Patent Publication Number: US-6222496-B1

Title: Modified inverted-F antenna

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to radio frequency antennas and, more particularly, to inverted-F antennas. 
     BACKGROUND OF THE INVENTION 
     Inverted-F antennas are commonly used in mobile transmitter/receivers, such as cellular telephones and wireless modems for portable computers. FIG. 1 illustrates a conventional inverted-F antenna  100 . As shown in FIG. 1, the inverted-F antenna  100  has a vertical ground  110  and a straight horizontal element  120 . Conventional inverted-F antennas, such as the inverted-F antenna  100  of FIG. 1 can be fabricated on a printed circuit board (PCB), or using a wire or plate construction, in a well-known manner. For a detailed discussion of conventional inverted-F antennas, see, for example, Kazuhiro Hirasawa and * 5  AsMisao Haneishi, “Analysis, Design, and Measurement of Small and Low-Profile Antennas,” Artech House, Norwood, Mass (1992); or Kyohei Fujimoto et al., “Small Antennas,” Research Studies Press, United Kingdom (1987), each incorporated by reference herein. 
     Inverted-F antennas are generally characterized by the distance, S, between the grounding element  110  and feeding element  130 ; the overall length, L, of the antenna  100 ; and the height, H, of the antenna  100 . Impedance matching for an inverted-F antenna is obtained by adjusting the distance, S, between the grounding and feeding elements. As the size of the devices in which inverted-F antennas are utilized has decreased, the space available for such inverted-F antennas has likewise decreased. For many applications, the distance, S, between the grounding element  110  and feeding element  130  has become so small that the tuner must be extremely sensitive. In particular, the impedance matching is very difficult or too sensitive due to the small distance, S, between the grounding  110  and the feeding elements  130 . In addition, the rectangular shape of conventional inverted-F antennas  100  does not provide sufficient mechanical strength for many applications. 
     A need therefore exists for an improved inverted-F antenna that exhibits improved impedance matching and mechanical strength. A further need exists for an improved inverted-F antenna that has a reduced overall dimension and an additional degree of freedom for tuning the impedance of the antenna. 
     SUMMARY OF THE INVENTION 
     Generally, a modified inverted-F antenna is disclosed that improves on conventional designs by incorporating a sloped grounding element at a fixed end of the horizontal element and a downward bend at a loose end of the horizontal element. According to one aspect of the invention, the sloped grounding element is connected in a triangular configuration with the feeding element and a ground plane of the antenna, to provide additional benefits. First, the triangular shape of the present invention decreases the distance, D, between the grounding plane and the feeding element relative to a conventional rectangular connection. Thus, the present invention exhibits improved impedance matching characteristics. The distance, D, between the grounding plane and the feeding element can be expressed as follows: 
     
       
         D={square root over (H 2 +L +S 2 +L )}. 
       
     
     where H is the height of the antenna and S is the horizontal spacing between the feeding element and where the sloped grounding element connects to the grounding plane. 
     In addition, the triangular shape provides increased mechanical strength relative to a conventional rectangular connection. According to another feature of the invention, the downward bend at the loose end of the antenna can be adjusted to thereby further adjust the impedance matching of the antenna. 
     The sloped grounding element and downward bend features of the modified inverted-F antenna also serve to reduce the overall dimension of the antenna. The total length, L T , of the disclosed antenna device can be expressed as follows: 
     
       
         L T ={square root over (H 2 +L +S 2 +L )}+L 1 {square root over (B h   2 +L +B V   2 +L )}. 
       
     
     where H is the height of the antenna, S is the horizontal spacing between the feeding element and point where the sloped grounding element connects to the grounding plane, L 1  is the length of a horizontal portion of said horizontal element, B v  is the vertical distance of said downward bend and B h  is the horizontal distance of said downward bend. 
     A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a conventional inverted-F antenna; 
     FIG. 2 illustrates a modified inverted-F antenna in accordance with the present invention; 
     FIGS. 3A and 3B illustrate a side and top view, respectively, of an implementation of a modified inverted-F antenna in accordance with the present invention; and 
     FIG. 4 illustrates the Voltage Standing Wave Ratio (VSWR) of the modified inverted-F antenna of FIGS. 3A and 3B on a small ground plate. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 2 shows the general configuration of a modified inverted-F antenna  200  in accordance with the present invention. As shown in FIG. 2, the modified inverted-F antenna  200  has a horizontal element  210  that includes a sloped grounding element  220  and a downward bend  230  that ensure the robustness of the antenna  200 . The a loped grounding element  220  at the fixed end of the inverted-F antenna  200  decreases the distance, D, between the grounding plane  240  and the feeding element  250 . The distance, D, between the grounding plane  240  and the feeding element  250  can be obtained as follows: 
     
       
         D={square root over (H 2 +L +S 2 +L )}. 
       
     
     Thus, unlike conventional inverted-F antennas, such as the antenna  100  shown in FIG. 1, the decreased distance to ground, D, of the modified inverted-F antenna  200  avoids impedance matching difficulties due to very small values of S. In addition, the triangular shape formed by the sloped grounding element  220 , the feed line  250  and the ground plane  240  provides increased mechanical strength for the antenna  200 . 
     As shown in FIG. 2, a downward bend  230  is used at the loose end of the inverted-F antenna  200 . The downward bend  230  serves two purposes. First, the bending  230  can change the impedance matching, and thereby provides another mechanism to tune the impedance of the antenna  200 . Second, the bending  230  will reduce the overall dimension occupied by the antenna  200 . As previously indicated, the overall dimension is very important for some applications, especially mobile applications. 
     Similar to the conventional inverted-F antenna  100  discussed above, the resonate frequency of the modified inverted-F antenna  200  is primarily determined by the total length of the antenna. Thus, the total length, L T , of the conventional inverted-F antenna  100  is obtained as follows: 
     
       
         L T =H+S+L. 
       
     
     Likewise, the total length, L T , of the modified inverted-F antenna  200  is obtained as follows: 
     
       
         L T ={square root over (H 2 +L +S 2 +L )}+L 1 +{square root over (B h   2 +L +B v   2 +L )}. 
       
     
     It is noted that increasing the height, H, of the antenna  200  will increase the antenna bandwidth. Thus, given an antenna height, H, the spacing, S, is adjusted to achieve impedance matching. 
     FIGS. 3A and 3B show a side view and a top view, respectively, of an implementation of a modified inverted-F antenna  300  stamped from a metal sheet, such as brass or copper. The two small bents  360 ,  370  at the bottom of the antenna  300  are used as soldering points. In this manner, the antenna  300  can be soldered to a printed circuit board (PCB) or some other metal structures. It is noted that the design of the implementation of FIGS. 3A and 3B only requires two soldering points. As shown in FIG. 3B, the width, W 1 , of the sloped grounding element  320  and the overall width, W, of the antenna  300  can be adjusted for maximum impedance bandwidth within given space availability. 
     FIG. 4 shows the Voltage Standing Wave Ratio (VSWR)  400  of the antenna  300 . With a proper design, a 2:1 frequency bandwidth can be as wide as 300 MHz, which is wide enough for 2.4 GHz ISM applications. The 2.4 GHz band is centered at 2.45 Ghz with a 100 MHz bandwidth. 
     It has been found that the total radiation pattern of the modified inverted-F antennas  200  of the present invention are close to omnidirectional. 
     It is to be understood that the embodiments and variations shown and described herein are merely illustrative of the principles of this invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.