Abstract:
The boxed-in slot antenna is provided with a conductive box, functioning as a waveguide, which is configured substantially parallel to the ground plane in which the slot is formed, thereby providing significant space savings relative to prior art designs wherein the box is positioned perpendicular to the conductive ground plane. The inventive antenna can be easily constructed using printed circuit board technology, by forming the ground plane as a coating on one side of a printed circuit board substrate, forming the main conductive plane of the conductive box structure on the other side of the printed circuit board, and interconnecting the two using plated through holes (that is, vias). The folded structure of the conductive box of the present invention makes it particularly suited for space-critical applications, such as may be found in laptop computers and other portable and handheld electronic devices, which it is desired to interconnect with a wireless local area network (wireless LAN).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to antennas, and more particularly relates to a boxed-in slot antenna having a folded, space-saving configuration, which can be employed in space-critical applications, such as a laptop computer.  
           [0003]    2. Brief Description of the Prior Art  
           [0004]    There is increased interest in enabling laptop computers and other portable electronic devices to interface with a wireless local area network (WLAN). WLANs may operate under a number of standards, for example, the so-called “Bluetooth standar”. In such systems, an antenna is required to send and receive data via radio frequency (RF) communications. In portable electronic devices, space is typically at a premium. Accordingly, it is desirable to minimize space to be occupied by an antenna in such devices. One prior art approach to providing an RE antenna which takes up minimal space is disclosed in World Intellectual Property Organization (WIPO) international publication number WO 9506338 published on Mar. 2, 1995. In this publication, a folded monopole antenna is discussed. The folding of the monopole reduces its height so as to enable it to fit into small areas. However, the folding of the monopole has undesirable effects on the electrical match, frequency bandwidth and electromagnetic fields, requiring the introduction of a shunt inductance between the monopole and the ground plane.  
           [0005]    Slot antennas are known in the prior art, and are useful for low-profile or flush installations, such as in high-speed aircraft. A traditional slot antenna is described in the book  Antennas  by John D. Kraus, at pages 624-632 (Second Edition, McGraw-Hill 1988). FIG. 1 shows a prior art slot antenna, designated generally as 10. A conductive ground plane  12 , typically metallic, is formed with a slot  14 . The slot has a length, L, which is typically equal to half of the electric wavelength λ e  The slot  14  typically also has a width, w, which is much less than the wavelength. Such an antenna will radiate equally from both sides of the ground plane  12 . It is typically fed by a coaxial cable  16 , which can be attached at an off-center feed point in order to obtain a 50 Ohm antenna impedance so as to match the characteristic impedance, typically 50 Ohms, of coaxial cables.  
           [0006]    In some applications, it is desirable to have a slot antenna which radiates in only one direction. This can be achieved with a fairly large conductive ground&amp;lane, with one side of the slot boxed-in, as shown in FIG. 2. This type of structure is also discussed in the aforementioned Kraus reference book. The prior-art boxed-in slot antenna of FIG. 2 is designated generally as  20 . The antenna  20  of FIG. 2 is also formed with a conductive ground plane  22 , and with a slot having dimensions L,w as before. The slot is designated as  24 . A box structure  26  is used to box-in the slot  24 , and typically extends a depth, h, below the surface of the conductive ground plane  22 . The distance h is typically one-quarter of the waveguide wavelength λ g  The box structure  26  blocks radiation in the rearward direction in FIG. 2, so that radiation in the forward direction is enhanced; further, it doubles the radiation resistance of the original slot antenna  10 . Feed can be via a coaxial cable  28 . The original slot antenna  10  is not appropriate for use in a handheld electronic device or a laptop computer because of the radiation in both directions, while the prior art boxed-in slot antenna of FIG. 2 is also unsuitable, as the distance h must be so large that the antenna occupies an unacceptably large space. Note that the Kraus reference uses “d” for “h”; the “h” terminology is used herein to avoid confusion with a “d” parameter referred to below with respect to the present invention.  
           [0007]    It will be appreciated that the prior art folded monopole approach of the aforementioned WIPO publication results in unadvantageous changes to the electrical match, frequency bandwidth and electromagnetic fields, necessitating the introduction of a shunt inductance. Further, the slot i antennas discussed immediately above are unsuitable due to either bi-directional radiation or excessive size.  
           [0008]    In view of the foregoing, there is a need in the prior art for a compact antenna suitable for use in laptop computers and other portable electronic devices. There is the need for such an antenna which takes up minimal space, can be easily fabricated, and has desirable electric characteristics.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention, which addresses the needs identified in the prior art, provides a boxed-in slot antenna wherein the conductive box structure has a folded, space-saving configuration suitable for use in space-limited locations such as a laptop computer. The inventive antenna is for radiation having a free-space wavelength λ a waveguide wavelength λ g , and an electric half-wavelength λ e /2. The antenna includes a conductive ground plane having a slot formed therein, with the slot having a length L which is at least substantially equal to the electric half-wavelength. The slot also has a width w which is less than the length L, and the slot further has a longitudinal axis and first and second longitudinal edges. The antenna also includes a conductive box structure. The conductive box structure in turn comprises a main conductive plane which is substantially parallel to the ground plane and which is spaced a distance d therefrom. The distance d is substantially less than ¼ of the waveguide wavelength λ g  The conductive box structure further includes first and second conductive structures which are substantially parallel to each other and which are spaced apart a distance g which is at least substantially equal to L. The first and second conductive structures are substantially perpendicular to the conductive ground plane and the main conductive plane and are also substantially perpendicular to the longitudinal axis of the slot.  
           [0010]    The conductive box structure yet further includes third and fourth conductive structures which are substantially parallel to each other and which are spaced apart a distance a, with the third and fourth conductive structures being substantially perpendicular to the conductive ground plane and the main conductive plane, and also being substantially parallel to the longitudinal axis of the slot.  
           [0011]    The distance a can preferably be substantially equal to one of: the width w plus ¼ of the waveguide wavelength, and the width w plus ½ of the waveguide wavelength. The first, second, third and fourth conductive structures form conductive paths between the conductive ground plane and the main conductive plane. When viewed in plan, the first, second, third and fourth conductive structures bound the slot. Accordingly, it will be appreciated that the inventive antenna is an improvement over the prior-art boxed-in slot antenna, inasmuch as the arrangement just described provides a folded, space-saving configuration for the conductive box structure which permits its incorporation into space-limited locations such as a laptop computer. In particular, the distance d can be much less than the distance h in the prior-art type of boxed-in slot antenna.  
           [0012]    These and other features and advantages of the present invention will be appreciated by reading the following specification, taken in conjunction with the accompanying drawings, and the scope of the invention will be set forth in the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a semi-schematic view of a prior-art slot antenna;  
         [0014]    [0014]FIG. 2 is a semi-schematic view of a prior-art boxed-in slot antenna,  
         [0015]    [0015]FIG. 3 is a semi-schematic view of one form of boxed-in slot antenna in accordance with the present invention;  
         [0016]    [0016]FIG. 4 is a plan view of an antenna according to the present invention, similar to that depicted in  
         [0017]    [0017]FIG. 3, and having the conductive box structure formed of conductive plates;  
         [0018]    [0018]FIG. 5 is a cross-sectional view taken along line V-V in FIG. 4FIG.  
         [0019]    [0019]FIG. 6 is a plan view of an antenna in accordance with the present invention, similar to that depicted in FIG. 3, wherein the conductive box structure is formed of a series plated through 20 holes;  
         [0020]    [0020]FIG. 7 is a cross-sectional view of the antenna of FIG. 6 taken along line VII-VII of FIG. 6;  
         [0021]    [0021]FIG. 8 is a view similar to FIG. 6 employing a microstrip rather than a coaxial feed structure;  
         [0022]    [0022]FIG. 9 is a cross-sectional view taken along line IX-IX in FIG. 8;  
         [0023]    [0023]FIG. 10 is a semi-schematic view showing another form of the present invention;  
         [0024]    [0024]FIG. 11 is a semi-schematic view showing yet another form of the present invention;  
         [0025]    [0025]FIG. 12 is a semi-schematic view showing still another form of the present invention;  
         [0026]    [0026]FIG. 13 is a plan view of an embodiment of the invention similar to that depicted in FIG. 12 wherein the conductive structures are conductive plates;  
         [0027]    [0027]FIG. 14 is a cross-sectional view taken along line XIV-XIV in FIG. 13;  
         [0028]    [0028]FIG. 15 is a plan view similar to FIG. 13, but showing an embodiment of the invention wherein the conductive structures are formed from plated through holes;  
         [0029]    [0029]FIG. 16 is a cross-sectional view taken along line XVI-XVI in FIG. 15;  
         [0030]    [0030]FIG. 17 is a view similar to FIG. 15, but showing an embodiment of the invention employing a microstrip feed structure rather that a coaxial cable;  
         [0031]    [0031]FIG. 18 is a cross-sectional view taken along line XVIII-XVIII of FIG. 17;  
         [0032]    [0032]FIG. 19 is a plot of antenna voltage standing wave ratio (VSWR) as a function of operating frequency, for one exemplary embodiment of the present invention;  
         [0033]    [0033]FIG. 20 shows the elevation plane radiation patterns of the antenna for φ=0° (slot width direction) and for φ=90° (slot length direction) respectively, again, for the exemplary embodiment for which the VSWk was shown in FIG. 19; and  
         [0034]    [0034]FIG. 21 is a semi-schematic perspective view of a portable electronic device having an antenna installation in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]    Reference should now be had to FIG. 3, which is a semi-schematic pictorial view of one form of boxed-in slot antenna for radiation having a free-space wavelength λ, a waveguide wavelength λ g , and an electric half-wavelength λ e /2, in accordance with the present invention. The inventive antenna is designated generally as  100 . The antenna  100  includes a conductive ground plane  102 , which can be metallic (for example), and which has a slot  104  formed therein. Ground plane  102  has first and second sides. The slot  104  has a length L which is at least substantially equal to the electric half-wavelength. As used herein, “at least substantially equal to” means that L is greater than the electric half-wavelength, or is substantially equal to the electric half-wavelength, where substantially equal is meant to include equal to, or slightly greater than or less than, so long as functionality can be maintained. Slot  104  also has a width w which is less (preferably much less) than the length L, and further has a longitudinal axis  106  and first and second longitudinal edges  108 ,  110  respectively. It is preferable that the width w satisfy the relationship w&lt;&lt;λ. Slot  104  can be configured and dimensioned for predetermined radiation performance, for example, for radiation as described above. Those of skill in the antenna art will appreciate how to develop desired dimensions for the slot in view of the guidelines presented herein.  
         [0036]    The present invention further includes a conductive box structure  112 , which is conductively secured to the conductive ground plane  102  and which is configured to cause the slot antenna  100  to radiate from only a single side (i.e., one of the first and second sides) of the conductive ground plane  102 ; as depicted in FIG. 3, the slot antenna  100  would radiate outwardly towards the viewer from the conductive ground plane  102 , whereas radiation into the paper would be prevented by the conductive box structure  112 . Conductive box structure  112  can thus be configured to function as a waveguide to achieve the desired single-sided radiation.  
         [0037]    It will be appreciated that the prior art boxed-in slot antenna  20  shown in FIG. 2 also includes a ground plane with slot and conductive box structure. However, the present invention is improved over the prior art device shown in FIG. 2, in that the conductive box structure  112  is provided with a folded, space-saving configuration (it can be configured in a folded manner parallel to the ground plane  102 ). In particular, to achieve this configuration, the conductive box structure  112  in turn includes a main conductive plane  114 , which is substantially parallel to the conductive ground plane  102  and which is spaced a distance d from the ground plane. The distance d is substantially less than ¼ of the waveguide wavelength λ g  and is selected so as to permit the antenna  100  to easily fit into space-limited locations such as a laptop computer. The distance d should be as thin as possible to reduce size, consistent with adequate bandwidth. If d is too small, the bandwidth will be narrow. Appropriate values for d All also be influenced by the properties of the substrate in PCB embodiments of the invention to be discussed below. Any value of the distance d which is substantially less than ¼ of the waveguide wavelength should be considered within the scope of the invention. For example, d could be less than 15% of λ g , or preferably less than 10% of λ g , or even more preferably less than 5% of kg, consistent with adequate bandwidth. In the Example discussed below, d is about 3.8% of λ h . In view of these guidelines, those of skill in the antenna art will be able to select appropriate values for the distance d. Conductive box structure  112  further comprises first and second conductive structures  116 ,  118  respectively, which are substantially parallel to each other and which are spaced apart a distance g which is at least substantially equal to L (i.e., either substantially equal to L or greater than L). It is believed preferable that g be at least slightly greater than L. The first and second conductive structures  116 ,  118  are substantially perpendicular to the conductive ground plane  102  and to the main conductive plane  114 , and are also substantially perpendicular to the longitudinal axis of the slot  106 .  
         [0038]    The conductive box structure  112  of the inventive antenna  100  yet further includes third and fourth conductive structures  120 ,  122  respectively, which are substantially parallel to each other and which are spaced apart a distance a. The third and fourth conductive structures  120 ,  122  are substantially perpendicular to the conductive ground plane and the main conductive plane and are also substantially parallel to the longitudinal axis  106  of the slot  104 . Note that FIG. 3, like FIGS. 1 &amp; 2, is semi-schematic in nature, in the sense that no thickness is shown for the conductive ground plane  102  or the main conductive plane  114 , or the conductive structures  116 ,  118 ,  120 ,  122  respectively. It will be appreciated that this is purely for purposes of illustrative convenience, and the physical thickness of these various items is depicted in the other figures. FIGS. 10, 11 and  12  are also semi-schematic in nature.  
         [0039]    The distance a should preferably be substantially equal to either: the width w plus ¼ of the waveguide wavelength, or the width w plus ½ of the waveguide wavelength, but other values can be used as discussed elsewhere herein. As used herein, “substantially equal to” is intended to include precisely equal to, with also a slight variation above and below, so long as functionality can be maintained. The first, second, third and fourth conductive structures  116 ,  118 ,  120 ,  122  respectively form conductive paths between the conductive ground plane  102  and the main conductive plane  114 . When viewed in plan, the first through fourth conductive structures bound the slot  104 . It will be appreciated that by having the short dimension d of the box structure be perpendicular to the ground plane  102 , with the longer dimensions of the conductive box structure a and g being parallel to the ground plane  102 , a folded configuration is obtained for the conductive box structure  112 , which affords significant space savings when compared with the prior art.  
         [0040]    As used herein, a “plan” view refers to a view wherein the conductive ground plane is parallel to the paper on which the view is drawn. Furthermore, “bounding” of the slot by the conductive structures refers to the structures surrounding, or being substantially coincident with, the slot.  
         [0041]    Still referring to FIG. 3, it will be appreciated that in the embodiment of the invention shown therein, the distance a is substantially equal to the width w plus ¼ of the waveguide wavelength λ g  The third conductive structure  120  can, as shown, substantially coincide with the first longitudinal edge  108  of the slot  104 . “Substantially coinciding” is intended to refer to a spatial orientation wherein the third conductive structure  120  is even with or only slightly displaced from the first longitudinal edge  108  of the slot  104 . Further, the fourth conductive structure  122 , in the embodiment depicted in FIG. 3, can be located beyond the second longitudinal edge  110  of the slot  104 , spaced from the third conductive structure  120  in a direction moving from the first longitudinal edge  108  of the slot  104  to the second longitudinal edge  110  of the slot  104 . Reference should now be had to FIGS. 4 and 5, which depict plan and cross-sectional views respectively of an embodiment of the invention similar to that shown in FIG. 3 wherein the reference characters associated with like components have received the same number as in FIG. 3 but incremented by the value of  100 , wherein the first through fourth conductive structures are formed as conductive plates, for example, metallic plates. It will be appreciated that the conductive ground plane  202  and main conductive plane  114  could also be formed as conductive, for example metallic, plates. The embodiment depicted in FIGS. 4 and 5 can be fed, for example, with a coaxial cable  224 , having a conventional center conductor  226 , insulator  228 , and outer conductor  230 , in a well-known fashion. Outer conductor  230  of coaxial cable  224  can be soldered to first longitudinal edge  208  of slot  204  through a solder bead  232 , while center conductor  226  of coaxial cable  224  can be soldered to second longitudinal edge  210  of slot  204  at a solder bead  234 . While the outer conductor  230  of coaxial cable  224  is shown as being spaced from conductive ground plane  202 , with conductive contact only at solder bead  232 , it should be appreciated that the outer conductor  230  can be maintained in contact with the conductive ground plane  202  if desired (such contact may be advantageous).  
         [0042]    It will be appreciated that no coaxial cable, microstrip feed structure, or other type of antenna feed device is depicted in FIG. 3; this is purely for purposes of illustrative convenience. Further, it will be appreciated that the feed, such as the coaxial cable  224 , can be located substantially centered in the slot  204 , as shown in FIG. 4, or can be displaced therefrom, which will result in a lower impedance.  
         [0043]    Reference should now be had to FIGS. 6 and 7 which depict an embodiment of the invention similar to that shown in FIG. 3, which employs printed circuit board (PCB) technology. Items in FIGS. 6 and 7 which are similar to those in FIGS. 4 and 5 have received the same reference character incremented by  100 . The embodiment of FIGS. 6 and 7 is designated generally as  300 , and can include a first printed circuit board substrate  336  having first and second generally planar surfaces  338 ,  340  respectively. The conductive ground plane  302  can be formed as a first conductive layer  342  which is deposited on the first generally planar surface  338  of the first PCB substrate  336 . The slot  304  can be etched in the first conductive layer  342 . The main conductive plane  314  can be formed as a second conductive layer  344  which is deposited on the second generally planar surface  340  of the first PCB substrate  336 . The first, second, third and fourth conductive structures  316 ,  318 ,  320 ,  322  respectively can each be formed as a series of plated through holes  346  which are formed in the first PCB substrate  336  using techniques well known in the art of printed circuit board fabrication. It will be appreciated that the plated through holes  346  provide an electrically conductive path between the first and second conductive layers  342 ,  344 . As best seen in FIG. 6, the plated through-holes  346 ;which form the conductive structures can be spaced apart by a distance Δ, which is preferably no more than substantially one tenth of the free-space wavelength λ. The foregoing terminology is meant to cover plated through holes which are spaced slightly more than one tenth of λ apart, but which are still functional, and any closer spacing of the through holes  346 . The second conductive layer  344  can extend over the entire second surface  340  of the first PCB substrate  336 , or, if desired, can extend only over the region where it serves as the main conductive plane  314 , that is, within the region defined by the plated through holes  346 . Note that the distances a and g can be measured from the center lines of the plated through holes in all PCB embodiments of the invention.  
         [0044]    Coaxial cable  324  can be located in a centered position (shown) or off-center, as discussed above with regard to cable  224 . This is generally true for all embodiments of the invention disclosed herein.  
         [0045]    Reference should now be had to FIGS. 8 and 9, which depict an embodiment of the invention similar to that depicted in FIGS. 6 and 7, but wherein a microstrip feed structure is employed in lieu of a coaxial cable. Items in FIGS. 8 and 9 similar to those in FIGS. 6 and 7 have received the identical reference character incremented by  100 . The embodiment shown in FIGS. 8 and 9 can include a second PCB substrate  448  having an inner side  450  and an outer side  452 . The inner side  450  of the second PCB substrate  448  can be located adjacent the conductive ground plane  402 . The antenna  400  can further include a conductive strip  454  which is located on the outer side  452  of the second PCB substrate  448 . The conductive strip  454  can have a width c and can have a longitudinal axis  456  (which is coincident with the cutting plane line IX-IX in FIG. 8) which is substantially perpendicular to the longitudinal axis  406  of the slot  404  (at least in the region close to the slot). The thickness of the conductive strip  454  can be any appropriate value as selected by those of skill in the art. The conductive strip  454  can be electrically interconnected to one of the first and second longitudinal edges  408 ,  410  of the slot  404 , and can extend from the longitudinal edge to which it is interconnected towards the other of the first and second longitudinal edges  108 ,  110  of the slot  104 . In the embodiment shown in FIGS. 8 and 9, the conductive strip  454  is electrically interconnected to the second longitudinal edge  410  of the slot  404 , and extends back towards, and beyond, the first longitudinal edge  408  of the slot  404 . It will be appreciated that the conductive strip  454 , the second PCB substrate  448 , and the conductive ground plane  402  are configured so as to form a microstrip feed structure for the antenna  400 . The strip  454  can be centered with regard to the slot  404 , as shown in FIG. 8, or can be displaced laterally therefrom, which will tend to lower the impedance Z.  
         [0046]    The conductive strip  454  can be electrically interconnected to the one of the first and second longitudinal edges  408 ,  410  of the slot  404  to which it is desired to be connected by a plated through hole connection  458  which is formed in the second PCB substrate  448 .  
         [0047]    Attention should now be given to FIG. 10, which is a semi-schematic view similar to FIG. 3, but depicting an alternative form of the present invention. Items in FIG. 10 similar to those in FIG. 3 have received the same reference character incremented by  400 . It will be appreciated that the antenna  500  of FIG. 10 is similar to the antenna  100  of FIG. 3, except that in FIG. 10 the distance L is substantially equal to the distance g, while in FIG. 3, g&gt;L. With air in the conductive box structure, g&gt;L is preferred to support the TE 10  mode; with a dielectric (such as a PCB substrate) within the box structure, g=L can be acceptable. Increasing g can reduce λ g .  
         [0048]    Reference should now be had to FIG. 11 which depicts an embodiment of the invention similar to that depicted in FIG. 10, wherein similar items have received the same reference character incremented by 100 As in FIG. 10, the embodiment of FIG. 11 is depicted with L substantially equal to g. However, unlike FIGS. 3 and 10, where a was substantially equal to λ g /4+w, the embodiment depicted in FIG. 11 shows a value of a which is substantially equal to w+λ g /2. The higher value of a yields a higher bandwidth.  
         [0049]    Attention should now be given to FIG. 12, which depicts a form of the invention similar to that shown in FIG. 11, but wherein g&gt;L. Items in FIG. 12 similar to those in FIG. 11 have received the same reference character incremented by 100. With reference to FIG. 12, and as previously discussed with respect to FIG. 11, it will be appreciated that as depicted therein, the distance a is substantially equal to the width w plus ½ of the waveguide wavelength λ g . Furthermore, the third conductive structure  620 ,  720  is spaced substantially ¼ of the waveguide wavelength λ g  from the first longitudinal edge  608 ,  708  of the slot  604 ,  704 , while the fourth conductive structure  622 ,  722  is spaced substantially ¼ of the waveguide wavelength λ g  from the second longitudinal edge  610 ,  710  of the slot  604 ,  704 .  
         [0050]    Just as for the embodiment of FIG. 3, in the embodiments just discussed, the first, second, third and fourth conductive structures  616 ,  618 ,  620 ,  622  and  716 ,  718 ,  720 ,  722  can be made of conductive plates, such as metallic plates. This is depicted in FIGS. 13 and 14, which are similar to FIGS. 4 and 5 except for the larger value of a. Items in FIGS. 13 and 14 similar to those in FIGS. 4 and 5 have received the same reference character incremented by  600 . Other than the larger value of a, the construction of the embodiments shown in FIGS. 13 and 14 is similar to that discussed above with respect to FIGS. 4 and 5, and need not be discussed again.  
         [0051]    In addition to the just-discussed embodiments wherein the first through fourth conductive structures were conductive plates, such as metallic plates, embodiments with the larger value of a can also be constructed using printed circuit board techniques, as discussed above with respect to the smaller value of a, and can be fed from either coaxial cables or microstrip feed structures, or in any other suitable manner.  
         [0052]    [0052]FIGS. 15 and 16 depict an embodiment of the invention similar to that shown in FIGS. 6 and 7, wherein similar items have received the same reference character as in FIGS. 6 and 7 incremented by 600. Except for the larger value of a, construction is similar to the earlier-discussed embodiments.  
         [0053]    Finally, attention should be given to FIGS. 17 and 18, which depict an embodiment of the invention similar to that shown in FIGS. 8 and 9, including a microstrip feed structure, but with the larger value of a. Items in FIGS. 17 and 18 similar to those in FIGS. 8 and 9 have received the same reference character as in FIGS. 8 and 9, incremented by 600. Other than the larger value of a, construction of the embodiment of FIGS. 17 and 18 is similar to that FIGS. 8 and 9, and need not be further discussed.  
         [0054]    In view of the foregoing descriptions, it will be appreciated that the present invention provides a conductive box structure which is parallel to the ground plane rather than perpendicular to the ground plane, as in the prior art, resulting in a design which can be easily constructed using printed circuit board technology, with a markedly reduced thickness compared to the prior art.  
         [0055]    Referring to those embodiments of the invention where a is substantially equal to λ g /2+w, it will be appreciated that, in effect, a second conductive box structure has been added in series with the conductive box structure of the embodiments with the smaller value of a. Thus, the larger value of a can improve the bandwidth of the slot antenna. For example, if the impedance provided by the box structure with the lower value of a is Z, the overall impedance provided by the box structure with the larger value of a (i.e., a=w+λ g /2) will be 2 Z. The larger the overall antenna impedance, the lower will be the effect on the antenna bandwidth which can be obtained from the conductive box structures. In those embodiments where the box structure dimension g is greater than the length L of the slot, a transverse electromagnetic wave having mode TE 10  (that is, a TE 10 wave) can exist. It will be appreciated that in all embodiments of the present invention, the conductive box structure functions as a waveguide and it is desirable to set up a standing wave within the conductive box structure. It is preferred that a should be equal to either w+λ g /4 or w+λ g /2, in order to obtain the best performance, but other values are functional, and such other values are also within the scope of the invention.  
         [0056]    It will be appreciated that the operating frequency, dielectric characteristics (i.e., dielectric constant ε r ) of the substrate materials, and the dimension g of the conductive box structure, as well as its depth d, will determine the waveguide wavelength λ g , with g andε r ,being most important. Similar considerations apply in other embodiments of the invention having air within the conductive box structure; of course, ε r , for air is near unity.  
         [0057]    With regard to the embodiments depicted in FIGS. 8, 9,  17  and  18 , it will be appreciated that the width of the conductive strip, c, can be selected so as to provide a desired characteristic impedance, such as, for example,  50  ohms. The first and second PCB substrates  436 ,  448 , and  1036 ,  1048  can be made of different materials having different dielectric constants, and can have different thicknesses  
         [0058]    In all of the embodiments presented, it will be appreciated that the dimension L has a minimum value of approximately the electric half wavelength, that is, λ e /2. Larger values can be employed. For example, a value of L=0.7 λ e could be used. Preferably, L&lt;λ e  to suppress higher-order transmission modes. Reference should be had to the aforementioned antenna reference text by Kraus, Chapter 13 thereof. It will be further appreciated that increases in the value of L will tend to lower the impedance Z. The impedance can also be lowered by using an off-center feed, but as shown in the drawings, the feed, whether microstrip or coaxial, could also be centered. In all embodiments, the axis of the feed, whether microstrip or coaxial, should be perpendicular to the slot, at least for some distance close to the slot, as will be appreciated by those of skill in the antenna art.  
         [0059]    In all embodiments, the conductive ground plane should be as large as possible, but any dimensions which yield a functional antenna are within the scope of the invention. Preferred minimum dimensions are approximately 0.75λ), in the direction parallel to the longitudinal axis of the slot and approximately 0.5λ) in the direction perpendicular to the longitudinal axis of the slot.  
         [0060]    Attention should now be given to FIG. 21. The present invention contemplates the combination of a portable electronic device, designated generally as 2000, with any type of antenna in accordance with the present invention. Such a device could be a laptop computer, personal digital assistant, or other device. As shown in FIG. 21, such a device could have a first portion  2002  with, for example, alphanumeric keys  2004  (only a few are shown for illustrative convenience) and a pointing device  2006 . A second portion  2008  could be secured to first portion  2002  at a hinged edge  2010 . Second portion  2008  could include a display  2012  for data of a textual and/or graphical nature  2014 ,  2016  respectively. One or more antennas  2020 , of any configuration in accordance with the present invention, can be employed in conjunction with device  2000 . Multiple antennas could be used, for example, where it was desired to communicate on different frequencies, or in a system where diversity was required or desired.  
         [0061]    A preferred location for the antenna is on the second portion  2008  which has the display  2012 , close to the top  2022 . A first antenna  2020  is shown adjacent the right edge  2026  of portion  2008 , facing sideways. A second antenna  2020  is shown adjacent the top  2022  of portion  2008  facing away from a user (not shown) who would be typing on keys  2004 . Due to reflections in the indoor environment, either of the indicated orientations should be functional. The preferred location is high up on portion  2008  (i.e., near the top  2022 ) and close to the top or one of the edges  2024 ,  2026 . When located adjacent an edge  2024 ,  026 , antenna  2020  should still be near the top  2022 , as shown. Preferably, the antenna(s) should face sideways or away from the user, but any other functional orientation (e.g., upwards) should be considered as within the scope of the present invention.  
         [0062]    The ground plane of antenna  2020  should be grounded to a conductive portion of the device  2000 , for example, an existing metallic structural portion (and can even be formed integrally therewith) No other portion of the antenna  2020  should touch any conductive or metallic portion of device  2000 .  
         [0063]    The disclosure of U.S. Patent application Ser. No. 09/598,719 filed Jun. 21, 2000 under IBM docket number YOR9-2000-0206US1, entitled “An Integrated Antenna for Laptop Applications” by Ephraim Bemis Flint, Brian Paul Gaucher and Duixian Liu is expressly incorporated herein by reference in its entirety.  
       EXAMPLE  
       [0064]    Performance of a boxed-in slot antenna, with the inventive folded, space-saving configuration for the conductive box structure, was predicted via simulation with Zeland&#39;s IE3D computer program. Performance of an embodiment of the invention similar to that shown in FIG. 10 was predicted (i.e., g=L, a=w+λ g /4), but for a printed circuit board configuration, fed by a coaxial cable, similar to that shown in FIGS. 6 and 7 (but, as noted, with g=L). The conductive ground plane had a dimension of 70 mm perpendicular to the slot and 99 mm parallel to the slot. The slot width was w=3 mm, with g=L=50.5 mm. The first PCB substrate had a thickness of 3 mm and a relative dielectric constant of 4.6. A value of λ g /4=19.75 mm was used such that a was 22.75 mm.  
         [0065]    [0065]FIG. 19 depicts the predicted voltage standing wave ration (VSWR) of the antenna. The 2:1 VSWVR bandwidth is 154 Mz, which is sufficiently wide for 2.4 GHz ISM applications. FIG. 20 shows the simulated elevation plane radiation patterns of the antenna for φ=0°, i.e., the slot width direction, and φ=90°, i.e., the slot length direction, respectively. The maximum predicted gain for the antenna is 6.4 dB.  
         [0066]    While there have been described what are presently believed to be the preferred embodiments of the invention, those skilled in the art will realize that various changes and modifications can be made to the invention without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall within the scope of the invention.