Radio frequency broadcast system for enclosed spaces

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.

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.

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.RTM. 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 1/4
 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.