Patent Publication Number: US-8125394-B2

Title: Electronic device antenna with quartered rectangular cavity

Description:
BACKGROUND 
     This invention relates to electronic devices and, more particularly, to antennas for electronic devices. 
     Portable computers and other electronic devices often use wireless communications circuitry. For example, wireless communications circuitry may be used to communicate with local area networks and remote base stations. 
     Wireless computer communications systems use antennas. It can be difficult to design antennas that perform satisfactorily in electronic devices such as portable computers. It is generally desirable to create efficient antennas. For example, efficient antennas are desirable for portable computers, because efficient antennas help conserve battery power during wireless operations. However, optimum antenna efficiency can be difficult to obtain, because portable computer designs restrict the possible locations for implementing the antennas and require that the antennas be constructed as small light-weight structures. For example, it can be difficult to implement efficient antennas in portable computers that contain conductive housing structures, because the conductive housing structures can block radio-frequency signals and thereby reduce the effectiveness of the antennas. 
     It would therefore be desirable to be able to provide improved antenna arrangements for electronic devices such as portable computers. 
     SUMMARY 
     An antenna for an electronic device such as a portable computer is provided. The antenna may use a cavity-backed configuration in which conductive cavity walls are placed in the vicinity of an antenna feed structure. The cavity walls may form a cavity structure that resembles a quartered rectangular cavity. The quartered cavity may be mounted within an electronic device. For example, the quartered cavity may be mounted in the corner of a portable computer housing or other electronic device housing. 
     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 portable computer in which an antenna may be implemented in accordance with an embodiment of the present invention. 
         FIG. 2A  is a diagram of a three-dimensional conductive cavity bisected by a vertical plane. 
         FIG. 2B  is a diagram showing how electric and magnetic fields may be distributed on the vertical bisecting plane in the cavity of  FIG. 2A . 
         FIG. 2C  is a diagram of the three-dimensional conductive cavity of  FIG. 2A  bisected by a horizontal plane. 
         FIG. 2D  is a diagram showing how electric and magnetic fields may be distributed on the horizontal bisecting plane of the cavity of  FIG. 2A . 
         FIG. 3A  is a diagram of a quartered rectangular conductive cavity of the type that may be used in forming a cavity-backed antenna in accordance with an embodiment of the present invention. 
         FIG. 3B  is a diagram of the X-Y plane associated with the quartered cavity of  FIG. 3A  showing how electric and magnetic fields may be distributed over the front vertical opening in the cavity antenna structure of  FIG. 3A  in accordance with an embodiment of the present invention. 
         FIG. 3C  is a diagram of the X-Z plane associated with the quartered cavity of  FIG. 3A  showing how electric and magnetic fields may be distributed over the left vertical opening in the cavity antenna structure of  FIG. 3A  in accordance with an embodiment of the present invention. 
         FIG. 4A  is a perspective view of a quartered cavity of the type shown in  FIG. 3A  with a reduced height that narrows the cavity openings in accordance with an embodiment of the present invention. 
         FIG. 4B  is a diagram of the Y-Z plane associated with the shortened quartered cavity of  FIG. 4A  showing how electric and magnetic fields may be distributed at the upper and lower conductive cavity faces in the cavity antenna structure of  FIG. 4A  in accordance with an embodiment of the present invention. 
         FIG. 4C  is a diagram of the X-Y plane associated with the shortened quartered cavity of  FIG. 4A  showing how electric and magnetic fields may be distributed at the front face opening in the cavity antenna structure of  FIG. 4A  in accordance with an embodiment of the present invention. 
         FIG. 4D  is a diagram of the X-Z plane associated with the shortened quartered cavity of  FIG. 4A  showing how electric and magnetic fields may be distributed at the left face opening in the cavity antenna structure of  FIG. 4A  in accordance with an embodiment of the present invention. 
         FIG. 5  is a graph of the calculated radiation efficiency of an antenna of the type shown in  FIG. 4A  as a function of operating frequency in accordance with an embodiment of the present invention. 
         FIG. 6  is a perspective view of a corner portion of an electronic device such as a portable computer that includes a cavity antenna in accordance with an embodiment of the present invention. 
         FIG. 7  is a perspective view of a portion of a cavity antenna structure having illustrative feed and impedance matching components in accordance with an embodiment of the present invention. 
         FIG. 8  is a cross-sectional side view of an antenna probe structure based on a meandering conductive element that may be used to feed a quartered rectangular antenna cavity in accordance with an embodiment of the present invention. 
         FIG. 9  is a perspective view of an antenna probe structure based on a spiral conductive element that may be used to feed a quartered rectangular antenna cavity in accordance with an embodiment of the present invention. 
         FIG. 10  is a cross-sectional side view of a dielectric-filled cavity-backed antenna structure that operates through a narrowed dielectric gasket window and that is formed as an integral portion of the housing of an electronic device such as a portable computer in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to antenna structures for electronic devices. The antennas may be used to convey wireless signals for suitable communications links. For example, an electronic device antenna may be used to handle communications for a short-range link such as an IEEE 802.11 link (sometimes referred to as WiFi®) or a Bluetooth® link. An electronic device antenna may also handle communications for long-range links such as cellular telephone voice and data links. 
     Antennas such as these may be used in various electronic devices. For example, an antenna 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. With one suitable arrangement, which is sometimes described herein as an example, the electronic devices in which the antennas are provided may be portable computers such as laptop (notebook) computers. This is, however, merely illustrative. Antennas may, in general, be provided in any suitable electronic device. 
     An illustrative electronic device such as a portable computer in which an antenna may be provided is shown in  FIG. 1 . As shown in  FIG. 1 , portable computer  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 have a main structural support member that is formed from a solid block of machined aluminum or other suitable metal. Multipart housings may be used in which two or more individual housing structures are combined to form housing  12 . The structures in housing  12  may include internal frame members, external coverings such as sheets of metal, etc. Housing  12  and its associated components may, in general, be formed from any suitable materials such as such as plastic, ceramics, metal, glass, etc. An advantage of forming housing  12  at least partly from metal is that metal is durable and attractive in appearance. Metals such as aluminum may be anodized to form an insulating oxide coating. 
     Case  12  may have an upper portion  26  and a lower portion  28 . Lower portion  28  may be referred to as the base unit housing or main unit of computer  10  and may contain components such as a hard disk drive, battery, and main logic board. Upper portion  26 , which is sometimes referred to as a cover or lid, may rotate relative to lower portion  28  about rotational axis  16 . Portion  18  of computer  10  may contain a hinge and associated clutch structures and may sometimes be referred to as a clutch barrel. 
     Lower housing portion  28  may have an opening such as slot  22  through which optical disks may be loaded into an optical disk drive. Lower housing portion  28  may also have touchpad  24 , keys  20 , and other input-output components. Touch pad  24  may include a touch sensitive surface that allows a user of computer  10  to control computer  10  using touch-based commands (gestures). A portion of touchpad  24  may be depressed by the user when the user desires to “click” on a displayed item on screen  14 . If desired, additional components may be mounted to upper and lower housing portions  26  and  28 . For example, upper and lower housing portions  26  and  28  may have ports to which cables can be connected (e.g., universal serial bus ports, an Ethernet port, a Firewire port, audio jacks, card slots, etc.). Buttons and other controls may also be mounted to housing  12 . 
     If desired, upper and lower housing portions  26  and  28  may have transparent windows through which light may be emitted from light-emitting diodes. Openings such as perforated speaker openings  30  may also be formed in the surface of housing  12  to allow sound to pass through the walls of the housing. 
     A display such as display  14  may be mounted within upper housing portion  26 . Display  14  may be, for example, a liquid crystal display (LCD), organic light emitting diode (OLED) display, or plasma display (as examples). A glass panel may be mounted in front of display  14 . The glass panel may help add structural integrity to computer  10 . For example, the glass panel may make upper housing portion  26  more rigid and may protect display  14  from damage due to contact with keys or other structures. 
     Portable computer  10  may contain circuitry  32 . Circuitry  32  may include storage and processing circuitry  32 A and input-output circuitry  32 B. 
     Storage and processing circuitry  32 A may include one or more different types of storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory), volatile memory (e.g., static or dynamic random-access-memory), etc. Storage and processing circuitry  32 A may be used in controlling the operation of computer  10 . Processing circuitry in circuitry  32 A may be based on processors such as microprocessors, microcontrollers, digital signal processors, dedicated processing circuits, power management circuits, audio and video chips, and other suitable integrated circuits. Storage and processing circuitry  32 A may be used to run software on computer  10 , such as operating system software, application software, software for implementing control algorithms, communications protocol software etc. 
     Input-output circuitry  32 B may be used to allow data to be supplied to computer  10  and to allow data to be provided from computer  10  to external devices. Examples of input-output devices that may be used in computer  10  include display screens such as touch screens (e.g., liquid crystal displays or organic light-emitting diode displays), buttons, joysticks, click wheels, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers and other devices for creating sound, cameras, sensors, etc. A user can control the operation of computer  10  by supplying commands through these devices or other suitable input-output circuitry  32 B. Input-output circuitry  32 B may also be used to convey visual or sonic information to the user of computer  10 . Input-output circuitry  32 B may include connectors for forming data ports (e.g., for attaching external equipment such as accessories, etc.). 
     Computer  10  may include one or more antennas. For example, computer  10  may include one or more cavity antennas that are located at the corners of housing  12  such as corner  36  and/or corner  34  (as examples). Computer  10  may also include one or more additional antennas. The antennas in computer  10  may be coupled to wireless communications circuitry (e.g., radio-frequency transceiver circuits) in input-output circuitry  32 B using coaxial cables, microstrip transmission lines, or other suitable transmission lines. 
     The antenna structures in computer  10  may be used to handle any suitable communications bands of interest. For example, antennas and wireless communications circuitry in circuitry  32 B of computer  10  may be used to handle cellular telephone communications in one or more frequency bands and data communications in one or more communications bands. Typical data communications bands that may be handled by the wireless communications circuitry in computer  10  include the 2.4 GHz band that is sometimes used for Wi-Fi® (IEEE 802.11) and Bluetooth® communications, the 5 GHz band that is sometimes used for Wi-Fi communications, the 1575 MHz Global Positioning System band, and 2G and 3G cellular telephone bands. These bands may be covered using single-band and multiband antennas. For example, cellular telephone communications can be handled using a multiband cellular telephone antenna. A single band antenna may be provided to handle Bluetooth® communications. Computer  10  may, as an example, include a multiband antenna that handles local area network data communications at 2.4 GHz and 5 GHz (e.g., for IEEE 802.11 communications), a single band antenna that handles 2.4 GHz IEEE 802.11 communications and/or 2.4 GHz Bluetooth® communications, or a single band or multiband antenna that handles other communications frequencies of interest. These are merely examples. Any suitable antenna structures may be used by computer  10  or other electronic device to cover communications bands of interest. 
     The antennas in computer  10  may be implemented using any suitable antenna configuration. For example, an antenna for computer  10  may be implemented as a cavity antenna, a monopole antenna, a dipole antenna, a patch antenna, an inverted-F antenna, an L-shaped antenna, a planar inverted-F antenna (PIFA), a slot antenna, a helical antenna, a hybrid antenna including two or more of these antenna structures, or any other suitable antenna structures. 
     With one suitable arrangement, which is described herein as an example, at least one of the antennas used in computer  10  is implemented using a cavity antenna arrangement. With this type of configuration, a conductive cavity is formed from conductive materials such as metal. The cavity may have top and bottom surfaces (sometimes referred to as walls) and sidewalls. Unlike a completely enclosed conductive cavity, which is unable to radiate and serve as an antenna, the antenna cavity for computer  10  may use cavities from which some of the sidewall structures have been removed to form openings. The openings in the cavity antenna may be filled with a gaseous dielectric such as air or a non-gaseous dielectric. An example of a non-gaseous dielectric is a solid such as plastic or epoxy. If desired, materials such as flexible printed circuit board materials (e.g., polyimide) and rigid printed circuit board materials (e.g., fiberglass-filled epoxy) may be used in the cavity antenna. 
     An advantage of filling a cavity antenna with a solid dielectric material is that this may help prevent intrusion of dust, liquids, or other foreign matter into portions of the antenna. Dielectric in the cavity antenna may also be used as a support structure (e.g., when supporting a flex circuit antenna element or a portion of a housing). Dielectric materials are transparent to radio-frequency signals, so dielectric materials may be used in portions of the cavity antenna where it is desired not to block radio-frequency signals. 
     In general, any suitable dielectric material can be used to form dielectric cavity antenna structures for computer  10 . Dielectric structures that surround or are located within the cavity of a cavity antenna may be formed from a completely solid dielectric, a porous dielectric, a foam dielectric, a gelatinous dielectric (e.g., a coagulated or viscous liquid), a dielectric with grooves or pores, a dielectric having a honeycombed or lattice structure, a dielectric having spherical voids or other voids, a combination of such non-gaseous dielectrics, etc. Hollow features in solid dielectrics may be filled with air or other gases or lower dielectric constant materials. Examples of dielectric materials that may be used in a cavity antenna and that contain voids include epoxy with gas bubbles, epoxy with hollow or low-dielectric-constant microspheres or other void-forming structures, polyimide with gas bubbles or microspheres, etc. Porous dielectric materials used in a cavity antenna in computer  10  can be formed with a closed cell structure (e.g., with isolated voids) or with an open cell structure (e.g., a fibrous structure with interconnected voids). Foams such as foaming glues (e.g., polyurethane adhesive), pieces of expanded polystyrene foam, extruded polystyrene foam, foam rubber, or other manufactured foams can also be used in a cavity antenna in computer  10 . If desired, the dielectric antenna materials can include layers or mixtures of different substances such as mixtures including small bodies of lower density material. 
     The conductive antenna elements that form the sidewalls and other portions of a cavity antenna may be formed from conductive portions of housing  12 , conductive sheets such as planar metal sheets, wires, traces on rigid printed circuit boards or flex circuit substrates, stamped metal foil patterns, milled or cast metal parts, or any other suitable conductive structures. 
     The operation of a quarter-cavity antenna may be understood with reference to  FIGS. 2 ,  3 ,  4 , and  5 . 
       FIG. 2A  is a diagram of a rectangular cavity with six conductive walls. Cavity  38  of  FIG. 2A  has dimensions of LX, LY, and LZ. Vertical plane  42  bisects rectangular cavity  38 , but does not correspond to any conductive structures. 
     Solid line  44  of  FIG. 2A  illustrates the location of an excitation source for cavity  38  (i.e., a probe that launches electromagnetic fields into cavity  38 ). Because each of walls  40  in cavity  38  is conductive, cavity  38  and probe  44  cannot function as an antenna. Nevertheless, electromagnetic radiation from probe  44  may fill cavity  38 . Because of the conductive nature of walls  40 , walls  40  impose boundary conditions for electromagnetic fields in cavity  38 .  FIG. 2B , which corresponds to vertical plane  42  of  FIG. 2A , shows how electric field vectors E are oriented parallel to the X-axis and magnetic field vectors H are oriented perpendicularly, parallel to the Z-axis (for the TE 011  mode). The electric field strength is greatest around the center of the cavity (Z=LZ/2) and is essentially zero at the cavity walls (i.e., E=0 at Z=0 and at Z=LZ). 
     Another cross-sectional view of cavity  38  may be taken along horizontal bisecting plane  46  of  FIG. 2C . The electric and magnetic fields associated with plane  46  are shown in  FIG. 2D . 
     When divided into quarters, a non-radiating cavity such as cavity  38  of  FIGS. 2A and 2C  may be converted into a radiating cavity structure of the type that may be used in forming a cavity antenna for computer  10 . An illustrative cavity antenna that has been formed from a quartered rectangular cavity in this way is shown in  FIG. 3A . As shown in  FIG. 3A , cavity antenna  48  may be fed by a probe structure  50  (sometimes referred to as an antenna feed or feed structure). In the  FIG. 3A  configuration, probe  50  is aligned with the front left corner of cavity antenna  48  and runs along the X axis. The top surface  54 , bottom surface  54 , right sidewall  54 , and rear sidewall  54  of cavity antenna  48  are conductive. Because cavity  48  of  FIG. 3A  is formed from a quarter of cavity  38  of  FIG. 2A  and  FIG. 2C , the left and front sidewalls of cavity  48  are open and form openings  52  (e.g., open planar faces filled with air or another dielectric). Planar openings  52  meet at a right angle along the X-axis. 
     The dimensions of cavity antenna  48  are LX/2, LY/2, and LZ/2 (i.e., half of the dimensions of cavity  48  of  FIGS. 2A and 2C ).  FIGS. 3B and 3C  show how the electric field vectors and magnetic field vectors in cavity  48  are oriented within the X-Y plane at Z=0 and within the X-Z plane at Y=0, respectively (for the TE 011  mode). The magnitude of the electric field is greatest near where Y=0 ( FIG. 3B ) and near where Z=0 ( FIG. 3C ) and is lowest at Y=LY/2 and at Z=LZ/2. 
     The dimensions LZ/2 and LY/2 preferably correspond to approximately a quarter of a wavelength at the operating frequency of interest (i.e., LZ/2 and LY/2 may each be equal to λg/4, where λg corresponds to the wavelength of the radio-frequency antenna signals within antenna cavity  48 ). It is not necessary for vertical cavity dimension LX/2 to be as large as lateral cavity dimensions LZ/2 and LY/2, because the electric field E is oriented perpendicular to the Z-Y plane. 
     This is illustrated in  FIGS. 4A ,  4 B,  4 C, and  4 D.  FIG. 4A  shows how dimension LX/2 may be reduced to a size that is significantly smaller than lateral dimension LY/2 and lateral dimension LZ/2 (e.g., less than this lateral dimension, less than half of this lateral dimension, less than a quarter of this lateral dimension, less than a tenth of this lateral dimension, etc.). At an operating frequency of about 2.4 GHz, LZ/2 and LY/2 may, as an example, be about 20-40 mm, whereas cavity height LX/2 may, as an example, be several millimeters (e.g., 1-5 mm or 2-20 mm, etc.). 
     The ability to configure the dimensions of the cavity for cavity antenna  48  so that cavity antenna  48  is relatively short and wide, allows cavity antenna  48  to be mounted within housings that are relatively thin. For example, a relatively thin cavity antenna such as cavity antenna  48  of  FIG. 4A  may be mounted at a corner of housing  12  such as corner  34  or corner  36  of  FIG. 1 . In the vicinity of housing corners such as corners  34  and  36 , it may be desirable for housing  12  to be relatively thin. The use of thin cavity antennas in housing  12  may allow the corners of housing  12  to be reduced in thickness. 
     A graph showing the radiation efficiency of a cavity antenna (e.g., a cavity antenna such as cavity antenna  48  of  FIG. 4A ) as a function of operating frequency is shown in  FIG. 5 . In the  FIG. 5  example, cavity antenna  48  has been configured for operation in the 2.4 GHz communications band and achieves a maximum efficiency of about 10%. 
       FIG. 6  shows a perspective view of an illustrative cavity antenna such as antenna  48  of  FIGS. 4A  that has been formed at the corner of housing  12  of portable computer  10 . Antenna  48  of  FIG. 6  may be formed in a corner of a computer housing lid, in a corner of a computer housing base unit, or within other suitable structures in computer  10 . Feed  50  may be formed from a metal wire or other suitable probe structure. Positive and ground antenna feed terminals (e.g., terminals associated with a coaxial cable or other transmission line) may be used in feeding probe  50  and antenna  48 . 
     When the wire or other conductive structure that makes up probe  50  is short, probe  50  will tend to have a relatively small real component to its impedance and will tend to have a negative (capacitive) imaginary impedance component. For satisfactory impedance matching between the antenna transmission line and cavity antenna  48 , it may be desirable to enhance the impedance of probe  50  (e.g., by adding an inductive characteristic to probe  50  through its construction and/or by adding other impedance matching network components to antenna  48 ). The addition of an inductive component to the impedance of probe  50  may help to counterbalance the capacitive nature of a short probe structure and may thereby facilitate transmission line impedance matching. 
       FIG. 7  shows an illustrative arrangement for probe  50 . In the configuration of  FIG. 7 , probe  50  has conductive member  62  (e.g., a coaxial cable center conductor or other conductor). Tip  72  of conductor  62  is loaded with conductive loading structure  70 . Conductive loading structure  70  may be formed from a wire, a planar conductive loading patch, or other conductive structures. The presence of the top-loading provided by loading conductor  70  helps to increase the effective length of conductor  62  without excessively increasing the vertical height of cavity  48  (i.e., the X dimension LX/2 of cavity  48 ). 
     As shown schematically in  FIG. 7 , a matching network such as matching network  64  may be used to help match the impedance of probe  50  and cavity antenna  48  to transmission line  56 . Matching network  64  may be based on a segment of a transmission line, or circuit components such as inductors, capacitors, and resistors, and may be connected in a shunt configuration (e.g., across terminals  66  and  68  as shown in  FIG. 7 ), in series with probe conductor  62 , or in a configuration in which some of the matching network components are connected in series and some of the matching network components are connected in a shunt configuration. Impedance matching structures for impedance matching network  64  may, if desired, be formed from the structures that make up cavity  48  (e.g., conductive and dielectric structures such as conductor  62 , loading structure  70 , cavity sidewalls, dielectric loading material, etc.). Impedance matching structures such as these may be combined with impedance matching circuits formed from circuit components such as capacitors, inductors, and resistors, if desired. 
     As shown in  FIG. 7 , coaxial cable  56  (or other suitable transmission line) may be coupled to cavity antenna  48  and probe  62  at terminals such as ground antenna feed terminal  58  and positive antenna feed terminal  60 . During radio-frequency signal transmission operations, radio-frequency signals that are provided to cavity antenna  48  via transmission line  56  may be transmitted from cavity antenna  48 . During radio-frequency signal reception operations, radio-frequency signals may be received by antenna  48  and passed to transmission line  56 . Transmission line  56  may be coupled to radio-frequency transceiver circuitry (e.g., circuitry in input-output circuit  32 B of  FIG. 1 ). Some or all of the cavity walls for cavity antenna  48  may be formed by portions of conductive case  12 . 
     If desired, conductive member  62  of probe  50  may be formed from a meandering conductor. As shown in  FIG. 8 , for example, conductor  62  may have bend such as bends  74 . Bends  74  may form right angles (i.e., bends  74  may be perpendicular bends), may form curves in conductive member  62 , or may have other shapes that form a meandering path for probe  50 . When conductive member  62  of probe  50  has a meandering path shape, the height H of probe  50  will be less for a given overall path length than would otherwise be possible. This allows the vertical dimension LX/2 of cavity  48  to be reduced to fit within the tight confines of a portable computer or other electronic device without excessively reducing the length of probe  50 . 
     Some or all of conductive member  62  may be provided with a spiral shape, as shown in  FIG. 9 . The use of a spiral shape for probe  50  may create inductance to offset the capacitive qualities of a short probe length. The spiral shape of conductive member  62  may also help to reduce the height H of probe  50  within the cavity antenna for a given probe length. 
     Cavity antenna  48  may exhibit good radiation efficiency and may therefore be suitable for transmitting and receiving radio-frequency signals that pass through a relatively small gap. As a result, it may be desirable to mount cavity antenna  48  within portable computer  10  in a configuration in which the openings of the cavity antenna transmit and receive radio-frequency signals through an opening in housing  12  (as an example). 
     An illustrative arrangement of this type is shown in  FIG. 10 . As shown in the cross-sectional view of FIG.  10 , cavity antenna  48  may be formed in housing  12  of computer  10 . Housing  12  may have housing portions such as upper portion  26  and lower portion  28 . Gasket  76  may be formed from a dielectric such as a soft elastomeric substance. Gasket  76  may help to cushion housing  12  when upper housing portion  26  (e.g., the computer lid) is closed and bears against lower housing portion  28  (e.g., the computer base). 
     Dielectric  78  may be used to provide dielectric loading for cavity antenna  48 . Dielectric  78  may be formed from any suitable dielectric such as epoxy, polyimide, void-filled solids, etc. Dielectric  78  may fill all or part of the cavity portion of cavity antenna  48 . When dielectric  78  is incorporated into the cavity of cavity antenna  48 , the dimensions of cavity  48  can be reduced for a given operating frequency, due to the dielectric loading provided by the dielectric. 
     Layer  80  may be a conductive layer such as a sheet of metal or may be a dielectric such as a sheet of glass or plastic. The dimensions of cavity  48  may be defined by the shape of housing portions  26  and  28  (e.g., where layer  80  is dielectric) or may be defined by the shape of housing portion  26  (on the top) and layer  80  (on the bottom). 
     Consider, as an example, the situation in which structures  26  and  80  are conductive and are filled with a nongaseous dielectric  78 . Cavity antenna  48  may be located at a corner of housing  12  and may be fed using a probe such as probe  50  of  FIGS. 7 ,  8 , and  9  or other suitable probe structure that serves as an antenna feed. Gasket  76  may cover openings  52  (e.g., the left and front planar open faces of the cavity of  FIG. 3A ). During operation, radio-frequency signals may be transmitted and received through gap  82  and the dielectric material of gasket  76 . Gasket  76  may be relatively thin (e.g., about  2  mm in dimension  84 ), so the presence of cavity antenna  48  may easily be concealed from view. This can help provide computer  10  with a pleasing appearance that is not interrupted by the presence of unsightly antenna structures. 
     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.