Passive / active antenna systems having high selectivity wideband frequency selective surfaces

Abstract

A base station antenna includes an array of lower frequency band radiating elements, an array of higher frequency band radiating elements, and a frequency selective surface. The frequency selective surface comprises a first plurality of first circuits that extend in a first direction. Each first circuit comprises a first sub-circuit that includes a first inductor and a first capacitor that are electrically in series, and a second sub-circuit that is electrically in parallel with the first sub-circuit. The second sub-circuit comprises a second capacitor.

Description

Attorney Docket No.9833.7532.WO PASSIVE / ACTIVE ANTENNA SYSTEMS HAVING HIGH SELECTIVITY WIDEBAND FREQUENCY SELECTIVE SURFACES CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application claims priority under 35 U.S.C. § 119 to U.S. Provisional Patent Application Serial No.63 / 699,872, filed September 27, 2024, the entire content of which is incorporated herein by reference. FIELD

[0002] The present disclosure relates to communications systems and, in particular, to base station antennas for cellular communications systems. 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. Each 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.

[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 of radiating elements or columns in which some of the radiating elements are staggered horizontally to narrow the beamwidths of the generated antenna beams in the azimuth plane. Most modern base station antennas include both "low-band" linear arrays of radiating elements that support service in someAttorney Docket No.9833.7532.WO or all of the 617-960 MHz frequency band and "mid-band" linear arrays of radiating elements that support service in some or all of the 1427-2690 MHz frequency band. These linear arrays are typically formed using dual-polarized radiating elements that have both first polarization and second polarization radiators. This allows each linear array to transmit and receive RF signals at two orthogonal polarizations.

[0005] Each of the above-described linear arrays of dual-polarized radiating elements is coupled to two ports of a radio (one port for each polarization). An RF signal that is to be transmitted by the first polarization radiators of the radiating elements of a linear array is passed from a first port of a radio to the antenna, where the RF signal is divided into a plurality of sub- components. Each sub-component of the RF signal is fed to the first polarization radiators of 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 first polarization radiators to generate a first polarization antenna beam that covers a generally fixed coverage area, such as a sector of a cell. The second polarization radiators of the radiating elements in the linear array are similarly fed by the second port of the radio to generate second polarization antenna beams. Typically the above-described linear arrays will have remote electronic tilt ("RET") capabilities which allow a cellular operator to electronically 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 that only change in shape due to occasional adjustments in the downtilt angle of the antenna beam, they are often referred to as "passive" linear arrays.

[0006] Cellular operators are 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 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, although there is also interest in beamforming arrays that operate in the mid-band frequency range. The radiating elements in each column of an active beamforming array are typically coupled to a respective port of aAttorney Docket No.9833.7532.WO 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 output at each port of the radio (and hence to each respective column of radiating elements in the multi-column beamforming array) in order to generate antenna beams that have narrowed beamwidths in the azimuth plane and / or elevation plane (and hence higher antenna gain). These narrowed antenna beams can be electronically steered by proper selection of the amplitudes and phases of the sub-components of an RF signal that are output by the beamforming radio.

[0007] In order to avoid having to increase the number of antennas at cell sites, the above-described 5G base station antennas also often include passive linear arrays that support legacy 2G, 3G and / or 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) behind a passive base station antenna that includes one or more 2G, 3G, and / or 4G passive linear arrays. Herein, a base station antenna that comprises a combination of a passive base station antenna that has an active antenna module mounted therebehind may be referred to as a "passive / active antenna system." The reflector of the passive base station antenna of a passive / active antenna system typically includes an opening therein so that the antenna beams generated by the 5G active beamforming array can be transmitted through the passive base station antenna. A frequency selective surface typically covers the opening and is designed to reflect RF radiation in the operating frequency bands of at least some of the 2G / 3G / 4G linear arrays while passing RF radiation in the operating frequency band of the beamforming array included in the active antenna module. Chinese Patent Application 114530675 discloses various frequency selective surfaces and base station antennas that include such frequency selective surfaces. SUMMARY

[0008] Pursuant to embodiments of the present invention, base station antennas are provided that comprise an array of lower frequency band radiating elements; an array of higher frequency band radiating elements; and a frequency selective surface that comprises a first plurality of first circuits that extend in a first direction, where each first circuit comprises a firstAttorney Docket No.9833.7532.WO sub-circuit that includes a first inductor and a first capacitor that are electrically in series and a second sub-circuit that is electrically in parallel with the first sub-circuit, the second sub-circuit comprising a second capacitor.

[0009] In some embodiments, each first sub-circuit further comprises a second inductor that is electrically in series with the first capacitor of the respective first sub-circuit. In some embodiments, the first capacitor in each first sub-circuit is electrically interposed in between the first inductor and the second inductor of the respective first sub-circuit. In some embodiments, a capacitance of the first capacitor in each first circuit is at least ten times greater than a capacitance of the second capacitor of the respective first circuit. In other embodiments, a capacitance of the first capacitor in each first circuit is between twenty and forty times greater than a capacitance of the second capacitor of the respective first circuit. In some embodiments, an inductance of the first inductor in each first sub-circuit is approximately the same as an inductance of the second inductor of the respective first sub-circuit.

[0010] In some embodiments, the array of lower frequency band radiating elements is mounted forwardly of the frequency selective surface and the array of higher frequency band radiating elements is mounted rearwardly of the frequency selective surface. In some embodiments, the frequency selective surface is configured to pass RF radiation in an operating frequency band of the higher frequency band radiating elements and to reflect RF radiation in an operating frequency band of the lower frequency band radiating elements. In some embodiments, the operating frequency band of the higher frequency band radiating elements is within the 1695-2690 MHz frequency range and the operating frequency band of the lower frequency band radiating elements is within the 696-960 MHz frequency range.

[0011] In some embodiments, for each first sub-circuit, the first inductor comprises a first trace that includes at least one meandered section, and the second inductor comprises a second trace that includes at least one meandered section. In such embodiments, the first trace may include a plurality of first protrusions and the second trace may include a plurality of second protrusions that are interdigitated with the first protrusions to form the first capacitor.

[0012] In some embodiments, the frequency selective surface further comprises a second plurality of first circuits that extend in the first direction, where the first circuits in the second plurality of first circuits are electrically connected in series.Attorney Docket No.9833.7532.WO

[0013] In some embodiments, the frequency selective surface further comprises a first plurality of second circuits, where each second circuit in the first plurality of second circuits comprises a third inductor that is electrically in parallel with a third capacitor, wherein each second circuit is electrically in series with a respective one of the first circuits in the first plurality of first circuits. The second circuits in the first plurality of second circuits extend in the first direction. In some embodiments, the second circuits in the first plurality of second circuits are physically positioned in between respective pairs of the first circuits in the first plurality of first circuits. In some embodiments, the second circuits in the first plurality of second circuits are coupled electrically in series in between respective pairs of the first circuits in the first plurality of first circuits. In some embodiments, each second circuit comprises a closed metal loop that has at least a partially open interior. The closed metal loop may form the third inductor and capacitances across the at least a partially open interior of the closed metal loop form the third capacitor.

[0014] In some embodiments, the frequency selective surface may further comprise a first plurality of third circuits that extend in a second direction that is different than the first direction, where each third circuit comprises a third sub-circuit that includes a fourth inductor and a fourth capacitor that are electrically in series and a fourth sub-circuit that is electrically in parallel with the third sub-circuit, the fourth sub-circuit comprising a fifth capacitor. In some embodiments, the second direction is perpendicular to the first direction. In some embodiments, the third sub-circuit further comprises a fifth inductor that is electrically in series with the fourth capacitor. In some embodiments, the fourth inductor comprises a third trace that includes at least one meandered section, and the fifth inductor comprises a fourth trace that includes at least one meandered section. In some embodiments, the third trace includes a plurality of third protrusions and the fourth trace includes a plurality of fourth protrusions that are interdigitated with the third protrusions to form the fourth capacitor.

[0015] In some embodiments, the plurality of third circuits are electrically connected in series. In some embodiments, the frequency selective surface further comprises a second plurality of third circuits that extend in the second direction, wherein the third circuits in the second plurality of third circuits are electrically connected in series. In some embodiments, the frequency selective surface further comprises a first plurality of fourth circuits, where each fourth circuit in the first plurality of fourth circuits comprises a sixth inductor that is electricallyAttorney Docket No.9833.7532.WO in parallel with a sixth capacitor, wherein each fourth circuit in the plurality of fourth circuits is electrically in series with a respective one of the third circuits. In some embodiments, the fourth circuits in the first plurality of fourth circuits extend in the second direction.

[0016] In some embodiments, the frequency selective surface further comprises a plurality of open circuit traces. In some embodiments, each open circuit trace has an electrical length that will make the open circuit trace present as a capacitance in an operating frequency band of the higher frequency band radiating elements. In some embodiments, a first of the open circuit traces extends from an interface between the first capacitor and the first inductor of one of the first sub-circuits.

[0017] Pursuant to further embodiments of the present invention, base station antennas are provided that comprise a frequency selective surface that comprises a first plurality of first circuits, where each first circuit comprises a first sub-circuit that includes a first inductor and a first capacitor that are electrically in series and a second sub-circuit that is electrically in parallel with the first sub-circuit, the second sub-circuit comprising a second capacitor, and a first plurality of second circuits, where each second circuit comprises a third inductor that is electrically in parallel with a third capacitor.

[0018] In some embodiments, the first circuits in the first plurality of first circuits and the second circuits in the first plurality of second circuits are electrically coupled in series in alternating fashion. In some embodiments, the first circuits in the first plurality of first circuits and the second circuits in the first plurality of second circuits extend in alternating fashion in a first direction.

[0019] In some embodiments, each first sub-circuit further comprises a second inductor that is electrically in series with the first capacitor of the respective first sub-circuit. In some embodiments, the first capacitor is electrically interposed in between the first inductor and the second inductor of the respective first sub-circuit.

[0020] In some embodiments, a capacitance of the first capacitor in each first circuit is at least ten times greater than a capacitance of the second capacitor of the respective first circuit.

[0021] In some embodiments, an inductance of the first inductor in each first sub-circuit is approximately the same as an inductance of the second inductor of the respective first sub- circuit.Attorney Docket No.9833.7532.WO

[0022] In some embodiments, each first inductor comprises a first trace that includes at least one meandered section, and each second inductor comprises a second trace that includes at least one meandered section. In such embodiments, for each first sub-circuit, the first trace includes a plurality of first protrusions and the second trace includes a plurality of second protrusions that are interdigitated with the first protrusions to form the first capacitor.

[0023] In some embodiments, an array of lower frequency band radiating elements is mounted forwardly of the frequency selective surface and an array of higher frequency band radiating elements is mounted rearwardly of the frequency selective surface.

[0024] In some embodiments, the frequency selective surface further comprises a second plurality of first circuits and a second plurality of second circuits, where the first circuits in the second plurality of first circuits and the second circuits in the second plurality of second circuits are electrically coupled in series in alternating fashion. In some embodiments, the first circuits in the first plurality of first circuits and the second circuits in the first plurality of second circuits extend in the first direction, and the first circuits in the second plurality of first circuits and the second circuits in the second plurality of second circuits extend in the first direction in parallel to the first circuits in the first plurality of first circuits and the second circuits in the first plurality of second circuits.

[0025] In some embodiments, each second circuit comprises a closed metal loop that has at least a partially open interior. In some embodiments, the closed metal loop forms the third inductor and capacitances across the at least a partially open interior of the closed metal loop form the third capacitor. In some embodiments, the frequency selective surface further comprises a first plurality of third circuits and a first plurality of fourth circuits that extend in alternating fashion in a second direction that is perpendicular to the first direction, where each third circuit in the first plurality of third circuits comprises a third sub-circuit that includes a fourth inductor, a fourth capacitor and a fifth inductor that are electrically in series and a fourth sub-circuit that is electrically in parallel with the third sub-circuit, the fourth sub-circuit comprising a fifth capacitor, and where each fourth circuit in the first plurality of fourth circuits comprises a fifth inductor that is electrically in parallel with a sixth capacitor.

[0026] In some embodiments, the frequency selective surface further comprises a plurality of open circuit traces. In some embodiments, each open circuit trace has an electrical length that configures the open circuit trace to present as a capacitance with respect to signals inAttorney Docket No.9833.7532.WO an operating frequency band of the higher frequency band radiating elements. In some embodiments, a first of the open circuit traces extends from an interface between the first capacitor and the first inductor of one of the first sub-circuits.

[0027] In some embodiments, the frequency selective surface is configured to pass RF radiation in an operating frequency band of the higher frequency band radiating elements and to reflect RF radiation in an operating frequency band of the lower frequency band radiating elements.

[0028] Pursuant to additional embodiments of the present invention, base station antennas are provided that comprise an array of lower frequency band radiating elements; an array of higher frequency band radiating elements; and a frequency selective surface implemented in a single metal layer, the frequency selective surface having a bandpass filter response.

[0029] In some embodiments, the frequency selective surface comprises a first plurality of first circuits that are electrically coupled in series, where each first circuit comprises a first sub-circuit that includes a first inductor, a first capacitor and a second inductor that are electrically in series and a second sub-circuit that is electrically in parallel with the first sub- circuit, the second sub-circuit comprising a second capacitor. In some embodiments, the frequency selective surface further comprises a first plurality of second circuits that are electrically coupled in series with the first plurality of first circuits so that the first circuits and the second circuits are arranged in alternating fashion in the series connection, wherein each second circuit in the first plurality of second circuits comprises a third inductor that is electrically in parallel with a third capacitor.

[0030] In some embodiments, the frequency selective surface further comprises a plurality of open circuit traces. In some embodiments, each open circuit trace has an electrical length that configures the open circuit trace to present as a capacitance with respect to signals in an operating frequency band of the higher frequency band radiating elements. In some embodiments, a first of the open circuit traces extends from an interface between the first capacitor and the first inductor of one of the first sub-circuits.

[0031] In some embodiments, the frequency selective surface is configured to pass RF radiation in an operating frequency band of the higher frequency band radiating elements and toAttorney Docket No.9833.7532.WO reflect RF radiation in an operating frequency band of the lower frequency band radiating elements.

[0032] Pursuant to still other embodiments of the present invention, base station antennas are provided that comprise a frequency selective surface that comprises a plurality of open circuit traces; an array of lower frequency band radiating elements mounted forwardly of the frequency selective surface; and an array of higher frequency band radiating elements mounted rearwardly of the frequency selective surface. In some embodiments, the frequency selective surface comprises a plurality of unit cells, and each unit cell includes at least one open circuit trace. In some embodiments, the frequency selective surface comprises a plurality of unit cells, and each unit cell includes a total of four open circuit traces.

[0033] In some embodiments, each open circuit trace has an electrical length that configures the open circuit trace to add a capacitance with respect to signals in an operating frequency band of the higher frequency band radiating elements.

[0034] In some embodiments, a first of the open circuit traces extends from an interface between the first capacitor and the first inductor of one of the first sub-circuits.

[0035] In some embodiments, the frequency selective surface further comprises a first plurality of first circuits that extend in a first direction, where each first circuit comprises a first sub-circuit that includes a first inductor, a first capacitor and a second inductor that are electrically in series and a second sub-circuit that is electrically in parallel with the first sub- circuit, the second sub-circuit comprising a second capacitor. In some embodiments, the first capacitor is electrically interposed in between the first inductor and the second inductor.

[0036] In some embodiments, the first inductor comprises a first trace that includes at least one meandered section, and the second inductor comprises a second trace that includes at least one meandered section. In some embodiments, the first trace includes a plurality of first protrusions and the second trace includes a plurality of second protrusions that are interdigitated with the first protrusions to form the first capacitor.

[0037] In some embodiments, the first plurality of first circuits are electrically connected in series. In some embodiments, the frequency selective surface further comprises a second plurality of first circuits that extend in the first direction, wherein the first circuits in the second plurality of first circuits are electrically connected in series.Attorney Docket No.9833.7532.WO

[0038] In some embodiments, the frequency selective surface further comprises a first plurality of second circuits, where each second circuit in the first plurality of second circuits comprises a third inductor that is electrically in parallel with a third capacitor, wherein each second circuit is electrically in series with a respective one of the first circuits. In some embodiments, the second circuits in the first plurality of second circuits extend in the first direction. In some embodiments, the second circuits in the first plurality of second circuits are coupled electrically in series in between respective pairs of the first circuits in the first plurality of first circuits. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG.1A is a schematic rear perspective view of a passive / active antenna system that comprises a passive base station antenna and a pair of active antenna modules.

[0040] FIG.1B is a schematic front view of an antenna assembly of the passive base station antenna of FIG.1A.

[0041] FIG.1C is a schematic front view of one of the active antenna modules of FIG. 1A with a radome thereof removed.

[0042] FIG.1D is a front view of a small portion of a frequency selective surface according to embodiments of the present invention that may be included in the passive / active antenna system of FIG.1A.

[0043] FIG.1E is a front view of a single unit cell of the frequency selective surface of FIG.1D that illustrates the non-negligible capacitive and inductive couplings for horizontally- flowing currents.

[0044] FIG.1F is a circuit diagram showing the equivalent circuit of the unit cell of FIG.1D.

[0045] FIG.1G is a front view of a small portion of the frequency selective surface of FIG.1D that illustrates the non-negligible capacitive and inductive couplings for vertically- flowing currents.

[0046] FIGS.2A-2C are graphs illustrating the simulated rejection and transmission properties of frequency selective surfaces having the design of FIG.1D.

[0047] FIG.3 is a circuit diagram showing the equivalent circuit of unit cells of frequency selective surfaces according to further embodiments of the present invention that may be used in the passive / active antenna system of FIGS.1A-1B.Attorney Docket No.9833.7532.WO

[0048] FIG.4A is a schematic front view of a portion of a frequency selective surface according to further embodiments of the present invention that has unit cells having the equivalent circuit of FIG.3.

[0049] FIG.4B is a schematic front view of a small portion of the frequency selective surface of FIG.4A that illustrates the non-negligible capacitive and inductive couplings for currents flowing in the direction of the arrow.

[0050] FIG.5A is a schematic front view of a portion of a frequency selective surface according to additional embodiments of the present invention that has unit cells having the equivalent circuit of FIG.3.

[0051] FIG.5B is a schematic front view of a small portion of the frequency selective surface of FIG.5A that illustrates the non-negligible capacitive and inductive couplings for currents flowing in the direction of the arrow.

[0052] FIG.6A is a schematic front view of a portion of a frequency selective surface according to still further embodiments of the present invention that has unit cells having the equivalent circuit of FIG.3.

[0053] FIG.6B is a schematic front view of a small portion of the frequency selective surface of FIG.6A that illustrates the non-negligible capacitive and inductive couplings for currents flowing in the direction of the arrow.

[0054] FIG.7A is a schematic front view of a portion of a frequency selective surface according to yet additional embodiments of the present invention that has unit cells having the equivalent circuit of FIG.3.

[0055] FIG.7B is a schematic front view of a small portion of the frequency selective surface of FIG.7A that illustrates the non-negligible capacitive and inductive couplings for currents flowing in the direction of the arrow.

[0056] FIGS.8A-8C are graphs illustrating the simulated rejection and transmission properties of frequency selective surfaces having the unit cell equivalent circuit of FIG.3.

[0057] FIG.9A is a schematic front view of a portion of a frequency selective surface according to still other embodiments of the present invention.

[0058] FIG.9B is a schematic front view of a small portion of the frequency selective surface of FIG.9A that illustrates the non-negligible capacitive and inductive couplings for currents flowing in the direction of the arrow.Attorney Docket No.9833.7532.WO

[0059] FIG.9C is a circuit diagram showing the equivalent circuit of the unit cells of the frequency selective surface of FIG.9A. DETAILED DESCRIPTION

[0060] Pursuant to embodiments of the present invention, base station antennas are provided that have improved frequency selective surfaces. The frequency selective surfaces according to embodiments of the present invention may be used, for example, in the passive / active antenna systems, and may be mounted behind a lower frequency band array of radiating elements and in front of a higher frequency band array of radiating elements. The lower frequency band array of radiating elements may be, for example, a 2G / 3G / 4G linear array of low-band radiating elements, while the higher frequency band array of radiating elements may be a multi-column beamforming array of mid-band or high-band radiating elements. The improved frequency selective surfaces disclosed herein may have very large passbands which, in some cases, may exceed 80% of the operating frequency range of the frequency selective surface (where the bandwidth is calculated as the passband divided by the center frequency of the passband multiplied by 100). Moreover, the frequency selective surfaces according to some embodiments of the present invention may have high selectivity. In some embodiments, these frequency selective surfaces may comprise a single metal layer and hence may be low cost; for example, in some cases the frequency selective surfaces may be stamped from sheet metal.

[0061] The frequency selective surfaces according to some embodiments of the present invention may have passband responses. The responses may exhibit good selectivity, and hence may be designed to substantially reflect RF energy in the low-band operating frequency range while substantially passing RF energy in the mid-band operating frequency range (or at least in the 1695-2690 MHz portion of the mid-band operating frequency range) in example embodiments.

[0062] The frequency selective surfaces that are disclosed herein may be, for example, incorporated into a passive base station antenna. The passive base station antenna may be combined with one or more active antenna modules to provide a passive / active antenna system according to embodiments of the present invention. The passive base station antenna may include an array of lower frequency band radiating elements such as, for example, an array of low-band radiating elements that are configured to operate in, for example, the 696-960 MHz frequency band. Each of the one or more active antenna modules may include a beamformingAttorney Docket No.9833.7532.WO array of higher frequency band radiating elements such as, for example, a multi-column array of mid-band radiating elements that are configured to operate in some or all of the 1695-2690 MHz frequency band or a multi-column array of high-band radiating elements that are configured to operate in some portion of the 3100-7200 MHz frequency band. Some or all of the one or more active antenna modules may be positioned behind a frequency selective surface according to embodiments of the present invention so that the beamforming array transmits and receives RF signals through the frequency selective surface. The frequency selective surface may be configured to be substantially transparent in the operating frequency band of the beamforming array, and may be configured to be substantially reflective in the operating frequency band of the array of lower frequency band radiating elements that are mounted forwardly of the frequency selective surface.

[0063] Example embodiments of frequency selective surfaces according to embodiments of the present invention and passive / active antenna systems that include such frequency selective surfaces will now be discussed in greater detail with reference to the accompanying drawings.

[0064] Embodiments of the present invention are directed to highly selective, wideband, low-cost frequency selective surfaces and to passive / active antenna systems that include such frequency selective surfaces. FIGS.1A-1C illustrate a passive / active antenna system 100 that includes a passive base station antenna 110 and first and second active antenna modules 150-1, 150-2. In particular, FIG.1A is a schematic rear perspective view of the passive / active antenna system 100, and FIG.1B is a schematic perspective view of an antenna assembly of the passive base station antenna 110 of FIG.1A. FIG.1C is a schematic front view of one of the active antenna modules 150 of FIG.1A with a radome thereof removed to show the multi-column beamforming array included in the active antenna module 150. As will be discussed below, the frequency selective surfaces according to embodiments of the present invention that are disclosed herein may be used to implement the frequency selective surfaces included in the passive / active antenna system 100 of FIGS.1A-1B.

[0065] 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 the passive base station antenna 110 and first and second active antenna modules 150-1, 150-2, which are mounted behind the passive base station antenna 110. The active antenna modules 150 may be mounted directly on a rear surface of the passive base stationAttorney Docket No.9833.7532.WO antenna 110, or may be held in place behind the passive base station antenna 110 by, for example, the mounting hardware 104. 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 (FIG.1B). 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 modules 150 may be removably mounted behind the passive base station antenna 110 so that the active antenna module 150 may later be replaced with different active antenna modules.

[0066] Referring to FIG.1B, the passive base station antenna 110 includes a reflector assembly 120 and first and second low-band linear arrays 130-1, 130-2 that extend forwardly from the reflector assembly 120. Each low-band linear array 130 comprises a vertically- extending column of low-band radiating elements 132 that are configured to operate in all or part of the 617-960 MHz frequency band. The low-band linear arrays 130 are passive linear arrays that generate 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).

[0067] Each of the low-band radiating elements 132 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 132 are used, each of the low-band linear arrays 130 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) and the first polarization radiators of the radiating elements 132 in the linear array 130, and the second RF port 118 is connected between a second port of a radio and the second polarization radiators of the radiating elements 132 in the linear array 130. RF signals that are to be transmitted by a selected one of the linear arrays 130 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 areAttorney Docket No.9833.7532.WO fed to the respective first or second radiators of the radiating elements 132 in the linear array 130.

[0068] The passive reflector assembly 120 includes a main reflector 122, spaced-apart first and second upper reflector strips 124-1, 124-2 that extend upwardly from respective first and second opposed upper sides of the main reflector 122, and first and second lower reflector strips 126-1, 126-2 that extend downwardly from respective first and second opposed lower sides of the main reflector 122. The passive reflector assembly 120 may further include a third upper reflector strip 124-3 that extends in the horizontal direction between distal ends of the first and second upper reflector strips 124-1, 124-2, and / or a third lower reflector strip 126-3 that extends in the horizontal direction between distal ends of the first and second lower reflector strips 126-1, 126-2. A first opening 128-1 is defined between a top portion of the main reflector 122 and the first through third upper reflector strips 124-1 through 124-3. The first opening 128-1 is not visible in FIG.1B as it is covered by a first frequency selective surface 140-1, which is described in greater detail below, but the location of the first opening 128-1 is shown by the dashed box labelled 128-1. A second opening 128-2 is defined between a top portion of the main reflector 122 and the first through third lower reflector strips 126-1 through 126-3. Some of the low-band radiating elements 132 are mounted on the main reflector 122, while other of the low- band radiating elements 132 are mounted on the upper reflector strips 124-1, 124-2 or the lower reflector strips 126-1, 126-2. The first and second upper reflector strips 124-1, 124-2 and the first and second lower reflector strips 126-1, 126-2 may be integral with the main reflector 122 so that they are maintained at the same ground voltage as the main reflector 122.

[0069] FIG.1C is a schematic front view of one of the active antenna modules 150 with a radome thereof remove. Each active antenna module 150 includes a multi-column beamforming array 160 and a beamforming radio (not visible in the figures). The multi-column beamforming array 160 is mounted behind a radome of the active antenna module 150, and the beamforming radio is mounted behind the beamforming array 160. The first beamforming array 160-1 may comprise, for example, a plurality of vertically-extending columns of first high-band radiating elements 162A that are configured to operate in at least the 2.5-2.7 GHz frequency band. The second beamforming array 160-2 may comprise, for example, a plurality of vertically-extending columns of second high-band radiating elements 162B that are configured to operate in at least the 1.7-2.2 GHz frequency band. The beamforming radios may be capable ofAttorney Docket No.9833.7532.WO electronically adjusting the amplitudes and / or phases of the subcomponents of an RF signal that are output to different radiating elements 162A, 162B of the respective beamforming arrays 160- 1, 160-2 so that the narrowed antenna beams may be generated that can be steered in desired directions in the azimuth plane.

[0070] Referring again to FIGS.1A-1B, it can be seen that the first active antenna module 150-1 is mounted behind the first opening 128-1 in the reflector assembly 120, and the second active antenna module 150-2 is mounted behind the second opening 128-2 in the reflector assembly 120. The openings 128-1, 128-2 in the reflector assembly 120 allows the antenna beams generated by the first and second beamforming arrays 160-1, 160-2 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.

[0071] As shown in FIG.1B, the first opening 128-1 in the passive reflector assembly 120 is covered by a first frequency selective surface 140-1, and the second opening 128-2 in the passive reflector assembly 120 is covered by a second frequency selective surface 140-2. Only the first frequency selective surface 140-1 is shown in FIG.1B so that the opening 128-2 is clearly shown. The first and second frequency selective surfaces 140-1, 140-2 may be positioned in front of, behind, or flush with the openings 128-1, 128-2. The dotted box labelled 128-1 shows the location of the first opening 128-1, and the dashed box 140-2 shows the location of the second frequency selective surface 140-2.

[0072] Each frequency selective surface 140 may act as a spatial filter that passes, or substantially attenuates and / or reflects RF energy, depending on the frequency of the RF energy. Frequency selective surfaces are known in the art, and typically comprise a grid pattern of unit cells such as a grid pattern of metal patches and / or other metal structures that form resonant circuits. The metal patches / structures may be arranged in one or more layers. The frequency selective surface 140 may be implemented, for example, as a piece of sheet metal with the grid structure punched or otherwise formed therein or as a dielectric substrate with one or more metal patterns formed therein (such as a printed circuit board). The first and second frequency selective surfaces 140-1, 140-2 may be configured to substantially pass RF energy that is incident thereon in a first frequency range (here the first frequency range may include the operating frequency band of the radiating elements included in the respective beamformingAttorney Docket No.9833.7532.WO arrays 160-1, 160-2), while partially or substantially not passing (e.g., reflecting) RF energy that is incident thereon in a second frequency range (here the second frequency range may include the operating frequency bands of the radiating elements included in the passive base station antenna 110). Examples of frequency selective surfaces are discussed in U.S. Patent No. 11,482,774 to Hou et al., the entire content of which is incorporated herein by reference.

[0073] FIG.1D is a front view of a small, representative portion of a frequency selective surface 200 according to embodiments of the present invention that may be used to implement either or both of the first and second frequency selective surfaces 140-1, 140-2 in the passive / active antenna system of FIG.1A. The frequency selective surface 200 has a unit cell structure in which a large number of (typically) identical unit cells 220 form the frequency selective surface 200. The three dashed boxes in FIG.1D illustrate the locations of seven unit cells 220-1 through 220-7. It will be appreciated that the frequency selective surface 200 will typically include hundreds or thousands of unit cells 220 that may be arranged in columns and rows. FIG.1E is a plan view of a first of the unit cells 220-1 of the frequency selective surface 200 of FIG.1D. FIG.1E also includes circuit symbols that show the inductors and capacitors formed in the first unit cell 220-1.

[0074] As shown in FIGS.1D-1E, the frequency selective surface 200 may be formed in a printed circuit board that includes a dielectric substrate and a metallization pattern that is formed thereon. Alternatively, the frequency selective surface 200 may be formed of stamped (and possibly bent) sheet metal so that a metallization pattern is provided without an underlying dielectric substrate. Other implementations are possible (e.g., metallization printed on plastic or deposited using laser direct sintering).

[0075] As can be seen in FIG.1D, the narrow metal traces 230 define a series of squares, where each square may be viewed as a unit cell 220. Each unit cell 220 may be a "sub- wavelength" structure, meaning that the major dimensions of each unit cell 220 are smaller than the wavelengths of the RF signals that are transmitted and received by the conventional passive / active antenna system 100 of FIG.1A. Typically, the major dimensions of each unit cell 220 are on the order of 1 / 3 to ¼ or less the wavelength corresponding to the center frequency of the operating frequency range of the high-band radiating elements 162A, 162B that are mounted rearwardly of the frequency selective surface 200. The unit cells 220 have capacitive and inductive couplings formed therein that are designed to operate as a spatial filter that allows RFAttorney Docket No.9833.7532.WO energy in certain frequency ranges that are incident on the frequency selective surface 200 to pass therethrough with little attenuation, while being substantially reflective to RF energy in other frequency bands

[0076] As shown in FIGS.1D-1E, each unit cell 220 has a square shape when viewed from the front (or rear). The metallization pattern comprises narrow metal traces 230 that are configured to form capacitors and inductors. Referring to FIG.1E, it can be seen that first and second narrow metal traces 230-1, 230-2 extend through the middle of the first unit cell 220-1. Each narrow metal trace 230 comprises a straight portion 232, a meandered portion 234, and a comb portion 236. Herein, a meandered portion of a metal trace (or portion thereof) refers to a non-linear metal trace that follows a meandered path to increase the path length thereof. The straight portion 232 of the first narrow metal trace 230-1 forms part of a first inductor L1, and the meandered portion 234 forms the remainder of the first inductor L1 (the meandered portion 234 of the first metal trace 230-1 provides most of the inductance). The comb portions 236 of the first and second narrow metal traces 230-1, 230-2 are interdigitated to form a first capacitor C1. The straight portion 232 of the second narrow metal trace 230-2 forms part of a second inductor L2, and the meandered portion 234 of the second first metal trace 230-2 forms the remainder of the second inductor L2. The first inductor L1, the first capacitor C1 and the second inductor L2 are electrically connected in series. Thus, currents flowing in the direction of the arrow in FIG.1E will pass through a series inductor-capacitor-inductor (LCPL) circuit that comprises the first inductor L1, the first capacitor C1 and the second inductor L2. Herein, this circuit may be referred to as a first sub-circuit SC1 of a first circuit EC1.

[0077] Third and fourth narrow metal traces 230-3, 230-4 extend vertically along the left side 222-1 of the first unit cell 220-1, and fifth and sixth narrow metal traces 230-5, 230-6 extend vertically along the right side 222-2 of the first unit cell 220-1. The third narrow metal trace 230-3 is partly within the first unit cell 220-1, partly within a second unit cell 220-2 that is to the left of the first unit cell 220-1, and partly with in a third unit cell 230-3 that is above the second unit cell 220-2 (see FIG.1D). The fourth narrow metal trace 230-4 is partly within the first unit cell 220-1, partly within a second unit cell 220-2, and partly within a fourth unit cell 230-4 that is below the second unit cell 220-2 (see FIG.1D). Similarly, the fifth narrow metal trace 230-5 is partly within the first unit cell 220-1, partly within a fifth unit cell 220-5 that is to the right of the first unit cell 220-1, and partly within a sixth unit cell 230-6 that is above the fifth unit cell 220-5Attorney Docket No.9833.7532.WO (see FIG.1D). The sixth narrow metal trace 230-6 is partly within the first unit cell 220-1, partly within the fifth unit cell 220-5, and partly within a seventh unit cell 230-7 that is below the fifth unit cell 220-5 (see FIG.1D). Each of the third through sixth narrow metal traces 230-3 through 230-6 includes a respective comb portion 236 that mates with a comb portion 236 of a respective one of another four narrow metal traces 230-7 through 230-10. A portion of each of these "matings" occur within the first unit cell 220-1. Portions of the seventh through tenth narrow metal traces 230-7 through 230-10 are visible within the first unit cell 220-1 in FIG.1E.

[0078] As can be seen from FIGS.1D-1E, each of the third through sixth narrow metal traces 230-3 through 230-6 may be identical to either the first narrow metal trace 230-1 or the second narrow metal trace 230-2 except that the third through sixth narrow metal traces 230-3 through 230-6 extend in the vertical direction rather than the horizontal direction. Each of the third through sixth narrow metal traces 230-3 through 230-6 mates with a respective one of the seventh through tenth narrow metal traces 230-7 through 230-10 to form a respective LCL circuit that may be identical to the first sub-circuit SC1 of the first circuit EC1.

[0079] Still referring to FIG.1E, it can be seen that, for currents flowing in the direction of the arrow in FIG.1E, the third narrow metal trace 230-3 will capacitively couple with the fifth narrow metal trace 230-5 and the ninth narrow metal trace 230-9 to form a first part C21of a second capacitor C2, and the sixth narrow metal trace 230-6 will capacitively couple with the fourth narrow metal trace 230-4 and the eighth narrow metal trace 230-8 to form a second part C22of the second capacitor C2. The second capacitor C2 forms a second sub-circuit SC2 of the first circuit. As can be seen in FIG.1E, the first sub-circuit SC1 is electrically in parallel with the second sub-circuit SC2. The first and second sub-circuits SC1, SC2 form the first circuit EC1. The frequency selective surface 200 includes a large number of first circuits EC1.

[0080] The left and right sides 222-1, 222-2 of the first unit cell 220 are spaced far apart as compared to, for example, the distance between the comb portions 236 of the first and second narrow metal traces 230-1, 230-2. As such, the capacitance of second capacitor C2 may be much less than the capacitance of the first capacitor C1. For example, the capacitance of the second capacitor C2 may be on the order of 3%-20% of the capacitance of the first capacitor C1. However, since the unit cells 220 are very small in size, the second capacitor C2 may have a non-negligible capacitance that can be used to materially impact the response of the frequency selective surface 200.Attorney Docket No.9833.7532.WO

[0081] FIG.1F is an equivalent circuit diagram of the first unit cell 220-1 for currents that flow in the direction of the arrow shown in FIG.1E (i.e., for horizontal currents). As shown in FIG.1F, and as discussed above, the first and second narrow metal traces 230-1, 230-2 form a first sub-circuit SC1 that comprises the first inductor L1, the first capacitor C1 and the second inductor L2 which are electrically coupled in series, and portions of the third through sixth and ninth and tenth narrow metal traces 230-3 through 230-6, 230-9, 230-10 form a second sub- circuit SC2 that comprises parallel capacitors C21and C22(collectively, C2). The second sub- circuit SC2 is disposed electrically in parallel with the first sub-circuit SC1 to form the first circuit EC1.

[0082] Referring again to FIG.1D, it can be seen that the first unit cell 220-1 is part of a first row R1 of unit cells 220 that extends in the horizontal direction. Since each unit cell 220 in the first row R1 comprises a first circuit EC1 that has the equivalent circuit that is shown in FIG.1F, it can be seen that the first row R1 of unit cells 220 comprises a first plurality of first circuits EC1 that are electrically connected to each other in series.

[0083] Still referring to FIG.1E, it can be seen that the inductance of the first and second inductors L1, L2 may be varied by, for example, increasing / decreasing the number of meandered section or by increasing / decreasing the length in the vertical direction of the meandered sections. Similarly, the capacitance of the first capacitor C1 may be varied by, for example, increasing / decreasing the number of protrusions or by increasing / decreasing the length of the protrusions.

[0084] In the embodiment of FIGS.1D-1E, a capacitance of the first capacitor C1 in each first sub-circuit SC1 may be at least ten times greater than a capacitance of the second capacitor C2 of the respective first sub-circuit SC2. For example, the capacitance of each first capacitor C1 may be between ten and forty times greater than a capacitance of each second capacitor C2. An inductance of the first inductor L1 in each first sub-circuit may be approximately the same as an inductance of the second inductor L2 of the respective first sub- circuit SC1.

[0085] FIG.1G is a front view of a portion 220V of the frequency selective surface 200 of FIG.1D that is within the dotted box. As can be seen, the portion 220V of the frequency selective surface 200 of FIG.1D that is reproduced in FIG.1G comprises the left half of the first unit cell 220-1 and the right half of the second unit cell 220-2. As shown in FIG.1G, theAttorney Docket No.9833.7532.WO portion 220V of the frequency selective surface 200 has the exact same equivalent circuit as is shown in FIG.1F with respect to currents flowing in the direction of the arrow in FIG.1G. In FIG.1G, the first sub-circuit SC1 of FIGS.1E-1F is renamed as a third sub-circuit SC3, and the second sub-circuit SC2 of FIGS.1E-1F is renamed as a fourth sub-circuit SC4 to distinguish therebetween. As shown in FIG.1G, the third sub-circuit SC3 comprises a fourth inductor L4, a fourth capacitor C4, and a fifth inductor L5 that are electrically connected in series, and the fourth sub-circuit SC4 comprises a fifth capacitor C5. The third sub-circuit SC3 is electrically in parallel to the fourth sub-circuit SC4. Referring to FIGS.1D, 1E and 1G, the frequency selective surface 200 may be viewed as having rows R1, R2, etc. of unit cells 220H that each form a first circuit EC1 for currents flowing in the horizontal direction and as having columns C1, C2, etc. of unit cells 220V that each form a first circuit EC1 for currents flowing in the vertical direction. The unit cells 220H, 220V in the horizontal and vertical direction are offset from each other by one-half a unit cell, as described above.

[0086] Referring to FIGS.1A-1B and 1D-1G, when the frequency selective surface 200 is incorporated into base station antenna 100, a base station antenna 100 is provided that comprises an array 130 of lower frequency band radiating elements 132, an array 160 of higher frequency band radiating elements 162A or 162B, and a frequency selective surface 200. The array 130 of lower frequency band radiating elements 132 is mounted forwardly of the frequency selective surface 200 and the array 160 of higher frequency band radiating elements 162 is mounted rearwardly of the frequency selective surface 200. The operating frequency band of the higher frequency band radiating elements 162 may, for example, be within the 1695-2690 MHz frequency range and the operating frequency band of the lower frequency band radiating elements 132 may, for example, be within the 696-960 MHz frequency range. The frequency selective surface 200 is configured to pass RF radiation in an operating frequency band of the higher frequency band radiating elements 162 and to reflect RF radiation in an operating frequency band of the lower frequency band radiating elements 132.

[0087] The frequency selective surface 200 comprises a first plurality of first circuits EC1 that extend in a first direction (in FIGS.1D-1E, the horizontal direction), where each first circuit EC1 comprises a first sub-circuit SC1 that includes a first inductor L1 and a first capacitor C1 that are electrically in series and a second sub-circuit SC2 that is electrically inAttorney Docket No.9833.7532.WO parallel with the first sub-circuit SC1, the second sub-circuit SC2 comprising a second capacitor C2.

[0088] Each first sub-circuit SC1 may further comprise (but need not include) a second inductor L2 that is electrically in series with the first capacitor C2 of the respective first sub- circuit SC1. The first capacitor C1 in each first sub-circuit SC1 may be electrically interposed in between the first inductor L1 and the second inductor L2 of the respective first sub-circuit SC1. In some embodiments, a capacitance of the first capacitor C1 in each first circuit EC1 is at least ten times greater than a capacitance of the second capacitor C2 of the respective first circuit EC1. In some embodiments, a capacitance of the first capacitor C1 in each first circuit EC1 is between twenty and forty times greater than a capacitance of the second capacitor C2 of the respective first circuit EC1. An inductance of the first inductor L1 in each first sub-circuit is approximately the same as an inductance of the second inductor L2 of the respective first sub- circuit SC1.

[0089] In some embodiments, the first inductor L1 of each first sub-circuit SC1 may be implemented as a first trace 230-1 that includes at least one meandered section 234, and the second inductor L2 of each first sub-circuit SC1 may be implemented as a second trace 230-2 that includes at least one meandered section 234. The first trace 230-1 may further include a plurality of first protrusions that form a first comb portion 236-1 and the second trace 230-2 may similarly include a plurality of second protrusions that form a second comb portion 236-2. The protrusions of the first and second comb portions 236-1, 236-2 are interdigitated to form the first capacitor C1.

[0090] The frequency selective surface 200 further comprises a second plurality of first circuits EC1 that extend in the first direction, where the first circuits EC1 in the second plurality of first circuits EC1 are electrically connected in series. The first plurality of first circuits EC1 may comprise a first row R1 of the frequency selective surface 200 and the second plurality of first circuits EC1 may comprise a second row R2 of the frequency selective surface 200.

[0091] Referring again to FIGS.1D and 1G, the frequency selective surface 200 may further comprise a first plurality of third circuits EC3 that extend in a second direction that is different than the first direction (e.g., perpendicular to the first direction), where each third circuit EC3 comprises a third sub-circuit SC3 that includes a fourth inductor L4 and a fourth capacitor C4 that are electrically in series and a fourth sub-circuit SC4 that is electrically inAttorney Docket No.9833.7532.WO parallel with the third sub-circuit SC3, the fourth sub-circuit SC4 comprising a fifth capacitor C5. The third sub-circuit SC3 may also include a fifth inductor L5 that is electrically in series with the fourth capacitor C4. The fourth inductor L4 may be implemented as a third trace 230-3 that includes at least one meandered section 234, and the fifth inductor L5 may be implemented as a fourth trace 230-4 that includes at least one meandered section 234. The third trace 230-3 may also include a plurality of third protrusions and the fourth trace 230-4 may also include a plurality of fourth protrusions that are interdigitated with the third protrusions to form the fourth capacitor C4. The plurality of third circuits EC3 may be electrically connected to one another in series. The frequency selective surface may further comprise a second plurality of third circuits EC3 that extend in the second direction, where the third circuits EC3 in the second plurality of third circuits EC3 are likewise electrically connected in series. The first plurality of third circuits EC3 may comprises a first column C1 of circuit elements in the frequency selective surface 200, and the second plurality of third circuits EC3 may comprises a second column C2 of circuit elements in the frequency selective surface 200.

[0092] FIGS.2A-2C are graphs illustrating the simulated rejection and transmission properties of the frequency selective surface 200 of FIG.1D. In FIGS.2A-2C, the curve with triangles illustrates the S11 (reflectivity) performance of the frequency selective surface 200 and the curve with squares illustrates the S21 (transmissivity) performance of the frequency selective surface 200. The capacitance values of the first and second capacitors C1, C2 and the inductance values of the first and second inductors L1, L2 may be varied to tune the response of the bandpass filter to have different passbands. In each case, the frequency selective surface 200 is configured to reject (reflect) RF radiation in the 696-960 MHz frequency band.

[0093] As shown in FIG.2A, in a first embodiment, the frequency selective surface 200 is configured to pass RF radiation in the 3.3-5.0 GHz frequency band. The frequency selective surface 200 has a return loss of less than -13.75 dB throughout this pass band, and has an insertion loss of greater than -11.8 dB throughout the 696-960 MHz stop band.

[0094] As shown in FIG.2B, in a second embodiment, the frequency selective surface 200 is configured to pass RF radiation in the 1.7-2.7 GHz frequency band. In this embodiment, the frequency selective surface 200 has a return loss of less than -8.8 dB throughout the 1.7-2.7 GHz pass band, and has an insertion loss of greater than -9.3 dB throughout the 696-960 MHz stop band.Attorney Docket No.9833.7532.WO

[0095] As shown in FIG.2C, in a third embodiment, the frequency selective surface 200 is configured to pass RF radiation in the 2.3-4.0 GHz frequency band. In this embodiment, the frequency selective surface 200 has a return loss of less than -7.1 dB throughout the 2.3-4.0 GHz pass band, and has an insertion loss of greater than -10.3 dB throughout the 696-960 MHz stop band.

[0096] FIG.3 is a circuit diagram showing an equivalent circuit of a row (or column) of unit cells of various frequency selective surfaces according to further embodiments of the present invention. Example frequency selective surfaces that have rows and columns that implement the equivalent circuit shown in FIG.3 are discussed in greater detail below with reference to FIGS. 4A-7B. The frequency selective surfaces of FIGS.4A-7B may also be used in the passive / active antenna system of FIGS.1A-1B.

[0097] As shown in FIG.3, each row of the frequency selective surface 300 includes alternating first and second circuits EC1, EC2 that are electrically connected to each other in series. The first circuit EC1 is identical to the first circuit EC1 discussed above with respect to FIGS.1D-1E and can be formed using the same arrangement of narrow metal traces 230 as described above. Thus, further description of the first circuit EC1 will be omitted here. The second circuit EC2 is a parallel LC circuit that comprises a third inductor L3 that is electrically in parallel with a third capacitor C3.

[0098] FIG.4A is a schematic front view of a portion of a frequency selective surface 300 according to embodiments of the present invention that has rows and columns of unit cells that form the equivalent circuit of FIG.3. FIG.4B is a schematic front view of a small portion of the frequency selective surface of FIG.4A that illustrates the non-negligible capacitive and inductive couplings for currents flowing in the direction of the arrow.

[0099] As can be seen by comparing FIGS.4A-4B to FIGS.1D-1E, the frequency selective surface 300 is identical to the frequency selective surface 200 of FIGS.1D-1E except that frequency selective surface 300 has narrow metal traces 330 that have first and second straight portions 332-1, 332-2, where the first straight portion 332-1 is shorter than but otherwise identical to the straight portion 232 of narrow metal trace 230, and the second straight portion 332-2 extends from the distal end of the first straight portion 332-1 at an angle of 135⁰. The second straight portions 332-2 of four adjacent narrow metal traces 330 thus form an open square 331 that interconnects the four adjacent narrow metal traces 330. In addition, four metal padsAttorney Docket No.9833.7532.WO 335 extend inwardly (i.e., into the center of the square 331) from the respective four second straight portions 332-2.

[0100] As shown in FIG.4B, the upper two second straight portions 332-2 of each square 331 form an inductor L31and the lower two second straight portions 332-2 of each square 331 form an inductor L32. The inductors L31, L32, which are electrically in parallel and hence have additive inductances, together form the inductor L3 of the second circuit EC2 in FIG.3. The opposed metal pads 335 extending inwardly from the upper left and lower right second straight portions 332-2 form a capacitor C31and the opposed metal pads 335 extending inwardly from the upper right and lower left second straight portions 332-2 form a capacitor C32. The capacitors C31, C32, which are electrically in parallel and hence have additive capacitances, together form the capacitor C3 of the second circuit EC2 in FIG.3.

[0101] FIG.5A is a schematic front view of a portion of a frequency selective surface 400 according to additional embodiments of the present invention that has rows and columns of unit cells that form the equivalent circuit of FIG.3. FIG.5B is a schematic front view of a small portion of FIG.5A that illustrates how the second circuit EC2 of FIG.3 is implemented in frequency selective surface 400.

[0102] The frequency selective surface 400 is very similar to the frequency selective surface 300 of FIGS.4A-4B, with the only differences being that (1) the second straight portions 332-2 of the narrow metal traces 330 of frequency selective surface 300 are replaced in frequency selective surface 400 with second traces 432-2 that include meandered portions and (2) the metal pads 335 of frequency selective surface 300 are omitted in frequency selective surface 400. As shown in FIG.5B, the upper two second traces 432-2 of each square 331 form an inductor L31and the lower two second traces 432-2 of each square 331 form an inductor L32, where the inductors L31, L32together form the inductor L3 of the second circuit EC2 in FIG.3. The opposed second traces 432-2 that form the upper left and lower right sides of the square 331 capacitively couple to form a capacitor C31and the opposed second traces 432- 2 that form the upper right and lower left sides of the square 331 capacitively couple to form a capacitor C32. The capacitors C31, C32, which are electrically in parallel and hence have additive capacitances, together form the capacitor C3 of the second circuit EC2 in FIG.3.

[0103] The inductor L3 of the frequency selective surface 400 of FIGS.5A-5B will have an increased inductance value as compared to the inductor L3 of the frequencyAttorney Docket No.9833.7532.WO selective surface 300 of FIGS.4A-4B since the meanders in the second traces 432-2 act to increase the inductance of the inductors L31, L32in frequency selective surface 400 as compared to the corresponding inductors L31, L32in the frequency selective surface 300 of FIGS.4A-4B and the metal pads 335 in frequency selective surface 300 act to reduce the inductance of the inductors L31, L32in frequency selective surface 300. The amount of inductance provided by the second traces 432-2 in the frequency selective surface 400 of FIGS.5A-5B may be modified by increasing or decreasing the amount of the meander (e.g., the number of meandered sections, the length of the meandered sections and / or the frequency of the meander). The capacitor C3 of the frequency selective surface 400 of FIGS.5A-5B will have a reduced capacitance value as compared to the capacitor C3 of the frequency selective surface 300 of FIGS.4A-4B since the inwardly-extending metal pads 335 are omitted in the frequency selective surface 400 of FIGS. 5A-5B. The size of the capacitor C3 may be varied by changing the size of the square 331 and the extent to which any inwardly-extending pads 335 extend inwardly.

[0104] FIG.6A is a schematic front view of a portion of a frequency selective surface 500 according to additional embodiments of the present invention that has rows and columns of unit cells that form the equivalent circuit of FIG.3. FIG.6B is a schematic front view of a small portion of FIG.6A that illustrates how the second circuit EC2 of FIG.3 is implemented in frequency selective surface 500.

[0105] The frequency selective surface 500 is identical to the frequency selective surface 200 of FIGS.1D-1E except that the frequency selective surface 500 includes four additional narrow metal traces 540. Each narrow metal trace 540 includes a first end section 542-1 that extends in parallel to the straight portion 232 of a first of the narrow metal traces 230 and a second end section 542-2 that extends in parallel to the straight portion 232 of a second of the narrow metal traces 230. Portions of a current flowing in the horizontal direction along the left-side narrow metal trace 230 in FIG.6B will capacitively couple to the right-side narrow metal trace 230 through the four additional narrow metal traces 540. The amount of current that capacitively couples from the left-side narrow metal trace 230 to the right-side narrow metal trace 230 will be small, as the two capacitively coupled paths each include four capacitors. The frequency selective surface 500 is designed so that the inductance of the inductor L3 and the capacitance of the capacitor C3 will both be small. It will be appreciated that the narrow metal traces 540 may be configured to couple to the narrow metal traces 230 in any fashion. FIG.6BAttorney Docket No.9833.7532.WO shows an example where capacitive connections are formed by bending a portion of each narrow metal trace 540 to run parallel to a one of the narrow metal traces 230. In other embodiments, the bent portions of the narrow metal traces 540 may be omitted so that the end of each narrow metal trace 540 edge couples with one of the narrow metal traces 230. In still other embodiments, the narrow metal traces may be narrowed further and directly connected to the narrow metal traces 230 to provide an inductive connection.

[0106] FIG.7A is a schematic front view of a portion of a frequency selective surface 600 according to additional embodiments of the present invention that has rows and columns of unit cells that form the equivalent circuit of FIG.3. FIG.7B is a schematic front view of a small portion of FIG.7A that illustrates how the second circuit EC2 of FIG.3 is implemented in frequency selective surface 600.

[0107] The frequency selective surface 600 is very similar to the frequency selective surface 300 of FIGS.4A-4B, with the only difference being that frequency selective surface 600 includes metal pads 635 that extend outwardly from the second straight portions 332-2 of the narrow metal traces 330 while frequency selective surface 300 includes metal pads 335 that extend inwardly from the second straight portions 332-2 of the narrow metal traces 330. The outwardly-extending metal pads 635 act to reduce the inductance of inductor L3 (as the width of the current path is increased by the metal pads 635) and hence the inductance of inductor L3 will be smaller in frequency selective surface 600 than in frequency selective surface 300. The outwardly-extending metal pads 635 generally do not affect the capacitance of capacitor C3.

[0108] Referring to FIGS.3-7B, pursuant to further embodiments of the present invention, base station antennas are provided that comprise a frequency selective surface 300, 400, 500, 600 that comprises a first plurality of first circuits EC1, where each first circuit EC1 comprises a first sub-circuit SC1 that includes a first inductor L1 and a first capacitor C1 that are electrically in series and a second sub-circuit SC2 that is electrically in parallel with the first sub- circuit SC1, the second sub-circuit SC2 comprising a second capacitor C2, and a first plurality of second circuits EC2, where each second circuit EC2 comprises a third inductor L3 that is electrically in parallel with a third capacitor C3. Each second circuit EC2 in the first plurality of second circuits EC2 may be electrically in series with a respective one of the first circuits EC1 in the first plurality of first circuits EC1. The first circuits in the first plurality of first circuitsAttorney Docket No.9833.7532.WO EC1 and the second circuits EC2 in the first plurality of second circuits may extend in a first direction (e.g., they may extend in alternating fashion in a row or column). Each second circuit EC2 in the first plurality of second circuits EC2 may be physically positioned in between a respective pair of the first circuits EC1 in the first plurality of first circuits EC1 so that the first and second circuits EC1, EC2 are electrically connected in series in alternating fashion.

[0109] In some embodiments, each second circuit EC2 may comprise a closed metal loop (e.g., the square 331 formed by the second portions of the narrow metal traces) that has at least a partially open interior. The closed metal loop forms the third inductor L3 and capacitances across the at least a partially open interior of the closed metal loop form the third capacitor C3.

[0110] FIGS.8A-8C are graphs illustrating the simulated rejection and transmission properties of frequency selective surfaces having the unit cell equivalent circuit of FIG.3. In FIGS.8A-8C, the curve with triangles illustrates the S11 (reflectivity) performance of the frequency selective surface and the curve with squares illustrates the S21 (transmissivity) performance of the frequency selective surface. The capacitance values of the capacitors C1, C2, C3 and the inductance values of the inductors L1, L2, L3 may be varied using the different techniques, discussed above with respect to FIGS.4A-7B to tune the response of the bandpass filter to have different passbands. In each case, the frequency selective surface is configured to reject (reflect) RF radiation in the 696-960 MHz frequency band.

[0111] As shown in FIG.8A, in a first embodiment, the frequency selective surface is configured to pass RF radiation in the 3.3-5.0 GHz frequency band. The frequency selective surface has a return loss of less than -14.4 dB throughout this pass band, and has an insertion loss of greater than -10.99 dB throughout the 696-960 MHz stop band.

[0112] As shown in FIG.8B, in a second embodiment, the frequency selective surface is configured to pass RF radiation in the 1.7-2.7 GHz frequency band. In this embodiment, the frequency selective surface has a return loss of less than -8.4 dB throughout the 1.7-2.7 GHz pass band, and has an insertion loss of greater than -8.7 dB throughout the 696-960 MHz stop band.

[0113] As shown in FIG.8C, in a third embodiment, the frequency selective surface is configured to pass RF radiation in the 2.3-4.0 GHz frequency band. In this embodiment, the frequency selective surface has a return loss of less than -9.8 dB throughout theAttorney Docket No.9833.7532.WO 2.3-4.0 GHz pass band, and has an insertion loss of greater than -10.3 dB throughout the 696-960 MHz stop band.

[0114] FIG.9A is a schematic front view of a portion of a frequency selective surface 700 according to still other embodiments of the present invention. FIG.9B is a schematic front view of a small portion of FIG.9A. One unit cell 720 of the frequency selective surface 700 (for horizontally-flowing currents) is highlighted by the dashed box in FIG.9A.

[0115] The frequency selective surface 700 is similar to the frequency selective surface 700 of FIGS.1D-1E, with the two differences being that (1) in frequency selective surface 700, the size of the meandered portion 734 of each narrow metal trace 730 is increased and (2) the frequency selective surface 700 includes open-circuit traces 750 that are provided at the interface between the meandered portion 734 and the comb portion 736 of each narrow metal trace 730. Each open-circuit stub 750 may have an electrical length that results in the open- circuit stub 750 presenting as a capacitance in the operating bandwidth of the higher frequency band radiating elements 162A, 162B. For example, each open-circuit stub 750 may have an electrical length that is less than a quarter of a wavelength of a center frequency of the operating bandwidth of the higher frequency band radiating elements 162A, 162B.

[0116] FIG.9C is a circuit diagram showing the equivalent circuit of the unit cells of the frequency selective surface 700 of FIG.9A. As can be seen from FIG.9C, each open-circuit stub 750 may act like a capacitor to ground. The capacitance of the open-circuit stubs 750 may be adjusted to a desired value by changing the length of the stubs 750. The amount that the length of the stub 750 is less than a quarter of a wavelength of a center frequency of the operating bandwidth of the higher frequency band radiating elements 162A, 162B changes the capacitance value, and if the length is made longer than a quarter wavelength the open-circuit stub 750 will act as an inductance to ground, and the length can again be varied to change the amount of inductance. It will also be appreciated that the electrical lengths where the open- circuit stub will appear as a capacitance repeats every half wavelength. Thus, the open circuit stubs 750 may also have lengths of, for example, between ½ and ¾ of a wavelength of a center frequency of the operating bandwidth of the higher frequency band radiating elements 162A, 162B.The addition of the open-circuit stubs 750 may increase the width of the passband and / or improve the amount of rejection in the stop band of the filter response of frequency selective surface 700.Attorney Docket No.9833.7532.WO

[0117] Referring to FIGS.9A-9C, pursuant to still further embodiments of the present invention, base station antennas are provided that comprise a frequency selective surface 700 that comprises a plurality of open circuit traces 750, an array 130 of lower frequency band radiating elements 132 mounted forwardly of the frequency selective surface 750, and an array 160 of higher frequency band radiating elements 162A or 162B mounted rearwardly of the frequency selective surface 750. The frequency selective surface 750 comprises a plurality of unit cells 720, and each unit cell 720 includes at least one open circuit trace 750. In the embodiment of FIGS.9A-9C, each unit cell 720 includes a total of four open circuit traces 750. Each open circuit trace 750 may have an electrical length that is approximately a quarter wavelength corresponding to a center frequency of an operating frequency band of the higher frequency band radiating elements 162A, 162B. In some embodiments, some or all of the open circuit traces 750 may extend from an interface between the first capacitor C1 and the first inductor L1 of a respective one of the first sub-circuits SC1 or from an interface between the first capacitor C1 and the second inductor L2 of a respective one of the first sub-circuits SC1.

[0118] As discussed above, the frequency selective surfaces according to embodiments of the present invention may have a bandpass filter response. Thus, according to further embodiments of the present invention, base station antennas are provided that comprise an array 130 of lower frequency band radiating elements 132, an array 160 of higher frequency band radiating elements 162, and a frequency selective surface implemented in a single metal layer, where the frequency selective surface has a bandpass filter response

[0119] The frequency selective surfaces according to embodiments of the present invention may be used in a wide variety of base station antennas including, for example, in the passive / active antenna system 100 of FIGS.1A-1B to implement either or both of the frequency selective surfaces 140-1, 140-2.

[0120] The frequency selective surfaces according to embodiments of the present invention may be used in many applications, including in passive / active antenna systems where the frequency selective surface may act as reflectors for low-band and / or mid-band linear arrays that are mounted in front of the frequency selective surface, and may be substantially transparent to RF radiation in the operating frequency band of a high-band beamforming array that is mounted rearwardly of the frequency selective surface.Attorney Docket No.9833.7532.WO

[0121] The present invention has been described above with reference to the accompanying drawings. The present invention is not limited to the illustrated embodiments. Rather, these embodiments are intended to fully and completely disclose the present 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.

[0122] 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 example 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.

[0123] Herein, the terms "attached," "connected," "interconnected," "contacting," "mounted," "coupled," and the like can mean either direct or indirect attachment or coupling between elements, unless stated otherwise.

[0124] 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.

[0125] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present 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.

Claims

Attorney Docket No.9833.7532.WO CLAIMS:

1. A base station antenna; comprising: an array of lower frequency band radiating elements; an array of higher frequency band radiating elements; and a frequency selective surface that comprises a first plurality of first circuits that extend in a first direction, where each first circuit comprises a first sub-circuit that includes a first inductor and a first capacitor that are electrically in series and a second sub-circuit that is electrically in parallel with the first sub-circuit, the second sub-circuit comprising a second capacitor.

2. The base station antenna of Claim 1, wherein each first sub-circuit further comprises a second inductor that is electrically in series with the first capacitor of the respective first sub-circuit.

3. The base station antenna of Claim 2, wherein the first capacitor in each first sub- circuit is electrically interposed in between the first inductor and the second inductor of the respective first sub-circuit.

4. The base station antenna of Claim 1, wherein a capacitance of the first capacitor in each first circuit is at least ten times greater than a capacitance of the second capacitor of the respective first circuit.

5. The base station antenna of Claim 1, wherein a capacitance of the first capacitor in each first circuit is between twenty and forty times greater than a capacitance of the second capacitor of the respective first circuit.

6. The base station antenna of Claim 2, wherein an inductance of the first inductor in each first sub-circuit is approximately the same as an inductance of the second inductor of the respective first sub-circuit.

7. The base station antenna of any of Claims 1-6, wherein the array of lower frequency band radiating elements is mounted forwardly of the frequency selective surface and the array of higher frequency band radiating elements is mounted rearwardly of the frequency selective surface.Attorney Docket No.9833.7532.WO 8. The base station antenna of Claim 3, wherein, for each first sub-circuit, the first inductor comprises a first trace that includes at least one meandered section, and the second inductor comprises a second trace that includes at least one meandered section.

9. The base station antenna of Claim 8, wherein the first trace includes a plurality of first protrusions and the second trace includes a plurality of second protrusions that are interdigitated with the first protrusions to form the first capacitor.

10. The base station antenna of any of Claims 1-6, wherein the frequency selective surface further comprises a second plurality of first circuits that extend in the first direction, wherein the first circuits in the second plurality of first circuits are electrically connected in series.

11. The base station antenna of any of Claims 1-6, wherein the frequency selective surface further comprises a first plurality of second circuits, where each second circuit in the first plurality of second circuits comprises a third inductor that is electrically in parallel with a third capacitor, wherein each second circuit is electrically in series with a respective one of the first circuits in the first plurality of first circuits.

12. The base station antenna of Claim 11, wherein the second circuits in the first plurality of second circuits extend in the first direction.

13. The base station antenna of Claim 11, wherein the second circuits in the first plurality of second circuits are physically positioned in between respective pairs of the first circuits in the first plurality of first circuits.

14. The base station antenna of Claim 11, wherein the second circuits in the first plurality of second circuits are coupled electrically in series in between respective pairs of the first circuits in the first plurality of first circuits.

15. The base station antenna of Claim 11, wherein each second circuit comprises a closed metal loop that has at least a partially open interior.Attorney Docket No.9833.7532.WO 16. The base station antenna of Claim 15, wherein the closed metal loop forms the third inductor and capacitances across the at least a partially open interior of the closed metal loop form the third capacitor.

17. The base station antenna of Claim 8, wherein the frequency selective surface further comprises a first plurality of third circuits that extend in a second direction that is different than the first direction, where each third circuit comprises a third sub-circuit that includes a fourth inductor and a fourth capacitor that are electrically in series and a fourth sub- circuit that is electrically in parallel with the third sub-circuit, the fourth sub-circuit comprising a fifth capacitor.

18. The base station antenna of Claim 17, wherein the second direction is perpendicular to the first direction.

19. The base station antenna of Claim 18, wherein the third sub-circuit further comprises a fifth inductor that is electrically in series with the fourth capacitor.

20. The base station antenna of Claim 19, wherein the fourth inductor comprises a third trace that includes at least one meandered section, and the fifth inductor comprises a fourth trace that includes at least one meandered section.

21. The base station antenna of Claim 20, wherein the third trace includes a plurality of third protrusions and the fourth trace includes a plurality of fourth protrusions that are interdigitated with the third protrusions to form the fourth capacitor.

22. The base station antenna of Claim 17, wherein the plurality of third circuits are electrically connected in series.

23. The base station antenna of Claim 17, wherein the frequency selective surface further comprises a second plurality of third circuits that extend in the second direction, wherein the third circuits in the second plurality of third circuits are electrically connected in series.

24. The base station antenna of Claim 19, wherein the frequency selective surface further comprises a first plurality of fourth circuits, where each fourth circuit in the first plurality of fourth circuits comprises a sixth inductor that is electrically in parallel with a sixth capacitor,Attorney Docket No.9833.7532.WO wherein each fourth circuit in the plurality of fourth circuits is electrically in series with a respective one of the third circuits.

25. The base station antenna of Claim 24, wherein the fourth circuits in the first plurality of fourth circuits extend in the second direction.

26. The base station antenna of any of Claims 1-6, wherein the frequency selective surface further comprises a plurality of open circuit traces.

27. The base station antenna of Claim 26, wherein each open circuit trace has an electrical length that configures the open circuit trace to add a capacitance with respect to signals in an operating frequency band of the higher frequency band radiating elements.

28. The base station antenna of Claim 26, wherein a first of the open circuit traces extends from an interface between the first capacitor and the first inductor of one of the first sub- circuits.

29. The base station antenna of any of Claims 1-28, wherein the frequency selective surface is configured to pass radio frequency ("RF") radiation in an operating frequency band of the higher frequency band radiating elements and to reflect RF radiation in an operating frequency band of the lower frequency band radiating elements.

30. The base station antenna of Claim 29, wherein the operating frequency band of the higher frequency band radiating elements is within the 1695-2690 MHz frequency range and the operating frequency band of the lower frequency band radiating elements is within the 696- 960 MHz frequency range.

31. A base station antenna, comprising: a frequency selective surface that comprises a first plurality of first circuits, where each first circuit comprises a first sub-circuit that includes a first inductor and a first capacitor that are electrically in series and a second sub-circuit that is electrically in parallel with the first sub- circuit, the second sub-circuit comprising a second capacitor, and a first plurality of second circuits, where each second circuit comprises a third inductor that is electrically in parallel with a third capacitor.Attorney Docket No.9833.7532.WO 32. The base station antenna of Claim 31, wherein the first circuits in the first plurality of first circuits and the second circuits in the first plurality of second circuits are electrically coupled in series in alternating fashion.

33. The base station antenna of Claim 32, wherein the first circuits in the first plurality of first circuits and the second circuits in the first plurality of second circuits extend in alternating fashion in a first direction.

34. The base station antenna of Claim 31, wherein each first sub-circuit further comprises a second inductor that is electrically in series with the first capacitor of the respective first sub-circuit.

35. The base station antenna of Claim 34, wherein the first capacitor is electrically interposed in between the first inductor and the second inductor of the respective first sub-circuit.

36. The base station antenna of Claim 31, wherein a capacitance of the first capacitor in each first circuit is at least ten times greater than a capacitance of the second capacitor of the respective first circuit.

37. The base station antenna of Claim 34, wherein an inductance of the first inductor in each first sub-circuit is approximately the same as an inductance of the second inductor of the respective first sub-circuit.

38. The base station antenna of Claim 34, wherein each first inductor comprises a first trace that includes at least one meandered section, and each second inductor comprises a second trace that includes at least one meandered section.

39. The base station antenna of Claim 38, wherein, for each first sub-circuit, the first trace includes a plurality of first protrusions and the second trace includes a plurality of second protrusions that are interdigitated with the first protrusions to form the first capacitor.

40. The base station antenna of Claim 31, wherein an array of lower frequency band radiating elements is mounted forwardly of the frequency selective surface and an array of higher frequency band radiating elements is mounted rearwardly of the frequency selective surface.Attorney Docket No.9833.7532.WO 41. The base station antenna of any of Claims 32-40, wherein the frequency selective surface further comprises a second plurality of first circuits and a second plurality of second circuits, where the first circuits in the second plurality of first circuits and the second circuits in the second plurality of second circuits are electrically coupled in series in alternating fashion.

42. The base station antenna of Claim 41, wherein the first circuits in the first plurality of first circuits and the second circuits in the first plurality of second circuits extend in the first direction, and the first circuits in the second plurality of first circuits and the second circuits in the second plurality of second circuits extend in the first direction in parallel to the first circuits in the first plurality of first circuits and the second circuits in the first plurality of second circuits.

43. The base station antenna of Claim 42, wherein each second circuit comprises a closed metal loop that has at least a partially open interior.

44. The base station antenna of Claim 43, wherein the closed metal loop forms the third inductor and capacitances across the at least a partially open interior of the closed metal loop form the third capacitor.

45. The base station antenna of Claim 42, wherein the frequency selective surface further comprises a first plurality of third circuits and a first plurality of fourth circuits that extend in alternating fashion in a second direction that is perpendicular to the first direction, wherein each third circuit in the first plurality of third circuits comprises a third sub- circuit that includes a fourth inductor, a fourth capacitor and a fifth inductor that are electrically in series and a fourth sub-circuit that is electrically in parallel with the third sub-circuit, the fourth sub-circuit comprising a fifth capacitor, and wherein each fourth circuit in the first plurality of fourth circuits comprises a fifth inductor that is electrically in parallel with a sixth capacitor.

46. The base station antenna of any of Claims 31-40, wherein the frequency selective surface further comprises a plurality of open circuit traces.Attorney Docket No.9833.7532.WO 47. The base station antenna of Claim 46, wherein each open circuit trace has an electrical length that configures the open circuit trace to add a capacitance with respect to signals in an operating frequency band of the higher frequency band radiating elements.

48. The base station antenna of Claims 46 or 47, wherein a first of the open circuit traces extends from an interface between the first capacitor and the first inductor of one of the first sub-circuits.

49. The base station antenna of any of Claims 40-48, wherein the frequency selective surface is configured to pass radio frequency ("RF") radiation in an operating frequency band of the higher frequency band radiating elements and to reflect RF radiation in an operating frequency band of the lower frequency band radiating elements.

50. A base station antenna; comprising: an array of lower frequency band radiating elements; an array of higher frequency band radiating elements; and a frequency selective surface implemented in a single metal layer, the frequency selective surface having a bandpass filter response.

51. The base station antenna of Claim 50, wherein the frequency selective surface comprises a first plurality of first circuits that are electrically coupled in series, where each first circuit comprises a first sub-circuit that includes a first inductor, a first capacitor and a second inductor that are electrically in series and a second sub-circuit that is electrically in parallel with the first sub-circuit, the second sub-circuit comprising a second capacitor.

52. The base station antenna of Claim 51, wherein the frequency selective surface further comprises a first plurality of second circuits that are electrically coupled in series with the first plurality of first circuits so that the first circuits and the second circuits are arranged in alternating fashion in the series connection, wherein each second circuit in the first plurality of second circuits comprises a third inductor that is electrically in parallel with a third capacitor.

53. The base station antenna of any of Claims 50-52, wherein the frequency selective surface further comprises a plurality of open circuit traces.Attorney Docket No.9833.7532.WO 54. The base station antenna of Claim 53, wherein each open circuit trace has an electrical length that configures the open circuit trace to add a capacitance with respect to signals in an operating frequency band of the higher frequency band radiating elements.

55. The base station antenna of Claims 53 or 54, wherein a first of the open circuit traces extends from an interface between the first capacitor and the first inductor of one of the first sub-circuits.

56. The base station antenna of any of Claims 50-55, wherein the frequency selective surface is configured to pass radio frequency ("RF") radiation in an operating frequency band of the higher frequency band radiating elements and to reflect RF radiation in an operating frequency band of the lower frequency band radiating elements.

57. A base station antenna; comprising: a frequency selective surface that comprises a plurality of open circuit traces; an array of lower frequency band radiating elements mounted forwardly of the frequency selective surface; and an array of higher frequency band radiating elements mounted rearwardly of the frequency selective surface.

58. The base station antenna of Claim 57, wherein the frequency selective surface comprises a plurality of unit cells, and each unit cell includes at least one open circuit trace.

59. The base station antenna of Claim 58, wherein the frequency selective surface comprises a plurality of unit cells, and each unit cell includes a total of four open circuit traces.

60. The base station antenna of any of Claims 57-59, wherein each open circuit trace has an electrical length that configures the open circuit trace to add a capacitance with respect to signals in an operating frequency band of the higher frequency band radiating elements.

61. The base station antenna of any of Claims 57-59, wherein a first of the open circuit traces extends from an interface between the first capacitor and the first inductor of one of the first sub-circuits.Attorney Docket No.9833.7532.WO 62. The base station antenna of any of Claims 57-59, wherein the frequency selective surface further comprises a first plurality of first circuits that extend in a first direction, where each first circuit comprises a first sub-circuit that includes a first inductor, a first capacitor and a second inductor that are electrically in series and a second sub-circuit that is electrically in parallel with the first sub-circuit, the second sub-circuit comprising a second capacitor.

63. The base station antenna of Claim 62, wherein the first capacitor is electrically interposed in between the first inductor and the second inductor.

64. The base station antenna of Claim 63, wherein the first inductor comprises a first trace that includes at least one meandered section, and the second inductor comprises a second trace that includes at least one meandered section.

64. The base station antenna of Claim 64, wherein the first trace includes a plurality of first protrusions and the second trace includes a plurality of second protrusions that are interdigitated with the first protrusions to form the first capacitor.

65. The base station antenna of Claim 62, wherein the first plurality of first circuits are electrically connected in series.

66. The base station antenna of Claim 65, the frequency selective surface further comprises a second plurality of first circuits that extend in the first direction, wherein the first circuits in the second plurality of first circuits are electrically connected in series.

67. The base station antenna of Claim 62, wherein the frequency selective surface further comprises a first plurality of second circuits, where each second circuit in the first plurality of second circuits comprises a third inductor that is electrically in parallel with a third capacitor, wherein each second circuit is electrically in series with a respective one of the first circuits.

68. The base station antenna of Claim 67, wherein the second circuits in the first plurality of second circuits extend in the first direction.Attorney Docket No.9833.7532.WO 69. The base station antenna of Claim 68, wherein the second circuits in the first plurality of second circuits are coupled electrically in series in between respective pairs of the first circuits in the first plurality of first circuits.

70. The base station antenna of any of Claims 57-69, wherein the frequency selective surface is configured to pass radio frequency ("RF") radiation in an operating frequency band of the higher frequency band radiating elements and to reflect RF radiation in an operating frequency band of the lower frequency band radiating elements.

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