Patent Publication Number: US-7586456-B2

Title: Broadband antenna

Description:
This application claims the benefit of Taiwan application Ser. No. 95121386, filed Jun. 15, 2006, the subject matter of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates in general to a broadband antenna, and more particularly to a broadband dipole antenna that is fed at one short side to enlarge the antenna bandwidth. 
   2. Description of the Related Art 
   Along with technological progress, the need for broadband antennas has been increasing rapidly. For example, people can easily watch digital TV via portable TV products. Digital TV is a television system which converts analog signals to digital signals, and Taiwan now uses a European digital video broadcasting terrestrial (DVB-T) system. The DVB-T system can effectively solve a multi-path interruption problem by using modulation standard signals. The European-specification system constructs a single frequency network (SFN) to increase accessible frequency spectrum resources. In addition, the European TV system has a feature of mobile reception and thus people can watch TV even in a car running at a high speed of 130 km/hr. 
   For example, in early periods, antenna technologies of advanced channel estimation and dual antenna reception for distribution and integration were developed in order to improve mobile reception function of the DVB-T receiver disposed in a car. However, these technologies resulted in system complexity, higher hardware costs, and higher antenna power consumption. In the present market, a conventional digital TV antenna used in a car is a monopole antenna. The monopole antenna uses the car shell as a grounded surface, but this greatly affects the appearance of the car and and decreases the car&#39;s attractiveness. Also, due to the shielding effects of the car&#39;s metal shell, the antenna&#39;s ability to receive signals becomes degraded. 
   Recently, technology of a dipole antenna for digital TV reception has been developed. Conventionally, owing that signals are fed into the central part of the dipole antenna symmetrically via a coaxial line, the coaxial line will not be parallel to the direction of two arms of the dipole antenna. Therefore, in practical application, the coaxial line needs to be perpendicular to the digital TV antenna, thereby increasing the volume of the antenna. When the digital TV antenna is disposed in a car, it will further affect the aethestic appearance of the car. In addition, due to the narrow width of the antenna&#39;s radiation conductor, the antenna&#39;s bandwidth may be inadequate. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the invention to provide a broadband antenna. By feeding signals to one short side of the antenna, the coaxial line can be oriented along the direction of a radiation conductor of the antenna. Therefore, the antenna can have a smaller size and thickness, larger bandwidth and more convenience in configuration without reducing antenna performance and attractiveness 
   The invention achieves the above-identified object by providing a broadband antenna including a dielectric substrate, a radiation conductor and a feeding gap. The radiation conductor is disposed on the dielectric substrate and has a first side and a second side. The first side is adjacent to the second side, and the first side is longer than the second side. The second side has a first feeding point and a second feeding point. The feed gap has a first end located at the first side and a second end located at the second side. The feed gap divides the radiation conductor into a first sub-radiation conductor and a second sub-radiation conductor. The first feeding point is located on the first sub-radiation conductor and the second feeding point is located on the second sub-radiation conductor. 
   Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a broadband antenna according to a preferred embodiment of the invention. 
       FIG. 2  is a schematic diagram of a current path of the antenna of  FIG. 1  in the first resonant mode. 
       FIG. 3  is a radiation pattern of the broadband antenna at 510 MHz according to the preferred embodiment of the invention. 
       FIG. 4  is a schematic diagram of a current path of the antenna of  FIG. 1  in the second resonant mode. 
       FIG. 5  is a radiation pattern of the broadband antenna at 740 MHz according to the preferred embodiment of the invention. 
       FIG. 6  is a comparison of the measured return loss for the broadband antenna according to the preferred embodiment of the invention and the corresponding conventional dipole antenna. 
       FIG. 7  is a schematic diagram of another broadband antenna according to the invention. 
       FIG. 8  is a schematic diagram of a third broadband antenna according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the broadband antenna of the invention, signals are fed into one short side of the antenna, and the coaxial line used as a feeding circuit oriented along the direction of the radiation conductor. Therefore, the antenna can have a smaller size and thickness, larger bandwidth and more convenience in configuration without reducing antenna performance and attractiveness. The broadband antenna of the invention can enlarge operating bandwidth by feeding signals to one short side, and can be applied to receive signals of various frequencies. Any antenna with a dipole antenna structure can use this method of feeding signals to one short side in the antenna to increase the bandwidth and flexibity in configuration no matter which frequency band the antenna is applied in. 
   Referring to  FIG. 1 , a schematic diagram of a broadband antenna according to a preferred embodiment of the invention is shown. The broadband antenna  100  includes a dielectric substrate  110 , a radiation conductor  120  and a feeding gap  130 . The radiation conductor  120  can be a radiation metal plate or other conductive materials, such as made of indium tin oxide (ITO) and is formed on the dielectric substrate  110  by a method of printing or etching. The radiation conductor  120  has a first side  121  and a second side  122  and the first side  121  is adjacent to the second side  122 . The first side  121  is longer than the second side  122  and the second side  122  has a first feeding point  125  and a second feeding point  126 . 
   The feeding gap  130  has a first end  131  located at the first side  121  and a second end  132  located at the second side  122 . The feeding gap divides the radiation conductor  120  into a first sub-radiation conductor  123  and a second sub-radiation conductor  124 . The first feeding point  125  is located in the area of the first sub-radiation conductor  126  at the second end  132  of the division gap  130  and the second feeding point  126  is located in the area of the second sub-radiation conductor  124  at the second end  132  of the division gap  130 . Besides, in practical application, the first end  131  of the division gap  130  can be positioned such that the second sub-radiation conductor  124  has a length close to one-third length of the first side  121 . 
   In the broadband antenna  100 , a radio-frequency (RF) signal is received at the first feeding point  125  and the second feeding point  126  via a coaxial line to excite a first resonant mode and a second resonant mode of the radiation conductor  120 . The second resonant mode is adjacent to the first resonant mode such that the antenna has one wide resonant mode formed by both the first and second resonant modes. Moreover, in practical application, the first end  131  of the feeding gap  130  can be positioned such that the length of the second sub-radiation conductor  124  is close to one-fourth wavelength of the second resonant mode. 
   In the embodiment, the first side  121  is 215 mm long, the second side  122  is 10 mm long and the second sub-radiation conductor  124  is 74 mm long for instance. Referring to  FIG. 2 , a schematic diagram of a current path of the antenna  100  of  FIG. 1  in the first resonant mode is shown. From FIG.  2 , it can be seen that the radiation field generated by the current flowing through the second sub-radiation conductor  124  is cancelled with the radiation field generated by the current flowing through the region of the first sub-radiation conductor  123  located under the second sub-radiation conductor  124 . According to the equation c=f×λ, it can be known that the first resonant mode is excited at about 510 MHz. Referring to  FIG. 3 , a radiation pattern of the broadband antenna  100  at 510 MHz according to the preferred embodiment of the invention is shown. From  FIG. 3 , at 510 MHz, the antenna can generate a near-omnidirectional radiation pattern in the x-z plane (the horizontal plane); this characteristic is very suitable for broadband antenna application. 
   Referring to  FIG. 4 , a schematic diagram of a current path of the antenna  100  of  FIG. 1  in the second resonant mode is shown. From  FIG. 4 , the radiation field generated by the current flowing through the second sub-radiation conductor  124  is cancelled by the radiation field generated by the current flowing through the region of the first sub-radiation conductor  123  located under the second sub-radiation conductor  124 . The resonant path of the second resonant mode is twice as long as that of the first resonant mode. According to the equation c=f×λ, it is known that the second resonant mode is excited at about 740 MHz. Referring to  FIG. 5 , a radiation pattern of the broadband antenna  100  at 740 MHz according to the preferred embodiment of the invention is shown. From  FIG. 5 , at 740 MHz, the antenna generates a near-omnidirectional radiation pattern in the x-z plane (the horizontal plane); this characteristic is very suitable for broadband antenna application. 
   Referring to  FIG. 6 , a comparison of the measured return loss for the broadband antenna according to the preferred embodiment of the invention and the corresponding conventional dipole antenna is shown. In  FIG. 6 , the curve  62  is a return loss curve of the broadband antenna  100  in the invention and the curve  61  is a return loss curve of the corresponding conventional dipole antenna. The point  63  corresponds to the first resonant mode and the point  64  corresponds to the second resonant mode. Under the condition that the voltage standing wave ratio (VSWR) is 2.5, the broadband antenna  100  of the invention has a bandwidth of 470˜860 MHz. This bandwidth is much larger than the bandwidth of 500˜600 MHz of the corresponding conventional dipole antenna. 
   In Table 1, performances of a monopole antenna, a conventional dipole antenna and the broadband antenna of the embodiment are shown for comparison. 
   
     
       
         
             
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
                 
                 
               Broadband 
             
             
                 
               Monopole 
               Conventional 
               Antenna of the 
             
             
                 
               antenna 
               dipole antenna 
               embodiment 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
               Volume (mm 3 ) 
               142 × 30 × 30 
               202 × 49 × 0.2 
               215 × 10 × 0.4 
             
             
               Bandwidth (MHz) 
               470~600 
               500~600 
               470~860 
             
             
               Gain (dBi) 
               2 
               2 
               2 
             
             
               VSWR 
               &lt;3 
               &lt;3 
               &lt;2.5 
             
             
                 
             
          
         
       
     
   
   From Table 1, the broadband antenna  100  of the invention has a lower VSWR, that is, a lower return loss and a larger bandwidth of 470˜860 MHz, without reducing the gain level. This bandwidth is suitable for digital TV operation in many different countries. Furthermore, the broadband antenna  100  of the invention has a smaller area and thickness than the monopole antenna and conventional dipole antenna. Therefore, when the broadband antenna  100  is applied as a digital TV antenna in a car, it is very suitable to be attached to the windshield in front of the driver without affecting the sight of the driver and the car&#39;s appearance. In the above-mentioned broadband antenna  100 , the feeding gap  130  is step-shaped. However, the shape of the feeding gap  130  is not restricted to just this. As long as the feeding gap  130  can divide the radiation conductor  120  into the first sub-radiation conductor  123  and the second sub-radiation conductor  124 , it will not depart from the scope of the invention. Referring to  FIG. 7 , a schematic diagram of another broadband antenna according to the invention is shown. In the broadband antenna  700 , the feeding gap  730  is line-shaped and divides the radiation conductor  720  into a first sub-radiation conductor  723  and a second sub-radiation conductor  724 . Referring to  FIG. 8 , a schematic diagram of a third broadband antenna according to the invention is shown. In the broadband antenna  800 , the feeding gap  830  is curve-shaped and divides the radiation conductor  820  into a first sub-radiation conductor  823  and a second sub-radiation conductor  824 . 
   In the broadband antenna disclosed by the invention, signals are fed into one short side of the antenna, and thus the coaxial line used as a feeding circuit oriented along the direction of the antenna radiation conductor. Therefore, the antenna can have a smaller area and thickness, and a larger bandwidth, which is suitable for antenna application in various bands. In addition, when applied as a digital TV antenna in a car, the broadband antenna of the invention can be attached to the car&#39;s windshield and still achieve good performance without affecting the sight of the driver and the car&#39;s appearance. 
   While the invention has been described by way of example and in terms of three preferred embodiments, it is understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.