Abstract:
A window antenna is provided for receiving electromagnetic radio waves from a motor vehicle. The vehicle has a metallic structure that forms an aperture with a window glass disposed within. The window glass has an upper region and a substantially horizontal top edge that interfaces with the metallic structure. The antenna is formed from an electrically conducting material affixed to the window glass. The antenna includes a horizontally elongate principal element that is substantially parallel to and spaced from the upper edge portion of the window. The principal element electrically connects to a connecting element which extends downwardly therefrom. The connecting element electrically connects to an auxiliary element that is spaced below the principal element by an amount sufficient that no significant coupling is formed therewith.

Description:
TECHNICAL FIELD 
     This invention relates to radio-frequency antenna systems that affix to a vehicle window. 
     BACKGROUND OF THE INVENTION 
     With the increasing development of communications devices and the expanding variety of those devices, there has been an increased demand to locate associated antennas in areas that improve the performance of the communication devices. In motor vehicles, concerns with aesthetics and not obstructing the view of the vehicle occupants further limit the range of desirable locations for antennas. Typical communication devices that are now installed in many vehicles include AM/FM radios, cellular phones, and vehicle remote keyless entry systems. Also, vehicle remote start-up systems are projected to enter the market in the near future. 
     Generally, traditional remote keyless entry systems include an internal antenna to receive radio waves that direct the system to lock or unlock the vehicle. The keyless entry module is usually located under the dashboard within the passenger compartment or other locations within the vehicle. Typically, these systems have provided satisfactory performance, however reception of the vehicle lock/unlock signal is limited due to attenuation of the signal by the intervening vehicle structure. Additionally, intermittent reception of the lock/unlock signal occurs depending on the direction from which the signal is transmitted due to differences in the amount of vehicle structure that intervene between the signal source and the internal antenna. For example, the keyless entry system might be unresponsive to a signal transmitted from near the back quarterpanel, but be responsive to a signal transmitted from near the driver&#39;s side door. Intermittent or inconsistent operation is undesirable because of the possibility of customer dissatisfaction. 
     An alternative to mounting an antenna in the passenger compartment is to mount the antenna in the windshield. Traditionally, mast or whip antennas have been used to receive and transmit radio waves from a motor vehicle. Recently, thin film antennas that are affixed to a vehicle window have been developed (see U.S. Pat. Nos. 5,083,135, 5,528,314, and 5,739,794). The thin film antennas have eliminated the need for mast or whip antennas for VHF and UHF reception. 
     U.S. Pat. No. 5,083,135 issued Jan. 21, 1992, describes a transparent film antenna for a vehicle window in the shape of a “T” with a horizontally elongate principal element spaced from and parallel to the upper horizontal edge of the window and a vertically elongate impedance matching element extending down the window from the center of the principal element. A disadvantage associated with film antennas wherein the elements extend into the main viewing area of the windshield is the impact on vehicle appearance resulting when sunlight reflects off of the surface of the windshield. The sunlight tends to have a shimmering effect on the area of the windshield where the film antenna is embedded. 
     U.S. Pat. No. 5,528,314 issued Jun. 18, 1996 and U.S. Pat. No. 5,739,794 issued Apr. 14, 1998, provide improved film antennas (referred to as solar-ray antennas) that do not degrade the appearance of the vehicle. Both solar-ray antennas include a horizontally elongate principal element spaced from and parallel to the upper horizontal edge of the window. Each antenna includes an impedance matching element coupled to the principal element and extending outward so that it is spaced from the edge of the window aperture so as to form a slot transmission line. The impedance matching elements therefore enclose or occupy almost the entire windshield with the exception of a strip across the top in the area that is normally tinted. 
     A vehicle that employs a solar-ray antenna for AM/FM reception precludes using a conventional film antenna for a vehicle keyless entry system because of physical interference. The solar-ray antenna encloses virtually the entire windshield except for the tinted area at the top. A vehicle keyless entry system that employed a conventional film antenna located in the tinted region of the windshield would require a vertical impedance matching element that would extend into the impedance matching element of the solar-ray antenna. If the solar-ray impedance matching element is shifted to a lower position on the windshield so as to avoid physical interference with the keyless entry system antenna, the appearance of the vehicle will be degraded due to the shimmering effect of the windshield area with the film antenna contrasting with the remainder of the windshield. Accordingly, what is desired is a film antenna that can be located entirely within the tinted area of a windshield. 
     SUMMARY OF THE INVENTION 
     A vehicle window antenna is provided for receiving electromagnetic radio waves. The vehicle has a metallic structure that forms an aperture with a window glass disposed within. The window glass has an upper region and a substantially horizontal top edge that interfaces with the metallic structure. The antenna is formed from an electrically conducting material affixed to the window glass. The antenna includes a horizontally elongate principal element that is substantially parallel to and spaced from the upper edge portion of the window. The principal element includes an upper edge and a lower edge. The lower edge electrically connects to a connecting element which extends downwardly. The lower portion of the connecting element electrically connects to an auxiliary element that has an upper edge spaced below the principal element by an amount sufficient that no significant coupling is formed therewith. 
    
    
     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 shows a vehicle having a window provided with an antenna according to the invention; 
     FIG. 2 shows a detailed view of a vehicle windshield provided with an antenna according to the invention; 
     FIG. 3 illustrates a presently preferred embodiment of the invention; 
     FIG. 4 illustrates an embodiment of the invention formed with ribbon conductors; and 
     FIG. 5 illustrates an embodiment of the invention formed with ribbon conductors that terminate in an open circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present embodiment of the invention is configured as a small high frequency antenna for a vehicle remote keyless entry system that operates at 315 MHz with a wavelength of 952 mm. However, the principles of the invention extend to antennas designed for other vehicle systems such as electronic toll, cellular, GPS, GMS, and PCS. The antenna is preferrably located in the upper tinted region of the vehicle windshield, although other locations on a vehicle window are within the scope of the invention. The vehicle in the preferred embodiment includes an AM/FM reception solar-ray antenna in addition to one or more high frequency antennas. The solar-ray antenna extends over a substantial portion of the main viewing area of the vehicle windshield limiting the potential locations for high frequency antennas. 
     Referring to FIG. 1, a vehicle body  10  including a window antenna  16  for a keyless entry system is illustrated. The vehicle body  10  is made of an electrically conducting metal such as steel or aluminum and having a window defined by a body window edge  11 . A window glass  12  overlaps the body window edge  11  around its periphery to provide a windshield for the vehicle body  10 . The windshield  12  is preferably a standard laminated automobile windshield formed of two layers of glass with an interposing thermoplastic or polyvinyl butyral layer. 
     With reference to FIG. 2, a detailed section of the windshield  12  is illustrated. The windshield  12  may optionally be provided with a longitudinally extending the tinted region  14  across the top. This tinted region  14 , commonly used in windshields for light and glare reduction, may be advantageously utilized in this invention to render the antenna less visible. Representing one embodiment of the present invention, the window antenna  16  is shown disposed in the tinted region  14  of the windshield  12 . For reception of AM/FM signals, a solar-ray antenna  30  is mounted in the main viewing area of the windshield  12 . Before describing the window antenna  16 , first the solar-ray antenna  30  will be briefly described. 
     The solar-ray antenna  30 , configured as described in U.S. Pat. No. 5,528,314, is shown disposed substantially in the main viewing area of the windshield  12 . The solar-ray antenna  30  is formed from an electrically conducting material affixed to the windshield  12 . The solar-ray antenna  30  is a planar antenna of substantially constant thickness, therefore, its reception and radiating characteristics are greatly affected by its planar shape. The antenna  30  may be described as comprising two basic elements: a principal element  32  and an impedance matching element  34 . The principal element  32  is centrally located within the tinted region  14  of the windshield  12 . The principal element  32  is essentially rectangular in shape and substantially parallel to and spaced from the horizontal upper portion of the body window edge  11 . The impedance matching element  34  substantially covers all of the windshield  12  below the tinted region  14  and thus most or all of the main viewing area of the windshield  12 . 
     The window antenna  16  for the keyless entry system is formed from an electrically conducting material affixed to the windshield  12 . Suitable conducting materials include non-transparent films and frit material such as silver and glass. Alternatively, the antenna  16  may be made of single or multi-layer transparent conductive films such as provided by indium-tin-oxide or layers of silver and titanium dioxide, which are essentially transparent to visible light and electrically conducting, preferably having a sheet resistance of 4 ohms per square or less. The scope of the invention is not limited to the aforementioned conducting materials, any film of material having suitable transparency and conductivity may used to form the antenna  16 . The basic requirement for the conducting material is to provide sufficient conductivity for the electric currents that run along the edges of the elements. For a given sheet resistance, this conductivity will vary with the width of the strip of material that conducts current. Visually transparent materials generally have larger sheet resistances and must therefore be made wider, but their transparency allows the wider elements to be placed on a window without compromising window viewing area constraints. The materials with lower sheet resistance, such as the frit material mentioned above, are generally non-transparent to visible light; and window antenna elements made therefrom must therefore be made narrower or located at the edge of the windshield  12  so as not to affect visibility through the window. 
     Referring to FIG. 3, a magnified view of the antenna  16  is provided. The antenna  16  is a planar antenna, therefore, its reception and radiating characteristics are greatly affected by its planar shape. The antenna  16  may be described as comprising three basic elements: a principal element  18 , a connecting element  20 , and an auxiliary element  22 . The connecting element  20  and auxiliary element  22  combine to provide the impedance matching element function of prior art antennas. In conventional window antennas, the impedance matching element is a vertically elongate member that attaches to the central portion of the lower edge of the principal element. An antenna employing such a configuration in the present application would extend downward into the main viewing area of the windshield and physically interfere with the solar-ray antenna impedance matching element. In the presently preferred embodiment the combination of the connecting element  20  and auxiliary element  22  provides an area efficient replacement for the prior art impedance matching element. By maintaining approximately the same circumference for the combination as would be required for the prior art impedance matching element, and extending a the auxiliary element  22  sidewardly instead of downwardly, approximately the same antenna impedance is attained while requiring less vertical length for the antenna  16 . Similar to a prior art impedance matching element, the connecting element  20  combined with the auxiliary element  22  behaves as a short inverted monopole with respect to the vehicle body  10 . The impedance of the antenna  16  is tuned by adjusting the current path length (outer circumference of the elements  20  and  22 ) of the combination of the connecting element  20  and the auxiliary element  22 . The combination has an associated impedance which varies primarily as a function of the current path length. By attaching the connecting element  20  and auxiliary element  22  to the principal element  18 , their respective impedances essentially combine in parallel and appear as the total impedance for the antenna  16  between the feed point and ground point. 
     The principal element  18  is similar in outer shape and orientation to the principal element of the referenced U.S. Pat. No. 5,083,135. The principal element  18  is located substantially parallel to and spaced from the upper horizontal portion of the body window ledge  11 . The principal element  18  is essentially rectangular, although its horizontal edges may follow any slight curvature of the upper horizontal portion of the body window ledge  11  and its corners may be rounded for a more pleasing appearance. Additionally, since most of the electrical currents flowing in the principal element  18  exist near its outer edges, it is not necessary for the principal element  18  to be continuous over the entire area within its outer dimensions. The principal element  18  is a tuned element having an effective horizontal length of an odd integer of one-quarter of the wavelength to which it is tuned. 
     The principal element  18  exhibits a zero reactive impedance at the tuned wavelength. In the presently preferred embodiment, the antenna  16  is designed for reception of a 315 MHz signal that controls a remote keyless entry system for a vehicle. The principal element  18  is tuned to a wavelength, 952 mm, corresponding to the operating frequency and thus has an effective horizontal length of about 238 mm. As with the antenna of U.S. Pat. No. 5,083,135, it has been found that the physical length of the principal element  18  at resonance is somewhat shorter than a measured one-quarter of the actual signal wavelength. The referenced patent provides a detailed description of the coupling effects that cause the reduced length of principal element  18 . In the presently preferred embodiment, the principal element  18  has been found to work well with an actual horizontal length of 152 mm and a vertical width of 12.7 mm. The principal element  18  is ideally spaced below the upper horizontal portion of the body window edge  11  by a distance which provides maximum signal gain, but this distance may be compromised to obtain other advantages for a particular vehicle design as will be described below. In the presently preferred embodiment, the principal element  18  is preferably spaced 6.4 mm from the upper horizontal portion of the body window edge  11 , although a spacing in the range 4.76 mm to 15.9 mm is also appropriate. 
     The connecting element  20  is preferably formed from an elongated strip or ribbon being substantially perpendicular to the principal element  18 . The connecting element  20  is electrically connected to the center portion of the lower edge of principal element  18 , extending therefrom downwardly. Although in the presently preferred embodiment the connecting element  20  electrically connects to the center portion of the principal element  18 , it is within the scope of the invention to connect the connecting element along other portions of the lower edge of the principal element  18 . The length of the connecting element  20  is selected so as to minimize transmission coupling effects between the principal element  18  and auxiliary element  22 . In the presently preferred embodiment, a length of preferably 50.8 mm is employed. Since most of the electrical currents flowing in the connecting element  20  exist near its outer edges, it is not necessary for the connecting element  20  to be continuous over the entire area within its outer dimensions. To simplify manufacture, the connecting element  20  is preferably formed of the same material and the same process as the principal element  18 . 
     The auxiliary element  22  is preferably comprised of a strip or ribbon extending from the connecting element  20  and made from the same material in the same process. The auxiliary element  22  is substantially horizontally elongate having an upper portion  24  connected to the lower portion of connecting element  20 , extending therefrom substantially equally towards each side. Although in the presently preferred embodiment the center of the upper portion of the auxiliary element  22  electrically connects to the lower portion of the connecting element  20 , it is within the scope of the invention to connect to other places along the upper edge of the auxiliary element  22  to the lower portion of the connecting element  20 . The angle at which the auxiliary element  22  connects to the connecting element  20  is selected so as to minimize transmission coupling effects between the principal element  18  and auxiliary element  22 . The lower edge of the auxiliary element  22  is spaced a sufficient distance from solar-ray antenna impedance matching element  34  so as to minimize transmission coupling effects between the auxiliary element  22  and the impedance matching element  34 . In the presently preferred embodiment, a spacing of preferably 19.0 mm is maintained, although a spacing approaching 12.8 mm is allowable. Since most of the electrical currents flowing in the auxiliary element  22  exist near its outer edges, it is not necessary for the auxiliary element  22  to be continuous over the entire area within its outer dimensions. To simplify manufacture, the auxiliary element  22  is preferably formed of the same material and the same process as the connecting element  20 . 
     The window antenna  16  is preferably fed at feed point  25  located along the upper edge of the principal element  18 ; preferrably approximately centered along the upper edge of the principal element  18 . The window antenna  16  may also be fed by a feed apparatus  29  extending upward from the upper edge. The feed apparatus  29  encompasses any of the feed arrangements described in U.S. Pat. No. 5,528,314 and U.S. Pat. No. 5,648,785 issued on Jul. 15, 1997, both of which are incorporated by reference with regard to such feed arrangements. 
     The connecting element  20  and auxiliary element  22  in combination behave as a short inverted vertical monopole with respect to the metallic structure of the vehicle  10 . The combination has an associated impedance which varies primarily as a function of the current path length. By attaching the connecting element  20  and auxiliary element  22  to the principal element  18 , the respective impedance of those elements essentially combine in parallel and appear as the total impedance for the antenna  16  between the feed point  25  and ground point  23 . As a result, the impedance of the antenna  16  can be tuned by adjusting the current path length of the connecting element  20  and auxiliary element  22 . This can be particularly useful in improving the impedance match between a particular coaxial cable and film antenna  16  to maximize antenna gain. 
     To maintain an aesthetically pleasing appearance, the upper edge of the solar-ray antenna impedance matching element  34  is preferably located within the tinted region  14  of the windshield  12 . The location of the impedance matching element upper edge constrains the location of the window antenna  16  to a region spaced sufficiently above the upper edge so as to minimize transmission line coupling between the window antenna  16  and the impedance matching element  34 . The overall height of the window antenna  16  is limited to the distance between the impedance matching element  34  and the window edge  11 . Generally, maximum antenna performance of the window antenna  16  is obtained when the principal element  18  is spaced a certain distance below the window edge  11  and the auxiliary element  22  is spaced a certain minimum distance away from the solar-ray impedance matching element  34 . However, it might not be possible to space the antenna  16  the optimum distance from the window edge  11  and impedance matching element  34  due to limitations on the vertical extent of the tinted region  14 . Restrictions imposed by design or governmental requirements on the extent of the tinted region  14  may limit the overall height of the antenna  16 . Therefore, the optimum spacings and element lengths of the window antenna  16  may have to be compromised, with a resulting decrease in antenna gain. 
     Referring to FIG. 4, another antenna  116  configured in accordance with the principles of the invention is illustrated. Antenna  116  is similar to antenna  16  in function with corresponding elements numbered in the range  100 - 199 , except that antenna  116  is non-symmetrical and the elements are constructed of electrically conducting ribbon material approximately 2 mm in width affixed to the windshield  12 . The principal element  118  is substantially parallel to and spaced from the upper horizontal portion of the window edge  123 . The length of the principal element lower edge  119   a  is substantially shorter than the principal element lower edge  119   b.  The connecting element  120  electrically connects to the lower edges  119   a  and  119   b  of the principal element  118  and extends downwardly therefrom. The lower portion of the connecting element  120  connects to the upper edges  124   a  and  124   b  of the auxiliary element  122 . The lengths of the upper edges  124   a  and  124   b  are set so that the upper edge  124   a  in combination with the principal element lower edge  119   a  is approximately equivalent in length to the combination of the upper edge  124   b  and the principal element lower edge  119   b.  The antenna  116  is fed by a feed apparatus  129  located at approximately the center of the upper edge of the principal element  118 . The feed apparatus  129  is similar to previously disclosed feed apparatus  29 . The null point  127  of antenna  116  is preferably located at the center of the lower edge of the auxiliary element  122 , although other locations along the smooth portion of the auxiliary element lower edge are within the scope of the invention. 
     Referring to FIG. 5, another antenna  216  configured in accordance with the principles of the invention is illustrated. Antenna  216  is similar to antenna  16  in function with corresponding elements numbered in the range 200-299. Antenna  216  differs from antenna  16  in that the elements of antenna  216  are constructed of electrically conducting ribbon material approximately 2 mm in width affixed to the windshield  12  and the ribbon material is electrically discontinuous along the lower edge of the auxiliary element  222  at the null point  227 . Current paths for the antenna  216  extend outward from the feed point  225  along the upper edge of the principal element  218 . The current paths continue along the ribbon conductor on either side of the connecting element  220  through to the auxiliary element lower edges  226   a  and  226   b  and terminate at the null point  227 . Current that flows from the feed point  225  through the principal element  218  and connecting element  220  flows through the respective lower edges  226   a  and  226   b  of the auxiliary element  222  and reflect back upon themselves at the null point  227 . 
     The present invention recognizes that the reflected impedance of an antenna impedance matching element is related to the current path length of that element. A space efficient impedance matching element can be constructed by extending sideways portions from a portion of the element while effectively maintaining similar current path lengths. Physical symmetry of the current paths is not required, however the null point of the current paths is preferably located on a flat edge of the auxiliary element to ensure a consistent antenna impedance with slight variations in path lengths caused by manufacturing tolerances. It has been found that a window antenna constructed in this manner for high frequency signals can be restricted to the upper tinted region of a vehicle window and still provide acceptable antenna performance. Because the window antenna is configured within the tinted region of a vehicle window, a solar-ray antenna for AM/FM reception can be disposed within the main viewing area of the window. 
     It should be understood that while this invention has been described in connection with particular examples thereof, no limitation is intended thereby since obvious modifications will become apparent to those skilled in the art after having the benefit of studying the foregoing specification, drawings and following claims.