PATENT DOCUMENT

Publication Number: US-8866692-B2
Application Number: US-34061008-A
Country: US
Kind Code: B2

Title: Electronic device with isolated antennas

Abstract:
Antennas for electronic devices are provided. First and second antennas may be mounted within an electronic device. Free-space coupling between the first and second antennas may give rise to interference. The first and second antennas may be coupled to a global ground. The global ground may be formed using a conductive member in the electronic device such as a conductive frame member. Signals that pass between the antennas through the global ground may serve as canceling signals that reduce the magnitude of free-space interference signals and thereby improve antenna isolation. The antennas may be coupled to the global ground using electrical paths or through near-field electromagnetic coupling. Coupling efficiency to the global ground may be enhanced by configuring the conductive traces of one or both of the antennas to form a resonant circuit.

Claims:
What is claimed is: 
     
       1. A computer, comprising:
 a plastic housing having a top and a bottom and having four sides; 
 a metal frame member within the plastic housing, wherein the metal frame member extends along at least two of the four sides of the plastic housing; 
 a first antenna having a first antenna resonating element and a first antenna local ground; and 
 a second antenna having a second antenna resonating element and a second antenna local ground, wherein the metal frame member forms a global ground structure that is coupled to the first antenna and that is coupled to the second antenna, wherein the electronic device is configured such that a first version of a transmitted antenna signal from the first antenna that is received at the second antenna through the global ground structure at least partially cancels a second version of the transmitted antenna signal from the first antenna that is received by the second antenna through a free-space path to increase isolation between the first and second antennas, wherein the first antenna resonating element comprises a first planar substrate that lies in a first plane, wherein the second antenna resonating element comprises a second planar substrate that lies in a second plane, wherein the first and second planar substrates each has first, second, and third dimensions, wherein the third dimension of the first planar substrate is smaller than the first and second dimensions of the first planar substrate, wherein the third dimension of the second planar substrate is smaller than the first and second dimensions of the second planar substrate, wherein the third dimension of the first planar substrate is perpendicular to the first plane, wherein the third dimension of the second planar substrate is perpendicular to the second plane, wherein the first plane is not parallel to the second plane, wherein the first and second antennas are disposed on a given one of the four sides of the plastic housing and are between the plastic housing and the metal frame member, wherein the metal frame member is planar and lies in a third plane along the given one of the four sides of the plastic housing, wherein the third plane is parallel to the first plane, wherein the first antenna comprises conductive traces that are configured to form an L-shaped antenna resonating element and wherein the L-shaped antenna resonating element has a maximum width of between 2 mm and 5 mm and has a maximum length of between 4 mm and 8 mm. 
 
     
     
       2. The computer defined in  claim 1  wherein the first antenna comprises conductive traces that are configured to form a resonant circuit. 
     
     
       3. The computer defined in  claim 1  wherein the first antenna local ground comprises a conductive trace having two ends spaced apart by a gap to form a series capacitance for a resonant circuit. 
     
     
       4. The computer defined in  claim 3  wherein the first antenna local ground comprises a C-shaped conductive trace. 
     
     
       5. The computer defined in  claim 1  wherein the first antenna local ground comprises a C-shaped conductive trace. 
     
     
       6. The computer defined in  claim 1  wherein the first antenna comprises conductive traces that are configured to electromagnetically couple to the global ground structure. 
     
     
       7. The computer defined in  claim 6  further comprising a conductive path between the second antenna and the global ground structure. 
     
     
       8. The computer defined in  claim 1  further comprising a conductive path between the second antenna and the global ground structure. 
     
     
       9. The computer defined in  claim 1  wherein the first antenna is a single band antenna that is configured to operate at 5 GHz and the second antenna is a dual band antenna that is configured to operate at 2.4 GHz and 5 GHz. 
     
     
       10. The computer defined in  claim 1  wherein the first dimension of the first planar substrate is larger than the second and third dimensions of the first planar substrate, wherein the first dimension of the second planar substrate is larger than the second and third dimensions of the second planar substrate, and wherein the first dimension of the first planar substrate is parallel to the first dimension of the second planar substrate. 
     
     
       11. The computer defined in  claim 1  wherein the metal frame member comprises an array of holes on the at least two of the four sides that the metal frame member extends along. 
     
     
       12. The computer defined in  claim 1  further comprising:
 a first conductive bracket that connects the first antenna to the metal frame member; and 
 a second conductive bracket that connects the second antenna to the metal frame member. 
 
     
     
       13. A computer comprising:
 a plastic housing having a top and a bottom and having four sides; 
 an internal metal frame member adjacent and parallel to a given side of the four sides of the plastic housing, wherein there is a gap between the internal frame member and the given side of the plastic housing; 
 first and second antennas disposed within the gap between the internal frame member and the given side of the plastic housing; 
 first and second brackets that short respective portions of the first and second antennas to the internal frame member, wherein the internal frame member comprises a global ground structure, wherein the first and second antennas are disposed at respective positions within the gap between the internal frame member and the given side of the plastic housing such that a first version of a transmitted antenna signal from the first antenna that is received at the second antenna through the internal metal frame member at least partially cancels a second version of the transmitted antenna signal from the first antenna that is received by the second antenna through a free-space path, wherein the first antenna comprises a local ground formed from a C-shaped conductive trace having two ends spaced apart by a gap that forms a series capacitance for a resonant circuit, wherein the C-shaped conductive trace has a maximum width of between 3 mm and 7 mm, wherein the C-shaped conductive trace has a maximum external length of between 20 mm and 30 mm, wherein the C-shaped conductive trace has a maximum internal length of between 10 mm and 15 mm, and wherein the gap is 0.2 mm to 3 mm between the two ends. 
 
     
     
       14. The computer defined in  claim 13  wherein the computer does not include a display. 
     
     
       15. The computer defined in  claim 14  wherein the internal frame member comprises a plurality of evenly-spaced holes along the given side of the plastic housing. 
     
     
       16. The computer defined in  claim 15  wherein the internal frame member also extends along at least a second given side of the four sides of the plastic housing and wherein there is a gap between the internal frame member and the second given side of the plastic housing. 
     
     
       17. The computer defined in  claim 16  wherein the internal frame member comprises a plurality of evenly-spaced holes along the second given side of the plastic housing. 
     
     
       18. The computer defined in  claim 17  further comprising:
 a base member adjacent and parallel to the bottom of the plastic housing, wherein the base member comprises an array of evenly-spaced holes. 
 
     
     
       19. The computer defined in  claim 18  wherein the first antenna comprises conductive traces that are configured to form an L-shaped antenna resonating element and wherein the L-shaped antenna resonating element has a maximum width of between 2 mm and 5 mm and has a maximum length of between 4 mm and 8 mm. 
     
     
       20. The computer defined in  claim 1  wherein the first antenna comprises a local ground formed from a C-shaped conductive trace having two ends spaced apart by a gap that forms a series capacitance for a resonant circuit, wherein the C-shaped conductive trace has a maximum width of between 3 mm and 7 mm, wherein the C-shaped conductive trace has a maximum external length of between 20 mm and 30 mm, wherein the C-shaped conductive trace has a maximum internal length of between 10 mm and 15 mm, and wherein the gap is 0.2 mm to 3 mm between the two ends.

Description:
BACKGROUND 
     This invention relates to electronic devices and, more particularly, to antennas for electronic devices. 
     Electronic devices often use wireless communications circuitry. For example, wireless communications circuitry is used in wireless base stations to support communications with computers and other wirelessly networked devices. 
     Some electronic devices use multiple antennas. For example, a device may use a first antenna to support operations in a first set of communications bands and may use a second antenna to support operation in a second set of communications bands. By using multiple antennas, band coverage may be increased or multiple-input multiple-output (MIMO) antenna schemes may be implemented. 
     Particularly in electronic devices of relatively small size, it may be necessary to locate different antennas in close proximity. This can cause undesirable coupling effects in which the operation of one antenna interferes with the operation of another antenna. It is therefore challenging to produce successful antenna arrangements in which multiple antennas operate in close proximity to each other without experiencing undesirable interference. 
     It would therefore be desirable to be able to provide improved antenna structures for wireless electronic devices. 
     SUMMARY 
     An electronic device is provided that has wireless communications capabilities. The electronic device may have a housing. The housing may contain storage and processing circuitry. A radio-frequency transceiver circuit may be coupled to the storage and processing circuitry. Multiple antennas may be coupled to the radio-frequency transceiver circuitry using respective transmission lines. For example, a first antenna may be coupled to the radio-frequency transceiver using a first coaxial cable and a second antenna may be coupled to the radio-frequency transceiver using a second coaxial cable. The first and second antennas may be single band or multiband antennas. For example, the first antenna may be a single band antenna that operates at 5 GHz, whereas the second antenna may be a dual band antenna that operates at 2.4 GHz and 5 GHz (as an example). 
     The electronic device may include a conductive structure such as a conductive frame member that serves as a global ground. The first and second antennas may each be electrically and/or electromagnetically coupled to the conductive structure. During operation, signals that are transmitted from one antenna may be received by the other antenna over a free-space path. These signals represent interference. The interference signal can be reduced using a deliberately created cancelling signal. The cancelling signal may be of comparable magnitude and opposite phase to that of the interference signal. The cancelling signal may be routed from one antenna to the other by coupling the antennas through the global ground. The presence of the global ground cancelling path serves to increase isolation between the first and second antennas. Increased isolation may, in turn, improve antenna performance in various modes of operation (e.g., single band and dual band operating modes and operating modes with both antennas transmitting, both antennas receiving, one antenna transmitting and the other antenna receiving, etc.). 
     To enhance coupling between the antennas and the global ground, one or both antennas may have traces that are configured to form a resonant circuit. For example, an antenna ground element may be formed from a C-shaped trace. The length of the ground element trace gives rise to an inductance for the resonant circuit. A gap in the ground element trace forms a capacitance in series with the inductance. 
     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 such as a wireless base station or computer in which isolated antennas may be implemented in accordance with an embodiment of the present invention. 
         FIG. 2  is schematic diagram of an illustrative electronic device such as a wireless base station or computer in which isolated antennas may be implemented in accordance with an embodiment of the present invention. 
         FIG. 3  is a schematic diagram of two isolated antennas that may be used in an electronic device such as a wireless base station or computer in accordance with an embodiment of the present invention. 
         FIG. 4  is a circuit diagram of an illustrative resonant circuit for an antenna structure in accordance with an embodiment of the present invention. 
         FIG. 5  is a diagram of illustrative antenna traces that may be used in an antenna that includes the resonant circuit of  FIG. 4  in accordance with an embodiment of the present invention. 
         FIG. 6  is a diagram of illustrative antenna structures that may be used in another antenna in accordance with an embodiment of the present invention. 
         FIG. 7  is a perspective view of an interior portion of an illustrative electronic device with isolated antennas in accordance with an embodiment of the present invention. 
         FIG. 8  is a perspective view of an illustrative antenna having an antenna element trace pattern of the type shown in  FIG. 5  and that may be used in a device of the type shown in  FIG. 7  in accordance with an embodiment of the present invention. 
         FIG. 9  is a cross-sectional perspective view of an illustrative antenna of the type shown in  FIG. 8  in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to antennas for electronic devices. The antennas may be used to convey wireless signals for wireless communications links in any suitable communications bands. For example, the antennas may be used to handle communications for local area network links such as an IEEE 802.11 links (sometimes referred to as WiFi® links) or Bluetooth® links. The antennas may also be used to handle other communications frequencies, such as 2G and 3G cellular telephone frequencies. The antennas may be single band antennas or multiband antennas. A given electronic device may have two or more antennas that are isolated from each other to improve antenna performance. 
     An illustrative configuration in which two antennas are used to handle local area network signals is sometimes described herein as an example. In this type of illustrative configuration, a first antenna of the two antennas may be a single band antenna that handles IEEE 802.11 communications in the 5 GHz band and a second of the two antennas may be a dual band antenna that handles IEEE 802.11 communications in the 2.4 GHz and 5 GHz bands. 
     Antennas such as these may be used in various electronic devices. For example, the antennas may be used in an electronic device such as a handheld computer, a miniature or wearable device, a portable computer, a desktop computer, a router, an access point, a backup storage device with wireless communications capabilities, a mobile telephone, a music player, a remote control, a global positioning system device, devices that combine the functions of one or more of these devices and other suitable devices, or any other electronic device. 
     As is sometimes described herein as an example, the electronic device in which the antennas are provided may be a wireless base station such as a router or may be a miniature computer with wireless capabilities. The base station or computer may include local storage such as hard drive storage or solid state drive storage. These are, however, merely illustrative examples. Antennas may, in general, be provided in any suitable electronic device. 
     An illustrative electronic device  10  such as a wireless base station or computer in which the antennas may be provided is shown in  FIG. 1 . As shown in  FIG. 1 , device  10  may have a housing  12 . Housing  12 , which is sometimes referred to as a case, may be formed from one or more individual structures. For example, housing  12  may include structural support members and cosmetic coverings that are made from plastic and metal parts. Metal parts may be grounded and may serve as part of the antennas of device  10 . Plastic parts and other dielectric parts are generally transparent to radio-frequency signals. Accordingly, it is generally desirable for the portions of housing  12  that enclose the antennas to be formed from dielectric materials. Conductive parts may be used for internal structures in device  10 . 
     Device  10  may have antennas such as antennas  14  and  16 . Radio-frequency transceiver circuitry  18  may include a radio-frequency receiver and a radio-frequency transmitter. Transmission line paths such as transmission lines  22  and  24  may be used to couple transceiver circuitry  18  to antennas  14  and  16 . In the  FIG. 1  example, transceiver circuitry  18  is connected to antenna  14  using transmission line  24  and is connected to antenna  16  by transmission line  22 . Transmission lines  22  and  24  may be implemented using any suitable transmission line structures (e.g., cables, microstrip transmission line structures, etc.). With one suitable arrangement, which is sometimes described herein as an example, transmission lines  22  and  24  are implemented using coaxial cables. 
     Transceiver circuitry  18  may be coupled to circuitry such as storage and processing circuitry  20  using paths such as path  26 . During data transmission operations, data from storage and processing circuitry  20  may be routed to transceiver  18  over path  26  and may be wirelessly transmitted to external equipment using transceiver  18  and antennas  14  and  16 . During data reception operations, incoming radio-frequency signals may be received using antennas  14  and  16 , paths  24  and  22 , and transceiver circuitry  18 . Transceiver circuitry  18  may provide received signals to storage and processing circuitry  20  over path  26 . 
     For optimum wireless performance, it is desirable for antennas such as antennas  14  and  16  to interfere with each other as little as possible. Antenna interference can lead to degraded signal-to-noise ratios and reduced data communications throughput. Undesirable levels of interference can arise when antennas such as antennas  14  and  16  are placed in close proximity to each other. Due to the relatively small size of electronic devices such as device  10 , it may be difficult or impossible to separate antennas  14  and  16  by extremely large distances. Nevertheless, satisfactory isolation between antennas  14  and  16  may be achieved by configuring the structures that make up antennas  14  and  16  so as to reduce interference. 
     With one suitable arrangement, antenna-to-antenna isolation levels of 30 dB or greater may be achieved (as an example). Isolation performance of this level may be achieved when operating antennas  14  and  16  in the same communications band (e.g., both in a first communications band) and may be achieved when operating antenna  14  in a first communications band and operating antenna  16  in a second communications band that is different than the first communications band. The first antenna, such as antenna  14  may, as an example, operate at a communications band of 5 GHz (e.g., for IEEE 802.11 communications), whereas the second antenna such as antenna  16  may operate at communications bands such as 2.4 GHz and 5 GHz bands (e.g., for IEEE 802.11 communications). While operating in this configuration, the first and second antennas may exhibit antenna isolations of more than 30 dB for both bands (2.4 GHz and 5 GHz) that are handled by the second antenna. 
     A schematic circuit diagram of an illustrative electronic device such as device  10  of  FIG. 1  is shown in  FIG. 2 . As shown in  FIG. 2 , device  10  may include storage and processing circuitry  20  and input-output devices  28 . Storage and processing circuitry  20  may include hard disk drives, solid state drives, optical drives, random-access memory, nonvolatile memory and other suitable storage. Storage may be implemented using separate integrated circuits and/or using memory blocks that are provided as part of processors or other integrated circuits. 
     Storage and processing circuitry  20  may include processing circuitry that is used to control the operation of device  10 . The processing circuitry may be based on one or more circuits such as a microprocessor, a microcontroller, a digital signal processor, an application-specific integrated circuit, and other suitable integrated circuits. Storage and processing circuitry  20  may be used to run software on device  10  such as operating system software, code for implementing the functions of a server with an array of one or more hard disk drives, solid state drives, or other server storage, software for implementing the functions of router or other communications hub, or other suitable software. To support wireless operations, storage and processing circuitry  20  may include software for implementing wireless communications protocols such as wireless local area network protocols (e.g., IEEE 802.11 protocols—sometimes referred to as Wi-Fi®), protocols for other short-range wireless communications links such as the Bluetooth® protocol, protocols for handling 3G communications services (e.g., using wide band code division multiple access techniques), 2G cellular telephone communications protocols, WiMAX® communications protocols, communications protocols for other bands, etc. 
     Input-output devices  28  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices such as electronic equipment  34 . Input-output devices  28  may include user input-output devices such as buttons, display screens, touch screens, joysticks, click wheels, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, cameras, etc. A user can control the operation of device  10  by supplying commands through the user input devices. This may allow the user to adjust settings such as security settings, etc. Input-output devices  28  may also include data ports, circuitry for interfacing with audio and video signal connectors, and other input-output circuitry. 
     As shown in  FIG. 2 , input-output devices  28  may include wireless communications circuitry  32 . Wireless communications circuitry  32  may include communications circuitry such as radio-frequency (RF) transceiver circuitry  18  formed from one or more integrated circuits such as a baseband processor integrated circuit and other radio-frequency transmitter and receiver circuits. Circuitry  32  may include power amplifier circuitry, passive RF components, antennas  30  (e.g., antennas such as antennas  14  and  16  of  FIG. 1 ), and other circuitry for handling RF wireless signals. 
     Device  10  may use wired data paths such as path  36  and wireless data paths such as path  38  to communicate with external equipment  34 . External equipment  34  may include any suitable electronic equipment such as desktop computers, handheld computers and other portable computers, cellular telephones (e.g., multifunction cellular telephones with IEEE 802.11 capabilities), music players, remote controllers, peer devices (i.e., other equipment such as device  10 ), network equipment (e.g., in a local area network or in a cellular telephone network), etc. Wired paths such path  36  may be formed using wired data cables. Wireless paths such as path  38  may be formed by transmitting and receiving radio-frequency signals using antennas  30 . 
     Any suitable technique may be used in device  10  to isolate antennas  14  and  16 . For example, antennas  14  and  16  may be isolated using blocking techniques in which conductive structures are interposed between antennas  14  and  16  to mitigate interference. Isolation may also be improved by reducing antenna scattering through proper antenna placement, by using orthogonal antenna polarizations, by reducing common mode resonances, etc. 
     An illustrative isolation scheme for antennas  14  and  16  is shown in the schematic diagram of  FIG. 3 . As shown in  FIG. 3 , antenna  14  and antenna  16  may be separated by a distance X. One way in which to improve the isolation between antenna  14  and antenna  16  is to increase distance X (e.g., to the largest distance possible within the confines of a desired device housing). When large values of distance X are used, the amount of radio-frequency signal coupling between antenna  14  and antenna  16  along free-space path  40  will generally be reduced. There may be scattering and reflective paths associated with the free-space coupling between antenna  14  and antenna  16 . In general, however, the largest component of the free-space coupling between antenna  14  and antenna  16  will be associated with a relatively direct free-space path between antenna  14  and antenna  16 . 
     With the configuration shown in  FIG. 3 , each antenna may have an antenna resonating element and an associated local antenna ground. A global ground such as ground  42  may be formed that spans both antennas. Antenna  14  may be formed from antenna resonating element  14 A and local ground  14 B. Antenna  16  may be formed from antenna resonating element  16 A and local ground  16 B. Antennas  14  and  16  may each interact with the conductive structures that make up global ground  42  (which may therefore be considered to form a part of antennas  14  and  16 ). 
     Antenna  14  may be coupled to global ground  42  by near-field electromagnetic coupling (illustrated by radio-frequency signal path  48  in  FIG. 3 ). Antenna  16  may also be coupled to global ground  42  by near-field electromagnetic coupling (illustrated by radio-frequency signal path  50  in  FIG. 3 ). If desired, conductive paths such as conductive paths  44  and  46  may be used to electrically couple antennas  14  and  16  to global ground  42 , respectively. 
     Isolation may be improved by coupling antenna  14  to antenna  16  through global ground  42  such that the antenna signals from antenna  14  that reach antenna  16  through ground  42  cancel the signals from antenna  14  that reach antenna  16  through free-space path  40  (and vice versa). With this type of arrangement, signals that travel from antenna  14  along path  44  and/or path  48 , path  42 , and path  46  and/or path  50  have equal magnitude and are 180° out of phase with the signals that travel from antenna  14  to antenna  16  over free-space path  40 . 
     The magnitude of the signal that reaches antenna  16  through path  42  can be increased by increasing the coupling between antenna  14  and ground  42  and by increasing the coupling between antenna  16  and ground  42 . The phase of the cancelling signal traveling through ground  42  can be adjusted using matching components (e.g., resistors, inductors, capacitors, antenna elements with resistive, inductive, and capacitive properties, etc.), by making adjustments to the lengths of structures such as global ground  42  and paths  48 ,  44 ,  50 , and  46 , etc. Magnitude and phase adjustments such as these may be used to ensure that the cancelling signal between antennas  14  and  16  that passes through global ground  42  cancels other signals such as the signals conveyed over free-space path  40 . Antenna  14  can be isolated from antenna  16  and antenna  16  can be isolated from antenna  14  in this way. 
     If desired, the antenna resonating element and local ground of antenna  14  and/or antenna  16  can be adjusted to create a resonating circuit (e.g., by adjusting inductive, capacitive, and resistive antenna components to form a circuit that resonates at frequencies associated with the operation of antennas  14  and/or  16 ). Resonant circuit behavior that is created in this way can enhance the coupling efficiency associated with antenna  14  and global ground  42  and the coupling efficiency associated with antenna  16  and global ground  42  to increase the magnitude of the cancelling signal. Resonant circuit effects can be used in combination with other antenna structure adjustments to adjust the amplitude and phase of the canceling signal provided through global ground path  42  to obtain maximum isolation between antennas  14  and  16 . 
     An illustrative resonant circuit  52  that may be used in an antenna such as antenna  14  or antenna  16  is shown in  FIG. 4 . In the example of  FIG. 4 , resonant circuit  52  has been formed from series-connected inductor  54  and capacitor  56  in loop  58 . This type of circuit will tend to resonate at frequencies around a given frequency f. By proper selection of the components of circuit  52 , the resonant frequency f can be made to coincide with an operating frequency in a communications band of interest (e.g., the IEEE 802.11 bands at 2.4 and 5 GHz, as examples). When loop  58  is placed parallel to global ground  42  and close to global ground  42 , near-field electromagnetic coupling (paths  48  and/or  50  in  FIG. 3 ) will cause signals to be coupled between the antenna and the global ground and vice versa. If desired, other resonant circuit configurations may be used. The illustrative L-C circuit of  FIG. 4  is merely illustrative. 
       FIG. 5  shows an illustrative layout that may be used for antenna  14 . As shown in  FIG. 5 , antenna  14  may have an antenna resonating element such as antenna resonating element  14 A and a local ground such as local ground element  14 B. Elements  14 A and  14 B may be formed from conductive traces such as copper traces or other metal traces on a supporting substrate such as a flex circuit, rigid printed circuit board, or plastic support structure. Any suitable dimensions may be used for the conductive structures that form elements  14 A and  14 B. For example, dimension D 1  may be about 2-5 mm, dimension D 2  may be about 4-8 mm, dimension D 3  may be about 20-30 mm, dimension D 4  may be about 10-15 mm, dimension D 5  may be about 3-7 mm, and dimension D 6  may be about 0.2-3 mm (as examples). 
     The dimensions of elements  14 A and  14 B can be selected to tune the electrical properties of antenna  14 . For example, ground element  14 B of  FIG. 5  has a series inductance associated with the length LT of the C-shaped loop formed by trace  68 . Ground element  14 B also has a series capacitance formed by gap  62  between opposing trace ends  60 . Ground element  14 B forms a resonant L-C circuit of the type shown in  FIG. 4 . The length LT of trace  68  influences the amount of inductance associated with element  14 B. If length LT is increased, the amount of inductance associated with element  14 B will increase. Decreases in length LT will reduce the inductance of element  14 B. The width D 6  of gap  62  and the lateral dimensions of end faces  60  influence the amount of capacitance associated with element  14 B. Larger end faces  60  (i.e., larger dimensions D) will exhibit more capacitance, whereas narrower end faces  60  will exhibit less capacitance. The size of dimension D 6  can be reduced to increase the capacitance associated with gap  62  and can be increased to decrease the capacitance associated with gap  62 . Adjustments can also be made to trace resistivity, substrate dielectric constant, etc. 
     Antenna  14  may be fed using any suitable feed arrangement. For example, a transmission line (transmission line  24  of  FIG. 1 ) such as a coaxial cable or a microstrip transmission line may have a positive path connected to positive antenna feed terminal  64  and a ground (negative) antenna path connected to ground antenna feed terminal  66 . Positive feed terminal  64  may be connected to antenna resonating element  14 A. Ground feed terminal  66  may be connected to local antenna ground element  14 B. To ensure optimum impedance matching between the antenna transmission line and antenna  14 , an optional impedance matching network may be interposed between the transmission line and feed terminals  64  and  66 . Impedance matching components may also be incorporated into the structures of antenna  14 . 
     A perspective view of an illustrative configuration for antenna  16  is shown in  FIG. 6 . As shown in  FIG. 6 , patterned conductive traces  94  may be formed on substrate  96 . Traces  94  may include planar trace patterns that define one or more branches, slots, or other antenna features for antenna resonating element  16 A. Substrate  96  may be formed from printed circuit board material or other suitable dielectric. For example, substrate  96  may be formed from rigid printed circuit board material such as fiberglass-filled epoxy or flex circuit material such as polyimide. Substrate  96  may be mounted on bracket  98  or other suitable mounting structures using conductive adhesive or other suitable mounting arrangements. 
     Antenna  16  may be fed by connecting coaxial cable conductors or other transmission line paths in a path such as path  22  of  FIG. 1  to antenna feed terminals such as positive antenna feed terminal  92  and ground antenna feed terminal  90 . An impedance mating network may be used to improve impedance matching between transmission line  22  and antenna  16 . 
     Bracket  98  may be formed from a conductive material such as metal and may be used in forming local ground  16 B. Bracket  98  may be mounted to conductive structures in device  10  such as conductive structures that form global ground  42  ( FIG. 3 ). Base portion  86  of bracket  98  may have screw holes such as hole  88 . Screws or other fasteners that pass through holes  88  may be used to attach bracket  98  and antenna  16  to global ground  42 . Conductive bracket  98  may form a conductive path between antenna  16  and global ground  42  such as path  46  in  FIG. 3 . If desired, a conductive bracket or other such conductive structure may also be used to electrically couple antenna  14  to global ground  42  (e.g., to form a path such as path  44  of  FIG. 3 ). 
       FIG. 7  is a perspective view of an interior portion of an illustrative electronic device  10  with isolated antennas  14  and  16 . As shown in  FIG. 7 , device  10  may have a base portion  70  and a frame portion  72 . Holes  74  may be formed in frame member  72  (e.g., to reduce weight). Base  70  may be formed from materials such as metal and plastic. Frame  72  may be formed from a conductive material such as metal and may serve as global ground  42  of  FIG. 3 . Frame  72  may be formed from one or more individual members and may have features such as brackets  76 . Brackets  76  may be used in supporting internal mounting structures such as antenna support structures. Brackets on frame  72  may also be used in attaching a top housing portion formed of metal or plastic or other housing structures to base structure  70  (e.g., to form a cube-shaped housing such as housing  12  of  FIG. 1 ). 
     As shown in  FIG. 7 , antennas  14  and  16  may be mounted in device  10  in the vicinity of frame  72  or other conductive structural members associated with housing  12  and device  10 . Transmission lines  78  and  80  may be grounded to frame  72  using brackets such as brackets  82  and  84 . If desired, brackets  84  and  82  may serve as mounting structures and may optionally be used to form conductive coupling paths to the global ground structure formed from frame  72 . Brackets  84  and  82  may be formed from a dielectric such as plastic, a conductive material such as metal, or other suitable materials. If desired, brackets  84  and  82  or portions of brackets  84  and  82  may be formed as integral parts of frame  72 . 
     Antennas  14  and  16  may have substantially planar substrates on which patterned traces are formed. The planes of the substrates may be oriented to be orthogonal to each other as shown in  FIG. 7  (e.g., to increase the amount by which the polarizations of the antennas differ and thereby increase isolation). Coaxial cable  78  may serve as transmission line  24  of  FIG. 1  and may be used to couple transceiver circuitry  18  ( FIG. 1 ) to antenna  14 . Coaxial cable  80  may serve as transmission line  22  of  FIG. 1  and may be used to couple transceiver circuitry  18  to antenna  16 . 
       FIG. 8  is a perspective view of antenna  14  of  FIG. 7  showing how antenna  14  may have patterned traces such as trace  68  and resonating element trace  14 A formed on substrate  100 . Substrate  100  may be formed from a rigid printed circuit board material, a flex circuit material such as polyimide, or other suitable dielectric materials. Adhesive  102  may be used to attach substrate  100  to an antenna mounting structure formed from plastic or other dielectric materials. Antenna  16  of  FIG. 7  may also be mounted in device  10  using a dielectric mounting structure and adhesive. 
     Transmission line  78  may be a coaxial cable having center conductor  104 , a dielectric layer  106 , an outer conductor  108 , and a plastic jacket  110 . Clip  112  may be used in attaching cable  78  to frame  72  (e.g., at portion  82  using a screw). Center conductor  104  may be connected to antenna resonating element  14 A at antenna feed terminal  66  ( FIG. 5 ). Outer conductor  108  may be connected to ground antenna feed terminal  66  on local ground element  14 B of antenna  14  ( FIG. 5 ). 
     An illustrative antenna mounting structure to which antenna  14  may be mounted in device  10  is shown in  FIG. 9 . As shown in  FIG. 9 , substrate  100  of antenna  14  may be mounted to antenna mounting structure  114  at planar surface interface  116  using adhesive  102 . Mounting structure  114  may be formed from a dielectric such as plastic or other suitable materials. Mounting structure  114  may form part of housing  12  and may be attached to frame  72  by bracket  76  (e.g., using screws, adhesive, or other suitable attachment structures). Antenna  16  may also be mounted in device  10  using a mounting structure such as mounting structure  114 . 
     When antennas  14  and  16  are mounted within device  10  as shown in  FIG. 7 , radio-frequency signals may be transmitted and received using antennas  14  and  16  and radio-frequency transceiver  18 . Antenna  14  may be configured to operate in one or more bands (e.g., at 5 GHz) and antenna  16  may be configured to operate in one or more bands (e.g., 2.4 GHz and 5 GHz). 
     Although antennas  14  and  16  are spaced apart to increase isolation, there will still be a free-space signal path such as path  40  of  FIG. 3  between antennas  14  and  16  that can lead to undesirable electromagnetic coupling and signal interference. Isolation between antennas  14  and  16  can be improved using a cancelling signal path between antennas  14  and  16  formed by global ground  42  (a structure that is formed, in this example, using metal frame member  72 ). As described in connection with  FIG. 3 , free-space signal path  40  serves as a relatively direct path between antennas  14  and  16  and can lead to antenna interference. The signal path through global ground  42  serves as an indirect path through which canceling signals pass. The presence of the cancelling path serves to increase isolation between antennas  14  and  16 , because cancelling path signals can cancel out signals that are coupled over free-space path  40 . 
     Consider, as an example, a situation in which one antenna is transmitting. In this scenario, the free-space signal path (path  40 ) serves to convey a first version of a transmitted signal from a first of the antennas to a second of the antennas, whereas the path through global ground  42  serves to convey a second version of the same transmitted signal between the first and second antennas. The first version of the signal can serve as a source of interference for the second antenna. However, when cancelling path  42  is present, the first and second versions of the signal cancel each other at the second antenna, thereby reducing interference from the first version of the signal. Because the amount of interfering signal that is received at the second antenna from the first antenna is reduced, the isolation between the antennas is improved. This allows antennas  14  and  16  to be placed closer to each other in device  10  than would otherwise be possible and/or improves the wireless performance of device  10 . The presence of path  42  can enhance antenna isolation regardless of the mode of operation of antennas  14  and  16  (e.g., transmitting, receiving, simultaneously transmitting and receiving, etc.). 
     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: 20081219
Publication Date: 20141021
Grant Date: 20141021
Priority Date: 20081219
Inventors: VAZQUEZ ENRIQUE AYALA
CHIANG BING
XU HAO
Assignee: APPLE INC
CPC Classifications: [{"code": "H01Q1/521", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01Q1/2291", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01Q1/48", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01Q1/48", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01Q1/2291", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01Q1/521", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 42265239