Abstract:
A polarized antenna for sending and receiving polarized radio frequency signals is disclosed which includes a dipole and a reflector plate. The dipole is formed as a single part including the radiating arms and feeding structures, thereby requiring minimum assembly. This dipole can be formed by molding conventional materials, such as copper, aluminum, and plastic, which can then be plated. The feeding structure through which the cable passes features a slotted aperture. The impedance of the dipole is based on the width of these apertures and the size of the cable conductor. By having a single-body construction, the dipole of the present invention provides, good impedance, low intermodulation distortion, good port-to-port isolation, and good pattern purity.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to dual polarized panel base-station antennae for use in mobile communication systems. More specifically, the invention relates to the structure of dipoles used with dual polarized panel base-station antennae.  
           [0003]    2. Description of Related Art  
           [0004]    Dipole antennae are common in the communications industry, and conventional structures, including half-wavelength dipoles with “bow tie” structures and “butterfly” structures, are described in several books, including  Banalis, Constantine A., “Antenna Theory Analysis and Design”,  Wiley, 1997.  
           [0005]    In particular, panel base-station antennae, such as those used in mobile communication systems, rely heavily on dual polarization antennae. In many cases, these antennae are constructed using single linear polarized elements, grouped in such a way that creates dual polarization. In this case, two separate arrays of radiating elements are required to radiate on both polarizations.  
           [0006]    Building antenna using this approach is undesirable, however, because creating the dual polarization effect with single linear polarized elements increases the labor cost and the number of parts involved in the antenna&#39;s manufacture, while reducing its overall performance. To overcome this, most dual polarization antennae are made with directly dual polarized elements, either by including a single patch element fed in such a manner as to create a dual polarized structure, or by combining two single linear polarized dipoles into one, thereby making a single, dual polarization element.  
           [0007]    Feeding signals to and from these dual polarization structures is usually accomplished by conventional coupling structures such as coaxial cables, microstrip or stripline transmission lines, or slits. The drawback to using these conventional coupling structures with the antennae and dipoles described above is that they increase the number of parts needed to construct the antenna, thereby generating undesired intermodulation distortions.  
           [0008]    In addition, manufacturing these panel antennae with dipoles that include numerous radiating elements often requires numerous solder joints and screw connections. The total number of parts required in such panel antennae, in addition to the cost of their assembly, makes them unsuitable for mass-production. In addition, solder, screws, and similar types of attachments between parts not only add to the manufacturing time and labor cost, but also generate undesired intermodulation distortions as well.  
           [0009]    In addition to avoiding these intermodulation distortions, it is necessary to achieve good port-to-port isolation between the two inputs of the radiating elements in the antenna in order to achieve an efficient communication system. This isolation is the measure of the ratio of power leaving one port and entering the other port. But using the air dielectric transmission lines that are common in conventional coupling structures creates distortions in the signal fed to and from the reflector. In these circumstances, it is prohibitively expensive and difficult to achieve the desired isolation, meaning that the antenna cannot be configured such that one port is used for transmission and the other port for reception.  
           [0010]    Finally, in addition to having good port-to-port isolation characteristics and a minimum of intermodulation distortions, it is also important for the dipoles in the antenna array to have a good impedance so that all of the dipoles in the array can be properly matched.  
           [0011]    In the view of the above, there is a need in the art for low-cost panel base-station antennae that are easy to assemble, that include a simple arrangement of radiating elements, and require a reduced number of parts and connections. In addition, such antennae must have good port-to-port isolation, good pattern purity, good impedance, and low intermodulation distortion.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention provides a new and useful single or dual polarized antenna for use in mobile communication systems.  
           [0013]    A first embodiment of the invention provides a polarized antenna for use in a mobile communication system comprising at least one dipole having a base portion and a plurality of radiating arms extending therefrom, wherein said dipole is formed as a single structure; and a reflector plate to which the base portion is attached, said reflector plate being a ground plane and reflecting polarized radio frequency signals. The dipole may include two sets of arms, including a first set and a second set respectively having a first polarization and a second polarization corresponding to two polarizations of said dipole. Each set of arms preferably includes two pairs of arms arranged in a V-shape and having a vertex portion. A first pair of arms in each set has a first slot at said vertex portion and a second pair of arms has a second slot at said vertex portion for receiving a feed cable, said first slot receiving a cable center conductor and said second slot receiving an insulating jacket. The dipole can also include a cavity for feeding the cable located at the vertex portion of the arms.  
           [0014]    The present invention further provides a method of manufacturing a dipole for use in a polarized antenna, comprising forming an entire dipole body as a single piece, including a base portion and a plurality of radiating arms. The dipole body is optimally molded from a conventional material such as plastic, aluminum or the like. In this case, the method of the present invention further comprises plating the molded dipole body with a metallic material that can be soldered.  
           [0015]    Accordingly, the invention comprises the features of construction, combination of elements and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The objects and advantages of the present invention will be made more clear with reference to the following drawings, in which like elements have been given like reference characters. In particular:  
         [0017]    [0017]FIG. 1 is a perspective view of an antenna using an array of dipoles.  
         [0018]    [0018]FIG. 2 is a perspective view of the dual polarization dipole (all parts assembled).  
         [0019]    [0019]FIG. 3 is a top view of the dual polarization dipole shown in FIG. 2.  
         [0020]    [0020]FIG. 4 is a view of an embodiment of an antenna using an array of dipoles having a variety of RF isolation devices.  
         [0021]    [0021]FIG. 5 is a plot of three radiation patterns of the first polarization having beamwidths of 65.4 degrees at 1.71 GHz, 62.2 degrees at 1.8 GHz and 60.5 degrees at 1.88 GHz respectively for a 1*9 antenna array using the subject matter of the invention shown in FIG. 4.  
         [0022]    [0022]FIG. 6 is a plot of three radiation patterns for the second polarization of a 1*9 arrayed antenna using the subject matter of the invention shown in FIG. 4. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    The invention will now be taught using a preferred exemplary embodiment. Although the embodiment is described in detail, it will be appreciated that the invention is not limited to just this embodiment, but has a scope that is significantly broader. The appended claims should be consulted to determine the true scope of the invention.  
         [0024]    A preferred embodiment of the invention will now be described with reference to FIGS.  1 - 6 . FIG. 1 shows a dual polarization antenna  14  of the present invention with a 1×9 array of dipoles  16  according to the present invention. The antenna  14  comprises the array of dipoles  16  and a reflector plate  12  to which the dipoles  16  are attached. Of course, it is understood that the invention is not limited to a particular array.  
         [0025]    [0025]FIG. 2 shows a dipole  16  of the present invention in greater detail. The dipole  16  is formed as a unitary structure including the base portion, arms, and feeding structures discussed below. The forming of the dipole can be accomplished by conventional methods, such as molding, casting, or carving. In addition, the dipole can be formed using conventional materials such as copper, bronze, plastic, aluminum, or zamak. If the material used is a type that cannot be soldered, such as plastic or aluminum, then the dipole, once formed, can be covered or plated, in part or in whole, with a metallic material that can be soldered, such as copper, silver, or gold.  
         [0026]    The dipole  16  includes four pairs of arms  18 ,  20 ,  22 , and  24  attached to a base portion  26 . The arms are arranged in pairs  18 ,  20 ,  22 , and  24  each having a V- or U-shape, with the arms radiating outward from the vertex portion  21  of the V or U. The base portion  26  of the dipole attaches to the reflector plate  12  shown in FIG. 1.  
         [0027]    The pairs of arms are arranged such that pair  18  is opposite pair  20 , and pair  22  is opposite pair  24 . The opposing pairs are wired and positioned with respect to the reflector plate  14  so as to transmit and/or receive RF energy at two polarizations: a first polarization of +45 degrees and a second polarization of −45 degrees. Opposing pairs  20  and  18  correspond to the first and second polarization of the antenna  14 , respectively. Likewise, opposing pairs  24  and  22  correspond to the first and second polarizations. The dipole of the present invention is not limited to these polarizations, and it is understood that changing the number, arrangement and position of the arm pairs can change both the number of polarizations and the polarization angles of the antenna.  
         [0028]    Each set of opposing pairs of arms includes a feeding structure  28  which is located at the vertex portion  21  of one of the arm pairs. This feeding structure  28  is a longitudinal cavity  23  running the length of the dipole body, allowing a cable  30  to be fed into the base portion  26  of the dipole, through the feeding structure, and out to the top of the dipole. A slot, discussed below, is placed in the vertex of the opposite arm pair. The conductor of the cable is soldered to this vertex via this slot.  
         [0029]    [0029]FIG. 2 and FIG. 3 show the relationship of these pairs of arms in greater detail. Focusing on a single arm set, including arm pairs  22  and  24 , the feeding structure  28  is defined by the cavity  23  that is provided in the vertex portion of one of the arms  22  of the pair. The cable  30  passes through the cavity  23 . This feeding structure  28  also includes a slotted aperture  32  that extends along the cavity and has a width m. The slotted aperture  32  exposes the insulating jacket  34  of the cable  30  running through the cavity  23 .  
         [0030]    Each arm set also includes first and second slots  31  and  38 , respectively, through which the cable is further fed. The first slot  31  is located at the vertex portion of a first pair of arms  22  and the second slot  38  is formed at the vertex portion of the second set of arms  24 . The cable is run such that the first slot  31  retains the entire cable (i.e., unstripped) and the second slot  38  retains the conductor portion  36  of the cable. The conductor  36  is then soldered to the vertex portion  21  of the second set of arms  24  proximate the second slot  38 .  
         [0031]    The arm set including arm pairs  18  and  20  is arranged in a similar fashion. The vertex portion  21  of the pair of arms  18  includes a feeding structure  28  through which is defined by the cavity  23 , through which a second cable  42  is passed. This feeding structure  28  also includes a slotted aperture  44  that extends along the cavity  23  and has a width m. The slotted aperture  44  exposes the insulating jacket  46  of the cable  42  running through the cavity  23 .  
         [0032]    Arm sets  18  and  20  also include first and second slots  47  and  50 , respectively, through which the cable is further fed. The first slot  47  is located at the vertex portion  21  of the first pair of arms  18  and the second slot  50  is formed at the vertex portion  21  of the second set of arms  20 . The cable is run such that the first slot  47  retains the entire cable (i.e., unstripped) and the second slot  50  retains the conductor portion  48  of the cable  42 . The conductor  48  is then soldered to the vertex portion  21  of the second set of arms  20  proximate the second slot  50 .  
         [0033]    An advantage of this dipole structure is that it allows the use of simple coaxial cables to serve as feed cables  30  and  42 , as discussed above. These coaxial cables typically include an inner conductor surrounded by an insulator of PTFE or similar material.  
         [0034]    Furthermore, the dipole and its internal feeding structure allows these cables  42  and  30  to directly pass through the body of the dipole  16  to the top and connect to the arm pairs  20 ,  18  and  24 ,  22  at slots  50  and  38 , respectively, without needing any grommets to insulate the conductors  36  and  48  from the conductive base portion  26  to which the arms  20  or  24  are attached. This reduces the overall number of parts needed to build the dipole, thereby lowering the manufacturing cost and improving the RF performance of the antenna.  
         [0035]    The signal performance of the dipole  16  can be further improved by placing conventional insulating separators  37  between adjacent arm pairs. These separators can be made of conventional insulating materials such as plastic or PTFE.  
         [0036]    Because the impedance of the dipole is determined by the sizes of the apertures, the center conductor of the cable, and the holes in the base portion  26  extending into the cavities  28 , these sizes can be chosen to provide the dipole with a desired impedance as well as to facilitate the forming and plating of the dipole. In particular, the size of these apertures can be made wide enough to ensure proper plating of the molded piece, but narrow enough to allow the dipole to provide good port-to-port isolation, good impedance, and good pattern purity. The scope of the invention is not intended to be limited to any particular shape of these apertures.  
         [0037]    Specifically, depending on the size m of the apertures in the feeding structure, the characteristic impedance Zo can be readily estimated as follows.  
         [0038]    First, in the case where apertures  32  and  44  are closed (where their width m is zero), the impedance, Zo, can be calculated by the following equation:  
         Zo   =       60       ɛ        r       ·     ln        [     D   d     ]           ,                         
 
         [0039]    where D is the diameter of the holes in the base portion  26  and the longitudinal cavities  28 , d is the diameter of the cable&#39;s center conductor, and εr is the dielectric constant of the cable insulator used.  
         [0040]    In the second case, where the width m of apertures  32  and  44  is very small, the impact of the width on the impedance is negligible. However, if the aperture is slanted at an angle along the length of the feeding structure, then characteristic impedance Zo can be more precisely approximated by the equation:  
         Zo   =         60       ɛ        r       ·     ln        [     D   d     ]         +     (     0.03                   θ   2       )         ,                         
 
         [0041]    where D is the diameter of the holes in the base portion  26  and the longitudinal cavities  28 , d is the diameter of the cable&#39;s center conductor, θ is the angle at which the aperture is slanted, and εr is the dielectric constant of the cable insulator used.  
         [0042]    In the third case, where the width m of apertures  32  and  44  is larger, thereby exposing the surface of the cable, then the characteristic impedance Zo can be approximated by the equation:  
         Zo   =       60       ɛ        r       ·     ln        [       4      h     d     ]           ,                         
 
         [0043]    where h is the radius of the longitudinal cavities, d is the diameter of the cable&#39;s center conductor, and εr is the dielectric constant of the cable insulator used.  
         [0044]    It is understood that the molded dipole of the present invention can be used in a variety of antenna configurations. Furthermore, the base portion  26  of the molded dipole can be designed and shaped to match a complimentary form on the reflector plate  12  so as to further facilitate the assembly of the antenna array. It would be obvious to one skilled in the art that the size and shape of the base portion can vary from antenna to antenna and still be within the scope of the invention.  
         [0045]    The present invention also provides for the isolation of inputs of a dipole  16  in antenna arrays that include a plurality of dipoles of the present invention. Dipoles  16  in the dual polarization antenna  14  can be isolated from each other using conventional radio frequency isolation devices, such as walls, H structures and I structures. For example, FIG. 4 shows a dual polarization antenna  70  in which the dipoles  16  are isolated using a number of different isolation devices including walls  60 , H isolators  62 , and I isolators  64 . It is understood that the dipole of the present invention can be used in conjunction with ordinary isolation devices and structures.  
         [0046]    FIGS.  5 - 6  show the performance characteristics of the antenna array shown in FIG. 4. FIGS. 5 and 6 show a plot of three radiation patterns of the first and second polarizations of the antenna array of FIG. 4 using dipoles  16  of the present invention. As shown, the antenna exhibits good port-to-port isolation of less than 30 dB at a variety of beamwidths and at high frequencies.  
         [0047]    The foregoing description is merely exemplary and is not to be construed in a limiting sense. Modifications will be readily apparent to those of ordinary skill in the art, and are considered to be within the scope of the invention, which is to be limited only by the following claims. For example, although reference is made to arm pairs being V-shaped, it is understood that these arm pairs could also be U-shaped without departing from the spirit of the invention. Indeed, reference to “V-shaped” is intended to include a U-shaped arrangement.