Patent Publication Number: US-2012026047-A1

Title: Monopole antenna, antenna assembly, and vehicle

Description:
TECHNICAL FIELD 
     The present invention relates to the field of antennas. 
     The present invention particularly relates to antennas used in the field of automobiles. 
     BACKGROUND ART 
     Antennas for automobiles called radio antennas are known. Although the efficiency for this type of antenna is favorable (70% or more), its appearance is comparatively poor. Furthermore, this antenna is easily susceptible to malicious damage, has the possibility of producing noise when traveling at high speeds, and only performs well in a limited bandwidth within the range of 87 to 108 MHz corresponding to radio frequency. 
     Meanwhile, the electromagnetic spectrum includes a plurality of frequency bands dedicated to various applications. In Europe, for example, the band from 88 to 108 MHz is dedicated to radio, while the band from 470 to 862 MHz is dedicated to television. 
     In the future, it will become possible to use many applications (such as TV, Wi-Fi, and mobile phones) inside an automobile. However, although a radio antenna like that described in Patent Literature 1 has good performance in a frequency range limited to the FM band, its performance suffers in other frequency ranges, and thus its performance is insufficient as an antenna for other applications inside an automobile. 
     PRIOR ART DOCUMENTS 
     Patent Literature 1: Unexamined Japanese Patent Application KOKAI Publication No. H4-260883 
     DISCLOSURE OF INVENTION 
     Problem to be Solved by the Invention 
     It is an object of the present invention to provide an antenna that makes many applications usable inside a vehicle. 
     Means for Solving the Problem 
     For this purpose, the present invention takes as an object to provide a monopole antenna having an ultra wide band, and configured to be able to receive and/or transmit at least a frequency-modulated signal in an FM band, which is a predetermined frequency band where the reflection coefficient is less than −6 dB. 
     An ultra wide band antenna is defined to be an antenna having a band whose bandwidth is greater than a minimum value among values equal to 20% of the center frequency of the band, or greater than 500 MHz. 
     In order to measure antenna performance, a parameter called S 11  is used. The parameter S 11  indicates an antenna&#39;s reflection coefficient. In the case where S 11 =0 dB, all power is reflected by the antenna, and no power is emitted from the antenna. In the case where S 11 =−10 dB, 90% of the power is emitted from the antenna, and 10% of the power is reflected. For this reason, in order to improve antenna performance in a given frequency band, it is desirable to decrease the algebraic value of the parameter S 11  as much as possible and increase its absolute value as much as possible. Consequently, an antenna having a parameter of S 11 =−40 dB for a given frequency has better performance than an antenna having S 11 =−10 dB at the same frequency. In the case where an antenna has a parameter S 11  that is nearly less than a performance threshold value in a given frequency range or equal to this performance threshold value, performance is seen as good for the given frequency range. In this case, the parameter S 11  has a frequency band corresponding to that frequency range. Typically, the performance threshold value is −6 dB. 
     In the case where a monopole antenna is of the ultra wide band type, it is possible to use the antenna for various applications such as mobile phones, Wi-Fi, TV, or DAB (Digital Audio Broadcasting). Since antennas also have good performance in the FM band, they can also be used for car radio. 
     In one embodiment, a monopole antenna includes an FM band pre-determiner able to determine the resonant frequency of the FM band and/or the bandwidth of the FM band in advance. 
     Such a pre-determiner make it possible to adjust the resonant frequency and/or bandwidth of the FM band when manufacturing the monopole antenna. This pre-determiner can adapt an antenna to various geographical zones where the antenna is to be used. Since in practice the FM band may be somewhat narrow depending on the geographical zone, it is possible to prioritize antenna performance in a comparatively narrow FM band, or alternatively, to prioritize antenna performance that, although not particularly good, is sufficient across a comparatively wide FM band, depending on such geographical zones. 
     In another embodiment, a monopole antenna includes an FM band adjuster able to adjust the resonant frequency of the FM band and/or the bandwidth of the FM band. 
     Such an adjuster exclusively enable commercialization of an antenna type where the user of a monopole antenna can modify the resonant frequency and FM bandwidth. The adjuster makes it possible to use an antenna in various geographical zones with different FM bands. 
     According to arbitrary characteristics of a monopole antenna, a monopole antenna may include emitter having a conductive supporter, preferably planar. 
     The conductive supporter is demarcated by an outer edge, and the monopole antenna includes 
     an aperture provided in the conductive supporter and demarcated by a resonant inner edge, and 
     a hole provided in the conductive supporter and demarcated by an edge that joins the resonant inner edge of the aperture with the outer edge of the conductive supporter. 
     The resonant inner edge has a resonant perimeter. A current flowing into a monopole antenna uses this edge. By appropriately selecting a perimeter value and consequently a configuration of the resonant edge, antenna performance in the FM band can be improved. This edge may typically be circular, polygonal, or elliptical. 
     The aperture and the hole make it possible to obtain transmission characteristics and/or receiving characteristics similar to those of a folded antenna while keeping a monopole antenna including a planar supporter. 
     The pre-determiner or the adjuster in the FM bandwidth includes a capacitor that joins two facing parts of the edge of the hole. 
     The capacitor can extend the resonant perimeter of the monopole antenna, and consequently improve performance in the FM band. 
     The monopole antenna accordingly includes a monopole antenna connecter provided with a supply port joined to the edge of the hole. 
     A desired, predetermined antenna impedance is obtained depending on the position of the connecter. Preferably, this impedance is nearly equal to the impedance of a coaxial cable, or in other words 50Ω. 
     In one embodiment, the external dimensions of the supporter are preferably larger than 0.1 m 2 , more preferably larger than 0.15 m 2 , and more preferably larger than 0.18 m 2 . 
     The external dimensions correspond to the product of the maximum vertical dimensions of the supporter. 
     In another embodiment, the external dimensions of the supporter are preferably smaller than 0.06 m 2 , more preferably smaller than 0.04 m 2 , and more preferably smaller than 0.03 m 2 . 
     Particularly, there are cases where it is desirable to use a monopole antenna of comparatively compact size for reasons of external dimensions or appearance. According to a monopole antenna in accordance with the present invention, it becomes possible to use a monopole antenna with an ultra wide band, good performance in the FM band, and furthermore comparatively compact. 
     Furthermore, since the monopole antenna is compact, a plurality of monopole antennas can be used. Such plural antennas improve the directional diversity and spatial diversity of the combined antenna obtained as a result compared to a single antenna. With such improvements in diversity, monopole antenna performance can be increased. 
     It is also an object of the present invention to provide an antenna assembly comprising an element formed of a plastic material for a vehicle, and a monopole antenna attached to the element formed of the plastic material. The antenna assembly may be provided with at least one monopole antenna, and may also be provided with two or three monopole antennas. 
     It is also an object of the present invention to provide a vehicle including at least one monopole antenna as above or an antenna assembly as above. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       A more complete understanding of the present invention can be obtained when the following detailed description, herein given as an example and not as limiting, is read with reference to the attached drawings. 
         FIG. 1  is a diagram illustrating a vehicle that includes an antenna in accordance with a first embodiment. 
         FIG. 2  is a schematic perspective view illustrating the spoiler of the vehicle in  FIG. 1 . 
         FIG. 3  is a diagram illustrating a supporter for the antenna in  FIG. 1 . 
         FIG. 4  is a diagram illustrating a vehicle that includes a plurality of antennas in accordance with a second embodiment. 
         FIG. 5  is a diagram illustrating one of the antennas in  FIG. 4 . 
         FIG. 6  is an enlarged view illustrating zone VI in  FIG. 5 . 
         FIG. 7  is a diagram similar to  FIG. 5  regarding an antenna in accordance with a third embodiment. 
         FIG. 8  is a diagram similar to  FIG. 6  regarding an antenna in accordance with a third embodiment. 
         FIG. 9  is a graph illustrating variation in performance parameters according to frequency for various types of antennas. 
         FIG. 10  is a graph illustrating variation in performance parameters according to frequency for various types of antennas. 
         FIG. 11  is a graph illustrating variation in performance parameters according to frequency for various types of antennas. 
         FIG. 12  is a graph illustrating variation in performance parameters according to frequency for various types of antennas. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     In several of these drawings, orthogonal coordinates X, Y, and Z are illustrated corresponding to the conventional transverse direction, lengthwise direction, and vertical direction of a vehicle. 
     In  FIG. 1 , the reference sign  10  is given to indicate the automobile as a whole. The automobile includes a part  12  consisting of plastic material such as reinforced polypropylene, and including an antenna  14 A. In this case, the part  12  is a spoiler or rear fin. The antenna  14 A is attached to the part  12 , for example, by being pasted, and is hidden below one or a plurality of coat layers. In a modified embodiment, the part  12  is the surface of the rear or front bumper, the roof, or the bonnet of the automobile. 
     In  FIG. 2 , the antenna  14 A and a metal part  16  of the automobile  10  are illustrated. The antenna  14 A includes a connecter  18  provided with a supply port  20  consisting of a coaxial cable  21  protected by a sheath  22 . The sheath  22  is affixed to the metal part  16 , while one end of the coaxial cable  21  is affixed to the antenna  14 A. 
     Referring to  FIG. 3 , the antenna  14 A is an ultra wide band monopole antenna. Additionally, the antenna  14 A is configured such that a frequency modulated signal can be received and/or transmitted in the predetermined frequency band that is the FM band. The antenna  14 A has a reflection coefficient of less than −6 dB in a predetermined frequency band. 
     The antenna  14 A includes an emitter  26  provided with a conductive supporter  28  composed of a conductive material such as copper, for example. The supporter  28  is a planar surface nearly parallel to the XY plane, and takes the form of an adhesive sheet. The supporter  28  is demarcated by an outer edge  30  and has a long and narrow shape overall, preferably forming a spindle elliptical shape. The outer edge  30  has two bowed parts  30 A and  30 B. The supporter  28  extends along the X direction. The supporter  28  is symmetrical about the center symmetry plane P parallel to the YZ plane. 
     The supply port  20  is affixed to the supporter  28  so as to obtain an impedance nearly equal to 50Ω. 
     The supporter  28  herein is 109 cm long and 17 cm wide. Consequently, its external dimensions are larger than 0.1 m 2 , additionally larger than 0.15 m 2 , and more accurately larger than 0.18 m 2 , being 0.1853 m 2 . 
       FIGS. 4 to 6  illustrate one or a plurality of antennas  14 B in accordance with a second embodiment. Like reference signs are given to elements like those illustrated in  FIGS. 1 to 3 . 
       FIG. 4  illustrates an automobile  10  that includes three antennas  14 B. The antennas  14 B are arranged in a row along the transverse direction X. 
     As illustrated in  FIGS. 5 and 6 , the supporter  28  is nearly a half-ellipse overall, and is symmetrical about the center symmetry plane P parallel to the YZ plane. The antenna  14 B is provided on the supporter  28 , and includes an aperture  32  demarcated by a resonant inner edge  34 . Meanwhile, the antenna  14 B includes a hole  36  (gap) provided in the supporter  28 . The hole  36  is demarcated by an edge  38  that joins the resonant inner edge  34  and the outer edge  30 . In this case, the aperture  32  is nearly circular. 
     The outer edge  30  has one part  30 A nearly parallel to the X direction, and two bowed parts  30 B. In other words, the outer edge  30  of the supporter  28  is provided with a linear part  30 A configured to extend in the X direction (widthwise direction) of the vehicle, and an arced part (composed of two bowed parts  30 B,  30 B) configured to join one end of the linear part  30 A with the other end. Additionally, the hole  36  (gap) is provided in the center of the arced part. Also, the aperture  32  is disposed at a position offset towards the arced part of the outer edge  30 . 
     In this embodiment, the antenna  14 B is inscribed in a rectangle having dimensions of 36 cm×15 cm. For this reason, the external dimensions of the supporter  28  are smaller than 0.06 m 2 , being 0.0525 m 2 . 
     The antenna  14 B includes a pre-determiner  40  for the FM band. The pre-determiner  40  includes a pre-determiner  42  for a resonant frequency F 0  in the FM band on one hand, and includes a pre-determiner  44  for a bandwidth A in the FM band on the other hand. In this case, the pre-determiner  42  and  44  act mutually, and include a capacitor  46  that joins two facing parts  38 A and  38 B on the edge  38  of the hole  36 . The supply port  20  is joined to the edge  38  which demarcates the hole  36  so as to obtain an impedance nearly equal to 50Ω. 
     In a modified embodiment, the antenna  14 B includes frequency F 0  and bandwidth A adjuster. With this device, the user of the antenna  14 B is able to modify the capacitance of the capacitor  46 . 
       FIGS. 7 and 8  illustrate an antenna  14 C in accordance with a third embodiment. Like reference signs are given to elements similar to those in the drawings illustrated hitherto. 
     In this embodiment, the supporter  28  takes the form of one-fourth of an ellipse overall. The antenna  14 C is inscribed in a rectangle having dimensions of 19 cm×15 cm. For this reason, the external dimensions of the supporter  28  are smaller than 0.04 m 2 , and additionally smaller than 0.03 m 2 , being 0.0285 m 2 . Since an antenna  14 C in accordance with the third embodiment is compact, it can be hidden inside the rear-view mirror of an automobile. 
     The outer edge  30  includes two linear parts  30 A and one bowed part  30 B. In other words, the outer edge  30  of the supporter  28  is provided with a first linear part  30 C, a second linear part  30 D joined to the first linear part  30 C and extending in a direction orthogonal to the linear part  30 C, and a bowed part  30 B configured to join the ends of the first linear part  30 C and the second linear part  30 D that are not joined together. Additionally, a hole (gap)  36  is provided at the part where the linear part  30 C and the bowed part  30 B intersect near the short axis of the supporter  28 . The hole  36  is demarcated by edges  38 A and  38 B that join the resonant inner edge  34  and the outer edges  30 C and  30 B. 
       FIG. 9  illustrates variation in the S 11  parameter according to frequency (logarithmic scale) for an antenna  14 A in accordance with the first embodiment (curve A, bold solid line), an antenna  14 B in accordance with the second embodiment (curve B, bold broken line), and an ultra wide band antenna in accordance with the related art (curve D, regular line). 
     The parameter S 11  for an antenna in accordance with the related art is nearly zero from 50 MHz to 170 MHz, and less than −6 dB from 180 MHz. For this reason, an antenna in accordance with the related art has good performance in the high frequency band past 180 MHz. However, the FM frequency band is not included in this band. 
     The parameter S 11  for the antenna  14 A is less than −6 dB from 80 MHz, and has the FM band with a resonant frequency f 0,A  at nearly 110 MHz. For this reason, the antenna  14 A has good performance in the frequency band from 80 MHz to 3 GHz. Consequently, the antenna  14 A has an ultra wide band, and particularly covers the FM frequency band. 
     The parameter S 11  of the antenna  14 B is less than −6 dB in a first range from approximately 88 MHz to 110 MHz, and subsequently in a second range from approximately 170 MHz to 3 GHz. The parameter S 11  has the FM band with a resonant frequency F 0,B  at nearly 100 MHz in the first range. Consequently, the antenna  14 B has an ultra wide band, and particularly covers the FM frequency band. 
     The parameter S 11  of the antennas  14 A and  14 B is less than −6 dB in a frequency range from 190 MHz to 3 GHz. 
       FIG. 10  illustrates variation in the parameter S 11  for an antenna  14 B of the type in the second embodiment (curve B, bold solid line), an antenna  14 B of the type in the second embodiment without an aperture  32  (curve E, regular broken line), and an antenna  14 B of the type in the second embodiment without an aperture  32  nor a hole  36  (curve F, regular line). 
     The external dimensions of the supporter  28  for the antennas  14 B whose parameter S 11  is illustrated in  FIG. 10  is smaller than 0.06 m 2 . 
     In the case of no aperture  32  and no hole  36 , the FM frequency band cannot be covered. Similarly, in the case of no hole  36 , FM frequency band coverage is insufficient despite the presence of an aperture  32 . Consequently, when the external dimensions are a given value, in this case smaller than 0.06 m 2 , the presence of an aperture  32  and a hole  36  are necessary in order for the antenna  14 B to have high performance in a predetermined FM frequency band. The parameter S 11  of the antennas  14 B illustrated in  FIG. 10  has a band with a resonant frequency F 0,B  nearly equal to 95 MHz. 
       FIG. 11  illustrates variation in the antenna parameter S 11  with respect to various values for the capacitance C of the capacitor  46 . 
     Each antenna has a parameter S 11  having one band in an FM frequency range from approximately 70 MHz to 100 MHz. Each band in this range has a resonant frequency F 0  corresponding to the value for the capacitance C of the capacitor. The resonant frequency F 0  decreases as the value for the capacitance C increases. In the case of the capacitances 4·10 −12  F, 7·10 −12  F, 1·10 −11  F, and 1.3·10 −11  F, the frequencies F 0,1 , F 0,2 , F 0,3 , and F 0,4  are nearly equal to 105 MHz, 98 MHz, 89 MHz, and 79 MHz, respectively. Consequently, the capacitor  46  can determine in advance and/or adjust the frequency F 0  of the band that covers FM band frequencies. 
     Furthermore, the bandwidth A decreases as the value for the capacitance C increases. For this reason, the capacitor  46  can determine in advance and/or adjust the bandwidth F 0  of the band that covers FM band frequencies. 
     Varying the capacitance C of the capacitor  46  effects little to no change in the resonant frequencies of other bands covering frequency ranges greater than 150 MHz. 
       FIG. 12  illustrates variation in the parameter S 11  for an antenna  14 B in accordance with the second embodiment (curve B, bold solid line) and an antenna  14 C in accordance with the third embodiment (curve C, regular line). 
     As similarly illustrated in  FIGS. 9 and 10 , the parameter S 11  of the antenna  14 B is less than −6 dB in a first frequency range from approximately 88 MHz to 110 MHz, and subsequently in a second frequency range from approximately 170 MHz to 3 GHz. Consequently, the antenna  14 B has an ultra wide band, particularly covers the FM frequency band, and has good performance in this band. 
     The external dimensions of the supporter  28  for the antenna  14 C are smaller than 0.03 m 2 . The parameter S 11  of the antenna  14 C is less than −6 dB in a first frequency range from approximately 87 MHz to 110 MHz, subsequently in a second frequency range from approximately 170 MHz to 360 MHz, and furthermore in a third frequency range from approximately 560 MHz to 3 GHz. Consequently, the antenna  14 C has an ultra wide band, particularly covers the FM frequency band, and has good performance in this band. 
     The electromagnetic spectrum includes a plurality of frequency bands dedicated to various applications. These dedicated frequency bands vary significantly by geographical zone. As an example, the frequency bands used for FM radio, DAB (Digital Audio Broadcasting), TV, inter-vehicular communication, Wi-Fi, and mobile phones in Europe, the United States, and Japan are indicated in Table 1. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Application 
                 Europe (MHz) 
                 USA (MHz) 
                 Japan (MHz) 
               
               
                   
               
             
            
               
                 FM radio 
                  88~108 
                 88~106 
                 70~90 
               
               
                 DAB 
                 174~237 
                 88~106 
                 470~770 
               
               
                 TV 
                 175~862 
                 175~806  
                 170~770 
               
               
                 Inter-vehicle comm. 
                 5900 
                 5900 
                 700 
               
               
                 Wi-Fi 
                 2400~5800 
                 2400~5800  
                 2400~5800 
               
               
                 Mobile phones 
                  900~1800 
                 700~2170 
                  810~2170 
               
               
                   
               
            
           
         
       
     
     Each of the antennas  14 A,  14 B, and  14 C can be used in various geographical zones depending on the application which is desired to be made useable. With the frequency F 0  and bandwidth A pre-determiner  42  and  44 , the FM frequency band can be adapted to a geographical zone. 
     The following Table 2 (antenna of a type in accordance with the second embodiment) and Table 3 (antenna of a type in accordance with the third embodiment) indicate the external dimensions, value for capacitance C, and ranges where the parameter S 11  is equal to or less than −6 dB, or in other words ranges where antenna performance is good, for the respective geographical zones of Europe, the United States, and Japan. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Europe 
                 USA 
                 Japan 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Ext. dimensions 
                 0.0525 
                 0.0525 
                 0.0525 
               
               
                 (m 2 ) 
               
               
                 Capacitance (pF) 
                 60 
                 60 
                 87 
               
               
                 Ultra wide band 
                 170 MHz~6 
                 170 MHz~6 GHz 
                 170 MHz~6 GHz 
               
               
                 range where S11 &lt; 
                 GHz 
               
               
                 −6 dB 
               
               
                 FM band range 
                 85 MHz~110 
                 85 MHz~110 
                 69 MHz~90 MHz 
               
               
                 where S11 &lt; −6 dB 
                 MHz 
                 MHz 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Europe 
                 USA 
                 Japan 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Ext. dimensions 
                 0.0285 
                 0.0285 
                 0.0285 
               
               
                 (m 2 ) 
               
               
                 Capacitance (pF) 
                 7.5 
                 7.5 
                 11 
               
               
                 Ultra wide band 
                 170 MHz~360 
                 170 MHz~360 
                 170 MHz~360 
               
               
                 range where S11 &lt; 
                 MHz 
                 MHz 
                 MHz 
               
               
                 −6 dB 
                 560 MHz~6 
                 560 MHz~6 
                 560 MHz~6 GHz 
               
               
                   
                 GHz 
                 GHz 
               
               
                 FM band range 
                 87 MHz~108 
                 87 MHz~108 
                 73 MHz~88 
               
               
                 where S11 &lt; −6 dB 
                 MHz 
                 MHz 
                 MHz 
               
               
                   
               
            
           
         
       
     
     The present application is based on French Patent Application No. 0952472 filed on Apr. 15, 2009, the specification, claims, and drawings of which are herein incorporated in their entirety by way of reference.