Abstract:
The invention provides a method of operating a patch antenna having a radiating element. The radiating patch is excited and generates a circularly polarized radiation beam solely in a higher order mode at a desired frequency. This allows for the radiating element to have a small surface area with the radiating beam tilted away from an axis perpendicular to the radiating element. Thus, the patch antenna provides a relatively small footprint and excellent RF signal reception from SDARS satellites at low elevation angles.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/868,436, filed Dec. 4, 2006. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The subject invention relates to a method of operating a patch antenna. 
         [0004]    2. Description of the Related Art 
         [0005]    Satellite Digital Audio Radio Service (SDARS) providers use satellites to broadcast RF signals, particularly circularly polarized RF signals, back to receiving antennas on Earth. The elevation angle between a satellite and an antenna is variable depending on the location of the satellite and the location of the antenna. Within the continental United States, this elevation angle may be as low as 20° from the horizon. Accordingly, specifications of the SDARS providers require a relatively high gain at elevation angles as low as 20° from the horizon. 
         [0006]    SDARS reception is primarily desired in vehicles. SDARS compliant antennas are frequently bulky, obtuse-looking devices mounted on a roof of a vehicle. SDARS compliant patch antennas typically have a square-shaped radiating element with sides about equal to ½ of the effective wavelength of the SDARS RF signal. These patch antennas typically also include a square-shaped ground plane that has a surface area larger than that of the radiating element. When the patch antenna is disposed on a window of the vehicle, the large “footprint” defined by the radiating element and ground plane often obstructs the view of the driver. Therefore, these patch antennas are not typically disposed on the windows of the vehicle. 
         [0007]    Various methods of operating patch antennas to receive RF signals are well known in the art. Examples of such methods are disclosed in the U.S. Pat. No. 4,887,089 (the &#39;089 patent) to Shibata et al. and U.S. Pat. No. 6,252,553 (the &#39;553 patent) to Soloman. 
         [0008]    The &#39;089 patent discloses a method of operating a patch antenna having a radiating element. The method includes the step of feeding a signal to the radiating element at either a first port or a second port, utilizing a switching mechanism. The method also includes the step of generating a horizontally polarized (i.e., linearly polarized) radiation beam in a higher order mode. The patch antenna of the &#39;089 patent does not generate a circularly polarized radiation beam and therefore is of little value in the reception of circularly polarized RF signals broadcast from satellites. 
         [0009]    The &#39;553 patent also discloses a method of operating a patch antenna having a radiating element. The method includes the step of shifting the phase of a base signal to produce at least one phase-shifted electromagnetic signal. The method continues by feeding the base signal and the phase-shifted signal to side feed ports of the radiating element and feeding the base signal to a central feed port of the radiating element. The method also includes the step of generating a circularly-polarized radiation beam in a fundamental mode and a higher order mode. The patch antenna of the &#39;553 patent does not generate the circularly polarized radiation beam solely in a higher order mode. As a result, the surface area defined by the radiation element is significantly large. 
         [0010]    There remains an opportunity to introduce a method of operating a patch antenna that aids in the reception of a circularly polarized RF signal from a satellite at a low elevation, especially when the patch antenna is disposed on an angled pane of glass, such as the window of a vehicle. There also remains an opportunity to introduce a method of operating a patch antenna which significantly reduces the required “footprint” of the antenna&#39;s radiating element when compared to other prior art patch antennas. 
       SUMMARY OF THE INVENTION AND ADVANTAGES 
       [0011]    The invention provides a method of operating a patch antenna at a desired frequency. The patch antenna includes a radiating element formed of a conductive material. The method includes generating a circularly polarized radiation beam solely in a higher order mode at the desired frequency by exciting the radiating element. 
         [0012]    By generating the circularly polarized radiation beam solely in a higher order mode the maximum gain of the radiation beam is tilted away from an axis perpendicular to the radiating element. This tilting-effect is very beneficial when attempting to receive the circularly polarized RF signals from a satellite at a low elevation angle. Furthermore, by generating the circularly polarized radiation beam solely in a higher order mode, the dimensions of the radiating element are much smaller than many prior art radiating elements. This is very desirable to automotive manufacturers and suppliers who wish to mount the radiating element on a window of a vehicle and still maintain good visibility for a driver through the glass. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
           [0014]      FIG. 1  is a perspective view a vehicle with a patch antenna supported by a pane of glass of the vehicle; 
           [0015]      FIG. 2  is a perspective view of the antenna showing a radiating element, a first dielectric layer, a feed network, a second dielectric layer, and a ground plane; 
           [0016]      FIG. 3  is a cross-sectional view of a preferred embodiment of the antenna with the radiating element disposed on the pane of glass and electromagnetic coupling of a feed line network to the radiating element; 
           [0017]      FIG. 4  is an electrical schematic block diagram of the preferred embodiment of the antenna showing the radiating element, a receiver, a low noise amplifier, a first phase shift circuit, and a plurality of feed lines; 
           [0018]      FIG. 5  is a chart showing a pattern of a left hand circularly polarized radiation beam resulting from operation of the preferred embodiment of the antenna; 
           [0019]      FIG. 6  is a cross-sectional view of the preferred embodiment of the antenna taken along line  6 - 6  of  FIG. 3  showing a feed line network disposed on the second dielectric layer; 
           [0020]      FIG. 7  is a cross-sectional view of an alternative embodiment of the antenna with the ground plane disposed between the dielectric layers and direct electrical connection of the feed line network to the radiating element; and 
           [0021]      FIG. 8  is a bottom view of the alternative embodiment of the antenna taken along line  8 - 8  of  FIG. 7  and showing the feed line network disposed on the second dielectric layer. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, a patch antenna  20  and associated method of operation are provided. 
         [0023]    The method of operation of the antenna  20  is described herein with reference to a preferred structural embodiment for the antenna  20 . Those skilled in the art realize that the method may be practiced with other antennas of alternative embodiments that differ in design and construction from that of the preferred embodiment. Therefore, the structure of the antenna  20  recited herein should not be read as limiting. 
         [0024]    In the preferred embodiment, the antenna  20  is utilized to receive a circularly polarized radio frequency (RF) signal from a satellite. Specifically, the antenna  20  may be utilized to receive a left-hand circularly polarized (LHCP) RF signal like those produced by a Satellite Digital Audio Radio Service (SDARS) provider, such as XM® Satellite Radio or SIRIUS® Satellite Radio. However, those skilled in the art understand that the antenna  20  may also receive a right-hand circularly polarized (RHCP) RF signal. Furthermore, in addition to receiving the LCHP and/or RHCP RF signals, the antenna  20  may also be used to transmit the circularly polarized RF signal. The antenna  20  will be described hereafter mainly in terms of receiving the LHCP RF signal, but this should not be read as limiting in any way. 
         [0025]    Referring to  FIG. 1 , the antenna  20  is preferably integrated with a window  22  of a vehicle  24 . This window  22  may be a part of a roof (such as a glass roof), a rear window (backlite), a front window (windshield), or any other window of the vehicle  24 . Those skilled in the art realize that the antenna  20  as described herein may be located at other positions on the vehicle  24 , such as on a sheet metal portion like the roof of the vehicle  24  or a side mirror of the vehicle  24 . The antenna  20  may also be implemented in other situations completely separate from the vehicle  24 , such as on a building or integrated with a radio receiver. The rear window  22  and the windshield are typically each disposed in the vehicle  24  at an angle, such that they define a surface that is not parallel to the ground (i.e., the surface of the Earth). Therefore, the antenna  20  disposed on these types of windows  22  are also not parallel to the ground. 
         [0026]    The window  22  preferably includes at least one pane of glass  28 . The pane of glass  28  is preferably automotive glass and more preferably soda-lime-silica glass, which is well known for use in panes of glass of vehicles  24 . The pane of glass  28  functions as a radome to the antenna  20 . That is, the pane of glass  28  protects the other components of the antenna  20 , as described in detail below, from moisture, wind, dust, etc. that are present outside the vehicle  24 . The pane of glass  28  defines a thickness between 1.5 and 5.0 mm, preferably 3.1 mm. The pane of glass  28  also has a relative permittivity between 5 and 9, preferably 7. Of course, the window  22  may include more than one pane of glass  28 . Those skilled in the art realize that automotive windows  22 , particularly windshields, include two panes of glass sandwiching a layer of polyvinyl butyral (PVB). 
         [0027]    Referring now to  FIG. 2 , the antenna  20  includes a radiating element  30  formed of an electrically conductive material described additionally below. The radiating element  30  is also commonly referred to by those skilled in the art as a “patch” or a “patch element”. The radiating element  30  of the preferred embodiment defines a generally rectangular shape, specifically a square shape. Each side of the radiating element  30  measures about ¼ of an effective wavelength λ of the RF signal to be received by the antenna  20 . RF signals transmitted by SDARS providers typically have a frequency from 2.32 GHz to 2.345 GHz. Specifically, XM Radio broadcasts at a center frequency of 2.338 GHz. Therefore, each side of the radiating element  30  measures about 24 mm. However, those skilled in the art realize alternative embodiments where the radiating element  30  defines alternative shapes and sizes based on the desired frequency and other considerations. 
         [0028]    The antenna  20  also includes a ground plane  32  formed of an electrically conductive material such as, but not limited to, copper. The ground plane  32  is disposed substantially parallel to and spaced from the radiating element  30 . It is preferred that the ground plane  32  also defines a generally rectangular shape, specifically a square shape. In the preferred embodiment, the ground plane  32  measures about 60 mm×60 mm. However, the ground plane  32  may be implemented with various shapes and sizes. 
         [0029]    At least one dielectric layer  34  is disposed between the radiating element  30  and the ground plane  32 . Said another way, the at least one dielectric layer  34  is sandwiched between the radiating element  30  and the ground plane  32 . The preferred embodiment of the at least one dielectric layer  34  is described in greater detail below. 
         [0030]    In the preferred embodiment, as shown in  FIG. 3 , the pane of glass  28  of the window  22  supports the radiating element  30 . The pane of glass  28  supports the radiating element  30  by the radiating element  30  being adhered, applied, or otherwise connected to the pane of glass  28 . Preferably, the radiating element  30  comprises a silver paste as the electrically conductive material disposed directly on the pane of glass  28  and hardened by a firing technique known to those skilled in the art. Alternatively, the radiating element  30  could comprise a flat piece of metal, such as copper or aluminum, adhered to the pane of glass  28  using an adhesive. 
         [0031]    Referring now to  FIG. 4 , the patch antenna  20  of the preferred embodiment also includes a plurality of feed lines  35 . Each feed line  35  is electrically connected to the radiating element  30  at a feed port  43 . Each feed port  43  is defined as the end point, or terminus, of each feed line  35 . In the preferred embodiment, the feed ports  43  are not in contact with the radiating element  30 . Instead, the electrical connection is produced by an electromagnetic coupling between the feed port  43  and the radiating element  30 . However, in alternative embodiments, the feed ports  43  (and accordingly, the feed lines  35 ) may come into direct contact with the radiating element  30 . 
         [0032]    In the preferred embodiment, the antenna  20  is implemented with four feed lines  36 ,  38 ,  40 ,  42  electrically connected to the radiating element  30  at four feed ports  44 ,  46 ,  48 ,  50 . Specifically, a first feed line  36  is electrically connected to the radiating element  30  at a first feed port  44 , a second feed line  38  is electrically connected to the radiating element  30  at a second feed port  46 , a third feed line  40  is electrically connected to the radiating element  30  at a third feed port  48 , and a fourth feed line  42  is electrically connected to the radiating element  30  at a fourth feed port  50 . 
         [0033]    The feed ports  44 ,  46 ,  48 ,  50  of the preferred embodiment are disposed with relationship to one another such that the feed ports  44 ,  46 ,  48 ,  50  define corners of a square shape. Of course, the square shape is merely a hypothetical construct for easily showing the physical relationship between the feed ports  44 ,  46 ,  48 ,  50 . Those skilled in the art realize that the feed ports  44 ,  46 ,  48 ,  50  of the preferred embodiment also define a circle shape with each feed port  44 ,  46 ,  48 ,  50  about equidistant along a periphery of the circle shape from adjacent feed ports  44 ,  46 ,  48 ,  50  and a diameter equal to the diagonals of the square shape. For ease in labeling, the feed ports  44 ,  46 ,  48 ,  50  are assigned sequentially counter-clockwise around the square or circle. For example, if the feed port  43  in the upper, left-hand corner of the square is the first feed port  44 , then the second feed port  46  is in the lower, left-hand corner, the third feed port  48  is in the lower, right-hand corner, and the fourth feed port  50  is in the upper, right-hand corner. 
         [0034]    The antenna  20  of the preferred embodiment also includes at least one phase shift circuit  51  for shifting the phase of a base signal. In the preferred embodiment, the base signal is provided to a low noise amplifier  25  and/or a receiver  26  from the antenna  20 . Of course, in other embodiments, in which the antenna  20  is used to transmit, the base signal is provided by a transmitter (not shown). The base signal, since it is not phase shifted, may be referred to as being offset by zero degrees (0°). 
         [0035]    In the preferred embodiment, as shown in  FIG. 4 , the at least one phase shift circuit  51  is implemented as a first phase shift circuit  52 . The first phase shift circuit  52  shifts the base signal by about ninety degrees (90°) to produce a first phase-shifted signal. Those skilled in the art realize that the 90° phase shift could vary by up to ten percent with little impact on overall performance. The first phase shift circuit  52  is electrically connected to the second feed line  38  and the fourth feed line  42 , and thus, provides the first phase-shifted signal (90°) to the second feed port  46  and the fourth feed port  50 . As a result, the first phase-shifted signal (90°) is applied at opposite corners of the square. The LNA  25  is electrically connected to the first feed line  36  and the third feed line  40 . Thus, the base signal (0°) is applied to the first feed port  44  and the third feed port  48 , also at opposite corners of the square. Application of the base signal and first phase-shifted signal in this manner produces a circularly polarized radiation beam. Those skilled in the art will realize alternate embodiments to produce the circularly polarized radiation beam using different configurations of phase shift circuits  51 . 
         [0036]    As stated above, the subject invention provides a method of operating the patch antenna  20 . This method includes the step of generating a circularly polarized radiation beam solely in a higher order mode at the desired frequency by exciting the radiating element  30 . Said another way, the circularly polarized radiation beam is not generated in a fundamental mode, but only in a higher order mode. That is, the operating mode of the antenna  20  consists of a higher order mode. Preferably, the higher order mode is a transverse magnetic mode. More preferably, the higher order mode is a TM22 mode. However, those skilled in the art realize that the other higher order modes besides the TM22 mode may achieve acceptable results. Furthermore, in other embodiments, the radiation beam may also be generated in both the higher order and fundamental modes. 
         [0037]    Generating the circularly polarized radiation beam solely in a higher order mode is accomplished due to the application of the base signal and the phase-shifted signals to the radiating element  30  along with the spacing of the feed ports  44 ,  46 ,  48 ,  50  with respect to one another. In the preferred embodiment, each side of the square defined by the feed ports  44 ,  46 ,  48 ,  50  measures about ⅙ of the effective wavelength of the resulting radiation beam. Said another way, each feed port  44 ,  46 ,  48 ,  50  is separated from two other adjacent feed ports  44 ,  46 ,  48 ,  50  by about ⅙ of the effective wavelength. The spacing between the feed ports  44 ,  46 ,  48 ,  50  is dependent on the desired operating frequency of the antenna  20 , which, in the preferred embodiment, is about 2.338 GHz. Within the teaching of the present invention, the dimensions may be modified by one skilled in the art for alternative operating frequencies. Furthermore, the effective wavelength depends on the window  22  and the dielectric layers  34 . As such, the permittivity and thickness of these elements has an effect on the size of the patch as is appreciated by those skilled in the art. 
         [0038]    By generating the circularly polarized radiation beam solely in a higher order mode, a null is established in the LHCP radiation beam at an axis perpendicular to the radiating element  30 . Said another way, the pattern of the radiation beam shows a null in the broadside direction, as is shown in  FIG. 5 . More importantly, the maximum gain of the LHCP radiation beam is about 40-50 degrees offset the axis perpendicular to the radiating element  30 . Thus, the LHCP radiation beam is “tilted” (or “steered”.) This tilting-effect is very beneficial when attempting to receive the LHCP RF signals from a satellite at a low elevation angle, e.g., an XM radio satellite. Furthermore, by generating the circularly polarized radiation beam solely in a higher order mode, the dimensions of the radiating element  30  are much smaller than many prior art radiating elements  30 . This is very desirable to automotive manufacturers and suppliers who wish to lessen the amount of obstruction on the windows  22  of the vehicle  24 . Additionally, the use of less conductive material in the radiating element  30  may also reduce manufacturing costs and enhance and improve aesthetics. 
         [0039]    The method of operating the patch antenna  20  also includes the step of shifting the phase of a base signal to produce at least one phase-shifted signal. This may be accomplished, as described above, with one or more phase shift circuits  51 . In the preferred embodiment, this step includes shifting the phase of the base signal by 90 degrees to produce a first phase-shifted signal. 
         [0040]    The method of operating the patch antenna  20  may also include the step of feeding the base signal to the radiating element  30  through at least one of the plurality of feed ports  44 ,  46 ,  48 ,  50  and feeding the at least one phase-shifted signal to the radiating element  30  through at least one of the other feed ports  44 ,  46 ,  48 ,  50 . In the first implementation, the step includes feeding the base signal through the first and third feed ports  44 ,  48  and feeding the first phase-shifted signal through the second and fourth feed ports  46 ,  50 . In the second implementation, the step includes feeding the base signal through the first feed port  44 , feeding the first phase-shifted signal through the second feed port  46 , feeding the second phase-shifted signal through the third feed port  48 , and feeding the third phase-shifted signal through the fourth feed port  50 . 
         [0041]    Referring again to  FIG. 2 , in the preferred embodiment, the at least one dielectric layer  34  is implemented as a first dielectric layer  60  and a second dielectric layer  62 . The first dielectric layer  60  is in contact with the ground plane  32 . The second dielectric layer  62  is in contact with the radiating element  30 . Preferably, the first and second dielectric layers  60 ,  62  are at least partially in contact with one another. The width of the dielectric layers  60 ,  62  is based, in part, on the dielectric constant of the dielectric layers  60 ,  62 . Preferably, the dielectric constant of both dielectric layers  60 ,  62  is about 4.5. The width of the second dielectric layer  62  is about 1/20 of the effective wavelength and the width of the first dielectric layer  60  is about 1/60 of the effective wavelength. 
         [0042]    The patch antenna  20  preferably includes a feed line network  58  formed of conductive strips  59  as shown in  FIG. 6 . The conductive strips  59  act as the feed lines  36 ,  38 ,  40 ,  42  and feed line ports  44 ,  46 ,  48 ,  50  described above. The feed line network  58  also defines an input port  64  which may be electrically connected to the receiver  26  and/or the LNA  25 . 
         [0043]    In the preferred embodiment, where the feed lines  36 ,  38 ,  40 ,  42  are electromagnetically coupled to the radiating element  30 , the feed line network  58  is sandwiched between the first and second dielectric layers  60 ,  62 . The conductive strips  59  of the feed line network  58  are disposed either on the first dielectric layer  60  or the second dielectric layer  62  at the junction of the dielectric layers  34 . The conductive strips  59  may be etched on one of the dielectric layers  34  by processes known to those skilled in the art. 
         [0044]      FIGS. 7 and 8  show an alternative embodiment where there is a direct connection between the feed lines  36 ,  38 ,  40 ,  42  and the radiating element  30 . In this alternative embodiment, the ground plane  32  is sandwiched between the first and second dielectric layers  60 ,  62 . The feed line network  58  is disposed on the first dielectric layer  60  on the opposite side from the feed line network  58 . A plurality of pins  64  electrically connects the feed lines to the ground plane  32 . Passage holes (not numbered) are defined in the ground plane  32  to prevent an electrical connection between the feed lines  36 ,  38 ,  40 ,  42  and the ground plane  32 . 
         [0045]    In both the preferred and alternative embodiments, the feed line network  58  is also utilized to shift the phase of a signal applied to the feed lines  36 ,  38 ,  40 ,  42 , thus, acting as the phase shift circuits  51  described above. This phase shifting is accomplished due to the inductive and capacitive properties of the conductive strips  59  of the feed line network  58 . The inductive and capacitive properties of the conductive strips  59  are determined by the impedance and length of each conductive strip  59 . The impedance of each conductive strip  59  is determined by the frequency of operation, the width of each conductive strip  59 , the dielectric constant of the first dielectric layer  60 , and the distance between the conductive strips  59  and the ground plane  32 . In the described embodiments, a conductive strip  59  width of about 1/60 of the effective wavelength yields an impedance of about 70.71 ohms and a width of about 1/35 of the effective wavelength yields an impedance of about 50 ohms. 
         [0046]    The feed line network  58  shown in  FIG. 6  implements the 0°, 90°, 0°, and 90° phase shifts. As can be seen, the conductive strips  59  form divergent paths which alternate between the various widths. Resistors  68  electrically connect between the divergent paths to ensure that an equal amount power is carried to or from each feed line port  44 ,  46 ,  48 ,  50 . Those skilled in the art realize that the feed line network  58  could be designed to perform other phase shifts or in a manner that does not perform any phase shifts. 
         [0047]    Those skilled in the art realize that many of the Figures are not drawn to scale. This is particularly evident in the cross-sectional representations of the various embodiments of the antenna  10  in  FIGS. 3 and 7 . Particularly, in these Figures, the width of the electrically conductive components, such as the radiating element  30 , the ground plane  32 , and the feed line network  58 , is exaggerated such that it may be seen from the cross-sectional view. Those skilled in the art also realize that the width of these electrically conductive components may be much less than 1 mm and therefore difficult to perceive from an actual cross-sectional view of the antenna. 
         [0048]    The present invention has been described herein in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. Obviously, many modifications and variations of the invention are possible in light of the above teachings. The invention may be practiced otherwise than as specifically described within the scope of the appended claims.