Patent Publication Number: US-9837708-B2

Title: Cobra antenna

Description:
TECHNICAL FIELD 
     The present invention relates to a cobra antenna that can be used as an antenna for a wide frequency band ranging from an FM band to a UHF band and that can be implemented with a simple structure. 
     BACKGROUND ART 
     Various forms of antennas have conventionally been used as an antenna for receiving various broadcast waves such as television broadcast or FM broadcast. For example, a dipole antenna or a Yagi-Uda antenna is often used for receiving television broadcast and FM broadcast. 
     On the other hand, the various broadcast waves have increasingly being received in a room, in a car or during travel on foot. The antenna used in such cases needs to be easily handled, for example, for assembly or installation. 
     Such an easily-assembled or easily-handled antenna is typified by a dipole antenna that is implemented by the antenna elements that are simply structured. A cobra antenna is known as an embodiment of the dipole antenna. The cobra antenna is used with some turns of a coaxial wire around a ferrite core (for example, Non-patent Document 1). 
       FIG. 5  is a view for showing an exemplary cobra antenna that has been produced by modifying a dipole antenna. As shown in  FIG. 5 , a cobra antenna  100  includes a central conductor (core wire)  300  and a ferrite core  400 . On the assumption that the radio wave to be received has a wavelength of λ, the central conductor  300  is λ/4 in length and is connected, as an upper element, on a feeding point  200 . The ferrite core  400  is provided under and λ/4 away from the feeding point  200 . A coaxial cable (coaxial wire)  500  is wound around the ferrite core  400 . Although the coaxial cable  500  is wound 3 times in  FIG. 5 , the number of turning (the number of winding) does not necessarily need to be three times. The number may be once or twice. 
     When the coaxial wire is wound around the ferrite core  400  three times or more, the impedance tends to drastically decrease regardless of the size of the ferrite over about the frequency of 100 MHz. For example, it has been reported that, when the number of winding is once, the impedance of the antenna tends to increase even though the frequency exceeds 100 MHz; however, when the number of winding is three times, the impedance drastically decreases. 
     In the cobra antenna shown in  FIG. 5 , a choke coil is formed by a ferrite core  300  and the coaxial cable  500  wound around the ferrite core. The choke coil separates a feeder part below the ferrite core  400  so that a λ/4 dipole antenna can easily be formed. An egg-shaped glass or the like is attached to the upper core wire  300  of the dipole antenna for insulation so that the antenna can be hung from a tree branch or a wooden frame. This can facilitate the installation of an antenna. A cobra antenna structured in such a manner can also be applied to an antenna of a car-mounted mobile device. 
     CITATION LIST 
     Non-Patent Document 
     
         
         Non-patent Document 1: Chapter 1 ANTENA NO KISO, p. 84 in “WIRE ANTENNA” edited by CQ ham radio HENSHU BU 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, when the cobra antenna shown in  FIG. 5  is used as an antenna for a wide frequency band ranging from an FM band to a UHF band, an interference of radio waves sometimes occurs depending on the length of the coaxial cable  500  from the ferrite core  400  to a receiver. In other words, there is a problem about radio wave interference in that the high-frequency current received by the upper part of the coaxial cable  500  leaks into the lower part of the coaxial cable  500 . The upper part extends from the ferrite core  400  to the feeding point  200 . The lower part extends from the ferrite core  400  and is connected to the receiver. The leakage of the high-frequency current is considered to occur due to the impedance mismatch between the upper side and the lower side across the ferrite core  400 . There is a disadvantage in that the leakage causes the gain characteristics as an antenna to become bad. 
     The occurrence of the leakage of the high-frequency current depends on the length of the coaxial cable  500  from the ferrite core  400  to the point connected to the receiver. Thus the occurrence becomes a strict limitation when the length of the part of the coaxial cable  500  is determined. In other words, in a conventional cobra antenna  100 , the length of the coaxial cable  500  from the ferrite core  400  to the receiver cannot freely be determined. It is considered that the interference due to the high-frequency current occurs because the cobra antenna  100  uses the outer sheath of the coaxial cable  500  as an antenna. Thus, there is a problem in that the required performance cannot be obtained when the cobra antenna  100  is connected to a connector of the receiver without modification. 
     The present invention has been made in light of the foregoing problems, and an object of the present invention is to provide a small cobra antenna that can be used as an antenna for a wide frequency band ranging from an FM band to a UHF band, and has a high performance as an antenna. The cobra antenna also minimizes the limitation on the length of the coaxial wire. 
     Solutions to Problems 
     To solve the above-mentioned problems and achieve the object of the present invention, the cobra antenna of the present invention includes a junction constituting the feeding point. An antenna element is electrically connected to one terminal of the junction. The antenna element has a length corresponding to the frequency of the broadcast wave to be received. A coaxial wire is connected to the other terminal of the junction. A ferrite core is positioned at a location a length identical to the length of the antenna element away from the other terminal of the junction connected to the coaxial wire. The coaxial wire is wound around the ferrite core about once to three times. A high frequency interrupting part is provided at the front side of a connecter of a receiver connected to the other terminal of the coaxial wire. The high frequency interrupting part is for interrupting the high-frequency current from the coaxial wire. 
     Note that the high frequency interrupting part is a second ferrite core that has high impedance against a high-frequency wave. The above-mentioned coaxial wire passes through the inside of, or is wound around, the second ferrite core. Further, on the assumption that the frequency to be received has a wavelength of λ, the antenna element is λ/4 in length and the length from the junction of the coaxial wire to the ferrite core is λ/4. 
     The cobra antenna of the present invention can prevent the high-frequency wave picked up by the coaxial wire from entering the receiver by including, in front of the connector of the receiver, the second ferrite core that has high impedance against a high-frequency wave. 
     Effects of the Invention 
     According to the present invention, the length of the part of the coaxial wire except the antenna wire can freely be determined. This reduces the limitation on the placement of the antenna. Thus, the cobra antenna according to the present invention can fully exert the performance as an antenna regardless of the equipment to be connected to the antenna, and regardless of the length of the coaxial wire of the antenna. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing the comparison between an exemplary embodiment of the cobra antenna of the present invention (B) and a conventional cobra antenna (A). 
         FIG. 2  is a schematic view showing the comparison between the frequencies-gain characteristics of the cobra antenna of the exemplary embodiment of the present invention (B) and the frequencies-gain characteristics of the conventional cobra antenna (A). 
         FIG. 3  is a view showing an example where the cobra antenna of the exemplary embodiment of the present invention is attached as a car-mounted antenna. 
         FIG. 4  is a view showing the route used for the field test for a car on which the cobra antenna of the exemplary embodiment of the present invention is mounted as a car-mounted antenna. 
         FIG. 5  is a view for describing a conventional cobra antenna. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     An exemplary embodiment of the present invention (hereinafter, sometimes referred to as the present example) will be described below based on  FIGS. 1 to 4 , and described in the following order. 
     1. Description of the basic structure and the basic principle of a cobra antenna 
     2. The structure and the characteristics of the cobra antenna of the exemplary embodiment of the present invention 
     3. The field test performed using the cobra antenna of the exemplary embodiment of the present invention 
     &lt;Description of the Basic Structure and the Basic Principle of a Cobra Antenna&gt; 
       FIG. 1A  shows the same cobra antenna as the conventional cobra antenna described in  FIG. 5 .  FIG. 1B  shows the cobra antenna of the present example. First, the commonalities between  FIGS. 1A and 1B  will be described. 
     Each of cobra antennas  10  shown in  FIGS. 1A and 1B  includes an antenna element  2 , a junction  3 , a coaxial wire  5 , and a ferrite core  4 . The length of the antenna element  2  is λ/4 on the assumption that the radio wave to be received has a wavelength of λ. The junction  3  is a feeding point. The length of the coaxial wire from the junction  3  to the ferrite core  4  is λ/4, which is the same as the length of the antenna element  2 . 
     An end of the coaxial wire  5  is connected to the antenna element  2  through the junction  3 . Further, the coaxial wire  5  is wound around the ferrite core  4  about once to three times. The other end of the coaxial wire  5  is connected to a connector  6  of a receiver  8 . In this case, a connector that has a low loss of the high-frequency signal is preferably selected as the connector  6 . At the antenna element  2 , an outer sheath (protective coating)  5   a  and a shield wire (external conductor)  5   b  of the coaxial wire  5  are removed. 
     At the junction  3 , the outer sheath  5   a  and the shield wire  5   b  of the coaxial wire  5  are removed, and a core material  5   c  (inductor) is exposed. A core wire  5   d  of the coaxial wire  5  is connected to a core wire of the antenna element  2  by means of, for example, soldering. The junction  3  is molded and formed on a substrate  7 . The junction  3  is a feeding point Fp of the cobra antenna  10 . 
     With this configuration, the coaxial wire  5  from the junction  3  (the feeding point) to the ferrite core  4  (λ/4 in length) and the antenna element  2  (λ/4 in length) form a dipole antenna of λ/2. 
     &lt;The Structure and the Characteristics of the Antenna of the Exemplary Embodiment of the Present Invention&gt; 
     As described above, the commonalities between the cobra antennas shown in  FIGS. 1A and 1B  have been described. However, the cobra antenna of the present example shown in  FIG. 1B  differs from the conventional cobra antenna shown in  FIG. 1A  in that the cobra antenna of the present invention is provided with a second ferrite core  4   a  in front of the connector  6  of the receiver  8 . 
     Hereinafter, the conventional cobra antenna shown in  FIG. 1A  will be referred to as a cobra antenna (one-core product) and the cobra antenna of the present invention will be referred to as a cobra antenna (two-core product). 
     In the conventional cobra antenna (one-core product), as already described, a high-frequency coupling occurs between the coaxial wire  5  from the ferrite core  4  to the junction  3  and the coaxial wire  5  from the ferrite core  4  to the connector  6 . This degrades the performance of the antenna. Because the degrading depends on the length to the coaxial wire  5  from the ferrite core  4  to the connector  6 , the length of the part becomes a limitation when this type of cobra antenna is used as a car-mounted antenna. 
     In the cobra antenna (two-core product) of the present example shown in  FIG. 1B , the second ferrite core  4   a  is provided at a position near the receiver  8 . Because the ferrite core  4   a  has high impedance against a high-frequency wave, the high-frequency current leaking from the antenna is not propagated to the receiver side. 
       FIG. 2A  and Table 1 are the graphs showing the peak gains of the vertical polarization (V) and of the horizontal polarization (H) of the conventional cobra antenna (one-core product) shown in  FIG. 1A . The horizontal axis of  FIG. 2A  denotes the frequencies (MHz) and the vertical axis denotes the peak gains (dBd). 
     The frequencies to be measured are set at FM/VHF bands (70 MHz to 220 MHz). The vertical polarization (V) is denoted by a dash line. The horizontal polarization (H) is denoted by a solid line. 
     Table 1 shows the value of the peak gain of the vertical polarization (V) and the value of the peak gain of the horizontal polarization (H) at each measurement point in the graph shown in  FIG. 2A . Note that, in Table 1, only the measured values of the frequencies from 76 MHz to 107 MHz are shown from among the frequencies shown in the horizontal axis of  FIG. 2A . 
     As shown in  FIG. 2A  and Table 1, the peak gain of the vertical polarization (V) becomes −11.50 dBd at 86 MHz and −10.85 dBd at 95 MHz. The peak gain of the horizontal polarization (H) becomes −16.70 dBd at 86 MHz and −14.85 dBd at 95 MHz. In other words, it is found that the conventional cobra antenna (one-core product) also can receive both of the vertical polarization and the horizontal polarization in the FM/VHF bands. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 Vertical polarization 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Freq[MHz] 
                 76 
                 78.5 
                 81 
                 83.5 
                 86 
                 95 
                 101 
                 107 
               
               
                   
               
               
                 Peak[dBd] 
                 −12.04 
                 −12.60 
                 −12.81 
                 −12.14 
                 −11.50 
                 −10.85 
                 −11.87 
                 −12.96 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 Horizontal polarization 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Freq[MHz] 
                 76 
                 78.5 
                 81 
                 83.5 
                 86 
                 95 
                 101 
                 107 
               
               
                   
               
               
                 Peak[dBd] 
                 −18.76 
                 −18.80 
                 −18.61 
                 −17.72 
                 −16.70 
                 −14.85 
                 −15.14 
                 −15.50 
               
               
                   
               
            
           
         
       
     
     On the other hand, the frequency gain characteristics of the cobra antenna (two-core product) of the present example are shown in  FIG. 2B  and Table 2. As is obvious from  FIG. 2B  and Table 2, both of the vertical polarization (V) and the horizontal polarization (H) reach maximum values near 95 MHz. The vertical polarization (V) is −8.25 dBd and the horizontal polarization (H) is −13.65 dBd. In comparison with the conventional type (one-core product) shown in  FIG. 2A  and Table 1, the peak gains at 95 MHz become higher. The frequency-gain characteristics are obviously improved. In other words, it is found that the performance of the cobra antenna (two-core product) of the present example is superior to that of the conventional cobra antenna (one-core product). 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
            
               
                   
                 Vertical polarization 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Freq[MHz] 
                 76 
                 78.5 
                 81 
                 83.5 
                 86 
                 95 
                 101 
                 107 
               
               
                   
               
               
                 Peak[dBd] 
                 −12.40 
                 −12.80 
                 −12.81 
                 −11.92 
                 −10.70 
                  −8.25 
                  −8.87 
                 −10.83 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 Horizontal polarization 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Freq[MHz] 
                 76 
                 78.5 
                 81 
                 83.5 
                 86 
                 95 
                 101 
                 107 
               
               
                   
               
               
                 Peak[dBd] 
                 −20.31 
                 −20.20 
                 −19.96 
                 −18.71 
                 −17.30 
                 −13.65 
                 −13.67 
                 −14.76 
               
               
                   
               
            
           
         
       
     
       FIGS. 2A and 2B  show the minimum values at about 130 MHz. It is indicated that setting the resonance frequency at 100 MHz causes the Q factor of the antenna to become high at about 130 MHz and causes antiresonance (mismatch) so that the frequency cannot be received. Note that the resonance frequency that has been set at 100 MHz resonates with a high frequency. Specifically, the odd multiples of the resonance frequency or, namely, even the triple, or quintuple of the basic resonance wavelength can be received. As for the cobra antenna (two-core product) of the present example, the resonance can occur even when the frequency is set at 200 MHz. 
     &lt;The Field Test Performed Using the Cobra Antenna of the Present Invention&gt; 
       FIG. 3  is a view showing an example where the cobra antenna (two-core product) of the present invention is mounted on the car belonging to the inventor to perform a field test for the cobra antenna (two-core product). Needless to say, the conventional cobra antenna (one-core product) has also been mounted on the car to perform the same measurement for comparison. 
     As shown in  FIG. 3 , the antenna element  2  from the junction  3  of the cobra antenna  10  to the tip is horizontally attached at the windshield from the rearview mirror. The coaxial wire  5  from the junction  3  to the ferrite core  4  is longitudinally attached at the left side. This forms the cobra antenna  10  as a V-shaped antenna having the junction  3  as a center (starting point). The junction  3  is a feeding point. 
     In consideration of the fact that an FM band of 90 MHz has a wavelength λ of 3.33 m, in each of the cobra antenna (two-core product) of the present example and the conventional cobra antenna (one-core product), the antenna element  2  is set as 0.83 m equal to λ/4 in length, the coaxial wire  5  from the junction  3  to the ferrite core  4  is similarly set as 0.83 m equal to λ/4 in length, and then the antenna is set as λ/2 (1.66 m) in length. 
     The coaxial wire  5  from the ferrite core  4  to the connector  6  of the receiver  8  is horizontally routed on the dashboard of the car. Note that, in the cobra antenna (two-core product)  10  of the present example, the second ferrite core  4   a  is inserted into the front (proximity) of the connector  6  of the receiver  8 . 
     The coaxial wire  5  can only pass through the hole of the second ferrite core  4   a.  However, the coaxial wire  5  can also be wound around the ferrite core  4   a  about once to three times and be connected to the connector  6 . As described above, in the cobra antenna (two-core product)  10  of the present example, the ferrite core  4   a  is positioned in front of the connector  6 . Accordingly, the receiver  8  side has high impedance against the high-frequency current picked up by the coaxial wire  5 . The coaxial wire  5  connects the ferrite core  4  to the connector  6 . Thus, even though the coaxial wire  5  from the first ferrite core  4  to the connector  6  picks up the leaked high-frequency current, the leaked high-frequency current does not adversely affect the receiver  8  side. 
     As shown in  FIG. 3 , the cobra antenna (two-core product) of the present example and the conventional cobra antenna (one-core product) have separately been mounted on the car to perform a field test. 
       FIG. 4  is a view showing the course for the test of each reception performance of the cobra antennas that have actually been mounted on the inventor&#39;s car by the inventor. The type of the car was Toyota Carolla (registered trademark). The equipment used as the receiver  8  was a personal navigation device (PND) manufactured by SANYO Electric Co., Ltd. (GORILLA NV-SD750FT) (GORILLA is a registered trademark). The received frequency was 81.9 MHz from VICS Yokohama and the output was 5 kW. 
     As for the sample of the cobra antenna  10 , the distance from the junction  3  to the tip of the antenna element  2  was 83 cm and the distance from the junction  3  to the ferrite core  4  was also 83 cm. Further, in the test, the second ferrite core  4   a  was provided about 5 cm away from a plug to be inserted into the connector  6  of the receiver  8 . However, the distance can be determined as needed. 
     As shown in  FIG. 4 , in the field test, the conventional cobra antenna (one-core product) was first mounted on the car and the car run on Nakahara-Kaido way shown in the drawing to append the VICS updated every five minutes in the running section. Next, the cobra antenna (two-core product) of the present example was mounted on the car and the car run on the same course to append the VICS every five minutes in the running section in the same manner. 
     The test results are the following. 
     the conventional cobra antenna (one-core product): 6/11 times, 54% reception rate 
     the cobra antenna (two-core product) of the present example: 12/14 times, 78% reception rate 
     As is obvious from the results, it can be confirmed that the cobra antenna (two-core product) of the present invention can almost certainly update the data every five minutes in comparison with the conventional type (one-core product). 
     As described above, the cobra antenna (two-core product) as the exemplary embodiment of the present invention has been described in comparison with the conventional cobra antenna (one-core product). In the above-mentioned description, the antenna using a coaxial wire (wire rod) has been described. However, an antenna constituted of a substrate, a film, and a metal wire can be used for the antenna element part to exert the same effect. Further, needless to say, the present invention can be used for the equipment in a room except a car although the present example has been described as an example that has been mounted on the car. 
     REFERENCE SIGNS LIST 
     
         
           10 ,  100  Cobra antenna 
           2 ,  300  Antenna element 
           3  Junction 
           4 ,  4   a,    400  Ferrite core 
           5 ,  500  Coaxial wire 
           5   a  Protective coating 
           5   b  Shield wire 
           5   c  Core material 
           5   d  Core wire 
         Fp,  200  Feeding point 
           6  Connector 
           7  Substrate 
           8  Receiver