PATENT DOCUMENT

Publication Number: US-9041619-B2
Application Number: US-201213452585-A
Country: US
Kind Code: B2

Title: Antenna with variable distributed capacitance

Abstract:
Electronic devices may be provided with antennas. An antenna may be formed from conductive antenna structures that include a frequency-dependent distributed capacitor. The antenna may include an antenna ground and an antenna resonating element that are separated by a gap. A low pass filter circuit may bridge the gap. The antenna resonating element may have antenna resonating element conductive structures that serve as first and second electrodes for the distributed capacitor. The second electrode may have first and second conductive elements coupled by a filter. The filter may be a low pass filter implemented using an inductor. The inductor may have a first terminal coupled to the first conductive element and a second terminal coupled to the second conductive element. A first antenna feed terminal may be coupled to the first conductive element and a second antenna feed terminal may be coupled to the antenna ground.

Claims:
What is claimed is: 
     
       1. An antenna for an electronic device, comprising:
 an antenna ground; and 
 an antenna resonating element having a distributed capacitor that exhibits a frequency-dependent capacitance, wherein the distributed capacitor has a capacitor electrode formed from first and second conductive elements and a low pass filter coupled between the first and second conductive elements, the antenna resonating element comprising:
 a conductive antenna resonating element structure that serves as a first capacitor electrode for the distributed capacitor; and 
 a second capacitor electrode for the distributed capacitor that is formed from the first and second conductive elements. 
 
 
     
     
       2. The antenna defined in  claim 1  wherein the low pass filter comprises an inductor. 
     
     
       3. The antenna defined in  claim 2  further comprising an antenna feed formed from first and second antenna feed terminals, wherein the first antenna feed terminal is coupled to one of the first and second conductive elements and wherein the second antenna feed terminal is coupled to the antenna ground. 
     
     
       4. The antenna defined in  claim 1  further comprising an antenna feed having a first antenna feed terminal coupled to the first conductive element and a second antenna feed terminal coupled to the antenna ground. 
     
     
       5. The antenna defined in  claim 4  wherein the first and second conductive elements are separated from the conductive antenna resonating element by a first gap and wherein the first and second conductive elements are separated from the antenna ground by a second gap. 
     
     
       6. The antenna defined in  claim 5  wherein the low pass filter comprises an inductor having a first terminal coupled to the first conductive element and a second terminal coupled to the second conductive element. 
     
     
       7. The antenna defined in  claim 6  further comprising low pass filter circuitry coupled between the conductive antenna resonating element structure and the antenna ground. 
     
     
       8. An antenna for an electronic device, comprising:
 a first conductive structure that serves as a first capacitor electrode; 
 second and third conductive structures that are separated from the first conductive structure by a gap; 
 a radio-frequency filter coupled between the second and third conductive structures, wherein the second and third conductive structures and the radio-frequency filter are configured to serve as a second capacitor electrode and the first and second capacitor electrodes form a frequency-dependent distributed capacitor; and 
 an antenna feed having first and second antenna feed terminals, wherein the first antenna feed terminal is coupled to the second conductive structure. 
 
     
     
       9. The antenna defined in  claim 8  further comprising an antenna ground, wherein the second antenna feed terminal is coupled to the antenna ground. 
     
     
       10. The antenna defined in  claim 9  wherein the radio-frequency filter comprises a low pass filter. 
     
     
       11. The antenna defined in  claim 9  wherein the radio-frequency filter comprises an inductor having a first terminal coupled to the second conductive structure and having a second terminal coupled to the third conductive structure. 
     
     
       12. An electronic device antenna, comprising:
 an antenna feed having first and second feed terminals; 
 an antenna ground structure, wherein the first antenna feed terminal is coupled to the antenna ground structure; and 
 an antenna resonating element having a first portion that forms a first capacitor electrode and having a second portion that forms a second capacitor electrode, wherein the second portion of the antenna resonating element includes first and second conductive elements and the first and second conductive elements are interposed between the first capacitor electrode and the antenna ground structure. 
 
     
     
       13. The electronic device antenna defined in  claim 12  further comprising a filter circuit coupled between the first and second conductive elements. 
     
     
       14. The electronic device antenna defined in  claim 13  wherein the filter circuit comprises a low pass filter. 
     
     
       15. The electronic device antenna defined in  claim 14  wherein the second antenna feed terminal is coupled to the first conductive element. 
     
     
       16. The electronic device defined in  claim 15  wherein the second portion of the antenna resonating element is separated from the first portion of the antenna resonating element by a first gap and wherein the second portion of the antenna resonating element is separated from the antenna ground structure by a second gap. 
     
     
       17. The electronic device antenna defined in  claim 13  wherein the filter circuit comprises an inductor coupled between the first and second capacitor electrodes. 
     
     
       18. The antenna defined in  claim 8 , wherein the first, second, and third conductive structures each form part of an antenna resonating element for the antenna. 
     
     
       19. The electronic device antenna defined in  claim 12 , further comprising:
 a band stop filter coupled between the first portion of the antenna resonating element that forms the first capacitor electrode and the antenna ground structure.

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 and electronic 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. 
     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. 
     An electronic device antenna may be formed from conductive antenna structures that include a variable distributed capacitor. The variable distributed capacitor may include a passive filter. The filter may be used to couple conductive structures to each other. Using the filter, the variable distributed capacitor may exhibit a frequency-dependent capacitance. The frequency-dependent capacitance may help match the impedance of the antenna to a desired impedance over a range of operating frequencies. 
     The antenna may include an antenna ground and an antenna resonating element that are separated by a gap. The antenna resonating element may have antenna resonating element conductive structures that serve as a first electrode of the variable distributed capacitor and may have a first and second conductive elements coupled by a filter that form a second electrode of the capacitor. 
     The filter may be a low pass filter implemented using an inductor. Low pass filters may also be implemented using multiple components such as capacitors and inductors. The inductor or other low pass filter circuit may have a first terminal coupled to the first conductive element and a second terminal coupled to the second conductive element. A first antenna feed terminal may be coupled to the first conductive element and a second antenna feed terminal may be coupled to the antenna ground. 
     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 cross-sectional side view of a portion of an electronic device showing how the device may be provided with an antenna in accordance with an embodiment of the present invention. 
         FIG. 4  is a diagram of an illustrative antenna coupled to a radio-frequency transceiver in accordance with an embodiment of the present invention. 
         FIG. 5  is a diagram of an illustrative antenna having an antenna resonating element and antenna ground in accordance with an embodiment of the present invention. 
         FIGS. 6A and 6B  are Smith charts in which antenna performance for an antenna of the type shown in  FIG. 5  and other antennas have been plotted in accordance with an embodiment of the present invention. 
         FIG. 7  is a diagram of an illustrative antenna having an antenna resonating element and antenna ground that are coupled by a low pass filter formed from an inductor in accordance with an embodiment of the present invention. 
         FIG. 8  is a diagram of an illustrative antenna that has an antenna resonating element and antenna ground that are coupled by a low pass filter such as a shunt inductor and that has a feed with a series capacitor in accordance with an embodiment of the present invention. 
         FIG. 9  is a diagram of an illustrative antenna that has an antenna resonating element and antenna ground that are coupled by a shunt inductor and that has a distributed variable capacitor in accordance with an embodiment of the present invention. 
         FIG. 10A  is a graph showing how a variable capacitor for an antenna may be configured to exhibit a decreasing capacitance value with increasing frequency to improve antenna performance over a range of operating frequencies in accordance with an embodiment of the present invention. 
         FIG. 10B  is a graph showing how a capacitor that has a decreasing capacitance value with increasing frequency of the type shown in  FIG. 10A  may be characterized by a reactance having a magnitude that is relatively constant as a function of frequency in accordance with an embodiment of the present invention. 
         FIG. 11  is a diagram of an illustrative antenna having an antenna resonating element and antenna ground that are coupled by a low pass filter and having a variable distributed capacitor such as a variable distributed capacitor with multiple segments coupled by filter circuitry in accordance with an embodiment of the present invention. 
         FIG. 12  is a diagram of an illustrative low pass filter formed from stacked band stop filters in accordance with an embodiment of the present invention. 
         FIG. 13A  is a graph showing how the stages in a stacked band stop filter of the type shown in  FIG. 12  may be characterized by overlapping stop bands in accordance with an embodiment of the present invention. 
         FIG. 13B  is a graph showing how the stacked band pass filter circuit of  FIG. 12  may be used in implementing a low pass filter over a range of low band and high band operating frequencies 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 one or more antennas. 
     The antennas can be formed from conductive structures on printed circuit boards or other dielectric substrates. If desired, conductive structures for the antennas may be formed from conductive electronic device structures such as portions of conductive housing structures. Examples of conductive housing structures that may be used in forming an antenna include conductive internal support structures such as sheet metal structures and other planar conductive members, conductive housing walls, a peripheral conductive housing member such as a display bezel, peripheral conductive housing structures such as conductive housing sidewalls, a conductive planar rear housing wall and other conductive housing walls, or other conductive structures. Conductive structures for antennas may also be formed from parts of electronic components, such as switches, integrated circuits, display module structures, etc. Shielding tape, shielding cans, conductive foam, and other conductive materials within an electronic device may also be used in forming antenna structures. 
     Antenna structures may be formed from patterned metal foil or other metal structures. If desired, antenna structures may be formed from conductive traces such as metal traces on a substrate. The substrate may be a plastic support 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. If desired, antenna structures may be formed using combinations of these approaches. For example, an antenna may be formed partly from metal traces (e.g., ground conductor) on a plastic support structure and partly from metal traces on a printed circuit (e.g., patterned traces for forming antenna resonating element structures). 
     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. An antenna for device  10  may be mounted adjacent to a dielectric housing wall or may be mounted under an antenna window structure so that the antenna window structure overlaps the antenna. During operation, radio-frequency antenna signals may pass through dielectric antenna windows and other dielectric structures in device  10 . If desired, device  10  may have a display with a cover layer. Antennas for device  10  may be mounted so that antenna signals pass through the display cover layer. 
     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 or may be insensitive to touch. A touch sensor for display  14  may be formed from capacitive touch sensor electrodes, a resistive touch array, touch sensor structures based on acoustic touch, optical touch, or force-based touch technologies, or other suitable touch sensors. 
     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 layer may cover the surface of display  14 . The cover layer may be formed from a transparent glass layer, a clear plastic layer, or other transparent member. As shown in  FIG. 1 , openings may be formed in the cover layer to accommodate components such as button  16 . 
     Display  14  may have an active portion and, if desired, may have an inactive portion. The active portion of display  14  may contain active image pixels for displaying images to a user of device  10 . The inactive portion of display  14  may be free of active pixels. The active portion of display  14  may lie within a region such as central rectangular region  22  (bounded by rectangular outline  18 ). Inactive portion  20  of display  14  may surround the edges of active region  22  in a rectangular ring shape. 
     In inactive region  20 , the underside of the display cover layer for display  14  may be coated with an opaque masking layer. The opaque masking layer may be formed from an opaque material such as an opaque polymer (e.g., black ink, white ink, a coating of a different color, etc.). The opaque masking layer may be used to block interior device components from view by a user of device  10 . The opaque masking layer may, if desired, be sufficiently thin and/or formed from a sufficiently non-conductive material to be radio transparent. This type of configuration may be used in configurations in which antenna structures are formed under inactive region  20 . As shown in  FIG. 1 , for example, antenna structures such as one or more antennas  40  may be mounted in housing  12  so that inactive region  20  overlaps the antenna structures. 
     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  12  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, antennas  40  may be mounted under the display cover layer for display  14  as shown in  FIG. 1  (e.g., under inactive region  20 ) and/or antennas  40  may be mounted adjacent to one or more dielectric antenna windows in housing  12 . During operation, radio-frequency antenna signals can pass through the portion of inactive region  20  of the display cover layer that overlaps antennas  40  and/or radio-frequency antenna signals can pass through other dielectric structures in device  10  such as antenna window structures. In general, antennas  40  may be located in any suitable location in device housing  12  (e.g., along the edges of display  14 , in corners of device  10 , under an antenna window or other dielectric structure on a rear surface of housing  12 , etc.). 
     Device  10  may have a single antenna or multiple antennas. In configurations in which multiple antennas are present, 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 also be used to implement an antenna diversity scheme in which device  10  activates and inactivates each antenna based on its real time performance (e.g., based on received signal quality measurements). In a device with wireless local area network wireless circuitry, the device may use an array of antennas  40  to transmit and receive wireless local area network signals (e.g., IEEE 802.11n traffic). Multiple antennas may be used together in both transmit and receive modes of operation or may only be used together during only signal reception operations or only signal transmission 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 wireless local area network communications such as IEEE 802.11 communications or Bluetooth® communications, voice and data cellular telephone communications, global positioning system (GPS) communications or other satellite navigation system 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 such as IEEE 802.11 protocols—sometimes referred to as WiFi® and protocols for other short-range wireless communications links such as the Bluetooth® protocol, cellular telephone protocols, etc. 
     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, near field communications circuitry, 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 one or more antennas  40 . Antennas  40  may, if desired, have distributed capacitor structures. The distributed capacitor structures may have portions that are coupled to each other using one or more passive radio-frequency filters such as low pass filters. Using low pass filter circuitry, the distributed capacitor structures may exhibit a capacitance value that decreases as a function of increasing frequency (i.e., the distributed capacitor structures may be configured to form a frequency-dependent variable distributed capacitor). An antenna such as one of antennas  40  may be provided with a variable distributed capacitor (e.g., to form a series capacitor for an antenna feed for antenna  40 ). The use of the variable distributed capacitor may help ensure that a transmission line is impedance matched to the antenna over a range of operating frequencies. 
       FIG. 3  is a cross-sectional side view of a portion of device  10 . In the illustrative configuration of  FIG. 3 , antenna  40  has been formed along one of the edges of device housing  12  under inactive portion  20  of display  14 . Display structures  52  (e.g., an array of image pixels for displaying images for the user of device  10 ) may be mounted under display cover layer  42  of display  14  in the center of device housing  12  (i.e., under active region  22  of display  14 ). In inactive display region  20 , the interior surface of display cover layer  42  may be covered with opaque masking material  44  to block internal structures such as antenna  40  from view by a user of device  10 . Housing  12  may have a planar rear housing wall. Housing  12  may have vertical sidewalls that run perpendicular to the planar rear housing wall or may, as shown in  FIG. 3 , have curved sidewalls that extend vertically upwards from the planar rear housing wall. 
     Device  10  may include one or more substrates such substrate  48  on which electrical components  50  are mounted. Electrical components  50  may include integrated circuits, discrete components such as resistors, inductors, and capacitors, switches, connectors, light-emitting diodes, and other electrical devices for forming circuitry such as storage and processing circuitry  28  and input-output circuitry  30  of  FIG. 2 . 
     Substrate  48  may be formed from a dielectric such as plastic. If desired, substrate  48  may be implemented using one or more printed circuits. For example, substrate  48  may be a flexible printed circuit (“flex circuit”) formed from a flexible sheet of polyimide or other polymer layer or may be a rigid printed circuit board (e.g., a printed circuit board formed from fiberglass-filled epoxy). Substrate  48  may include conductive interconnect paths such as one or more layers of patterned metal traces for routing signals between components  50 , antennas such as antenna  40 , and other circuitry in device  10 . 
     Antenna  40  may include patterned conductive structures such as patterned metal traces on a printed circuit or plastic carrier. The conductive structures for antenna  40  may be located on upper surface  54 T, on sidewall surfaces such as sidewall surface  54 S, or elsewhere in antenna  40 . If desired, portions of device  10  such as portions of conductive housing  12 , shielding structures such as structures  46  (e.g., conductive tape, conductive foam, etc.), portions of internal conductive components such as display structures  52 , components  50 , and printed circuit  48  may form conductive antenna structures for antenna  40  (e.g., antenna ground structures). 
     During operation, antenna  40  may transmit and receive radio-frequency signals. These signals may pass through opaque masking layer  44  and display cover layer  42  in inactive region  20  and/or may pass through dielectric portions of housing  12  such as a dielectric antenna window formed in region  12 ′ of housing  12 . 
       FIG. 4  is a diagram showing how antenna  40  may be coupled to radio-frequency transceiver circuitry  56  using transmission line structures such as transmission line path  58 . Radio-frequency transceiver circuitry  56  may include transceiver circuits such as satellite navigation system receiver circuitry  35 , wireless local area network transceiver circuitry  36 , and cellular telephone transceiver circuitry  38 . Antenna  40  may have an antenna feed such as antenna feed  64  to which transmission line  58  is coupled. Antenna feed  64  may have a positive antenna feed terminal such as positive antenna feed terminal  60  that is coupled to positive transmission line conductor  58 P in transmission line  58 . Antenna feed  64  may also have a ground antenna feed terminal such as ground antenna feed terminal  62  that is coupled to ground transmission line conductor  58 G in transmission line  58 . 
     Transmission line  58  may be formed from a coaxial cable, a microstrip transmission line structure, a stripline transmission line structure, a transmission line structure formed on a rigid printed circuit board or flexible printed circuit board, a transmission line structure formed from conductive lines on a flexible strip of dielectric material, or other transmission line structures. If desired, one or more electrical components such as components  60  may be interposed within transmission line  58  (i.e., transmission line  58  may have two or more segments). Components  60  may include radio-frequency filter circuitry, impedance matching circuits (e.g., circuits to help match the impedance of antenna  40  to that of transmission line  58 ), switches, and other circuitry. 
     In electronic devices such as devices with compact layouts, it can be challenging to satisfy antenna design requirements. The relatively small amount of space that is sometimes available for forming antenna structures may make it desirable to place ground plane structures in close proximity to antenna resonating element structures. The presence of ground structures within close proximity to antenna resonating element structures may, however, tend to reduce antenna bandwidth and make it difficult to achieve desired antenna bandwidth goals. 
     An antenna design that can be used in device  10  to overcome these challenges may have an antenna feed with a variable distributed capacitor. The presence of the variable distributed capacitor may help impedance match transmission line  58  to antenna  40  over a relatively wide range of frequencies, thereby enhancing antenna performance. 
       FIG. 5  is a diagram of an illustrative antenna. Antenna  40  of  FIG. 5  may have antenna resonating element  68  and antenna ground  66 . The antenna of  FIG. 5  may have an antenna feed such as antenna feed  64  that is formed from positive antenna feed terminal  60  and ground antenna feed terminal  62 . In the example of  FIG. 5 , antenna resonating element  68  has been implemented using resonating element structure  70  (e.g., a rectangular metal trace or a conductive structure having other suitable shapes). Positive antenna feed terminal  60  may be coupled to antenna resonating element structure  70 . Ground antenna feed terminal  62  may be formed on an opposing portion of antenna ground structure  66 . Antenna resonating element structure  70  and antenna ground structure  66  may be separated by a gap such as gap  71 . 
       FIGS. 6A and 6B  are Smith charts in which antenna impedance has been plotted for the illustrative antenna of  FIG. 5  and for antennas with configurations of the types shown in  FIGS. 7 ,  8 ,  9 , and  11 . The Smith chart of  FIG. 6A  contains impedance plots for operation in a first illustrative communications band of interest (e.g., a low band B 1  that extends from a first frequency of f 1  to a second frequency of f 2  and that is centered on a low band frequency of f L ). The Smith chart of  FIG. 6B  contains impedance plots for operation in a second communications band of interest (e.g., a high band B 2  that extends from a third frequency of f 3  to a fourth frequency of f 4  and that is centered on a high band frequency of f H ). Antennas for device  10  may operate in other bands, if desired. 
     Transmission line  58  ( FIG. 4 ) may be characterized by an impedance. The impedance of transmission line  58  may, as an example, be 50 Ohms. For optimum antenna performance, it is desirable to match the impedance of antenna  40  to the impedance of transmission line  58  (i.e., it is desirable to configure antenna  40  so that antenna  40  exhibits an impedance of 50 Ohms to match the 50 Ohm impedance of transmission line  58 ). 
     An ideal antenna impedance of 50 Ohms is represented by point  72  in the Smith charts of  FIGS. 6A and 6B . In practice, it can be challenging to configure antenna  40  to exhibit the desired 50 Ohm impedance represented by point  72 . For example, an antenna of the type shown in  FIG. 5  may exhibit a complex impedance such as impedance  74  of  FIG. 6  when operating in low band B 1 . Impedance  74  may be characterized by a first impedance value  74 . 1  at low band operating frequency f 1  (i.e., at the lower end of the low band) and a second impedance value  74 . 2  at low band operating frequency f 2  (i.e., at the upper end of the low band). 
     As shown in  FIG. 6A , impedance  74  (corresponding to a configuration for antenna  40  of the type shown in  FIG. 5 ) may be too capacitive, leading to a non-negligible mismatch between actual antenna impedance  74  and desired antenna impedance  72 . Impedance  74  may, for example, be too capacitive in configurations in which antenna  40  is implemented in a restricted volume (e.g., in a compact electronic device having dimensions that are limited relative to a quarter of a wavelength at operating frequencies of interest). To address this mismatch, a shunt inductance such as a thin copper trace or a discrete component such as a shunt inductor or other shunt low pass filter circuit (in which frequencies f 1  to f 2  lie within the pass band) may be added to antenna  40  that spans gap  71  between antenna resonating element  68  and antenna ground  66 . 
     A configuration of the type that may be used for antenna  40  in which a low pass filter such as a shunt inductor has been incorporated into the antenna is shown in  FIG. 7 . As shown in  FIG. 7 , antenna  40  may have a shunt inductance such as low pass filter circuitry (inductor)  76 . Low pass filter  76  may have a first terminal that is coupled to resonating element structure  70  and an opposing second terminal that is coupled to antenna ground  66  across gap  71 . Low pass filter  76  may be formed from a discrete component such as a surface mount technology (SMT) component, may be formed from metal traces (e.g., a metal line coupled between resonating element structure  70  and antenna ground  66 ), may be formed from one or more SMT components that are coupled to antenna  40  using metal traces that exhibit an inductance, or may be formed using other filter circuitry. When antenna  40  is modified to incorporate a shunt inductance such as low pass filter  76  of antenna  40  in  FIG. 7  (in which frequencies f 1  to f 2  lie within the pass band), antenna  40  may exhibit an impedance such as impedance  78  of  FIG. 6A . Impedance  78  may be characterized by a first impedance value  78 . 1  at low band operating frequency f 1  (i.e., at the lower end of the low band) and a second impedance value  78 . 2  at low band operating frequency f 2  (i.e., at the upper end of the low band). Low pass filter  76  in a shunt configuration may behave more like a short circuit at frequency f 1  than at frequency f 2  (i.e., impedance  78 . 1  may be changed more significantly from impedance  74 . 1  by the presence of low pass filter  76  than impedance  78 . 2  is changed from impedance  74 . 2 ). 
     To counteract the larger movement of impedance  74 . 1  to  78 . 1  when incorporating low pass filter  76  into antenna  40 , a series capacitor can also be introduced into antenna  40 . For example, antenna  40  may be configured as shown in  FIG. 8 . In the illustrative configuration of  FIG. 8 , a series capacitance has been interposed in feed  64  of antenna  40  (i.e., series capacitor  80  has been formed between antenna resonating element structure  70  and antenna feed terminal  60 ). Including a capacitor such as capacitor  80  into the feed of antenna  40  may alter the impedance of antenna  40 . 
     In particular, when antenna  40  is modified to incorporate an inductor such as inductor  76  of antenna  40  in  FIG. 7  and an antenna feed such as antenna feed  64  of  FIG. 8  that includes a series capacitance such as series capacitor  80 , antenna  40  may exhibit an impedance such as impedance  82  of  FIG. 6A . Impedance  82  may be characterized by a first impedance value  82 . 1  at low band operating frequency f 1  (i.e., at the lower end of the low band) and a second impedance value  82 . 2  at low band operating frequency f 2  (i.e., at the upper end of the low band). Capacitor  80  of antenna  40  of  FIG. 8  may behave more like an open circuit at frequency f 1  than at frequency f 2 . Impedance  82 . 1  may therefore be changed more significantly from impedance  78 . 1  by the presence of capacitor  80  than impedance  82 . 2  is changed from impedance  78 . 2 ), as shown in  FIG. 6A . The resulting values of impedance for antenna  40  of  FIG. 8  (impedance values  82 ) may be sufficiently close to desired impedance  72  to be satisfactory during operation of antenna  40  in device  10  in low band B 1 . 
     High band performance may be understood with reference to the Smith chart of  FIG. 6B . When operating in high band B 2  (e.g., at operating frequencies ranging from lower high band frequency f 3  to upper high band frequency f 4 ), an antenna of the type shown in  FIG. 5  may exhibit impedance  74 . As shown in  FIG. 6B , impedance  74  may be characterized by an impedance value  74 . 3  at high band operating frequency f 3  (i.e., at the lower end of the high band) and impedance value  74 . 4  at high band operating frequency f 4  (i.e., at the upper end of the low band). Impedance  74  may not be too capacitive relative to desired operating impedance  72  during high band operations. Nevertheless, when shunt low pass filter  76  of  FIG. 7  (in which frequencies f 3  to f 4  lie within the stop band) is added to antenna  40  to ensure satisfactory low band performance, high band impedance  74  may change into high band impedance  78 . Impedance  78  may be characterized by an impedance value  78 . 3  at high band operating frequency f 3  (i.e., at the lower end of the high band) and impedance value  78 . 4  at high band operating frequency f 4  (i.e., at the upper end of the low band). Because shunt low pass filter  76  behaves more like an open circuit in high band B 2  than in low band B 1 , there ideally would be minimal impact on antenna impedance due to the presence of low pass filter  76 . However, due to the presence of thin traces that are generally used when coupling the components of low pass filter  76  between antenna resonating element  70  and ground  66  and due to imperfections in the low pass filter&#39;s stop band, low pass filter will appear as a small shunt inductance and there will generally be movement from impedance  74  to impedance  78  in high band B 2  when low pass filter  76  is incorporated into antenna  40 . 
     To counteract the movement of impedance  74  to impedance  78  in high band B 2  due to the non-zero contribution of shunt inductance from low pass filter  76 , series feed capacitor  80  in an antenna of the type shown in  FIG. 8  may be implemented using a variable capacitor design that exhibits a decreasing capacitance with increasing frequency of operation. When a variable capacitor is used in implementing capacitor  80  of antenna  40  in an arrangement of the type shown in  FIG. 8 , antenna  40  may exhibit satisfactory impedance  82  in high band B 2 . Impedance  82  may be characterized by an impedance value  82 . 3  at high band operating frequency f 3  (i.e., at the lower end of the high band) and impedance value  82 . 4  at high band operating frequency f 4  (i.e., at the upper end of the high band). Because impedance  82  is well matched to desired impedance  72 , antenna  40  of  FIG. 8  may, when capacitor  80  is implemented using a variable capacitor, exhibit satisfactory operation in high band B 2  while simultaneously exhibiting satisfactory operation in low band B 1 , as described in connection with impedance  82  of  FIG. 6A . The variable capacitor for antenna  40  may be implemented using one or more discrete capacitors (e.g., surface mount technology capacitors), a distributed capacitor formed from traces on an antenna substrate such as a plastic support, a flexible printed circuit, a rigid printed circuit board, or other substrate, or a combination of discrete and distributed capacitor structures. 
       FIG. 9  is a diagram of a configuration of the type that may be used when implementing a series feed capacitance for antenna  40  using a fixed distributed capacitor configuration. As shown in  FIG. 9 , in a distributed capacitor arrangement, the capacitance of capacitor  80  of  FIG. 8  may be implemented using a conductive antenna structure such as antenna structure  88  in antenna resonating element  68 . Structure  88  may be formed from a metal trace on a substrate such as a plastic carrier or other dielectric support structure, a flexible printed circuit, a rigid printed circuit board, or other substrate. Structure  88  may, for example, be formed from a metal trace. Structures  88  and  70  and, if desired, some or all of ground  66  and structures for forming inductor  76  may be mounted on a common substrate. 
     Antenna resonating element structure  70  and structure  88  may be separated by a gap such as gap  92 . Gap  92  may be characterized by a length L and width W. Structures  88  and  70  may serve as capacitor electrodes that form series capacitance  80  for antenna feed  64 . The magnitude of the capacitance exhibited by structures  88  and  70  may be directly proportional to length L and indirectly (inversely) proportional to width W. In the illustrative configuration of  FIG. 9 , structures  88  and  70  have rectangular shapes and width W of gap  92  is uniform along its length. This is merely illustrative. Structures  88  and  70  may have other shapes (e.g., shapes with bends, shapes with curved edges, shapes with curved and straight edges, or other suitable shapes) and gap  92  may have other shapes (e.g., gap shapes with straight edges, curved edges, combinations of straight and curved edges, shapes characterized by variable widths W, etc.). 
     As with capacitor  80  of  FIG. 8 , the capacitance exhibited by distributed capacitor  80  of  FIG. 9  may be used to change impedance  78  into impedance  82  in low band B 1 . Because the distributed capacitance arrangement of  FIG. 9  may be used to avoid or reduce reliance on discrete components in antenna  40 , the arrangement of  FIG. 9  may help reduce the cost and complexity of antenna  40  while helping to improve reliability. 
     The impedance of an antenna with a fixed series capacitance such as antenna  40  of  FIG. 9  will tend to vary as a function of frequency, because the reactance X of a fixed capacitor varies inversely with operating frequency, decreasing with increasing frequency. To counteract this decrease in reactance at higher operating frequencies, a variable capacitor design may be used for capacitor  80 . For example, a distributed capacitor for antenna  40  may be implemented using a frequency-dependent variable capacitance configuration. With this type of configuration, the capacitance C of the distributed capacitor may decrease as a function of increased operating frequency, as indicated by variable capacitance C in the graph of  FIG. 10A . As shown in  FIG. 10A , when the variable capacitor is operated at relatively low frequencies such as frequencies in lower communications band B 1  centered at lower frequency f L  and extending from lower frequency f 1  to upper frequency f 2  the capacitor may exhibit a relatively high capacitance value of about C H . When the capacitor is operated at relatively high frequencies such as frequencies in higher communications band B 2  centered at higher frequency f H  and extending between lower frequency f 3  and upper frequency f 4  the capacitor may exhibit a relatively low capacitance value of about C L . The decrease in capacitance C with increasing operating frequency f that is exhibited by the variable capacitance configuration may help ensure that the reactance associated with the capacitor remains relatively constant over a range of operating frequencies (e.g., at both low band B 1  and high band B 2 ), as illustrated in  FIG. 10B . The relatively constant value of reactance that is exhibited by the variable capacitor configuration of capacitor  80  can be used to help ensure that the impedance of antenna  40  will be well matched to desired impedance  72  over this range of operating frequencies. When incorporating a fixed capacitance value for capacitor  80  into antenna  40 , impedance  74  may change to undesirable (mismatched) impedance  78  of  FIG. 6B . Impedance  78  of  FIG. 6B  is not desirable, because impedance  78  is less matched to desired impedance  72  than impedance  74 . To match antenna impedance to desired impedance  72  in high band B 2 , it may be desirable for the reactive contribution from capacitor  80  to not be significantly lower in high band B 2  in comparison to the reactive contribution from capacitor  80  in low band B 1  that was successfully used in producing matched impedance  82  for low band operations. This can be accomplished by configuring a variable capacitor to exhibit a sufficiently decreasing capacitance at high frequencies to maintain the reactance from capacitor  80  at a relatively similar magnitude during high band and low band operations. 
     A frequency-dependent variable capacitance configuration for a distributed variable capacitor may be implemented by forming one or more of the electrodes for the distributed from discrete segments that are coupled together using filter circuitry (e.g., passive filter circuitry). An illustrative configuration for antenna  40  in which antenna  40  includes a frequency-dependent distributed variable capacitor (capacitor  80 ′) that is based on a passive filter is shown in  FIG. 11 . 
     In the arrangement of  FIG. 11 , capacitor  80 ′ has a first electrode formed from structure  70  and a second electrode (electrode  88 ). Structure  70  and electrode  88  may form part of antenna resonating element  68  and may be separated from each other by gap  92 . 
     As shown in  FIG. 11 , distributed capacitor electrode  88  may include multiple individual conductive elements such as conductive electrode element  88 A and conductive electrode element  88 B. Elements  88 A and  88 B may be separated from antenna ground  66  by gap  71 . 
     A passive radio-frequency filter such as filter  90  may be interposed between elements  88 A and  88 B. In the example of  FIG. 11 , filter  90  has been implemented using a series inductor (i.e., filter  90  is a low pass filter formed from an inductor). One terminal of the inductor may be coupled to element  88 A and the other terminal of the inductor may be coupled to element  88 B. Other types of filters (e.g., other low pass filter circuits) may be coupled between elements  88 A and  88 B if desired. The inductor or other components that form filter  90  may be formed from discrete components (e.g., an SMT inductor and/or other SMT components) and/or patterned metal traces. 
     Conductive element  88 A and conductive element  88 B may have respective lengths of L 1  and L 2  (as an example). The magnitude of lengths L 1  and L 2  may be used to tune the low frequency capacitance and high frequency capacitance exhibited by frequency-dependent variable distributed capacitor  80 ′. 
     At lower operating frequencies such as frequencies associated with band B 1  of  FIG. 10 , filter  90  will exhibit a low impedance because the inductor that forms filter  90  will effectively be a short circuit. As a result, conductive elements  88 A and  88 B will be shorted together and will serve as a single unitary capacitor electrode (i.e., electrode  88  of  FIG. 11  will include both element  88 A and element  88 B). Capacitor electrode  88  in this situation will have a length L (L=L 1 +L 2 ). The magnitude of capacitance C of capacitor  80 ′ will therefore be inversely proportional to width W of gap  92  and directly proportional to length L (i.e., capacitance C of capacitor  80 ′ will be equal to C H  of  FIG. 10  when operated in band B 1 ). Because capacitor  80 ′ is configured to exhibit a capacitance of C H  during low band operations in band B 1 , antenna  40  of  FIG. 11  may exhibit an impedance such as satisfactory low band impedance  82  of  FIG. 6A  in low band B 1 . 
     At higher operating frequencies such as frequencies associated with band B 2  of  FIG. 10 , filter  90  will exhibit a high impedance because the inductor that forms filter  90  will effectively be an open circuit. As a result of the open circuit between conductive elements  88 A and  88 B, conductive element  88 A and  88 B will be electrically isolated from each other. In this situation, capacitor electrode  88  will effectively include only conductive element  88 B of length L 2 . Conductive element  88 A will be electrically isolated from conductive element  88 B and antenna feed terminal  60  on conductive element  88 B. The isolation of element  88 A prevents element  88 A from contributing to the capacitance of capacitor  80 ′. When operated at higher operating frequencies such as frequencies in band B 2  of  FIG. 10 , capacitor electrode  88  will therefore have a length L 2 . The magnitude of capacitance C of capacitor  80 ′ will thus be inversely proportional to width W of gap  92  and directly proportional to length L 2  (i.e., capacitance C of capacitor  80 ′ will be equal to C L  of  FIG. 10  when operated in band B 2 ). Because capacitor  80 ′ is configured to exhibit a capacitance of C L  during high band operations in band B 2 , antenna  40  of  FIG. 11  may exhibit an impedance such as satisfactory high band impedance  82  of  FIG. 6B  in high band B 2 . 
     If desired, the electrodes for frequency-dependent distributed capacitance  80 ′ may be formed from more than two conductive elements and a corresponding number of filters for coupling the elements together. The arrangement in which capacitor electrode  88 ′ has two conductive elements ( 88 A and  88 B) coupled using a single filter is merely illustrative. Moreover, the sizes and shapes of the conductive elements that form the capacitor electrodes and resonating element structure  70  may be different than shown in the example of  FIG. 11 . These elements may, for example, have curved edges, bends, shapes with straight and curved elements and/or bent portions, etc. The filters that are used in coupling the elements together may be formed from inductors and other electrical components and may have different filter characteristics (e.g., different low pass filter cutoff frequencies). 
     By using a distributed capacitor such as capacitor  80 ′ of  FIG. 11  that exhibits a frequency dependent capacitance such as capacitance C of  FIG. 10A , antenna  40  may be impedance matched to a desired impedance value (e.g., desired impedance value  72  of  FIGS. 6A and 6B ) over an expanded range of operating frequencies when compared to an antenna such as antenna  40  of  FIG. 9  that has a distributed capacitor that exhibits a fixed capacitance as a function of frequency. As an example, antenna  40  of  FIG. 11  may exhibit an impedance such as impedance  82  of  FIG. 6A  in low band B 1  and an impedance such as impedance  82  of  FIG. 6B  in high band B 2 . In low band B 1 , capacitance value C H  may be used to impedance match the impedance of antenna  40  to desired impedance  72  (e.g., by exhibiting impedance  82  of  FIG. 6A  or other suitable impedance that is close to the value of impedance  72 ). At higher operating frequencies, such as frequencies in band B 2 , the reactance of capacitor  80 ′ may be maintained at a value similar to the reactance of capacitor  80 ′ in low band B 1  due to the presence of filter  90 . Filter  90  is a low pass filter that exhibits a relatively large impedance in band B 2 , which removes element  88 A from electrode  88  and thereby reduces the value of C to C L . Because the reactance of capacitance  80 ′ is inversely proportional to operating frequency (which is higher in band B 2  than in band B 1 ) and is inversely proportional to capacitance C (which is lower in band B 2  than in band B 1 ), the reactance of capacitance  80 ′ (and therefore the impedance of antenna  40 ) may be relatively unchanged at band B 2  relative to band B 1  as shown in  FIG. 10B  (i.e., antenna  40  may exhibit impedance  82  of  FIG. 6B  or other suitable impedance that is close to the value of impedance  72  when operating in band B 2  in addition to exhibiting impedance  82  when operating in band B 1 ). 
     If desired, low pass filter  76  (and, if desired, low pass filters such as low pass filter  90 ) may be implemented using multiple discrete components. As an example, filter  76  may be formed from multiple band stop filters coupled in series between terminal T 1  (i.e., a first terminal that is coupled to resonating element  70 ) and terminal T 2  (i.e., a second terminal that is coupled to ground  66 ), as shown in  FIG. 12 . In the example of  FIG. 12 , low pass filter  76  has been implemented using four band stop filters coupled in series (i.e., band stop filters  76 - 1 ,  76 - 2 ,  76 - 3 , and  76 - 4 ). Other numbers of band stop filters (e.g., fewer than four or more than four) or other types of filter circuits may be used in forming filter  76 , if desired. 
     Each series-connected band stop filter in filter  76  may include a different inductor and capacitor. The values of inductances L 1 , L 2 , L 3 , and L 4  and respective capacitances C 1 , C 2 , C 3 , and C 4  in  FIG. 12  may, for example, be selected to tune the stop bands of each band stop filter stage in filter  76 . As shown in  FIG. 13A , the individual stages of filter  76  of  FIG. 13A  may exhibit overlapping resonances at slightly offset frequencies, resulting in low pass filter performance of the type shown in  FIG. 13B . The use of band stop filters to implement low pass filter  76  may help improve the performance of low pass filter  76  relative to a design that uses a single inductor by lowering the impedance of filter  76  in low band B 1 , by raising the impedance of filter  76  in high band B 2 , and/or by otherwise helping to ensure that the transition between the low and high band impedances closely follows an ideal step function response. Other types of low pass filter may be used for filter  76  or elsewhere in antenna  40  if desired. The use of multiple series-connected band stop filters is merely illustrative. 
     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.

Metadata:
Filing Date: 20120420
Publication Date: 20150526
Grant Date: 20150526
Priority Date: 20120420
Inventors: MCMILIN EMILY B.
LI QINGXIANG
SCHLUB ROBERT W.
Assignee: APPLE INC
CPC Classifications: [{"code": "H01Q5/314", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01Q9/0457", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01Q1/243", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01Q23/00", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01Q5/314", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01Q9/0457", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01Q1/243", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01Q5/314", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01Q1/243", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01Q1/24", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01Q9/0457", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01Q1/48", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 48143018