Patent Publication Number: US-7224319-B2

Title: Multiple-element beam steering antenna

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The subject invention relates to an antenna, specifically a multi-element antenna in an array-type configuration, for receiving a circularly polarized radio frequency (RF) signal from a satellite and a linearly polarized RF signal from a terrestrial source. 
   2. Description of the Prior Art 
   Vehicles have long implemented glass to enclose a cabin of the vehicle while still allowing visibility for the driver of the vehicle. Automotive glass is typically either a tempered (or toughened) glass or a laminated glass which is produced by bonding two or more panes of glass together with a plastic interlayer. The interlayer keeps the panes of glass together even when the glass is broken. 
   Recently, antennas have been integrated with the glass of the vehicle. This integration helps improve the aerodynamic performance of the vehicle as well to help provide the vehicle with an aesthetically-pleasing, streamlined appearance. Integration of antennas for receiving linearly polarized RF signals, such as those generated by AM/FM terrestrial broadcast stations, has been the principal focus of the industry. 
   However, that focus is shifting to integrating antennas for receiving RF signals from Satellite Digital Audio Radio Service (SDARS) providers. SDARS providers use satellites to broadcast RF signals, particularly circularly polarized RF signals, back to Earth. SDARS providers use multiple satellites in a geostationary orbit or in an inclined elliptical constellation. The elevation angle between the respective satellite and the 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°. Accordingly, specifications of the SDARS providers require a relatively high gain at elevation angles as low as 20°. SDARS providers also use terrestrial “repeater” stations to rebroadcast their satellite signal. These terrestrial stations operate at an elevation angle of 0° and are useful in urban environments where tall buildings may obstruct signals from the satellites. Linear polarization is used for these terrestrial rebroadcasts. 
   Additionally, automotive manufacturers and vehicle drivers demand that the antenna integrated with the glass does not obstruct the view of the driver. Therefore, it is typically a requirement that the antenna occupy less than a certain surface area, or “footprint”, when integrated with the glass. 
   Various antennas for receiving both circularly polarized and linearly polarized RF signals are known in the art. Examples of such antennas are disclosed in the U.S. Pat. No. 6,697,019 (the &#39;019 patent) to Hyuk-Joon et al and U.S. Pat. No. 6,545,647 (the &#39;647 patent) to Sievenpiper et al. The &#39;019 patent discloses an antenna system installable on the roof of a vehicle for receiving RF signals produced by circularly polarized transmitters and linearly polarized transmitters. The antenna includes four linear polarized radiation elements and four circularly polarized radiation elements arranged symmetrically about a center. The antenna includes a circuit board for supporting the linear polarized radiation elements and a dielectric substrate. The linear polarized radiation elements each have a brick shape and include a microstrip resonator having a length of one-quarter wavelength λ. The circularly polarized radiation elements are microstrip patches disposed on the dielectric substrate. The circularly polarized radiation elements each have a square shape that is geometrically different from that of the linearly polarized radiation elements. The antenna system also includes a 90-degree hybrid. The 90-degree hybrid shifts the signal to two of the circularly polarized radiation elements by 90 degrees while the signal to the other two circularly polarized radiation elements is unshifted. The antenna requires separate feed lines for the linear and circular polarized signals. 
   Since the antenna of the &#39;019 patent is a large, bulky array of antenna elements for mounting on the roof of the vehicle, it is not suitable for integration with a window of the vehicle. If the antenna of the &#39;019 patent were to be mounted onto the window, the eight separate elements would occupy a large surface area and obstruct the view of a driver of the vehicle. Furthermore, the antenna does not significantly aid in reception of RF signals from low elevation angles. 
   The &#39;647 patent discloses an antenna for receiving RF signals produced by circularly polarized transmitters and linearly polarized transmitters. The antenna includes four radiation elements arranged symmetrically about a center and disposed on a high impedance surface. The high impedance surface acts as a ground plane and is typically mounted on a large metallic object, such as a roof of a vehicle. The radiation elements are formed of an electrically conductive material and implemented either as pieces of wire or metallic patches. Various connections of phase-shift circuits to the radiation elements give the antenna its circular and linear polarizations. The antenna requires separate feed lines for a receiver to receive the linear and circular polarized signals. The antenna of the &#39;647 patent does not significantly aid in reception of RF signals from low elevation angles. 
   There remains an opportunity to introduce an antenna that aids in the reception of the RF signal from a satellite. Particularly, there remains an opportunity for an antenna that aids in reception of the RF signal from elevation angles as low as 20°. Furthermore, there remains an opportunity for an antenna that does not significantly obstruct the view of the driver of the vehicle and provides both circular and linear polarized signals on a single feed line. 
   SUMMARY OF THE INVENTION AND ADVANTAGES 
   The subject invention provides a window having an integrated antenna. The window includes a nonconductive pane. A circularly polarized radiation element is disposed on the nonconductive pane. A linearly polarized radiation element is also disposed on the nonconductive pane and spaced from the circularly polarized radiation element. The linearly polarized radiation element has a geometric shape different from that of the circularly polarized radiation element. 
   The structure of the antenna produces a directional radiation beam with a highest gain portion at a certain elevation angle. The spacing between the radiation elements affects a relative phasing between the two different radiation elements. As a result of this relative phasing, the elevation angle of the radiation beam tilts; thus also tilting the highest gain portion of the radiation beam. This tilt is particularly important when receiving an RF signal broadcast from a satellite of a Satellite Digital Audio Radio Service (SDARS) provider. Specifications of the SDARS providers require a relatively high gain at elevation angles as low as 20°. The antenna of the subject invention produces a relatively high gain of the RF signal even at these low elevation angles. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIG. 1  is a perspective view of a vehicle with an antenna supported by a pane of glass of the vehicle; 
       FIG. 2  is a cross sectional side view of a first embodiment of the antenna taken along line  2 - 2  of  FIG. 3  showing the pane of glass, radiation elements, a ground plane, and a circuit board; 
       FIG. 3  is a cross-sectional bottom view of the first embodiment of the antenna taken along line  3 - 3  of  FIG. 2  showing the radiation elements and the pane of glass; 
       FIG. 4  is a schematic block diagram of the antenna showing electrical connections between the radiation elements, an amplifier, a 90 degree hybrid, and a phase shift circuit; 
       FIG. 5  is a cross-sectional bottom view of a second embodiment of the antenna showing the radiation elements and the pane of glass; 
       FIG. 6  is a cross-sectional bottom view of a third embodiment of the antenna showing the radiation elements and the pane of glass; and 
       FIG. 7  is a chart showing a radiation pattern produced by the first embodiment of the antenna. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the Figures, wherein like numerals indicate like parts throughout the several views, an antenna is shown generally at  10 . The antenna  10  is utilized to receive a circularly polarized radio frequency (RF) signal from a satellite and a linearly polarized RF signal from a terrestrial source. Specifically, the first embodiment of the antenna  10  receives 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, and their associated linearly polarized terrestrial repeater broadcasts. However, it is to be understood that the antenna  10  may also receive a right-hand circularly polarized (RHCP) RF signal. Also, the antenna  10  may also be configured to receive linearly polarized RF signals that are either vertically or horizontally orientated. XM® Satellite Radio produces a vertically orientated linearly polarized signal. Furthermore, those skilled in the art realize that the antenna  10  may also be used to transmit the circularly and linearly polarized RF signals 
   Referring to  FIG. 1 , the antenna  10  is preferably integrated with a window  12  of a vehicle  14 . This window  12  may be a rear window  12  (backlite), a front window  12  (windshield), or any other window  12  of the vehicle  14 . The antenna  10  may also be implemented in non-window portions of the vehicle, such as a roof or mirror. Furthermore, the antenna  10  may be implemented in other situations completely separate from the vehicle  14 , such as on a building or integrated with a radio receiver. The window  12  includes at least one nonconductive pane  16 . The term “nonconductive” refers to a material, such as an insulator or dielectric, that when placed between conductors at different potentials, permits only a small or negligible current in phase with the applied voltage to flow through material. Typically, nonconductive materials have conductivities on the order of nanosiemens/meter. 
   In the first embodiment, the nonconductive pane  16  is implemented as at least one pane of glass  18 . Of course, the window  12  may include more than one pane of glass  18 . Those skilled in the art realize that automotive windows  12 , particularly windshields, may include two panes of glass  18  sandwiching a layer of polyvinyl butyral (PVB). 
   The pane of glass  18  is preferably automotive glass  18  and more preferably soda-lime-silica glass  18 . The pane of glass  18  defines a thickness between 1.5 and 5.0 mm, preferably 3.1 mm. The pane of glass  18  also has a relative permittivity between 5 and 9, preferably 7. Those skilled in the art, however, realize that the nonconductive pane  16  may be formed from plastic, fiberglass, or other suitable nonconductive materials. 
   For descriptive purposes only, the subject invention is referred to below only in the context of the most preferred nonconductive pane  16 , which is the pane of automotive glass  18 . This is not to be construed as limiting, since, as noted above, the antenna  10  can be implemented with nonconductive panes  16  other than panes of glass  18 . 
   Referring now to  FIG. 2 , the pane of glass  18  functions as a radome to the antenna  10 . That is, the pane of glass  18  protects the other components of the antenna  10 , as described in detail below, from moisture, wind, dust, etc. that are present outside the vehicle  14 . The pane of glass  18  is disposed at a mounting angle φ relative to the ground. Depending on the mounting angle φ required by the vehicle  12 , it may be desirous to tilt the elevation angle of a radiation beam upwards or downwards to increase the gain of the RF signal transmitted by a satellite or terrestrial source and received by the antenna. The antenna  10 , as explained more fully below, performs this beam tilting. 
   Referring now to  FIG. 3 , the antenna  10  includes a circularly polarized radiation element  20  disposed on the pane of glass  18 . The circularly polarized radiation element  20  preferably has a rectangular shape and most preferably has a square shape. The circularly polarized radiation element  20  preferably receives and/or transmits an RF signal having a circular polarization by using a 90° phase shift as described in detail below. The circularly polarized radiation element  20  is commonly referred to by those skilled in the art as a “patch” or a “patch element” and formed of an electrically conductive material. Preferably, the circularly polarized radiation element  20  comprises a silver paste as the electrically conductive material that is disposed directly on the pane of glass  18  and hardened by a firing technique known to those skilled in the art. Alternatively, the circularly polarized radiation element  20  could comprise a flat piece of conductive metal, such as copper or aluminum, adhered to the pane of glass  18  using an adhesive. 
   The circularly polarized radiation element  20  has a first edge  22  and a second edge  24 , with the second edge  24  perpendicular to the first edge  22 . The first edge  22  defines a first width W 1  and the second edge  24  defines a first length L 1 . In the first embodiment, the first width W 1  and the first length L 1  of the circularly polarized radiation element  20  each measure about ½ of a wavelength λ of a base signal to be received or transmitted by the antenna  10 . Since the first width W 1  and the first length L 1  are preferably equal in length, the circularly polarized radiation element  20  preferably has a square shape. In the first embodiment, the desired frequency to be received is about 2,338 MHz, which corresponds to the center frequency used by XM® Satellite Radio. Therefore, in the first embodiment, the first and second edges  22 ,  24  of the circularly polarized radiation element  20  each measure about 64 mm. 
   The antenna  10  also includes a linearly polarized radiation element  26  formed of an electrically conductive material and disposed on the nonconductive pane  16 . The linearly polarized radiation element  26  receives and/or transmits an RF signal having a linear polarization. The linearly polarized radiation element  26  may be implemented as a monopole by utilizing a segment of wire, a line of silver paste, or a rectangular-shaped section of electrically conductive material. Alternatively, the linearly polarized radiation element  26  may be implemented as a portion of electrically conductive material defining a slot. 
   The geometric shape of the linearly polarized radiation element  26  is different from that of the circularly polarized radiation element  20 . As mentioned above, the circularly polarized radiation element  20  is preferably square-shaped. Another square-shaped element in combination with such a circularly polarized radiation element  20  would be unacceptable to automotive manufacturers and drivers based on the resulting size of the antenna  10  and the obstruction of the view of the driver, as is understood by those skilled in the art. Thus, the linearly polarized radiation element  26  must be of a different geometric shape than the circularly polarized radiation element  20 , as well as occupying a smaller surface area, to satisfy the needs of the automotive manufacturers and drivers. 
   In the first embodiment, and as shown in  FIG. 3 , the linearly polarized radiation element  26  comprises a silver paste as the electrically conductive material that is disposed directly on the pane of glass  18  and hardened by a firing technique known to those skilled in the art. The linearly polarized radiation element  26  preferably has a rectangular shape with a third edge  28  and a fourth edge  30 . The third edge  28  is perpendicular to the fourth edge  30 . The third edge  28  defines a second width W 2  and the fourth edge  30  defines a second length L 2 . The second width W 2  measures about 1/20 of the wavelength λ and the second length L 2  measures about ½ of the wavelength λ. Therefore, at the desired frequency of 2,338 MHz, the second width W 2  measures about 6 mm and the second length L 2  measures about 64 mm. The linearly polarized radiation element  26  is spaced from the circularly polarized radiation element  20  by a distance D. The distance D is preferably in a range of 1/20 to ½ of the wavelength λ. More preferably, and in the first embodiment, the distance D measures about ⅕ of the wavelength λ, which is about 26 mm at the desired frequency of 2,338 MHz. 
   The radiation elements  20 ,  26  are preferably co-planar with one another. That is, the radiation elements  20 ,  26  lie generally in a single plane defined by a surface of the nonconductive pane  16 . Said another way, the radiation elements  20 ,  26  are not one on top of the other and are conformal with a surface of the pane of glass  18 . 
   In the first embodiment, the third edge  28  of the linearly polarized radiation element  26  is generally parallel to the first edge  22  of the circularly polarized radiation element  20 . In this alignment, the linearly polarized radiation element  26  produces a vertically-oriented linearly polarization. The radiation elements  20 ,  26  have a combined surface area of about 4,250 mm 2 . Therefore, the antenna  10  will not create a significant obstruction to the view of the driver of the vehicle  12 . 
   Referring again to  FIG. 2 , the antenna  10  preferably includes a ground plane  32  for enhancing the performance of the antenna  10 . The ground plane  32  is formed of a generally flat electrically conductive material, such as a conductive metal like copper or aluminum. The ground plane  32  is spaced from and preferably parallel to the radiation elements  20 ,  26 . The ground plane  32  preferably has a rectangular shape with a first side  34  and a second side  36 . The first side  34  faces the radiation elements  20 ,  26 . Those skilled in the art realized that other shapes of the ground plane  32  may be implemented. Furthermore, the antenna  10  may function without the ground plane  32  whatsoever. 
   A dielectric  38  is sandwiched between the first side  34  of the ground plane  32  and the radiation elements  20 ,  26 . In the first embodiment, the dielectric  38  is air, which has a relative permittivity of 1. However, depending on the particularly performance characteristics of the antenna  10 , the dielectric  38  may be formed of one or more alternate materials having an alternate relative permittivity. The thickness T of the dielectric can be up to ¼ of the wavelength λ, which is about 32 mm at the frequency of 2,338 MHz. 
   The antenna  10  also preferably includes a circuit board  40 . The circuit board  40  is connected to the second side  36  of the ground plane  42 . This location of the circuit board  40  is for convenience of connection to the radiation elements  20 ,  26  of the antenna  10  and compactness of the entire antenna  10 . Those skilled in the art realized that the circuit board  40  may be implemented at a location distant from the radiation elements  20 ,  26 . Alternatively, the antenna  10  could be implemented without a circuit board  40  whatsoever. 
   Referring now to  FIG. 4 , the antenna  10  also includes a base signal line  42 , a 90°-shifted signal line  44 , and a phase-shifted signal line  46 . The base signal line  42  is electrically connected to the circularly polarized radiation element  20  adjacent the first edge  22  of the element  20 , preferably near a center of the first edge  22 . The 90°-shifted signal line  44  is electrically connected to the circularly polarized radiation element  20  adjacent the second edge  24 , preferably near a center of the second edge  24 . The base signal line  42  carries a base signal having a phase angle β. The 90°-shifted signal line  44  carries a signal shifted 90° from the base signal and therefore having a phase angle β+90°. Preferably, but not necessarily, the 90° shift is accomplished by a 90° hybrid  54 , which is further described below. 
   The combination of the base signal and the 90°-shifted signal fed to perpendicular edges  22 ,  24  give the circularly polarized radiation element  20  a circular polarization. Those skilled in the art realize alternative techniques of generating circular polarization without use of a 90°-shifted signal line  44 . These techniques, include, but are not limited to, a square-shaped radiation element with two opposite corners being truncated, a radiation element with a cross-shaped slot whose legs have unequal lengths, a radiation element with a 45° offset feed and trim tabs, a square-shaped radiation element with trim tabs. However, these techniques may or may not work effectively with the linearly polarized radiation element  26  to achieve the desired beam tilting, as described in more detail below. 
   The phase-shifted signal line  46  is electrically connected to the linearly polarized radiation element  26 . Preferably, the phase-shifted signal line  46  is electrically connected adjacent the third edge  28 , preferably near a center of the third edge  28 . The phase-shifted signal line  46  carries a phase-shifted signal that is shifted from the base signal β by a certain angle Δβ. The phase angle of the phase-shifted signal is therefore β+Δβ. Preferably, but not necessarily, the phase shift is accomplished by a phase shift circuit  56 , which is further described below. 
   The circularly and linearly polarized radiation beams produced by the antenna  10  are tilted (or steered) by both the spacing, i.e., the distance D, between the radiation elements  20 ,  26  and the phase-shifted signal feeding the linearly polarized radiation element  26 . The combination of these two techniques enhances the beam tilting effect. As mentioned previously, this tilt is particularly important when receiving an RF signal broadcast from a satellite of an SDARS provider. The magnitude of tilt is based on the relative phase angle γ between the circularly polarized radiation element  20  and the linearly polarized radiation element  26 . The relative phase angle γ, in turn, is determined by the both a certain angle Δβ of phase shift on the phase-shifted signal line  46  and the spacing distance D between the radiation elements  20 ,  26 . 
   The signal lines  42 ,  44 ,  46  are each formed of an electrically conductive material. In the first embodiment, the signal lines  42 ,  44 ,  46  are implemented as microstrip lines disposed on the circuit board  40 . A plurality of pins  48  electrically connects each of the signal lines  42 ,  44 ,  46  to their respective positions on the radiation elements  20 ,  26 . The pins  48  are formed of an electrically conductive material, such as a conductive metal. The ground plane  32  and the circuit board  40  each define a plurality of holes  50 . The holes  50  accommodate the pins  48  as they extend perpendicularly from the radiation elements  20 ,  26  to the signal lines  42 ,  44 ,  46  disposed on the circuit board  40 . The pins  48  are preferably soldered to both the radiation elements  20 ,  26  and the signal lines  42 ,  44 ,  46 . As such, the pins could also act to support the circuit board  40  and the ground plane  32 . Alternatively, the overall packaging of the antenna  10  could also support the circuit board  40  and the ground plane  32 . Of course, other alternative techniques of connecting the signal lines  42 ,  44 ,  46  to the radiation elements  20 ,  26  will be obvious to those skilled in the art. While direct electrical connection of the signal lines  42 ,  44 ,  46  to the radiation elements  20 ,  26  is preferred, the electrically connection may be accomplished by electromagnetically coupling the signal lines  42 ,  44 ,  46  to the radiation elements  20 ,  26 . 
   Preferably, an amplifier  52  is electrically connected to the base signal line  42  for amplifying the base signal to generate an amplified signal. In configurations where the antenna  10  is implemented to receive RF signals, the amplifier  52  is a preferably a low-noise amplifier (LNA). The amplifier  52  is preferably disposed on the circuit board  40 . A single feed line  53  is electrically connected to the amplifier  52  for carrying the amplified signal to a receiver. The amplified signal carried by the single feed line  53  provides a single source for RF signals received by the linearly and circularly polarized radiation elements  20 ,  26 . Those skilled in the art realize that in configurations where the antenna  10  is used to transmit RF signals, the amplifier  52  would be implemented as a power amplifier. 
   The 90° hybrid  54  mentioned above is electrically connected between the base signal line  42  and the 90°-shifted signal line  50  for phase shifting the base signal by 90° to achieve the 90°-shifted signal. The 90° hybrid  54  is also preferably disposed on the circuit board  40 . 
   The phase shift circuit  56  also mentioned above is electrically connected between the base signal line  42  and the phase-shifted signal line  36 . The phase shift circuit  56  shifts the base signal by the certain angle Δβ to achieve the phase-shifted signal having the phase angle β+Δβ. The phase shift circuit  56  is preferably disposed on the circuit board  40 . 
   Other dimensions, alignments, and configurations of the radiation elements  20 ,  26  are possible, depending on the desired performance and dimensional area requirements of the antenna  10 . In a second embodiment, as shown in  FIG. 5 , the dimensions of the circularly polarized radiation element  20  are the same as in the first embodiment. However, the linearly polarized radiation element  26  defines a slot  58 . A length L 3  of the slot  58  is defined as ½ of the wavelength λ. The fourth edge  30  of the linearly polarized radiation element  26  is parallel to the first edge  22  of the circularly polarized radiation element  20 . The electrical connection of the phase-shifted signal line  36  to the linearly polarized element is adjacent a center of the slot  58 . The spacing distance D between the elements remains at the most preferred ⅕ of the wavelength λ. 
   A third embodiment is shown in  FIG. 6 . This embodiment is similar to the first embodiment, except that the second length L 2  of the linearly polarized element  26  is ¼ of the wavelength λ. Again, the spacing distance D between the elements remains at the most preferred ⅕ of the wavelength λ. The third embodiment further reduces the surface area of the window  12  that is occupied by the antenna  10 . 
   The tilt of the radiation beam is perhaps best understood by reviewing results of a computerized simulation of the antenna  10  of the first embodiment.  FIG. 7  shows the LHCP and vertically linearly polarized radiation beams of the subject invention. The highest gain portion of the radiation beams are tilted by about 20°. Conventional non-beam steering antennas provide no such tilt, having their highest gain portion at about 0°. As such, the antenna  10  according to the subject invention produces a higher gain for the RF signal received from the satellite at relatively low elevation angles than conventional non-beam steering antennas. 
   Multiple antennas  10  may be implemented as part of a diversity system of antennas  10 . For instance, the vehicle  14  of the first embodiment may include a first antenna  10  on the windshield and a second antenna  10  on the backlite. These antennas  10  would each have separate amplifiers  52  that are electrically connected to the receiver within the vehicle  14 . Those skilled in the art realize several processing techniques may be used to achieve diversity reception. In one such technique, a switch is used to select the antenna  10  that is currently receiving the strongest RF signal from the satellites or terrestrial source. 
   Obviously, many modifications and variations of the present 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.