Abstract:
An antenna system for an automobile uses an external antenna that is either connected to the cell phone directly, or connected to a passive antenna that communicates with the cell phone.

Description:
BACKGROUND 
     Cellular telephones are often used in automobiles. 
     A cellular telephone can be used in a car kit. The car kit holds the telephone and has connections to provide power to the cellular telephone. The car kit often operates in hands-free mode, so that the call is carried out in speakerphone mode. 
     The car kit can be placed up high, above the chassis of the automobile. This puts the cell phone closer to the glass and allows better reception. However, this location can be less aesthetically pleasing. 
     A less-intrusive mounting places the cell phone down lower—below the auto chassis, e.g. in an arm rest. Reception would often be impaired at this location because of interference from the chassis of the automobile. Therefore, this type of mounting can include a connection to an external antenna. 
     This external antenna can provide additional gain for the cellular transmission and reception, e.g. 6 dbi of gain. 
     The user often wants to conduct a private conversation. Therefore, the user may remove the handset from the car kit for a private conversation in the vehicle. The handset&#39;s connection to the external antenna is lost when the phone is removed from its cradle. For a short time, until the phone is brought up to the user&#39;s ear, the shielding of the vehicle cab could shield the signal and reduce the signal strength. This could cause the call to be dropped. 
     SUMMARY 
     The present system uses an in-vehicle passive antenna, within the car kit, to connect signal to the external antenna when the portable phone is not resting within the kit. 
     The in-vehicle antenna is passive in the sense that it couples RF transmissions and receptions to and from the cell phone when the handset is not docked. Therefore, when the cell phone is removed from the car kit, the connection to the external antenna can still assist handset transmission and reception. The passive connection is the effective when the cell phone antenna is closest to the car kit, e.g. when the cell phone is being docked and undocked. 
     According to a disclosed mode, an electronic or mechanical switch switches the connection between electrical connection to the external antenna and passive connection to the external antenna, depending on whether the cell phone is connected or removed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects will now be described with reference to accompanying drawings, wherein: 
     FIG. 1 shows an overall diagram of the car showing the shaded reception area; 
     FIG. 2 shows a first switched connection; and 
     FIG. 3 shows a switched connection with a purely mechanical switch. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment is shown in FIG. 1, which shows an automobile including a glass area  100  e.g. a windshield, a metal chassis area  102  defining an RF-shaded area  101  therein, and cell phone car kit  110 . The car kit  110  can be for any type of portable telephone including but not limited to cellular, FM, PCS or any type of digital telephone. The car kit  110  has a housing  109  with a cell phone cradle which is preferably located within the RF-shaded area  101  of the automobile. This portion  101  is shaded by the metal chassis  102  of the car. Therefore, the reception in this area  101  can be relatively attenuated. 
     Car kit  110  is connected via a transmission line  114  to an external antenna  116 . The external antenna  116  may provide a typical 6 dbi of gain, while the transmission line  114  may provide 1 to 1½ dbi of loss. 
     The car kit  110  also includes a passive antenna  120  connected as shown in FIG.  2 . 
     FIG. 2 shows a more detailed diagram of the switching system used in the car kit  110  of FIG.  1 . Antenna  120  is here shown as a tuned passive dipole. A switch  200  is connected at one end  210  to external antenna  116  via transmission line  114 . The other end of the switch  200  has two connections  202  and  204 . Connection  202  is connected to a contact  220  of the cellular telephone  222 . 
     The cellular telephone provides its RF transmission and reception over the contact  220 . When the cell phone is in place on its cradle, therefore, transmission and reception occur via contact  220  through switch contact  202  to switch contact  210 , to transmission line  114  to external antenna  116 . 
     Other contacts to the cell phone are also made shown generically as contact  224 . In this embodiment, contact  224  can include a source of DC power. 
     A processor  230  within the car kit detects power consumption of the cell phone  222 , and controls the position of the switch accordingly. When the cell phone is consuming power, the switch is in the position shown in FIG.  2 . When no power is consumed, the switch is quickly switched to its other position. 
     As shown, the other position connects contact  210  to contact  204  and thereby connects the internal dipole  120  to the external antenna  116 . 
     An alternative embodiment is shown in FIG.  3 . In this alternative embodiment, the switch  299  is a purely mechanical switch. The switch is actuated by the act of physically inserting the cell phone  222  into the car kit. Contact  300  is mounted on a leaf spring  310 . When the cell phone is inserted, the act of insertion presses the contact  300  against the contact  302 . When the cell phone is removed, contact  300  is pressed against contact  304  which is connected to the internal dipole  120 . 
     The dipole antenna  120  can simply be a line of metalized foil, set into a desired shape. As shown in FIG. 2, the antenna  120  is preferably etched into a metal foil on either the outside, inside or mid portion of the physical car kit. Therefore, the cost of this antenna is relatively minimal. 
     While this embodiment is described using a passive dipole, the internal antenna could alternatively be a helix type antenna, a patch type antenna, an inverted F type antenna or any other type of antenna known in the art.