Patent Publication Number: US-7724196-B2

Title: Folded dipole multi-band antenna

Description:
FIELD OF THE INVENTION 
     This invention relates in general to wireless devices, and more particularly, to an internal multi-element multi-band antenna for use in hand-held devices. 
     BACKGROUND OF THE INVENTION 
     Wireless communication is the transfer of information over a distance without the use of electrical conductors or “wires.” This transfer is actually the communication of electromagnetic waves between a transmitting entity and remote receiving entity. The communication distance can be anywhere from a few inches to thousands of miles. 
     Wireless communication is made possible by antennas that radiate and receive the electromagnetic waves to and from the air, respectively. The function of the antenna is to “match” the impedance of the propagating medium, which is usually air or free space, to the source that supplies the signals sent or interprets the signals received. 
     Antenna designers are constantly balancing antenna size against antenna performance. Unfortunately, these two characteristics are generally inversely proportional. To make matters more difficult, consumers are now favoring cellular phones with internal antennas. The ever-shrinking size of cellular phones leaves little space inside the phone for these antennas. To add even more complexity to this communication problem, phones are needed that offer communication in multiple modes and in multiple frequency ranges, requiring multiple and differening antenna elements within the phone. With the reduction in antenna element real estate, communication performance suffers. 
     Therefore, a need exists to overcome the problems with the prior art as discussed above. 
     SUMMARY OF THE INVENTION 
     A loop antenna, in accordance with an embodiment of the present invention includes a ground plane and a conductive element with a first C-shaped element portion having an open end and a closed end, with only the open end extending directly above a first portion of the ground plane, a second C-shaped element portion having an open end and a closed end, with only the open end extending directly above a second portion of the ground plane, and a transmission line element disposed between the first C-shaped element portion and the second C-shaped element portion and positioned directly above a third portion of the ground plane. 
     In accordance with another feature of the present invention, the first C-shaped element portion has a first end, the second C-shaped element portion has a second end and the transmission line element is in a series connection between the first end of the first C-shaped element portion and the second end of the second C-shaped element portion. 
     In accordance with a further feature of the present invention, the first C-shaped element portion is symmetrical with the second C-shaped element portion. 
     In accordance with a yet another feature, the present invention includes a stub element coupled to the conductive element at a feedpoint of the conductive element and one of generally follows the shape of one of the C-shaped element portions and meanders in a proximity of one of the C-shaped element portions. 
     In accordance with a yet another feature, the present invention includes a handset supporting and containing the element and the ground plane, the handset having a first side to face a user&#39;s head during use and a second side to face a user&#39;s hand during use, wherein the ground plane is disposed between the first side and the second side and the element is disposed between the first side and the ground plane. 
     The present invention, according to an embodiment, is a wireless communication device that includes a first side with user-interactable components, a second side to be supported by a user&#39;s hand, a ground plane disposed between the first side and the second side and having outer edges, and a conductive element located between the first side and the ground plane. The conductive element includes a first element portion forming a C-shape with an open end directly above a first portion of the ground plane and a closed end extending beyond at least one edge of the ground plane, a second element portion forming a C-shape with an open end directly above a second portion of the ground plane and a closed end extending beyond at least one edge of the ground plane, and a transmission line element connecting the first element portion to the second element portion and positioned directly above a third portion of the ground plane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views, and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention. 
         FIG. 1  is a perspective view of a prior-art cellular device. 
         FIG. 2  is a perspective view of a dual-element/multi-frequency band antenna, according to an embodiment of the present invention. 
         FIG. 3  is a fragmentary, partially hidden, perspective view of the dual-element/multi-frequency band antenna of  FIG. 2  diagrammatically placed within a cellular communication device, according to an embodiment of the present invention. 
         FIG. 4  is a perspective view of the cellular communication device and internal dual-element/multi-frequency band antenna of  FIG. 3  placed within a “C” block, according to an embodiment of the present invention. 
         FIG. 5  is a perspective view of a dual-element/multi-frequency band antenna, according to another embodiment of the present invention. 
         FIG. 6  is a graph showing the performance of the present invention over the Cellular and GPS frequency bands. 
         FIG. 7  is a set of graphs showing the frequency response of the present invention vs. the frequency response of a prior art internal λ/4 wire antenna when placed in the C-block of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention. 
     The terms “a” or “an”, as used herein, are defined as one or more than one. The term “plurality”, as used herein, is defined as two or more than two. The term “another”, as used herein, is defined as at least a second or more. The terms “including” and/or “having”, as used herein, are defined as comprising (i.e., open language). The term “coupled”, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically. 
     The present invention provides a novel and efficient multi-band antenna structure that includes an asymmetrical λ/2 (half wavelength) folded dipole element and a λ/4 (quarter wavelength) resonant stub element. The elements share a common feeding point and utilize a common grounding plane. The invention is advantageous in that it allows for a reduction of the area normally needed for a λ/2 antenna element, without interfering with Radio Frequency (RF) performance. 
     An antenna is a transducer designed to transmit or receive radio waves, which are a class of electromagnetic waves. In other words, antennas convert radio frequency electrical currents into electromagnetic waves, and vice versa. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, wireless LAN, radar, and space exploration. 
     Physically, an antenna is a conductor that generates a radiating electromagnetic field in response to an applied alternating voltage and the associated alternating electric current. Alternatively, an antenna can be placed in an electromagnetic field so that the field will induce an alternating current in the antenna and a voltage between its terminals. It is through these antennas that electronic wireless communication is made possible. 
     The electromagnetic (EM) “spectrum” is the range of all possible electromagnetic radiation. This spectrum is divided into frequency “bands,” or ranges of frequencies, that are designated for specific types of communication. Many radio devices operate within a specified frequency range, which limits the frequencies on which the device is allowed to transmit. The lower and upper-bound frequencies are the points at which signal strength of the device falls off by 3 dB. 
     EM energy at a particular frequency (f) has an associated wavelength (λ). The relationship between wavelength and frequency is expressed by:
 
λ= c/f  
 
where c is the speed of light (299,792,458 m/s). It therefore follows that high-frequency EM waves have a short wavelength and low-frequency waves have a longer wavelength.
 
     The Integrated Digital Enhanced Network (iDEN) is a mobile telecommunications technology, developed by Motorola, Inc., of Schaumberg, Ill., which provides its users the benefits of a trunked radio and a cellular telephone. iDEN places more users in a given spectral space, as compared to analog cellular and two-way radio systems, by using speech compression and Time Division Multiple Access (TDMA). iDEN is designed and licensed to operate in the frequency band starting at 806 MHz up to and including 941 MHz. In addition to the iDEN band there are other bands in the 825-960 MHz frequency range that are used by cellular systems. For purposes of lexography the combined range of frequencies from 806-960 MHz will be designated as the “cellular band.” The present invention provides a λ/2 antenna element that efficiently operates in the cellular band frequency range. 
     The Global Positioning System (GPS) is currently the only fully-functional Global Navigation Satellite System (GNSS). Utilizing a constellation of at least 24 Earth orbiting satellites that transmit precise microwave signals, the GNSS enables a GPS receiver to determine its location, speed, and direction. The present invention includes a GPS element that that is tuned to receive GPS signals at the frequency of 1575.42 MHz and can also be tuned (if desired) to the 1800-1990 MHz Personal Communications System (PCS) frequency bands. 
       FIG. 1  shows a cellular phone  100 , also referred to as a “handset.” The phone  100  has user-interactable components, such as a keypad  102  for dialing numbers and entering characters and digits, a display screen  106 , and a selection pad  104  for making selections, entering responses, navigating graphical user interface menus, and otherwise interacting with the device  100 . The view of the phone  100  in  FIG. 1  shows a first side  108  of the phone that is intended to be placed against a user&#39;s head during use. The cellular phone  100  has a speaker  110  and a microphone  112  and is intended to be oriented so that, in use, the microphone  112  is positioned in proximity to the user&#39;s mouth and the speaker  110  is positioned in proximity to the user&#39;s ear. The back side of the phone  100 , which cannot be seen in the view in  FIG. 1 , is the side of the phone  100  that faces the user&#39;s hand during use. 
     The particular cellular phone  100  is a well-known “clamshell” device, where the term “clamshell” refers to the way the phone  100  folds  114  and places the display screen  106  directly above the keypad  102  when closed. This folding feature not only makes the device smaller and easily transportable, it also protects the display screen from damage. The present invention, however, is not limited to clamshell designs or to any particular type or configuration of cellular phone. 
       FIG. 2  shows a first embodiment of an antenna in accordance with the present invention. The antenna  200  includes a ground plane  202  and a first cellular band element  204  spaced above the ground plane  202  with portions of the cellular band element  204  positioned directly above the ground plane  202  and other portions extending away from the ground plane  202 . The term “directly above,” as used herein, is defined as intersecting with any line in a set of lines that are orthogonal to the surface of the ground plane  202  and extend from a perimeter of the ground plane  202 . 
     The element  204  can be of any suitable radiating material. The cellular band element  204  includes two generally C-shaped element portions  206  and  208 , which, together, efficiently operate in frequency ranges covering the cellular band. A C-shaped portion, as defined herein, is any shape where two points along the length of the element cross a single plane, with a curved portion being disposed between the two points. In addition to the curved portion, the length between the two points that cross the plane can also include one or more line segments or other curves. 
     Each of the generally C-shaped portions  206  and  208  is oriented so that the open end  218  of the C-shape is positioned directly above a portion of the ground plane  202  and the closed, or curving, part  220  of the C-shape extends away from the ground plane  202 . More specifically, the open end  218  of the first C-shaped portion  206  extends above a first portion of the ground plane  202 , the first portion of the ground Diane being that part of the around plane  202  shown in  FIG. 2  that overlaps with the dotted line indicating the first C-shaped portion  206 . Likewise, the open end of the second C-shaped portion  208  extends above a second portion of the ground plane  202 . The second portion of the ground plane is that Part of the ground plane  202  shown in  FIG. 2  that overlaps with the dotted line indicating the second C-shaped portion  208 . In this configuration, only part of the cellular band element  200  feels the full effect of the ground plane  202 . The cellular band element  200  has a feed point  212 , where the element  200  is energized, and a ground point  214 , where the element  200  is shorted to the ground plane  202 . The ground point  214  is the only place where the element  204  makes direct electrical contact with the ground plane  202 . 
     The ground plane  202  is defined as “partial” because it is smaller than the element  204  to which it is coupled in the region where the antenna element portions reside. The ground plane  202  is further defined as having a connecting end  222  where the antenna  200  connects to the ground plane  202  and an opposite end  224  that extends over a region beyond the antenna element  204 . In the region  224  beyond the antenna elements  204 , the ground plane  202  can take on any arbitrary shape and can be larger than the antenna element. 
     A transmission line element  210  is provided between the first C-shaped portion  206  and the second C-shaped portion  208  and is positioned so that the entire transmission line element  210  is directly above a third portion of the ground plane  202 . The third portion of the ground plane  202  is shown in  FIG. 2  as that portion of the ground plane defined by the dashed line indicating transmission line element  210  and located between the first portion of the ground plane  202  and the second portion of the ground plane  202 . As can be seen in  FIG. 2 , the transmission line element  210  is sandwiched between the two C-shaped element portions  206  and  208  and is in an electrical series path between the feed point  212  and the ground point  214 . 
     The transmission line element  210  is a reactive distributive element that provides length to the overall element  200  as well as electromagnetic coupling to the ground plane  202 . The added length provided by the transmission line element  210  extends the overall element length closer to the desirable λ/2 dimension. The electromagnetic coupling provided by the transmission line element  210  also makes the element electrically appear taller than it is, which helps “match” the antenna element  200  to the impedance of air. In this particular embodiment, the transmission line element  210  is rectangular in shape, although the invention is not so limited and can be, for example, square or curved. 
     In the particular embodiment shown in  FIG. 2 , the two general C-shaped portions  206  and  208  are substantially mirror symmetrical. However, symmetry between the C-shapes is not necessary and the present invention is not so limited. The dotted lines in  FIG. 2  generally defines the boundaries of the C-shapes, but are not meant to be exact. 
       FIG. 3  shows a partially hidden perspective view of a phone  300  with an antenna  200  located internally within the phone  300 , the antenna being illustrated only diagrammatically. As shown in detail in  FIG. 2 , the exemplary embodiment of the antenna  200  is a folded dipole implemented on top of a partial ground plane  202 . The primary resonance, Transmission Line Mode (TLM), covers the low band. The primary resonance or TLM is defined as the mode of operation where the current distribution along the structure exhibits one maximum and the feed point impedance is real or resistive. This establishes the lowest frequency of operation for the antenna which is designated as the low band. In this embodiment, the low band constitutes the iDEN frequencies ranging from 806-941 MHz, however, the invention is not so limited. The antenna can also be configured to cover the GSM bands from 825-960 MHz. This particular configuration (λ/2 size and positioning within the phone) presents advantages over traditional λ/4 antennas. One advantage is that the antenna excites fewer currents on the chassis of the radio  300  and is, therefore, subjected to less detuning from handling of the phone  300 . Another advantage arises from positioning the antenna  200  near the front side  108  of the phone  300  backed by the “partial” ground plane  202 , which subjects the antenna  200  to less power dissipation caused by a user&#39;s hand due to increased distance and isolation from the user&#39;s palm on the back side of the phone  300 . 
     Table 1 below presents the results of a simulation that compares radiation and system efficiency between the folded dipole antenna of the present invention and a prior-art internal λ/4 wire antenna. The comparison is performed, as is shown in  FIG. 4 , by placing the phone  400  in a C-shaped block  402  that surrounds the sides  404  and  406  and back (not shown) of a lower portion  408  of the phone  400 . The C-shaped block  402  is configured to mimic or simulate loading caused by the presence of a user&#39;s hand, i.e., the Dispatch Position. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Radiation 
                   
                   
               
               
                 Antenna 
                 Efficiency (%) 
                 System Efficiency (%) 
                 RL (dB) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Folded Dipole 
                 31.77 
                 30.92 
                 −15 
               
               
                 P.A. λ/4 wire antenna 
                 21.46 
                 15.12 
                 −5.3 
               
               
                   
               
            
           
         
       
     
     There are two metrics that quantify antenna performance for cellular phones. One metric is Radiation efficiency defined as the radiated efficiency of the antenna excluding mismatch loss. The radiation efficiency metric indicates mainly the effect of detuning and dissipation from a user&#39;s hand. The second metric is System efficiency which is the radiation efficiency including mismatch loss. System efficiency indicates the effect of mismatch loss to the antenna. The simulation comparison in the Dispatch Position shows that the folded dipole of the present invention provides an increased radiation efficiency of 1.7 dB (10*LOG (31.77/21.46)) over the prior art internal λ/4 wire antenna design. The folded dipole of the present invention also provides an increased system efficiency of 3.1 dB (10*LOG(30.92/15.12)) over the prior art internal λ/4 wire antenna design.  FIG. 7  shows this frequency response  704  of the present invention operating in the C-block  402  compared to the frequency response  702 , of a prior-art the prior art internal λ/4 wire antenna operating in the C-block  402 . 
     Referring now back to  FIG. 2 , a second element  216  is included and is part of the antenna  200 . The second element  216  is a λ/4 resonance stub that is tuned to efficiently receive data at the GPS frequency of 1575.42 MHz. As can easily be seen in  FIG. 2 , the GPS element  216  is also fed at the feed point  212 . The second element  216  extends out away from the ground plane  202  for part of its length and then returns back over the ground plane  202 , generally following the C-shaped of the second general C-shaped portion  208 . The extension away from and then back towards the ground plane  202  provides a variable coupling with the ground plane  202 . 
     The second element  216  is connected to element  204  at the feed point  212  and is electromagnetically coupled to element  204  along its length to match the impedance of element  216  to the desired feed point impedance (typically 50 Ohm). Element  216  can also be a constructed with a meandering conductor in the region of an outer edge of area  208 , which is electromagnetically coupled to element  204  and whose overall electrical length is λ/4 at the GPS frequency. The term “meandering,” as user herein, means a winding path or course. Element  216  can also be disposed above element  204 . 
       FIG. 6  shows an exemplary multi-band frequency response graph of the present invention, as tested. The graph shows that the efficiency of the inventive antenna  200  advantageously peaks in the cellular band and again in the GPS band and has nulls outside of these frequency bands. 
       FIG. 5  shows another embodiment of the present invention. In  FIG. 5 , a first element  506  of the antenna  500  is fed through an input  502  that is adjacent, but electrically isolated from, a partial ground plane  504 . The ground plane  504  is defined as “partial” because it is, similar to ground plane  202  in  FIG. 2 , smaller than the element to which it is coupled. 
     An element  506  is positioned directly above the ground plane  504  and is spaced away from the ground plane  504 , but extends beyond the ground plane  504  on both sides. The element  506  can be made of any suitable radiating material. The element  506  includes two generally C-shaped portions  508  and  510  substantially defining operation in frequency ranges covering the cellular band. In this embodiment, there is no discontinuity between the two generally C-shaped portions  508  and  510 . The dotted lines in  FIG. 5  generally defines the boundaries of the C-shapes, but are not meant to be exact. 
     Each of the generally C-shaped portions  508  and  510  is oriented so that the open end of the C-shape is positioned directly above the ground plane  504  and the closed, or curving, part of the C-shape extends away from the ground plane  504 . In this configuration, only part of the element  506  feels the affect of the ground plane  504 . The antenna  500  also has a ground point  512 , where the element  506  is shorted to the ground plane  504 . 
     A transmission line element  514  is provided between the first general “C” shape portion  508  and the second general “C” shape portion  510  and is positioned so that the entire transmission line element  514  is directly above the ground plane  504 . In the embodiment illustrated, the transmission line element  514  is rectangular, but the invention is not so limited and can be, for example, square or curved. As can be seen in  FIG. 5 , the transmission line element  514  is provided in series between the feed point  502  and the ground point  512  and is sandwiched between the two general “C” shaped portions  508  and  510 . 
     The transmission line element  514  is a reactive distributive element that provides length to the overall element  506  as well as electromagnetic coupling to the ground plane  504 . The added length provided by the transmission line element  514  extends the overall element length closer to the desirable λ/2. The electromagnetic coupling provided by the transmission line element  514  also makes the element electrically appear taller than it is, which helps “match” the antenna element  506  to the impedance of air. 
     One noticeable difference from the embodiment of  FIG. 2  is that in the embodiment of  FIG. 5 , the first general C-shaped portion  508  of the element  506 , the second general C-shaped portion of the first element  506 , and the transmission line element  514  form a continuous closed electrical loop. In contrast, in the embodiment of  FIG. 2 , there is a discontinuity in the direct path between the feed point  212  and the ground point  214 . 
     In the particular embodiment shown in  FIG. 5 , similar to the embodiment of  FIG. 2 , the two general C-shaped portions  508  and  510  are substantially mirror symmetrical with each other. However, symmetry between the C-shapes is not necessary and the present invention is not so limited. 
     The embodiment of  FIG. 5  also includes a GPS element  516 . The GPS element  516  is a λ/4 resonance stub that is tuned to efficiently communicate in the GPS frequency range. As can be easily seen in  FIG. 5 , the GPS element  516  is also fed at the feed point  502 . The GPS element  516  extends out away from the ground plane  504  for part of its length and then returns back over the ground plane  504 , generally following the C-shape of the second general C-shaped portion  510 . The extension away from and then back towards the ground plane  504  provides variable coupling with the ground plane  504 . 
     CONCLUSION 
     As should now be clear, embodiments of the present invention provide a multi-band antenna that exceeds cellular band and UPS antenna performance specifications, as well as the performance of traditional antennas, such as the prior art internal λ/4 wire antenna. The inventive antenna advantageously provides a half wavelength cellular band element that is fits within the interior of a phone and is minimally impacted by the user&#39;s hand during operation. In addition, the shape of the antenna does not interfere with existing component located within several models of cellular phones. 
     Non-Limiting Examples 
     Although specific embodiments of the invention have been disclosed, those having ordinary skill in the art will understand that changes can be made to the specific embodiments without departing from the spirit and scope of the invention. The scope of the invention is not to be restricted, therefore, to the specific embodiments, and it is intended that the appended claims cover any and all such applications, modifications, and embodiments within the scope of the present invention.