Abstract:
An antenna element is mounted on a glass of a rear quarter window of a vehicle along the edge thereof. A coupling element is provided midway along said antenna element for coupling between two segmented elements with phase adjustment between reception signals thereof in a specific frequency range. The coupling element connects the two segmented elements in lower frequency range, and disconnects the two elements in higher frequency range. The length of the antenna element is automatically corrected in respective low and high frequency ranges so as to cover a wide range.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates o a window antenna for a vehicle and, more particularly, to a window antenna which can reliably receive a TV broadcast wave. 
     2. Description of the Prior Art 
     As is conventionally known, a radio-reception antenna wire can be provided on a surface on the passenger compartment side of the rear windshield of a vehicle, in addition to a plurality of defogging heater wires. Such an antenna wire is normally tuned to sensitively receive a radio broadcast signal in the AM and FM bands, but cannot receive a TV broadcast signal. Therefore, in order to receive both radio and TV broadcast signals in the vehicle compartment, a special-purpose antenna for receiving a TV broadcast signal must be additionally provided. 
     It is generally difficult to mount a broad-band Yagi antenna for receiving a TV broadcast signal on a vehicle. Therefore, a TV antenna wire must be arranged on a surface of the vehicle, as in the case of an AM/FM window antenna. However, since the vehicle body is formed mainly of an iron plate and the rear windshield is occupied by the radio antenna wire, possible mounting areas for the TV antenna are limited. It is undesirable to mount the antenna wire on the front windshield, with regard to the field of view. 
     Even if the TV antenna can be mounted, a TV broadcasting frequency extends over a wide range, for example in Japan, from a low frequency range (90 to 108 MHz of channel Nos. 1 to 3) to a high frequency range (170 to 222 MHz of channel Nos. 4 to 12). Therefore, it is very difficult to cover such a broad bandwidth as well as a UHF band with a single antenna. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the above situation, and has as its object to provide a window antenna which effectively utilizes a limited antenna mounting area, and which can reliably receive a TV broadcast signal. 
     A window antenna for a vehicle according to the present invention is characterized by including an antenna wire, having a coupling wire (folded portion) for coupling antenna elements, provided along edges of a window glass mounted in a rear quarter window frame, so as to adjust phases of the antenna elements in a specific frequency range. 
     In a window antenna for a vehicle according to a preferred aspect of the present invention, an antenna wire having a pair of antenna elements provided along different edges of a window glass, a coupling wire which couples the respective elements so that their phases are adjusted in a specific frequency range, and a feed point provided at an end of one antenna element is provided on the window glass. 
     According to another preferred aspect of the present invention, the window antenna is provided on rear quarter windows on both sides of the vehicle. 
     For a low TV broadcasting band, a sum of the two antenna elements serves as an effective antenna length, and for a high TV broadcasting band, the coupling wire loses its phase adjustment function and serves to isolate the elements, so that the one antenna element connected to the feed point is enabled. 
     These and other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to 5 show patterns of an antenna wire provided on a rear side window of a vehicle according to an embodiment of the present invention; 
     FIGS. 6A to 6I are directivity diagrams of the window antenna according to the embodiment of the present invention; and 
     FIG. 7 shows the details of a diversity reception system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 to 6 show an embodiment of the present invention, in which a TV antenna wire 2 is provided on a window glass (rear quarter window) 1, which is mounted in a rear quarter window frame portion of a vehicle, by printing and baking a conductive paste material thereon. The rear quarter window 1 may be of a fitted type, or a pivoted vent or a slidable vent. The antenna wire 2 is provided on one or both rear side windows 1. When two antenna wires 2 are symmetrically provided on two side window glasses 1, they are combined in a diversity reception system which selects the reception signal having the higher reception level between the signals from the two antenna wires 2. 
     In the operation of the well-known diversity reception system, shown in FIG. 7, the reception signals obtained at feed points 3a, 4a are fed to a changeover switch through preamplifiers (not shown). A selected reception signal, e.g. at 3a, is then supplied ot a tuning circuit 12 which includes a front-end unit, an IF circuit, a detector circuit etc. The tuning circuit 12 outputs a detection signal to a next stage such as a video amplifier and a color decoder. 
     The tuning circuit 12 includes a level detector for detecting a level or an amplitude of the reception signal and outputs a level detection signal a to a threshold level controller 14. The threshold level control sets and holds a threshold level b for discriminating a decrease or increase of the level detection signal a. The level detection signal a and threshold level b are fed to a comparator 16 to be compared therein. 
     When the level detection signal a decreases below the threshold level b, a change-over signal c is generated in synchronism with a clock pulse e.g. 100 KHz supplied by a clock pulse circuit 18. The change-over signal c is fed to a trigger terminal of a steering flip-flop 20 to switch over. The switched Q and Q outputs of the flip-flop 20 are supplied to change-over switch 10 to change over the reception signal at the feed point 3a to a signal at the feed point 4a. Then, the reception signal from the antenna 4 having a higher level than the former is processed by the tuning circuit 12 and the signal level thereof is now discriminated at the comparator 16. 
     The Q output of the flip-flop 20 is supplied to a threshold level controller 14 to modify automatically the threshold level b in accordance with a change-over frequency. The threshold b is finely modified either downwardly or upwardly so as to increase the change-over frequency. Thus, the threshold level is finely adjusted just on the generally crossing level of two reception signal form the feed points 3a and 4a. At the crossing level of two reception signals, the change-over occurs most frequently. 
     The antenna wire 2 consists of a pair of antenna elements 2a and 2b, which are provided along an edge 1a of the window glass 1 and are connected through a U-shaped coupling wire (folded wire) 2c. The elements 2a and 2b can be constituted by a combination of two L-shaped (vertical and horizontal) segments, as shown in FIG. 1; alternatively by a combination of only vertical segments, as shown in FIG. 2; by a combination of an L-shaped and a vertical segment, as shown in FIG. 3; or by a combination of L-shaped segments and another auxiliary antenna element 2d, as shown in FIG. 4. The antenna elements 2a and 2b can be either symmetrical or asymmetrical. A feed point 3 is provided for one antenna element, e.g., at one end of one of the antenna elements 2a and 2b. 
     The coupling wire 2c is tuned so that of reception signals of the antenna elements 2a and 2b are matched and the signals are added. In a low frequency range (e.g., TV channel Nos. 1 to 3), the elements 2a and 2b serve as a single antenna, and a good reception output can be obtained. An effective antenna length at this time corresponds to a sum of the lengths of the elements 2a and 2b, i.e., 500-600 mm, excluding that of the coupling wire 2c. In other words, terminals A and B of the elements 2a and 2b, respectively, are substantially short-circuited when tuned. 
     In a high frequency range (i.e., TV channel Nos. 4 to 12), the terminal B of the element 2b connected to the feed point 3 serves as a reflection point of a standing wave, and an impedance after the terminal B (on the element 2a side) is much increased, thus providing reception characteristics which are shifted to a short wavelength, as if the element 2a was disconnected. The effective antenna length at that time corresponds to the length of the element 2b (i.e., 250 to 300 mm), and a high gain can be obtained in the high frequency range. 
     In this way, good reception characteristics can be obtained over all the VHF TV broadcasting bands. When the window antenna of the present invention is provided on both rear side windows of a vehicle, a variation in reception level corresponding to a change in vehicle running direction can be stabilized by the diversity reception system using the two antennas. 
     Alternatively, the length adjustment of the elements 2a and 2b, and tuning of the coupling wire 2c can be performed so that the VHF band is received by the sum of the elements 2a and 2b, and the UHF band is received only by the element 2b. 
     The coupling wire 2c can be folded several times in order to obtain a predetermined length within a limited area, and can be an arc (ellipse) or a part of a secondary curve connecting the terminals A and B. Alternatively, the wire 2c can be replaced with a choke coil exhibiting a high impedance in the high frequency range. 
     A window antenna of the pattern shown in FIG. 5 was arranged on a fixed rear quarter window on the driver&#39;s side of a vehicle, and directivities of this antenna in various frequency ranges were examined, obtaining the results shown in FIGS. 6A to 6I. 
     In each directivity diagram, a vehicle C, on which the window antenna was mounted, was rotated through 360° with respect to a reference position S0, and a TV broadcast signal (a horizontally polarized wave) was transmitted thereto from the direction of the reference position S0 in a uniform electric field of 60 dBμ, so as to plot reception sensitivity levels (in units of: dBμ) corresponding to respective positions of the vehicle C. Each directivity diagram represents a mean reception sensitivity level (MEAN), a minimum reception sensitivity level (MIN), a maximum reception sensitivity level (MAX), and a directivity (DIR.=(MEAN)-(MIN)). 
     As can be easily understood from the directivity diagrams, in the TV broadcast signal range of 90 to 755 MHz, a virtually satisfactory reception sensitivity can be obtained, and the directivities are relatively small (5.0 dBμ to 12.9 dBμ). Therefore, the window antenna of this embodiment serves as a non-directivity antenna having a relatively stable reception sensitivity. Note that if an amplifier is added, the reception sensitivity can be increased to a given level. 
     Since the window antenna of the present invention has the characteristics described above, it can be provided in one or both rear quarter windows of a vehicle. When the window antenna of the present invention is provided in both the rear quarter windows, they can be individually used as antennas for the diversity reception system. Alternatively, when the window antenna of the present invention is provided in one rear quarter window and an antenna having another pattern is provided in the other rear quarter window, they can also constitute the diversity reception system. Moreover, when the window antenna of the present invention is provided in one rear quarter window, it can be combined with at least one other antenna provided on another portion of the vehicle, such as a rear-rod antenna, a front-rod antenna, a rear-whip antenna, and a front-whip antenna, so as to constitute the diversity reception system. 
     A window glass with an antenna of the present invention can be fitted in a rear quarter window frame, or can be mounted thereon to be open or closed horizontally or vertically by means of a hinge, or can be slidably mounted thereon. 
     According to the present invention as described above, in the low frequency range, a sum of the lengths of a pair of antenna elements 2a and 2b can be an effective antenna length through a phase adjustment function of the coupling wire 2c, and in the high frequency range, the coupling wire 2c loses its phase adjustment function and only the element 2b connected to the feed point 3 is enabled. Therefore, good reception characteristics over a wide range, from low to high frequencies, can be obtained and, more specifically, a TV broadcast signal can be received in the compartment of a vehicle.