Patent Publication Number: US-6211831-B1

Title: Capacitive grounding system for VHF and UHF antennas

Description:
TECHNICAL FIELD 
     This invention relates generally to a vehicle antenna system and, more particularly, to a vehicle antenna system including an improved capacitive grounding system for a VHF and UHF vehicle antenna system. 
     BACKGROUND OF THE INVENTION 
     Most modern vehicles include a vehicle radio that requires an antenna system to receive amplitude modulation (AM) and frequency modulation (FM) broadcasts from various radio stations. Many vehicle antenna systems include a dipole antenna that extends from a vehicle fender, vehicle roof, or some applicable location on the vehicle to receive these broadcasts. Improvements in vehicle antenna systems have included the development of backlite antenna systems, where antenna elements are formed on a rear window of the vehicle. The antenna elements in the backlite antenna systems are typically made of a conductive frit deposited on an inside surface of the window. Additionally, vehicle windshield antennas, such as the Solar-Ray antenna disclosed in U.S. Pat. No. 5,528,314, have also been developed. The Solar-Ray antenna includes a transparent conductive film laminated between the inner and outer glass sheets of the windshield. The windshield and backlite antenna systems provide a number of advantages over mast antenna systems, including no wind noise, reduced drag on the vehicle, elimination of corrosion of the antenna elements, no performance change with time, limited risk of vandalism, and reduced cost and installation. 
     Advancements in vehicle communication technologies has led to the need for various high frequency antennas to provide reception for different communication systems, such as radio frequency accessory (RFA) and key-less entry systems, cellular telephone, global positioning systems (GPS), personal communication systems (PCS), toll systems, garage door openers, etc. Because these antenna systems operate at higher frequencies than the AM and FM frequency bands, the size of the antenna is reduced from AM and FM antenna systems. These high frequency antennas must be positioned on a vehicle at a location where the antenna radiation is not adversely effected by the conductive vehicle body. It has been suggested to incorporate high frequency antennas in the vehicle windshield or backlite glass in combination with the existing AM/FM antennas. In one design, the high frequency antennas are mounted on an inside surface of the inside glass sheet of the windshield along a tinted top edge of the windshield so that they do not obstruct the view of the vehicle operator. 
     Each of the various vehicle window antennas typically include an antenna feed that is usually a coaxial cable connected to the antenna at a location suitable for optimum performance. The cable can be either directly connected to the antenna element or capacitively coupled to the antenna element through the vehicle glass. The center conductor of the cable is electrically connected to the antenna element and the outer conductor or outer shield of the cable is electrically connected to the vehicle ground, usually a vehicle body panel. An antenna design providing optimum performance would require that the outer shield of the coaxial feed cable be terminated and DC grounded to the vehicle body panel as close as possible to the edge of the window opening. This location provides a low impedance path for both DC and RF signals. However, antenna performance measurements indicate that the actual ground point for the feed cable can be located a distance of up to one-twentieth of the desirable reception wavelength S without significantly affecting the reception characteristics of the antenna. 
     In one known design, the DC ground for the Solar-Ray antenna is provided by soldering a metal bracket to the outer shield of the feed cable. The bracket is attached to the vehicle body by a ground screw about five inches from the window opening, or about S/25 of the middle of the FM frequency band. It has been determined that this type of feed arrangement would be unacceptable for proposed UHF and VHF antennas because of certain manufacturing concerns. 
     It is an object of the present invention to provide an alternate grounding system for these UHF and VHF vehicle antennas. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a capacitive grounding system is disclosed for a VHF and UHF vehicle antenna. The outer shield of a coaxial feed cable is capacitively coupled to a vehicle body panel at an edge of the vehicle body panel proximate a vehicle window opening. The feed cable is attached to the vehicle body panel by taping, gluing, etc. at this location. The resulting capacitance between the outer shield and the vehicle body panel results in a very low impedance path for RF signals. 
     Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a vehicle windshield including an RFA antenna and a Solar-Ray antenna, and associated coaxial feed cables, according to an embodiment of the present invention; 
     FIG. 2 is plan view of an end of the RFA feed cable shown in FIG. 1 mounted at an edge of a vehicle body panel, according to an embodiment of the present invention; and 
     FIG. 3 is a cross-sectional view of the RFA feed cable mounted to the vehicle body. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following discussion of the preferred embodiments directed to capacitively coupling an outer shield of a coaxial feed cable for a VHF and UHF vehicle antenna is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. Particularly, the discussion below will refer to a feed cable for an RFA antenna in a vehicle. However, the invention can be used for other vehicle antennas as well as non-vehicle antennas. 
     FIG. 1 is a front plan view of a vehicle windshield  10  removed from the vehicle (not shown). The windshield  10  includes an upper tinted region  12  and a shaded border  14 . The windshield  10  further includes a Solar-Ray antenna  18  for providing AM and FM reception of the type disclosed in the &#39;314 patent referred to above. The Solar-Ray antenna  18  includes a conductive film configured as shown to include a tuning element  20  and an impedance element  22 , where the tuning element  20  runs along an upper edge  28  of the windshield  10 . The antenna  18  further includes a vertical connecting portion  24  connecting the impedance element  22  to the tuning element  20 . A connecting tab  26  extends vertically up from tuning element  20  almost to the upper edge  28  of the windshield  10 . The conductive film is formed on an inside surface of an outer glass layer  30  of the windshield  10  so that it is positioned between the outer glass layer  30  and an inner glass layer  32  (see FIG. 3) of the windshield  10 . 
     A coaxial feed cable  40  provides the feed connection to the antenna  18 , and is connected to the tuning element  20  at a desirable location to provide optimum antenna performance. A center conductor  42  of the cable  40  is connected to a conductive patch  44  formed on an inside surface of the outer glass layer  30  of the windshield  10 . The patch  44  is positioned at a location so that it is electrically coupled to the connecting tab  26  on the outer glass layer  30 . An outer shield  46  of the cable  40  is grounded to a vehicle body panel at a suitable location close to the edge of the vehicle body panel, as discussed above. 
     The windshield  10  also includes a “T-shaped” patch antenna  50 , also formed between the inner layer  32  and the outer glass layer  30  of the windshield  10  in the tinted region  12 . The patch antenna  50  is a high frequency antenna for receiving VHF and UHF signals, and has a particular application for use as an RFA antenna. The antenna  50  has other high frequency applications, including being used as a GPS antenna, toll antenna, garage door opener antenna, PCS antenna, etc. A coaxial feed cable  52  provides the feed to the antenna  50 . The cable  52  includes a center conductor  54  that is electrically connected to a conductive patch  56  formed on an inside surface of the inner glass layer  32  of the windshield  10 . The conductive patch  56  is capacitively coupled through the inner glass layer  32  to the antenna  50  in this embodiment. In an alternate embodiment, the antenna  50  can be formed on an inside surface of the inner glass layer  32 . In this embodiment, the center conductor  54  would be directly connected to the antenna  50 , and, for example, the patch  56  shown in FIG. 3 would be the antenna  50 . 
     FIG. 2 is a perspective view of a portion of the cable  52  mounted to a vehicle body panel  60 . FIG. 3 is a cross-sectional view of a portion of the body panel  60  and the windshield  10  showing the connection of the cable  52  to the antenna  50 . The windshield  10  rests on a flange  62  of the body panel  60 , and is sealed thereto by a sealing strip  64 , such as a urethane sealing strip, as is known in the art. The cable  52  can be mounted to the body panel  60  by any suitable fastening device, such as glue, tape, etc. In this embodiment, the cable  52  is mounted to the flange  62  by an adhesive strip  66  over a distance of about 100 mm along the length of the cable  52 . In addition to the center conductor  54 , the cable  52  includes a coaxial arrangement of an internal dielectric layer  68 , an outer shield  70  and an outer dielectric covering  72 , as shown. As is apparent, the outer shield  70  is positioned flush with an edge  74  of the body panel  60  proximate the opening for the windshield  10 . 
     In this embodiment, the outer shield  70  is positioned flush with the edge  74 , so that it is as close to the window opening as possible. In alternate designs for different antennas, the cable  52  can be mounted to the body panel  60  slightly away from the edge  74 , with the requirement that it be within one-twentieth of the wavelength of the frequency band of interest. The resulting capacitance between the outer shield  70  and the body panel  60  provides a very low impedance path for RF signals. This low impedance path for FM and RFA signals is much less than their respective cable impedances, i.e., 125 Ohms for the FM cable and 50 Ohms for the RFA cable. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.