PATENT DOCUMENT

Publication Number: US-9178278-B2
Application Number: US-201113299123-A
Country: US
Kind Code: B2

Title: Distributed loop antennas with extended tails

Abstract:
Electronic devices may be provided with antenna structures such as distributed loop antenna resonating element structures. A distributed loop antenna may be formed on an elongated dielectric carrier and may have a longitudinal axis. The distributed loop antenna may include a loop antenna resonating element formed from a sheet of conductive material that extends around the longitudinal axis. A gap may be formed in the sheet of conductive material. The gap may be located under an opaque masking layer on the underside of a display cover glass associated with a display. The loop antenna resonating element may have a main body portion that includes the gap and may have an extended tail portion that extends between the display and conductive housing structures. The main body portion and extended tail portion may be configured to ensure that undesired waveguide modes are cut off during operation of the loop antenna.

Claims:
What is claimed is:  
     
       1. A loop antenna configured to operate in at least one communications band, comprising:
 a loop antenna resonating element formed from a sheet of conductive material that is wrapped around an axis to form a conductive loop, wherein the loop antenna resonating element has a main body portion and an extended tail portion, and the main body portion and the extended tail portion are configured so that the loop antenna does not support waveguide modes for signals in the at least one communications band; 
 a dielectric support structure on which the sheet of conductive material is formed that supports the sheet of conductive material and comprises a wall of dielectric material that extends around the axis to surround a cavity, wherein the wall is characterized by a thickness that has different thickness values at different positions along the axis; and 
 an antenna feed structure that that is configured to indirectly feed the loop antenna resonating element, wherein the antenna feed structure comprises a loop-shaped metal trace on the dielectric support structure, and the sheet of conductive material has a ground plane recess adjacent to the loop-shaped metal trace. 
 
     
     
       2. The loop antenna defined in  claim 1  wherein the loop antenna resonating element is characterized by a width perpendicular to the axis and wherein the width has different values at different positions along the axis. 
     
     
       3. The loop antenna defined in  claim 1  wherein the loop antenna resonating element is characterized by a width perpendicular to the axis, wherein the main body portion is characterized by a first thickness perpendicular to the width and the axis, wherein the extended tail portion is characterized by a second thickness perpendicular to the width and the axis, and wherein the second thickness is less than the first thickness. 
     
     
       4. The loop antenna defined in  claim 3  wherein the second thickness is at least 1 mm. 
     
     
       5. The loop antenna defined in  claim 4  wherein the width is greater than 10 mm. 
     
     
       6. A loop antenna configured to operate in at least one communications band, comprising:
 a loop antenna resonating element formed from a sheet of conductive material that is wrapped around an axis to form a conductive loop, wherein the loop antenna resonating element comprises dielectric support structures that support the sheet of conductive material, the loop antenna resonating element has a main body portion and an extended tail portion, and the main body portion and the extended tail portion are configured so that the loop antenna does not support waveguide modes for signals in the at least one communications band; and 
 an antenna feed structure that is configured to indirectly feed the loop antenna resonating element using near-field electromagnetic coupling, wherein the antenna feed structure comprises a loop-shaped metal trace on the dielectric support structure, the sheet of conductive material has a ground plane recess adjacent to the loop-shaped metal trace, and the feed structure is configured to exhibit resonance at frequencies of the at least one communications band supported by the loop antenna resonating element. 
 
     
     
       7. The loop antenna defined in  claim 6  wherein the conductive loop has a perimeter, wherein the sheet of material has at least one gap with an associated capacitance that is interposed within the perimeter, wherein the support structure comprises a plastic structure with an air-filled cavity. 
     
     
       8. The loop antenna defined in  claim 7  wherein the at least one communications band comprises a 2.4 GHz communications band. 
     
     
       9. The loop antenna defined in  claim 8  wherein the loop antenna resonating element is configured to operate in the 2.4 GHz communications band and is configured to operate in a 5 GHz communications band. 
     
     
       10. A loop antenna configured to operate in at least one communications band, comprising:
 a loop antenna resonating element formed from a sheet of conductive material that is wrapped around an axis to form a conductive loop, wherein the loop antenna resonating element has a main body portion and an extended tail portion, and the main body portion and the extended tail portion are configured so that the loop antenna does not support waveguide modes for signals in the at least one communications band; and 
 an antenna feed structure that comprises a loop shaped structure and that is configured to indirectly feed the loop antenna resonating element, wherein the loop shaped structure comprises a metal trace on a dielectric support structure, the sheet of conductive material is supported by the support structure, and the sheet of material has a ground plane recess adjacent to the loop-shaped structure. 
 
     
     
       11. The loop antenna defined in  claim 10  wherein the main body portion is used to transmit and receive signals in a first communications band, wherein the extended tail portion is used to transmit and receive signals in a second communications band, and wherein the first communications band is a lower frequency band than the second communications band. 
     
     
       12. The loop antenna defined in  claim 10  further comprising:
 a dielectric support structure on which the sheet of conductive material is formed. 
 
     
     
       13. The loop antenna defined in  claim 12  wherein the dielectric support structure comprises walls that surround a cavity. 
     
     
       14. The loop antenna defined in  claim 13  wherein the walls are characterized by a thickness, and the thickness has different values at different positions along the axis.

Description:
BACKGROUND 
     This relates generally to electronic devices and, more particularly, to electronic devices with antennas. 
     Electronic devices such as computers are often provided with antennas. For example, a computer monitor with an integrated computer may be provided with antennas that are located along an edge of the monitor. 
     Challenges can arise in mounting antennas within an electronic device. For example, the relative position between an antenna and surrounding device structures and the size and shape of antenna structures can have an impact on antenna tuning and bandwidth. If care is not taken, an antenna may become detuned or may exhibit an undesirably small efficiency bandwidth at desired operating frequencies. 
     It would therefore be desirable to be able to provide improved antennas for use in electronic devices. 
     SUMMARY 
     Electronic devices may be provided with antenna structures such as distributed loop antenna resonating element structures. A distributed loop antenna may be formed on an elongated dielectric support structure and may have a longitudinal axis. The distributed loop antenna may include a loop antenna resonating element formed from a sheet of conductive material that extends around the longitudinal axis. A gap may be formed in the sheet of conductive material. The gap may be located under an opaque masking layer on the underside of a display cover glass associated with a display. 
     The loop antenna resonating element may have a main body portion on which the gap is formed and may have an extended tail portion. The extended tail portion may extend between the display and conductive housing structures for an electronic device. The main body portion and extended tail portion may be configured to ensure that undesired waveguide modes are cut off during operation of the loop antenna. 
     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 antenna structures in accordance with an embodiment of the present invention. 
         FIG. 2  is a cross-sectional side view of illustrative antenna structures mounted within an illustrative electronic device in accordance with an embodiment of the present invention. 
         FIG. 3  is a diagram of illustrative wireless circuitry for an electronic device including a transceiver circuit and antenna coupled by a transmission line path in accordance with an embodiment of the present invention. 
         FIG. 4  is a perspective view of conductive structures forming an illustrative antenna resonating element for a distributed loop antenna in accordance with an embodiment of the present invention. 
         FIG. 5  is a cross-sectional end view of an illustrative distributed loop antenna having a cross-sectional shape with a main body portion and an extending tail portion in accordance with an embodiment of the present invention. 
         FIG. 6  is a graph in which propagation constant has been plotted as a function of cutoff frequency for antenna structures with a tail portion and for antenna structures with a truncated version of the tail in accordance with an embodiment of the present invention. 
         FIG. 7  is a perspective view of an illustrative distributed loop antenna having a main body portion and an extending tail portion in accordance with an embodiment of the present invention. 
         FIG. 8  is a perspective view of a portion of a distributed loop antenna in which a portion of the conductive ground plane structures of the antenna have been removed to form a ground plane recess in the vicinity of a loop-shaped indirect antenna feeding element in accordance with an embodiment of the present invention. 
         FIG. 9  is a top view of a portion of a loop antenna having high band and low band portions of different sizes in accordance with an embodiment of the present invention. 
         FIG. 10  is a top view of a portion of a loop antenna having high band and low band portions of different sizes that are joined by a continuously varying section in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices may be provided with antennas and other wireless communications circuitry. The wireless communications circuitry may be used to support wireless communications in multiple wireless communications bands. One or more antennas may be provided in an electronic device. For example, antennas may be used to form an antenna array to support communications with a communications protocol such as the IEEE 802.11(n) protocol that uses multiple antennas. 
     An illustrative electronic device of the type that may be provided with one or more antennas is shown in  FIG. 1 . Electronic device  10  may be a computer such as a computer that is integrated into a display such as a computer monitor. Electronic device  10  may also be a laptop computer, a tablet computer, a somewhat smaller portable device such as a wrist-watch device, pendant device, headphone device, earpiece device, or other wearable or miniature device, a cellular telephone, a media player, or other electronic equipment. Illustrative configurations in which electronic device  10  is a computer formed from a computer monitor are sometimes described herein as an example. In general, electronic device  10  may be any suitable electronic equipment. 
     Antennas may be formed in device  10  in any suitable location such as location  26 . The antennas in device  10  may include loop antennas, inverted-F antennas, strip antennas, planar inverted-F antennas, slot antennas, cavity antennas, hybrid antennas that include antenna structures of more than one type, or other suitable antennas. The antennas may cover cellular network communications bands, wireless local area network communications bands (e.g., the 2.4 and 5 GHz bands associated with protocols such as the Bluetooth® and IEEE 802.11 protocols), and other communications bands. The antennas may support single band and/or multiband operation. For example, the antennas may be dual band antennas that cover the 2.4 and 5 GHz bands. The antennas may also cover more than two bands (e.g., by covering three or more bands or by covering four or more bands). 
     Conductive structures for the antennas may, if desired, be formed from conductive electronic device structures such as conductive housing structures, from conductive structures such as metal traces on plastic carriers, from metal traces in flexible printed circuits and rigid printed circuits, from metal foil supported by dielectric carrier structures, from wires, and from other conductive materials. 
     Device  10  may include a display such as display  18 . Display  18  may be mounted in a housing such as electronic device housing  12 . Housing  12  may be supported using a stand such as stand  14  or other support structure. 
     Housing  12 , which may sometimes be referred to as a case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of these materials. In some situations, parts of housing  12  may be formed from dielectric or other low-conductivity material. In other situations, housing  12  or at least some of the structures that make up housing  12  may be formed from metal elements. 
     Display  18  may be a touch screen that incorporates capacitive touch electrodes or other touch sensor components or may be a display that is not touch sensitive. Display  18  may include image pixels formed from light-emitting diodes (LEDs), organic LEDs (OLEDs), plasma cells, electrophoretic ink elements, electrowetting display elements, liquid crystal display (LCD) components, or other suitable image pixel structures. 
     A display cover layer such as a layer of cover glass or a plastic cover layer may cover the surface of display  18 . Rectangular active region  22  of display  18  may lie within rectangular boundary  24 . Active region  22  may contain an array of image pixels that display images for a user. Active region  22  may be surrounded by an inactive peripheral region such as rectangular ring-shaped inactive region  20 . The inactive portions of display  18  such as inactive region  20  are devoid of active image pixels. Display driver circuits, antennas (e.g., antennas in regions such as region  26 ), and other components that do not generate images may be located under inactive region  20 . 
     The cover layer for display  18  may cover both active region  22  and inactive region  20 . The inner surface of the cover layer in inactive region  20  may be coated with a layer of an opaque masking material such as opaque plastic (e.g., a dark polyester film) or black ink. The opaque masking layer may help hide internal components in device  10  such as antennas, driver circuits, housing structures, mounting structures, and other structures from view. 
     Antennas mounted in region  26  under an inactive portion of the display cover layer may transmit and receive signals through the display cover layer. This allows the antennas to operate, even when some or all of the structures in housing  12  are formed from conductive materials. For example, mounting the antenna structures of device  10  in region  26  under part of inactive region  20  may allow the antennas to operate even in arrangements in which some or all of the walls of housing  12  are formed from a metal such as aluminum or stainless steel (as examples). 
     A cross-sectional side view of an illustrative antenna mounted in an electronic device such as device  10  of  FIG. 1  is shown in  FIG. 2 . As shown in  FIG. 2 , display  18  may be covered with display cover layer  100  (e.g., a layer of cover glass or plastic). Opaque masking layer  102  may be formed on the underside of display cover layer  100  to cover internal device components such as antenna  28  from view. Display module  104 , which may contain an array of active image pixels, may be used to generate images in active region  22 . 
     Internal device components such as display module  104 , conductive foam  106 , integrated circuits, discrete components such as resistors, capacitors, and inductors, connectors, sensors, audio components such as microphones and speakers, components mounted on one or more printed circuits, other electronic equipment, and other structures in device  10  may, in combination with portions of housing  12  such as the curved sidewalls of housing  12  that are shown in  FIG. 2  or other device structures, create a volume within which antenna  28  may be mounted. The volume may have conductive interior surfaces such as metal portions of display module  104 , conductive foam  106 , and conductive sidewall  12 . The volume may have a main portion that receives main antenna body  28 M and a thinner extended portion that receives extended tail portion  28 T of antenna  28 . The inclusion of tail portion  28 T may allow antenna  28  to be mounted within compact interior locations within device  10  such as locations in which extended tail portion  28 T is interposed between display  18  (e.g., display module  104 ) and housing  12  (e.g., a metal housing wall), while exhibiting a satisfactory efficiency bandwidth at desired operating frequencies. 
     One or more antennas such as antenna  28  may be mounted within housing  12 . In the illustrative configuration of  FIG. 2 , antenna  28  has a shape that allows antenna  28  to fit within the confines of housing  12  and surrounding device components so that main portion  28 M is located under black ink  102  and inactive region  20  (e.g., region  26  of  FIG. 1 ) of display  18  and so that tail portion  28 T is located between display module  104  and housing  12 , under module  104  in active region  22  of display  18 . Other suitable mounting locations in device  10  include positions behind dielectric antenna windows, etc. In configurations in which device  10  uses a curved housing sidewall shape or other shapes that provide relatively small amounts of interior volume for antenna  28 , the shape of antenna  28  may be adjusted accordingly (e.g., so that the antenna has a cross-sectional outline that lies within the small interior volume). If desired, device  10  may have sidewalls that are not curved (e.g., planar and perpendicular sidewall structures). 
     Antenna  28  may, if desired, include metal or other conductive material such as conductive structures  52 . Conductive structures  52  may be supported by support structures  58 . Support structures  58  may be formed from a dielectric such as plastic, glass, or ceramic, and may, if desired, have one or more air-filled interior cavities such as air-filled chamber  108 . Conductive structures  52  may form a loop around axis  40  (i.e., an axis that runs parallel to the Z-axis of  FIG. 2 ). A gap such as gap  50  may be interposed within the loop. Gap  50  may be straight or may have other shapes (e.g., gap  50  may follow a meandering path). Discrete capacitors and other components may, if desired, bridge gap  50 . 
     The conductive loop formed from structures  52  may form a loop antenna resonating element for antenna  28  (i.e., antenna  28  may be a loop antenna). The loop antenna resonating element may be fed directly or indirectly. As shown in  FIG. 3 , for example, loop antenna resonating element L 2  may be indirectly fed using loop-shaped indirect feed structure L 1 . Feed structure L 1  may be formed from a loop of conductive material (loop-shaped conductor  56 ). As illustrated by electromagnetic fields  54  of  FIG. 3 , feed structure L 1  and loop-shaped antenna resonating element L 2  may be coupled using near-field electromagnetic coupling. If desired, loop-shaped antenna resonating element L 2  may be directly feed. 
     Wireless circuitry  38  for electronic device  10  may include radio-frequency transceiver circuitry  36  (e.g., one or more receivers, one or more transmitters, etc.). One or more antennas such as antenna  28  may be used in device  10 . Each antenna  28  may be coupled to transceiver circuitry  36  using a radio-frequency communications path such as transmission line  34 . Transmission line  34  may include one or more portions of transmission lines such as coaxial cable transmission lines, microstrip transmission lines, stripline transmission lines, edge coupled microstrip transmission lines, edge coupled stripline transmission lines, or other suitable transmission line structures. Transmission line  34  may include one or more portions of different types of transmission line structures (e.g., a segment of coaxial cable, a segment of a microstrip transmission line formed on a printed circuit board, etc.). Transmission line  34  may contain a positive conductor (+) and a ground conductor (−). The conductors in transmission line may be formed from wires, braided wires, strips of metal, conductive traces on substrates, planar metal structures, housing structures, or other conductive structures. 
     In the illustrative configuration of  FIG. 3  in which the (+) and (−) terminals of transmission line  34  are coupled to feed structure L 1 , antenna resonating element L 2  may be indirectly fed. If desired, antenna resonating element L 2  may be directly fed by coupling transmission line  34  across pairs of terminals in element L 2  such as terminals T 1  and T 2  or terminals T 3  and t 4  (as examples). Indirect feeding arrangements for loop antenna  28  may sometimes be described herein as an example. This is, however, merely illustrative. In general, any suitable feeding arrangement may be used for feeding antenna  28  if desired. 
     Loop antenna  28  may be formed using conductive antenna resonating element structures such as metal traces  52  on a dielectric carrier such as a plastic support structure (e.g., support structures  58 ). If desired, the conductive structures such as structures  52  that form loop antenna  28  may include wires, metal foil, conductive traces on printed circuit boards, portions of conductive housing structures such as conductive housing walls and conductive internal frame structures, and other conductive structures. 
     Loop antenna  28  may have resonating element conductive structures that are spread out (“distributed”) along the longitudinal axis of loop L 2 . Loop antenna  28  may therefore sometimes be referred to as a distributed loop antenna. As shown in  FIG. 4 , antenna resonating element L 2  may have a longitudinal axis such as axis  40 . Axis  40  may extend parallel to dimension Z of  FIG. 4 , so dimension Z may sometimes be referred to as the longitudinal axis of loop antenna  28 . 
     Conductive structures  52  in resonating element loop L 2  of antenna  28  may include a sheet of conductor that has a first dimension that is wrapped around longitudinal axis  40  and a second dimension ZD that extends along the length of longitudinal axis  40 , as shown in  FIG. 4 . Conductive structures  52  may wrap around axis  40 . During operation, antenna currents can flow within sheet  52  around axis  40 . In effect, sheet  52  forms a wide strip of conductor in the shape of a loop that is characterized by a perimeter P (see, e.g., perimeter P of  FIG. 3 ). The antenna currents flowing in sheet  52  tend to lie in planes parallel to the X-Y plane of  FIG. 4 , as indicated by arrows  44 . As a result, the “loop” of loop antenna  28  effectively lies in the X-Y plane, whereas the longitudinal axis  40  that runs along the center of the wrapped conductive sheet (sheet  52 ) lies parallel to the Z-axis (and perpendicular to the X-Y plane of the antenna loop). In a typical installation arrangement, longitudinal axis  40  of antenna  28  may extend parallel to an adjacent edge of housing  12  in electronic device  10 . 
     It may be desirable to form antenna  28  from conductive structures that exhibit a relatively small dimension P. In a loop without any break along periphery P, the antenna may resonate at signal frequencies where the signal has a wavelength approximately equal to P. In compact structures with unbroken loop shapes, the frequency of the communications band covered by antenna  28  may therefore tend to be high. By incorporating a gap or other structure into the loop, a capacitance C can be introduced into antenna  28 . With the presence of a capacitance within the perimeter of the loop antenna, the resonant frequency of the antenna may be reduced to a desired frequency of operation without enlarging the perimeter. 
     Any suitable structure may be used to interpose a capacitance within the loop of conductor formed by conductive sheet  52 . For example, one or more gaps such as gap  50  of  FIGS. 2 ,  3 , and  4  may be formed. Gap  50  may be filled with dielectric (e.g., a solid dielectric such as plastic, etc. or a dielectric such as air). The gap width GW of gap  50  may affect the value of the capacitance formed by gap  50  (e.g., the capacitance of the gap may tend to increase as gap width GW is decreased). To avoid creating a situation in which the upper portions of sheet  52  are effectively shorted to the lower portions of sheet  52 , antenna resonating element L 2  (e.g., the tail portion of antenna resonating element L 2  associated with antenna tail portion  28 T) may be characterized by a minimum separation MN between the upper portion of sheet  52  and the lower portion of sheet  52 , as shown in  FIG. 4 . The magnitude of MN (and therefore the minimum thickness of the tail portion of the antenna) may be, for example, 1 mm, 1.5 mm, 2 mm, or other suitable thickness. The main body portion of antenna  28  may be 3 mm or more, 4 mm or more, 6 mm or more, less than 1 cm, or 1 cm or more (as examples). 
     Conductive sheet  52  may be formed by metal traces on a dielectric carrier, metal on a wrapped flex circuit, metal foil that has been bent into a desired shape, or other suitable conductive structures. In the example of  FIG. 4 , metal sheet  52  has a constant dimension ZD as sheet  52  wraps around axis  40 . If desired, metal layer  52  may have a dimension ZD parallel to longitudinal antenna axis  40  that varies as a function of position around axis  40  (i.e., ZD need not be constant at all portions of the loop antenna). The  FIG. 4  arrangement is merely illustrative. 
     The size and shape of the conductive structures in antenna  28  influence the frequency response of antenna  28 . In some frequencies of operation such as the high band frequencies associated with dual band IEEE 802.11(n) signals, there is a potential for loop antenna resonating element L 2  to support undesired waveguide modes that can consume power and thereby decrease high-band efficiency. In resonating element configurations such as resonating element L 2  of  FIG. 4 , additional waveguide modes will be supported if width W of resonating element L 2  (i.e., the larger of the two lateral dimensions that are perpendicular to axis  40 ) is excessive. 
     The potential for resonating element L 2  to support waveguide modes as a function of various sizes of width W is illustrated in connection with  FIGS. 5 and 6 .  FIG. 5  is a cross-sectional side view of a configuration for loop-based antenna resonating element L 2  in which element L 2  has a width W 0 . The thickness (i.e., the vertical dimension perpendicular to width W 0  and the Z axis) of main portion  28 M is greater than the thickness of extended tail portion  28 T, but both may be considered to form portions of a common cavity with the potential to support waveguide modes if sufficiently wide. As illustrated by line  120 , which represents electric field strength as a function of position within the antenna, element L 2  of length W 0  may support a TE10 waveguide mode (i.e., a first order transverse electric mode). In this configuration, some of the radio-frequency energy associated with the antenna will not be radiated, but rather will propagate within the waveguide structure formed by conductive structures  52 , thereby reducing antenna efficiency. 
     To minimize efficiency losses due to waveguide modes, the size of antenna resonating element L 2  may be shortened to width W 1  by removing structures  122  in the portion of antenna resonating element L 2  that is associated with tail portion  28 T. 
       FIG. 6  is a graph in which waveguide propagation constant for the structures of  FIG. 5  have been plotted as a function of operating frequency f. When the propagation constant is real, radio-frequency signals can propagate (i.e., element L 2  will support undesired waveguide modes). When the propagation constant is imaginary, radio-frequency signals are attenuated (and antenna performance will tend not to be degraded by the undesired support of waveguide modes because waveguide modes will be cut off). Curve  122  is associated with the TE10 waveguide mode performance of element L 2  of  FIG. 5  in a configuration in which L 2  has a width of W 0 . As shown in  FIG. 6 , in this configuration, element L 2  exhibits a cutoff frequency of 4.3 GHz. When operated at a high-band frequency such as 5.0 GHz, the value of the propagation constant will be V 1 . Because V 1  is real, antenna  28  will support the TE10 waveguide mode and the efficiency of antenna  28  will be reduced by the presence of TE10 waveguide mode signals. At frequencies f below cutoff frequency 4.3 GHz, the propagation constant will be imaginary, so low-band performance such as performance at a frequency of 2.4 GHz for the low band of an IEEE 802.11(n) system, may be satisfactory. 
     When performance at a high-band frequency such as 5.0 GHz is desired, it may not be acceptable to use a width of W 0  when forming element L 2 , because this would allow undesired TE10 modes to be supported within element L 2 . By reducing the size of element L 2 , however, the TE10 waveguide cutoff frequency for element L 2  may be shifted to higher frequencies. In particular, reduction in the width of element L 2  to width W 1 , may result in the propagation constant values of curve  124 . As shown in  FIG. 6 , curve  124  may be characterized by a cutoff frequency of 6.7 GHz (as an example). Signals at a frequency of 5.0 GHz (e.g., high-band signals in an 802.11(n) system), may have a propagation constant of V 2 . Because propagation constant V 2  is imaginary, the TE10 mode will be cut off at 5.0 GHz (i.e., antenna efficiency will not be undesirably reduced due to waveguide-type signal propagation in element L 2 ). 
     Higher-order modes such as the TE20 mode will not generally be supported in element L 2  except at very high frequencies. For example, when the width of element L 2  is W 0 , element L 2  may be characterized by a TE20 propagation constant curve such as curve  126 . As shown in  FIG. 6 , curve  126  may have a cutoff frequency of 9.7 GHz (as an example), which is above both low-band and high-band IEEE 802.11(n) bands and other bands typically used in wireless communications (e.g., cellular telephone bands from 700-2100 MHz, the Bluetooth® band at 2.4 GHz, etc.). When the cutoff frequency is 9.7 GHz, element L 2  will tend not to support TE20 waveguide propagation for signals at frequencies less than 9.7 GHz. In a configuration for element L 2  in which the width of element L 2  is W 1 , the TE20 cutoff frequency will be even larger than 9.7 GHz. 
       FIG. 7  is a perspective view of an illustrative configuration that may be used for distributed loop antenna  28 . The arrangement shown in  FIG. 7  include antenna feed structure L 1  and loop antenna resonating element L 2 . As shown in  FIG. 7 , support structures  58  may be covered with patterned conductive structures  52 . Conductive structures  52  may be patterned to form distributed loop antenna resonating element L 2  and loop-shaped antenna feed structure L 1 . If desired, support structures  58  may be hollow (see, e.g., interior cavity  108  of support structures  58  of  FIG. 2 ). In situations in which support structures  58  have an air-filled cavity such as cavity  108 , support structures  58  may have a wall of plastic or other dielectric material that extends around axis  40  under patterned conductive structures  52 . 
     Antenna  28  of  FIG. 7  may support dual band operations (e.g., operations at a low band of 2.4 GHz and a high band of 5.0 GHz, or other suitable low and high communications bands). With a configuration of the type shown in  FIG. 7 , loop antenna resonating element may exhibit a resonance at 2.4 GHz and a second harmonic resonance near 5.0 GHz. Antenna feed element L 1  may tend to exhibit a resonance at 5.0 GHz that helps enhance the performance of L 2  at 5.0 GHz. With this type of configuration, high-band portion HB of antenna  28  may be primarily used in handling high-band signals (e.g., signals in the 5.0 GHz band) and low-band portion LB may be used in handling low band signals (e.g., signals in the 2.4 GHz band) and some high-band signals. 
     Portion  132  of antenna  28  in high band section HB may help couple element L 1  and L 2  (and may therefore help element L 1  serve as a satisfactory indirect feeding structure for antenna  28 ). Nevertheless, excessive conductive material in portion  132  may give rise to a possibility that portion HB of antenna  28  will support undesired waveguide modes that could reduce antenna efficiency. As shown in  FIG. 8 , this possibility can be reduced by removing some of conductive structures  52  in the vicinity of element L 1 . For example, portions of conductive structures  132  in region  128  near ground plane portion  130  of structures  52  may be removed from antenna  28  to produce a recessed ground plane portion  130 . The presence of the recess in ground plane  130  may increase the cutoff frequency for signals in high band portion HB and reduce the likelihood of supporting undesired waveguide modes in high band portion HB. 
     Another way to ensure that waveguide modes are cut off effectively at operating frequencies of interest in the high band portion of antenna  28  involves locally changing the dimension (width) W of antenna  28  in the low band and high band portions of antenna  28 . As shown in the top view of the illustrative configuration of  FIG. 9 , for example, antenna  28  may be provided with a low band portion that has a larger width (WLB) and a high band portion that has a smaller width (WHB&lt;WLB). The smaller size of WHB relative to WLB may help cut off waveguide modes in high band portion HB of antenna  28 . In the  FIG. 10  example, there is a gradual change in width between the low band and high band portions of antenna  28 . The arrangements of  FIGS. 9 and 10 , other configurations that selectively vary the size (e.g., the lateral width dimension perpendicular to axis  40  and/or the lateral thickness dimension perpendicular to axis  40 ) of antenna  28  in the vicinities of elements L 1  and L 2  may be used with or without using localized metal removal arrangements such as the arrangement of  FIG. 8 . 
     Another way in which to ensure that waveguide modes are cut off at desired operating frequencies involves control of the effective dielectric constant ∈ r  of the environment surrounding conductive structures  52  (e.g., the effective dielectric constant of support structures  58  and cavity  108 ). The cutoff frequency f cutoffTE10  for the TE10 waveguide mode in antenna  28  may be given by equation 1.
 
 f   cutoffTE10 =[2 W (μ c ∈ o ∈ r ) 1/2 ] −1   (1)
 
In equation 1, W is the width of antenna structure  28  (i.e., in a configuration in which the width W is larger than the thickness of antenna structures  28 ), μ o  is the permeability of free space, ∈ o  is the permittivity of free space, and ∈ r  is the relative permittivity (sometimes referred to as the dielectric constant) of structure  28  in the vicinity of structures  58 . An example of a material for forming structures  58  is plastic (e.g., PC/ABS, which is a blend of polycarbonate and acrylonitrile butadiene styrene plastics). With this type of dielectric material, the relative permittivity of structures  58  may be about 2.9 (as an example). The permittivity of structures  58  may be decreased by enlarging cavity  108  and may be decreased by decreasing cavity  108 . Different materials and support structure shapes may also be used to adjust the relative permittivity of the support structures used in forming antenna  28 . Adjustments to the value of ∈ r  may be made locally (e.g., so that antenna  28  has a lower values of ∈ r  in high band portion HB than in low band portion LB) in combination with making localized adjustments such as localized width adjustments (adjustments to W), and/or adjustments to the amount of metal near element L 1  (e.g., to remove and/or include metal in region  128  of  FIG. 8 ).
 
     An example of a size for W that may be used in antenna  28  to support operation at 2.4 GHz is 20 mm. This value may be too large for maximized efficiency when operating at 5.0 GHz. To ensure that antenna  28  operates satisfactorily, it may therefore be desirable to reduce ∈ r , remove metal from portion  128  of structures  52  (as described in connection with  FIG. 8 ), and/or to reduce W in region HB of antenna  28 . 
     The use of a relatively large perimeter value P for antenna  28  may allow the value of C to be decreased (for a given efficiency). The ability of C to be decreased may allow the width GW ( FIG. 4 ) of gap  50  to be increased. Gap  50  may serve as an aperture for antenna  28 , so larger gap sizes GW may help improve efficiency and bandwidth for antenna  28 . 
     There is generally a tradeoff between low band and high band performance. For satisfactory low band performance, larger values of perimeter P may be desirable to allow smaller C values and larger GW values. For satisfactory high band performance, excessively large W values (helpful for enlarging P) cannot be used without giving rise to a risk of supporting undesired waveguide modes that can consume power and decrease high band efficiency. 
     One possible design for antenna  28  involves the use of a compromise size for W. As an example, a value of W of about 15 mm may be sufficient to ensure that high band waveguide modes are cut off, without decreasing perimeter P excessively. Antenna width W may have other values if desired (e.g., greater than 10 mm, greater than 12 mm, 12-19 mm, etc.). 
     The need to compromise on design parameters such as width W may be minimized by using locally varying structures in antenna  28 , such as localized variations in lateral dimension W and/or localized variations in ∈ r  (by locally varying the composition of the material used in forming structures  58 , and/or by locally varying the size of chamber  108 —e.g., by enlarging the size of chamber  108  in high band region HB so that structures  58  are thinner in region HB than in region LB). Localized changes such as removing metal from region  128  in high band portion HB of antenna  28  may also be used. 
     By changing ∈ r , W, and other antenna attributes as a function of length along axis  40  of antenna  28 , efficiency can be maximized. Tail portion  28 T of antenna  28  may protrude under components such as display module  104 , thereby allowing perimeter P to be relatively large and allowing gap width GW to be relatively large, even when antenna  28  is installed in a device such as device  10  of  FIG. 1  in which interior volume for mounting antennas is scarce. At the same time, localized changes in antenna  28  and/or appropriate selection of attributes (e.g., ∈ r , W, etc.) may help ensure that undesired waveguide modes are not supported so as to enhance antenna efficiency. 
     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: 20111117
Publication Date: 20151103
Grant Date: 20151103
Priority Date: 20111117
Inventors: ZHU JIANG
GUTERMAN JERZY
PASCOLINI MATTIA
SCHLUB ROBERT W.
NATH JAYESH
AYALA VAZQUEZ ENRIQUE
HAYLOCK JONATHAN
SHIU BOON W.
CABALLERO RUBEN
Assignee: APPLE INC
CPC Classifications: [{"code": "H01Q7/00", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01Q1/2266", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01Q1/2266", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01Q7/00", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 48426249