Abstract:
An antenna system for a radio telephone in a vehicle that has a primary antenna to realize telephone communication with optimal connection quality connected to the radio telephone by an antenna wire, a secondary antenna that at least allows for telephone communication with limited connection quality when the primary antenna fails and an electric switch that connects the secondary antenna to the radio telephone after the primary antenna fails. This is realized in that the primary antenna is mounted on the vehicle with a breakable fastening element and that the switch automatically connects the secondary antenna to the antenna wire by mechanical means in place of the primary antenna when the fastening element is broken off.

Description:
BACKGROUND OF THE INVENTION 
     Antenna system for a radio telephone in a vehicle that has a primary antenna to realize telephone communication with optimal connection quality and a secondary antenna that will at least allow telephone communication with lower connection quality when the primary antenna becomes damaged. The primary antenna is advantageously located on the outside of the vehicle, e.g. on the surface of the body of the vehicle or near the window, which provides a higher level of performance for sending as well as for receiving, but which can lead to complete failure if the vehicle is involved in an accident. The secondary antenna serves as an emergency antenna after the failure of the primary antenna, the main antenna, especially after an accident, to establish a telephone connection in order to call for help. As a radio telephone is only ready for operation when all of its components are functioning properly and an antenna is particularly sensitive mechanically when placed on the exterior of a vehicle, the solution according to the invention significantly increases the reliability of a radio telephone in an emergency. The application of the invention is not limited to conventional radio telephones with one operating frequency band. The invention can also be applied to antennas utilizing various operating frequency bands for mobile communication. 
     Car phones are usually equipped with an external or window antenna. The location of this antenna is primarily determined by the requirements that need to be met to achieve optimal sending and receiving quality. 
     One disadvantage of selecting such a location is that the probability of damaging the antenna to the point of total failure is high when the vehicle is involved in an accident or when other external forces act on the antenna. In particular, these other external forces acting on external antennas include, for example, the intentional destruction of the antenna by a stranger or the breaking off of the antenna while passing under an obstacle with low clearance. It is not possible to establish a radio telephone connection to call for help after a traffic accident or vehicle malfunction, for example, after the total failure of the antenna. 
     To eliminate this shortcoming an emergency or back-up antenna is installed in a protected location such as the passenger compartment of the vehicle as stated in publication EP-A1-0 859 237. This secondary antenna is then used for sending/receiving operations after the external antenna used as the main antenna fails. Each antenna is connected to the radio telephone via a separate coaxial cable. The radio telephone has two separate antenna connections for this purpose. 
     To obtain the maximum transmission quality and to prevent interference during normal send/receive operations, the emergency antenna is not to be used while the main antenna is functioning. This means that the emergency antenna and the corresponding wire are only to be put into operation in an emergency by the manual or automatic initiation of an emergency call. To accomplish this, an emergency call button is activated or the air bag and/or seat belt mechanism controller sends a corresponding control signal to the radio telephone when triggered to switch the radio telephone to the secondary antenna connection. 
     In principle, there are various solutions used to switch the radio telephone to the emergency antenna: 
     In simple solutions, the initiation of an emergency call in the radio telephone will automatically force the radio telephone to switch to the connection for the emergency antenna regardless of whether or not the main antenna is still operational. One requirement for this to occur is that there must be a high probability that the emergency antenna and its separate antenna wire still function due to installation in a protected location. 
     However, malfunctions or damage to the antenna feed cable leading to the emergency antenna can arise when connecting the antenna or operating the vehicle that remain undetected because the emergency antenna is not used during normal operation. Under certain circumstances, this antenna may not function properly in an emergency. Additionally, its efficiency is generally lower that that of the main antenna due to its installation location. This may also lead to the inability to connect to the base station using the less powerful emergency antenna when the vehicle is in an unfavorable position although the connection could be made using an intact main antenna. 
     To avoid this shortcoming, radio telephones with several antenna connections and other accessories periodically perform a test procedure in which the antennas are operated alternately and checked to see if they are functioning properly. This can be done, for example, by comparing the signal strength of the signal received or, in accordance with publication EP 0 859 237 A1, by comparing the signal strengths of the signal supplied and the signal reflected back by the antenna wire. 
     In this manner, malfunctions and damage to the antennas and the wires will be detected and indicated, and the unit can quickly switch to a functioning branch of the antenna. The test procedure is also generally performed when an emergency call is triggered so that the unit only switches to the less powerful emergency antenna when the main antenna has failed due to breakage, for example. 
     However, one disadvantage of these two solutions is that the radio telephone requires several antenna connections for the separate branches of the antennas and that an antenna feed cable must be installed for each antenna. In addition, the radio telephone must have special test circuitry to automatically perform the test procedure in the latter solution. The requirements stated therefore result in additional undesired complexity. 
     An antenna switching circuit to selectively use the internal or external antenna of a radio telephone is known from publication DE 197 19 657 A1. In contrast to the solution according to the invention, the internal antenna is an antenna that is built into the radio telephone and the external antenna is an automobile antenna that is connected manually to the radio telephone via a flexible feed cable and an antenna connector plug. A switching device on the HF module of the radio telephone disconnects the internal antenna from the module and connects the radio telephone to a vehicle antenna as soon as the antenna connector plug is inserted into a radio telephone socket equipped with contact springs. The contact springs of the socket form a switch that supplies the switching unit with a corresponding control voltage. The known solution only allows for the manual changing of antenna connections. There is no automatic switching performed in an emergency. 
     Based on the shortcomings of the known solutions, it is the task of the invention to create an antenna system with several antennas for a radio telephone in a vehicle in which, with little or no effort, the unit only switches to an emergency antenna when the main antenna is not present any more. 
     SUMMARY OF THE INVENTION 
     The task of the invention is accomplished for a radio telephone by alternately operating the primary and secondary antenna over the same antenna wire and by operating a switch near the primary antenna that is activated when the primary antenna is missing in order to connect the secondary antenna to the antenna wire. To do this, the primary antenna is mounted on the vehicle on a holder with a breakable fastening element and is connected mechanically to a contact spring in the switch. This will hold the contact spring in place under tension in a direction opposite the direction of force of the spring. The secondary antenna is connected to a contact element that forms the opposing contact for the tensed contact spring. The primary antenna is advantageously manufactured using an unbreakable material while the fastening element is made of a material with a lower breaking strength. Alternatively, the fastening element could also be designed so that it has a breaking point. This breaking point will cause the antenna to break off near the holder when an unusually high external force acts on the antenna, for example when the vehicle rolls over. The contact spring is mounted in the holder such that when the antenna breaks off or is removed, the contact spring is released and the secondary antenna is connected to the radio telephone instead of the primary antenna. 
    
    
     The invention will be explained in more detail using the following examples and drawings. The corresponding drawings show cross-sectional diagrams of: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 An initial design of an antenna system for a radio telephone in a vehicle according to the invention in its original functional state 
     FIG. 2 The antenna system according to FIG. 1 after the main antenna has broken off due to an accident, for example 
     FIG. 3 Another design for the antenna system according to the invention in its original functional state 
     FIG. 4 The antenna system according to FIG. 3 with the main antenna not mounted 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows the cross-sectional diagram of an antenna system for a radio telephone according to an initial design of the invention in its original functional state, i.e. with no mechanical damage to the system. 
     In the present example a primary antenna  12 , in the form of a conventional rod antenna, is mounted externally on the roof of the vehicle  10  or on another part of the vehicle body, and a secondary antenna  14  is mounted under the roof of the vehicle  10 . Antenna  12  is fastened to the roof of the vehicle by a holder  16 . The holder  16  is, as usual, made of a nonconductive material and contains a breakable fastening element in the form of a hollow cylinder  18  with an externally threaded section  20  onto which the antenna  12  is screwed. The hollow cylinder  18  is either made of metal, at least near the externally threaded section  20 , or it contains a conductor that establishes an electrical connection between the antenna  12  and the antenna wire  22 . Antenna wire  22  connects the antenna system to the radio telephone. 
     In this example a mechanical contact spring  24 , which can be retracted, is mounted directly on the hollow cylinder  18 , which contains a strain relief  26  under tension in the interior of the hollow cylinder  18 . The strain relief  26  is advantageously fastened directly to the externally threaded section  20  of the holder. This has the advantage that, on the one hand, the holder  16  can be completely prefabricated as a module together with the hollow cylinder  18 , the contact spring  24  and the strain relief  26 . On the other hand, the antenna  12  can be temporarily removed without any problems when going through a car wash, for example, without having to release contact spring  24 . 
     The hollow cylinder  18  is designed mechanically so that it breaks instead of the body of the antenna when antenna  12  is subject to extreme mechanical stress such as the stress resulting from the bending moment M shown, for example. To accomplish this, antenna  12  is advantageously made of conductive rubber or elastic metal, and the hollow cylinder  18  is made of rigid metal or rigid plastic. The fastening element can also be a direct part of the holder  16  as shown in FIG.  3  and can have a breaking point in the form of a reduced cross-sectional area of material at that point. 
     The antenna  14 , which is designed as a sheet antenna, for example, and which is mounted directly under antenna  12  in the interior of the vehicle, has a contact element  28 . Together with the contact spring  24 , this forms a switch  30  that is held open in its normal operating state by the strain relief  26 . The entire interior part of the antenna system is embedded together with the end of the antenna wire  22  that is connected to the antenna in an insulated housing  24  in the body panel of the roof of the vehicle  10  so that when the roof of the vehicle  10  has sufficient mechanical stability, the danger of destroying the antenna is very low even if an accident occurs. 
     FIG. 2 shows the system according to the first example in the destroyed state, i.e. after the antenna  12  has broken off near the hollow cylinder  18 . When the hollow cylinder  18  is broken and the antenna  12  correspondingly fails, the externally threaded section  20  of the hollow cylinder  18  remains inside of the antenna  12 . At the same time, the strain relief  26  tears or breaks off and releases the tension on the contact spring  24 . The spring releases and then makes contact with the contact element  28 . This causes switch  30  to establish a connection between antenna wire  22  and antenna  14 , and this antenna is used automatically instead of the failed antenna  12 . 
     In the present design example, antenna  14  is advantageously designed as a printed circuit board. The contact element  28  is an electroplated contact surface on the surface of the circuit board. In another design according to the invention, the antenna  14  can be mounted at a different location in the vehicle and can be connected to the contact element  28  via an additional antenna wire. 
     In another design according to the invention, an antenna used for a different type of signal, for example for receiving AM/FM radio or satellite navigation signals, can be used for antenna  14  instead of an additional antenna for the radio telephone. In this case the selection circuit for separating the frequency bands can be advantageously placed on the surface of the circuit board. 
     FIG.  3  and FIG. 4 show another type of design of the invention. This design has the additional advantage that the antenna system switches to antenna  14  even when antenna  12  has only been unscrewed and removed. This feature allows one to make an emergency call even if antenna  12  was stolen or forgotten. 
     In the solution according to FIGS. 3 and 4, the primary antenna  12  is also mounted as a rod antenna on the roof of the vehicle  10 , and the secondary antenna  14  is mounted under the roof of the vehicle  10 . The antenna  12  is fastened to the roof of the vehicle  10  using the holder  16 . The antenna  12  and the holder  16  are different from those in the first solution in that the antenna  12  has an externally threaded section  34  that screws into the threaded head  36  of the holder  16 . An elongated contact blade spring  38  that reacts to pressure and that is clamped at one end in a holder  40  is used instead of the retractable contact spring  22 . In this case there is a bent contact element  42  connected to the antenna  14 , which is formed as a sheet. The end of this bent contact is placed within the range of motion of the movable end of the contact blade spring  38 . In this design, the contact element  42  and the contact blade spring  38  form switch  30 . 
     There is an electrically conductive, vertically adjustable contact pin  44  placed in the holder  16  instead of the strain relief  26  in the hollow cylinder  18  from FIG.  1 . This pin forms the electric connection between antenna  12  and the antenna wire  22  and in doing so presses, as shown in FIG. 3, the contact blade spring  38  and the contact element  42  apart when the antenna system is in its original functional state. This keeps switch  30  open and antenna  14  disconnected from the antenna wire  22 . 
     FIG. 4 shows the antenna system according to FIG. 3 with antenna  12  removed, for example to drive through a car wash. In this state the contact blade spring  38  is in contact with the contact element  42  because the contact blade spring  38  pushes the contact pin  44  upwards because antenna  12  is missing. Now the antenna  14  is connected to the antenna wire  22  via the contact blade spring  38  and the contact element  42  instead of being connected to the missing antenna  12 . This function is makes sense when someone has forgotten to screw the antenna  12  back on or when the antenna has been lost. 
     In this design the threaded head  36  in conjunction with the breaking point  46 , which is located on the holder  16  underneath the threaded head  36 , serves as the breakable fastening element for the desired emergency function. The breaking point  46  is formed by significantly reducing the cross-sectional diameter of the material at this location. When an extreme mechanical load is placed on the antenna  12 , the holder  16  breaks at the breaking point  46 , whereby the antenna  12  remains in the threaded head  36  that was broken off. In doing so, the contact pin  44  moves away from its original position and releases the pressure contact blade spring  38 . The antenna  14  is connected to antenna wire  22  so that the emergency call function can be performed. 
     It must be expressly pointed out that the solution according to the invention allows for numerous variations of the mechanical elements. For example, the contact spring or the contact element could be formed differently or mounted at different locations. The contact pin  44  can also be fixed on one end. Combinations of individual elements from both designs also result in favorable solutions.