Patent Publication Number: US-6339403-B1

Title: Vehicle antenna system for multiple vehicle electronic components

Description:
This application claims benefit to U.S. Provisional No. 60/141,790 filed Jun. 30, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to vehicle electronic components which communicate in the electromagnetic spectrum. More particularly, this invention relates to a single vehicle antenna system which receives signals for a plurality of electronic components which operate in different spaced apart frequency bands. 
     Vehicles typically include a multiple of various electronic components. Each of the components typically operates in different spaced apart frequency bands. Examples of such electronic components include an FM radio, which operates at approximately 100 MHz, and remote keyless entry (RKE) systems, which operate at approximately 315 MHz. Each component must therefore include its own frequency specific antenna system. 
     Known antenna systems may include a diplexer which allows a single antenna to receive a different frequency for each associated vehicle electronic component. The diplexer typically has at least two branches. Each branch includes a band pass filter for passing signals in a portion of the frequency spectrum different from the portions passed by the other branches. Each component thereby receives only its particular operating signal. Each diplexer branch band pass filter passes only its particular signal while blocking all other signals. Each band pass filter also tends to attenuate the signals for the other branches. A transmitter of a higher power output or a more sensitive receiver must therefore be used for each vehicle component. The expense involved in increasing vehicle electrical components receiver sensitivity typically outweighs the benefits of providing a single antenna system. 
     Accordingly, it is desirable to combine a single antenna system with a plurality of vehicle components while minimizing the attenuation of the passed signals in one branch by the filter in another branch. 
     SUMMARY OF THE INVENTION 
     In general terms, this invention includes an antenna system connected to at least two vehicle electronic components which communicate through signals in spaced apart frequency bands. In one disclosed embodiment the first component is associated with a resonant frequency of an FM radio, which operates at approximately 100 MHz, and the second component is associated with a resonant frequency of an Remote Keyless Entry system (RKE), which operates at approximately 315 MHz. 
     The antenna is coupled to a diplexer at a common branch. The radio is connected to a first equipment branch of the diplexer by a first transmission line and the RKE system is connected to a second equipment branch of the diplexer unit by a second transmission line. In a preferred embodiment, the first transmission line is of a length equal to one quarter the wavelength of the second signal (λ/4 at 315 MHz), while the second transmission line is of a length equal to one quarter the wavelength of the first signal (λ/4 at 100 MHz). 
     The first transmission line which carries the 100 MHz first signal is matched to be a short circuit at 315 MHz and the second transmission line which carries the 315 MHz second signal is matched to be a short circuit at 100 MHz. At the diplexer, the 100 MHz signal of the first signal, is not affected by the 315 MHz signal of the second signal as the 315 MHz signal appears as an open circuit because of the λ/4 at 315 MHz length of the first transmission line. Conversely, the 315 MHz signal of the second signal, is not affected by the 100 MHz signal of the first signal as the 100 MHz signal appears as an open circuit because of the λ/4 at 100 MHz length of the second transmission line. 
     Accordingly, by relating the length of the first and second transmission lines, each operates as a short in relation to the signal carried by the other which substantially eliminates attenuation. Thus, instead of the passed signals being attenuated, only the passed signal is “seen” by the junction. 
    
    
     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 diagrammatic, perspective illustration of a vehicle including an antenna system designed according to this invention; and 
     FIG. 2 is a block diagram of an antenna system of FIG. 1 designed according to this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 diagrammatically illustrates an antenna system  10  in accordance with the present invention. The arrangement illustrated in FIG. 1 is suitable for a number of applications in which multiple electronic components such as a vehicle radio (shown schematically at  12 ) and a remote keyless entry (RKE) system (shown schematically at  14 ) are required together. 
     The antenna system  10  comprises an antenna  16  in the form of an elongated member typically installed at an external location on a vehicle  18 . The antenna  18  has two modes of resonance in spaced apart frequency bands. In the disclosed embodiment the first mode of resonance is associated with a resonant frequency of an FM radio  12 , which operates at approximately 100 MHz, and a second mode of resonance at approximately 315 MHz typically used by the vehicle RKE system  14 . 
     As is well known an “RF link budget” provides an effective model of how system components which transmit in the electromagnetic spectrum perform. The link budget can be analyzed using the following relationship: 
     
       
           P   R   =P   T   +G   A1   +G   A2   −G   C1   −G   C2 −10 log [(4*π* R )/λ] 2   
       
     
     Wherein: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 P T  = transmitted power in dBW 
                 P R  = received power in dBW 
               
               
                 G A1  = transmitter antenna gain in dB 
                 G A2  = receiver antenna gain in dB 
               
               
                 G C1  = transmitter line loss in dB 
                 G C2 = receiver line loss in dB 
               
               
                   
               
            
           
         
       
     
     R=range between antennae in meters 
     λ=Carrier Frequency wavelength in meters 
     In an RKE system  14  it is desirable to decrease the size of the RKE remote  15 . As such, the RKE remote  15  typically includes a relatively small printed circuit board transmitter antenna. As it is not desirable to increase the RKE remote  15  antenna, and as can seen by the above link budget relationship, it is preferable to make use of the relatively larger vehicle antenna  18  commonly used for the vehicle radio system. 
     Referring to FIG. 2, the antenna system  10  is schematically illustrated. The antenna  18  is mounted to a conductive surface  20  which, in this embodiment, is coupled to a diplexer  22  at a common branch  22 A. The radio  12  is connected to a first equipment branch  22 B of the diplexer unit  22  by a first transmission line  24  and the RKE system  14  is connected to a second equipment branch  22 C of the diplexer unit  22  by a second transmission line  26  to communicate with the antenna  18 . The diplexer unit  22  provides impedance matching of the radio  12  and RKE system  14  to the antenna  18  in its different modes of resonance, and isolates the two units  12  and  14  so that they may be operated substantially independently. In other words, each equipment branch  22 B,  22 C acts as a filter to the other  22 C,  22 B, however, the diplexer also tends to attenuate the passed signals. It should also be realized that when the signals are far apart (approximately F 1 &lt;2*F 2 ) coaxial cable can be used to form the diplexer  22 . 
     A first signal (schematically illustrated at  28 ) is to the radio  12  at 100 MHz and a second signal (schematically illustrated at  30 ) is to the RKE system  14  at 315 MHz. Although particular frequencies are presented in the disclosed embodiment it should be realized that other components operating at other frequencies will benefit from the present invention. The first transmission line  24  is of a length equal to one quarter the wavelength of the second signal  30 , (λ/4 at 315 MHz) while the second transmission line  26  is of a length equal to one quarter the wavelength of the first signal  28 , (λ/4 at 100 MHz). In a preferred embodiment, this results in a first transmission line  24  of approximately 23.8 cm in length and a second transmission line  26  of approximately 45 cm in length. 
     By relating the length of the first and second transmission line  24 ,  26 , each operates as a short in relation to the signal carried by the other to substantially eliminate attenuation. The first transmission line  24  which carries the 100 Mhz first signal  28  is matched to be a short circuit at 315 MHz and the second transmission line  26  which carries the 315 Mhz second signal  30  is matched to be a short circuit at 100 MHz. At the diplexer  22 , the 315 MHz signal of the second signal  30  is not affected by the 100 MHz band pass filter. The first transmission line  24  appears as an open circuit En z because it is λ/4 at 315 MHz. Conversely, the 100 MHz signal of the first signal  28  is not affected by the 315 MHz band pass filter. The second transmission line appears as an open circuit because it is λ/4 at 100 MHz. Thus, instead of the passed signals being attenuated, only the passed signal is “seen” by the junction. 
     The foregoing description is exemplary rather than defined by the limitations within. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.