Abstract:
Uniform and concentrated radio broadcast signals are obtained employing an antenna structure which uses separate antenna conductors on opposite sides of a radio reception area. The antenna conductors perform similarly to the plates of a capacitor. Highly efficient antenna performance results. In a preferred embodiment wherein the antennas are used for radio testing inside a vehicle assembly plant, the track or carrier structure of the vehicle assembly line provides one of the antenna conductors.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates in general to broadcasting radio frequency signals to an enclosed area, and more specifically, to providing radio broadcast signals having a concentrated intensity which are particularly useful in testing radio systems during their installation into vehicles on an assembly line. 
     Wireless broadcasting from radio towers, such as in standard AM and FM broadcasting, transmits radio frequency (RF) signals through the air to individual receivers. The RF signals have a limited ability to penetrate into tunnels, buildings, and other structures. Receivers in these locations may be unable to receive a usable signal. Therefore, rebroadcast systems are used which employ an external antenna on the outside of the building and transmission-line wiring (possibly including an amplifier) for bringing RF signals inside the building without attenuation and then rebroadcasting with an internal antenna to receivers located in the structure. 
     It may also be desirable to provide only an internal antenna for a system broadcasting dedicated signals within a structure. In other words, the source signal for such a broadcasting system need not be externally derived radio broadcast signals. Nevertheless, in any such a system, it is important to restrict broadcast of signals to be within the structure and minimize external radiation which could interfere with other broadcasts outside the structure. 
     One application of rebroadcast type systems is in the testing of radio receivers and audio systems in automobile manufacturing plants. During manufacture of an automobile, antenna connections and speaker connections to the audio system must be checked. In a typical process, after installation of the radio and all of its interconnections, the radio is powered up and an operator presses the seek button to perform a seek tuning operation which stops at a received broadcast station of sufficient strength. If the radio fails to stop at any frequency (even though a sufficiently strong broadcast signal is present), then the antenna connection needs to be checked. Once a station is received, the audio is played through the speakers so that each speaker may be listened to, thereby permitting its speaker connections and proper operation to be verified (this process is often referred to as a speaker “walkaround” test). 
     A test area for performing these checks is typically inside a large building having a large amount of metal structure which results in highly attenuated RF signals penetrating the building. Furthermore, during the vehicle manufacturing process, a full radio antenna is typically not installed. In order to avoid antenna breakage during shipping of vehicles to their point of sale, only the antenna stub or base is present during manufacture. The full whip antenna is installed after shipping of the vehicle (e.g., at the dealer). Since only a partial antenna is present, the radio is even less sensitive to RF signals. 
     Typical rebroadcast systems in a building use a long-wire antenna inside the building which spreads the broadcast RF signals over a large area and fails to provide uniform transmission fields. Therefore, typical broadcast systems have had difficulty providing sufficient field strength for testing of radio systems in vehicle assembly plants. Furthermore, the exact location of radio testing on a vehicle assembly line may change from time to time, which may lead to problems due to the lack of uniformity in the broadcast field. 
     SUMMARY OF THE INVENTION 
     The present invention has the advantage of providing a concentrated RF radio broadcast signal in an enclosed space, such as a test area inside a vehicle assembly plant, with a simple inexpensive antenna structure. 
     More specifically, the invention provides a broadcast system for broadcasting signals in an enclosed space to a radio reception area. A first antenna conductor forms a first emission surface along a side of the radio reception area. A second antenna conductor forms a second emission surface along an opposite side of the radio reception area. An RF amplifier is coupled across the first and second antenna conductors to produce concentrated RF radio broadcast signal between the first and second emission surfaces and in the radio reception area. The first and second antenna conductors act like two plates of a capacitor which concentrates the electric field in the area between the plates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing a rebroadcast system in a vehicle assembly plant. 
     FIG. 2 is a schematic diagram showing the manner of concentrating RF signals employed in the present invention. 
     FIG. 3 is a schematic diagram showing a vehicle manufacturing test system in greater detail. 
     FIG. 4 is a plot showing an example of receivable broadcast signals in a particular region. 
     FIG. 5 is a plot showing the frequency response of a notch filter as used in FIG.  3 . 
     FIG. 6 is a perspective view showing an antenna connection of the present invention to the rail structure in a vehicle assembly plant. 
     FIG. 7 shows an alternative embodiment for creating a larger test area. 
     FIG. 8 is a flow chart showing a radio test process using the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a vehicle manufacturing building  10  including a vehicle assembly line  11 . A rail or track  12  moves vehicles along at least a portion of the assembly line. In particular, a vehicle  13  supported by a fixture or sled  14  is pulled along track  12 . At the point shown in FIG. 1, vehicle  13  already has an audio system and electrical system installed, including a radio  15 , an antenna stub  16 , speakers  17 , and wiring to connect these three components. In some assembly plants, radio testing might not be performed until a vehicle actually has wheels and is rolling. 
     An external antenna  20  mounted outside vehicle manufacturing building  10  receives RF broadcast signals  21  from transmitting towers over the local geographic region. RF signals  21  picked up by antenna  20  are amplified by an amplifier  22  and are rebroadcast inside vehicle manufacturing building  10  by an internal antenna  23 . The resulting RF broadcast signals  24  are transmitted in the vicinity of vehicle  13  during radio testing. However, since broadcast signals  24  are not well controlled, a relatively large amount of power may be necessary in order to provide sufficient field strength for reliable testing of radio  15  and its interconnections. 
     The present invention achieves a concentrated and localized electric field to perform radio testing as shown in FIG.  2 . An RF signal (e.g., from an external antenna) is provided to an amplifier  25 . The amplified RF signals are transmitted through a transmission line  26  to a pair of antenna conductors  27  and  28 . Antenna conductors  27  and  28  follow elongated paths which are disposed on opposite sides of a test area  30 . The antenna conductors create field emission surfaces similar to plates of a capacitor so that a concentrated RF radio broadcast signal is produced between the emission surfaces. The surfaces may be lines (i.e., the conductors are formed by straight wires) or can be planes if planar conducting surfaces (i.e., flat plates) or grids are employed. Antenna conductors  27  and  28  may preferably be provided above and below the radio test area, although locations on opposite lateral sides of the test area are also acceptable. 
     A test system for rebroadcasting AM and FM signals and for broadcasting AM and FM specialized (i.e., dedicated) test signals is shown in FIG.  3 . An AM antenna  31  and an FM antenna  32  are located on the exterior of building  10 . AM antenna  31  is connected to a notch filter  33  and the filtered AM broadcast signals are amplified in an amplifier  34 . The amplified AM signals are coupled to one input of a summing amplifier  35 . An audio source  36  such as a compact disc player or a waveform generator generates other test signals which may be used in radio testing and are coupled to an AM transmitter  37  which provides AM broadcast signals to a second input of summing amplifier  35 . Audio source  36  may provide special tones, for example, for specialized vehicle testing. 
     The output of summing amplifier  35  is connected to a coaxial transmission line  38  which transmits the summed AM broadcast signals to the antenna via a matching network  40 . Matching network  40  is optional and would be used only if needed to provide sufficient energy coupling to the antenna. For the AM antenna, a first conductor is provided by the shield conductor of a coaxial cable  41 . The second antenna conductor is provided by connection to a metallic rail or track structure  42  which is associated with the assembly line along which a vehicle  43  is moving. If necessary, a dummy load  44  may be connected between first and second antenna conductors  41  and  42 . Coaxial cable  41  is installed in the ceiling of building  10  directly over rail  42  to create the concentrated AM broadcast signal of the present invention. If a metal rail or track  42  is not available in the radio test area, then a conductor can be laid on or within the floor along the assembly line in the radio test area. 
     FM broadcast signals picked up by FM antenna  32  are coupled through a notch filter  45  and an amplifier  46  to one input of a summing amplifier  47 . In a manner similar to the AM signals, an audio source  48  provides test signals through an FM transmitter  49  to a second input of summing amplifier  47 . FM broadcast signals are coupled through a transmission line  50  (typically a coaxial cable) to coaxial cable  41  which acts by itself as a long wire FM broadcast antenna. As is known in the art, a leaky coaxial cable can be employed as the FM antenna  41  (such as Radiax® cable available from Andrew Corporation). If necessary, a dummy load  51  may be connected between the end of coaxial cable  41  and ground. 
     By employing the shield conductor of coaxial cable  41  as the first antenna conductor for the AM broadcast antenna, the antenna hardware is reduced and installation is made easier. However, to avoid shorting AM signals supplied to the shield conductor to ground through the FM summing amplifier  47 , a DC blocking circuit  52  is connected between the shield conductor of coaxial cable  41  and the shield conductor of transmission line  50 . DC blocking circuit  52  can simply be comprised of a blocking capacitor. 
     Coaxial cable  41  is preferably coextensive with the section of rail  42  which is connected as an antenna conductor. Rail  42  also has a ground connection corresponding with the termination of cable  41 . Preferably, the length of the two antenna conductors are less than or equal to about ¼ of a wavelength of an AM signal. Restricting the length of the antenna helps maintain field uniformity throughout the radio test area. 
     The purpose of notch filters  33  and  45  will be described with reference to FIGS. 4 and 5. FIG. 4 shows a typical spectrum within the AM broadcast band wherein the channels of the AM broadcast band are not all used in a particular geographic area. At beginning  50  of the AM broadcast band the first available channel is shown as being unoccupied. The second and forth channels respectively contain AM broadcast signals  51  and  52 . A relatively stronger AM broadcast signal  53  is shown occupying a higher frequency channel. If amplifiers  34  and  46  of FIG. 3 were to amplify the entire broadcast bands including all transmissions present, there is a danger that the strongest broadcast signals may overload the amplifiers. Since the antenna connection test is comprised of a scan tune which begins at the beginning  50  of the broadcast band, and since it is desirable to complete the test in the shortest amount of time possible, it is desirable to have sufficient field strength from AM broadcast signal  51  to activate the stop sequence of the scan tune operation. However, if equal amplification of the entire broadcast band is performed, then the amount of amplification of AM signal  51  may be limited because of the presence of AM signal  53  and the rest of the signal in the AM band. To compensate for this, notch filters  33  and  45  are inserted having a characteristic as shown in FIG.  5 . In particular, the filter provides a relatively great amount of attenuation except at the frequencies of an acceptable broadcast signal at the low end of the band, such as the second channel in FIG.  4 . It may also be desirable to have two notches or a single notch wide enough to pass signals  51  and  52  without attenuation so that the radio will stop at signal  52  in the event that signal  51  were to inadvertently go off the air. 
     FIG. 6 shows a further embodiment which integrates an antenna conductor with the track system of the assembly line. An elongated rail system  55  supports and pulls a fixture or sled  56  which is attached to a vehicle  57  being assembled. Track  55  may be in contact with a continuous series of metal plates  58  which cover some of the track mechanisms. Track  55  and plates  58  form one continuous electrically conductive structure whereby the emission surface or radiation surface of the antenna conductor is in the form of the plane rather than just a straight line. The antenna connection can be made as shown at point  60  by clipping, screwing, soldering or other electrically conductive means. Two connections are made to track  55  in this same manner to provide both the transmission line and the ground connections. 
     In a further embodiment of the invention, the radio test area may be made larger (e.g. not restricted to a single-file line) by employing conductive grid structures  61  and  62  for the first and second antenna conductors. Grids  61  and  62  may be constructed of wires or metal pipes and concealed in the ceiling and floor respectively. 
     FIG. 8 shows a preferred method for utilizing the antenna of the present invention in vehicle manufacturing. In step  65 , radio broadcast signals are generated from prerecorded test signals or as a rebroadcast of commercial services. In step  66 , the radio broadcast signals are coupled to the antenna conductors to produce a concentrated broadcast signal in the test area while providing a uniform field intensity along the full length of the antenna. A vehicle having an audio system to be tested is moved into the test area in step  67 . In step  68 , the audio system is activated either manually or under automatic control. In step  69 , the response of the audio system is monitored to various test actions such as a scan tune or a speaker walkaround test.