port A or port B while simultaneously coupling antenna  40 Y to either port B or port A. 
     An illustrative configuration that may be used for antenna  40 N is shown in  FIG. 4 . As shown in  FIG. 4 , antenna  40 N may include conductive structures that form antenna resonating element  50  and antenna ground  52 . Antenna resonating element  50  may, for example, be formed from patterned metal traces on a rigid or flexible printed circuit substrate or patterned metal traces on a molded plastic substrate (as examples). Antenna ground  52  may be formed from metal traces on a printed circuit, metal traces on a molded plastic substrate, and/or other conductive structures such as metal portions of housing  12 . Antenna resonating element  50  in the example of  FIG. 4  is an inverted-F antenna resonating element. This is merely illustrative. Antennas  40 N may be based on any suitable type of antenna (e.g., a loop antenna, a strip antenna, a planar inverted-F antenna, a slot antenna, a hybrid antenna that includes antenna structures of more than one type, or other suitable antennas). 
     Antenna resonating element  50  may include a main resonating element arm such as arm  60 . Short circuit branch  62  may be coupled between antenna resonating element arm  60  and antenna ground  52 . Antenna  40 N may have an antenna feed formed from feed terminals  54  and  56  in antenna feed branch  58 . Antenna feed branch  58  may be coupled between arm  60  and ground  52 . Signal path  44  may include positive path  64  and ground path  66 . Positive path  64  may be coupled to positive antenna feed terminal  54 . Ground signal path  66  may be coupled to ground antenna feed terminal  56 . If desired, antenna  40 N may include matching circuits, additional conductive structures, etc. Antenna  40 N of  FIG. 4  is switchless and does not contain potentially non-linear components such as radio-frequency switches (e.g., switches implemented from transistor circuitry on an integrated circuit). 
     Antenna  40 N may have any suitable size and shape. In the illustrative example of  FIG. 4 , antenna  40 N has a length L 1  (e.g., a first lateral dimension associated with the length of main resonating element arm  60 ) and a height H 1  (e.g., an orthogonal second lateral dimension associated with the length of short circuit branch  62 ). The overall area of antenna  40 N in the illustrative configuration of  FIG. 4  (e.g., the area associated with antenna resonating element  50 ) is approximately equal to L 1 *H 1 . The volume occupied by antenna  40 N may be L 1 *H 1 *T1, where T1 is the thickness of the antenna resonating element. 
     An illustrative configuration that may be used for antenna  40 Y is shown in  FIG. 5 . As shown in  FIG. 5 , antenna  40 Y may include conductive structures that form antenna resonating element  80  and antenna ground  52 . As with antenna resonating element  50  of antenna  40 N, antenna resonating element  80  may be formed from patterned metal traces on a rigid or flexible printed circuit substrate or patterned metal traces on a molded plastic substrate (as examples). Antenna ground  52  of antenna  40 Y may be formed as part of the same conductive structures that form antenna ground  52  of antenna  40 N or may be formed from other conductive structures. As an example, antenna ground  52  may be formed from metal traces on a printed circuit, metal traces on a molded plastic substrate, and/or other conductive structures such as metal portions of housing  12 . Housing  12  may, for example, form a common antenna ground for both antennas  40 N and  40 Y. 
     Antenna resonating element  80  in the example of  FIG. 5  is an inverted-F antenna resonating element. This is merely illustrative. Antennas  40 Y may be based on any suitable type of antenna (e.g., a loop antenna, a strip antenna, a planar inverted-F antenna, a slot antenna, a hybrid antenna that includes antenna structures of more than one type, or other suitable antennas). 
     Antenna resonating element  80  may include a main resonating element arm such as arm  82 . Short circuit branch  78  may be coupled between antenna resonating element arm  80  and antenna ground  52 . Antenna  40 Y may have an antenna feed formed from feed terminals  72  and  74  in antenna feed branch  76 . Antenna feed branch  76  may be coupled between arm  82  and ground  52 . Signal path  44  may include positive path  70  and ground path  68 . Positive path  70  may be coupled to positive antenna feed terminal  72 . Ground signal path  68  may be coupled to ground antenna feed terminal  74 . If desired, antenna  40 Y may include matching circuits, additional conductive structures, etc. 
     Antenna  40 Y may include adjustable circuitry. The adjustable circuitry may be adjusted in real time in response to control signals from control circuitry such as a baseband processor or other circuitry (see, e.g., storage and processing circuitry  28  of  FIG. 2 ). The adjustable circuitry may be placed in different states to support different modes of operation. In each mode of operation, the antenna may be tuned to exhibit a different frequency response. By adjusting the antenna to cover different signal frequencies of interest, antenna  40 Y can cover a desired range of operating frequencies. Antenna  40 Y may, as an example, uses its different frequency response settings to cover substantially the same frequency range as antenna  40 N (as an example), even in configurations in which antenna  40 Y has been implemented using a more compact (and narrower bandwidth) resonating element. 
     The adjustable circuitry that is used in tuning antenna  40 Y may be coupled between respective portions of antenna resonating element  80 , between respective portions of ground  52 , or between resonating element  80  and ground  52 . As shown in  FIG. 5 , for example, antenna  40 Y may have an adjustable antenna tuning circuit such as adjustable circuit  86  that is coupled between tip portion  84  of antenna resonating element arm  82  in antenna resonating element  80  and antenna ground  52  (i.e., an adjustable circuit having a first terminal coupled to antenna resonating element arm  82  and a second terminal coupled to antenna ground  52 ). Adjustable circuits such as adjustable circuit  86  may also be incorporated into other portion of antenna  40 Y, if desired. The example of  FIG. 5  is merely illustrative. 
     In the  FIG. 5  example, adjustable circuit  86  is a switch-based adjustable circuit that includes radio-frequency switch  88 . Radio-frequency switch  88  may be adjusted using control signals (e.g., control signals from control circuitry in device  10  that are received via control signal path  102 ). Other types of control mechanisms may be used, if desired. 
     Switch  88  may be coupled between arm  84  and ground  52  in series with multiple electrical components such as parallel inductors  96 ,  98 , and  100 . Switch  88  may have a terminal such a terminal  104  that is coupled to antenna ground  52 . Switch  88  may also have terminals  90 ,  92 , and  94  that are coupled respectively to inductors  96 ,  98 , and  100 . Each of inductors  96 ,  98 , and  100  may have a different respective inductance value. When it is desired to couple the inductance of inductor  96  between resonating element arm  82  and antenna ground  52 , control signals may be provided to switch  88  (e.g., via control path  102 ) to couple terminal  104  to terminal  90 . When it is desired to couple the inductance of inductor  98  between resonating element arm  82  and antenna ground  52 , control signals may be provided to switch  88  to couple terminal  104  to terminal  92 . Terminal  104  may be coupled to terminal  94  by switch  88  when it is desired to couple the inductance of inductor  100  between resonating element arm  82  and antenna ground  52 . 
     Antenna  40 Y may have any suitable size and shape. In the illustrative example of  FIG. 5 , antenna  40 Y has a length L 2  (e.g., a first lateral dimension associated with the length of main resonating element arm  82 ) and a height H 2  (e.g., an orthogonal second lateral dimension associated with the length of short circuit branch  78 ). The overall area of antenna  40 Y in the illustrative configuration of  FIG. 5  (e.g., the area associated with antenna resonating element  80 ) is approximately equal to L 2 *H 2 . The volume occupied by antenna  40 Y may be L 1 *H 1 *T2, where T2 is the thickness of the antenna resonating element. The magnitude of T2 may be comparable to the magnitude of thickness T1 of antenna  40 N. 
     Because of the antenna tuning capabilities provided by adjustable circuit  86 , antenna  40 Y may, if desired, be implemented in a smaller volume than antenna  40 N while exhibiting a comparable bandwidth (i.e., L 2 *H 2  may be less than L 1 *H 1 , L 2  may be less than L 1 , and/or L 2 *H 2 *T2 may be less than L 1 *H 1 *T1). Antennas  40 Y and  40 N may also be implemented in the same volume (or  40 Y may be larger than  40 N), in which case antenna  40 Y may exhibit a larger bandwidth than antenna  40 N. 
     Antenna  40 Y of  FIG. 5  contains adjustable circuit  86 . Adjustable circuit  86  of  FIG. 5  is an adjustable inductor based on switch  88  and three associated inductors. The graph of  FIG. 10  shows how this type of antenna may be tuned. In  FIG. 10 , antenna performance (standing wave ratio) has been plotted as a function of frequency. Curve  106  corresponds to the performance of antenna  40 N (in this example). Curves  108 ,  110 , and  112  correspond to the performance of antenna  40 Y as switch  88  is adjusted between each of its three positions to produce three respective inductance values for adjustable circuit  86 . Antenna  40 N may exhibit a relatively large bandwidth and may cover the communications band centered at frequency f1, as indicated by curve  106 . Curves  108 ,  110 , and  112  may cover narrower frequency ranges (centered, respectively, at fa, f1, and fb). Using tuning, antenna  40 Y may be placed into any of three configurations. The overall amount of frequency coverage of antenna  40 Y may be comparable to that of antenna  40 N due to the ability of antenna  40 Y to operate in different tuning states. As this example demonstrates, the resonating structures of antenna  40 Y may exhibit a narrower bandwidth than antenna  40 N in the absence of tuning, but, using tuning, may be adjusted to cover the same bandwidth as antenna  40 N. 
       FIG. 11  shows how the size of antenna  40 Y may be reduced (taking advantage of its tuning capabilities) so that antenna  40 Y is smaller than antenna  40 N. As shown in  FIG. 11 , antenna  40 N may be formed from antenna resonating element  50  and antenna ground  52 , whereas antenna  40 Y may be formed from tunable antenna resonating element  80  and antenna ground  52 . Antenna ground  52  may at least partly be formed from a metal device housing for electronic device  10  (as an example) and may be common to both antenna  40 N and antenna  40 Y. Antenna  40 N may be used for transmitting and receiving signals (serving as a primary antenna for device  10 ). Antenna  40 Y may be used exclusively for receiving signals or may be used for transmitting and receiving signals (serving as a secondary antenna for device  10 ). 
       FIG. 12  shows how antenna  40 N (i.e., a switchless, non-adjustable antenna) may be coupled to a port of transceiver circuitry  46  that is associated with a transmitter (TX) and a receiver (RX), whereas antenna  40 Y (i.e., an antenna with switch-based tuning) may be coupled to a port of transceiver circuitry  46  that is associated with a receiver (RX). In this type of configuration, no transmitter need be associated antenna  40 Y. Antenna  40 N may be used in transmitting and receiving radio-frequency signals for device  10 , whereas in this type of configuration, antenna  40 Y may be used exclusively for receiving antenna signals (and not transmitting antenna signals). 
     As shown in the illustrative arrangement of  FIG. 13 , antenna  40 N (i.e., a switchless non-tunable antenna) and antenna  40 Y (i.e., a tunable antenna that includes switch-based adjustable circuitry  86 ) may be associated with transceiver circuitry that includes a transmitter (TX) and a receiver (RX) for antenna  40 N and a transmitter (TX) and receiver (RX) for antenna  40 Y. In this type of configuration both antenna  40 N and  40 Y may be used in both transmitting and receiving radio-frequency signals. To avoid issues associated with the non-linear behavior of adjustable circuitry  86  in antenna  40 Y, the maximum power P TX-MAX  that is allowed during signal transmissions using antenna  40 Y (i.e., power P 2 ) may be maintained at a lower level than the maximum power P TX-MAX  that is allowed during signal transmissions using antenna  40 N (i.e., power P1). For example, maximum transmit power P2 may be 70% (or less) of maximum transmit power P1, maximum transmit power P2 may be 30% (or less) of maximum transmit power P1, maximum transmit power P2 may be 15% (or less) of power P1, or maximum transmit power P2 may be 5% (or less) than of power P1 (as examples). 
     In situations in which use of adjustable circuitry  86  to handle transmitted signal powers is acceptable in some communications bands but not others, control circuitry  28  of device  10  can be used to transmit any desired transmit powers using antenna  40 N, while restricting the use of antenna  40 Y so that antenna  40 Y is only used to transmit signals in a selected acceptable subset of communications bands. If desired, antenna  40 Y may be used to transmit signals at different acceptable maximum power levels for different communications bands (e.g., power levels that are lower than those used for antenna  40 N in the same bands). 
     If desired, front-end circuitry (e.g., filters, impedance matching networks, switches, and other circuitry) may be coupled between antennas  40  and transceiver circuitry  46  in  FIGS. 11 ,  12 , and  13 . 
     The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention.