Abstract:
An antenna system for a vehicle including a rear windshield is disclosed. The antenna system comprises a global positioning system (GPS) antenna unit including a radiating element electromagnetically coupled to an excitation element about the rear windshield glass.

Description:
RELATED APPLICATIONS 
   The present application claims priority to U.S. provisional application Ser. No. 60/550,280 filed on Mar. 5, 2004. 

   TECHNICAL FIELD 
   The present invention generally relates to vehicular glass-mount antennas having improved radiation characteristics. 
   BACKGROUND OF THE INVENTION 
   It is known in the art that automotive vehicles are commonly equipped with audio radios that receive and process signals relating to amplitude modulation/frequency modulation (AM/FM) antennas, satellite digital audio radio systems (SDARS) antennas, global positioning system (GPS) antennas, digital audio broadcast (DAB) antennas, dual-band personal communication systems digital/analog mobile phone service (PCS/AMPS) antennas, Remote Keyless Entry (RKE) antennas, Tire Pressure Monitoring System antennas, and other wireless systems. 
   Currently, patch antennas are employed for reception and transmission of GPS [i.e. right-hand-circular-polarization (RHCP) waves] and SDARS [i.e. left-hand-circular-polarization (LHCP) waves]. Patch antennas may be considered to be a ‘single element’ antenna that incorporates performance characteristics of ‘dual element’ antennas that essentially receives terrestrial and satellite signals. SDARS, for example, offer digital radio service covering a large geographic area, such as North America. Satellite-based digital audio radio services generally employ either geo-stationary orbit satellites or highly elliptical orbit satellites that receive uplinked programming, which, in turn, is re-broadcasted directly to digital radios in vehicles on the ground that subscribe to the service. SDARS also use terrestrial repeater networks via ground-based towers using different modulation and transmission techniques in urban areas to supplement the availability of satellite broadcasting service by terrestrially broadcasting the same information. The reception of signals from ground-based broadcast stations is termed as terrestrial coverage. Hence, an SDARS antenna is required to have satellite and terrestrial coverage with reception quality determined by the service providers, and each vehicle subscribing to the digital service generally includes a digital radio having a receiver and one or more antennas for receiving the digital broadcast. GPS antennas, on the other hand, have a broad hemispherical coverage with a maximum antenna gain at the zenith (i.e. hemispherical coverage includes signals from 0° elevation at the earth&#39;s surface to signals from 90° elevation up at the sky). Emergency systems that utilize GPS, such as OnStar™, tend to have more stringent antenna specifications. 
   Unlike GPS antennas which track multiple satellites at a given time, SDARS patch antennas are operated at higher frequency bands and presently track only two satellites at a time. Thus, the mounting location for SDARS patch antennas makes antenna reception a sensitive issue with respect to the position of the antenna on a vehicle. As a result, SDARS patch antennas are typically mounted exterior to the vehicle, usually on the roof, or alternatively, inside the vehicle in a hidden location, for example, within an instrument panel. In some instances, such as cellular telephone mast antennas, have been located on the exterior surface of automotive glass and the received signals are electromagnetically coupled through the glass to the vehicle&#39;s receiver. Electromagnetically coupling such antennas in an SDARS application, without an external amplifier, is very difficult due to inherent loss and distorted radiation patterns associated with front windshield glass composition, which includes an intermediate plastic layer sandwiched between inner and outer glass layers. Additionally, external antennas are highly visible, prone to being damaged, and not aesthetically pleasing. 
   With respect to GPS antenna performance, GPS antennas mounted on a location other than the roof of the vehicle suffer degradation at lower elevation angles and rely on peak antenna gain to capture signals from multiple-tracked satellites. This feature of the antenna performance can be exploited to place the antenna at any desirable location inside the vehicle, such as on the rear-windshield glass. Although GPS antennas may be located on the front windshield glass as well, the front glass may introduce losses in addition to losses associated with the intermediate plastic layer of the front windshield glass. For example, the front windshield glass may include a high degree of curvature that causes the front glass to act as a lens that distorts the received radiation pattern by focusing waves at different locations other than the antenna. 
   SUMMARY OF THE INVENTION 
   The inventors of the present invention have recognized these and other problems associated with glass-mount antennas. To this end, the inventors have developed an antenna system associated with rear windshield. The antenna system comprises an global positioning system (GPS) antenna unit including a radiating element electromagnetically coupled to an excitation element. According to one embodiment of the invention, the radiating element may be coupled to the front windshield glass, and the excitation element may be positioned on a passenger compartment interior surface of the front windshield glass. The radiating element and/or the excitation element may also be located within the rear windshield glass. The antenna system also comprises a high-gain dual element antenna unit including a first radiating element, a second radiating element, a 90-degree phase shift circuit, and a low noise amplifier that is directly pin-feed coupled to the phase shift circuit. The radiating elements receive signals through the rear windshield glass. The antenna unit and the high-gain duel element antenna unit may function in a diversity antenna configuration. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  illustrates a general side view of the vehicle glass mount antenna system; 
       FIG. 2  illustrates a passenger compartment view of a front windshield glass mount antenna according to one embodiment of the invention; 
       FIG. 3  illustrates a passenger compartment view of a rear glass mount antenna according to one embodiment of the invention; 
       FIG. 4A  illustrates a cross-sectional view of the front windshield glass mount antenna according to one embodiment of the invention; 
       FIG. 4B  illustrates a top view of a first element of the front windshield glass mount antenna according to  FIG. 4A ; 
       FIG. 4C  illustrates a top view of a second element of the front windshield glass mount antenna according to  FIG. 4A ; 
       FIG. 5A  illustrates a cross-sectional view of the rear windshield glass mount antenna according to one embodiment of the invention; 
       FIG. 5B  illustrates a schematic top view of the rear windshield glass mount antenna according to  FIG. 5A ; 
       FIG. 6A  illustrates a cross-sectional view of a rear-view mirror assembly and the front windshield glass mount antenna according to one embodiment of the invention; 
       FIG. 6B  illustrates a cross-sectional view of a rear-view mirror assembly and the front windshield glass mount antenna according to another embodiment of the invention; 
       FIG. 7A  illustrates a cross-sectional view of the front windshield glass mount antenna according to another embodiment of the invention; 
       FIG. 7B  illustrates a cross-sectional view of the front windshield glass mount antenna according to another embodiment of the invention; 
       FIG. 8A  illustrates a cross-sectional view of the front windshield glass mount antenna according to another embodiment of the invention; 
       FIG. 8B  illustrates a cross-sectional view of the front windshield glass mount antenna according to another embodiment of the invention; 
       FIG. 9A  illustrates a cross-sectional view of a rear backglass glass mount GPS antenna according to one embodiment of the invention; 
       FIG. 9B  illustrates a top view of a first element of the rear backglass glass mount GPS antenna according to  FIG. 9A ; 
       FIG. 9C  illustrates a top view of a second element of the rear backglass glass mount GPS antenna according to  FIG. 9A ; 
       FIG. 10A  illustrates a cross-sectional view of a rear windshield glass mount GPS antenna according to one embodiment of the invention; 
       FIG. 10B  illustrates a top view of a first element of the rear windshield glass mount GPS antenna according to  FIG. 10A ; 
       FIG. 10C  illustrates a top view of a second element of the rear windshield glass mount GPS antenna according to  FIG. 10A ; and 
       FIGS. 11A–11E  illustrate cross-sectional views of rear windshield glass mount GPS antenna assemblies according to multiple embodiments of the invention that may include the antenna elements of  FIGS. 9B ,  9 C or  10 B,  10 C. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The above described disadvantages are overcome and a number of advantages are realized by inventive antenna systems, which are generally illustrated at  10   a ,  10   b  in  FIGS. 1–3 . As illustrated in  FIG. 1 , a vehicle, V, includes a front windshield glass  12   a  and rear windshield glass  12   b  each including antenna units  14   a ,  14   b , respectively. Referring to  FIG. 2 , the antenna unit  14   a  is shown proximate a rear-view mirror assembly  13  mounted via an adjacent arm  17  at a top portion  15  of the front windshield glass  12   a  that meets a headliner (not shown). The location of headliner provides the shortest path to route and hide wires  16  extending from the antenna unit  14   a  and rear-view mirror assembly  13 . When implemented near the top portion  15 , the antenna unit  14   a  should not come into direct contact with the vehicle body so as to ensure that the antenna unit  14   a  is not shorted out. As seen in  FIG. 3 , the antenna unit  14   b  is located near a corner  18  of the rear windshield glass  12   b  such that defroster wires  19  are routed about the mounting location of the antenna unit  14   b . Although the antenna unit  14   b  is shown near the corner  18 , the antenna unit  14   b  can be located at any desirable location on the rear windshield glass  12   b , but more preferably, in a location that is less visible to the vehicle passengers and driver. For example, in an alternative embodiment, the antenna unit  14   b  may be located between the rear windshield glass  12   b  and a rear brake light housing  21  so as to completely hide the antenna unit  14   b  from the passengers and driver. 
   Referring now to  FIGS. 4A and 5A , it is illustrated that the front windshield glass  12   a  ( FIG. 4A ) includes a layer of plastic film  11   c  that is sandwiched between an outer glass layer  11   a  and an inner glass layer  11   b , whereas, conversely, the rear windshield glass  12   b  ( FIG. 5A ) does not comprise an intermediate plastic film layer  11   c  ( FIG. 4A ), but rather a unit of glass defined by a thickness, T. Because the outer layer of glass  11   a  is exposed to the elements, which may undesirably result in failure and cracking, the inner layer of glass  11   b  is separated and shielded from the outer glass  11   a  by the intermediate plastic film layer  11   c . Although sufficient in preventing complete physical failure of the front windshield glass  12   a  as described above, the plastic film layer  11   c  introduces losses and distorted radiation patterns related to antenna performance, which may significantly degrade the electromagnetic coupling characteristics of conventional on-glass antennas related to GPS applications. 
   As seen in  FIGS. 4A–4C , the antenna unit  14   a , which is hereinafter referred to as an aperture coupled, slot-wave antenna  14   a , provides a vehicular glass mount patch antenna while also improving electromagnetic coupling performance over conventional front windshield-mount antennas. The aperture coupled, slot-wave antenna  14   a  is essentially a two-element antenna system such that the radiation element is electromagnetically coupled through the front windshield glass  12   a  to an excitation part located on the interior surface of the front windshield glass  12   a.    
   As illustrated, the first element of the aperture coupled, slot-wave antenna  14   a  includes a substantially rectangular top metallization  20  (i.e. the radiation element). The substantially rectangular top metallization  20  is linearly polarized (i.e. to receive terrestrial signals) and may include any desirable conducting material, such as, for example, a silver conducting film. In an alternative embodiment, the top metallization  20  may include an optically transparent conducting film comprising, for example, indium peroxide, to reduce the appearance of the aperture-couple slot-wave antenna  14   a  located about the front windshield glass  12   a . The second element of the aperture coupled, slot-wave antenna  14   a  includes a bottom portion  22  (i.e. the excitation element) that is electromagnetically coupled through at least one layer  11   a – 11   c  of the three-layered windshield glass  12   a.    
   The bottom portion  22  includes a substantially rectangular metal layer  24  and low noise amplifier (LNA) circuit  26 . As illustrated, the metal layer  24  is further defined to include an absence of material in the form of a substantially off-centered rectangular slot  28 . Additionally, the metal layer  24  is excited by a microstrip line  30  (shown in phantom in  FIG. 4C ) located adjacent the LNA circuit  26 . In operation, circular polarization is built into the antenna  14   a  as a result of the combination of the slot  28  and microstrip line  30 , which excites electromagnetic waves received by the top metallization  20 . In an alternative embodiment, the circular polarization may be achieved by providing a cross-aperture in the metal layer  24  in place of the substantially rectangular slot  28 . In yet another alternative embodiment, circular polarization may be built into the top metallization  20  by moving the slot  28  and microstrip line  30  into the top metallization  20 . 
   Referring to  FIG. 6A , a first implementation of the aperture-coupled slot-wave antenna  14   a  on the front windshield glass  12   a  is shown according to one embodiment of the invention. The aperture-coupled slot-wave antenna  14   a  is shown in a generally similar configuration as that in  FIG. 4A  expect that a radome  32  is located over the top metallization  20  so as to protect the top metallization  20  from the elements. The radome  32  is a thin, plastic element that has a low dielectric constant, which, as a result, appears transparent to electromagnetic waves received by the top metallization  20 . To reduce the appearance of the aperture-coupled slot-wave antenna  14   a , the bottom portion  22  of the slotted patch antenna array  14   a  is located on the passenger-compartment interior surface  23  of the glass layer  11   b  near an adjustment arm  25  of the rear-view mirror assembly  13 . The bottom portion  22  may be affixed to the inner glass layer  11   b  by an adhesive and covered by a plastic closeout (not shown). As a result, the bottom portion  22  may be hidden by positioning the rear-view mirror assembly  13  proximate the bottom portion  22 . 
   In an alternative embodiment, as seen in  FIG. 6B , the rear-view mirror assembly  13  may include a bezeled portion  27  located about the adjustment arm  25  that provides an adequate volume for housing the bottom portion  22 . In this embodiment, the radome  32  covers the top metallization  20 . In this implementation, the bezel  27  performs the dual function of completely hiding the bottom portion  22 , but may also provide a routing of wires  16  from the bottom portion  22  with other wires  16  associated with and extending from the rear-view mirror assembly in a tube  29  to the headliner. 
   As seen in  FIGS. 7A and 7B , another embodiment of the antenna system  10   a  includes bezeled portions, illustrated generally at  31  and  33 , in the intermediate plastic film layer  11   c . As seen in  FIG. 7A , the bezeled portion  31  is located adjacent to the outer glass layer  11   a , and conversely as shown in  FIG. 7B , the bezeled portion  33  is located adjacent the inner glass layer  11   b . In yet another alternative embodiment, the glass layers  11   a ,  11   b  may each include bezeled portion, which are illustrated generally at  35  and  37  in  FIGS. 8A and 8B , respectively. As seen in  FIG. 8A , the bezeled portion  35  is located in the inner glass layer  11   b  adjacent the intermediate plastic film layer  11   c , and conversely, as shown in  FIG. 8B , the bezeled portion  37  is located in the outer glass layer  11   a  adjacent the intermediate plastic film layer  11   c.    
   The alternative embodiments illustrated in  FIGS. 7A–8B  function in eliminating the radome  32  because the top metallization  20  is protected from the elements by integrating the top metallization  20  within any one of the layers  11   a – 11   c  of the front windshield glass  12   a . Additionally, the alternate embodiments illustrated in  FIGS. 7A–8B  locates the top metallization  20  closer to the bottom portion  22  to reduce the distance that the received signal has to travel via the electromagnetic coupling between the front windshield glass  12   a . As a result, electromagnetic coupling through the intermediate plastic film layer  11   c  may be passed completely when the bezeled portion is located as illustrated in  FIGS. 7B and 8A  when the inner glass layer  11   b  or plastic layer  11   c  is bezeled out at  33  and  35  such that the top metallization  20  is positioned directly adjacent the inner glass layer  11   b . Although bezeled portions  31 ,  33 ,  35 ,  37  are illustrated in  FIGS. 7A–8B , the top metallization  20  may include a reduced thickness such that the top metallization  20  is sandwiched between any one of the layers  11   a – 11   c  without including a bezeled portion  31 ,  33 ,  35 ,  37 . However, if the top metallization  20  is sandwiched between the layers  11   a – 11   c  without the bezeled portion  31 ,  33 ,  35 ,  37 , the material comprising top metallization  20  and/or the layers  11   a – 11   c  may have to be altered so as to compensate for material expansion considerations. Additionally, although the alternate embodiments illustrated in  FIGS. 7A–8B  do not show the combination of a bezel  31 ,  33 ,  35 ,  37  used in conjunction with the mounting of the bottom portion  22  within the adjustment arm  25  of the rear-view mirror assembly  13 , any one of the illustrated bezels  31 ,  33 ,  35 ,  37  may be used in combination with the location of the bottom portion  22  within the adjustment arm  25  as shown in  FIG. 6B . 
   Referring now to  FIGS. 5A and 5B , the antenna unit  14   b , which is hereinafter referred to as an antenna array  14   b , illustrates another embodiment of a vehicular glass mount patch antenna. The antenna array  14   b  includes a 90-degree phase shift circuit  34   c  intermediately disposed between the two patch elements  34   a ,  34   b  adjacent the interior surface  39  of the rear windshield glass  12   b . As illustrated, a dielectric layer  38  and a bottom metal layer  36  are disposed below the patch antenna elements  34   a ,  34   b  and phase shift circuit  34   c.    
   Referring to  FIG. 5B , the antenna array  14   b  is essentially a high-gain dual element antenna such that the dual elements are spatially orientated by 90-degrees with respect to each other so as to provide better axial ratio and more radiation to compensate the inherent losses due to the dielectric constant of the rear windshield glass  12   b . As illustrated, the antenna elements  34   a ,  34   b  include symmetrically cut corners  40  to create left-hand circular polarization for the antenna array  14   b . Alternatively, if the opposing corners  42  were to be cut, the antenna array  14   b  would be a right-hand circular polarized antenna. 
   As seen in  FIGS. 9A–9C , an antenna system  10   c  includes an aperture coupled, slot-wave GPS antenna unit  14   c , provides a vehicular glass mount patch antenna while also improving electromagnetic coupling performance over conventional rear windshield-mount GPS antennas. The aperture coupled, slot-wave antenna  14   c  is essentially a two-element antenna system such that the radiation element is electromagnetically coupled through the rear windshield glass  12   b  to an excitation part located on the interior surface of the rear windshield glass  12   a.    
   As illustrated, the first element of the aperture coupled, slot-wave antenna  14   c  includes a right-hand circularly polarized top metallization  44  (i.e. the radiation element). Because the top metallization  44  is right-hand circularly polarized, the top metallization receives GPS signals and may include any desirable conducting material, such as, for example, a silver conducting film. In an alternative embodiment, the top metallization  44  may include an optically transparent conducting film comprising, for example, indium peroxide, to reduce the appearance of the aperture-couple slot-wave antenna  14   c  located about the rear windshield glass  12   c . The second element of the aperture coupled, slot-wave antenna  14   c  includes a bottom portion  46  (i.e. the excitation element) that is electromagnetically coupled through the rear windshield glass  12   b . The bottom portion  46  includes a substantially rectangular metal layer  48  and low noise amplifier (LNA) circuit  50 . As similarly described with respect to the bottom portion  22  in  FIG. 4C , the metal layer  48  is further defined to include an absence of material in the form of a substantially off-centered rectangular slot  52 . Additionally, the metal layer  48  is excited by a microstrip line  54  (shown in phantom in  FIG. 9C ) located adjacent the LNA circuit  50 . In operation, the combination of the slot  52  and microstrip line  54  excites electromagnetic waves received by the top metallization  44 . 
   Referring to  FIGS. 10A–10C , another embodiment of the invention includes an antenna system  10   d  includes a GPS antenna unit  14   d  defined by a co-planar-type feed comprising a top metallization  56  including a cross-aperture-shaped slot  58  and a bottom metallization  60  including a pair of parallel slots  62 . 
   Both embodiments of the invention described in  FIGS. 9A and 10A  include the top metallization  44 ,  56 , which is covered by a radome  32  and located on the exterior surface  64  of the glass  12   b . The bottom portion  46  is located on the interior surface  66  of the glass  12   b  and may be protected by a plastic cover (not shown), or, alternatively, the bottom portion may be housed within the rear-brake-light housing bezel (not shown). According to another embodiment of the invention as shown in  FIGS. 11A–11E , antenna systems  10   c – 10   i  may include any desirable location of the top metallization  44 ,  56  and bottom portion  46  about the rear windshield glass  12   b . Although the antenna unit  14   c  (refer  FIG. 9A ) is shown located within the glass  12   b  in  FIGS. 11A–11E , the antenna unit  14   d  (refer  FIG. 10A ) or any other desirable antenna unit may be located within the rear windshield glass  12   b  as shown. 
   As seen in  FIG. 11A , the top metallization may be located within a pocket  68  formed in the glass  12   b . Alternatively, as seen in  FIG. 11B , the bottom portion  46  may be located within the pocket  68 . According to yet another embodiment of the invention as shown in  FIG. 11C , a pair of pockets  70 ,  72  formed in the glass  12   b  may maintain the top metallization  44 ,  56  and bottom portion  46  in an opposing relationship and spaced at a distance, D 1 , within the glass  12   b . According to yet another embodiment of the invention as shown in  FIG. 11D , a single pocket  74  is formed in the glass  12   b  to maintain the top metallization  44 ,  56  and bottom portion  46  in an opposing relationship with an intermediate air gap  76  defined by a separation distance, D 2 . Alternatively, as seen in  FIG. 11E , rather than including an air gap  76  within the single pocket  74 , a dielectric material  78  may be intermediately located between the top metallization  44 ,  56  and bottom portion  46 . If desired, any embodiment of the invention described above may be incorporated into a diversity antenna configuration if a diversity GPS receiver (not shown) is incorporated into the vehicle. 
   The present invention has been described with reference to certain exemplary embodiments thereof. However, it will be readily apparent to those skilled in the art that it is possible to embody the invention in specific forms other than those of the exemplary embodiments described above. This may be done without departing from the spirit of the invention. The exemplary embodiments are merely illustrative and should not be considered restrictive in any way. The scope of the invention is defined by the appended claims and their equivalents, rather than by the preceding description.