Abstract:
An improved log periodic dipole antenna, adapted for use in a cellular telephone system, has a plurality of radiating elements with differing lengths. The sequence of lengths selected results in a horizontal beam width of about 65 degrees and a front-to-back signal strength ratio exceeding 45 dB. This combination of characteristics reduces interference among adjacent cellular telephone transmitter sites, and reduces waste of transmission energy from the back of the antenna. A preferred sequence of radiating element lengths is long-short-long-short-long, which may be described as a “double stacked hourglass” configuration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. Ser. No. 08/807,560, POWELL &amp; YARSUNAS, filed Feb. 28, 1997, now abandoned, the disclosure of which is incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to log periodic dipole antennas (LPDA) and, more particularly, to an improved log periodic dipole antenna which is particularly well adapted for use at a cell transmitter site in a cellular telephone system. 
     BACKGROUND 
     Dipole antennas have long been used in various communications systems, including radio, television, and radiotelephone systems. It is well known that the lengths of the dipole arms on the antenna should be adapted to the wavelengths (λ) of the signals transmitted and received. Typically, a plurality of arms having different lengths are used, in order to cover a predetermined range of frequencies. The sequence and spacing of these arms, and of any reflector behind them, determines various characteristics of the resulting beam or radiation field. These characteristics include vertical beam width, horizontal beam width, and front-to-back (F/B) ratio, i.e. the ratio of signal strength in front of the antenna to signal strength in back of the antenna. When a number of different arms are used, each arm makes its own contribution to the resulting field, and the overall expected result rapidly becomes difficult to calculate mathematically in advance. Therefore, considerable experimentation is often needed to achieve desired beam characteristics. 
     A well-known log periodic dipole antenna (LPDA) design is the “tree” configuration, in which parallel arms extend sideways from a central “trunk” or “standoff,” the bottom arm near the base is the longest, and each successive arm is shorter toward the top of the antenna. Such LPDA designs typically result in a front-to-back (F/B) ratio less than 40 dB. This F/B ratio is considered insufficient for use in current PCS (Personal Communication System) cellular telephone sites, since radiation emanating out the back of the antenna tends to cause interference among adjacent sites. A horizontal beam width of 90 degrees is typical. However, in highly congested urban environments, it is preferable to have horizontal beamwidth of 65 degrees, which is obtained by using two parallel columns of dipoles, spaced 0.25 λ to 0.30 λ apart. The wavelength lambda (λ) is the inverse of the frequency. The frequency band allotted for PCS use in the United States is between 1.85 GigaHertz and 1.99 GigaHertz, with a center frequency 1.92 GHz. The PCS band allotted in Europe has a center frequency 1.78 GHz, meaning that the wavelength is about 8% greater. Accordingly, antenna dimension examples stated for the U.S. should be scaled up about 8% for use in Europe. 
     My earlier LPDA design work has included an “hourglass” dipole strip configuration, in which top and bottom arms are longer than one or more middle arms. This design works well for generating a 90 degree beamwidth, but when used for generating a 65 degree beamwidth, typically results in F/B ratios in the range between 37 dB and 42 dB, better than provided by the “tree” configuration, but still insufficient. Reference is made to pending application U.S. Ser. No. 08/807,560 by myself and a colleague. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an improved log periodic dipole antenna in which the horizontal beam width is 65 to 70 degrees and the front-to-back ratio is at least 45 dB. 
     Briefly, this combination of beam characteristics has been achieved by a “double stacked hourglass” configuration, in which, from the antenna base outwards, the lengths of the dipole arms follow a sequence long-short-long-short-long. The antenna is center-fed with a radio frequency signal. An air dielectric microstrip carries a transmission signal from a feedpoint, where a cable is connected, to the dipoles. There are two columns of parallel radiating elements, spaced about 0.27 λ apart. The spacing between adjacent pairs of radiating elements is about 0.9 λ to 1.0 λ. The horizontal beam width is about 65 degrees. 
    
    
     BRIEF FIGURE DESCRIPTION 
     FIGS. 1A-1E illustrate a dipole array configuration of 8 radiating elements for an antenna having a sixty-five degree beamwidth; 
     FIG. 2 shows a “tree” dipole radiating element; 
     FIG. 3A shows the radiation pattern of the tree dipole at 1.85 GHz; 
     FIG. 3B shows the radiation pattern of the tree dipole at 1.92 GHz; 
     FIG. 3C shows the radiation pattern of the tree dipole at 1.99 GHz; 
     FIG. 4 shows an “hourglass” dipole radiating element, in which the top and bottom arms are longer than the middle arms; 
     FIG. 5A shows the radiation pattern of the hourglass dipole at 1.85 GHz; 
     FIG. 5B shows the radiation pattern of the hourglass dipole at 1.92 GHz; 
     FIG. 5C shows the radiation pattern of the hourglass dipole at 1.99 GHz; 
     FIG. 6 shows a “double-stacked hourglass” dipole radiating element in accordance with the present invention; 
     FIG. 7A shows the radiation pattern of the double-stacked hourglass dipole at 1.85 GHz; 
     FIG. 7B shows the radiation pattern of the double-stacked hourglass dipole at 1.92 GHz; 
     FIG. 7C shows the radiation pattern of the double-stacked hourglass dipole at 1.99 GHz. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1A illustrates a log periodic dipole antenna configuration  100  adapted to produce a beam about 65 degrees wide in azimuth when the antenna configuration is oriented with its longer dimension perpendicular to the earth. It includes a left column of radiating elements  11 ,  13 ,  15 ,  17  and a right column of radiating elements  12 ,  14 ,  16 ,  18 , all mounted on a metallic reflector plate  19 . The left and right columns are suitably spaced about 0.27 λ apart horizontally, where λ is the wavelength of the intended central operating frequency of the antenna, e.g. 1.92 GHz in North America for the PCS (Personal Communications System) band 1.85-1.99 GHz. Alternatively, a single column could be used, with a wide reflector. The vertical spacing between the rows of radiating elements is suitably about 0.9 to 1 λ. Multiple rows are used, in order to narrow the vertical beamwidth, since most cellphone users are in a plane along the horizon, and the beam should be directed there. 
     A signal is fed to the antenna via a feedpoint  20 , which may be a coaxial connector extending through an opening in reflector plate  20 , for connecting a coaxial cable (not shown) on the side of the reflector plate remote from the radiating elements. Preferably, a microstrip feedline  22  extends from feedpoint  20  to all of the radiating elements. However, it is known in the antenna art to feed the dipoles in other ways, e.g. by cables or printed circuit board tracks. Each radiating element consists of two parallel dipole strips, one active and one passive, e.g.  11 A &amp;  11 P, and a center feed conductor  24  (shown in FIGS. 1C &amp; 1E) between the dipole strips. Center feed conductor  24  has a bottom end connected to microstrip feedline  22 , and a top end connected to one of the dipole strips. The connected strip is the active dipole strip, since it is supplied with the signal from feedpoint  20 . The unconnected dipole strip is the passive strip. In FIG. 1A, the active strips are designated with the suffix “A” and the passive strips are designated with the suffix “P.” Preferably, there is an alternation, from row to row, in whether the left strip or the right strip is active. This helps to produce a radio beam whose center is directly perpendicular to the reflector. 
     FIG. 1B is a side view, showing four radiating elements extending from the reflector. 
     FIG. 1C is another side view, showing two radiating elements edgewise, each with a center feed conductor  24  connecting about halfway up the active dipole strip. The dipole strips can be made of aluminum sheet having a thickness of about 0.063 inches (1.6 mm). Preferably, a dielectric spacer is provided between upper ends of the active and passive dipole strips to provide mechanical stability. A suitable spacer material is polytetrafluoroethylene (PTFE), also known by the trademark TEFLON. 
     FIG. 1D is an enlarged detail view, showing in section a metal ring or nut  26  which is bolted or screwed between center feed conductor  24  and the active strip. FIG. 1E is another enlarged detail view, showing how the dipole strip is connected to the reflector plate. 
     As shown in FIG. 2, each dipole strip has a central “trunk” or “standoff”  28  which extends outward from a base at reflector plate  19 , and has a plurality of arms or branches  31 - 35  extending perpendicularly sideways from the standoff. The arms extend alternately to left and to right from the standoff. In each radiating element, respective arms of the active and passive dipole strips extend in opposite directions. For example, if the bottom-most arm of the active strip extends left, the bottom-most arm of the passive strip extends right. In a conventional “tree” dipole, the arms become progressively shorter as the distance from reflector plate  19  increases. 
     FIG. 3A illustrates the azimuth radiation pattern at a frequency of 1.85 GHz of a “tree” dipole antenna according to FIG.  2 . As shown, the beamwidth is about 66 degrees and the front-to-back ratio is about 35 dB, which today is considered inadequate. FIG. 3B illustrates the azimuth radiation pattern of the same antenna at 1.92 GHz. The beamwidth is about 65 degrees and the F/B ratio is not quite 40 dB. FIG. 3C illustrates the azimuth radiation pattern of the same antenna at 1.99 GHz. The beamwidth is about 63 degrees and the F/B ratio is about 36 dB. 
     FIG. 4 shows an “hourglass” dipole strip structure, as disclosed in FIG. 9 of my earlier U.S. patent application 08/807,560, filed Feb. 28, 1997. That application was directed primarily to production of a 90 degree azimuth beamwidth, but the same radiating elements can arranged in an array for production of a 65 degree azimuth beamwidth. As shown, the five dipole arms  128 ( a ),  128 ( b ),  128 ( c ),  128 ( d ) and  128 ( e ) have respective lengths whose ratios are 1.53, 1.257, 0.93, 0.98 and 1.047, i.e. the middle arm is shorter than the bottom and top arms. The outer contour of this structure is shaped like an hourglass, which is the reason for the name given to the structure. This structure provides a better F/B ratio than the “tree” dipole structure, but the result is still less favorable than desired. 
     FIG. 5A illustrates the azimuth radiation pattern at a frequency of 1.85 GHz of an “hourglass” dipole antenna according to FIG.  4 . As shown, the beamwidth is about 70 degrees and the front-to-back ratio is about 37 dB. FIG. 5B illustrates the azimuth radiation pattern of the same antenna at 1.92 GHz. The beamwidth is about 69 degrees and the F/B ratio is not quite 40 dB. FIG. 5C illustrates the azimuth radiation pattern of the same antenna at 1.99 GHz. The beamwidth is about 65.5 degrees and the F/B ratio is about 42 dB. 
     FIG. 6 shows a “double stacked hourglass” dipole strip structure in accordance with the present invention. As shown, the five dipole arms  61 - 65  have respective lengths in the sequence long-short-long-short-long. In a preferred embodiment, their ratios are 1.598, 1.139, 1.25, 0.795, and 0.817, i.e. the second arm  62  is shorter than the bottom arm  61  and middle (third) arm  63 , and the fourth arm  64  is shorter than the middle (third) arm  63  and top (fifth) arm  65 . 
     FIG. 7A illustrates the azimuth radiation pattern at a frequency of 1.85 GHz of a “double stacked hourglass” dipole antenna according to FIG.  6 . As shown, the beamwidth is about 70 degrees and the front-to-back ratio is about 50 dB. FIG. 7B illustrates the azimuth radiation pattern of the same antenna at 1.92 GHz. The beamwidth is about 68 degrees and the F/B ratio is over 57 dB. FIG. 7C illustrates the azimuth radiation pattern of the same antenna at 1.99 GHz. The beamwidth is about 66.5 degrees and the F/B ratio is about 46 dB. 
     These F/B ratios are much greater than the “tree” dipole F/B ratios of 35, 40, and 37, (FIGS. 3A-3C) and are a major improvement over the F/B ratios of 37, 40, and 42 (FIGS. 5A-5C) ratios of my earlier “hourglass” design. This improved F/B ratio reduces interference among adjacent cell sites, and conserves energy by preventing wasted emissions out the back of the antenna. 
     The relevant data for the plots shown in FIGS. 3A-3C,  5 A- 5 C and  7 A- 7 C is summarized in the following table: 
     
       
         
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 BEAM 
                 BEAM 
                 F/B 
                 SIDELOBE 
                 SIDELOBE 
                 SIDELOBE 
                 SIDELOBE 
               
               
                 FIG 
                 FREQ 
                 PEAK 
                 WIDTH 
                 RATIO 
                 DEGREE 
                 DB 
                 DEGREE 
                 DB 
               
               
                   
               
             
             
               
                 3A 
                 1.850 
                 0.16 
                 66.30 
                 −34.466 dB 
                 −140.50 
                 −39.74 
                 146.25 
                 −31.86 
               
               
                   
                 GHz 
                 deg. 
                 deg. 
               
               
                 3B 
                 1.920 
                 0.37 
                 64.73 
                 −39.578 dB 
                 −135.41 
                 −36.06 
                 144.59 
                 −33.42 
               
               
                   
                 GHz 
                 deg. 
                 deg 
               
               
                 3C 
                 1.990 
                 −0.38 
                 62.82 
                 −36.361 dB 
                   
                   
                 138.69 
                 −33.28 
               
               
                   
                 GHz 
                 deg 
                 deg. 
               
               
                 5A 
                 1.850 
                 −1.04 
                 69.74 
                 −36.709 dB 
                 −142.75 
                 −35.41 
                 177.75 
                 −36.58 
               
               
                   
                 GHz 
                 deg. 
                 deg. 
               
               
                 5B 
                 1.920 
                 −0.01 
                 68.94 
                 −39.578 dB 
                 −147.00 
                 −38.48 
                 147.50 
                 −37.66 
               
               
                   
                 GHz 
                 deg. 
                 deg. 
               
               
                 5C 
                 1.990 
                 −0.57 
                 65.51 
                 −42.491 dB 
                 −105.75 
                 −26.10 
                 145.75 
                 −36.03 
               
               
                   
                 GHz 
                 deg. 
                 deg 
               
               
                 7A 
                 1.850 
                 −0.92 
                 70.13 
                 −49.855 dB 
                 −144.75 
                 −40.52 
                 154.75 
                 −43.11 
               
               
                   
                 GHz 
                 deg. 
                 deg. 
               
               
                 7B 
                 1.920 
                 −0.12 
                 68.44 
                 −57.642 dB 
                 −158.00 
                 −45.48 
                 133.75 
                 −38.59 
               
               
                   
                 GHz 
                 deg. 
                 deg. 
               
               
                 7C 
                 1.990 
                 1.24 
                 66.43 
                 −46.038 dB 
                 −148.75 
                 −39.63 
                 161.25 
                 −40.32 
               
               
                   
                 GHz 
                 deg. 
                 deg. 
               
               
                   
               
             
          
         
       
     
     Those skilled in the art will appreciate that various changes and modifications are possible within the scope of the present invention, in order to adapt to other frequency bands or to other terrain conditions. Therefore, the invention is not limited to the particular embodiments shown and described, but rather is defined by the following claims.