Abstract:
An antenna system for use in cellular and other wireless communication includes a dual polarized compact antenna array. In one embodiment, the antenna system includes four T-shaped dipole antenna elements mounted on a ground plane, forming a side of a square shaped array. In another embodiment, the antenna system includes seven T-shaped dipole antenna elements mounted on a ground plane to form two side by side square arrays, wherein the square arrays share a common T-shaped dipole antenna element.

Description:
INTRODUCTION 
     This application pertains to the field of antennas and antenna systems and more articularly pertains to antennas for use in wireless communication systems. 
     BACKGROUND OF THE INVENTION 
     Urban and suburban RF environments typically possess multiple reflection, scattering, and diffraction surfaces that can change the polarity of a transmitted signal and also create multiple images of the same signal displaced in time (multipath) at the receiver location. Within these environments, the horizontal and vertical components of the signal will often propagate along different paths, arriving at the receiver decorrelated in time and phase due to the varying coefficients of reflection, transmission, scattering, and diffraction present in the paths actually taken by the signal components. Note that the likely polarization angle of an antenna on a handset used in cellular communication systems to the local earth nadir is approximately 60° towards horizontal (this may be readily verified by drawing a straight line between the mouth and ear of a typical human head and measuring the angle that the line makes with respect to the vertical). The resulting offset handset antenna propagates nearly equal amplitude horizontal and vertical signals subject to these varying effects of an urban/suburban RF environment. As a mobile phone user moves about in such an environment, the signal amplitude arriving at the antenna on the base station antenna the handset is communicating with will be a summation of random multiple signals in both the vertical and horizontal polarizations. 
     The summation of the random multiple signals results in a signal having a Rayleigh fading characterized by a rapidly changing amplitude. Because the signal arriving at the base station often has nearly identical average amplitude in the vertical and horizontal polarizations that are decorrelated in time and/or phase, the base station receiver may choose the polarization with the best signal level at a given time (selection diversity) and/or use diversity combining techniques to achieve a significant increase in the signal to noise ratio of the received signal. 
     Prior art base station antennas that may be used in a selection diversity or diversity combining system often use two separate linearly polarized antennas. This makes for a bulky and unwieldy arrangement because of the space required for each antenna and its associated hardware. U.S. Pat. No. 5,771,024, the contents of which are incorporated by reference, discloses a compact dual polarized split beam or bi-directional array. There is a need in the art, however, for a compact dual polarized boresight array. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a dual polarized antenna array for use in wireless communication systems. The antenna array of the present invention may be deployed in relatively small, aesthetically appealing packages and, because the arrays are dual polarized, the arrays may be utilized to provide substantial mitigation of multipath effects. 
     In one innovative aspect, the present invention is directed to an antenna array comprising a first and a second T-shaped dipole antenna mounted on a ground plane wherein the first and second T-shaped dipoles are aligned along mutually parallel axes such that the first and second dipoles transmit and receive a first polarization. A third and a fourth T-shaped dipole antennas are mounted on the ground plane wherein the third and fourth T-shaped dipoles are aligned along mutually parallel axes such that the third and fourth dipoles are aligned to transmit and receive a second polarization, the second polarization being orthogonal to the first polarization. A first equal phase power divider is coupled to the first and second T-shaped dipoles and a second equal phase power divider is coupled to the third and fourth T-shaped dipoles. The first and second T-shaped dipoles are preferably spaced apart broadside to one another approximately a half wavelength of an operating frequency. Similarly, the third and fourth T-shaped dipoles are preferably spaced apart broadside to one another approximately a half wavelength of the operating frequency. Such an array produces a boresight beam with equal elevation and azimuth (E and H plane) beatnwidths in both the vertical and horizontal polarizations. 
     In another innovative aspect of the invention, additional antenna elements are added to produce unequal elevation and azimuth beamwidths. For example, a first and a second T-shaped dipole are mounted along a first axis of a ground plane. A third and a fourth T-shaped dipole are mounted along a second axis of the ground plane wherein the first and second axes are mutually parallel. A fiftih, sixth, and a seventh T-shaped dipole are mounted on a third, fourth, and fifth axis of the ground plane, respectively, wherein the third, fourth, and fifth axes are orthogonal to the first and second axes. The fifth, sixth, and seventh T-shaped dipoles are positioned between the first and second axes and the sixth antenna element is positioned between the first and second T-shaped dipoles. 
     In a preferred embodiment, the first and second T-shaped dipoles are spaced apart a half wavelength of an operating frequency along the first axis. Similarly, the third and fourth T-shaped dipoles are spaced apart a half wavelength of the operating frequency along the second axis that, in turn, is spaced apart a half wavelength from the first axis. Finally, the third, fourth, and fifth axes are spaced apart from one another a half wavelength of the operating frequency. If the first and second axes are positioned to extend in the direction defining vertical polarization, the elevation (E plane) beamwidth of the array is 30° whereas the azimuth beamwidth is 65° for both the vertically and the horizontally polarized signals. Additional antenna elements can be added along the first and second axes to further narrow the elevation beamwidth. 
    
    
     DESCRIPTION OF FIGURES 
     FIG. 1 a  is an illustration of the main radiating element of a T-shaped dipole antenna element according to the present invention. 
     FIG. 1 b  is an illustration of a reactive feed element of the T-shaped dipole antenna shown in FIG. 1 a.    
     FIG. 2 a  is a plan view of the bottom surface of the ground plane of an array having four T-shaped dipole antenna elements according to one embodiment of the invention. 
     FIG. 2 b  illustrates the ground pads and microstrips for bottom surface of the ground plane of the antenna array of FIG. 2 a.    
     FIG. 3 is a plan view of the top surface of the ground plane of the array of FIG. 2 a.    
     FIG. 4 is a perspective view of the bottom surface of the ground plane of the array of FIG. 2 a.    
     FIG. 5 is a perspective view of the enclosure for the array of FIG. 2 a.    
     FIG. 6 a  is an illustration of the horizontally polarized E-plane cut radiation pattern of the array of FIG. 2 a.    
     FIG. 6 b  is an illustration of the horizontally polarized H-plane cut radiation pattern of the array of FIG. 2 a.    
     FIG. 6 c  is an illustration of the vertically polarized E-plane cut radiation pattern of the array of FIG. 2 a.    
     FIG. 6 d  is an illustration of the vertically polarized H-plane cut radiation pattern of the array of FIG. 2 a.    
     FIG. 7 is a perspective view of the top surface of a ground plane having seven T-shaped dipole antenna elements mounted thereon according to one embodiment of the invention. 
     FIG. 8 is a perspective view of the bottom surface of the ground plane of FIG.  7 . 
     FIG. 9 a  is an illustration of the horizontally polarized E-plane cut radiation pattern of the array of FIG.  7 . 
     FIG. 9 b  is an illustration of the horizontally polarized H-plane cut radiation pattern of the array of FIG.  7 . 
     FIG. 9 c  is an illustration of the vertically polarized E-plane cut radiation pattern of the array of FIG.  7 . 
     FIG. 9 d  is an illustration of the vertically polarized H-plane cut radiation pattern of the array of FIG.  7 . 
    
    
     DETAILED DESCRIPTION 
     Turning to the figures, in one innovative aspect the present invention is directed to the implementation of a square T-shaped dipole antenna. As shown in FIGS. 1 a - 1   b , a T-shaped dipole antenna element  5  comprises a large T-shaped radiating element  10  having a longitudinally extending stem  15  and a pair of laterally extending arms  20 . The T-shaped radiating element  10  and a reactive feed strip  40  are formed on opposite sides of a PC board substrate  30 . The reactive feed strip  40  is arranged to produce an antipodal excitation across a longitudinally extending slot  35  in the stem  15 . The reactive feed strip has a first portion  41  extending from the base of the stem to an end along a first side of the slot  35 . A second portion  42  of the reactive feed strip crosses the slot  35  to connect the end of the first portion  41  to a third portion  44  of the reactive feed strip. The third portion  44  extends downwardly on a second side of the slot  35 . In this fashion, the reactive feed strip  40  includes an antipodal excitation across the slot  35 , thereby making a dipole antenna. It will be appreciated that the radiating element  10  and the reactive feed strip  40  may be and are preferably manufactured by depositing copper cladding in a conventional manner over opposite surfaces of the printed circuit board substrate  30 , followed by etching portions of the copper cladding away to form the radiating element  10  and the feed strip  40 . The printed circuit board may be manufactured from woven TEFLON® having a thickness of approximately 0.03 inches and an epsilon value (or delectric constant) between 3.0 and 3.3. 
     The upper edge of the arms  20  are aligned with the top of the stem  15 . The lower edge of each arm  20  comprises a first arcuate segment having a radius R 1  and a second arcuate segment having a radius R 2  wherein the first arcuate segment merges with the edge of the stem  15 . In a preferred embodiment of the T-shaped antenna  5 , the T-shaped radiating element  10  is 2.8 inches across the top and 1.97 inches high. The width of the stem is 0.6 inches. The radius R 1  is 0.2 inches, and the radius R 2  is 1.82 inches. The slot  35  is 0.15 inches wide and 0.95 inches long. The reactive feed strip  40  is 0.07 inches wide. The second portion  42  of the feed strip is located 0.4 inches from the top of the T-shaped radiating element  10 . The third portion  44  has a length of 0.3 inches. While these dimensions are optimal for transmission at a center frequency of 1850 MHZ, those of ordinary skill in the art will appreciate that the dimensions of the various elements will vary depending upon the operational characteristics desired for a particular application. 
     Turning now to FIGS. 2 a  through  5 , in another innovative aspect the present invention is directed to a dual polarized array of four T-shaped dipole antenna elements  5  arranged in a square configuration on a ground plane  50 . The T-shaped dipole antenna elements are preferably formed as described with respect to FIGS. 1 a  and  1   b . The ground plane  50  may comprise a printed circuit board substrate having opposing coplanar surfaces (i.e., a top surface illustrated in FIG. 3 and a bottom surface illustrated in FIG. 5) whereon respective layers of copper cladding are deposited. Features on the ground plane, such as microstrip feed lines  60  located on the bottom surface are preferably formed by etching away portions of the deposited copper cladding. The dipole antenna elements  5  mount to the ground plane  50  by inserting tabs  32  into slots  34 . The tabs are soldered to the top surface of the ground plane  50  and to grounding pads  36  located on the bottom surface of the grounding plane  50 . 
     The reactive feed strip  40  of the dipole antenna is preferably connected to microstrips  60  by feed pins (not illustrated) that extend through insulated holes  62 . The microstrips  60  are arranged so as to form two equal phase power dividers  67  wherein each power divider  67  is excited at a center pad  68 . A power source (not illustrated) couples to the dipole antennas through coaxial connectors  70 . The coaxial connectors  70  may be standard type N coax connectors sized to receive 0.082 inch diameter coaxial cable. The inner conductor of the coaxial connector couples to center pads  68  (and ultimately, the equal phase power dividers  67 ) adjacent to center ground pads  69  through wires  75 . As can be seen from inspection of FIG. 2 a , the sections of microstrip  60  that couple from the center pads  68  to the insulated holes  62  are of equal length in each equal phase power divider  67 . In this fashion, the reactive feed strips  30  of each dipole antenna element  5  attached to a given equal phase power divider are fed in phase with one another because the electrical energy will have traveled the same electrical length at each reactive feed strip. 
     As can be seen from FIGS. 3 and 4, four dipole antenna elements  5  are arranged in pairs wherein each pair of antenna elements is coupled to an equal phase power divider  67 . A first pair of antenna elements are aligned on mutually parallel axes  77 . Because the arms  20  of the first pair of dipole antenna elements  5  are aligned on the axes  77 , the electric field produced by this first pair will be polarized parallel to axes  77 . A second pair of dipole antenna elements are aligned on mutually parallel axes  78  wherein the axes  78  are orthogonal to the axes  77 . In this fashion, the electric field produced by the second pair of antenna elements will be orthogonally polarized to the field produced by the first pair of antenna elements. Thus, the resulting antenna array forms a square wherein the pairs of dipole antenna elements form opposing sides of the square. 
     The outer conductors of the coaxial connectors  70  are coupled to the copper cladding coating the upper surface of the ground plane  50 . In addition, an array of small perforations (not shown) are distributed around the periphery  65  and on the center ground pads  69  as well as holes  71  act as ground vias. This insures that the respective copper cladding layers form a single, unified ground plane. To provide an impedance match between the microstrips  60  and the reactive feed strips  30 , a quarter wave length transition section of microstrip line  72  is implemented. The dimensions that follow correspond to a center frequency of 1850 MHZ. Those of ordinary skill in the art will appreciate that the dimensions would be altered accordingly for a differing center frequency. In one embodiment, the microstrip line is 0.020 inches wide whereas the quarter wave length transition section is 0.031 inches wide and 0.97 inches long. 
     In order to provide a half-wavelength spacing between identically polarized dipole elements  5 , the pair of mutually parallel axes  77  are spaced apart a half wavelength. Similarly, the pair of mutually parallel axes  78  are also spaced apart a half wavelength. At the preferred operating frequency of 1710 to 1990 MHZ, the axes are spaced apart a distance of substantially 3.3 inches. 
     Turning now to FIG. 5, in a preferred form the dual polarized four T-shaped antenna element array may be mounted in a casing comprising an aluminum base  80  and a plastic cover  82 . The aluminum base  80  is formed such that the ground plane  50  containing the antenna elements  5  may be mounted within a step (not illustrated) formed in the outer wall of the base  80 , and such that the ground plane  50  is coupled to the base  80  by means of a set of screws (not illustrated) through the periphery  65  of the ground plane  50  insuring that the base  80  remains grounded during operation of the antenna array. The base  80  also has formed therein a pair of mounts for the coaxial connectors  70  and a series of threaded holes for receiving a plurality of screws  85  that secure the cover  82  to the base  80 . Those of ordinary skill in the art will appreciate that, to avoid possible intermodulation effects, the cover  82  may be glued to the base  80  using an adhesive such as RTV, rather than using screws  85  to secure the cover  82  to the base  80 . 
     The dual polarized four T-shaped antenna element array embodiment of the present invention produces a single boresight beam which projects orthogonally from the ground plane  50  through the cover  82 . In the field, the antenna element would be mounted on the wall of a building or on a light pole or other structure. One pair of the antenna elements, for example that illustrated on axes  77 , could be aligned with the vertical direction such that the antenna elements aligned with axes  77  will transmit and receive vertically polarized fields. Conversely, the antenna elements aligned on axes  78  would then transmit and receive horizontally polarized fields. FIGS. 6 a  through  6   d  illustrate the elevation beamwidth (E-Plane) and azimuth beamwidths (H-Plane) for the horizontally polarized and vertically polarized components, respectively. Inspection of the figures reveals that the azimuth and elevation beamwidths for the vertical and horizontal polarized components are equal to approximately 65°. 
     In another innovative aspect of the invention, the present invention is directed to a dual polarized compact antenna array having unequal elevation and azimuth beamwidths by adding extra T-shaped dipole antenna elements to the square array of FIGS. 3 and 4. Turning now to FIGS. 7-8, in one embodiment such an array comprises two vertically polarized T-shaped dipole antenna element pairs and three horizontally polarized T-shaped antenna elements. A first and a second T-shaped dipole antenna elements  5  are mounted on axis  90  on ground plane  51 . A third and a fourth T-shaped dipole antenna elements  5  are mounted on axis  92  on ground plane  51  wherein axes  90  and  92  are mutually parallel. A fifth, sixth, and a seventh T-shaped dipole are mounted on axes  94 ,  96 , and  98  on ground plane  51 , respectively wherein axes  94 ,  96 , and  98  are orthogonal to axes  92  and  90 . The fifth, sixth, and seventh T-shaped dipoles antenna elements are positioned between axes  90  and  92  and the sixth antenna element is positioned between the first and second T-shaped dipoles. Because the first, second, third, fourth and sixth T-shaped dipole antenna elements are positioned between the fifth and seventh dipoles, the resulting antenna array is rectangular, comprising two of the square antenna arrays of FIGS. 3 and 4 wherein the two square arrays share the sixth dipole antenna element as can be seen from inspection of FIG.  7 . Preferably, the axes  90  and  92  are spaced apart approximately a half wavelength of the center frequency. The first and second T-shaped dipoles on axis  90  are spaced apart approximately a half wavelength as are the third and fourth T-shaped dipoles on axis  92 . Similarly, axes  94 ,  96 , and  98  are spaced apart approximately a half wavelength of the center frequency. At the preferred center frequency of 1850 MHZ, this spacing equals 3.3 inches. 
     Other than having additional T-shaped dipole elements, the array of FIGS. 7 and 8 is very similar to the square array already described with respect to FIGS. 3 and 4. Thus, the ground plane  51  may comprise a printed circuit board substrate having opposing coplanar surfaces (i.e., a top surface illustrated in FIG. 7 and a bottom surface illustrated in FIG. 8) whereon respective layers of copper cladding are deposited. Features on the ground plane, such as microstrip feed lines  100  located on the bottom surface are preferably formed by etching away portions of the deposited copper cladding. 
     The set of horizontally polarized T-shaped dipole antenna elements are fed by a first equal phase power divider  105 . Similarly, the set of vertically polarized T-shaped dipole antenna elements are fed by a second equal phase power divider  110 . Each of the equal phase power dividers  105  and  110  comprises equal lengths of microstrip feed lines  100  attaching to the various T-shaped dipole antenna elements. The equal phase power dividers  105  and  110  are coupled through wires  120  to center conductors of coaxial connectors  125 . 
     The outer conductors of the coaxial connectors  125  are coupled to the copper cladding coating the upper surface of the ground plane  51 . In addition, as described with respect to the square antenna array of FIGS. 3 and 4, an array of small perforations (not shown) are distributed around the periphery of the ground plane  51  as well as on ground pads and holes act as ground vias. This insures that the respective copper cladding layers form a single, unified ground plane. To provide an impedance match between the microstrips  100  and the reactive feed strips  30 , a quarter wave length transition section of microstrip line is implemented. The ground plane  51  with the mounted T-shaped dipole antenna array is secured within a housing similarly to the housing depicted in FIG. 5 for the corresponding square antenna array. It is to be noted that the present invention produces a dual polarized antenna array such that the labeling of antenna elements as vertically or horizontally polarized is arbitrary and depends upon the ultimate orientation of the housing with respect to the horizon. FIGS. 9 a  through  9   d  illustrate the elevation beamwidth (E-Plane) and azimuth beamwidths (H-Plane) for the horizontally polarized and vertically polarized components, respectively. Inspection of the figures reveals that the azimuth and elevation beamwidths for the vertical and horizontal polarized components are unequal. The vertically polarized component has an elevation and azimuth beamwidth of 30° whereas the horizontally polarized component has a 30° elevation beamwidth and a 65° azimuth beamwidth. 
     While those of ordinary skill in the art will appreciate that this invention is amenable to various modifications and alternative embodiments, specific examples thereof have been shown by way of example in the drawings and are herein described in detail. It is to be understood, however, that the invention is not to be limited to the particular forms or methods disclosed, but to the contrary, the invention is to broadly cover all modifications, equivalents, and alternatives encompassed by the spirit and scope of the appended claims.