Patent Publication Number: US-6339402-B1

Title: Low profile tunable circularly polarized antenna

Description:
This application claims the benefit of U.S. Provisional Application No. 60/171,765 filed Dec. 22, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an antenna assembly suitable for wireless transmission of analog and/or digital data, and more particularly to an antenna assembly for providing a conformal circularly polarized antenna. 
     BACKGROUND OF THE INVENTION 
     Recent advances in wireless communications devices have renewed interest in antennas suitable for such systems. Several factors are usually considered in selecting an antenna for a wireless telecommunications device. Significant among these factors are the size, the bandwidth, and the radiation pattern of the antenna. 
     Currently, monopole antennas, patch antennas and helical antennas are among the various types of antennas being used in wireless communications devices. These antennas, however, have several disadvantages, such as limited bandwidth and large size. Also, these antennas exhibit significant reduction in gain at lower elevation angles (for example, 10 degrees), which makes them undesirable in some applications. 
     One type of antenna is an external half wave single or multi-band dipole. This antenna typically extends or is extensible from the body of a wireless communication device in a linear fashion. Because of the physical configuration of this type of antenna, electromagnetic waves radiate equally toward and away from a user. Thus, there is essentially no front-to-back ratio and little or no specific absorption rate (SAR) reduction. With multi-band versions of this type of antenna, resonances are achieved through the use of inductor-capacitor (LC) traps. With this antenna, gains of +2 dBi are common. While this type of antenna is acceptable in some wireless communication devices, it has drawbacks. One significant drawback is that the antenna is external to the body of the communication device. This places the antenna in an exposed position where it may be accidentally or deliberately damaged. 
     A related antenna is an external quarter wave single or multi-band asymmetric wire dipole. This antenna operates much like the aforementioned antenna, but requires an additional quarter wave conductor to produce additional resonances. This type of antenna has drawbacks similar to the aforementioned antenna. 
     Yet another type of antenna is a Planar Inverted F Antenna (PIFA). A PIFA derives its name from its resemblance to the letter “F” and typically includes various layers of rigid materials formed together to provide a radiating element having a conductive path therein. The various layers and components of a PIFA are typically mounted directly on a molded plastic or sheet metal support structure. Because of their rigidity, PIFAs are somewhat difficult to bend and form into a final shape for placement within the small confines of radiotelephones. In addition, PIFAs may be susceptible to damage when devices within which they are installed are subjected to impact forces. Impact forces may cause the various layers of a PIFA to crack, which may hinder operation or even cause failure. Various stamping, bending and etching steps may be required to manufacture a PIFA because of their generally non-planar configuration. Consequently, manufacturing and assembly is typically performed in a batch-type process which may be somewhat expensive. In addition, PIFAs typically utilize a shielded signal feed, such as a coaxial cable, to connect the PIFA with the RF circuitry within a radiotelephone. During assembly of a radiotelephone, the shielded signal feed between the RF circuitry and the PIFA typically involves manual installation, which may increase the cost of radiotelephone manufacturing. 
     SUMMARY OF THE INVENTION 
     An antenna assembly for a wireless communications device. The antenna assembly is mountable onto a printed wiring board (PWB) and consists of first and second conducting elements. The first conducting element is both capacitively coupled via a matchable shunt and operatively connected to a ground plane of the PWB, while the second conducting element is operatively connected to the ground plane of the PWB at two locations. The first and second conducting elements are operatively connected to each other by a tunable bridge capacitor to form orthogonal magnetic dipole elements. The antenna assembly provides substantially circular polarization within a hemisphere by virtue of the geometry and orientation of the two magnetic dipole elements which are fed with equal amplitude, but in-phase quadrature. The matchable shunt acts as an impedance transformer to yield a low voltage standing wave ratio (VSWR) of less than two-to-one at the operating frequency. The antenna assembly includes a single feed point which is capacitively coupled to and in parallel with the matchable shunt to ensure that the magnet dipole elements do not present a direct current (DC) ground to any radio frequency (RF) circuit connected to the antenna assembly. The single feed point permits RF energy to be distributed to both conducting elements without a required power splitter or phase shifter(s). 
     It is an object of the present invention to provide an antenna assembly which may be incorporated into a wireless communication device. 
     It is another object of the present invention to provide polarization diversity which can enhance radio performance in multipath environments, such as inside buildings or within metro areas. 
     It is yet another object of the present invention to provide frequency agility by adjusting the value of a bridge capacitor. 
     It is a further object of the present invention to enhance operation of an antenna assembly over a range of frequencies. 
     A feature of the present invention is the provision of orthogonally oriented magnetic dipole elements. 
     Another feature of the present invention is that there is a single feed point for radio frequencies. 
     Another feature of the present invention is that the antenna assembly is tunable over a range of frequencies. 
     An advantage of the present invention is that the antenna assembly has a low profile which enables it to be used in small articles such as wireless communication devices. 
     Another advantage of the present invention is that various components of a transciever device may be positioned within interior regions of the antenna assembly to reduce the overall size of the electronic device. 
     These and other objects, features and advantages will become apparent in light of the following detailed description of the preferred embodiments in connection with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a wireless communication device incorporating an antenna assembly according to the present invention; 
     FIG. 2 is a fragmentary perspective view of the antenna assembly according to the present invention; 
     FIG. 3 is a fragmentary top plan view of the antenna assembly according to the present invention; 
     FIG. 4 is a an end view of the antenna assembly according to the present invention; 
     FIG. 5 is a plan view of another embodiment of the first and second conducting elements of the antenna assembly of the present invention prior to forming and attaching onto the ground plane of a printed wiring board. 
     FIG. 6 is a fragmentary perspective view of the antenna assembly of the present invention illustrating a first magnetic dipole element; and, 
     FIG. 7 is a fragmentary perspective view of the antenna assembly of the present invention illustrating a second magnetic dipole element. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like numerals depict like parts throughout, FIG. 1 illustrates a wireless communications device  10 , such as a cellular telephone, utilizing an antennas assembly  20  according to the present invention. As depicted, the antenna assembly  20  is disposed at an upper corner of a printed wiring board (PWB)  12  which, in turn, is positioned so that the antenna assembly is adjacent the top  18  and projects away from the front surface  16  of the wireless communications device  10 . 
     As depicted in FIG. 2, the antenna assembly  20  is comprised of two main portions, a first conducting element  22  and a second conducting element  42 . The first conducting element  22  includes a first conductive surface  24  which is coupled at two regions to ground plane  14  of the printed wiring board  12  by first and second leg elements  26 ,  36 . The first leg element  26  extends between the first conductive surface  24  and the ground plane  14  in a generally orthogonal orientation. The leg element  26  includes a foot  28 . Dielectric element  30  is disposed between the foot  28  and the ground plane  14 . Together foot  28 , dielectric element  30  and ground plane  14  form a shunt matching capacitor  32 . Shunt capacitor  32  could alternatively be a discrete capacitor coupled between the ground plane  14  and the leg element  26 . For GPS frequencies (1575 MHz), the shunt matching capacitor has a capacitance value of around 1.0 pF. 
     The second leg element  36  extends between and operatively connects the first conductive surface  24  and the ground plane  14 . In the preferred embodiment, and as best depicted in FIG. 4, the second leg element  36  is diagonally oriented with respect to the conductive surface  24  and the ground plane  14 . The diagonal orientation of the second leg element  36  may be varied depending on the particular application, e.g., a different device housing, etc. Together, the first and second leg elements  26 ,  36  position the first conductive surface  24  of the first conducting element  22  at a predetermined distance or spaced relation from the ground plane  14  of the printed wiring board  12 . Note that in doing so, an interior region  40  is defined. This interior region  40  may be used to receive various components of the wireless communication device to form a more compact structure. 
     The conductive surface  24  also includes a feed point  34  which is coextensive with the plane of the first conductive element  24  and which extends away therefrom towards the second conductive surface  44  of the second conducting element  42 . The feed point  34  is operatively connected via a conductive post or other conductor to a radio frequency (RF) signal connection or port on the printed wiring board  12 . Preferably, the feed point  34  is capacitively coupled to ensure that the magnetic dipole elements do not present a DC ground to any RF circuit connected thereto. In operation, RF energy is distributed to both of the conducting elements  22 ,  42  without the need of a power splitter of phase shifter(s). 
     The second conducting element  42  includes a second conductive surface  44  which is operatively connected at two points to ground plane  14  of the printed wiring board  12  by a leg element  46  and a conducting member  70 . In the preferred embodiment and as best depicted in FIG. 4, leg element  46  is diagonally oriented with respect to the conductive surface  44  and the ground plane  14 . The leg element  46  positions the second conductive surface  44  of the second conducting element  42  a predetermined distance or spaced relation from the ground plane  14  of the printed wiring board  12 . Note that in positioning the second conductive surface  44  a predetermined distance from the ground plane  14 , an interior region  50  is defined as illustrated in FIG.  2 . As with the interior region  40 , this interior region  50  may be used to house various components of the wireless communication device to form a more compact structure. 
     The second conducting element  42  is also operatively connected to the ground plane  14  by a conductive connecting member  70  and forms one of the electromagnetic dipole elements. 
     The connecting member  70  may be located at other locations, however, as will be appreciated by one skilled in the arts this may alter the operating characteristics of the antenna assembly as a whole. 
     As can be seen, the first and second conductive surfaces  24 ,  44  of the first and second conducting elements  22 ,  42  are capacitively coupled to each other by a bridge capacitor  60 . The bridge capacitor  60  has a tuning range of ±30%. For GPS frequency operation, the bridge capacitor has a capacitance value of around 0.65 pF and an adjustable range of around 0.3-0.9 pF to yield the aforementioned ±30% bandwidth. 
     Referring now to FIGS. 3 and 4, it can be seen that the first conductive surface  24  is generally rectangular and substantially planar. However, the first conductive surface  24  may assume other configurations. For example, they could trapezoidal, circular, etc. ; or they may have different thicknesses; or they may be non-planar; or the feed point may be angled and/or non-aligned with the first conductive surface. 
     As seen in FIG. 2, the first leg element  26  includes a foot  28  which is adjacent a dielectric element  30 , with the foot  28 , dielectric element  30  and the ground plane  14  forming a shunt matching capacitor  32 . The dielectric element  30  is of conventional material having a dielectric constant of between 1.0 and 1.0, and preferably around 3.0. The shunt matching capacitor  32  acts as an impedance transformer to yield a low voltage standing wave ratio (less than 2:1) at the operating frequency (1575.42 MHz). Alternative capacitor structures or types may also be appreciated. 
     As illustrated in FIG. 4, the second leg element  36  of the first conducting element  22  extends generally diagonally in a plane perpendicular to the ground plane  14  to an attachment point  38  located at a corner portion of the printed wiring board  12 . 
     The second conductive surface  44  of the second conducting element  42  is positioned a predetermined distance from the first conductive surface  24  so that there is a gap therebetween. Preferably, the second conductive surface  44  is trapezoidal, planar and aligned with the first conductive surface  24 , as shown in FIGS. 2 and 3. However, the second conductive surface  44  may assume other configurations as discussed above for the first conductive surface  24 . 
     Again referring to FIG. 4, and as with the second leg element  36  of the first conducting element  22 , the leg element  46  of the second conducting element  42  extends generally diagonally in a plane perpendicular to the ground plane  14  to an attachment point  48  located at a corner portion of the printed wiring board  12 . 
     FIG. 5, in conjunction with Table 1, discloses dimensions for a preferred embodiment of the antenna assembly of the present invention. This figure depicts the conducting elements  22 ,  42  as they may appear during the process of formation by stamping, after initial separation from a blank of material such as brass, but prior to the steps of bending the leg elements and the foot to the desired orientations, and attaching the conducting elements to the printed wiring board. A variety of other conductive materials may be utilized to form the conducting elements  22 ,  42 , including but not limited to, sheet metal elements, plated plastic or dielectric elements, selectively etched structures, etc. Here, the angled leg elements  36 ,  46  can be readily discerned. After the conducting element  22 ,  42  have been separated from a sheet of material, they are formed to the desired shape by manipulation along bend lines  54 ,  56 ,  64  and  66 . Note that the end portions  58 ,  68  formed at the end of leg elements  36 ,  46  may be manipulated along bend lines  56 ,  66 , respectively, to form feet which are attached to the ground plane or they may be left alone and the end elements are attached to the edge of the printed wiring board in a conventional manner (not shown). Although the preferred material used in the conducting elements is patterned brass having a thickness of around 0.020 inch, it will be appreciated that other materials may be used. Although the preferred method of fabrication is a single piece metal stamping adaptable to high volume production, it is understood that other methods of fabrication may be used, including but not limited to injection molding over conductive surfaces, etc. 
     Particular dimension for the embodiment of FIG. 5 according to the present invention are included as Table 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Dimension 
                 Inch 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 a 
                 0.263 
               
               
                   
                 b 
                 1.575 
               
               
                   
                 c 
                 0.240 
               
               
                   
                 d 
                 0.125 
               
               
                   
                 e 
                 0.200 
               
               
                   
                 f 
                 0.120 
               
               
                   
                 g 
                 0.245 
               
               
                   
                 h 
                 0.195 
               
               
                   
                 I 
                 0.278 
               
               
                   
                 j 
                 0.102 
               
               
                   
                 k 
                 0.067 
               
               
                   
                 l 
                 0.255 
               
               
                   
                 m 
                 0.340 
               
               
                   
                 n 
                 0.411 
               
               
                   
                   
               
            
           
         
       
     
     Generally, it should be noted that the antenna assembly as depicted in the preferred embodiments is for a right hand circularly polarized global positioning satellite (GPS) operating at a frequency of 1575.42 MHz, with overall dimensions of 1.14 inches in length, by 0.79 inches in width, and 0.45 inches in height. As mounted on a corner of a printed wiring board (PWB), the antenna assembly yields a right hand circular polarization with hemispherical coverage and an axial ration of 2.5 dB at the zenith. 
     FIGS. 6 and 7 illustrate the first and second magnetic dipole elements  80 ,  90  that are formed as part of the antenna assembly. In FIG. 6, the first magnetic dipole element  80  is depicted as a dashed line which follows a circuit defined by the first conductive surface  24  and the second leg element  36  of the first conducting element  22 , the ground plane  14  of the printed wiring board  12 , the leg element  46  and the second conductive surface  44  of the second conducting element  42 , and the bridge capacitor  60 . The first magnetic dipole element  80  thus formed defines two substantially orthogonally oriented planes. 
     In FIG. 7, the second magnetic dipole element  90  is depicted as a dashed line which follows a circuit defined by the second conductive surface  44  and the leg element  46  of the second conducting element, the ground plane  14  of the printed wiring board  12 , and the conducting member  70 . The second magnetic dipole element  90  thus formed defines a third plane which is substantially orthogonal to the planes of the first magnetic dipole element  80 . 
     Additional advantages and modifications will readily occur to those skilled in the art. 
     The invention in its broader aspects is, therefore, not limited to the specific details, representative apparatus and illustrative examples shown and described. Accordingly, departures from such details may be made without departing from the spirit or scope of the applicant&#39;s general inventive concept.