Abstract:
A multiplexer circuit for CB, AM/FM, and television splits the radio and television signals, adding impedance to the split lines, and providing an amplifier upstream of the split.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention pertains to antennas and multiplexers more particularly to multiplexers for use with antennas and receiving apparatus operating in the AM/FM, CB and television frequency ranges. 
     2. Prior Art 
     It is known to use multiband antennas coupled to multiplexers in systems capable of receiving AM/FM and weather band signals and transmitting and receiving CB and cellular phone signals. See, for example, commonly assigned U.S. Pat. No. 6,297,711 to Seward, et al. In recent years, the technology of cellular telephones has progressed to the point where coupling a cellular telephone to a vehicular antenna is unnecessary. 
     At the same time, there is increased demand to provide television reception in vehicles, particularly in commercial trucking and in marine environments. In the trucking industry, separate “Vee” antennas are typically installed in the headliner of composite bodied trucks, and in the marine industry, similar separate television antennas are used for television reception, apart from the other communication antennas and antennas for conventional AM/FM reception. 
     There remains a need to minimize the number of antennas in use, and the now unused cellular port in known multiplexers provides a convenient port for a television signal from a multiband antenna. But the solution is not as simple as replacing the cellular port on the multiplexer with a television port. One problem with incorporating television reception on existing multiband antenna systems is that FM frequencies are between television channels 6 and 7, so that splitting like frequencies over two receivers (one FM and one television) reduces the signal to each. In addition, the receivers tend to load the system which results in greater losses and degradation of the signals. The problem of incorporating television reception into existing multiband antenna systems remains. 
     SUMMARY OF THE INVENTION 
     These and other problems of the prior art are overcome in accordance with this invention by means of a multiplexer circuit for coupling at least one antenna to a CB receiving apparatus, to an FM receiving apparatus, and to a television receiving apparatus. The multiplexer circuit includes an input conductor adapted to connect to at least one antenna, a first output conductor adapted to connect to a CB radio apparatus, a second output conductor adapted to connect to a television receiving apparatus, and a third output conductor adapted to connect to an FM receiving apparatus. A first inductor and a first capacitor are connected in series to form a series LC circuit between the input conductor and the first output conductor. The series LC circuit is configured to provide low impedance to signals inside the CB frequency range and high impedance to signals outside the CB frequency range. A second inductor and a second capacitor are connected in parallel to form a parallel LC circuit between the input conductor and a line extending to the second and third output conductors. The parallel LC circuit is configured to provide high impedance to signals inside the CB frequency range. The multiplexer also has a node in the line where the line is split into the second and third conductors. 
     Preferably, the third conductor is adapted to connect to an FM/AM receiving apparatus connected to the third output conductor. The multiplexer can further have a resistor in each of the second and third conductors. As well, the multiplexer can further have an amplifier between the parallel L-C circuit and the node. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An illustrative embodiment of the invention is described below with reference to the drawing in which: 
         FIG. 1  is a diagrammatic representation of a dual CB/AM/FM/television antenna system incorporating the principles of the invention; and 
         FIG. 2  is a circuit diagram of the multiplexer of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an antenna system  100  comprising a pair of identical antennas  101 ,  102 . The antennas  101 ,  102  are connected to a multiplexer  103 , according to the invention, via input conductors  104 ,  105 , respectively. The multiplexer  103  serves to connect the antennas to an AM/FM receiver  107  via conductor  106 , to a television receiver  109  via conductor  108  and to a CB transceiver  111  via conductor  110 . Each of the antennas is mounted by means of a mounting nut  126  on a bracket  127  which may, for example, be a side mirror mounting bracket of a truck. The overall antenna is preferably on the order of 54 inches in length. The antennas each may comprise an enamel coated conductive antenna wire  130  wound around an essentially cylindrically shaped core  131 . The core  131  may be a solid core of fiberglass or the like material having a diameter of ¼ inch. The wire of each antenna preferably extends continually from the top of the core  131  to the mounting nut  126  where each antenna is connected to the multiplexer  103  via one of the conductors  104 ,  105 . The wire section from the mounting nut  126  to the upper end of the rod  131  has an electrical length of one-quarter wavelength in the CB frequency range. 
     The overall length of the wire  130  includes a tightly wound loading coil  120  at the top of each antenna as well as the wire section  121  extending between the loading coil  120  and an FM self-resonant circuit  122 . In the FM self-resonant circuit the successive turns of the wire  130  are immediately adjacent each other. The successive turns of the wire  130  are spaced apart in the area  123  between the FM self-resonant circuit  122  and mounting nut  126 . The electrical length of the wire section from the mounting nut  126  to the lower end of the FM self-resonant circuit  122  has an electrical length of preferably one-quarter wavelength in the FM frequency range. This electrical length works also for television because the television band is generally from 54 to about 890 MHz, both above and below the standard FM band of 88-108 MHz. 
     It will be understood that the antenna is exemplary only, for showing the environment in which the invention operates. More or less than two antennas may be used depending on performance requirements or expectations. As well, different types of antennas may be used. For example, a loop or bow tie antenna may be added or equivalently incorporated into an existing antenna to improve TV reception in the UHF range. 
       FIG. 2  schematically shows an exemplary circuit of the multiplexer  103  which provides an interface to the CB transceiver  111  via conductor  110 , to AM/FM receiver  107  via conductor  106 , and to the television  109  via conductor  108 . A series LC circuit  141  is configured to offer low impedance to the CB signal and high impedance to the AM/FM and television signals to avoid loading the AM/FM receiver or the television when the CB transceiver is operating. A parallel LC circuit  144  is configured to provide high impedance at 27 MHZ, thereby isolating the CB transmitter from the AM/FM receiver and the television, and preventing damage to them when transmitting with the CB. Since both the AM/FM receiver  107  and the television are at high impedances when not tuned to the frequency of interest, these units could extend from the same line  150  by way of a splitter at node  152 . However, since both the television  109  and the AM/FM receiver  107  will typically be several feet from the node  150 , several feet of coaxial cable are typically used for each device. If the unused device is an odd multiple of quarter wavelengths away from the node  150 , it would present very low impedance in parallel and therefore greatly degrade the performance of the device in use. In order to minimize this problem, a resistor  154  is placed in series with both the FM/AM receiver  107  and the television  109 , preferably about 75 Ohms. This, however, creates a secondary problem because it reduces the performance of both the AM/FM receiver  107  and the television  109  by about 3 dB when the cable&#39;s length is an even number of quarter wavelengths and by about 8 dB when the cable&#39;s length is a multiple of odd quarter wavelengths. In order to solve this problem, an amplifier  156  is disposed in the cable  150  upstream of the node split  152 . The resistors  154  and the amplifier  156  sufficiently isolate the television  109  and the AM/FM receiver  107  and maintain optimum signal levels.