Patent Publication Number: US-9431711-B2

Title: Broadband multi-strip patch antenna

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to antennas and, more particularly, to multi-layer patch antennas. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Wireless communication requires the use of an antenna to transmit and receive electromagnetic signals. Several antenna types are available for a variety of purposes and the choice of selecting one type of antenna or another typically depends upon the particular application of the antenna. To select an antenna, various operating characteristics of the antennas may be evaluated and compared to determine the type of antenna that provides the most benefit or is best suited for a specific application. 
     Occasionally, one antenna having all or most of the desired operating characteristics for a particular application may not exist and there may be several antennas having varying combinations of favorable and unfavorable aspects. For instance, a small antenna with a low profile and a wide bandwidth may generally be preferred for modern wireless communication. A microstrip or patch antenna is a relatively inexpensive antenna that is capable of being easily integrated with many electronic devices. Although the patch antenna may feature a low-profile, its relatively large size (approximately one-half wavelength) and narrow bandwidth (approximately 5%) may be a disincentive for its use in some wireless applications. However, various techniques have been developed to significantly reduce the size of the patch antenna. For example, by shorting one edge of the patch antenna and/or folding the patch antenna over itself, a reduction to one-fourth its original size may be achieved. Unfortunately, reducing the size of the patch antenna in this manner may also significantly reduce its bandwidth, e.g., 1.3% fractional bandwidth. The bandwidth of current patch antennas is therefore too narrow for practical use in short to medium range wireless communication systems, e.g., wireless microphones, wireless audio monitoring systems, local wireless data networks, wireless medical devices. 
     SUMMARY 
     Example apparatus and methods to provide an antenna for use in a wireless system are herein described. In one example embodiment, the antenna includes a main patch, a parasitic patch, and a ground plane having a ground strip extending from the ground plane. The main patch includes a first strip and a second strip, wherein at least a portion of the first strip of the main patch is positioned above the ground strip and forms a first radiating edge with the ground strip, and at least a portion of the second strip of the main patch is positioned below the ground strip and forms a second radiating edge with the ground plane. The parasitic patch is coupled to the main patch along at least a portion of a non-radiating edge of the main patch. The parasitic patch includes a first strip and a second strip, wherein at least a portion of the first strip of the parasitic patch is disposed above the ground strip and at least a portion of the second strip of the parasitic patch is disposed below the ground strip. 
     If desired, the antenna may include a tuning strip directly coupled to the parasitic patch and the ground strip. The antenna may further include at least a portion of the first strip of the main patch and at least a portion of the first strip of the parasitic patch lie in a first plane, and at least a portion of the second strip of the main patch and at least a portion of the second strip of the parasitic patch lie in a second plane, wherein the first plane and the second plane are different and the first plane may or may not be parallel to the second plane. Additionally, a second parasitic patch may be coupled to the main patch along at least a portion of a second non-radiating edge of the main patch. The second parasitic patch includes a first strip and a second strip, at least a portion of the first strip of the second parasitic patch is disposed above the ground strip and at least a portion of the second strip of the second parasitic patch disposed below the ground strip. The main patch, first parasitic patch, and second parasitic patch each include a length and a width. The lengths of the main patch, first parasitic patch, and second parasitic patch may be the same or different, and the widths of the main patch, first parasitic patch, and second parasitic patch may be the same or different. 
     Another example embodiment of the antenna may include a flexible printed circuit board including the main patch and one or both of the first and second parasitic patches. The flexible printed circuit board is folded about the ground strip and a stiffener to support the flexible circuit board and may be attached to one or more supports. An alternative implementation of the antenna may include a plurality of printed circuit boards, wherein a first printed circuit board includes the first strip of the main patch and the first strip of one or both of the first and second parasitic patches, a second printed circuit board includes the ground strip, and a third printed circuit board includes the second strip of the main patch and the second strip of one or both of the first and second parasitic strips. A first connector operatively couples the first strip of the main patch to the second strip of the main patch and a second connector operatively couples the first strip of the parasitic patch to the second strip of the parasitic patch. If a second parasitic patch is used, a third connector operatively couples the first and second strips of the second parasitic patch. One or more spacers and one or more supports may be utilized to separate and arrange the first, second, and third printed circuit boards in a layered, low-profile configuration. 
     An additional example embodiment is directed to providing an antenna for use in a wireless system. The method includes providing a ground strip extending from a ground plane and providing a main patch including a first strip and a second strip. The method positions the main patch about the ground strip, wherein at least a portion of the first strip of the main patch is positioned above the ground strip and forms a first radiating edge with the ground strip, and at least a portion of the second strip of the main patch is positioned below the ground strip and forms a second radiating edge with the ground plane. The method couples a parasitic patch to the main patch along at least a portion of a non-radiating edge of the main patch, wherein the parasitic patch includes a first strip and a second strip, and wherein at least a portion of the first strip of the parasitic patch is positioned above the ground strip and at least a portion of the second strip of the parasitic patch is positioned below the ground strip. The method provides for adjusting the bandwidth of the antenna by performing one or more of the following steps: attaching a tuning strip between that parasitic patch and the ground strip, changing a size of the tuning strip, changing a position of the tuning strip between the parasitic patch and the ground strip, changing a position of a feeding pin; directly coupling the main patch to the parasitic patch; gap-coupling the main patch to the parasitic patch; adjusting a spatial relationship between a gap-coupled main patch and parasitic patch; maintaining a constant spatial relationship between the first strip of the main patch and the second strip of the main patch, maintaining a constant spatial relationship between the first strip of the parasitic patch and the second strip of the parasitic patch, varying a spatial relationship between at least a portion of the first strip of the main patch and at least a portion of the second strip of the main patch, varying a spatial relationship between at least a portion of the first strip of the parasitic patch and at least a portion of the second strip of the parasitic patch, varying a spatial relationship between at least a portion of the second strip of the main patch and a ground plane, modifying a width of the main patch to be different in comparison to a width of the parasitic patch, and modifying a length of the main patch to be different in comparison to a length of the parasitic patch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are perspective views illustrating one example of a broadband multi-strip antenna. 
         FIG. 2A  is a perspective view illustrating the example broadband multi-strip antenna shown in  FIGS. 1A and 1B  wherein the driven strip, or main patch, is highlighted. 
         FIG. 2B  is a perspective view illustrating the example broadband multi-strip antenna shown in  FIGS. 1A and 1B  wherein the feeding pin is highlighted. 
         FIG. 3  is a perspective view illustrating the example broadband multi-strip antenna shown in  FIGS. 1A and 1B  wherein the ground strip is highlighted. 
         FIG. 4A  is a perspective view illustrating the example broadband multi-strip antenna shown in  FIGS. 1A and 1B  wherein one of two parasitic patches is highlighted. 
         FIG. 4B  is a perspective view illustrating the example broadband multi-strip antenna shown in  FIGS. 1A, 1B, and 4A  wherein the other of two parasitic patches is highlighted. 
         FIG. 5  is a perspective view illustrating the example broadband multi-strip antenna shown in  FIGS. 1A and 1B  wherein the tuning strip(s), or current modifying strip(s) is highlighted. 
         FIG. 6A  is a plan view of the example broadband multi-strip antenna shown in  FIGS. 1A and 1B . 
         FIG. 6B  is a left-side elevation view of the example antenna shown in  FIG. 6A ; 
         FIG. 6C  is a front-side elevation view of the example antenna shown in  FIG. 6A  taken along cross-sectional line  6 C- 6 C; 
         FIG. 6D  is a right-side elevation view of the example antenna shown in  FIG. 6A . 
         FIGS. 7A and 7B  are perspective views of one example embodiment of a broadband multi-strip antenna. 
         FIGS. 8A-8C  are perspective views of another example embodiment of a broadband multi-strip antenna. 
         FIGS. 9A-9C  are various views of another example embodiment of a broadband multi-strip antenna. 
         FIG. 10  is a table showing antenna sizes for several example embodiments of the broadband multi-strip antenna. 
         FIG. 11  is a table showing various antenna performance parameters for several example embodiments of the broadband multi-strip antenna. 
         FIG. 12  is a graph of voltage standing wave ratio (VSWR) versus frequency for an example broadband multi-strip antenna. 
         FIG. 13  is a graph of VSWR versus frequency illustrating effects of a gradual increase in ground plane separation for an example broadband multi-strip antenna. 
         FIG. 14A  is a graph of VSWR versus frequency for an example antenna in free space and for the example antenna mounted on a metal surface. 
         FIG. 14B  is a polar chart of radiation patterns for the example antenna of  FIG. 14A  operating in free space and for the example antenna mounted on a metal surface. 
         FIG. 15A  is a graph of VSWR versus frequency for another example antenna assembled in accordance with the teachings of the present invention operating in free space and for the example antenna mounted on a metal surface. 
         FIG. 15B  is a polar chart of radiation patterns for the example antenna of  FIG. 15A  operating in free space and for the example antenna mounted on a metal surface. 
     
    
    
     For purposes of clarification and ease of illustration, it is to be understood that certain portions of the several example embodiments of the antenna have been depicted in the figures in shading and/or hidden lines, which may or may not be present in other corresponding views and/or figures. 
     DETAILED DESCRIPTION 
     The disclosed apparatus and method provide for a low profile, compact, broadband antenna for use in modern wireless applications. In general, a multi-layer multi-strip configuration is utilized to overcome the known conflict in patch antenna design between size reduction and bandwidth broadening. In particular, the disclosed apparatus and method incorporate various combinations of a folded main patch with two radiating edges, one or more parasitic patches coupled to the main patch, and/or one or more shorting strips coupled between the one or more parasitic strips and a ground plane to achieve a significant size reduction in all dimensions and a significant broadening of the fractional bandwidth with respect to a conventional patch antenna. 
       FIGS. 1-6  generally depict an example broadband multi-strip antenna  100 . More specifically,  FIGS. 1A and 1B  depict the antenna  100  including an antenna block  110  and a ground plane  106 . The antenna block  110  includes a main patch  101  (shown highlighted in  FIG. 2A ) positioned about a ground strip  102  (shown highlighted in  FIG. 3 ) extending from a ground plane  106 . A feeding pin  203 , shown in  FIGS. 2A and 2B , extends through an opening in the ground plane  106  (see  FIGS. 6B, 6C, and 6D ) and is coupled to the main patch  101  to transfer energy to and from the antenna block  110 . 
     The antenna block  110  further includes a first parasitic patch  103  (shown highlighted in  FIG. 4A ) coupled to the main patch  101  along at least a portion of a first non-radiating edge of the main patch  101 , and a second parasitic patch  104  (shown highlighted in  FIG. 4B ) coupled to the main patch  101  along at least a portion of a second non-radiating edge of the main patch  101 . One or both of the parasitic patches  103 ,  104  may also be directly coupled to the ground strip  102  by a tuning strip  105 - 1 ,  105 - 2  (shown highlighted in  FIG. 5 ), respectively. 
     In the example embodiment shown in  FIG. 1-6 , the main patch  101  is positioned very close to the parasitic patches  103 ,  104  and is considered to be gap-coupled to the parasitic patches. In this gap-coupled configuration, there is not a direct coupling between the main patch  101  and the parasitic patches  103 ,  104  and surface current is therefore not able to flow between the main patch and the parasitic patches. However, because of the proximity of the parasitic patches  103 ,  104  to the main patch  101 , RF energy is able to be transferred from the main patch  101  to the parasitic patches  103 ,  104  through the electromagnetic field that emanates from the main patch. Due to the gap coupling, the RF energy potential at the main patch  101  may be slightly different than the RF energy potential at each of the parasitic patches  103 ,  104 . For example, the gap coupling between the main patch  101  and the parasitic patches  103 ,  104  may provide for amplitude and phase difference in RF energy potentials at the main patch and the parasitic patches. By adjusting the distance or spacing of the gap(s) between the main patch  101  and the parasitic patches  103 ,  104 , it may be possible to attain certain amplitude and phase differences in RF energy potentials at the patches  101 ,  103 ,  104  and broaden the antenna&#39;s bandwidth. 
     Alternatively, the main patch  101  may be directly coupled to one or both of the parasitic patches  103 ,  104 . In a direct coupling configuration, a conductor, e.g., conductive metal, connects the main patch  101  to one or both of the parasitic patches  103 ,  104 . RF energy is propagated from the main patch  101  to the parasitic patches  103 ,  104  via the conductor and the RF energy potential at the point of coupling contact on the main patch may be very similar to the RF energy potential at the point of coupling contact on the parasitic patches. The location of the direct coupling determines the surface current pattern on the parasitic patches. By adjusting the location of the conductor that connects the main patch to the parasitic patches, it may be possible to attain a certain surface current distribution on the parasitic patches and broaden the antenna&#39;s bandwidth. 
     Referring briefly to  FIG. 3 , to provide direct coupling of the ground strip  102  to the ground plane  106 , the ground strip  102  may include a horizontal portion  102 - 1  and a vertical portion  102 - 2 . The horizontal portion  102 - 1  is disposed between the upper  101 - 1  and the lower  101 - 2  strips of the main patch  101  and the vertical portion  102 - 2  extends down from the horizontal portion  102 - 1  and couples the ground strip  102  to the ground plane  106 . 
     In  FIG. 2A , the main patch  101  of the antenna  100  includes a first, upper strip  101 - 1 , having at least a portion disposed above the ground strip  102  and a second, lower strip  101 - 2 , having at least a portion disposed below the ground strip  102 . The main patch  101  has a width and a length and forms a pair of radiating edges with the ground strip  102  and the ground plane  106  at opposing sides of the main patch above and below the ground strip  102 . More specifically, a first radiating edge  201  includes a first radiating slot  601  (shown in  FIG. 6C ) formed by the upper strip (or segment)  101 - 1  of the main patch  101  and the ground strip  102 , and a second radiating edge  202  includes a second radiating slot  602  (shown in  FIG. 6C ) formed by the lower strip (or segment)  101 - 2  of the main patch  101  and the ground plane  106 . Incorporating two radiating edges  201 ,  202  in the folded arrangement of the main patch  101 , the ground strip  102 , and the ground plane  106  increases radiation efficiency and reduces the quality factor (Q) of the antenna  100  as compared to a folded patch antenna assembly having one of its radiating edges shorted to ground. As a result, the dual radiating edges  201 ,  202  of the main patch  101  of the antenna  100  allows for a broader band impedance match to be achieved by the antenna  100 , which leads to the broadband operation of the antenna  100 . 
     As known, a patch antenna generally resonates at a frequency determined by the length of its driven patch, and the resonant length of the driven patch is approximately λ 0 /(2√{square root over (∈ r )}), where λ 0  is the free space wavelength of the lowest operating frequency of the antenna and ∈ r  is the relative permittivity of the dielectric material between the patch and the ground plane or the ground strip. When the dielectric material is air, its ∈ r  equals to 1. The length of the main patch  101  is therefore selected according to the lowest operating frequency of the desired operating frequency range of the antenna  100 . However, due to the folded arrangement of the main patch  101 , the overall length of the antenna element  110  may be reduced. 
     The width of a patch antenna generally affects the input impedance of the antenna and the dimension of the width may be selected to provide a good impedance match at the antenna input. Due in part to the coupling of the parasitic patches  103 ,  104  to the patch antenna  100 , the width of the main patch  101  may be reduced for a particular desired bandwidth. The width of the main patch  101  may further be reduced through the implementation of the one or more tuning strips  105 . Through the combination of one or more of these size reduction techniques, the width and length of the antenna block  110  may be reduced to approximately λ 0 /6. 
     The parasitic patches  103 ,  104  are provided in the antenna  100  to enhance the broadband performance of the antenna  100 . To this end, a length and a width for each of the parasitic patches  103 ,  104  are selected to achieve a suitable input impedance match for the antenna  100  in a suitably wide frequency band. Although the size of the antenna  100  will generally increase with the addition of the parasitic patches  103 ,  104 , the size increase may be offset, at least partially, by using a folded arrangement of the parasitic patches  103 ,  104  similar to the folded arrangement of the main patch  101 . Accordingly, each of the parasitic patches  103 ,  104  may be folded about the ground strip  102  as illustrated in  FIGS. 4A and 4B . As shown in  FIG. 4A , the first parasitic patch  103  includes a first, upper strip (or segment)  103 - 1  and a second, lower strip (or segment)  103 - 2 . At least a portion of the upper strip  103 - 1  of the first parasitic patch  103  is disposed above the ground strip  102  and at least a portion of the lower strip  103 - 2  of the first parasitic patch  103  is disposed below the ground strip  102 . Similarly, as shown in  FIG. 4B , the second parasitic patch  104  includes a first, upper strip (or segment)  104 - 1  and a second, lower strip (or segment)  104 - 2 . At least a portion of the upper strip  104 - 1  of the second parasitic patch  104  is disposed above the ground strip  102  and at least a portion of the lower strip  104 - 2  of the second parasitic patch  104  is disposed below the ground strip  102 . It is to be noted that the antenna  100  is not limited to the two-parasitic patch implementation illustrated in  FIGS. 1-6 , and, in some embodiments, the antenna  100  may include any other suitable amount (e.g., 1, 3, 4, etc.) of parasitic patches. For example, either one of the parasitic patches  103 ,  104  may be omitted from the antenna  100 . 
     The tuning strips  105 - 1 ,  105 - 2  shown in  FIG. 5  may be utilized to modify the distribution of electric current (or magnetic field) on the parasitic patches  103 ,  104  to further enhance broadband performance of the antenna  100 . To this end, at least one tuning strip  105 - 1 ,  105 - 2  may be arranged such that a suitable impedance matching for the antenna  100  is achieved over a wider frequency range compared to the frequency range provided by non-modified current distribution on the parasitic patches  103 ,  104 . As a result, the tuning strips  105 - 1 ,  105 - 2  further increase the fractional bandwidth of the antenna  100 . To achieve a desired electrical current distribution on each of the parasitic patches  103 ,  104 , the location and the width of each of the tuning strips  105 - 1 ,  105 - 2  are selected based on the standing wave current distribution on the corresponding parasitic patch  103 ,  104 . By selecting a desired shorting location along the standing wave current pattern, and by controlling the length of the shorting element (i.e., the tuning strip  105 - 1 ,  105 - 2 ), current distribution is shaped in a controlled manner and a desired current distribution is thereby achieved. The location and the width of either of the tuning strips  105 - 1 ,  105 - 2  may be determined empirically and/or through the use of an electromagnetic analysis software tool. For example, the desired antenna bandwidth for the antenna  100  may be achieved by positioning each of the tuning strips  105 - 1 ,  105 - 2  near or nearer the vertical portion  102 - 2  of the ground strip  102  extending from the ground plane  106 . 
       FIG. 6A  illustrates a plan view of the antenna  100  shown in  FIGS. 1A and 1B . In particular, the lengths and the widths of the patches  101 ,  103 ,  104  need not be the same. For example, the length of each of the patches  101 ,  103 ,  104  may be selected such that each patch resonates at a slightly different frequency with respect to each other. Selecting different lengths for the patches  101 ,  103 ,  104  leads to a broader bandwidth of the antenna  100 . As an example, the length of the first parasitic patch  103  may be slightly less than the length of the main patch  101 , which may lead to a slightly higher resonant frequency of the first parasitic patch  103  relative to the resonant frequency of the main patch  101 , thereby possibly extending the impedance bandwidth of the antenna  100  in a frequency band above the center operational frequency of the antenna  100 . The length of the second parasitic patch  104 , on the other hand, may be slightly greater than the length of the main patch  101 , which may lead to a slightly lower resonant frequency of the second parasitic patch  104  relative to the resonant frequency of the main patch  101 , thereby possibly extending the impedance bandwidth of the antenna  100  in a frequency band below the center operational frequency of the antenna  100 . The widths of the patches  101 ,  103 ,  104  may also be selected to further optimize the impedance bandwidth by providing a suitable impedance match for the antenna  100  over a wider frequency band. It should be noted that the location of the feeding pin  203  may also be altered, providing an additional tuning parameter for achieving a desired broadband performance of the antenna  100 . 
       FIGS. 6B, 6C, and 6D  illustrate, respectively, a left-side elevation view, a front-side cross-sectional elevation view, and a right-side elevation view of the antenna  100  shown in  FIG. 6A . As can be seen in the cross-sectional view of  FIG. 6C  taken along lines  6 C- 6 C of  FIG. 6A , the first radiating edge  201  of the antenna  100  includes the first radiating slot  601  formed between the upper strip  101 - 1  of the main patch  101  and the ground strip  102 , and the second radiating edge  202  of the antenna  100  includes the second radiating slot  602  formed between the lower strip  101 - 2  of the main patch  101  and the ground plane  106 . When the length of the main patch  101  is approximately λ 0 /2, the current and voltage distribution along the main patch  101  is such that the current at each of the radiating edges  201  and  202  is at approximately zero and the voltage is at a maximum. 
     In the layered arrangement illustrated in  FIGS. 1-6 , the upper strips  101 - 1 ,  103 - 1 ,  104 - 1  lie in a first plane in space and the lower strips  101 - 2 ,  103 - 2 ,  104 - 2  lie in a second plane in space. The horizontal portion  102 - 1  of the ground strip  102  lies in a third plane in space, and the ground plane  106  lies in a fourth plane in space. The first, second, third, and fourth planes are parallel with respect to each other, in the illustrated embodiment. As will be explained in more detail below in connection with  FIGS. 9A-9C , in some embodiments, at least a portion of the second plane (i.e., the plane that includes the lower strips  101 - 2 ,  103 - 2 ,  104 - 2  of the patches  101 ,  103 ,  104 ) may be angled with respect to the first, third, and fourth planes, providing a gradual change or increase of separation between the ground plane  106  and the antenna block  110  in the angled portion of the antenna block  110 . Providing such a gradual increase in separation between the antenna block  110  and the ground plane  106  may further increase the bandwidth of the antenna  100 , in at least some configurations (see the discussion of  FIGS. 9A-9C ). 
       FIGS. 7A and 7B  depict one embodiment of an antenna structure  700  utilizing printed circuit boards  702  to implement the antenna  100  of  FIGS. 1-6 . A first circuit board  702 - 1  includes the upper strip  101 - 1  of the main patch  101 , the upper strip  103 - 1  of the first parasitic patch  103 , and the upper strip  104 - 1  of the second parasitic patch  104 . A second circuit board  702 - 2  includes the lower strip  101 - 2  of the main patch  101 , the lower strip  103 - 2  of the first parasitic patch  103 , and the lower strip  104 - 2  of the second parasitic patch  104 . A third circuit board  702 - 3  includes the horizontal portion  102 - 1  of the ground strip  102 . In combination, the circuit boards  702  form the antenna block  110  of the antenna  100 . The circuit board  702 - 3  may comprise a sheet of suitable metal, such as copper or aluminum attached to a suitable non-conductive substrate, such as layered fiberglass epoxy FR4, for example. The patch strips  101 ,  103 , and  104  may be printed on circuit boards  702 - 1  and  702 - 2 , or generated on the circuit boards  702 - 1 ,  702 - 2  using any other suitable process, such as, for example, etching. 
     In the embodiment shown in  FIG. 7A , the circuit boards  702  are mounted to the ground plane  706  using one or more, e.g., a set, of non-conductive screws and/or spacers  701  disposed between the layers of the antenna structure  700 . For example, spacers  701  may be positioned near the corners of the circuit boards  702 - 1 ,  702 - 2 ,  702 - 3  between each layer of the antenna structure  700 . An advantage of using spacers  701  for arranging the layers of the antenna structure  700  is that, in this arrangement, the separation between the layers can be easily and precisely controlled. Alternatively, another assembly process for the antenna structure  700  may use one or more non-conductive walls extending from the ground plane  706  to arrange the circuit boards  702 . In such embodiments, one or more screws and/or spacers  701  may be omitted from the antenna structure  700 . 
     Referring now to  FIG. 7B , each of the upper strips  101 - 1 ,  103 - 1 ,  104 - 1  is coupled to the corresponding lower strip  101 - 2 ,  103 - 2 ,  104 - 2  with a respective connector  703 . In particular, a connector  703 - 2  couples the upper strip  101 - 1  with the lower strip  101 - 2  of the main patch  101 , a connector  703 - 1  couples the upper strip  103 - 1  with the lower strip  103 - 2  of the first parasitic patch  103 , and a connector  703 - 3  couples the upper strip  104 - 1  with the lower strip  104 - 2  of the second parasitic patch  104 . Similarly, a connector  703 - 4  couples the ground PCB  702 - 3  with the ground plane  706 , as shown in  FIG. 7A . If desired, one or more tuning strips  707  may be connected between the first parasitic patch  103  and the ground PCB  702 - 3  and/or the second parasitic patch  104  and the ground PCB  702 - 3 . 
       FIGS. 8A-8C  depict an antenna structure  800  implementing the antenna  100  of  FIG. 1-6 , according to another embodiment, wherein the main patch  101 , the first parasitic patch  103 , and the second parasitic patch  104  are printed on a flexible circuit board  801 . The flexible circuit board  801  is folded about a ground strip  802  which extends from or is connected to a ground plane  804 . The folded flexible circuit board  801  and the ground strip  802  may be held in place with one or more non-conductive supports  803 , for example, walls. In some embodiments, as illustrated in  FIG. 8C , the antenna assembly  800  may also include one or more stiffeners  805  to produce a desired shape of the folded flexible circuit board  801 . If desired, one or more tuning strips  807  may be connected between the first parasitic patch  103  and the ground strip  802  and/or the second parasitic patch  104  and the ground strip  802 . Using a flexible circuit board instead of two separate boards for the lower and the upper strips generally simplifies the manufacturing process of the antenna  100  by eliminating the need to separately connect respective lower and upper strips of the antenna patches  101 ,  103 , and  104 . 
       FIGS. 9A-9C  depict another embodiment of an antenna structure  900  implementing the antenna  100  of  FIG. 1-6 , wherein the main patch  101 , the first parasitic patch  103 , and the second parasitic patch  104  are printed on a flexible circuit board  905 . The flexible circuit board  905  is folded about the ground strip  908  which extends from or is connected to the ground plane  904 . The folded flexible circuit board  905  and the ground strip  908  may be held in place with one or more non-conductive supports  909 , for example, walls. In some embodiments, the antenna assembly  900  may also include one or more stiffeners  906  to produce a desired shape of the folded flexible circuit board  905 . If desired, one or more tuning strips  907  may be connected between the first parasitic patch  103  and the ground strip  908  and/or the second parasitic patch  104  and the ground strip  908 . 
     In the antenna structure  900 , at least a portion of the upper strip of the main patch  101  and at least a portion of the upper strips of the first and second parasitic patches  103 ,  104  of the flexible circuit board  905  lie in a first plane  901  in space. At least a portion of the lower strip of the main patch  101  and at least a portion of the lower strips of the first and second parasitic patches  103 ,  104  of the flexible circuit board  905  lie in a second plane  902  and a third plane  903 , in space. The second plane  902  is not parallel to the first plane  901  or the ground plane  904 . Accordingly, in this arrangement, the distance between the ground plane  904  and the non-parallel portion of the patch antenna element lying within the second plane  902  (as well as the portions of the lower strips  101 - 2 ,  103 - 2 ,  104 - 2  of the respective patches  101 ,  103 ,  104 ) is gradually increased in one direction. Increasing the ground separation generally improves radiation efficiency of the antenna, thereby decreasing the Q factor of the antenna and broadening the bandwidth of the antenna. Thus, gradual increase of the degree of separation between the ground plane  904  and the lower strips  101 - 2 ,  103 - 2 ,  104 - 2  of the respective patches  101 ,  103 ,  104  included within the non-parallel portion of the flexible circuit board  905  lying within the second plane  902  increases the bandwidth of the antenna without increasing the overall antenna height. It should be noted that the gradual separation feature in the antenna  900  is not limited to a flexible circuit board implementation and can be implemented in any other suitable manner (e.g., using several non-flexible circuit boards). 
     It can be appreciated from the description above that the antenna&#39;s operational frequency characteristics, in particular, the bandwidth, may be adjusted by performing one or more of the following steps: attaching a tuning strip between the parasitic patch and the ground strip; changing a size of the tuning strip; changing a position of the tuning strip between the parasitic patch and the ground strip; changing a position of a feeding pin; directly coupling the main patch to the parasitic patch; gap-coupling the main patch to the parasitic patch; adjusting a spatial relationship between a gap-coupled main patch and parasitic patch; maintaining a constant spatial relationship between the first strip of the main patch and the second strip of the main patch; maintaining a constant spatial relationship between the first strip of the parasitic patch and the second strip of the parasitic patch; varying a spatial relationship between at least a portion of the first strip of the main patch and at least a portion of the second strip of the main patch; varying a spatial relationship between at least a portion of the first strip of the parasitic patch and at least a portion of the second strip of the parasitic patch; varying a spatial relationship between at least a portion of the second strip of the main patch and a ground plane, modifying a length of the main patch to be different in comparison to a length of the parasitic patch; and modifying a width of the main patch to be different in comparison to a width of the parasitic patch. 
     Table  1000  in  FIG. 10  shows a comparison of the size of antenna  100  relative to a conventional single resonator patch antenna at several operating frequencies according to several embodiments. As can be seen from table  1000 , a significant reduction in size relative to the size of a conventional patch antenna is achieved by utilizing the techniques described herein. 
     Table  1100  in  FIG. 11  shows a comparison of antenna performance of the antenna  100  relative to a conventional single resonator patch antenna at several operating frequencies, according to several embodiments. Table  1100  shows that the gain, directivity, and mismatch loss of the antenna  100 , although slightly degraded, are still comparable with the corresponding parameters of a conventional single resonator patch antenna, making the antenna  100  suitable for many applications that require or can benefit from the reduced size of the antenna  100 . 
       FIG. 12  is a voltage standing wave ratio (VSWR) graph  1200  showing VSWR versus frequency for an example embodiment of the antenna  100 . Graph  1200  shows that a suitable input impedance match (VSWR&lt;6) is achieved over a fractional bandwidth of approximately 40%, in the illustrated embodiment. 
       FIG. 13  is a graph  1300  of VSWR versus frequency for two example embodiments of the antenna, i.e., with and without a gradual increase in ground plane separation (discussed above in connection to  FIGS. 9A-9C ). In the graph  1300 , VSWR for an example antenna without gradual ground plane separation is indicated by the solid line, while an example antenna having gradual separation from ground plane is indicated by the dashed line. As can be seen from the graph  1300 , the dashed line shows a low VSWR (e.g., &lt;6) region spanning a larger frequency band compared to the frequency band spanned by a low VSWR (e.g., &lt;6) region indicated by the solid line. Accordingly, graph  1300  shows that antenna bandwidth is enhanced when a gradual increase in ground plane separation is introduced. 
       FIGS. 14A and 14B  depict a VSWR plot  1400  and a polar chart  1410  of radiation pattern, respectively, comparing an example antenna operating in free space with the same example antenna mounted on a metal surface, according to an embodiment In the embodiment depicted in  FIGS. 14A and 14B , the example antenna operates in a relatively low frequency range in the ultra-high frequency (UHF) band, with an operation frequency range of approximately 470 MHz-790 MHz. In  FIGS. 14A and 14B , the dashed lines correspond to an example antenna operating in free space, while the solid lines correspond to the same example antenna, but mounted on a large metal surface. As can be seen from plots  1400  and  1410 , the mounting surface does not have a significant effect on the performance of the antenna. 
       FIGS. 15A and 15B  depict a VSWR plot  1500  and a polar chart  1510  of radiation pattern, respectively, comparing an example antenna operating in free space with the same example antenna mounted on a metal surface, according to another embodiment. In the embodiment depicted in  FIGS. 15A and 15B , the example antenna operates in a relatively high frequency range in the UHF frequency band, with an operation frequency range of approximately 680 MHz-980 MHz. In  FIGS. 15A and 15B , the dashed lines correspond to an example antenna operating in free space, while the solid lines correspond to the same example antenna, but mounted on a large metal surface. As can be seen from plots  1500  and  1510 , the mounting surface does not have a significant effect on the performance of the antenna operating in the higher frequency range. 
     The configurations and techniques described above provide several tuning options for reducing the size of a patch antenna as well as increasing the bandwidth, such as, using a folded main patch with two radiating edges, gap-coupling a parasitic patch to the main patch along at least a portion of a non-radiating edge of the main patch, using one or more tuning strips to couple one or more parasitic patches to the ground strip, gradually increasing the separation between the main and the parasitic patch(es) and the ground plane, and modifying the length and width of the main patch and one or more parasitic patches. Through the use of one or more of these tuning options, an improved patch antenna having a 40% fractional bandwidth and a 50% size reduction in all dimensions over current patch antennas was able to be attained. Such a patch antenna is suitable for short to medium range wireless communication systems, for example, wireless microphones, wireless audio monitoring systems, local wireless data networks, and wireless medical devices. In addition, the low profile, significantly reduced size, and insensitivity to mounting surfaces makes the antenna of the present invention compatible for permanent indoor installations. 
     While the disclosed methods and apparatus have been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, it will be apparent to those of ordinary skill in the art that changes, additions or deletions may be made to the disclosed embodiments without departing from the spirit and scope of the invention. This patent therefore covers all methods, apparatus and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.