Abstract:
A phased-array antenna radiator for a super economical cellular communication system is provided. The phased-array antenna radiator comprises two dipole radiators. The first dipole radiator includes a first monopole radiating element supported by a first outer conductor, a second monopole radiating element supported by a second outer conductor, a first inner conductor, disposed within the first outer conductor and extending therethrough, having an upper termination, a first feed strap, attached to the upper termination of the first inner conductor, and a first stub, disposed within the second outer conductor and attached to the first feed strap. The second dipole radiator includes a third monopole radiating element supported by a third outer conductor, a fourth monopole radiating element supported by a fourth outer conductor, a second inner conductor, disposed within the third outer conductor and extending therethrough, having an upper termination, a second feed strap, attached to the upper termination of the second inner conductor, and a second stub, disposed within the fourth outer conductor and attached to the second feed strap.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 61/046,765 (filed on Apr. 21, 2008, entitled “Phased-Array Antenna Radiator for a Super Economical Broadcast System”), the contents of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates, generally, to cellular communication systems. More particularly, the present invention related to a radiator for a phased-array antenna. 
     BACKGROUND OF THE INVENTION 
     Cellular radiotelephone system base transceiver stations (BTSs), at least for some United States (U.S.) and European Union (EU) applications, may be constrained to a maximum allowable effective isotropically radiated power (EIRP) of 1640 watts. EIRP, as a measure of system performance, is a function at least of transmitter power and antenna gain. As a consequence of restrictions on cellular BTS EIRP, U.S., EU, and other cellular system designers employ large numbers of BTSs in order to provide adequate quality of service to their customers. Further limitations on cells include the number of customers to be served within a cell, which can make cell size a function of population density. 
     One known antenna installation has an antenna gain of 17.5 dBi, a feeder line loss of 3 dB (1.25″ line, 200 ft mast) and a BTS noise factor of 3.5 dB, such that the Ga−NFsys=17.5−3.5−3.0=11 dBi (in uplink). Downlink transmitter power is typically 50 W. With feeder lines, duplex filter and jumper cables totaling −3.5 dB, the Pa input power to antenna is typically 16 W, such that the EIRP is 16 W+17.5 dB=1,000 W. 
     In many implementations, each BTS is disposed near the center of a cell, variously referred to in the art by terms such as macrocell, in view of the use of still smaller cells (microcells, nanocells, picocells, etc.) for specialized purposes such as in-building or in-aircraft services. Typical cells, such as those for city population density, have radii of less than 3 miles (5 kilometers). In addition to EIRP constraints, BTS antenna tower height is typically governed by various local or regional zoning restrictions. Consequently, cellular communication providers in many parts of the world implement very similar systems. 
     Restrictions on cellular BTS EIRP and antenna tower height vary within each country. Not only is the global demand for mobile cellular communications growing at a fast pace, but there are literally billions of people, in technologically-developing countries such as India, China, etc., that currently do not have access to cellular services despite their willingness and ability to pay for good and inexpensive service. In some countries, government subsidies are currently facilitating buildout, but minimization of the cost and time for such subsidized buildout is nonetheless desirable. In these situations, the problem that has yet to be solved by conventional cellular network operators is how to decrease capital costs associated with cellular infrastructure deployment, while at the same time lowering operational expenses, particularly for regions with low income levels and/or low population densities. An innovative solution which significantly reduces the number of conventional BTS site-equivalents, while reducing operating expenses, is needed. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide a phased-array antenna radiator for a super economical broadcast system. 
     In one embodiment, the phased-array antenna radiator comprises two dipole radiators. The first dipole radiator includes a first monopole radiating element supported by a first outer conductor, a second monopole radiating element supported by a second outer conductor, a first inner conductor, disposed within the first outer conductor and extending therethrough, having an upper termination, a first feed strap, attached to the upper termination of the first inner conductor, and a first stub, disposed within the second outer conductor and attached to the first feed strap. The second dipole radiator includes a third monopole radiating element supported by a third outer conductor, a fourth monopole radiating element supported by a fourth outer conductor, a second inner conductor, disposed within the third outer conductor and extending therethrough, having an upper termination, a second feed strap, attached to the upper termination of the second inner conductor, and a second stub, disposed within the fourth outer conductor and attached to the second feed strap. 
     There have thus been outlined, rather broadly, certain embodiments of the invention, in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below, and which will form the subject matter of the claims appended hereto. 
     In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as in the abstract, are for the purpose of description and should not be regarded as limiting. 
     As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods, and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a perspective view of a base transceiver station antenna, in accordance with an embodiment of the present invention. 
         FIG. 2  depicts a perspective view of a partial antenna panel, in accordance with an embodiment of the present invention. 
         FIG. 3  depicts a group of four crossed-dipole radiators, in accordance with an embodiment of the present invention. 
         FIG. 4  depicts an exploded view of crossed-dipole radiator, in accordance with an embodiment of the present invention. 
         FIG. 5  depicts a crossed dipole radiator, in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention provide a phased-array antenna radiator for a super economical broadcast system. 
     According to one aspect of the present invention, cell spacing, i.e., the distance between adjacent BTSs, is advantageously increased relative to conventional cellular systems while providing a consistent quality of service (QoS) within each cell. Preferred embodiments of the present invention increase the range of each BTS. Conventional macrocells typically range from about ¼ mile (400 meters) to a theoretical maximum of 22 miles (35 kilometers) in radius (the limit under the GSM standard); in practice, radii on the order of 3 to 6 mi (5-10 km) are employed except in high-density urban areas and very open rural areas. The present invention provides full functionality at the GSM limit of 22 mi, for typical embodiments of the invention, and extends well beyond this in some embodiments. Cell size remains limited by user capacity, which can itself be significantly increased over that of conventional macrocells in some embodiments of the present invention. 
     Commensurate with the increase in cell size, the BTS antenna tower height is increased, retaining required line-of-sight (for the customary 4/3 diameter earth model) propagation paths for the enlarged cell. Preferred embodiments of the present invention increase the height of the BTS antenna tower from about 200 feet (60 meters) anywhere up to about 1,500 ft (about 500 m). In order for the transmit power and receive sensitivity of a conventional cellular transceiver (user&#39;s hand-held mobile phone, data terminal, computer adapter, etc.) to remain largely unchanged, both the EIRP and receive sensitivity of the tower-top apparatus for the SEC system are increased at long distances relative to conventional cellular systems and reduced near the mast. These effects are achieved by the phased-array antenna and associated passive components, as well as active electronics included in the present invention. 
     Standard BTS equipment, such as transceivers, electric power supplies, data transmission systems, temperature control and monitoring systems, etc., may be advantageously used within the SEC system. Generally, from one to three or more cellular operators (service providers) may be supported simultaneously at each BTS, featuring, for example, 36 to 96 transceivers and 216 to 576 Erlang of capacity. Alternatively, more economical BTS transmitters (e.g., 0.1 W transmitter power) may be used by the cellular operators, further reducing cost and energy consumption. These economical BTSs have a smaller footprint and lower energy consumption than previous designs, due in part to performance of transmitted signal amplification and received signal processing at the top of the phased-array antenna tower rather than on the ground. 
       FIG. 1  presents a perspective view of a BTS antenna, in accordance with an embodiment of the present invention. 
     The base transceiver station  10  includes an antenna tower  12  and a phased-array antenna  14 , with the latter disposed on an upper portion of the tower  12 , shown here as the tower top. The antenna  14  in the embodiment shown is generally cylindrical in shape, which serves to reduce windload, and has a number of sectors  16 , such as, for example, 6 sectors, 8 sectors, 12 sectors, 18 sectors, 24 sectors, 30 sectors, 36 sectors, etc., that collectively provide omnidirectional coverage for a cell associated with the BTS. Each sector  16  includes a number of antenna panels  18  in a vertical stack. Each elevation  20  includes a number of antenna panels  18  that can surround a support system to provide 360° coverage at a particular height, with each panel  18  potentially belonging to a different sector  16 . Each antenna panel  18  includes a plurality of vertically-arrayed radiators, which are enclosed within radomes that coincide in extent with the panels  18  in the embodiment shown. 
     Feed lines, such as coaxial cable, fiber optic cable, etc., connect cellular operator equipment to the antenna feed system located behind the respective sectors  16 . At the input to the feed system for each sector  16  are diplexers, power transmission amplifiers, low-noise receive amplifiers, etc., to amplify and shape the signals transmitted from, and received by, the phased-array antenna  14 . In one embodiment, the feed system includes rigid power dividers to interconnect the antenna panels  18  within each sector  16 , and to provide vertical lobe shaping and beam tilt to the panels  18  in that sector. In another embodiment, flexible coaxial cables may be used within the feed system. 
       FIG. 2  depicts a perspective view of a partial antenna panel  100 , in accordance with an embodiment of the present invention. A single rectangular box extrusion  102  has four internal chambers  104 , operative as discrete, grounded signal line outer conductors, in addition to any number of structural chambers  106 , functional at least as stiffeners. Outer surfaces of the chambers  106  further serve, along with external surfaces of the signal line chambers  104 , to establish a continuous reflector face (backplane)  108  proximal to a plurality of radiators  110 . 
       FIG. 3  depicts an arbitrary group of four, proximate crossed-dipole radiators  110 , in accordance with an embodiment of the present invention. Radiators  110 , including transverse quadrilateral crossed dipoles  140 ,  142 , are mounted on a face  108  of the antenna panel  100  (shown in  FIG. 2 ), and arranged in a staggered configuration. In at least one embodiment, radiators  110  are similar, in some respects, to radiators disclosed within U.S. Patent Application Publication No. 2007-0254587 (published Nov. 1, 2007), which is incorporated herein by reference in its entirety. Radiators  110  advantageously exhibit intrinsic low cross coupling between their respective dipoles  140 ,  142 . When spaced vertically about a wavelength apart, they further exhibit intrinsic low mutual coupling between proximal radiators  110 . In one preferred embodiment, radiators  110  transmit and receive signals in the 900 MHz frequency range. 
     Radiators  110  are arranged in two staggered vertical rows  144 ,  146  of radiators  110 , so that the dipoles  140 ,  142  in each row are, in some instances, oriented end-to-end with dipoles on proximal radiators  110  in the other row, or oriented orthogonally thereto; these dipoles are substantially non-interacting. The remaining dipoles  140 ,  142  in alternate rows  144 ,  146  are parallel, and spaced between 0.5 and 0.7 wavelengths apart. These dipoles are sufficiently close to affect impedance of one another. In compensation, the termination impedance of the feed system may be altered, by a process such as that described below. Vertical spacing between the radiators  110  is substantially equal and uniform within each of the staggered rows  144 ,  146 . Spacing may be selected to provide maximum radiative efficiency, to provide beam shaping, or for other purposes. Horizontal spacing between rows  144 ,  146  may be selected to maintain isolation between orthogonal dipoles, which can be realized using a 45 degree angle between radiators  110  as shown. Vertical separation between radiators  110  may be greater or less in some embodiments, provided horizontal spacing is adjusted along with vertical spacing to control impedance and coupling characteristics. Excessive separation can produce grating lobes in some embodiments. 
     The modified quadrilateral, or “cloverleaf,” construction of the dipoles  140 ,  142  and their spacing further provides a voltage standing wave ratio (VSWR) that is low over at least a bandwidth required for cellular telephony, namely about 7.6% for the basic 900 MHz GSM band, or up to 9.1% for the P-, E-, or R-extended versions of that band. For the 1.8 GHz GSM band, bandwidth is again about 9.1%, with the gap between transmit and receive frequencies roughly equal to that of the E-GSM band. The individual monopoles of each dipole have straight portions parallel to straight portions of adjacent monopoles of the other dipole; spacing and length of these parallel portions can be selected to cause them to function as transformers with particular values of coupling. This can control an extent of isolation between the orthogonal dipoles within a radiator. 
     Design variants can be configured to realize specific azimuth beam widths. For example, 30 degree and 45 degree widths are readily implemented, and the design further supports beam narrowing to 22.5 degrees or less and broadening to 60 degrees or more. Beam width is determined by details of the “clover leaf” shape of the dipoles  140 ,  142 , by the spacing, number, and size of parasitics  170 , supported by spacer insulators  168 , by implementation of alternate backplane  108  geometries, such as basket, lip, or curved surfaces of different widths, and by other alterations. These variants permit the number of sectors making up an omnidirectional antenna to be at least 12-around or 8-around, for 30 degree and 45 degree radiator beam widths, respectively, with greater and lesser numbers likewise realizable. Selection of azimuth beam width, as well as selection of a total number of sectors serving a cell, such as eight, 12, 16, or 24 sectors, for example, may be determined by requirements such as the number of service providers operating within a cell and sharing the antenna, the number of mobile units to be served, a preferred limit of frequency reuse, and the like. 
       FIG. 4  depicts an exploded view of crossed-dipole radiator  110 , in accordance with an embodiment of the present invention. Coupling from the suspended stripline terminations within the backplane to the respective dipoles  140 ,  142  is by outer conductors  154  and inner conductors  152  that cross over in the form of unbalanced feed straps  166  and tuned stubs  150  that jointly form balanced terminations. 
     Advantageously, embodiments of the present invention include feed lines, such as, for example, rigid coaxial line feeding each dipole  140 ,  142  within the radiators  110 , each of which includes an inner conductor  152  which, after passing out through the end of an outer conductor  154 , which also provides structural support, crosses the center of the dipole  140 ,  142  by a feed strap  166  and couples by a tuned conductive feed stub  150  to another outer conductor  156 , which also provides structural support. The respective inner conductors  152  and outer conductors  154  form coaxial feed lines having characteristic impedances based on diameter ratios between the inner  152  and outer  154  conductors and the dielectric constants of any insulators/fill materials  158 . The feed stubs  150  likewise have diameter ratios with the outer conductors  156 , lengths, and dielectric fillers  160  chosen to establish termination impedances that couple signal energy to the first monopoles  162  over the selected frequency range. The feed straps  166  are unbalanced, and the spacing between the radiators further affects input impedance, so the selected lengths of the feed stubs  150  are factors in termination matching at the level of the entire antenna. 
     In one preferred embodiment, radiators  110  transmit and receive signals in the 900 MHz range. In this embodiment, the outer conductors  154 ,  156  are approximately 3.4″ long, 0.07″ thick and 0.5″ in diameter, the inner conductors  152  are approximately 4.4″ long and 0.15″ in diameter, the feed straps  166  are approximately 1.5″ long, and the stubs  150  are approximately 2.4″ long and 0.15″ in diameter. The monopole radiating elements  162 ,  164  are generally rectangular in shape, with one truncated corner, are approximately 2.6″ long on each side and have a square cross section of approximately 0.2″. These dimensions are, of course, not intended to be limiting and may be adjusted by one skilled in the art, in accordance with the teachings of the present invention, to accommodate other applications, frequency ranges, etc. 
     Advantageously, embodiments of the present invention have appreciably lower transmit signal levels and has receive functionality, each of which increases PIM product susceptibility. As a consequence, both highly smoothed component shape and uniformity of material composition within each component are potentially beneficial, while electromechanical joints are potential sources of PIM products. 
     For example, prototyping of the antenna embodiments illustrated in the figures can result in PIM products being manifested repeatedly and to some extent unpredictably. Construction of the parts shown from larger numbers of simple screw-machine formed and/or cut and stamped parts, assembled with screws, is associated with PIM production. Disassembly/reassembly activities that eliminate one PIM may introduce another. Slightly-damaged screw slots, variations in assembly torque, traces of oils in connection points, and the like all represent potential sources of PIM-related defects detectable at the receiver, requiring prolonged troubleshooting to overcome. 
     In a preferred embodiment, subgroups of the parts making up each radiator and each panel may be candidates for consolidated into single parts as shown, and enhanced processes for realizing connection uniformity may be adopted with a view to preventing generation of PIM products. For example, each of the outer conductors  154 ,  156  may be formed as a single piece with its associated monopole  162 ,  164 , such as by investment casting or a comparable high-precision metal forming process. Indeed, all four may be cast with a common base in some embodiments. Similarly, the inner conductors  152  and stubs  150 , along with feed straps  166 , may be one piece as shown, whether cast, forged, molded from a powder-metal slurry and fired to final size, or the like. The extruded backplane  108 , shown in  FIG. 3 , is likewise a product of such reduction in PIM vulnerability, since preferred embodiments have unitized construction with a continuous, substantially smooth interior that functions as a stripline reference ground. It is to be observed that any holes drilled through the extruded backplane  108  for radiator connection or stripline mounting require rigorous deburring on blind sides thereof (i.e., removal of burrs formed on interior surfaces of the extruded backplane  108  as a result of drilling inward from an external surface thereof) to suppress still other PIM product sources. 
     Materials for configurations addressed herein may vary. As previously noted, copper, copper-bearing alloys, and aluminum alloys are generally usable for at least some parts of apparatus incorporating the invention. For casting, forging, and related processes, some zinc-rich alloys exhibit desirable properties, subject to further enhancement by tin, copper, and/or alloy plating, similar to present processes for manufacturing U.S. one-cent pieces (pennies). Zinc&#39;s lower conductivity (than copper, aluminum, and some other alloys) may be of little effect in view of the low surface current densities of antennas according to the invention. For other forming processes, other materials may be preferred. Plating of conductive materials over less-conductive cores may be practical, such as electrodeposition of copper over cores molded from carbon fiber reinforced epoxy. Indeed, carbon fiber-reinforced units may be sufficiently conductive for use alone in some embodiments. Climate-driven degradation of metallic structural and bond integrity from electronegativity differences has been shown in previous applications to be a minor aspect of at least some combinations of materials in typical environments, but may require verification. Insulating coatings may be beneficial, with the understanding that effects on transmitting and receiving characteristics from applying thin layers of dielectrics may require compensation. 
     Joining conductive or conductive-surface parts is required in substantially all embodiments. In the instance of copper-over-tin plated cast zinc feeds joined to copper striplines, conventional soft or hard soldering can provide rapid, high-yield, reworkable joints. Brazing or welding processes may narrow material choices, while conventional practice for such processes introduces positioning challenges and may tend to produce spatter that can be difficult to find and remove in enclosed spaces. Screw assembly, such as in the prototype assembly procedure described above, may require more extensive testing to verify that PIM products are absent. 
       FIG. 5  depicts a crossed dipole radiator, in accordance with another embodiment of the present invention. In this embodiment, crossed-dipole radiator  210  transmits and receives signals in the 1.8 GHz frequency range. Similar in configuration to radiator  110 , the size of the constituent components is respectively reduced to accommodate the higher frequency. So, for example, crossed-dipole radiator  210  includes, inter alia, inner conductors  252 , outer conductors  254 , feed straps  266 , monopole radiating elements  264 , parasitic elements  270 , etc. 
     The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.