Abstract:
A glass antenna assembly for receiving and transmitting cellular telephone signals includes a two pair of dipole antennas, each pair mounted on a vehicle window. Space diversity is achieved by placing the vehicle windows with the antenna pair on opposite sides of the vehicle. This results in an improved omni-directional antenna pattern. Each dipole antenna is tunes, and employs at least three elements to broad band the dipole antenna. Coaxial feeders leading from the antenna assembly can be concealed under the roof lining for improved aesthetics.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to an antenna apparatus, system and method for receiving and transmitting cellular telephone signals. More particularly, the invention relates to a dipole antenna coupled to a transmission line that is printed on a vehicle window. 
     BACKGROUND 
     A number of apparatus and methods exist for an antenna that utilizes the surface of a glass. For example, one type of antenna has been used exclusively for reception in the VHF band, having a low gain and an unfavorably high voltage standing wave ratio (VSWR). For practical reasons, pole or rod antennas have been used for portable communications services such as cellular telephones and for receiving global positioning satellite (GPS) signals. 
     Rod and pole antenna typically extend outward from the automobile, and generally create noise at high speed, interfere with washing of the vehicle, can be snagged on low branches, and adversely affect the overall aesthetics of the vehicle. 
     Dipole antennas typically appear as a metal rectangle on the end of a short mounting beam, and is the basic antenna for fixed point communications. Dipole antennas are omni-directional when vertically polarized and have relatively low gain. It is not common to use a dipole antenna in a horizontally polarized system because other antennas having higher gain and lower cost are readily available. 
     As depicted in FIG. 1, shielded dipole antennas  10  are also known, for example, U.S. Pat. No. 4,746,925. The coaxial cable  12  must run across a window glass  14 , which is aesthetically unappealing and obscures driver or passenger visibility. Moving the antenna  10  closer to pillars or trim area  16  degrade performance as the dipole radiation pattern is severely distorted by the proximity of the surrounding metal, as well as significantly radiating into the vehicle. 
     Consequently, there is a need for a dipole antenna that provides improved antenna performance and as well as improved aesthetic qualities. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary object of the present invention to provide an antenna system for the reception of cellular telephone signals and transmission of the cellular telephone signals to a cellular receiver, as well as the transmission of cellular signals from a cellular transmitter to external cellular receivers over a transmission line having an improved omni-directional antenna pattern when mounted on a vehicle window. 
     Another object of the invention is to provide a dipole antenna mounted on the surface of a vehicle window that is in a clear path RF environment. 
     Another object of the invention utilizes two sets of dipole antennas with a modified feed length, each antenna positioned on opposite sides of the vehicle providing enhancement of the radiation pattern. 
     The present invention is directed to an automotive on glass antenna having parallel tuned feeders. Two sets of antenna elements are printed on a vehicle window and are tuned to an upper part of the desired frequency band and to a lower part of the desired frequency band. The antenna elements can be printed on the glass using techniques known in the art for printing rear defogger elements and AM/FM radio antennas onto glass. For example, in a cellular telephone application having a bandwidth of approximately 70 MHz, a VSWR of less than 2:1 can be maintained. Each tuned dipole antenna employs three elements to broad band the dipole antenna. A parallel tuned feeder for each antenna is a multiple electrical half wavelength to transfer the approximately 50 Ohm impedance of the dipole. Parallel tuned feeders transform the impedance of the coaxial cable to match the impedance of the antenna. The parallel tuned feeder allows for the placement of the printed modified dipole antenna in a clear path RF environment, resulting in a well-defined omni-directional antenna pattern. 
     The printed antenna elements are connected to one end of a coaxial cable, which forms a coaxial transmission line. This coaxial transmission line has an impedance of approximately 75 Ohms and odd multiple electrical quarter wavelengths. One hundred-ohm transmission line combines in parallel to 50 ohms, feeding into a 50-ohm transmission line matching the impedance of the transmitter. This results in the power supplied at the feed point to be split and each antenna receives one-half of the input power. 
     A relatively symmetrical radiation pattern is achieved by placing one of these dipoles on each side window of a vehicle having stationary window glass, resulting in space diversity. Additionally, by splitting the power equally between the antennas, the field strength is also divided, and the amount of RF exposure to the interior of the vehicle is reduced. 
     One advantage of using two dipoles with space diversity is an improved radiation pattern versus a single dipole pattern. 
     Use of window mount dipole antenna of this invention virtually eliminate rain leakage, are less costly that roof installed antennae, improves vehicle appearance, and can be utilized on all vehicles having a stationary or partially stationary window. Vehicle appearance is also improved by concealing the coaxial transmission line going to the transmitter, for example, beneath the roof liner. 
     These and other features and advantages of this invention are described in or are apparent from the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of this invention will be described in detail, with reference to the following figures, wherein: 
     FIG. 1 is a planar view of a single dipole antenna of the prior art; 
     FIG. 2 is a perspective view of a dipole antenna of the invention on a side window of a vehicle; 
     FIG. 3 is a close-up planar view from the outside of the vehicle of the antenna shown in FIG. 2 attached to the window; 
     FIG. 4 is a diagram depicting a passive diversity antenna system with parallel tuned feeders of the invention; 
     FIG. 5 depicts the min-max horizontal directional diagram of a prior art collinear antenna taken at 0 degrees with respect to the bottom of the antenna; 
     FIG. 6 depicts the average gain of a prior art collinear antenna taken at 0 degrees with respect to the bottom of the antenna; 
     FIG. 7 depicts the min-max horizontal directional diagram of a prior art collinear antenna taken at 20 degrees with respect to the bottom of the antenna; 
     FIG. 8 depicts the average gain of a prior art collinear antenna taken at 20 degrees with respect to the bottom of the antenna; 
     FIG. 9 depicts the min-max horizontal directional diagram of a prior art collinear antenna taken at 30 degrees with respect to the bottom of the antenna; 
     FIG. 10 depicts the average gain of a prior art collinear antenna taken at 30 degrees with respect to the bottom of the antenna; 
     FIG. 11 depicts the min-max horizontal directional diagram of a prior art collinear antenna taken at 40 degrees with respect to the bottom of the antenna; 
     FIG. 12 depicts the average gain of a prior art collinear antenna taken at 40 degrees with respect to the bottom of the antenna; 
     FIG. 13 depicts the min-max horizontal directional diagram of a passive diversity antenna system with parallel tuned feeders of the invention taken at 0 degrees with respect to the bottom of the antenna; 
     FIG. 14 depicts the average gain of a passive diversity antenna system with parallel tuned feeders of the invention taken at 0 degrees with respect to the bottom of the antenna; 
     FIG. 15 depicts the min-max horizontal directional diagram of a passive diversity antenna system with parallel tuned feeders of the invention taken at 20 degrees with respect to the bottom of the antenna; 
     FIG. 16 depicts the average gain of a passive diversity antenna system with parallel tuned feeders of the invention taken at 20 degrees with respect to the bottom of the antenna; 
     FIG. 17 depicts the min-max horizontal directional diagram of a passive diversity antenna system with parallel tuned feeders of the invention taken at 30 degrees with respect to the bottom of the antenna; 
     FIG. 18 depicts the average gain of a passive diversity antenna system with parallel tuned feeders of the invention taken at 30 degrees with respect to the bottom of the antenna; 
     FIG. 19 depicts the min-max horizontal directional diagram of a passive diversity antenna system with parallel tuned feeders of the invention taken at 40 degrees with respect to the bottom of the antenna; and 
     FIG. 20 depicts the average gain of a passive diversity antenna system with parallel tuned feeders of the invention taken at 40 degrees with respect to the bottom of the antenna. 
    
    
     Throughout the drawing figures, like reference numerals will be understood to refer to like parts and components. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As seen if FIGS. 2-4, which depict various details of the antenna assembly  20  of this invention, the antenna assembly  20  has two tuned dipole antenna  22 ,  24 , each antenna  22 ,  24  having at least two wires  26  mounted on a vehicle window  28 , and a parallel tuned feeder  30 ,  32  electrically connected at a first end to each of the antenna  22 ,  24 . Each parallel tuned feeder  30 ,  32  is electrically connected at a second end to a coaxial cable  34 ,  36  at combining points  38 ,  40 . Both coaxial cables  34 ,  36  are combined together at a combining point  42 , where another coaxial cable  44  electrically connects the two tuned dipole antenna  22 ,  24  to a transceiver  46 . 
     In the preferred embodiment, the dipole antenna  22 ,  24  utilize three antenna wires, or elements  26  to broad band the dipole antenna. This method of broad banding is also known in the art as diversity feed, and two or mores wires are typically used to broad band. In FIG. 3, the two sets of antenna elements  26  are tuned for the upper and lower part of the desired frequency band. For a cellular telephone, when the bandwidth is 70 MHz, a VSWR of less than 2:1 can be maintained. 
     The antenna  22 ,  24  are preferably omni-directional in an elevation plane between 0 degrees and 60 degrees from the horizontal. The voltage standing wave ratio, VSWR, preferably has a value of 2 or less, where 1 is a perfect 50 ohm antenna. 
     Parallel tuned feeders  30 ,  32  are multiple electrical half wavelengths used to transfer the 50 Ohm impedance of the dipole at the combining points  38 ,  40 . 
     The dipole antennas  22 ,  24  and the parallel tuned feeders  30 ,  32  are preferably printed on the vehicle window  28  using existing technology, for example, printing automobile rear defogger elements and AM/FM radio antennas on glass. 
     Coaxial transmission lines  34 ,  36  have an impedance of 75 Ohms each and are odd multiple electrical quarter wavelengths. The coaxial transmission lines  34 ,  36  combine at the combining point  42  at 100 ohms each, combining in parallel to 50 ohms. The parallel tuned feeders  30 ,  32  transforms the impedance of the coaxial cables  34 ,  36  to match the impedance of the antenna. 
     The coaxial transmission line  44 , located inside the vehicle  54 , is connected to a transceiver  46 , transferring the RF signals to the transceiver  46  for conversion to audio. The coaxial transmission line  44  is 50 ohms to match the impedance of the transceiver  46 . In this manner, the power supplied at the transceiver feed point  48  is split at the combining point  42  and each dipole antenna  22 ,  24  receives one-half of the power input. 
     The transceiver  46  can be any radio frequency transceiver. In the preferred embodiment, the transceiver  46  is a cellular telephone, either analog, digital, or PCS, using any frequency assigned for the service. In the preferred embodiment, the transceiver  46  is a cellular telephone operating in the frequency range of approximately 820 to 900 MHz. 
     In this manner, a relatively symmetrical radiation pattern is achieved by placing one of the dipole antennas  22 ,  24  on each side window  50 ,  52  of a vehicle  54  where the glass on the side windows  50 ,  52  is stationary. Additionally, since the power is split equally, the field strength at each antenna  22 ,  24  is also divided. 
     On the reciprocal, the received signal can be added or subtracted at the combining point  42 . A total received signal of plus or minus 3 dB over a single dipole antenna  22 ,  24  is possible, due to the combinations of instantaneous phase relationship at the antennas  22 ,  24 . This equates to an amount equal to or slightly less than the received signal at the transceiver  46  when compared to a traditional roof mount antenna. 
     The use of two dipole antennas  22 ,  24  have the advantage of seeing both sides of the vehicle without obstruction versus a single dipole antenna on one side window. This is also known as space diversity. 
     As depicted in FIGS. 2 and 3, the antennas  22 ,  24  are attached to the vehicle side windows  50 ,  52  near the center of the viewing area  56 . This effectively places the antennas  22 ,  24  farthest away from any metal that can interfere with the operation of the antennas  22 ,  24 , such as door trim  56 . The coaxial cables  34 ,  36 ,  44  are located beneath the headliner, not shown, for improved vehicle aesthetics. Alternatively, the coaxial cables  34 ,  36 ,  44  can be concealed beneath any interior panel, carpet, trim, and the like to effectively conceal and route the cables to the transmitter. 
     FIGS. 5 to  12  show antenna patterns and average gain plots for a collinear antenna mounted on a vehicle known in the art. FIGS. 13-20 show antenna patterns and average gain plots for a dipole antenna of the present invention. The reported angle is with respect to the horizon, but referenced to the bottom of each antenna. Measurements were taken at 0 degrees, 20 degrees, 30 degrees and 40 degrees. All measurements were taken with vertical polarization. 
     Antenna gain is a measure of how well the antenna will send or receive an RF signal. Gain is typically measured in decibels-isotropic, dBi, or in decibels-dipole, dBd. When using dBi, performance is a determination of how much better the antenna is compared to an isotropic radiator. An isotropic radiator is an antenna that sends signals equally in all directions. A true isotropic antenna has a 0 dBi gain. The higher the decibel figure, the higher the gain. For example, an antenna having a 6 dBi gain will receive a signal better than a 3 dBi antenna. Dipole antennas typically have a 2.4 dBi gain as dipole antennas are better than isotropic radiators. Additionally, dipole antennas are omni-directional when vertically polarized. 
     The average gain for each antenna at each elevation angle is given as average gain and linear average gain. The average gain is determined as the average measured gain. The linear average gain is determined by taking the average gain values in dBi, converting those values to linear equivalent, averaging the linear values, and converting back to dBi. When the antenna pattern is perfectly symmetrical, the average gain and the linear average gain will be identical. When the antenna pattern is not symmetrical, the linear average gain will always be higher than the average gain. This in a result of the average gain not being indicative of the actual power under the curve. 
     As seen in FIGS. 5 and 13, the prior art collinear antenna performed better than the dipole antenna of this invention at 0 degrees. In contrast, as seen in FIGS. 11 and 19, the dipole antenna of this invention performed better than the collinear antenna of the prior art as the angle increased. It will be readily understood, for example, that the dipole antenna of this invention performs better than the collinear antenna in hilly areas because the radiated energy approaches the antenna from elevated transmitters, resulting in an increased elevation angle. 
     While advantageous embodiments have been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention, as defined in the appended claims. For example, the parallel tuned feeder is not limited to the broadband dipole antenna, as many different types of antennas could be placed in the center area of a window while concealing the coaxial cable. Other antenna designs also using a tuned feeder could be used to steer the radiation pattern is desired. The transceiver can be any two-way communications device, including a wireless modem.