Abstract:
A slot antenna comprising: a first and second boards each having an edge attached to respective opposite edges of a center board to form an essentially C-shaped open-ended channel having an inner surface and an outer surface; the center board having a slot for radiating signals defined in an electrically conductive layer bonded to an outer surface thereof and an electrically conductive feed line bonded to an inner surface thereof; the slot having a drive point being a portion of the feed line undercrossing the slot between opposite edges of the slot; the first and second boards each having an electrically conductive layer bonded to an outer surface thereof to reduce nulls, thereby providing an essentially omni-directional radiation pattern for the slot; and, a connector for coupling the signals to the antenna.

Description:
This application claims priority from U.S. patent application Ser. No. 60/331,765, filed Nov. 21, 2001, and Canadian Patent Application No. 2,363,519, filed Nov. 21, 2001 and incorporated herein by reference. 

   The invention relates to the field of slot antennas, and more specifically to horizontally polarized slot antennas for mobile communication systems, wireless LAN systems, and the like. 
   BACKGROUND OF THE INVENTION 
   There has been an increase in the use of horizontally polarized slot antennas for mobile communication systems. While directional horizontally polarized slot antennas are relatively easy to be design and manufacture, omni-directional and sectorial horizontally polarized slot antennas are more difficult to design and manufacture and hence are more expensive. This has an adverse effect on cost of establishing base stations for systems including mobile communication, wireless LAN (e.g. at 2.4 GHz and 5.8 GHz), UNII (Unlicensed National Information Infrastructure), MMDS (Multichannel Multipoint Distribution Service), and WLL (Wireless Local Loop) systems. 
   A need therefore exists for an improved horizonally polarized slot antenna capable of omni-directional and sectorial radiation patterns that can be manufactured cost effectively. Consequently, it is an object of the present invention to obviate or mitigate at least some of the above mentioned disadvantages. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the invention there is provided a slot antenna comprising: first and second boards each having an edge attached to respective opposite edges of a center board to form an essentially C-shaped open-ended channel having an inner surface and an outer surface; the center board having a slot for radiating signals defined in an electrically conductive layer bonded to an outer surface thereof and an electrically conductive feed line bonded to an inner surface thereof; the slot having a drive point being a portion of the feed line undercrossing the slot between opposite edges of the slot; the first and second boards each having an electrically conductive layer bonded to an outer surface thereof to reduce nulls, thereby providing an essentially omni-directional radiation pattern for the slot; and, a connector for coupling the signals to the antenna. 
   According to another aspect of the invention the slot antenna includes a reflector having first and second panels each having an edge attached to respective opposite edges of a center panel to form an essentially C-shaped open-ended channel having an inner surface and an outer surface; the center panel having an outer surface spaced from and parallel to the inner surface of the center board to sectorialize the radiation pattern of the slot by approximately 180 degrees. 
   According to another aspect of the invention the slot antenna includes first and second reflector panels each spaced successively from and parallel to the inner surface of said center board to sectorialize the radiation pattern of the slot by approximately 120 degrees. 
   Advantageously, the present invention provides an improved horizonally polarized slot antenna for omni-directional and sectorial applications that can be manufactured cost effectively. Communication capacity may be almost doubled, without increasing interference, by using this slot antenna in mobile communications systems. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention may best be understood by referring to the following description and accompanying drawings. In the description and drawings, like numerals refer to like structures and/or processes. In the drawings: 
     FIGS.  1 ( a ),  1 ( b ), and  1 ( c ) are front, right-hand side, and bottom views, respectively, illustrating a single-element omni-directional slot antenna in accordance with an embodiment of the invention; 
       FIG. 2  is a front view illustrating a two-element omni-directional slot antenna in accordance with an embodiment of the invention; 
     FIGS.  3 ( a ) and  3 ( b ) are front and right-hand side views, respectively, illustrating a centre-fed four-element omni-directional slot antenna array in accordance with an embodiment of the invention; 
     FIGS.  4 ( a ),  4 ( b ), and  4 ( c ) are front, right-hand side, and bottom views, respectively, illustrating a single-element 180 degree sectorial slot antenna in accordance with an embodiment of the invention; and, 
     FIGS.  5 ( a ),  5 ( b ), and  5 ( c ) are front, right-hand side, and bottom views, respectively, illustrating a single-element 120 degree sectorial slot antenna in accordance with an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following description, numerous specific details are set forth to provide a thorough understanding of the invention. However, it is understood that the invention may be practiced without these specific details. In other instances, well-known structures and/or processes have not been described or shown in detail in order not to obscure the invention. In the description and drawings, like numerals refer to like structures and/or processes. 
   In general, the invention described herein provides a slot antenna that includes a slot on the surface of a dielectric substrate, a feed network on the bottom of the substrate, and two pieces of parallel conductor panel located on each side of the slot. The slot antenna is horizontally polarized, omni-directional, and may be fitted with reflectors for sectorial applications. Communication capacity may be almost doubled, without increasing interference, by using this slot antenna in mobile communication systems. 
   FIGS.  1 ( a ),  1 ( b ), and  1 ( c ) are front, right-hand side, and bottom views, respectively, illustrating a single-element omni-directional slot antenna in accordance with an embodiment of the invention. In FIGS.  1 ( a ),  1 ( b ), and  1 ( c ), the single-element slot antenna is shown generally by numeral  100 . The slot antenna  100  is horizontal polarized and can be used to provide omni-directional and sectorial radiation patterns as will be described below. The slot antenna  100  includes two main assemblies. The first is a slot antenna assembly  110  which is fed by a microstrip line with a ground plane  111 . The second is a conductor/PCB assembly  120  consisting of two pieces of conductor panel  121  and  122  which may be supported by dielectric material  123  and  124  respectively (which collectively, may, for example, be single sided PCB  1 ( a ),  1 ( b ). And the conductor assembly  120  is attached to the slot antenna assembly  110  mechanically, so that conductors  121  and  122  and ground plate  111  are held together but remain DC isolated from each other electrically. The slot antenna assembly  110  and the conductor/PCB assembly  120  define a single-element omni-directional slot antenna  100 . 
   Theoretically, a slot antenna having an infinite ground plane may be considered as a magnetic dipole antenna. As such, its radiation pattern should be omni-directional. However, in practice, due to the limited size of its ground plane, there are typically two nulls at 90° and 270° in a slot antenna&#39;s horizontal radiation pattern. In order to eliminate these nulls, the present invention provides a PCB assemble  120 , as shown in FIGS.  1 ( a ),  1 ( b ), and  1 ( c ), that includes two pieces of single-sided PCB  1 ( a ),  1 ( b ) attached to opposite sides of the slot antenna assembly  110 . By optimizing the size of the PCBs  1 ( a ),  1 ( b ), and the distance between them, an omni-directional radiation pattern is achieved in which the difference between the maximum and minimum radiation levels is less than 2 dB. Based on the single-element omni-directional slot antenna  100  of FIGS.  1 ( a ),  1 ( b ), and  1 ( c ), two-element and four-element omni-directional antenna arrays are provided as described below. In addition, by adding metal reflectors behind the single-element omni-directional slot antenna  100  of FIGS.  1 ( a ),  1 ( b ), and  1 ( c ), 180° and 120° sectorial slot antennas are provided as described below. 
   Again referring to FIGS.  1 ( a ),  1 ( b ), and  1 ( c ), the slot antenna assembly  110  includes a rectangular, low-loss dielectric RF-35 (e.g. a ceramic filled, low cost PTFE substrate from Taconic) PCB  3  having thin copper sheets adhered to both sides  111 ,  112 . Conductive segments  5 ,  7  are formed on the RF-35 PCB  3  by etching or milling. These conductive segments include a ground plane  7  formed on the front-side  111  of the RF-35 PCB  3  and a microstrip feed line  5  formed on the back-side  112  of the RF-35 PCB  3 . A rectangular slot  6  for radiating radio frequency (“RF”) signals is formed in the ground plane  7  by removing copper through an etching or milling process. The slot  6  may be centred in the ground plane  7  along the horizontal and vertical centre-lines (i.e. Ref. Line A and Ref. Line B in FIG.  1 ( a )) of the ground plane  7  or PCB  3 . A 50-ohm connector  4  located along the bottom edge of the RF-35 PCB  3  couples RF signals to the slot antenna assembly  100  via the microstrip feed line  5  and the ground plane  7 . Typically, the 50-ohm connector  4  is a coaxial cable having a conventional inner conductor, insulator, and an outer conductor or shield. The outer conductor is connected to the ground plane  7  and the inner conductor is connected to the feed line  5 . A portion  105  of the feed line  5  crosses the slot  6  along the horizontal centre-line (i.e. Ref Line A in FIG.  1 ( a )) of the slot  6 . This portion  105  of the feed line drives the slot  6 . The midpoint O of this portion  105  of the feed line  105  may be considered the drive point O of the slot  6 . In this embodiment, the drive point O lies at the centre of the slot, that is, at the crossing of the slot&#39;s horizontal and vertical centre-lines Ref. Line A, Ref Line B. 
   The PCB assembly  120  includes two pieces of rectangular, single-sided FR-4 (i.e. epoxy glass laminate substrate) PCB  1 ( a ),  1 ( b ) attached to opposite sides of the slot antenna assembly  110 . Each FRA PCB  1 ( a ),  1 ( b ) includes a layer of copper on one side. Each FR-4 PCB  1 ( a ),  1 ( b ) is composed of one-ounce FR-4 material. Advantageously, performance is improved by locating the copper layer on the outer side  121 ,  122  of each FR-4 PCB  1 ( a ),  1 ( b ). The FR-4 PCBs  1 ( a ),  1 ( b ) are attached to opposite edges of the RF-35 PCB  3  by gluing. The FR-4 PCBs  1 ( a ),  1 ( b ) are generally parallel to each other and perpendicular to the RF-35 PCB  3 . 
   It is known that for a conventional quarter-wavelength slot antenna, the input impedance at the feed point is approximately 500 ohms, which is difficult to match to a 50-ohm connector. According to an embodiment of the invention, a slot  6  approximately 88 mm in length (i.e. 0.715 wavelength) is used to lower the input impedance to approximately 200 ohms and to increase the gain of the slot  6  to approximately 3.5 dB. 
   In addition, the two FR-4 PCBs  1 ( a ),  1 ( b ) of the PCB assembly  120  provide several advantages. Firstly, by optimizing the design and spacing of the two FR-4 PCBs  1 ( a ),  1 ( b ), the input impedance of the slot antenna  100  is further reduced to approximately 70 ohms. This allows the slot antenna  100  to be more easily matched to the 50-ohm connector  4 . Secondly, the two FR-4 PCBs  1 ( a ),  1 ( b ) function to remove the nulls at 90° and 270° in the horizontal radiation pattern of the slot antenna  100  so that an omni-directional radiation pattern can be achieved. Based on tests performed by the applicants, a difference between maximum and minimum radiation levels of less than 2 dB may be achieved with the slot antenna  100  of the present invention. 
   According to this embodiment of the invention, the width of the slot  6  is approximately 3.6 mm (i.e. 0.028 wavelength) at 2.4 GHz, which corresponds to a free-space wavelength at 2.4 GHz of approximately 123 mm. This width is wide enough to achieve an operational frequency bandwidth of 83 MHz (i.e. a frequency band from 2.4 to 2.483 GHz) and is also narrow enough to ensure a low cross-polarization radiation level of approximately −20 dB. In this embodiment, the length and width of the RF-35 PCB  3  are 140.8 mm and 23.4 mm, respectively. The length and width of each single-sided FR-4 PCB  1 ( a ),  1 ( b ) arc approximately 140.8 mm and 24 mm, respectively. And, the inner sides  123 ,  124  of the two single-sided FR-4 PCBs  1 ( a ),  1 ( b ) are spaced approximately 20.3 mm apart. 
     FIG. 2  is a front view illustrating a two-element omni-directional slot antenna array  200  in accordance with an embodiment of the invention. The array  200  includes two slot antenna elements  201 ,  202  that are similar to the single-element slot antenna  100  of FIGS.  1 ( a ),  1 ( b ), and  1 ( c ). A RF signal is delivered to or received from the first element  201  via a 50-ohm connector  8 . A portion of the RF signal is further delivered to or received from the second element  202  though a microstrip line  9 . From the connector  8  to Ref. Line E, the microstrip feed line  9  has a first common width. Between Ref. Line E and Ref. Line D, the microstrip feed line  9  has a second common width that is narrower that the first common width. According to one embodiment of the invention, the first and second common widths are approximately 3.6 mm and 2.4 mm, respectively. The width of the microstrip feed line  9  is designed to provide appropriate matching and phase shifting so that the RF signal delivered to both the first and second elements  201 ,  202  will have approximately the same amplitude and will be approximately in-phase. In this embodiment, the maximum antenna gain achieved is approximately 5.5 dBi. Note that the array  200  includes two continuous rectangular FR-4 PCBs  10 ( a ),  10 ( b ), a continuous rectangular ground plane  207 , and two vertically-spaced rectangular slots  203 ,  204 . At 2.4 GHz, the slotts  203 ,  204  are spaced vertically by approximately 129.6 mm (i.e. from Ref. Line D to Ref. Line E). 
   FIGS.  3 ( a ) and  3 ( b ) are front and right-hand side views, respectively, illustrating a centre-fed four-element omni-directional slot antenna array  300  in accordance with an embodiment of the invention. The array  300  includes two two-element slot antenna arrays  301 ,  302  that are similar to the two-element slot antenna array  200  of FIG.  2 . The second two-element slot antenna array  302  is the mirror image of the first two-element slot antenna array  301  with respect to Ref. Line F. Both the first and second arrays  301 ,  302  share a common feed point B along Ref. Line F. The array  300  has a 50-ohm connector  11  that is connected to a 50-ohm low-loss semi-rigid cable  12 . The cable  12  is terminated along Ref. Line F on the RF-35 PCB  16 . The cable  12  may be a coaxial cable having a conventional inner conductor, insulator, and an outer conductor or shield. The outer conductor of the cable  12  is connected (e.g. soldered) to the ground plane  307  on the front-side of the RF-35 PCB  16 . The inner conductor of the cable  12  is connected (i.e. soldered) to feed point B on the microstrip feed line  15  on the back-side of the RF-35 PCB  16 . The inner conductor of the cable  12  passes through the RF-35 PCB  16  at point A along the horizontal centre-line Ref. Line F of the RF-35 PCB  16 . Note that the array  300  includes two continuous rectangular FR-4 PCBs  13 ( a ),  13 ( b ), a continuous rectangular ground plane  307 , and four vertically-spaced rectangular slots  303 ,  304 ,  305 ,  306 . 
   In operation, a RF signal is fed into the 50-ohm connector  11 . The signal travels through the 50-ohm low-loss semi-rigid cable  12  to the feed point B along Ref. Line F. This coaxial cable-to-printed microstrip food line transition provides an effective 50-ohm match for the RP signal. The signal is then equally distributed between the first and second arrays  301 ,  302 . That is, half of the signal energy will be distributed to the first array  301  via that portion of the stripline extending downward from Ret. Line F and half of the signal energy will be distrbuted ot the second array  302  via that portion of the stripline extending upward from Ref. Line F. Therefore, both the first and second arrays  301 ,  302  are fed with signals of approximately the same amplitude and phase. The cable  12  attached to the microstrip feed line  15  has little effect on the radiation pattern. Thus, the four-element slot antenna array  300  provides an effective omni-directional radiation pattern. 
   FIGS.  4 ( a ),  4 ( b ), and  4 ( c ) are front, right-hand side, and bottom views, respectively, illustrating a single-element 180 degree sectorial slot antenna  400  in accordance with an embodiment of the invention. The 180 degree sectorial slot antenna  400  includes the single-element omni-directional slot antenna  100  of FIGS.  1 ( a ),  1 ( b ), and  1 ( c ). In addition, the 180 degree sectorial slot antenna  400  includes a bent metal reflector assembly  17 . The reflector  17  includes a rectangular centre panel  410 , a first rectangular side panel  411  joined to one vertical edge  413  of the centre panel  410 , and a second rectangular side panel  412  joined to the opposite vertical edge  414  of the centre panel  410 . According to this embodiment, the width W 1  of the centre panel  410  is approximately 70 mm. The width W 2  of each side panel  411 ,  412  is approximately 70 mm. The angle X between the plane defined by the centre panel  410  and the planes defined by each of the side panels  411 ,  412  is approximately 135 degrees. The reflector  17  is mounted behind the single-clement slot antenna  100  such that the planes defined by the centre panel  410  and the slot antenna assembly  110  are parallel. The mounting of the reflector  17  may be accomplished in several ways. The slot antenna  100  and reflector  17  may be mounted in a common enclosure or radome. Or, the slot antenna  100  may be mounted in an enclosure or radome and the reflector  17  may be attached to this enclosure by screws and spacers, for example. In this embodiment, the spacing S between the reflector  17  and the single-element slot antenna  100  is approximately 32 mm. The single-element slot antenna  100  is centred along the horizontal centre-line Ref. Line G of the centre panel  410  of the reflector  17 . In general, the length and width of the centre panel  410  of the reflector  17  are greater than the length and width of the slot antenna assembly  110  of the single-element slot antenna  100 . 
   The reflector  17  provides electromagnetic reflection for the slot antenna  100  such that a horizontal beamwidth of approximately 180 degrees is achieved. Hence, the antenna  400  is referred to as a 180 degree sectorial slot antenna. At 2.4 GHz, a horizontal beamwidth of approximately 180 degrees is achieved with a spacing S of approximately 32 mm between the reflector  17  and the slot antenna  100 , an angle X of approximately 135 degrees between the centre panel  410  and each side panel  411 ,  412  of the reflector, and centre and side panel widths W 1 , W 2  of approximately 70 mm each. By modifying the spacing S between the reflector  17  and the slot antenna  100 , the angle X between the centre panel  410  and side panels  411 ,  412  of the reflector  17 , and/or the width W 1 , W 2  of each reflector panel  410 ,  411 ,  412 , different sectorial ranges can be achieved. These modifications affect the electromagnetic field distribution and hence the horizontal beamwidth of the antenna  400 . According to another embodiment of the invention, the widths W 1 , W 2  of the centre and side panels  410 ,  411 ,  412  are set to approximately 52 mm and 48 mm, respectively. 
   FIGS.  5 ( a ),  5 ( b ), and  5 ( c ) are front, right-hand side, and bottom views, respectively, illustrating a single-element 120 degree sectorial slot antenna  500  in accordance with an embodiment of the invention. The 120 degree sectorial slot antenna  500  includes the single-element omni-directional slot antenna  100  of FIGS.  1 ( a ),  1 ( b ), and  1 ( c ). In addition, the 120 degree sectorial slot antenna  500  includes first and second flat rectangular metal reflectors  20 ,  21 . The width W 3  of the first reflector  20  is approximately 51 mm. The width W 4  of the second reflector  21  is approximately 106 mm. The first reflector  20  is mounted behind the single-element slot antenna  100  such that the planes defined by the first reflector  20  and the slot antenna assembly  110  are parallel. The second reflector  21  is mounted behind the first reflector  20  such that the planes defined by each reflector  20 ,  21  are parallel. The mounting of the reflectors  20 ,  21  may be accomplished in several ways. The slot antenna  100  and reflectors  20 ,  21  may be mounted in a common enclosure or radome. Or, the slot antenna  100  may be mounted in an enclosure or radome and the reflectors  20 ,  21  may be attached to this enclosure by screws and spacers, for example. In this embodiment, the spacing S 1  between the first reflector  20  and the single-element slot antenna  100  is approximately 12.7 mm. The spacing S 2  between the first reflector  20  and the second reflector  21  is approximately 19.1 mm. The single-element slot antenna  100  is centred along the horizontal centre-line Ref. Line H of the first reflector  20 . The first reflector  20  is centred along the horizontal centre-line Ref. Line H of the second reflector  21 . In general, the length and width of the first reflector  20  are greater than the length and width of the slot antenna assembly  110  of the single-element slot antenna  100 . And, the length and width of the second reflector  21  are greater than the length and width of the first reflector  20 . 
   The reflectors  20 ,  21  provide electromagnetic reflection for the slot antenna  100  such that a horizontal beamwidth of approximately 120 degrees is achieved. Hence, the antenna  500  is referred to as a 120 degree sectorial slot antenna. At 2.4 GHz, a horizontal beamwidth of approximately 120 degrees is achieved with spacings S 1 , S 2  between the reflectors  20 ,  21  and the slot antenna  100  of approximately 12.7 mm and 19.1 mm, respectively, and reflector panel widths W 3 , W 4  of approximately 51 mm and 106 mm, respectively. By modifying the spacings S 1 , S 2  between the reflectors  20 ,  21  and the slot antenna  100 , and/or the widths W 3 , W 4  of each reflector  20 ,  21 , different sectorial ranges can be achieved. These modifications affect the electromagnetic field distribution and hence the horizontal beamwidth of the antenna  400 . In general, the beamwidth of the antenna  500  will be narrowed by increasing the width W 3  of the first reflector  20  while keeping the width W 4  of the second reflector  21  constant. The beamwidth of the antenna  500  will be widened by increasing the width W 4  of the second reflector  21  while keeping the width W 3  of the first reflector  20  constant. Experiments conducted by the applicant have found that, in general, the beamwidth is affected more by varying the width W 3  of the first reflector  20  than by varying the width W 4  of the second reflector  21 . According to another embodiment of the invention, the spacings S 1 , S 2  between the reflectors  20 ,  21  and the slot antenna  100  are set to approximately 16 mm and 32 mm, respectively, and the reflector panel widths W 3 , W 4  are set to approximately 48 min and 52 mm, respectively. 
   To reiterate and expand, the present invention provides a slot antenna having a slot that is optimized for maximum gain and improved cross-polarization performance. The slot length is approximately 73% of the desired wavelength and the width is approximately 3% of the desired wavelength. The slot antenna includes two conductive sheets (i.e. single-sided FR-4 PCBs) mounted on each side of the slot, and symmetric with respect to the slot, which provide an omni-directional radiation pattern for the antenna by removing the nulls at 90° and 270° that are due to limited ground plane size. In addition, the invention provides a series-fed two-element antenna array and a coaxial cable-to-microstrip transition parallel-fed four-element antenna array. Furthermore, by adding a bent metal reflector behind the slot antenna, the invention provides a sectorial antenna with approximately 180 degree beamwidth shaping. Moreover, by adding two flat meter reflectors behind the slot antenna, the invention provides a sectorial antenna with approximately 120 degree beamwidth shaping. 
   Although preferred embodiments of the invention have been described herein, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.