Patent Publication Number: US-2022216619-A1

Title: Base station antenna including fabrey-perot cavities

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Chinese Patent Application No. 201910593734.8, filed Jul. 3, 2019, the entire content of which is incorporated herein by reference as if set forth fully herein 
     FIELD 
     The present invention relates to cellular communication systems and, more particularly, to base station antennas. 
     BACKGROUND 
     Each cell in a cellular communication system has one or more antennas that are configured to provide two-way wireless radio frequency (RF) communication to mobile users geographically located within the cell. While a single antenna may be used to provide cellular service throughout the cell, multiple antennas are typically used and each antenna is configured to provide service to a respective sector of the cell. Typically, the multiple sector antennas are arranged on a tower and serve respective sectors by forming radiation beams that face outwardly in different directions in the horizontal or “azimuth” plane. 
       FIG. 1  is a schematic diagram of a conventional base station  10 . As shown in  FIG. 1 , base station  10  includes an antenna  20  that may be mounted on raised structure  30 . In the depicted embodiment, the raised structure  30  is a small antenna tower, but it will be appreciated that a wide variety of mounting locations may be used including, for example, utility poles, buildings, water towers and the like. As is further shown in  FIG. 1 , the base station  10  also includes base station equipment, such as baseband units  40  and radios  42 . A single baseband unit  40  and a single radio  42  are shown in  FIG. 1  to simplify the drawing, but it will be appreciated that more than one baseband unit  40  and/or radio  42  may be provided. Additionally, while the radio  42  is shown as being co-located with the baseband unit  40  at the bottom of the raised structure  30 , it will be appreciated that in other cases the radio  42  may be a remote radio head that is mounted on the raised structure  30  adjacent the antenna  20 . The baseband unit  40  may receive data from another source such as, for example, a backhaul network (not shown) and may process this data and provide a data stream to the radio  42 . The radio  42  may generate radio frequency (“RF”) signals that include the data encoded therein and may amplify and deliver these RF signals to the antenna  20  for transmission via a cabling connection  44 . It will also be appreciated that the base station  10  of  FIG. 1  will typically include various other equipment (not shown) such as, for example, a power supply, backup batteries, a power bus, Antenna Interface Signal Group (“AISG”) controllers and the like. 
     Typically, a base station antenna includes one or more phase-controlled arrays of radiating elements, with the radiating elements arranged in one or more vertical columns (a “column” herein, unless otherwise specified, refers to a column oriented in a vertical direction) when the antenna is mounted for use. Herein, “vertical” refers to a direction that is perpendicular relative to the plane defined by the horizon. Elements in the antenna that are referred to as being arranged, disposed or extending in a vertical direction means that when the antenna is mounted on a support structure for operation and there is no physical tilt, the elements are arranged, disposed or extending in a direction that is perpendicular relative to the plane defined by the horizon. 
     In a cellular base station having a conventional “3-sector” configuration, each sector antenna typically has a beamwidth of about 65° (a “beamwidth” herein, unless otherwise specified, refers to a half-power (−3 dB) beamwidth in an azimuth plane), as shown in  FIG. 2A . A base station may alternatively have a 6-sector configuration that may be used to increase system capacity. In a 6-sector cellular configuration, each sector antenna may have a narrower beamwidth, for example, a beamwidth of about 33° or 45° that is typically used in a cell with 6 sectors. Multiple sectors in a 6-sector cellular configuration may be covered by a multi-beam antenna that generates multiple antenna beams having different azimuth boresight pointing directions. A dual-beam antenna is one type of multi-beam antenna. An exemplary radiation pattern in the azimuth plane for a dual-beam antenna is shown in  FIG. 2B . As shown in  FIG. 2B , the radiation pattern has two antenna beams that have different azimuth boresight pointing directions, and each antenna beam has a narrower beamwidth of about 33°. The two antenna beams cover  2  adjacent sectors in a cell with 6 sectors. 
     A narrower beamwidth may be obtained by using multiple columns of radiating elements in a base station antenna, for example 3 or 4 columns of radiating elements. It is also feasible to obtain a narrower beamwidth by using an RF lens in a base station antenna. 
     SUMMARY 
     A first aspect of this invention is to provide a base station antenna. The base station antenna may comprise: a first array of radiating elements configured to emit first electromagnetic radiation; a second array of radiating elements configured to emit second electromagnetic radiation; a first backplane, the first array of radiating elements being disposed on an outer surface of the first backplane, and the first backplane being configured to reflect the first electromagnetic radiation outwardly; a second backplane, the second array of radiating elements being disposed on an outer surface of the second backplane, and the second backplane being configured to reflect the second electromagnetic radiation outwardly, wherein the first and second backplanes are positioned with a mechanical tilt relative to each other such that a direction of the first electromagnetic radiation is different from a direction of the second electromagnetic radiation in an azimuth plane; a first plate assembly configured to reflect a first portion of received electromagnetic radiation inwardly while allowing a second portion of the received electromagnetic radiation to pass outwardly through the first plate assembly, the first plate assembly being positioned to form, with the first backplane, a first Fabry-Perot cavity for the first electromagnetic radiation; and a second plate assembly configured to reflect a first portion of received electromagnetic radiation inwardly while allowing a second portion of the received electromagnetic radiation to pass outwardly through the second plate assembly, the second plate assembly being positioned to form, with the second backplane, a second Fabry-Perot cavity for the second electromagnetic radiation. 
     A second aspect of this invention is to provide a base station antenna. The base station antenna may comprise: a first array of radiating elements that are configured to emit first electromagnetic radiation; a second array of radiating elements that are configured to emit second electromagnetic radiation; a first backplane comprising a first conductor plane disposed on an inner surface of the first backplane, the first array of radiating elements being disposed on an outer surface of the first backplane; a second backplane comprising a second conductor plane disposed on an inner surface of the second backplane, the second array of radiating elements being disposed on an outer surface of the second backplane, wherein the first and second backplanes are positioned with a mechanical tilt relative to each other such that an emission direction of the first electromagnetic radiation is different from an emission direction of the second electromagnetic radiation in an azimuth plane; a first plate assembly comprising a first substrate and a plurality of first units arranged in an array disposed on the first substrate, a dimension of the first unit being a sub-wavelength of the first electromagnetic radiation, wherein the first plate assembly is positioned such that the array in which the plurality of first units are arranged receives the first electromagnetic radiation and forms, with the first conductor plane, a first Fabry-Perot cavity for the first electromagnetic radiation; and a second plate assembly comprising a second substrate and a plurality of second units arranged in an array disposed on the second substrate, a dimension of the second unit being a sub-wavelength of the second electromagnetic radiation, wherein the second plate assembly is positioned such that the array in which the plurality of second units are arranged receives the second electromagnetic radiation and forms, with the second conductor plane, a second Fabry-Perot cavity for the second electromagnetic radiation. 
     A third aspect of this invention is to provide a base station antenna. The base station antenna may comprise: a first array of radiating elements that are configured to emit first electromagnetic radiation; a second array of radiating elements that are configured to emit second electromagnetic radiation and positioned with a mechanical tilt relative to the first array of radiating elements such that an emission direction of the first electromagnetic radiation is different from an emission direction of the second electromagnetic radiation in an azimuth plane; a first reflector that is configured to reflect the first electromagnetic radiation outwardly; a second reflector that is configured to reflect the second electromagnetic radiation outwardly; a first plate assembly that is configured to reflect a first portion of received electromagnetic radiation inwardly while allowing a second portion of the received electromagnetic radiation to pass outwardly through the first plate assembly, the first plate assembly being positioned to form, with the first reflector, a first Fabry-Perot cavity for the first electromagnetic radiation; and a second plate assembly that is configured to reflect a first portion of received electromagnetic radiation inwardly while allowing a second portion of the received electromagnetic radiation to pass outwardly through the second plate assembly, the second plate assembly being positioned to form, with the second reflector, a second Fabry-Perot cavity for the second electromagnetic radiation. 
     A fourth aspect of this invention is to provide a base station antenna. The base station antenna may comprise: a first array of radiating elements that is configured to emit first electromagnetic radiation; a second array of radiating elements that is configured to emit second electromagnetic radiation; a first backplane, the first array of radiating elements being disposed on an outer surface of the first backplane, and the first backplane being configured to reflect the first electromagnetic radiation outwardly; a second backplane, the second array of radiating elements being disposed on an outer surface of the second backplane, and the second backplane being configured to reflect the second electromagnetic radiation outwardly, wherein the first and second backplanes are positioned with a mechanical tilt relative to each other such that a direction of the first electromagnetic radiation is different from a direction of the second electromagnetic radiation in an azimuth plane; and a first plate assembly that is configured to reflect a first portion of received electromagnetic radiation inwardly while allowing a second portion of the received electromagnetic radiation to pass outwardly through the first plate assembly, the first plate assembly being positioned to form, with the first backplane, a first Fabry-Perot cavity for the first electromagnetic radiation. 
     Further features of the present invention and advantages thereof will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a simplified schematic diagram showing a conventional base station in a cellular communication system. 
         FIG. 2A  is an exemplary radiation pattern in the azimuth plane of a sector antenna that is suitable for use in a conventional 3-sector cellular configuration. 
         FIG. 2B  is an exemplary radiation pattern in the azimuth plane of a dual-beam antenna that is suitable for use in a conventional 6-sector cellular configuration. 
         FIG. 3A  is a highly simplified horizontal cross-sectional view of a base station antenna according to an embodiment of the present invention. 
         FIG. 3B  is a highly simplified horizontal cross-sectional view of a base station antenna according to a further embodiment of the present invention. 
         FIG. 3C  is a highly simplified horizontal cross-sectional view of a base station antenna according to a further embodiment of the present invention. 
         FIGS. 4A and 4B  are schematic diagrams of distances between plate assemblies and backplanes in base station antennas according to some embodiments of the present invention. 
         FIGS. 5A through 5G  are plan views of plate assemblies in base station antennas according to some embodiments of the present invention. 
         FIGS. 6A through 6F  are schematic views of backplanes in base station antennas according to some embodiments of the present invention, in which arrays of radiating elements are shown. 
     
    
    
     Note that, in some cases the same elements or elements having similar functions are denoted by the same reference numerals in different drawings, and description of such elements is not repeated. In some cases, similar reference numerals and letters are used to refer to similar elements, and thus once an element is defined with reference to one figure, it need not be further discussed with reference to subsequent figures. 
     The position, size, range, or the like of each structure illustrated in the drawings may not be drawn to scale. Thus, the invention is not necessarily limited to the position, size, range, or the like as disclosed in the drawings. 
     DETAILED DESCRIPTION 
     The present invention will be described with reference to the accompanying drawings, which show a number of example embodiments thereof. It should be understood, however, that the present invention can be embodied in many different ways, and is not limited to the embodiments described below. Rather, the embodiments described below are intended to make the disclosure of the present invention more complete and fully convey the scope of the present invention to those skilled in the art. It should also be understood that the embodiments disclosed herein can be combined in any way to provide many additional embodiments. 
     The terminology used herein is for the purpose of describing particular embodiments, but is not intended to limit the scope of the present invention. All terms (including technical terms and scientific terms) used herein have meanings commonly understood by those skilled in the art unless otherwise defined. For the sake of brevity and/or clarity, well-known functions or structures may be not described in detail. 
     Herein, when an element is described as located “on” “attached” to, “connected” to, “coupled” to or “in contact with” another element, etc., the element can be directly located on, attached to, connected to, coupled to or in contact with the other element, or there may be one or more intervening elements present. In contrast, when an element is described as “directly” located “on”, “directly attached” to, “directly connected” to, “directly coupled” to or “in direct contact with” another element, there are no intervening elements present. In the description, references that a first element is arranged “adjacent” a second element can mean that the first element has a part that overlaps the second element or a part that is located above or below the second element. 
     Herein, the foregoing description may refer to elements or nodes or features being “connected” or “coupled” together. As used herein, unless expressly stated otherwise, “connected” means that one element/node/feature is electrically, mechanically, logically or otherwise directly joined to (or directly communicates with) another element/node/feature. Likewise, unless expressly stated otherwise, “coupled” means that one element/node/feature may be mechanically, electrically, logically or otherwise joined to another element/node/feature in either a direct or indirect manner to permit interaction even though the two features may not be directly connected. That is, “coupled” is intended to encompass both direct and indirect joining of elements or other features, including connection with one or more intervening elements. 
     Herein, terms such as “upper”, “lower”, “left”, “right”, “front”, “rear”, “high”, “low” may be used to describe the spatial relationship between different elements as they are shown in the drawings. It should be understood that in addition to orientations shown in the drawings, the above terms may also encompass different orientations of the device during use or operation. For example, when the device in the drawings is inverted, a first feature that was described as being “below” a second feature can be then described as being “above” the second feature. The device may be oriented otherwise (rotated 90 degrees or at other orientation), and the relative spatial relationship between the features will be correspondingly interpreted. 
     Herein, the term “A or B” used through the specification refers to “A and B” and “A or B” rather than meaning that A and B are exclusive, unless otherwise specified. 
     The term “exemplary”, as used herein, means “serving as an example, instance, or illustration”, rather than as a “model” that would be exactly duplicated. Any implementation described herein as exemplary is not necessarily to be construed as preferred or advantageous over other implementations. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the detailed description. 
     Herein, the term “substantially”, is intended to encompass any slight variations due to design or manufacturing imperfections, device or component tolerances, environmental effects and/or other factors. The term “substantially” also allows for variation from a perfect or ideal case due to parasitic effects, noise, and other practical considerations that may be present in an actual implementation. 
     Herein, certain terminology, such as the terms “first”, “second” and the like, may also be used in the following description for the purpose of reference only, and thus are not intended to be limiting. For example, the terms “first”, “second” and other such numerical terms referring to structures or elements do not imply a sequence or order unless clearly indicated by the context. 
     Further, it should be noted that, the terms “comprise”, “include”, “have” and any other variants, as used herein, specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. 
     Herein, reference coordinates used to describe a length, width and thickness of a base station antenna are the Cartesian coordinates with x′, y′ and z′ axes shown in  FIG. 3A . The direction of the x′ axis is the width direction of a base station antenna, the direction of the y′ axis is the length direction of the base station antenna, and the direction of the z′ axis is the thickness direction of the base station antenna. Further, the direction of the y′ axis is also described as a vertical direction, the plane defined by the x′ and z′ axes is described as a horizontal plane or a horizontal direction, and the positive direction of the z′ axis is described as the outer side of the base station antenna. Reference coordinates used to describe lengths, widths, and thicknesses of the plate assembly  131 , the backplane  121 , and the array of radiating elements  111  are the Cartesian coordinates with x, y and z axes shown in  FIG. 3A . The direction of the x axis is the width direction, the direction of the y axis is the length direction, and the direction of the z axis is the thickness direction of these components. Further, the positive and negative directions of the z axis are described as the outer side and the inner side of these components, respectively. It will be appreciated that reference coordinates used to describe lengths, widths, and thicknesses of the plate assembly  132 , the backplane  122 , and the array of radiating elements  112  in  FIG. 3A  are Cartesian coordinates (not shown) that is symmetric with the Cartesian coordinates with x, y and z axes about the plane defined by y′ and z′ axes; and reference coordinates used to describe lengths, widths, and thicknesses of plate assemblies, the backplanes, and arrays of radiating elements in other figures are similar to the Cartesian coordinates with x, y and z axes shown in  FIG. 3A . 
     According to an embodiment of the present invention, a multi-beam (e.g., dual-beam) base station antenna in which Fabry-Perot cavities are formed is provided. 
     Base station antennas according to embodiments of the present invention may include first and second arrays of radiating elements that are configured to respectively emit first and second electromagnetic radiation; and first and second backplanes on which the first and second arrays of radiating elements are respectively disposed. The first and second backplanes are positioned with a mechanical tilt relative to each other such that directions in which the first and second electromagnetic radiation are emitted are different in the azimuth plane. The first and second backplanes are configured to reflect inwardly-directed portions of the first and second electromagnetic radiation outwardly, respectively. The base station antenna further includes first and second plate assemblies, each of which is configured to reflect a first portion of its received electromagnetic radiation inwardly while allowing a second portion of the received electromagnetic radiation to pass outwardly therethrough. The first and second plate assemblies are positioned to form, respectively with the first and second backplanes, first and second Fabry-Perot cavities for the first and second electromagnetic radiation, respectively. The first and second plate assemblies are operated as Partially Reflective Surfaces of the respective Fabry-Perot cavities. After the first portion of the received electromagnetic radiation is reflected inwardly by a plate assembly, the first portion of the electromagnetic radiation travels inwardly to the corresponding backplane and is reflected outwardly by the backplane so as to reach the plate assembly again. Portions of the electromagnetic radiation are in-phase in the maximum radiation direction of the electromagnetic radiation, and out-of-phase in other directions. Accordingly, the electromagnetic radiation emitted by the array of radiating elements is gathered (focused) toward the maximum radiation direction so that the beam formed by the electromagnetic radiation is narrowed. Since the plate assembly may be relatively thin (for example, 1 to 2 mm), the base station antennas according to the embodiments of the present invention, as compared to conventional base station antennas having a spherical lens, a hemispherical lens or a cylindrical lens with a circular or semi-circular cross section, may have a reduced size (e.g., thickness) and improved heat dissipation. Since the Fabry-Perot cavity has an effect on focusing electromagnetic radiation, an array of radiating elements that each have, for example, a nominal 65° beamwidth in the azimuth plane may need to include only 2 columns or even 1 column of radiating elements so as to achieve a narrower beamwidth in the azimuth plane (for example, a beamwidth of 33°). Moreover, a conventional non-lensed base station antenna would typically include an array of radiating elements having 3 or 4 columns of radiating elements in order to achieve electromagnetic radiation patterns (also referred to as “antenna beams”) having azimuth beamwidths of about 33°. Accordingly, the base station antennas according to embodiments of the present invention may advantageously be smaller in size (e.g., width) as compared to conventional base station antennas with comparable capabilities, and may also advantageously have simplified feed networks. The width and length of each plate assembly may be designed according to requirements. The wider the plate assembly is, the more it narrows the antenna beam in the azimuth plane; and the longer the plate assembly is, the more it narrows the antenna beam in the elevation plane. 
     In some embodiments, the plate assembly includes a plurality of units that are arranged in an array so as to reflect the first portion of the received electromagnetic radiation inwardly while allowing the second portion to travel outwardly therethrough, where a dimension of each unit is a sub-wavelength of the received electromagnetic radiation. As long as the number of units arranged in the width direction of the plate assembly is more than a specific number, the plate assembly may have a narrowing effect on the antenna beam in the azimuth plane. For example, if the number of units arranged along the width direction of the plate assembly is not less than 10, a significant narrowing effect on the antenna beam may be achieved. The greater the number of units arranged along the width direction, the stronger the narrowing effect on the antenna beam in the azimuth plane may be achieved. The narrowing effect on the antenna beam in the elevation plane is similar to that in the azimuth plane. In the case where the dimension of each unit is a sub-wavelength such as, for example, one tenth of the wavelength, the width of the array in which the plurality of units are arranged is slightly more than one wavelength, which is obviously advantageous for reducing the size (e.g., width) of the base station antenna. 
     In some embodiments, the plate assembly may be fabricated using a mature manufacturing process such as printed circuit board (PCB) manufacturing technology, which facilitates manufacturing the plate assembly. In some embodiments, the plate assembly may be formed as at least a portion of the radome that houses the one or more arrays of radiating elements, which may facilitate simplifying the configuration and assembly of the base station antenna, further reducing the size of the base station antenna, and which may also improve heat dissipation. 
     According to further embodiments of the present invention, a multi-band base station antenna in which Fabry-Perot cavities are formed is provided. In one example embodiment of such a base station antenna, first and second arrays of radiating elements are provided that operate in a first frequency band, and third and fourth arrays of radiating elements are provided that operate in a second frequency band that is different than the first frequency band. The first and third arrays extend forwardly from the outer surface of a first backplane. The second and fourth arrays extend forwardly from the outer surface of a second backplane. The base station antenna further includes first and third plate assemblies disposed opposite the first backplane, and second and fourth plate assemblies disposed opposite the second backplane. The first and third plate assemblies respectively receive electromagnetic radiation from the first and third arrays of radiating elements, and respectively form, with the first backplane, first and third Fabry-Perot cavities for electromagnetic radiation from the first and third arrays of radiating elements, respectively. The second and fourth plate assemblies respectively receive electromagnetic radiation from the second and fourth arrays of radiating elements, and respectively form, with the second backplane, second and fourth Fabry-Perot cavities for electromagnetic radiation from the second and fourth arrays of radiating elements, respectively. Since different plate assemblies for respective arrays of radiating elements operating in different frequency bands may be arranged in multiple layers (e.g., two layers), the overall impact of adding the plate assemblies on the size of the base station antenna may be relatively small. Consequently, the multi-band base station antenna according to embodiments of the present invention may be smaller than a comparable conventional base station antenna having a radio frequency lens. 
     According to an additional embodiment of the present invention, another multi-band base station antenna is provided that includes Fabry-Perot cavities. The base station antenna includes first through third backplanes, where the first and second backplanes are positioned such that an angle between outer surfaces of the first and second backplanes is greater than 180 degrees, and the third backplane is positioned between the first and second backplanes. The first and second arrays of radiating elements extend forwardly from outer surfaces of respective the first and second backplanes. The first and second plate assemblies are respectively positioned to receive electromagnetic radiation from the first and second arrays of radiating elements, and form first and second Fabry-Perot cavities with the first and second backplanes for respective electromagnetic radiation, respectively. A third array of radiating elements whose operation frequency band is different from those of the first and second arrays of radiating elements is extends forwardly from an outer surface of the third backplane, such that the peak emission direction of the electromagnetic radiation of the third array of radiating elements in the azimuth plane is between the peak emission directions of the electromagnetic radiation of the first and second arrays of radiating elements. Since the first and second arrays of radiating elements each include only 2 columns or even 1 column of radiating elements so as to achieve a narrower beam, there may be sufficient space between the first and second arrays of radiating elements to place the third array of radiating elements, even if radiating elements in the third array of radiating elements have relatively large sizes when the array operates in a lower frequency band. 
       FIG. 3A  schematically shows the configuration of a base station antenna according to an embodiment of the present invention. The base station antenna includes first and second arrays of radiating elements  111  and  112  (only a single radiating element of each array is visible in the view of  FIG. 3A ) that extend forwardly from outer surfaces of respective first and second backplanes  121  and  122 . The backplanes  121  and  122  are configured to reflect the electromagnetic radiation from the arrays of radiating elements  111  and  112 , respectively. The arrays of radiating elements  111  and  112  each include a plurality of radiating elements that are arranged in a vertical column. The array of radiating elements  111  is configured to emit first electromagnetic radiation to generate a first antenna beam having a first pointing direction in the azimuth plane. The array of radiating elements  112  is configured to emit second electromagnetic radiation to generate a second antenna beam having a second pointing direction in the azimuth plane. The backplanes  121  and  122  are positioned with a mechanical tilt relative to each other such that the first and second pointing directions are different. 
     In the depicted embodiment, the backplanes  121  and  122  are positioned such that the angle between the outer surface of the backplane  121  and the outer surface of the backplane  122  is greater than 180 degrees. It will be appreciated that since each backplane  121 ,  122  has a physical thickness, the angle between the outer surfaces of the two backplanes refers to an angle that does not pass through the thickness of either of the backplanes  121 ,  122 . Since the angle between the outer surfaces of the backplanes  121  and  122  is greater than 180 degrees, interference between the electromagnetic radiation from the arrays of radiating elements  111  and  112  may be reduced. It will be appreciated, however, that the backplanes  121  and  122  may be positioned such that the angle between the outer surfaces of the two backplanes is less than 180 degrees, as long as there is a mechanical tilt between the two backplanes and the first and second directions are different. In the depicted embodiment, the base station antenna includes only two backplanes  121  and  122 . It will be appreciated that in other cases the base station antenna may include more backplanes with mechanical tilts therebetween. For example, additional backplanes may be provided so that the backplanes are arranged in a cylindrical shape such as, for example, a cylinder having a triangular, rectangular, or other polygonal horizontal cross section. 
     In the depicted embodiment, each of the arrays of radiating elements  111  and  112  includes a column of radiating elements. However, in some embodiments, each of the arrays of radiating elements  111  and  112  may include more than one column of radiating elements. In the depicted embodiment, the radiating elements in the first array of radiating elements  111  and the radiating elements in the second array of radiating elements  112  may be identical to each other. It will be appreciated that radiating elements in the respective first and second arrays may be different in other embodiments. In the depicted embodiment, the radiating elements in the first array  111  and the radiating elements in the second array  112  are each arranged in a single respective column to form first and second vertically-extending linear arrays  111 ,  112 . However, it will be appreciated that the radiating elements forming the respective first and second arrays  111 ,  112  may be disposed on their corresponding backplanes in any known pattern; for example, the plurality of radiating elements in a column may be staggered in the horizontal direction. In the depicted embodiment, the radiating elements in the two arrays are crossed dipole radiating elements. It will be appreciated that each of the arrays may use other suitable radiating elements including, for example, dipoles, slot radiating elements, horn waveguides, patch radiating elements, or the like. 
     The base station antenna further includes plate assemblies  131  and  132 . The plate assemblies  131  and  132  are configured to reflect a first portion of their received electromagnetic radiation inwardly and to allow a second portion of the received electromagnetic radiation to pass therethrough. In the depicted embodiment, the plate assembly  131  includes a substrate  131 - 1  and a plurality of units  131 - 2  arranged in an array that are disposed on an inner surface of the substrate  131 - 1 . The dimension of each unit  131 - 2  is a sub-wavelength of the electromagnetic radiation that is emitted by the first array of radiating elements  111 , such that the plate assembly  131  may reflect the first portion of the electromagnetic radiation received from the first array  111  inwardly while allowing the second portion of the received electromagnetic radiation to pass outwardly through the plate assembly  131 . The plate assembly  131  is positioned to form a first Fabry-Perot cavity with the backplane  121 . The first Fabry-Perot cavity is for the electromagnetic radiation from the first array of radiating elements  111 . The plate assembly  132  includes a substrate  132 - 1  and a plurality of units  132 - 2  arranged in an array that are disposed on an inner surface of the substrate  132 - 1 . The dimension of each unit  132 - 2  is a sub-wavelength of the electromagnetic radiation that is emitted by the second array of radiating elements  112 , such that the plate assembly  132  may reflect the first portion of the electromagnetic radiation received from the second array  112  inwardly while allowing the second portion of the received electromagnetic radiation to pass outwardly through the plate assembly  132 . The plate assembly  132  is positioned to form a second Fabry-Perot cavity with the backplane  122 . The second Fabry-Perot cavity is for electromagnetic radiation from the second array of radiating elements  112 . 
     The dimension of the units  131 - 2  or  132 - 2  refers to a dimension of the units  131 - 2  or  132 - 2  in at least one direction in a plan view that is parallel to the main surface of the respective plate assembly  131  or  132 . The sub-wavelength of electromagnetic radiation refers to a wavelength that is equal to or less than the wavelength corresponding to the center frequency of the emitted electromagnetic radiation. In the depicted embodiment, the array in which the plurality of units  131 - 2  are arranged and the array in which the plurality of units  132 - 2  are arranged are disposed on the inner surfaces of the substrates  131 - 1  and  132 - 1 , respectively. However, it will be appreciated that the two arrays may both be disposed on the outer surfaces of the respective substrates  131 - 1  and  132 - 1 , or one may be disposed on the inner surface of the corresponding substrate and the other disposed on the outer surface of the corresponding substrate. In other embodiments, the arrays may be arranged within interiors of the respective substrates  131 - 1 ,  132 - 1 . In still other embodiments, although not shown in the drawings, the plurality of units arranged in an array may not be disposed on either surface of the substrate. For example, the substrate may be formed of a conductive material and the plurality of units may be a plurality of apertures arranged in an array that are formed in the substrate. 
     In some embodiments, in the length directions of the plate assemblies  131  and  132 , the dimensions of the arrays, in which the plurality of units are arranged, may be slightly smaller than, substantially equal to, or larger (maybe slightly) than the lengths of respective arrays of radiating elements  111  and  112 . In some embodiments, in the width directions of the plate assemblies  131  and  132 , the dimensions of the arrays, in which the plurality of units are arranged, may be slightly smaller than, substantially equal to, or larger (maybe slightly) than the widths of respective backplanes  121  and  122 . In some embodiments, in the width direction of the plate assemblies  131  and  132 , the dimensions of the arrays, in which the plurality of units are arranged, may be related to the widths of respective arrays of radiating elements  111  and  112 , for example, the widths of the arrays of units may be 5-8 times the widths of the respective arrays of radiating elements  111  and  112 . 
     The plate assemblies  131  and  132  are positioned substantially parallel to and spaced apart from the respective backplanes  121  and  122  by a specific distance h so as to form respective Fabry-Perot cavities. According to the resonant condition of a Fabry-Perot cavity, the distance h between a plate assembly and a corresponding backplane is determined by: 
         h =(φ 1 +φ 2   −N 2π)λ/4π  Equation (1)
 
     In Equation (1), φ 1  denotes the reflection phase of the backplane with respect to the electromagnetic radiation, φ 2  denotes the reflection phase of the plate assembly with respect to the electromagnetic radiation, λ is the wavelength of the electromagnetic radiation, and N is a non-negative integer, i.e., N=0, 1, 2, . . . . 
     The distance h between the plate assembly and the corresponding backplane will be described below in connection with  FIGS. 4A and 4B  and taking the plate assembly  131  and the backplane  121  for example. As shown in  FIG. 4A , in some embodiments, the backplane  121  includes a dielectric substrate  121 - 1  and a conductor ground plane  121 - 2  formed on an inner surface of the dielectric substrate  121 - 1 . A patch radiating element  161  is disposed on an outer surface of the dielectric substrate  121 - 1 . The plate assembly  131  includes a substrate  131 - 1  formed of a dielectric material and a plurality of conductor units  131 - 2  arranged in an array on an inner surface of the substrate  131 - 1 . A dimension of the conductor unit  131 - 2  is a sub-wavelength of electromagnetic radiation that is emitted by the patch radiating element  161 . The reflection phase of the backplane  121  (for example, the conductor ground plane  121 - 2  having a reflection function included in the backplane  121 ) with respect to the electromagnetic radiation that is emitted by the patch radiating element  161  is π, the reflection phase of the plate assembly  131  (for example, the array in which the plurality of conductor units  131 - 2  are arranged having a reflection function included in the plate assembly  131 ) with respect to the electromagnetic radiation that is emitted by the patch radiating element  161  is also π, that is, φ 1 =φ 2 =π in the Equation (1). Then, according to Equation (1), the distance h between the plate assembly  131  and the backplane  121  when satisfying the resonant condition of the Fabry-Perot cavity is calculated to be Nλ/2. Therefore, in these embodiments, the plate assembly  131  is positioned such that the distance h between the plate assembly  131  and the backplane  121  (for example, the array in which the plurality of conductor units  131 - 2  are arranged and the conductor ground plane  121 - 2 ) is substantially an integer multiple of a half wavelength of the electromagnetic radiation emitted by the patch radiating element  161 . 
     Changing nature of the surface having the reflection function in the backplane affects the reflection phase of the backplane with respect to the electromagnetic radiation, that is, making φ 1 ≠π so that the distance h between the plate assembly and the backplane when satisfying the resonant condition of the Fabry-Perot cavity changes. As shown in  FIG. 4B , in some embodiments, the backplane  121  includes a dielectric substrate  121 - 1 , a conductor ground plane  121 - 2  that is formed on an inner surface of the dielectric substrate  121 - 1 , and a plurality of conductor units  121 - 3  arranged in an array that are disposed on an outer surface of the dielectric substrate  121 - 1 . A dimension of the conductor unit  121 - 3  is a sub-wavelength of the electromagnetic radiation that is emitted by the patch radiating element  161 . The reflection phase of the backplane  121  (for example, the array in which the plurality of conductor units  121 - 3  are arranged and the conductor ground plane  121 - 2  having reflection functions included in the backplane  121 ) with respect to the electromagnetic radiation that is emitted by the patch radiating element  161  is zero, the reflection phase of the plate assembly  131  (for example, the array in which the plurality of conductor units  131 - 2  are arranged having a reflection function included in the plate assembly  131 ) with respect to the electromagnetic radiation that is emitted by the patch radiating element  161  is still π, that is, φ 1 =0 and φ 2 =π in the Equation (1). Then, according to Equation (1), the distance h between the plate assembly  131  and the backplane  121  when satisfying the resonant condition of the Fabry-Perot cavity is calculated to be Nλ/4. Therefore, in these embodiments, the plate assembly  131  is positioned such that the distance h between the plate assembly  131  and the backplane  121  (for example, the array in which the plurality of conductor units  131 - 2  are arranged and the conductor ground plane  121 - 2 ) is substantially an integer multiple of a quarter wavelength of the electromagnetic radiation from the radiating element  161 . 
     In the depicted embodiment, the radiating element  161  is a patch radiating element, the array in which the plurality of conductor units  131 - 2  are arranged is disposed on the inner surface of the substrate  131 - 1 , and the conductor ground plane  121 - 2  is disposed on the outer surface of the dielectric substrate  121 - 1 . However, it will be appreciated that the radiating element  161  may be any suitable radiating element, the array in which the plurality of conductor units  131 - 2  are arranged may be disposed on either surface of the substrate  131 - 1 , and the conductor ground plane  121 - 2  may be disposed on either surface of the dielectric substrate  121 - 1 . 
       FIGS. 6A through 6F  schematically illustrate backplanes in base station antennas according to some embodiments of the present invention, where arrays of radiating elements  111  are disposed on outer surfaces of backplanes.  FIGS. 6A and 6B  are highly simplified side view and front view, respectively, of a backplane in a base station antenna according to an embodiment of the present invention. In this embodiment, feed boards  172  for feeding radiating elements are disposed inside a reflector  171 . The radiating element may be mounted on the feed board  172  through a hole formed in the reflector  171 . A plurality of feed boards  172  may be provided, each of which may feed a row of radiating elements in the array  111 . Although each row includes only one radiating element in the depicted embodiment, it will be appreciated that each row may include more radiating elements. In this embodiment, the backplane  121  that forms the Fabry-Perot cavity with the plate assembly  131  may be the reflector  171 . 
       FIGS. 6C and 6D  are highly simplified side view and front view, respectively, of a backplane in a base station antenna according to another embodiment of the present invention. In this embodiment, feed boards  172  for feeding radiating elements are disposed outside a reflector  171 . The radiating element is mounted on the feed board  172 . A plurality of feed boards  172  may be provided, each of which may feed a row of radiating elements in the array  111 . In this embodiment, the backplane  121  that forms the Fabry-Perot cavity with the plate assembly  131  may be the plurality of feed boards  172 , wherein the conductor plane that is disposed on the inner surface of the backplane  121  may be the whole of ground planes that are respectively disposed on the inner surfaces of the plurality of feed boards  172 . The size of the gap between adjacent feed boards  172  may be configured to be much smaller than the wavelength of the electromagnetic radiation of the radiating elements so as to avoid the electromagnetic radiation passing through the gap. 
       FIGS. 6E and 6F  are highly simplified side view and front view, respectively, of a backplane in a base station antenna according to another embodiment of the present invention. In this embodiment, a feed board  172  for feeding radiating elements is disposed outside a reflector  171 . The radiating elements are mounted on the feed board  172 . In this embodiment, a single feed plate  172  feeds each radiating elements in the array  111 . In this embodiment, the backplane  121  that forms the Fabry-Perot cavity with the plate assembly  131  may be the feed board  172 , wherein the conductor plane that is disposed on the inner surface of the backplane  121  may be the ground plane that is disposed on the inner surface of the feed board  172 . This is easier to be implemented in the case where the array  111  operates in a higher frequency band, because the dimensions of the radiating element and the feed board  172  (usually implemented by a printed circuit board PCB) are relatively small when the operating frequency band of the array  111  is higher. Therefore, it is easier to feed all of the radiating elements in the array  111  by a single feed board  172 . 
     In the embodiment depicted in  FIG. 3A , the distance between the plate assembly  131  and the backplane  121  is substantially equal to the distance between the plate assembly  132  and the backplane  122 . However, it will be appreciated that the two distances may be unequal, and either may be designed according to actual requirements. The base station antenna further includes a radome  141  that houses the first and second arrays of radiating elements  111  and  112 . At least one of the plate assemblies  131  and  132  may be formed as at least a portion of the radome  141 . 
       FIGS. 5A through 5G  are plan views schematically showing example implementations of the plate assembly  131  in base station antennas according to some embodiments of the present invention. In some embodiments, the substrate  131 - 1  of the plate assembly  131  is formed of a dielectric material, and the plurality of units  131 - 2  arranged in an array are formed of a conductive material on a surface of the substrate  131 - 1 . In some embodiments, the substrate  131 - 1  of the plate assembly  131  is formed of a conductive material, and the plurality of units  131 - 2  arranged in an array are apertures formed in the substrate  131 - 1 . Each of the units  131 - 2  shown in each of  FIGS. 5A through 5G  may be the above-described conductive material formed on a surface of the dielectric material substrate  131 - 1 , or may be the above-described apertures formed in the conductive material substrate  131 - 1 . For example, in  FIG. 5A , each unit  131 - 2  is rectangular, which may be either a solid conductor or a hollow aperture. The shape of each unit  131 - 2  is not limited to those shown in the drawings, as long as the dimension of the unit  131 - 2  is a sub-wavelength, and the plurality of units  131 - 2  are arranged in an array to form a periodic structure. For example, the unit  131 - 2  may be a solid shape (such as the shape shown in  FIG. 5A or 5B ), a hollow shape (such as the shape shown in  FIG. 5C or 5D ), a stripe (such as the shape shown in  FIG. 5G ), an unclosed shape (such as the shape shown in  FIG. 5E ), an irregular shape (such as the shape shown in  FIG. 5F ), or the like. 
     In some embodiments, the dimension of the unit is equal to about one tenth of the wavelength of the electromagnetic radiation received by the plate assembly. The dimension of the unit refers to the dimension of the unit along at least one direction (including but not limited to the length direction, width direction, diagonal direction, etc. of the plate assembly) in a plan view that is parallel to the main surface of the plate assembly. It will be appreciated that in other embodiments, the dimension of the unit may be smaller than one tenth of the wavelength, but smaller dimension always causes higher cost. In some embodiments, the number of units arranged in an array is greater than or equal to 10 along at least one direction in the plan view.  FIGS. 5A through 5G  also show dimensions d1 and d2 of the unit  131 - 2  in first and second directions (e.g., a width direction and a length direction) of the plate assembly  131 . In the example shown in  FIG. 5G , a plurality of units  132 - 2  are arranged along the first direction of the plate assembly  131 , and only one unit  132 - 2  is arranged along the second direction. Therefore, the plate assembly  131  may achieve the effect on narrowing the beam in the first direction, but may not achieve the effect on narrowing the beam in the second direction. In the case where the first direction is the width direction, the plate assembly  131  shown in  FIG. 5G  may focus the electromagnetic radiation in the azimuth plane. In the case where the first direction is the length direction, the plate assembly  131  shown in  FIG. 5G  may focus the electromagnetic radiation in the elevation plane. 
       FIG. 3B  schematically shows a configuration of a base station antenna according to a further embodiment of the present invention. The base station antenna includes arrays of radiating elements  113  through  115  which are respectively disposed on and extend forwardly from outer surfaces of the respective backplanes  121  through  123 . The backplanes  121  and  122  are configured to respectively reflect the electromagnetic radiation from the arrays of radiating elements  113  and  114  outwardly. Each of the arrays of radiating elements  113  through  115  includes a column of radiating elements. The array of radiating elements  113  is configured to emit first electromagnetic radiation within all or a portion of a first frequency band (e.g., 1710˜2690 MHz band and/or 3300˜6000 MHz band), the array of radiating elements  114  is configured to emit second electromagnetic radiation within all or a portion of the first frequency band as well, and the array of radiating elements  115  is configured to emit third electromagnetic radiation within all or a portion of a second frequency band (e.g., 694˜960 MHz band) that is different from the first frequency band. In the depicted embodiment, the second frequency band is lower than the first frequency band such that sizes of radiating elements in the array  115  are larger than sizes of radiating elements in the arrays  113  and  114 . The base station antenna further includes plate assemblies  131  and  132 , and a radome  141  that houses the arrays of radiating elements  113  through  115 . Since each of the plate assemblies  131  and  132  may be similar to that described above, duplicate descriptions will be omitted. In some embodiments, at least one of the plate assemblies  131  and  132  may be formed as at least a portion of the radome  141 . 
     The backplanes  121  and  122  are positioned with a mechanical tilt relative to each other such that the directions in which the first and second electromagnetic radiation are emitted are different. The backplane  123  is positioned between the backplanes  121  and  122 . Two vertical sides of the backplane  123  are mechanically coupled to respective sides of the backplanes  121  and  122 , respectively. The backplane  123  is oriented substantially along the width direction of the base station antenna, and the angle between the outer surface of the backplane  121  and the outer surface of the backplane  123  is substantially equal to the angle between the outer surface of the backplane  122  and the outer surface of the backplane  123 . Thus, in the azimuth plane, the direction of the third electromagnetic radiation may be about midway between the directions of the first and second electromagnetic radiation. 
     In the depicted embodiment, since the second frequency band in which the array of radiating elements  115  operates is lower than the first frequency band in which the arrays of radiating elements  113  and  114  operate, the radiating elements in the array of radiating elements  115  are larger than the radiating elements in the arrays of radiating elements  113  and  114 . The distance from the radiating arms (or surfaces, apertures, etc.) of the radiating elements in the array of radiating elements  115  to the outer surface of the backplane  123  is greater than the distances of the plate assemblies  131  and  132  to the outer surfaces of the respective backplanes  121  and  122 . That is, the radiating arms of each radiating element in the array of radiating elements  115  are located on outer sides of the plate assemblies  131  and  132 . This configuration may prevent the plate assemblies  131  and  132  from receiving electromagnetic radiation from the array of radiating elements  115 . In the depicted embodiment, each of the arrays of radiating elements  113  through  115  includes only one column of radiating elements. However, it will be appreciated that each array may include more columns of radiating elements in other embodiments. 
       FIG. 3C  schematically shows a configuration of a base station antenna according to a further embodiment of the present invention. The base station antenna includes arrays of radiating elements  116  through  119 . The arrays of radiating elements  116  and  117  are disposed on an outer surface of the backplane  121 , and the arrays of radiating elements  118  and  119  are disposed on an outer surface of the backplane  122 . The backplane  121  is configured to reflect the electromagnetic radiation from the arrays of radiating elements  116  and  117  outwardly, and the backplane  122  is configured to reflect the electromagnetic radiation from the arrays of radiating elements  118  and  119  outwardly. In the depicted embodiment, the array  116  includes two columns of radiating elements and the array  117  includes one column of radiating elements. The one column of radiating elements in array  117  is disposed between the two columns of radiating elements in array  116 , such that the arrays of radiating elements  116  and  117  are interdigitated on the outer surface of the backplane  121 . The array  118  includes two columns of radiating elements and the array  119  includes one column of radiating elements. The one column of radiating elements in array  119  is disposed between the two columns of radiating elements in array  118 , such that the arrays of radiating elements  118  and  119  are interdigitated on the outer surface of the backplane  122 . It will be appreciated, however, that each array of radiating elements may include any suitable number of columns of radiating elements, and the arrangement of the two arrays that are disposed on the same backplane may be designed as needed. The arrays of radiating elements  116  and  118  are configured to operate in all or a portion of a first frequency band (e.g., 1710˜2690 MHz band and/or 3300˜6000 MHz band), and the arrays of radiating elements  117  and  119  are configured to operate in all or a portion of a second frequency band (e.g., 694˜960 MHz band). In the depicted embodiment, the second frequency band is lower than the first frequency band such that the radiating elements in the arrays  117  and  119  are larger than the radiating elements in the arrays  116  and  118 . It will be appreciated, however, that the second frequency band may be higher than the first frequency band such that the radiating elements in the arrays  117  and  119  may be smaller than the radiating elements in the arrays  116  and  118  in other embodiments. 
     The base station antenna further includes plate assemblies  131  through  134 . The plate assemblies  131  through  134  are each configured to reflect a first portion of received electromagnetic radiation inwardly and to pass a second portion of the received electromagnetic radiation outwardly through the respective plate assemblies. In the depicted embodiment, the plate assembly  131  includes a substrate  131 - 1  and a plurality of units  131 - 2  arranged in an array that are disposed on an inner surface of the substrate  131 - 1 , and the plate assembly  133  includes a substrate  133 - 1  and a plurality of units  133 - 2  arranged in an array that are disposed on an inner surface of the substrate  133 - 1 . The plate assembly  132  includes a substrate  132 - 1  and a plurality of units  132 - 2  arranged in an array that are disposed on an inner surface of the substrate  132 - 1 , and the plate assembly  134  includes a substrate  134 - 1  and a plurality of units  134 - 2  arranged in an array that are disposed on an inner surface of the substrate  134 - 1 . 
     The plate assemblies  131  and  133  are each substantially parallel to the backplane  121  and are positioned at respective distances h 1  and h 2  from the backplane  121 , such that the plate assemblies  131  and  133  and the backplane  121  form Fabry-Perot cavities for the electromagnetic radiation emitted by the respective arrays of radiating elements  116  and  117 . For example, the plate assembly  131  and the backplane  121  may form a first Fabry-Perot cavity for electromagnetic radiation emitted by the array of radiating elements  116 , where the distance h 1  between the plate assembly  131  and the backplane  121 , and the dimension of the unit  131 - 2  are both related to the wavelength of the electromagnetic radiation emitted by the array of radiating elements  116 . The plate assembly  133  and the backplane  121  may form a second Fabry-Perot cavity for electromagnetic radiation emitted by the array of radiating elements  117 , where the distance h 2  between the plate assembly  133  and the backplane  121 , and the dimension of the unit  133 - 2  are both related to the wavelength of the electromagnetic radiation emitted by the array of radiating elements  117 . It will be appreciated that the plate assembly  131  may be used for the array of radiating elements  117 , where the distance h 1  and the dimension of the unit  131 - 2  may be related to the wavelength of the electromagnetic radiation emitted by the array of radiating elements  117 ; and the plate assembly  133  may be used for the array of radiating elements  116 , where the distance h 2  and the dimension of the unit  133 - 2  may be related to the wavelength of the electromagnetic radiation emitted by the array of radiating elements  116 . Similarly, the plate assemblies  132  and  134  are each substantially parallel to backplane  122  and are positioned to form, with the backplane  122 , Fabry-Pero cavities for the electromagnetic radiation emitted by the respective arrays of radiating elements  118  and  119 . 
     The arrays of radiating elements  116  and  117  are interdigitated on the outer surface of the backplane  121 , and therefore, the plate assemblies  131  and  133  that are configured to respectively receive the electromagnetic radiation from the arrays of radiating elements  116  and  117  are parallel to and overlap each other in a plan view parallel to the main surface of one of the plate assemblies  131  and  133 . The arrays of radiating elements  118  and  119  are interdigitated on the outer surface of the backplane  122 , and therefore, the plate assemblies  132  and  134  that are configured to respectively receive the electromagnetic radiation from the arrays of radiating elements  118  and  119  are parallel to and overlap each other in a plan view parallel to the main surface of one of the plate assemblies  132  and  134 . 
     The base station antenna further includes a radome  141  that houses the arrays of radiating elements  116  through  119 . At least one of the plate assemblies  131  through  134  may be formed as at least a portion of the radome  141 . In some embodiments, at least a portion of the radome  141  has a multi-layered structure, e.g., a structure with at least two layers that are parallel to each other. For example, the plate assembly  131  is formed as a first layer in the multi-layered structure of the at least a portion of the radome  141 , and the plate assembly  133  is formed as a second layer in the multi-layered structure. 
     In addition, the base station antenna may further include other conventional components not shown in  FIGS. 3A through 3C , such as a reflector assembly and a plurality of circuit components and other structures mounted therein. These circuit components and other structures may include, for example, phase shifters for one or more arrays of radiating elements, remote electronic tilt (RET) actuators for mechanically adjusting the phase shifters, one or more controllers, cable connections, RF transmission lines, etc. A mounting bracket (not shown) may also be provided for mounting the base station antenna to another structure, such as an antenna tower or utility pole. 
     Embodiments are described herein primarily with respect to operations of base station antennas in a transmitting mode in which an array of radiating elements emits electromagnetic radiation. It will be appreciated that base station antennas according to embodiments of the present invention may operate in a transmitting mode and/or a receiving mode in which an array of radiating elements receives electromagnetic radiation. The plate assemblies and backplanes described herein may form Fabry-Perot cavities for such received electromagnetic radiation in order to narrow the beamwidth of the antenna beam for received electromagnetic radiation. 
     Although some specific embodiments of the present invention have been described in detail with examples, it should be understood by a person skilled in the art that the above examples are only intended to be illustrative but not to limit the scope of the present invention. The embodiments disclosed herein can be combined arbitrarily with each other, without departing from the scope and spirit of the present invention. It should be understood by a person skilled in the art that the above embodiments can be modified without departing from the scope and spirit of the present invention. The scope of the present invention is defined by the attached claims.