Patent Publication Number: US-2015061949-A1

Title: Broadband antenna with adjustable resonant frequency band

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of Taiwanese Application No. 102132002, filed on Sep. 5, 2013. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a broadband antenna, more particularly to a broadband antenna with adjustable resonant frequency bands. 
     2. Description of the Related Art 
     With the rapid development of mobile communication technology toward the fourth generation of mobile phone mobile communication technology standards (4G), mobile communication devices are now required to support frequency band requirements of the 4G standards. In order to achieve broadband communication under a limited size specification, a conventional antenna as disclosed in U.S. Pat. No. 8,373,607 is configured to be capable of adjusting a resonant frequency band thereof. However, the conventional antenna generally has only one adjustable resonant frequency band with a narrow adjustable range. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a broadband antenna with multiple adjustable frequency bands that may alleviate the above drawbacks of the prior art. 
     Accordingly, a broadband antenna with adjustable resonant frequency bands of the present invention includes a grounding element, a first radiating conductor, a second radiating conductor, and a variable capacitor. 
     The first radiating conductor includes a feed-in portion and a radiating portion connected electrically to the feed-in portion. The feed-in portion and the radiating portion are spaced apart from the grounding element. The feed-in portion includes a feed-in end that is adjacent to the grounding element and that is configured to be fed with a radio frequency (RF) signal. 
     The second radiating conductor includes a short-circuit portion and a coupling portion. The short-circuit portion is connected electrically to the grounding element. The coupling portion is connected electrically to the short-circuit portion. The coupling portion is parallel to and couples with the radiating portion of the first radiating conductor. 
     The variable capacitor is connected electrically between the radiating portion of the first radiating conductor and the coupling portion of the second radiating conductor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments with reference to the accompanying drawings, of which: 
         FIG. 1  is a fragmentary schematic view of a first preferred embodiment of a broadband antenna with adjustable resonant frequency bands according to the present invention; 
         FIG. 2  is a plot showing voltage standing wave ratio (VSWR) of the first preferred embodiment; and 
         FIG. 3  is a fragmentary schematic view of a second preferred embodiment of the broadband antenna with adjustable resonant frequency bands according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before the present invention is described in greater detail, it should be noted that like elements are denoted by the same reference numerals throughout the disclosure. 
     Referring to  FIG. 1 , a first preferred embodiment of a broadband antenna  100  with adjustable resonant frequency bands according to the present invention includes a grounding element  1 , a first radiating conductor  2 , a second radiating conductor  3 , a variable capacitor (C V ) and a direct current blocking (DC-blocking) unit  4 . 
     The first radiating conductor  2  includes a feed-in portion  21 , a radiating portion  22  connected electrically to the feed-in portion  21 , and a grounding portion  23  connected electrically to the radiating portion  22 . The feed-in portion  21 , the radiating portion  22  and the grounding portion  23  are spaced apart from the grounding element  1 . The feed-in portion  21  includes a feed-in end  211  that is adjacent to the grounding element  1  and that is configured to be fed with a radio frequency (RF) signal and a direct current (DC) control signal. The radiating portion  22  extends, in a first direction (X) as shown in  FIG. 1 , from one end of the feed-in portion  21  opposite to the feed-in end  211 . The grounding portion  23  extends from the radiating portion  22  toward the grounding element  1  in a second direction (−Y) perpendicular to the first direction (X) as shown in  FIG. 1 . The radiating portion  22  is substantially perpendicular to the feed-in portion  21 , and the grounding portion  23  is substantially perpendicular to the radiating portion  22 . 
     The second radiating conductor  3  includes a short-circuit portion  31  and a coupling portion  32 . The short-circuit portion  31  is connected electrically between the grounding element  1  and the coupling portion  32 . The short-circuit portion  31  extends from the grounding element  1  in a third direction (Y) perpendicular to the first direction (X) and opposite to the second direction (−Y) as shown in  FIG. 1 . The coupling portion  32  extends in the first direction (X) from one end of the short-circuit portion  31  that is distal from the grounding element  1 . The coupling portion  32  is substantially perpendicular to the short-circuit portion  31 , and is parallel to and spaced apart from the radiating portion  22  of the first radiating conductor  2  so as to generate coupling effect therebetween. 
     The variable capacitor (Cv) is connected electrically between the radiating portion  22  of the first radiating conductor  2  and the coupling portion  32  of the second radiating conductor  3 . In this preferred embodiment, the variable capacitor (Cv) is a voltage-controlled variable capacitor, and capacitance thereof is controllable by the DC control signal. 
     The DC-blocking unit  4  is connected electrically between the grounding portion  23  of the first radiating conductor  2  and the grounding element  1 . The DC-blocking unit  4  is for blocking the DC control signal and for allowing passage of the RF signal from the feed-in end  211  of the feed-in portion  21  of the first radiating conductor  2  into the grounding element  1 . Accordingly, the DC control signal may not be grounded directly, and a loop of the RF signal may be formed between the first radiating conductor  2  and the grounding element  1 . In this preferred embodiment, the DC-blocking unit  4  is a DC-blocking capacitor (C B ) having a capacitance of 100 pF. 
     Through control of the capacitance of the variable capacitor (C V ), the coupling effect between the coupling portion  32  of the second radiating conductor  3  and the radiating portion  22  of the first radiating portion  2  may be adjusted so as to adjust resonant frequencies of the first and second radiating conductors  2 ,  3 . Referring further to  FIG. 2 , the first radiating conductor  2  resonates in a first frequency band (B 1 ), and the second radiating conductor  3  resonates in a second frequency band (B 2 ) that is lower than the first frequency band (B 1 ). The six curve lines shown in  FIG. 2  represent voltage standing wave ratios (VSWR) of the broadband antenna  100  when the capacitance of the variable capacitor (C V ) is 0 pF, 0.2 pF, 0.4 pF, 0.8 pF, 1.2 pF and 2.2 pF, respectively. 
     As illustrated in  FIG. 2 , both of the resonant frequencies of the first and second radiating conductors  2 ,  3  decrease while the capacitance of the variable capacitor (Cv) increases. That is to say, resonant frequency bands of the first and second radiating conductors  2 ,  3  are adjustable, thereby achieving broadband communication. It is noted that adjustable ranges of the resonant frequencies of the first and second radiating conductors  2 ,  3  in this preferred embodiment are greater than 250 MHz. In other words, by virtue of the variable capacitor (C V ) according to the present invention, resonant frequency bands of the first and second radiating conductors  2 ,  3  may be significantly adjusted. 
     In addition, one end of the variable capacitor (C V ) is connected electrically to an end part  221  of the radiating portion  22  proximate to the feed-in portion  21  such that a better effect of adjustment may be obtained. 
     Moreover, in other embodiments, the variable capacitor (C V ) maybe a mechanically controlled variable capacitor that is not controlled using electrical signals. As a result, the DC-blocking unit  4  may be omitted, and the grounding portion  23  of the first radiating conductor  2  may extend from the radiating portion  22  in the second direction (−Y) to connect electrically to the grounding element  1 . 
     Referring to  FIG. 3 , a second preferred embodiment of the preferred embodiment is shown to be similar to the first preferred embodiment. The differences reside in that the first radiating conductor  2 ′ does not include the grounding portion  23 , and the DC-blocking unit  4  is omitted. Specifically, the first radiating conductor  2  of the first preferred embodiment is an inverted-F antenna, and the first radiating conductor  2 ′ is a monopole antenna in this embodiment. 
     To conclude, by controlling the capacitance of the variable capacitor (C V ), the coupling effect between the coupling portion  32  of the second radiating conductor  3  and the radiating portion  22  of the first radiating conductor  2 ,  2 ′ may be adjusted, so that the resonant frequencies of the first and second radiating conductors  2 ,  2 ′,  3  of the broadband antenna  100  can be adjusted simultaneously according to the present invention. Furthermore, the adjustable ranges of the resonant frequencies of the first and second radiating conductors  2 ,  2 ′,  3  are relatively large, such that broadband communication may be achieved under the condition of a compact size. 
     While the present invention has been described in connection with what are considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.