Patent Publication Number: US-6211832-B1

Title: Windowpane antenna apparatus for vehicles

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a windowpane antenna apparatus for vehicles which is mounted on a windowpane of a vehicle such as an automobile. 
     There is a windowpane antenna apparatus for automobiles as the most typical one of conventional windowpane antenna apparatuses for vehicles. The typical antenna apparatus includes a thin, narrow, strip conductor provided on a window (usually a rear window) of an automobile, and the strip conductor is employed as an antenna. 
     In recent automobiles, a defogger is provided almost all over the rear window to serve as a heater for defogging the window. The antenna therefore has to be mounted in a limited space between the defogger and the window frame. 
     FIG. 8 shows an example of a prior art automobile windowpane antenna apparatus. As shown, a defogger  110  is mounted on a rear window  100 , and a loop-shaped antenna  120  constituted of a strip conductor is formed in a region above the defogger  110 . 
     A DC power supply voltage is applied to the defogger  110  from a car-mounted battery  111  through a noise filter  112  (which is constituted of, e.g., a choke coil and a capacitor) for eliminating high-frequency noise (in the AM band) and a power supply voltage application line  113 . 
     A reception signal of the antenna  120  is transmitted to a receiver set such as a radio from a feeding point  121  through a feeding cable (not shown). 
     FIGS. 9 to  11  are illustrations for explaining the performance of the prior art automobile windowpane antenna apparatus described above. These illustrations are used to describe an automobile windowpane antenna apparatus according to an embodiment of the present invention in comparison with the prior art apparatus. 
     The prior art antenna apparatus has the problem that its reception sensitivity in the AM and FM bands is not obtained sufficiently since a space for mounting the antenna  120  is limited. The apparatus also has the problem that since frequency characteristics are not flattened within a receiving band, tuning for optimizing the reception performance is difficult and a long period of time is required for performing the tuning operation. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a windowpane antenna apparatus for vehicles whose reception sensitivity is high in a wide bandwidth and whose tuning operation is easy to perform. 
     To attain the above object, a vehicle windowpane antenna apparatus according to the present invention has the following features in constitution. The other features will be clarified later in the Description of the Invention. 
     A windowpane antenna apparatus for vehicles comprises a defogger mounted on a window of a vehicle, for defogging the window, means for causing the defogger to serve as a slot antenna, and a driven antenna arranged close and opposite to the defogger with a given gap therebetween in such a manner that one side of the driven antenna is mutually coupled to one side of the defogger. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a view showing the constitution of a windowpane antenna apparatus for vehicles according to an embodiment of the present invention; 
     FIG. 2 is an illustration for explaining the principle of the windowpane antenna apparatus according to the embodiment of the present invention; 
     FIG. 3 is a view showing a modification to the windowpane antenna apparatus according to the embodiment of the present invention in which a driven antenna is shaped like a loop; 
     FIG. 4 is an illustration for explaining the principle of the modification shown in FIG. 3; 
     FIG. 5 is an equivalent circuit diagram of the windowpane antenna apparatus according to the embodiment of the present invention to describe its performance and function in an FM band; 
     FIG. 6 is a simplified equivalent circuit diagram of the windowpane antenna apparatus according to the embodiment of the present invention; 
     FIG. 7 is an equivalent circuit diagram showing the windowpane antenna apparatus according to the embodiment of the present invention to describe its reception performance in an AM band; 
     FIG. 8 is a view illustrating the constitution of a prior art windowpane antenna apparatus for vehicles; 
     FIG. 9 is an equivalent circuit diagram of the prior art windowpane antenna apparatus to describe its performance and function in an FM band; 
     FIG. 10 is a simplified equivalent circuit diagram of the prior art windowpane antenna apparatus; and 
     FIG. 11 is an equivalent circuit diagram of the prior art windowpane antenna apparatus to describe its reception performance in an AM band. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     (Embodiment) 
     FIG. 1 illustrates the constitution of a windowpane antenna apparatus for vehicles (automobiles) according to an embodiment of the present invention. As shown in FIG. 1, a defogger  10  is formed almost all over a rear window  100  of a vehicle (e.g., an automobile) to serve as a heater for defogging the window. 
     The defogger  10  includes a plurality of horizontal lines  10   a  arranged in parallel with each other and several (three in this embodiment) vertical lines  10   b  which cross the horizontal lines. The horizontal and vertical lines  10   a  and  10   b  are each constituted of a very thin, narrow, strip conductor. 
     The defogger  10  therefore has a mesh pattern including a number of meshes (openings) as shown in FIG.  1 . The mesh pattern is so formed that the length of a longer side of each mesh is set considerably small for the wavelength (1 [m] or more) of a VHF band or it is set not more than {fraction (1/10)} to {fraction (1/20)} of the wavelength. The defogger  10  can thus be considered to be equivalently a single metal thin plate for a received wave. 
     A DC power supply voltage is applied to the defogger  10  as a heat source from a car-mounted battery  11  through a noise filter  12  (which is constituted of, e.g., a choke coil and a capacitor) for eliminating high-frequency noise (in the AM band), a DC power supply voltage application line  13 , and a pair of FM choke coils  14 A and  14 B. 
     The FM choke coils  14 A and  14 B separate the DC power supply voltage application line  13  from both ends of the defogger  10  to render the ends in a high-frequency state and thus serve as inductance elements. 
     A non-loop driven antenna  20 , which is obtained by cutting part (upper central part) of a rectangular loop, is provided in a region above the defogger  10  or a region of the window between the uppermost portion of the defogger and the upper frame of the window. Like the defogger  10 , the driven antenna  20  is constituted of a very thin, narrow, strip conductor. The driven antenna  20  is formed close and opposite to the defogger  10  with a given gap Gm (about 1 [cm] to 2 [cm]) therebetween in such a manner that one side of the antenna  20  or a bottom  20   a  thereof is mutually coupled to one side of the defogger  10  or the uppermost one of the horizontal lines  10   a  (coupling index K is approximately 1). A feeding section  21  is set in position P, which is slightly shifted to the right (in FIG. 1) from the middle of the bottom  20   a  of the driven antenna  20 , and connected to a receiver set (not shown) through a feeding cable (not shown). 
     In FIG. 1, reference symbol MC indicates a mutual coupling portion between the defogger  10  and driven antenna  20 , and reference numerals  20   b  and  20   c  denote open ends of the driven antenna  20 . 
     FIG. 2 is an illustration for explaining the principle of the antenna apparatus shown in FIG.  1 . As described above, the defogger  10  is formed in a mesh pattern and considered to be equivalently a single thin metal plate for a received wave. Both ends of the defogger  10  are rendered in a high-frequency state by the paired FM choke coils  14 A and  14 B each serving as an inductance element. Therefore, the entire rear window  100  serves as an opening area  31 ,  32 ,  33  of a slot antenna surrounded with a metal body  30  of a car body which is considered to be an ideal ground (ground plane) and accordingly the defogger  10  functions as a slot antenna SA in the AM/FM band. 
     The coupling capacitance CX of a mutual coupling section MC of the defogger  10  and driven antenna  20  arranged close to each other, is set equal to or larger than 20 PF (CX≧20 PF). The driven antenna  20  is thus coupled to the slot antenna SA of the defogger  10  by relatively great force and their interaction decreases a radiation impedance of the driven antenna  20  or an output impedance. Consequently, the frequency characteristics are flattened within a receiving band and the band is broadened. Since the feeding section  21  of the driven antenna  20  is located in the position P slightly shifted from the middle of the antenna  20 , impedance matching between the feeding section  21  and a feeding cable  22  is easily performed. 
     (Modification to the Embodiment) 
     FIG. 3 is a view of a modification to the windowpane antenna apparatus according to the embodiment described above, and FIG. 4 is a view showing the principle of the modification. The modification differs from the embodiment in that a loop-shaped driven antenna  20 ′ is used in place of the antenna  20 . The other constituting elements are the same as those of the above embodiment and thus their descriptions are omitted. 
     Various shapes as well as the above ones can be applied to the driven antenna. 
     (Operation of the Embodiment) 
     Performance and Function in FM Band: 
     FIG. 5 is an equivalent circuit diagram of the windowpane antenna apparatus according to the above embodiment to describe its performance and function in an FM band, and FIG. 6 is a simplified equivalent circuit diagram of the windowpane antenna apparatus shown in FIG.  5 . Referring to FIGS. 5 and 6 and comparing them with FIGS. 9 and 10 showing a prior art antenna apparatus, the performance of the antenna apparatus of the present invention will be described. In FIG. 5, SA indicates a slot antenna and Zo represents an output impedance. 
     1) Gain of Antenna 
     As illustrated in FIGS. 5 and 6, impedance matching between the slot antenna SA, which is formed chiefly of the defogger  10 , and the driven antenna  20  arranged close thereto is performed satisfactorily by means of the mutual coupling section MC. Therefore, most power received by the slot antenna SA is supplied to the receiver set (not shown) such as a radio through a feeding cable  22 . It is thus thought that the antenna gain Ga of the present antenna apparatus is almost proportional to the area SWG of the whole window glass. 
     In the prior art antenna apparatus as shown in FIGS. 9 and 10, region of the defogger  110  is short-circuited with a ground plane  130  in a high-frequency region of the FM band, so that the effective area of the antenna  120  is limited to a small region existing above the defogger  110 . It is thus thought that the antenna gain GA is almost proportional to the area SC of the above region. 
     The ratio of SWG to SC is 4:1 to 5:1. It will be understood from this ratio that the antenna gain Ga of the antenna apparatus of the present invention is considerably higher than that GA of the prior art antenna apparatus. 
     2) Impedance of Antenna 
     In FIG. 6, Zb shows an impedance of the driven antenna obtained by converting a radiation impedance ZSA of the slot antenna SA, which is almost proportional to the inverse 1/SSA of the area SSA of a region where the defogger  10  is formed, using a coupling coefficient k of the mutual coupling section MC. As illustrated in FIG. 6, an equivalent resonant circuit ERC of the antenna apparatus of the present invention includes an impedance component of the driven antenna  20  and the above impedance Zb which is connected in parallel to the component. 
     In the prior art antenna apparatus shown in FIG. 10, an equivalent resonant circuit ERC does not include any equivalent for the above impedance Zb. 
     Since, in the present invention, the impedance Zb is contained in the equivalent resonant circuit ERC, the output impedance Zo of the antenna apparatus is lowered and so is a Q (sharpness of resonance) value thereof. Consequently, a reception frequency characteristic within a desired frequency band becomes constant and the frequency band is broadened. 
     Since, moreover, the impedance zb is almost inversely proportional to the product of the area SSA and the square (k 2 ) of coupling coefficient k of the mutual coupling section MC, the frequency characteristic can properly be determined if the coefficient k is set to an adequate value. 
     The coupling index K, which is equal to k×[square root of (Q of slot antenna)×(Q of driven antenna including a feeding cable of a load)], is close to 1. Inevitably, the coupling coefficient k becomes considerably smaller than 1. 
     The Q (=QS) in the slot antenna SA is almost proportionate to the inverse 1/SSA. QS is therefore given as follows: 
     
       
         QS≈X/SSA  (1) 
       
     
     where X is a coefficient. 
     If the Q (=QL) in the equivalent resonant circuit ERC is the following equation is given: 
     
       
         QL=(QE·QS) ½   (2) 
       
     
     Where QE is Q in the driven antenna. 
     The Q (=QC) in the antenna  120  is almost proportionate to 1/SC. QC is thus expressed by: 
     
       
         QC≈X/SC  (3) 
       
     
     where X is a coefficient. 
     If QE is equal to QC considering that the effective area of the driven antenna  20  and that (SC) of the windowpane antenna  120  are approximately equal to each other, the following is derived from the above equation (2): 
     
       
         QL≈(QC·QS) ½   (4) 
       
     
     Substituting the expressions (1) and (3) into the expression (4), QL is given as follows: 
     
       
         QL≈[(X/SC)·(X/SSA)] ½   (5) 
       
     
     
       
         where SSA is approximately equal to N·SC (N=an integral multiple, 4 or 5).  (6) 
       
     
     Substituting the expression (6) into the expression (5), the following is given as follows: 
     
       
         QL≈(X/SC)[1/N] ½   (7) 
       
     
     Applying the expression (3) into the expression (7), the following is given by: 
     
       
         QL≈QC[1/N] ½   (8) 
       
     
     Applying N (=4 to 5) to the expression (8), QL is expressed by: 
     
       
         QL≈QC/2  (9) 
       
     
     The Q (=QL) in the antenna apparatus of the present invention is equal to or smaller than half the Q (=QC) in the windowpane antenna  120 . It is thus understood that the passing frequency band (having a bandwidth of 3 dB) of the FM band of the antenna apparatus is two or more times greater than that of the prior art antenna apparatus. 
     As described above, it is evident that the antenna apparatus of the above embodiment is excellent in that its reception sensitivity (which is proportionate to the antenna gain) almost corresponds to the effective area of the antenna. Since, moreover, the output impedance Zo of the antenna can be lowered and the value Q of the antenna can be decreased, the frequency characteristic is made constant and the frequency band is broadened. The tuning operation (adjustment and modification) of the antenna is thus very easy to perform. 
     Reception Performance (Sensitivity) in AM Band: 
     FIG. 7 is an equivalent circuit diagram showing the windowpane antenna apparatus according to the above embodiment to describe its reception performance (sensitivity) in the AM band. Referring to FIG.  7  and comparing it with FIG. 11 corresponding thereto and showing a prior art antenna apparatus, the reception performance (sensitivity) of the antenna apparatus of the present invention will now be described. Since, however, the shape of the driven antenna  20  of the present antenna apparatus and that of the antenna  120  of the prior art antenna apparatus are nearly equal to each other, the effective lengths Lp and Lc of the antennas  20  and  120  are substantially equal to each other as basic conditions, as are the antenna capacitances CE and CG thereof. 
     As illustrated in FIG. 7, the capacitance CE of the driven antenna  20  is connected in parallel with a combined capacitance CT (a combination of antenna capacitance CSA of the slot antenna SA and coupling capacitance CX of the mutual coupling section MC). The antenna-received output voltage EP of the antenna apparatus is therefore increased by a voltage corresponding to the combined capacitance CT. 
     In contrast, the prior art antenna apparatus shown in FIG. 11 does not include any equivalent for the above combined capacitance CT but has only the antenna capacitance CG (which is substantially equal to the antenna capacitance CE of the driven antenna  20 ) of the windowpane antenna  120 . The antenna-received output voltage EC of the prior art apparatus is therefore low. 
     Consequently, the antenna apparatus of the present invention can output a voltage which is higher than that of the prior art antenna apparatus and thus improves in reception performance (sensitivity). 
     The capacitance CF of the feeding cable  22  is considerably larger than the antenna capacitances CE and CG and the combined capacitance CT. The antenna-received output voltage is thus calculated based on the fact that the capacitance Co or CG of the dominator of an equation for calculating the antenna-received output voltage can be ignored with respect to the capacitance CF. Since, furthermore, the coupling capacitance CX is not lower than 20 pF, it is predicted that the combined capacitance CT becomes 10 pF or higher and equal to or higher than the antenna capacitances CE and CG. For this reason, the antenna-received output voltage EP of the antenna apparatus of the present invention is two or more times higher than that EC of the prior art antenna apparatus, and its reception sensitivity is 6 dB or higher and excellent as compared with that of the prior art apparatus. 
     The above results have been confirmed together with the performance in the FM band in the trial-development stage and in the experimental stage for evaluation of measured values of the present antenna apparatus. 
     (Features of the Embodiment) 
     [1] A windowpane antenna apparatus for vehicles as described in the embodiment comprises a defogger ( 10 ) mounted on a window ( 100 ) of a vehicle, for defogging the window ( 100 ), means for causing the defogger ( 10 ) to serve as a slot antenna (SA), and a driven antenna ( 20 ) arranged close and opposite to the defogger ( 10 ) with a given gap (Gm) therebetween in such a manner that one side ( 20   a ) of the driven antenna ( 20 ) is mutually coupled to one side ( 10   a ) of the defogger ( 10 ). 
     In the foregoing windowpane antenna apparatus, the defogger ( 10 ) serves as a slot antenna (SA) and is mutually coupled to the given antenna ( 20 ). Since, therefore, the antenna apparatus is improved in sensitivity in the FM band and the frequency band can be broadened within a receiving band, tuning of the antenna apparatus can be very simplified. In the AM band, too, the reception performance (sensitivity) of the antenna apparatus is considerably higher than that of the prior art antenna apparatus. 
     [2] In the windowpane antenna apparatus for vehicles as described in the above paragraph [1], the means for causing the defogger ( 10 ) to serve as a slot antenna (SA) includes means for separating the defogger ( 10 ) from a power supply voltage application line ( 13 ) in a high-frequency manner by interposing an inductance element ( 14 A,  14 B) between each of both ends of the defogger ( 10 ) and the power supply voltage application line ( 13 ) and means for causing the defogger ( 10 ) to equivalently serve as a single metal thin plate for a received wave by forming the defogger ( 10 ) so as to have a mesh pattern including meshes whose long side is equal to or shorter than the wavelength of the received wave. 
     In the foregoing windowpane antenna apparatus, the defogger ( 10 ) can be caused to serve as a slot antenna (SA) more exactly. 
     [3] The windowpane antenna apparatus for vehicles as described in the embodiment includes a combination of the limitations recited in above paragraphs [1] and [2]. 
     (Modification) 
     The present invention is not limited to the above-described embodiment. In the embodiment, the present invention is applied to a radio receiving antenna apparatus used in both AM and FM bands. However, it can be applied widely to a TV receiving antenna apparatus in the VHF band and the like. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.