WO2025042680A1

WO2025042680A1 - Base station antenna systems having frequency selective surfaces

Info

Publication number: WO2025042680A1
Application number: PCT/US2024/042437
Authority: WO
Inventors: Xiangyang Ai; Chengcheng Tang; Peter J. Bisiules
Original assignee: Outdoor Wireless Networks LLC
Current assignee: Outdoor Wireless Networks LLC
Priority date: 2023-08-21
Filing date: 2024-08-15
Publication date: 2025-02-27
Anticipated expiration: 2026-02-21

Abstract

An antenna system includes a radiating element layer including a plurality of radiating elements, a reflective frequency selective surface (FSS) layer having a resonance transmission frequency range, and an absorptive FSS layer between the reflective FSS layer and the radiating element layer and having a resonance absorption frequency range. The resonance transmission frequency range is greater than the resonance absorption frequency range.

Description

Attorney Docket No.9833.7119.WO BASE STATION ANTENNA SYSTEMS HAVING FREQUENCY SELECTIVE SURFACES CROSS-REFERENCE TO RELATED APPLICATION [0001] The present application claims priority to and the benefit of U.S. Provisional Patent Application Serial No. 63/520,791, filed August 21, 2023, the disclosures of which are hereby incorporated by reference in their entireties. FIELD OF THE INVENTION [0002] The present invention generally relates to radio communications and, more particularly, to base station antenna systems that include frequency selective surfaces. BACKGROUND [0003] Cellular communications systems are well known in the art. In a cellular communications system, a geographic area is divided into a series of regions that are referred to as "cells" which are served by respective base stations. The base station may include one or more base station antennas that are configured to provide two-way radio frequency ("RF") communications with mobile subscribers that are within the cell served by the base station. Typically, the base station antennas are mounted on a tower or other raised structure, with the radiation patterns (also referred to herein as "antenna beams") that are generated by the base station antennas directed outwardly. [0004] A common base station configuration is the three sector configuration in which a cell is divided into three 120º "sectors" in the azimuth (horizontal) plane. A separate base station antenna provides coverage (service) to each sector. Typically, each base station antenna will include multiple vertically-extending columns of radiating elements that operate, for example, using second generation ("2G"), third generation ("3G") or fourth generation ("4G") cellular network protocols. These vertically-extending columns of radiating elements are typically referred to as "linear arrays," and may be straight columns or columns in which some of the radiating elements are staggered horizontally. Most modern base station antennas include both "low-band" linear arrays of radiating elements that support service in some or all of the 617- 960 MHz frequency band and "mid-band" linear arrays of radiating elements that support service Attorney Docket No.9833.7119.WO in some or all of the 1427-2690 MHz frequency band. These linear arrays are typically formed using dual-polarized radiating elements, which allows each array to transmit and receive RF signals at two orthogonal polarizations. [0005] Each of the above-described linear arrays is coupled to two ports of a radio (one port for each polarization). An RF signal that is to be transmitted by a linear array is passed from the radio to the antenna, where it is divided into a plurality of sub-components, with each sub- component fed to a respective subset of the radiating elements in the linear array (typically each sub-component is fed to between one and three radiating elements). The sub-components of the RF signal are transmitted through the radiating elements to generate an antenna beam that covers a generally fixed coverage area, such as a sector of a cell. Typically, these linear arrays will have remote electronic tilt ("RET") capabilities, which allow a cellular operator to change the pointing angle of the generated antenna beams in the elevation (vertical) plane in order to change the size of the sector served by the linear array. Since the antenna beams generated by the above-described 2G/3G/4G linear arrays generate static antenna beams, they are often referred to as "passive" linear arrays. [0006] Most cellular operators are currently upgrading their networks to support fifth generation ("5G") cellular service. One important component of 5G cellular service is the use of so-called multi-column "active" beamforming arrays that operate in conjunction with active beamforming radios to dynamically adjust the size, shape and pointing direction of the antenna beams that are generated by the active beamforming array. These active beamforming arrays are typically formed using "high-band" radiating elements that operate in higher frequency bands, such as some or all of the 3.3-4.2 GHz and/or the 5.1-5.8 GHz frequency bands. Each column of such an active beamforming array is typically coupled to a respective port of a beamforming radio. The beamforming radio may be a separate device, or may be integrated with the active antenna array. The beamforming radio may adjust the amplitudes and phases of the sub- components of an RF signal that are fed to each port of the radio to generate antenna beams that have narrowed beamwidths in the azimuth plane (and hence higher antenna gain). These narrowed antenna beams can be electronically steered in the azimuth plane by proper selection of the amplitudes and phases of the sub-components of an RF signal. [0007] To avoid having to increase the number of antennas at cell sites, the above- described 5G antennas also often include passive linear arrays that support legacy 2G, 3G and/or Attorney Docket No.9833.7119.WO 4G cellular services. In some cases, both the active beamforming arrays and the passive linear arrays may be included in a single base station antenna. Another solution for providing an antenna that supports both 2G/3G/4G and 5G cellular service is to mount a 5G active antenna module (i.e., a module that includes an active beamforming array and associated beamforming radio) on the rear surface of a passive base station antenna that includes a plurality of 2G, 3G, and/or 4G passive linear arrays. [0008] However, the antenna systems may be subjected to passive intermodulation (PIM) products, which generally refers to undesired RF signals that can be generated within the antenna structure when currents generated by two or more different RF signals pass through a non-linear junction. PIM products may be caused by nonlinearities in mechanical components of the antenna system (e.g., nonlinearities of the antenna connectors, junctions of dissimilar materials, etc.), mechanical stress or deformation of mechanical components of the antenna system, and/or electromagnetic coupling between the passive and active antenna arrays. In some embodiments, PIM products may also be caused by metal objects that are proximate to the antenna system (e.g., guy wires, anchors, roof flashings, and pipes), rust, corrosion, loose connections, dirt, and/or oxidation within the antenna system. Accordingly, PIM products may increase noise and reduce the signal quality and the coverage range of the antenna system. SUMMARY [0009] Pursuant to embodiments of the present invention, an antenna system includes a radiating element layer comprising a plurality of radiating elements, a reflective frequency selective surface (FSS) layer having a resonance transmission frequency range, and an absorptive FSS layer between the reflective FSS layer and the radiating element layer and having a resonance absorption frequency range, where the resonance transmission frequency range is greater than the resonance absorption frequency range. [0010] In some embodiments, the resonance transmission frequency range is within at least a portion of a 3,300-4,000 megahertz band. [0011] In some embodiments, the resonance absorption frequency range is within at least a portion of a 700-2,300 megahertz band. [0012] In some embodiments, the reflective FSS layer may include a plurality of frequency selective reflective elements, and where each frequency selective reflective element defines an opening. Attorney Docket No.9833.7119.WO [0013] In some embodiments, the plurality of frequency selective reflective elements comprise copper. [0014] In some embodiments, a dimensional characteristic of the opening is based on the resonance transmission frequency range. [0015] In some embodiments, the absorptive FSS layer comprises a plurality of frequency selective absorptive elements, and where each frequency selective absorptive element of the plurality of frequency selective absorptive elements comprises a dielectric substrate and at least one metal element on the dielectric substrate. [0016] In some embodiments, the dielectric substrate is a printed circuit board. [0017] In some embodiments, each metal element of the at least one metal element defines an opening, and where a dimensional characteristic of the opening is based on the resonance absorption frequency range. [0018] In some embodiments, the at least one metal element comprises a first metal element on a first surface of the dielectric substrate and a second metal element on a second surface of the dielectric substrate. [0019] In some embodiments, each metal element of the at least one metal element comprises a plurality of perimeter elements on an edge of the dielectric substrate, a plurality of extension elements that extend substantially perpendicular from the perimeter elements toward a center of the metal element, and a plurality of resistive elements. [0020] In some embodiments, adjacent extension elements of the plurality of extension elements are coupled by a given resistive element of the plurality of resistive elements. [0021] In some embodiments, the antenna system further includes a radio layer comprising at least one radio frequency (RF) port that is coupled to the plurality of radiating elements. [0022] In some embodiments, the antenna system further includes an additional radiating element layer comprising a plurality of additional radiating elements, where the reflective FSS layer is between the additional radiating element layer and the absorptive FSS layer. [0023] In some embodiments, the plurality of radiating elements comprises a plurality of beamforming radiating elements. [0024] Pursuant to embodiments of the present invention, a base station antenna includes a first radiating element layer comprising a first plurality of radiating elements configured to Attorney Docket No.9833.7119.WO operate in a first frequency range, a second radiating element layer comprising a second plurality of radiating elements configured to operate in a second frequency range, and an absorptive FSS layer between the first radiating element layer and the second radiating element layer and configured to absorb radio frequency (RF) signals in a resonance absorption frequency range. [0025] In some embodiments, the resonance absorption frequency range is within at least a portion of a 700-2,300 megahertz band. [0026] In some embodiments, the second frequency range is within at least a portion of a 1,700-2,700 megahertz band. [0027] In some embodiments, the first frequency range is within at least a portion of a 3,300-4,000 megahertz band. [0028] In some embodiments, the resonance absorption frequency range is within at least a portion of a 700-2,300 megahertz band, the first frequency range is within at least a portion of a 3,300-4,000 megahertz band, and the second frequency range is within at least a portion of a 1,700-2,700 megahertz band. [0029] In some embodiments, the absorptive FSS layer comprises a plurality of frequency selective absorptive elements, and where each frequency selective absorptive element of the plurality of frequency selective absorptive elements comprises a dielectric substrate and at least one metal element on the dielectric substrate. [0030] In some embodiments, the dielectric substrate is a printed circuit board. [0031] In some embodiments, each metal element of the at least one metal element defines an opening, and where a dimensional characteristic of the opening is based on the resonance absorption frequency range. [0032] In some embodiments, the at least one metal element comprises a first metal element on a first surface of the dielectric substrate and a second metal element on a second surface of the dielectric substrate. [0033] In some embodiments, each metal element of the at least one metal element comprises a plurality of perimeter elements on an edge of the dielectric substrate, a plurality of extension elements that extend substantially perpendicular from the perimeter elements toward a center of the metal element, and a plurality of resistive elements. [0034] In some embodiments, adjacent extension elements of the plurality of extension elements are coupled by a given resistive element of the plurality of resistive elements. Attorney Docket No.9833.7119.WO [0035] In some embodiments, the antenna system includes a radio layer comprising at least one radio frequency (RF) port that is coupled to the first plurality of radiating elements. [0036] In some embodiments, the absorptive FSS layer includes a plurality of resistive elements. [0037] In some embodiments, the first plurality of radiating elements comprises a plurality of beamforming radiating elements. [0038] Pursuant to embodiments of the present invention, a base station antenna comprises an array of radiating elements and an absorptive FSS layer comprising a plurality of frequency selective absorptive elements positioned behind the array of radiating elements, where each frequency selective absorptive element of the plurality of frequency selective absorptive elements comprises a dielectric substrate and at least one metal element on the dielectric substrate. [0039] In some embodiments, the dielectric substrate is a printed circuit board. [0040] In some embodiments, each metal element of the at least one metal element defines an opening. [0041] In some embodiments, a dimensional characteristic of the opening is based on a resonance absorption frequency range of the absorptive FSS. [0042] In some embodiments, the resonance absorption frequency range is within at least a portion of a 700-2,300 megahertz band. [0043] In some embodiments, each metal element of the at least one metal element comprises a plurality of perimeter elements on an edge of the dielectric substrate, a plurality of extension elements that extend substantially perpendicular from the perimeter elements toward a center of the metal element, and a plurality of resistive elements. [0044] In some embodiments, adjacent extension elements of the plurality of extension elements are coupled by a given resistive element of the plurality of resistive elements. [0045] In some embodiments, the plurality of perimeter elements and the plurality of extension elements comprise copper. [0046] In some embodiments, the plurality of resistive elements comprise a plurality of integrated passive resistive devices. [0047] In some embodiments, the plurality of resistive elements comprise a plurality of surface mount resistors. Attorney Docket No.9833.7119.WO [0048] In some embodiments, a resistance value of the plurality of the perimeter elements is less than a resistance value of the plurality of resistive elements. [0049] In some embodiments, the at least one metal element comprises a first metal element on a first surface of the dielectric substrate and a second metal element on a second surface of the dielectric substrate. [0050] In some embodiments, the first metal element defines a first opening, and where the second metal element defines a second opening. [0051] In some embodiments, a first dimensional characteristic of the first opening is based on a first frequency range, and where a second dimensional characteristic of the second opening is based on a second frequency range. [0052] In some embodiments, the first frequency range and the second frequency range at least partially overlap with a resonance frequency range of the absorptive FSS layer. [0053] In some embodiments, the first frequency range is about 700-1,700 megahertz, and the second frequency range is about 700-2,700 megahertz. [0054] Pursuant to embodiments of the present invention, an antenna system includes a base station antenna, an absorptive FSS layer comprising a plurality of frequency selective absorptive elements and configured to absorb radio frequency (RF) signals in a resonance absorption frequency range, and a reflector assembly on the absorptive FSS layer and comprising a plurality of frequency selective reflective elements. [0055] In some embodiments, the resonance absorption frequency range is within at least a portion of a 700-2,300 megahertz band. [0056] In some embodiments, each frequency selective absorptive element of the plurality of frequency selective absorptive elements comprises a dielectric substrate and at least one metal element on the dielectric substrate. [0057] In some embodiments, the dielectric substrate is a printed circuit board. [0058] In some embodiments, each metal element of the at least one metal element defines an opening, and where a dimensional characteristic of the opening is based on the resonance absorption frequency range. [0059] In some embodiments, the at least one metal element comprises a first metal element on a first surface of the dielectric substrate and a second metal element on a second surface of the dielectric substrate. Attorney Docket No.9833.7119.WO [0060] In some embodiments, each metal element of the at least one metal element comprises a plurality of perimeter elements on an edge of the dielectric substrate, a plurality of extension elements that extend substantially perpendicular from the perimeter elements toward a center of the metal element, and a plurality of resistive elements. [0061] In some embodiments, adjacent extension elements of the plurality of extension elements are coupled by a given resistive element of the plurality of resistive elements. [0062] In some embodiments, the plurality of frequency selective reflective elements comprise a main reflector and at least one reflector strip. [0063] In some embodiments, the at least one reflector strip includes longitudinally- extending first and second reflector strips that extend from the main reflector and are spaced apart from each other in a transverse direction that is perpendicular to a longitudinal direction, and a transversely-extending third reflector strip that extends between the first and second reflector strips. [0064] Pursuant to embodiments of the invention, a base station antenna includes a reflective frequency selective surface (FSS) layer comprising a plurality of frequency selective reflective elements and an absorptive FSS layer comprising a plurality of frequency selective absorptive elements, and where each frequency selective absorptive element of the plurality of frequency selective absorptive elements comprises a dielectric substrate and at least one metal element on the dielectric substrate, where a return loss of the reflective FSS layer is different than a return loss of the absorptive FSS layer. [0065] In some embodiments, the dielectric substrate is a printed circuit board. [0066] In some embodiments, each frequency selective reflective element of the plurality of frequency selective reflective elements defines an opening, and where a dimensional characteristic of the opening is based on a resonance transmission frequency range of the reflective FSS. [0067] In some embodiments, the resonance transmission frequency range is within at least a portion of a 3,300-4,000 megahertz band. [0068] In some embodiments, the metal element defines an opening, and where a dimensional characteristic of the opening is based on a resonance absorptive frequency range of the absorptive FSS. Attorney Docket No.9833.7119.WO [0069] In some embodiments, the resonance absorption frequency range is within at least a portion of a 700-2,300 megahertz band. [0070] In some embodiments, each metal element of the at least one metal element comprises a plurality of perimeter elements on an edge of the dielectric substrate, a plurality of extension elements that extend substantially perpendicular from the perimeter elements toward a center of the metal element, and a plurality of resistive elements. [0071] In some embodiments, adjacent extension elements of the plurality of extension elements are coupled by a given resistive element of the plurality of resistive elements. BRIEF DESCRIPTION OF THE FIGURES [0072] FIG.1A is a schematic rear perspective view of a passive/active antenna system that comprises a passive base station antenna that has an active antenna module mounted thereon. [0073] FIG.1B is a schematic perspective view of the passive/active antenna system of FIG.1A with a radome of the passive base station antenna removed. [0074] FIG.1C is a perspective view of the active antenna module of the passive/active antenna of FIGS.1A-1B. [0075] FIG.2A is a schematic cross-sectional view of an antenna system according to embodiments of the present invention. [0076] FIG.2B is a perspective view of a frequency selective surface (FSS) that is included in an antenna system of FIG.2A. [0077] FIG.2C is a perspective view of a unit cell of the FSS of FIG.2B. [0078] FIG.2D is a schematic perspective view of a unit cell of an antenna system according to further embodiments of the present invention. [0079] FIGS.3A, 3B, and 3C are example S-parameter plots of the antenna system illustrated in FIGS.2A-2D. [0080] FIG.4A is a schematic cross-sectional view of an antenna system according to still further embodiments of the present invention. [0081] FIG.4B is a perspective view of an FSS that is included in the antenna system of FIG.4A. [0082] FIG.4C is a schematic perspective view of a unit cell of the FSS of FIG.4B. Attorney Docket No.9833.7119.WO [0083] FIG.5 illustrates an example absorptive frequency selective surface layer of an antenna system according to still further embodiments of the present invention. DETAILED DESCRIPTION [0084] Pursuant to embodiments of the present invention, base station antennas are provided that are suitable for use in passive/active antenna systems. The base station antennas according to embodiments of the present invention may incorporate frequency selective surface layers that reduce the presence of PIM products within the base station antenna. [0085] PIM products can negatively impact the RF performance of a base station antenna for at least two reasons. First, if PIM products are generated in response to RF signals that are transmitted by the base station antenna, the generated PIM products act to reduce the magnitude of the signals to be transmitted. Second, and more importantly, PIM products may arise at frequencies that are within one or more of the receive bands of the base station antenna, where the PIM products will appear as noise and hence will reduce the signal quality of the received RF signals. Accordingly, a base station antenna according to embodiments of the present invention includes absorptive and reflective frequency selective surface (FSS) layers. In example embodiments, the absorptive FSS layer may be located within the active antenna system of the base station antenna, and the reflective FSS layer may be located within the passive antenna system of the base station antenna. In other example embodiments, both the absorptive FSS layer and the reflective FSS layer may be located within the passive antenna system of the base station antenna. [0086] The absorptive FSS layer is configured to absorb RF energy incident thereon that is within a frequency band in which the passive antenna system operates. The absorptive FSS layer may improve performance by absorbing PIM products and/or by absorbing RF signals that otherwise might generate PIM products. The absorptive FSS layer may be located between an array of radiating elements in the passive antenna system and an array of radiating elements in the active antenna system. The absorptive FSS layer may be transparent to RF energy that is within a frequency band in which the active antenna system operates. The reflective FSS layer reflects RF signals radiated toward the active antenna assembly by the passive antenna assembly and/or other undesirable/external signals. As such, the reflective and absorptive FSS layers substantially reduce the number/magnitude of PIM products that are generated in response to RF signals emitted by the passive antenna system and/or in response to external RF signals

RF Attorney Docket No.9833.7119.WO signals from other antennas on a cell tower that are incident on, for example, the active antenna assembly) while being substantially transparent to the RF signals generated by the active antenna assembly. Furthermore, the reflective and absorptive FSS layers enable an active antenna system to be integrated with a passive antenna system while satisfying one or more PIM product performance criteria of the passive antenna system without having to substantially modify the configuration and/or materials of the passive antenna system. [0087] According to some embodiments of the present invention, an antenna system includes a radiating element layer including a plurality of radiating elements, a reflective FSS layer having a resonance transmission frequency range (e.g., 3,300-4,000 MHz), and an absorptive layer between the reflective FSS layer and the radiating element layer and having a resonance absorption frequency range (e.g., 700 MHz-2,300 MHz). [0088] According to some embodiments of the present invention, a base station antenna includes a first radiating element layer that operates in a first frequency range (e.g., 3,300-4,000 MHz), a second radiating element layer that operates in a second frequency range (e.g., 1,700- 2,700 MHz), and an absorptive FSS layer between the first radiating element layer and the second radiating element layer and configured to absorb RF signals in a resonance absorption frequency range (e.g., 700-2,300 MHz). [0089] In other embodiments of the present invention, an antenna system includes a base station antenna and an absorptive FSS layer comprising a plurality of absorptive structures, where each absorptive structure of the plurality of absorptive structures comprises a dielectric substrate and at least one metal structure on the dielectric substrate. [0090] In additional embodiments of the present invention, an antenna system includes a base station antenna, an absorptive FSS layer comprising a plurality of absorptive structures and configured to absorb RF signals in a resonance absorption frequency range, and a reflector assembly on the absorptive FSS layer. [0091] In further embodiments of the present invention, a base station antenna includes a reflective FSS layer and an absorptive FSS layer, where a first transfer function corresponding to a signal reflection of a first signal propagating from the reflective FSS layer to the absorptive FSS layer is different than a second transfer function corresponding to a signal reflection of a second signal propagating from the absorptive FSS layer to the reflective FSS layer. Attorney Docket No.9833.7119.WO [0092] Example embodiments of the present invention will now be discussed in further detail with reference to the drawings. [0093] FIGS.1A-1C illustrate a passive/active antenna system 100 that includes both a passive base station antenna and an active antenna module. In particular, FIG.1A is a schematic rear perspective view of the passive/active antenna system 100. FIG.1B is a schematic perspective view of the passive/active antenna system 100 of FIG.1A with a radome of the passive base station antenna omitted. FIG.1C is a perspective view of the active antenna module. In FIGS.1A and 1B, the axes illustrate the longitudinal (L), transverse (T) and forward (F) directions of the base station antenna system 100. [0094] Referring to FIG.1A, the passive/active antenna system 100 may be mounted, for example, on an antenna tower 102 using mounting hardware 104. The passive/active antenna system 100 includes a passive base station antenna 110 and an active antenna module 150 that is mounted behind the passive base station antenna 110. The active antenna module 150 may be mounted directly on a rear surface of the passive base station antenna 110, or may be held in place behind the passive base station antenna 110 by the mounting hardware 104 that is used to mount the passive/active antenna system 100 on the antenna tower 102 (or other structure). The front surface of the passive/active antenna system 100 may be opposite the antenna tower 102 facing toward a coverage area of the passive/active antenna system 100. The passive base station antenna 110 includes a tubular radome 112 that surrounds and protects an antenna assembly that is mounted inside the radome 112. A top end cap 114 covers a top opening in the radome 112 and a bottom end cap 116 covers a bottom opening in the radome 112. A plurality of RF ports 118 extend through the bottom end cap 116 and are used to connect the passive base station antenna 110 to one or more external radios (not shown). The active antenna module 150 may be removably mounted behind the passive base station antenna 110 so that the active antenna module 150 may later be replaced with a different active antenna module, preferably without removing the passive base station antenna 110 from the antenna tower 102. [0095] Referring to FIG.1B, the passive base station antenna 110 includes a reflector assembly 120 and a plurality of passive linear arrays of radiating elements that extend forwardly from the passive reflector assembly 120. The reflector assembly 120 may be referred to herein as a "passive reflector assembly" since it is part of the passive base station antenna 110. The linear arrays may support, for example, 3G and/or 4G cellular service. In the example passive base Attorney Docket No.9833.7119.WO station antenna 110 shown in FIGS.1A-1B, the linear arrays include first and second low-band linear arrays 130-1, 130-2 that are configured to operate in all or part of the 617-960 MHz frequency band. Each low-band linear array 130 comprises a vertically-extending column of low-band radiating elements 132. The passive base station antenna 110 further includes first through fourth mid-band linear arrays 140-1 through 140-4 that are configured to operate in all or part of the 1427-2690 MHz frequency band. Each mid-band linear array 140 comprises a vertically-extending column of mid-band radiating elements 142. Each of the low-band and mid- band linear arrays 130, 140 may generate relatively static antenna beams that provide coverage to a predefined coverage area (e.g., antenna beams that are each configured to cover a sector of a base station), with the only change to the coverage area occurring when the electronic downtilt angles of the generated antenna beams are adjusted (e.g., to change the size of the cell). [0096] Each of the low-band and mid-band radiating elements 132, 142 may be implemented as dual-polarized radiating elements that include first and second radiators that transmit and receive RF energy at orthogonal polarizations. When such dual-polarized radiating elements are used, each of the low-band and mid-band linear arrays 130, 140 may be connected to a pair of the RF ports 118. The first RF port 118 is connected between a first port of a radio (e.g., a remote radio head mounted on the antenna tower 102 near the passive base station antenna 110) and the first polarization radiators of the radiating elements in the array, and the second RF port 118 is connected between a second port of a radio and the second polarization radiators of the radiating elements in the array. RF signals that are to be transmitted by a selected one of the linear arrays 130, 140 are passed from the radio to one of the RF ports 118, and passed from the RF port 118 to a power divider (or, alternatively, a phase shifter assembly that includes a power divider) that divides the RF signal into a plurality of sub-components that are fed to the respective first or second radiators of the radiating elements in the linear array, where the sub- components are radiated into free space. [0097] The passive reflector assembly 120 includes a main reflector 122 and spaced- apart first and second reflector strips 124-1, 124-2 that extend longitudinally from respective first and second opposed sides of the main reflector 122. The passive reflector assembly 120 may further include a third reflector strip 124-3 that extends in a transverse direction between the first and second reflector strips 124-1, 124-2. An opening 126 is defined between the first and second reflector strips 124-1, 124-2. For example, the opening 126 may be bounded by a top portion of Attorney Docket No.9833.7119.WO the main reflector 122, the first and second reflector strips 124-1, 124-2, and the third reflector strip 124-3. Most of the low-band and mid-band radiating elements 132, 142 are mounted to extend forwardly from the main reflector 122. However, low-band linear arrays 130-1, 130-2 extend substantially the full length of the passive/active antenna system 100 and hence extend beyond the main reflector 122. The first and second reflector strips 124-1, 124-2 provide mounting locations for low-band radiating elements 132 that are positioned above the main reflector 122. The first and second reflector strips 124-1, 124-2 may be integral with the main reflector 122 so that the first and second reflector strips 124-1, 124-2 and the main reflector 122 will be maintained at a common ground voltage, which may be important for the performance of the low-band linear arrays 130-1, 130-2. [0098] Each low-band radiating element 132 may comprise a slant -45⁰/+45⁰ cross- dipole radiating element that includes a -45⁰ dipole radiator 134-1 and a +45⁰ dipole radiator 134-2 that are arranged to form a cross when the radiating element 132 is viewed from the front. The dipole radiators 134 may (but need not) extend in a plane that is parallel to a plane defined by the main reflector 122. The dipole radiators 134-1, 134-2 may be mounted on a feed stalk 136 of the radiating element 132. Conventionally, cross-dipole radiating elements extend forwardly from a main reflector surface of a reflector assembly with the feed stalks the radiating elements extending perpendicularly to the main reflector surface. The feed stalk may be configured to pass RF signals between the dipole radiators and an associated feed network, and may also be used to support the dipole radiators forwardly of the reflector assembly. The radiating elements 132 that extend forwardly from the main reflector 122 may have a conventional design where the feed stalks extend perpendicularly to the main reflector 122. However, the centers of the low-band radiating elements 132 that are mounted on the first and second reflector strips 124-1, 124-2 are above the opening 126, and hence conventional radiating elements cannot be readily used. Thus, the three uppermost low-band radiating elements 132 have so-called "tilted" feed stalks 136 that extend forwardly from the reflector strips 124-1, 124-2 at oblique angles. In particular, the base of each feed stalk 136 is mounted on one of the reflector strips 124-1, 124-2, and the feed stalk 136 extends at an angle so that the center of the cross defined by the dipole radiators 134-1, 134- 2 is above the opening 126. In example embodiments, the feed stalks 136 may extend at an angle of about 30⁰-60⁰ with respect to the front surface of the reflector strips 124-1, 124-2. Attorney Docket No.9833.7119.WO [0099] Referring to FIGS.1B and 1C, the active antenna module 150 includes a multi- column beamforming array 160 of radiating elements 162 and a beamforming radio (shown below). The multi-column beamforming array 160 may be mounted in a forward portion of a radome 152 of the active antenna module 150, and the beamforming radio may be mounted behind the multi-column beamforming array 160. The beamforming array 160 may, for example, comprise a plurality of vertically-extending columns of high-band radiating elements 162 that are configured to operate in all or part of the 3.1-4.2 GHz frequency band. The high-band radiating elements 162 are mounted to extend forwardly from a reflector 154 of the active antenna module 150 (herein the "active reflector"). The beamforming radio is capable of electronically adjusting the amplitude and/or phase of the subcomponents of an RF signal that are output to different radiating elements 162 of the multi-column beamforming array 160. For example, each port of the beamforming radio may be coupled to a column of the beamforming array 160, and the amplitudes and phases of the sub-components of the RF signal that are fed to each column may be adjusted so that the generated antenna beam is narrowed in the azimuth plane and pointed in a desired direction in the azimuth plane. The active antenna module 150 may further include other components such as filters, a calibration network, an antenna interface signal group (AISG) controller and the like. [00100] As is shown in FIG.1B, the beamforming array 160 of active antenna module 150 is mounted behind the opening 126 in the passive reflector assembly 120. The beamforming array 160 is visible in FIG.1B as the radomes 112, 152 of both the passive base station antenna 110 and the active antenna module 150 are removed in the view of FIG.1B. The opening 126 in the passive reflector assembly 120 allows the antenna beams generated by the beamforming array 160 to pass through the passive base station antenna 110 and out of the front of the radome 112 of the passive base station antenna 110 to provide service to the coverage area of the passive/active antenna system 100. [00101] While the example passive/active antenna system 100 is shown in FIGS.1A-1B, it should be understood that the passive/active antenna system may have varying components and/or configurations in other embodiments and is not limited to the example passive/active antenna system 100 described herein. As another example and referring to FIGS.6A-6B, another example passive/active antenna system 600 that includes both a passive base station antenna and an active antenna module is shown. The passive/active antenna system 600 is shown as a base Attorney Docket No.9833.7119.WO station antenna that includes a housing 600h. The housing 600h may be substantially rectangular with a flat rectangular cross-section. The housing 600h may be provided to define at least part of a radome 611 with at least the front side 611f configured as a dielectric cover that allows RF energy to pass through in certain frequency bands. The housing 600h may also be configured so that the rear 600r defines a rear side 611r radome opposite the front side radome 611f. Optionally, the housing 600h and/or the radome 611 can also comprise two (narrow) sidewalls 600s, 611s facing each other and extending rearwardly between the front side 611f and the rear side 611r. Typically, the top side 600t of the housing 600h may be sealed in a waterproof manner and may comprise an end cap 620 and the bottom 600b of the housing 600h may be sealed with a separate end cap 630. The front side 611f, the sidewalls 611s and typically at least part of the rear side 611r of the radome 611 are substantially transparent to RF energy within the operating frequency bands of the passive/active antenna system 600 and active antenna module 610. The radome 611 may be formed of, for example, fiberglass or plastic. In some embodiments, an active antenna module 610 can attach to the base station antenna 600 using a frame 612 and accessory mounting brackets 613, 614. The rear 611r of the housing 600h may be a flat surface extending along a common plane over an entire longitudinal extent thereof or along at least a portion of the longitudinal extent thereof. [00102] FIG.6B illustrates that the rear surface 600r can comprise a recessed and/or stepped segment 602 facing the active antenna module 610. The stepped segment 602 resides closer to a front 600f of the housing than the back wall that is defined by a primary segment of the rear 600r of the housing 600h. The stepped segment 602 can have a lateral and longitudinal extent that is the same or greater than a lateral and longitudinal extent of the active antenna module 610. The rear surface 600r can also comprise a pair of spaced apart longitudinally extending rails 618 that engage an adapter mounting bracket 618 on the active antenna module 610 to attach the active antenna module 610 to the base station antenna housing 600h. [00103] Referring again to FIG.6A, in another embodiment, the rear surface 600r can comprise a plurality of longitudinally spaced apart mounting structure brackets, shown as upper, medial, and lower brackets, 615, 616, 617, respectively, that extend rearwardly from the housing 600h. In some embodiments, the mounting structure brackets 615, 616, 617 may be configured to couple to one or more mounting structures such as, for example, a tower, pole or building (not shown). At least two of the mounting structure brackets 615, 616 can also be configured to Attorney Docket No.9833.7119.WO attach to the frame 612 of the base station antenna arrangement, where used. The frame 612 may extend over a sub-length of a longitudinal extent L of base station antenna 600, where the sub- length is shown in FIG.3A as being at least a major portion thereof (at least 50% of a length thereof). The frame 612 can comprise a top 612t, a bottom 612b and two opposing long sides 612s that extend between the top 612t and the bottom 612b. The frame 612 can have an open center space 612c extending laterally between the sides 612s and longitudinally between the top 612t and bottom 612b. [00104] The frame 612, where used, may be configured so that a variety of different active antenna modules 610 can be mounted to the frame 612 using appropriate accessory mounting brackets 613, 614. As such, a variety of active antenna modules 610 may be interchangeably attached to the same base station antenna 600. While the frame 612 is shown by way of example, other mounting systems may be used. In some embodiments, a plurality of active antenna modules 610 may be concurrently attached to the same base station antenna 600 at different longitudinal locations using one or more frames 612. Such active antenna modules 610 may have different dimensions, for example, different lengths and/or different widths and/or different thicknesses. [00105] FIGS.2A-2D are schematic views of an example antenna system 200 according to embodiments of the present invention. In particular, FIG.2A is a schematic cross-sectional view of the antenna system 200. FIG.2B is a perspective view of a reflective frequency selective surface (FSS) layer and an absorptive FSS layer of the antenna system 200. FIG.2C is a perspective view of a unit cell of the FSS of FIG.2B. FIG.2D is a perspective view of the antenna system 200 illustrating the reflective FSS layer, the absorptive FSS layer, multiple radiating element layers, and a radio layer. [00106] As shown in FIG.2A-2D, the antenna system 200 may include, in part, an absorptive FSS layer 210, a reflective FSS layer 220, a radio layer 230, a first radiating element layer 240, and a second radiating element layer 250. While the components of the antenna system 200 are conceptually illustrated as layers in FIGS.2A-2D, it should be understood that the components of the antenna system 200 may not be arranged or stacked as layers in some embodiments. For example, the absorptive FSS layer 210 and the reflective FSS layer 220 may be provided on two sides of a single substrate as opposed to being provided on separate substrates (e.g., separate layers). Attorney Docket No.9833.7119.WO [00107] In various embodiments, the radio layer 230 and the first radiating element layer 240 are collectively implemented by the active antenna module 150 described herein. Specifically, the radio layer 230 may be implemented by the beamforming radio of the active antenna module 150 or the active antenna module 610 described above, and the first radiating element layer 240 may be implemented by the beamforming array 160 or a beamforming array of the passive/active antenna system 600. Furthermore, the second radiating element layer 250 may be implemented by at least one of the linear arrays 130, 140 of the passive base station antenna 110 or an array of passive radiating elements of the passive passive base station antenna of the passive/active antenna system 600 described herein. [00108] In some embodiments, the absorptive FSS layer 210, the radio layer 230, and the first radiating element layer 240 are provided within the radome 152 of the active antenna module 150. As an example, the first radiating element layer 240 (e.g., the multi-column beamforming array 160) may be mounted behind the radome 152, the radio layer 230 (e.g., the beamforming radio of the active antenna module 150) may be mounted behind the first radiating element layer 240, and the absorptive FSS layer 210 may be mounted in front of the first radiating element layer 240 so that it is between the first radiating element layer 240 and the radome 152. Furthermore, the reflective FSS layer 220 and the second radiating element layer 250 may be provided within the radome 112 of the passive base station antenna 110. As an example, the reflective FSS layer 220 may be mounted in a portion of the radome 112 that is between the active antenna module 150 and the second radiating element layer 250 (e.g., at least one of the linear arrays 130, 140). It should be understood that the absorptive FSS layer 210 may not be positioned within the radome 152 (or the radome 611) in some embodiments and may be positioned outside of or on an external surface of the radome 152 (or the radome 611). As an example, the absorptive FSS layer 210 may be provided externally to the radome 152 when the absorptive FSS layer 210 and the reflective FSS layer 220 are provided on two sides of a single substrate. [00109] In some embodiments, the absorptive FSS layer 210 includes a plurality of frequency selective absorptive elements 212 (hereinafter referred to as “absorptive elements 212”). As shown in FIGS.2B and 2C, the absorptive elements 212 may be collectively arranged in an array or grid shape, where each absorptive element is provided in a given unit cell 205. Each absorptive element 212 includes a dielectric substrate 214 and a metal element 216 on a Attorney Docket No.9833.7119.WO first surface 214A of the dielectric substrate 214. While the metal element 216 is shown on the first surface 214A of the dielectric substrate 214, it should be understood that an additional metal element 216 may be provided on the second surface 214B of the dielectric substrate in some variations. Additional details regarding the implementation of two metal elements on each surface of the dielectric substrate 214 are described below with reference to FIG.5. [00110] In one embodiment, the dielectric substrate 214 may be a printed circuit board, and as such, the metal element 216 may be formed on a dielectric substrate of the printed circuit board using known printed circuit board fabrication techniques. It should be understood that the dielectric substrate 214 may be provided by other known dielectric substrates (e.g., a dielectric sheet including a ceramic material, Teflon, Kapton, and/or fiberglass) and is not limited to the example embodiment described herein. It will also be appreciated that the dielectric substrate may be omitted and the absorptive FSS layer may be formed as a metal layer (e.g., a punched sheet metal). [00111] The metal element 216 may include a conductive loop element 217 and a plurality of resistive elements 218. The conductive loop element 217 may include perimeter elements 217-1 that are arranged proximate to an edge of the dielectric substrate 214 and extension elements 217-2 that extend substantially perpendicular from the perimeter elements 217-1 and toward a center of the metal element 216. The perimeter elements 217-1 and the extension elements 217-2 may be integrally formed (i.e., the perimeter elements 217-1 and the extension elements 217-2 are formed from a single piece of sheet metal and are therefore "monolithic") as copper traces using known printed circuit board fabrication processes. Adjacent pairs of extension portions 217-2 may be coupled by a given resistive element of the plurality of resistive elements 218, which may be an integrated passive resistive device or a surface mount resistor. As used herein, “adjacent pairs of extension portions 217-2” refer to pairs of extension portions 217-2 that extend from a same edge of the dielectric substrate 214. While an example metal element 216 is shown, it should be understood that the metal element 216 may have other shapes in other embodiments (e.g., a dipole, square-spiral, cross shapes, other looped geometries, solid interiors, or combinations thereof) and is not limited to the example described herein. [00112] In some embodiments, the conductive loop element 217 and the resistive elements 218 collectively define an opening 219. In some embodiments, a dimensional characteristic of the opening 219 and/or the metal element 216 is based on a resonance Attorney Docket No.9833.7119.WO absorption frequency range of the absorptive FSS layer 210. That is, the opening 219 and/or metal element 216 may have one or more dimensional characteristics such that the absorptive FSS layer 210 (and more specifically, the resistive elements 218) absorbs RF energy proximate to the antenna system 200 that is within the resonance absorption frequency range of the absorptive FSS layer 210, such as RF signals that are within all or part of the 617-2700 MHz frequency band. As used herein, “dimensional characteristic” refers to any quantitative or qualitative representation of a dimensional feature of the opening 219 and/or metal element 216. Example dimensional characteristics include, but are not limited to, an area, volume, perimeter, length, width, height, shape and/or the like. [00113] In one embodiment, the reflective FSS layer 220 includes a plurality of frequency selective reflective elements 222 (hereinafter referred to as “reflective elements 222”) that each define a respective opening 224. As shown in FIG.2B, the reflective elements 222 may be collectively arranged in an array or grid shape, where each reflective element 222 is proximate to a corresponding absorptive element 212 within the given unit cell 205. Each reflective element 222 may include perimeter elements 226-1 that are arranged proximate to an edge of the reflective element 222 and extension elements 226-2 that extend substantially perpendicular from the perimeter elements 226-1 and toward a center of the reflective element 222. The perimeter elements 226-1 and the extension elements 226-2 may each include copper (or another metal) and may be integrally formed (i.e., the perimeter elements 226-1 and the extension elements 226-2 are formed from a single piece of sheet metal and are therefore "monolithic") using known metal die punching or opening fabrication processes. In some embodiments, the extension elements 226-2 may have one or more chamfered or filleted edges. While an example reflective element 222 is shown, it should be understood that the reflective element 222 may have other configurations in other forms and is not limited to the example described herein. For example, the reflective element 222 may be provided by a dielectric substrate having a metal element on at least one surface of the dielectric substrate in other variations. [00114] In some embodiments, a dimensional characteristic of the opening 224 and/or reflective element 222 is based on a resonance transmission frequency range of the reflective FSS layer 220. That is, the opening 224 and/or reflective element 222 may have one or more dimensional characteristics such that the reflective FSS layer 220 transmits or redirects RF Attorney Docket No.9833.7119.WO energy proximate to the antenna system 200 and within the resonance transmission frequency range of the reflective FSS layer 220. As an example, the dimensional characteristics may be defined such that reflective FSS layer 220 transmits signals that are within all or part of the 3,100-4,200 MHz frequency band and reflects RF signals in other selected frequency bands, such as all or part of the 617-2,700 MHz frequency band. While not shown, it should be understood that the reflective FSS layer 220 may include one or more bandpass filters such that the reflective FSS layer 220 transmits signals that are within part of the 3,100-4,200 MHz frequency band, such as a 3,300 MHz and 4,000 MHz frequency band. [00115] As described above, the absorptive FSS layer 210 is configured to absorb RF energy proximate to the antenna system 200 that is within the resonance absorption frequency range of the absorptive FSS layer 210 (e.g., all or part of the 617-2700 MHz frequency band, such as 700-2,300 MHz). Furthermore, and as described above, the reflective FSS layer 220 transmits or redirects RF energy proximate to the antenna system 200 that is within the resonance transmission frequency range of the reflective FSS layer 220 (e.g., all or part of the 3,100-4,200 MHz frequency band, such as 3,300 MHz and 4,000 MHz) and reflect all or part of 617-2,700 MHz frequency band, such as 700-2,300 MHz from a front side of the reflective FSS layer 220. That is, the absorptive FSS layer 210 is configured to absorb RF energy from the rear side of the absorptive FSS layer 210, and the reflective FSS layer 220 is configured to redirect RF energy from the front side of the reflective FSS layer 220. As such, the absorptive FSS layer 210 and the reflective FSS layer 220 are collectively configured to transmit (or are transparent to) a predetermined frequency range, such as all or part of the 3,100-4,200 MHz frequency band, reflect all or part of the 617-2,700 MHz frequency band from the front side of reflective FSS layer 220, and absorb all or part of the 617-2,700 MHz frequency band from the rear side of absorptive FSS layer 210. [00116] Accordingly, the reflective FSS layer 220 substantially reflects RF signals radiated toward a frontside of the reflective FSS layer 220 by the second radiating element layer 250 (e.g., at least one of the linear arrays 130, 140) and/or other undesirable/external signals (collectively illustrated by arrows 260) that are not within the resonance transmission frequency range of the reflective FSS layer 220. As an example, the reflective FSS layer 220 may reflect at least 90% of RFs signals outside of the resonance transmission frequency range that are radiated toward a frontside of the reflective FSS layer 220. As such, the reflective FSS layer 220 Attorney Docket No.9833.7119.WO substantially reduces the number/magnitude of PIM products 265 that are generated from RF signals emitted by the radiating elements in the second radiating element layer 250 or from RF signals that enter the antenna system 200 from external sources. [00117] Furthermore, the absorptive FSS layer 210 may substantially absorb RF signals radiated toward a frontside of the absorptive FSS layer 210 by the second radiating element layer 250 and/or other undesirable/external signals that pass through or around the reflective FSS layer 220 to be incident on the absorptive FSS layer 210. These signals are collectively shown by arrows 270. As an example, the absorptive FSS layer 210 may absorb at least 90% of signals incident thereon that are within the resonance absorption frequency range. By absorbing these RF signals so that they do not pass to potentially PIM generating surfaces/junctions within the active antenna assembly, the absorptive FSS layer 210 further minimizes the number/magnitude of PIM products 265 resulting from RF signals that “leak” or permeate through the frontside of the reflective FSS layer 220. Moreover, the absorptive FSS layer 210 will also absorb RF signals (and/or PIM products) from external sources that would otherwise enter the active antenna assembly (e.g., environmental noise) during operation, as illustrated by arrows 280. [00118] Accordingly, the absorptive FSS layer 210 and the reflective FSS layer 220 collectively minimize the PIM products 265 generated within the antenna system 200 as a whole and, most importantly, PIM products that would otherwise by generated in the active antenna assembly. That is, the resonance absorption frequency range is less than the frequency range in which the first radiating element layer 240 operates, and the resonance absorption frequency range overlaps the frequency range in which second radiating element layer 250 operates and/or in which undesirable signals are radiated. Furthermore, the resonance transmission frequency range overlaps the frequency range in which the first radiating element layer 240 operates, and the resonance transmission frequency range is greater the frequency range in which second radiating element layer 250 operates and/or in which undesirable signals are radiated. As such, the absorptive FSS layer 210 and the reflective FSS layer 220 may be completely transparent to the RF signals 245 transmitted by the first radiating element layer 240 while absorbing or reflecting signals that are radiated toward the first radiating element layer 240, thereby minimizing the number/magnitude of PIM products 265 generated by RF signals passing into the active antenna assembly. Example S-parameter plots illustrating the improved transmission, Attorney Docket No.9833.7119.WO reflection, and absorption characteristics of the antenna system 200 are shown below with reference to FIGS.3A-3C. [00119] FIG.3A illustrates an example S-parameter plot of the reflective FSS layer 220, where plot 310 indicates the percentage of RF energy that passes through the reflective FSS layer 220 (e.g., an S12 scattering parameter) in a frequency band of 500 MHz to 4,000 MHz. The S12 parameter is often referred to as "insertion loss" as it represents how much power is lost passing through a medium. The S12 parameter also represents the power transferred between a pair of ports. Plot 320 indicates the percentage of RF energy that is incident on a surface of the reflective FSS layer 220 that is reflected back toward the RF source (e.g., an S22 scattering parameter) in the corresponding frequency band. The S22 parameter is often referred to as "return loss" as it represents how much power is reflected back to a source by a given medium. Both plots in FIG.3A are expressed as a ratio of incident and output power in decibels (dB). [00120] As shown by the plot 310, in the 700-2,300 MHz frequency band, only -17 dB to -5 dB of the incident RF energy will pass forwardly through the reflective FSS layer 220 (i.e., the insertion loss is between -5 to -17 dB). In the 2,300-3,300 MHz frequency band, between the insertion loss ranges from -5 dB and -0.38 dB. Furthermore, in the 3,300--4,000 MHz frequency band only about 0.4 dB, and within this frequency band, the peak insertion loss is near 0 dB. As such, the plot 310 indicates that the reflective FSS layer 220 (without the absorptive FSS layer 210) transmits less than 50% of RF signals at frequencies below 2,630 MHz and at least 90.8% of RF signals at frequencies between 3,300-4,000 MHz. [00121] As shown by plot 320, the return loss ranges from approximately 0 dB to 1.8 dB in the 700-2,300 MHz frequency band, between approximately 0 dB to -1.8 dB of the incident RF energy is reflected. In the 2,300-3,300 MHz frequency band, about -1.8 dB to -10.74 dB of the incident RF energy is reflected. Furthermore, at 3,300 MHz about -10.74 dB of the incident RF energy is reflected , while at 4,000 MHz -10.42 dB of the incident RF energy is reflected. Within the 3,300-4,000 MHz frequency band, the amount of RF energy reflected may be as low as approximately -30 dB (at 3,600 MHz). As such, the plot 320 indicates that the reflective FSS layer 220 (without the absorptive FSS layer 210) reflects more than 50% of incident RF energy at frequencies below 2,630 MHz and between 0.01% and 9.1% of incident RF energy at frequencies between 3,300 MHz and 4,000 MHz. Attorney Docket No.9833.7119.WO [00122] FIG.3B illustrates an example S-parameter plot of only the absorptive FSS layer 210, where plot 330 indicates an insertion loss of the absorptive FSS layer 210 (e.g., an S12 scattering parameter) in a frequency band of 500-4,000 MHz, and plot 340 indicates a percentage of RF energy that is incident on a surface of the absorptive FSS layer 210 that will be reflected back toward the RF source (e.g., an S22 scattering parameter) in the corresponding frequency band. [00123] As shown by plot 330, the insertion loss at 700 MHz is approximately -2 dB, the insertion loss at 2,300 MHz is approximately -5 dB, and within this frequency band, the peak insertion loss is near -7 dB at approximately 1,700 MHz. In the 2,300-3,300 MHz frequency band, the insertion loss ranges from -5 dB to -1 dB. Furthermore, the insertion loss at 3,300 MHz is -1 dB, the insertion loss at 4,000 MHz is -0.5 dB, and within this frequency band, the peak insertion loss is near -0.1 dB at 3,700 MHz. As such, the plot 330 indicates that the absorptive FSS layer 210 (without the reflective FSS layer 220) absorbs between 37% and 80% of incident RF energy at frequencies between 700 MHz and 2,300 MHz, between 20.6% and 69.4% of incident RF energy at frequencies between 2,300 MHz and 3,300 MHz, and between 10.9% and 19.6% of incident RF energy at frequencies between 3,300 MHz and 4,000 MHz. [00124] As shown by plot 340, the return loss at 700 MHz is approximately -7.47 dB, the return loss at 2,300 MHz is approximately -6.42 dB, and within this frequency band, the peak return loss is near -5 dB at 1,500 MHz. The return loss at 3,300 MHz is approximately -19 dB, the return loss at 4,000 MHz is approximately -9.5 dB, and within this frequency band, the peak return loss is near -24 dB at 3,500 MHz. As such, the plot 340 indicates that the absorptive FSS layer 210 (without the reflective FSS layer 220) reflects between 17.9% and 31.6% of incident RF energy at frequencies below 2,300 MHz, between 0.4% and 23.0% of incident RF energy at frequencies between 2,300 MHz and 3,500 MHz, and between 0.4% and 11.2% of incident RF energy at frequencies between 3,500 MHz and 4,000 MHz. [00125] Pursuant to embodiments of the present invention and referring to FIG.3C, an example S-parameter plot of the antenna system 200 is shown and, more specifically, the example S-parameter plot of the absorptive FSS layer 210 and the reflective FSS layer 220 provided within the antenna system 200. Plot 350 indicates an insertion loss of signals propagating through the combination of reflective FSS layer 220 and absorptive FSS layer 210 (e.g., an S12 scattering parameter) in a frequency band of 500 MHz to 4,000 MHz. Plot 360 Attorney Docket No.9833.7119.WO indicates a return loss of the absorptive FSS layer 210 (e.g., an S22 scattering parameter) in a frequency band of 500 MHz to 4,000 MHz. Plot 370 indicates a return loss of the reflective FSS layer 220 (e.g., an S11 scattering parameter) in a frequency band of 500 MHz to 4,000 MHz. Thus, plots 360 and 370 measure return loss from different sides of the FSS 210/220. [00126] As shown by plot 350, the insertion loss at 700 MHz is approximately -17 dB, the insertion loss at 2,300 MHz is approximately -8.25 dB, and within this frequency band, the peak insertion loss is near -20 dB at approximately 1,300 MHz. In the 2,300 MHz and 3,300 MHz frequency band, the insertion loss ranges from -8.25 dB to -1.29 dB. Furthermore, the insertion loss at 3,300 MHz is -1.29 dB, the insertion loss at 4,000 MHz is -0.58 dB, and within this frequency band, the peak insertion loss is near -0.1 dB at 3,600 or 3,700 MHz. More specifically, within the 3,500 MHz to 4,000 MHz frequency band, the insertion loss ranges from -0.5 dB to -0.1 dB. As such, the plot 350 indicates that the antenna system 200 transmits between 74.7% and 97.7% of incident RF energy at frequencies between 3,300 MHz and 4,000 MHz, and the plot 350 indicates that the antenna system 200 transmits between 89.1% and 97.7% of incident RF energy at frequencies between 3,600 MHz and 4,000 MHz. Furthermore, the plot 350 indicates that the antenna system 200 transmits between 2.0% and 15.0% of incident RF energy at frequencies between 700 MHz and 2,300 MHz. Accordingly, the plot 350 indicates that the antenna system 200 is substantially transparent in the 3,300 MHz to 4,000 MHz range (i.e., the resonant transmission frequency range) and substantially absorptive in the 700 MHz to 2,300 MHz range (i.e., the resonant absorption frequency range). Specifically, and with reference to FIGS.3A and 3C, the antenna system 200 demonstrates minimized insertion losses in the resonant absorption frequency range while substantially maintaining approximately the same insertion losses in the resonant transmission frequency range compared to when only the reflective FSS layer 220 is employed (as illustrated by plots 310, 350). [00127] As shown by plot 360, the return loss at 700 MHz is approximately -7.67 dB, the return loss at 2,300 MHz is approximately -10 dB, and within this frequency band, the peak return loss is near -7.3 dB at 1,500 MHz. Additionally, the return loss at 3,300 MHz is approximately -9.5 dB, and within the 2,300 MHz to 3,300 MHz frequency band, the peak return loss is near -3.8 dB at 2,800 MHz. The return loss at 4,000 MHz is approximately -9.2 dB, and within the 3,300 MHz to 4,000 MHz frequency band, the peak return loss is near -23.2 dB at 3,600 MHz. As such, the plot 360 and plot 350 indicate that the absorptive FSS layer 210 Attorney Docket No.9833.7119.WO absorbs between 75% and 81% of incident RF energy at frequencies between 700 MHz and 2,300 MHz, and between 0.5% and 14.5% of incident RF energy at frequencies between 3,300 MHz and 4,000 MHz. Accordingly, and with reference to FIGS.3A and 3C, the antenna system 200 demonstrates increased absorption in the resonant absorption frequency range while substantially maintaining approximately the same return losses in the resonant transmission frequency range compared to when only the absorptive FSS layer 210 or the reflective FSS layer 220 is employed (as illustrated by plots 320, 350). [00128] As shown by plot 370, which depicts a different return loss/transfer function as the plot 360, the return loss at 700 MHz is approximately 0 dB and the return loss at 2,300 MHz is approximately -2 dB. Additionally, the return loss at 3,300 MHz is approximately -9.50 dB, the return loss at 4,000 MHz is approximately -9.2 dB, and within the 3,300 MHz to 4,000 MHz frequency band, the peak return loss is near -21.5 dB at 3,600 MHz. As such, the plot 370 indicates that the reflective FSS layer 220 reflects between 63.1% and 99% of incident RF energy at frequencies between 700 MHz and 2,300 MHz, and between 0.7% and 12.0% of incident RF energy at frequencies between 3,300 MHz and 4,000 MHz. Accordingly, and with reference to FIGS.3B-3C, the antenna system 200 demonstrates increased reflection in the resonant absorption frequency range while substantially maintaining approximately the same return losses in the resonant transmission frequency range compared to when only the reflective FSS layer 220 is employed (as illustrated by plots 340, 350). [00129] While FIGS.2A-2D describe the antenna system 200 as including both a passive antenna system and an active antenna system, it should be understood that the antenna systems described herein may not include the active antenna system in other embodiments. As an example, and referring to FIGS.4A-4C, an example antenna system 400 that does not include an active antenna system according to embodiments of the present invention is shown. In particular, FIG.4A is a schematic cross-sectional view of an antenna system 400. FIG.4B is a perspective view of the antenna system 400 illustrating an absorptive FSS layer, a reflector assembly, and a plurality of radiating elements. FIG.4C is a perspective view of a unit cell 412 of the antenna system 400, where each unit cell 412 includes an absorptive element of the absorptive FSS layer 210. While the components of the antenna system 400 are conceptually illustrated as layers in FIGS.4A-4C, it should be understood that the components of the antenna system 400 may not be arranged or stacked as layers in some embodiments. Attorney Docket No.9833.7119.WO [00130] As shown in FIG.4A-4C, the antenna system 400 may include, in part, the absorptive FSS layer 210, the second radiating element layer 250, and a reflector assembly 410, and the antenna system 400 does not include the radio layer 230 and the first radiating element layer 240 that collectively form the active antenna system. In some embodiments, the reflector assembly 410 may be implemented by the reflector assembly 120 described herein, but it should be understood that other known reflector assemblies may be employed, and as such, the reflector assembly 410 should not be construed as being limited to the reflector assembly 120. [00131] The radiating element layer 250 emits RF energy both forwardly and rearwardly towards reflector assembly 410, which is configured to reflect RF energy to be in-phase with the forwardly emitted RF energy. RF energy radiated toward the reflector assembly 410 may form currents thereon that travel from a front surface of the reflector assembly 410 to a back surface. The current on the back surface of the reflector assembly 410 may result in RF energy that is emitted from the back side of the reflector assembly 410, as shown by arrows 290. Larger amounts of RF energy emitted from the reflector assembly 410 may increase the “front-to-back ratio” of the antenna system 400, and larger front-to-back ratios are generally associated with increased amounts of external PIM signals. [00132] Accordingly, the absorptive FSS layer 210 is positioned behind the reflective layer 410 to absorb signals emitted by the reflector assembly 410 so that they do not pass to potentially PIM generating surfaces/junctions within the antenna assembly 400, such as one or more components of the tower 102. Moreover, the absorptive FSS layer 210 will also absorb RF signals from external sources that would otherwise enter the passive antenna assembly (e.g., environmental noise) during operation, as illustrated by arrows 280. [00133] Referring to FIG.5, a variation of the absorptive element 212 of the absorptive FSS layer 210 that can be provided as part of the antenna systems 200, 400 is shown. Specifically, an absorptive element 500 is shown and may replace or supplement at least a set of the absorptive elements 212 of the antenna systems 200, 400 described herein. Similar to the absorptive elements 212, the absorptive element 500 includes the dielectric substrate 214 and the metal element 216 on the first surface 214A of the dielectric substrate 214. However, in this variation, the absorptive element 500 also includes a second metal element 516 on the second and opposite surface 214B of the dielectric substrate 214. Attorney Docket No.9833.7119.WO [00134] The second metal element 516 may include a conductive loop element 517 and a plurality of resistive elements 518 that are similar to the conductive loop element 217 and the plurality of resistive elements 218, respectively. The conductive loop element 517 and the resistive elements 518 may also collectively define an opening 519 that is similar to the opening 219, but in this variation, the opening 519 has a different dimensional characteristic than the dimensional characteristic of the opening 219. As an example, the opening 219 may have a dimensional characteristic that corresponds to a first resonance absorption frequency range of 700 MHz to 1,900 MHz, and the opening 519 may have a dimensional characteristic that corresponds to a second resonance absorption frequency range of 700 MHz to 2,700 MHz. As such, the multi-sided absorptive FSS layer 500 may absorb additional undesirable/external RF signals that are not within the resonance transmission frequency range of the reflective FSS layer 220 or the reflector assembly 410 compared to the single-sided absorptive FSS layer 210. Stated differently, the multi-sided absorptive FSS layer 500 widen/increase the bandwidth in which RF signals that are not within the resonance transmission frequency range of the reflective FSS layer 220 are absorbed. [00135] While the present invention has been described above primarily with reference to the accompanying drawings, it will be appreciated that the invention is not limited to the illustrated embodiments; rather, these embodiments are intended to fully and completely disclose the invention to those skilled in this art. In the drawings, like numbers refer to like elements throughout. Thicknesses and dimensions of some components may be exaggerated for clarity. [00136] Spatially relative terms, such as "under", "below", "lower", "over", "upper", "top", "bottom" and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "under" or "beneath" other elements or features would then be oriented "over" the other elements or features. Thus, the exemplary term "under" can encompass both an orientation of over and under. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. Attorney Docket No.9833.7119.WO [00137] Well-known functions or constructions may not be described in detail for brevity and/or clarity. As used herein the expression "and/or" includes any and all combinations of one or more of the associated listed items. [00138] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises", "comprising", "includes" and/or "including" when used in this specification, specify the presence of stated features, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, operations, elements, components, and/or groups thereof. [00139] Herein, the terms "attached", "connected", "interconnected", "contacting", "mounted" and the like can mean either direct or indirect attachment or contact between elements, unless stated otherwise. [00140] It will also be appreciated that the various embodiments described above may be combined in any and all ways to provide additional embodiments. [00141] The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.

Claims

Attorney Docket No.9833.7119.WO That Which is Claimed is: 1. An antenna system, comprising: a radiating element layer comprising a plurality of radiating elements; a reflective frequency selective surface (FSS) layer having a resonance transmission frequency range; and an absorptive FSS layer between the reflective FSS layer and the radiating element layer and having a resonance absorption frequency range, wherein the resonance transmission frequency range is greater than the resonance absorption frequency range. 2. The antenna system of Claim 1, wherein the resonance transmission frequency range is within at least a portion of a 3,300-4,000 megahertz band. 3. The antenna system of Claim 1, wherein the resonance absorption frequency range is within at least a portion of a 700-2,300 megahertz band. 4. The antenna system of Claim 1, wherein the reflective FSS layer comprises a plurality of frequency selective reflective elements, and wherein each frequency selective reflective element defines an opening. 5. The antenna system of Claim 4, wherein the plurality of frequency selective reflective elements comprise copper. 6. The antenna system of Claim 4, wherein a dimensional characteristic of the opening is based on the resonance transmission frequency range. 7. The antenna system of Claim 1, wherein the absorptive FSS layer comprises a plurality of frequency selective absorptive elements, and wherein each frequency selective absorptive element of the plurality of frequency selective absorptive elements comprises a dielectric substrate and at least one metal element on the dielectric substrate. 8. The antenna system of Claim 7, wherein the dielectric substrate is a printed circuit board. Attorney Docket No.9833.7119.WO 9. The antenna system of Claim 7, wherein each metal element of the at least one metal element defines an opening, and wherein a dimensional characteristic of the opening is based on the resonance absorption frequency range. 10. The antenna system of Claim 7, wherein the at least one metal element comprises a first metal element on a first surface of the dielectric substrate and a second metal element on a second surface of the dielectric substrate. 11. The antenna system of Claim 7, wherein each metal element of the at least one metal element comprises a plurality of perimeter elements on an edge of the dielectric substrate, a plurality of extension elements that extend substantially perpendicular from the perimeter elements toward a center of the metal element, and a plurality of resistive elements. 12. The antenna system of Claim 11, wherein adjacent extension elements of the plurality of extension elements are coupled by a given resistive element of the plurality of resistive elements. 13. The antenna system of Claim 1, further comprising a radio layer comprising at least one radio frequency (RF) port that is coupled to the plurality of radiating elements. 14. The antenna system of Claim 1, further comprising an additional radiating element layer comprising a plurality of additional radiating elements, wherein the reflective FSS layer is between the additional radiating element layer and the absorptive FSS layer. 15. The antenna system of Claim 1, wherein the plurality of radiating elements comprises a plurality of beamforming radiating elements. 16. A base station antenna, comprising: a first radiating element layer comprising a first plurality of radiating elements configured to operate in a first frequency range; a second radiating element layer comprising a second plurality of radiating elements configured to operate in a second frequency range; and Attorney Docket No.9833.7119.WO an absorptive FSS layer between the first radiating element layer and the second radiating element layer and configured to absorb radio frequency (RF) signals in a resonance absorption frequency range. 17. The base station antenna of Claim 16, wherein the resonance absorption frequency range is within at least a portion of a 700-2,300 megahertz band. 18. The base station antenna of Claim 16, wherein the second frequency range is within at least a portion of a 1,700-2,700 megahertz band. 19. The base station antenna of Claim 16, wherein the first frequency range is within at least a portion of a 3,300-4,000 megahertz band. 20. The base station antenna of Claim 16, wherein the resonance absorption frequency range is within at least a portion of a 700-2,300 megahertz band, the first frequency range is within at least a portion of a 3,300-4,000 megahertz band, and the second frequency range is within at least a portion of a 1,700-2,700 megahertz band. 21. The base station antenna of Claim 16, wherein the absorptive FSS layer comprises a plurality of frequency selective absorptive elements, and wherein each frequency selective absorptive element of the plurality of frequency selective absorptive elements comprises a dielectric substrate and at least one metal element on the dielectric substrate. 22. The base station antenna of Claim 21, wherein the dielectric substrate is a printed circuit board. 23. The base station antenna of Claim 21, wherein each metal element of the at least one metal element defines an opening, and wherein a dimensional characteristic of the opening is based on the resonance absorption frequency range. 24. The base station antenna of Claim 21, wherein the at least one metal element comprises a first metal element on a first surface of the dielectric substrate and a second metal element on a second surface of the dielectric substrate. Attorney Docket No.9833.7119.WO 25. The base station antenna of Claim 24, wherein each metal element of the at least one metal element comprises a plurality of perimeter elements on an edge of the dielectric substrate, a plurality of extension elements that extend substantially perpendicular from the perimeter elements toward a center of the metal element, and a plurality of resistive elements. 26. The base station antenna of Claim 25, wherein adjacent extension elements of the plurality of extension elements are coupled by a given resistive element of the plurality of resistive elements. 27. The base station antenna of Claim 16, further comprising a radio layer comprising at least one radio frequency (RF) port that is coupled to the first plurality of radiating elements. 28. The base station antenna of Claim 27, wherein the absorptive FSS layer includes a plurality of resistive elements. 29. The base station antenna of Claim 16, wherein the first plurality of radiating elements comprises a plurality of beamforming radiating elements. 30. A base station antenna, comprising: an array of radiating elements; and an absorptive FSS layer comprising a plurality of frequency selective absorptive elements positioned behind the array of radiating elements, wherein each frequency selective absorptive element of the plurality of frequency selective absorptive elements comprises a dielectric substrate and at least one metal element on the dielectric substrate. 31. The base station antenna of Claim 30, wherein the dielectric substrate is a printed circuit board. 32. The base station antenna of Claim 30, wherein each metal element of the at least one metal element defines an opening. 33. The base station antenna of Claim 32, wherein a dimensional characteristic of the opening is based on a resonance absorption frequency range of the absorptive FSS. Attorney Docket No.9833.7119.WO 34. The base station antenna of Claim 33, wherein the resonance absorption frequency range is within at least a portion of a 700-2,300 megahertz band. 35. The base station antenna of Claim 30, wherein each metal element of the at least one metal element comprises a plurality of perimeter elements on an edge of the dielectric substrate, a plurality of extension elements that extend substantially perpendicular from the perimeter elements toward a center of the metal element, and a plurality of resistive elements. 36. The base station antenna of Claim 35, wherein adjacent extension elements of the plurality of extension elements are coupled by a given resistive element of the plurality of resistive elements. 37. The base station antenna of Claim 35, wherein the plurality of perimeter elements and the plurality of extension elements comprise copper. 38. The base station antenna of Claim 35, wherein the plurality of resistive elements comprise a plurality of integrated passive resistive devices. 39. The base station antenna of Claim 35, wherein the plurality of resistive elements comprise a plurality of surface mount resistors. 40. The base station antenna of Claim 35, wherein a resistance value of the plurality of the perimeter elements is less than a resistance value of the plurality of resistive elements. 41. The base station antenna of Claim 30, wherein the at least one metal element comprises a first metal element on a first surface of the dielectric substrate and a second metal element on a second surface of the dielectric substrate. 42. The base station antenna of Claim 41, wherein the first metal element defines a first opening, and wherein the second metal element defines a second opening. 43. The base station antenna of Claim 42, wherein a first dimensional characteristic of the first opening is based on a first frequency range, and wherein a second dimensional characteristic of the second opening is based on a second frequency range. Attorney Docket No.9833.7119.WO 44. The base station antenna of Claim 43, wherein the first frequency range and the second frequency range at least partially overlap with a resonance frequency range of the absorptive FSS layer. 45. The base station antenna of Claim 43, wherein the first frequency range is about 700-1,700 megahertz, and the second frequency range is about 700-2,700 megahertz. 46. An antenna system, comprising: a base station antenna; an absorptive FSS layer comprising a plurality of frequency selective absorptive elements and configured to absorb radio frequency (RF) signals in a resonance absorption frequency range; and a reflector assembly on the absorptive FSS layer and comprising a plurality of frequency selective reflective elements. 47. The antenna system of Claim 46, wherein the resonance absorption frequency range is within at least a portion of a 700-2,300 megahertz band. 48. The antenna system of Claim 46, wherein each frequency selective absorptive element of the plurality of frequency selective absorptive elements comprises a dielectric substrate and at least one metal element on the dielectric substrate. 49. The antenna system of Claim 48, wherein the dielectric substrate is a printed circuit board. 50. The antenna system of Claim 48, wherein each metal element of the at least one metal element defines an opening, and wherein a dimensional characteristic of the opening is based on the resonance absorption frequency range. 51. The antenna system of Claim 48, wherein the at least one metal element comprises a first metal element on a first surface of the dielectric substrate and a second metal element on a second surface of the dielectric substrate. Attorney Docket No.9833.7119.WO 52. The antenna system of Claim 51, wherein each metal element of the at least one metal element comprises a plurality of perimeter elements on an edge of the dielectric substrate, a plurality of extension elements that extend substantially perpendicular from the perimeter elements toward a center of the metal element, and a plurality of resistive elements. 53. The antenna system of Claim 52, wherein adjacent extension elements of the plurality of extension elements are coupled by a given resistive element of the plurality of resistive elements. 54. The antenna system of Claim 46, wherein the plurality of frequency selective reflective elements comprise a main reflector and at least one reflector strip. 55. The antenna system of Claim 54, wherein the at least one reflector strip includes longitudinally-extending first and second reflector strips that extend from the main reflector and are spaced apart from each other in a transverse direction that is perpendicular to a longitudinal direction, and a transversely-extending third reflector strip that extends between the first and second reflector strips. 56. A base station antenna, comprising: a reflective frequency selective surface (FSS) layer comprising a plurality of frequency selective reflective elements; and an absorptive FSS layer comprising a plurality of frequency selective absorptive elements, and wherein each frequency selective absorptive element of the plurality of frequency selective absorptive elements comprises a dielectric substrate and at least one metal element on the dielectric substrate, wherein a return loss of the reflective FSS layer is different than a return loss of the absorptive FSS layer. 57. The base station antenna of Claim 56, wherein the dielectric substrate is a printed circuit board. 58. The base station antenna of Claim 56, wherein each frequency selective reflective element of the plurality of frequency selective reflective elements defines an opening, and Attorney Docket No.9833.7119.WO wherein a dimensional characteristic of the opening is based on a resonance transmission frequency range of the reflective FSS. 59. The base station antenna of Claim 58, wherein the resonance transmission frequency range is within at least a portion of a 3,300-4,000 megahertz band. 60. The base station antenna of Claim 56, wherein the metal element defines an opening, and wherein a dimensional characteristic of the opening is based on a resonance absorptive frequency range of the absorptive FSS. 61. The base station antenna of Claim 60, wherein the resonance absorption frequency range is within at least a portion of a 700-2,300 megahertz band. 62. The base station antenna of Claim 56, wherein each metal element of the at least one metal element comprises a plurality of perimeter elements on an edge of the dielectric substrate, a plurality of extension elements that extend substantially perpendicular from the perimeter elements toward a center of the metal element, and a plurality of resistive elements. 63. The base station antenna of Claim 62, wherein adjacent extension elements of the plurality of extension elements are coupled by a given resistive element of the plurality of resistive elements.