WO2024145734A1

WO2024145734A1 - Radiating elements having feed stalks with frequency selective surfaces and base station antennas including such radiating elements

Info

Publication number: WO2024145734A1
Application number: PCT/CN2023/070056
Authority: WO
Inventors: Jian Zhang; Changfu Chen; Bin Sun; Cheng XUE; Pengfei Guo
Original assignee: Commscope Technologies Llc
Priority date: 2023-01-03
Filing date: 2023-01-03
Publication date: 2024-07-11

Abstract

A radiating element comprises a feed stalk and a radiator mounted on the feed stalk. The feed stalk includes a signal line and a ground line, the ground line comprising a frequency selective surface.

Description

RADIATING ELEMENTS HAVING FEED STALKS WITH FREQUENCY SELECTIVE SURFACES AND BASE STATION ANTENNAS INCLUDING SUCH RADIATING ELEMENTS

BACKGROUND

The present invention generally relates to radio communications and, more particularly, to base station antennas for cellular communications systems and to radiating elements for such base station antennas.

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. Typically, the base station antennas are mounted on a tower or other raised structure, with the radiation patterns (also referred to herein as "antenna beams" ) that are generated by the base station antennas directed outwardly.

A common base station configuration is the three sector configuration in which a cell is divided into three 120° "sectors" in the azimuth (horizontal) plane. A separate base station antenna provides coverage (service) to each sector. Typically, each base station antenna will include multiple vertically-extending columns of radiating elements that operate, for example, using second generation ( "2G" ) , third generation ( "3G" ) or fourth generation ( "4G" ) cellular network protocols. These vertically-extending columns of radiating elements are typically referred to as "linear arrays, " and may be straight columns or columns in which some of the radiating elements are staggered horizontally. Most modern base station antennas include both "low-band" linear arrays of radiating elements that support service in some or all of the 617-960 MHz frequency band and "mid-band" linear arrays of radiating elements that support service in some or all of the 1427-2690 MHz frequency band. These linear arrays are typically formed using dual-polarized radiating elements, which allows each linear array to transmit and receive RF signals at two orthogonal polarizations.

Each of the above-described linear arrays is coupled to two ports of a radio (one port for each polarization) . An RF signal that is to be transmitted by a linear array is passed from the radio port to the antenna where it is divided into a plurality of sub-components, with each sub-component fed to a respective subset of the radiating elements in the linear array (typically each sub-component is fed to between one and three radiating elements) . The sub-components of the RF signal are transmitted through the radiating elements to generate an antenna beam that covers a generally fixed coverage area, such as a sector of a cell. Since the antenna beams generated by the above-described 2G/3G/4G linear arrays generate static antenna beams, they are often referred to as "passive" linear arrays.

Most cellular operators are currently upgrading their networks to support fifth generation ( "5G" ) cellular service. One important component of 5G cellular service is the use of so-called "active" beamforming arrays that operate in conjunction with active beamforming radios to dynamically adjust the size, shape and pointing direction of the antenna beams that are generated by the active beamforming array. These active beamforming arrays include multiple columns of radiating elements, with eight columns being the most common. Active beamforming arrays are typically formed using "high-band" radiating elements that operate in higher frequency bands, such as some or all of the 3.1-4.2 GHz and/or the 5.1-5.8 GHz frequency bands, although active beamforming arrays may also be provided that operate in the upper portion of the mid-band frequency range (e.g., 2300-2690 MHz) . Each column of radiating elements of such an active beamforming array is typically coupled to a respective port of a beamforming radio. The beamforming radio may be a separate device, or may be integrated with the active antenna array. The beamforming radio may adjust the amplitudes and phases of the sub-components of an RF signal that are fed to each port of the radio in order to generate antenna beams that have narrowed beamwidths in the azimuth plane (and hence higher antenna gain) . These narrowed antenna beams can be electronically steered in the azimuth plane by proper selection of the amplitudes and phases of the sub-components of an RF signal.

In order to avoid having to increase the number of antennas at cell sites, the above-described 5G antennas often include passive linear arrays that support legacy 2G, 3G and/or 4G cellular services. In one popular solution, a 5G active antenna module (i.e., a module that includes an active beamforming array and associated beamforming radio) is mounted on the rear surface of a passive base station antenna that includes a plurality of 2G, 3G, and/or 4G passive linear arrays. An opening is provided in the reflector of the passive base station antenna so that the antenna beams generated by the active beamforming array can be transmitted through the passive base station antenna. Typically, some of the radiating elements of the 2G/3G/4G passive linear arrays are mounted in front of the radiating elements of the beamforming array. The above-described antenna design is advantageous as the active antenna module may be removable, and hence as enhanced 5G capabilities are developed, a cellular operator may replace the original active antenna module with an upgraded active antenna module without having to replace the passive base station antenna. Herein, the combination of a passive base station antenna that has an active antenna module mounted thereon is referred to as a "passive/active antenna system. "

SUMMARY

Pursuant to embodiments of the present invention, radiating elements are provided that comprise a feed stalk having a signal line and a ground line, the ground line comprising a frequency selective surface and a radiator mounted on the feed stalk.

In some embodiments, the signal line may overlap the ground line.

In some embodiments, the frequency selective surface comprises at least two spaced-apart metal layers.

In some embodiments, the signal line and the ground line together comprise a RF feed line that is configured to feed RF signals to the radiator.

In some embodiments, the frequency selective surface comprises a bandpass filter.

In some embodiments, the frequency selective surface comprises a plurality of unit cells, each unit cell comprising a first metal pattern and a second metal pattern that is positioned behind the first metal pattern. In some embodiments, the first metal pattern comprises a cross-shaped pattern and/or the second metal pattern comprises a polygonal pad. In some embodiments, the first metal pattern of each unit cell is galvanically coupled to the first metal patterns of respective adjacent unit cells. In some embodiments, the second metal pattern of each unit cell is capacitively edge coupled to the second metal patterns of respective adjacent unit cells.

In some embodiments, the frequency selective surface may be configured to, on average, be at least 25%transparent (or at least 40%transparent) to RF energy in the higher operating frequency band.

In some embodiments, the ground line may comprise a plurality of longitudinally-extending traces and a plurality of transversely-extending traces that are interconnected to form a grid.

Pursuant to further embodiments of the present invention, radiating elements are provided that comprise a dipole radiator and a feed stalk printed circuit board having a first metal layer that includes a signal line that is configured to be coupled to a first conductor of a RF transmission line and a second metal layer that is configured to be coupled to a second conductor of the RF transmission line, and a third metal layer.

In some embodiments, the feed stalk printed circuit board further includes a first dielectric substrate that is between the first metal layer and the second metal layer and a second dielectric substrate that is between the second metal layer and the third metal layer. In some embodiments, the second metal layer and the third metal layer together form a frequency selective surface.

In some embodiments, the second metal layer comprises a ground line that is configured to be coupled to a second conductor of the RF transmission line. In some embodiments, the signal line and the ground line together comprise an RF feed line. In some embodiments, the frequency selective surface comprises a bandpass filter.

In some embodiments, the frequency selective surface comprises a plurality of unit cells, each unit cell comprising a first metal pattern and a second metal pattern that is positioned behind the first metal pattern.

In some embodiments, the first metal pattern comprises a cross-shaped pattern and the second metal pattern comprises a polygonal pad.

In some embodiments, the first metal pattern of each unit cell is galvanically coupled to the respective first metal patterns of unit cells that are adjacent thereto, and the second metal pattern of each unit cell is capacitively edge coupled to the respective second metal patterns of the unit cells that are adjacent thereto.

Pursuant to further embodiments of the present invention, base station antennas are provided that include a first radiating element that is configured as any of the above-described radiating elements, as well as a second radiating element that is configured to operate in a higher operating frequency band than the first radiating element. The frequency selective surface of included in the first radiating element may comprise a bandpass filter having a passband that encompasses at least a portion of the higher operating frequency band.

In some embodiments, the first radiating element may be mounted forwardly of the second radiating element.

Pursuant to still further embodiments of the present invention, base station antennas are provided that comprise a first radiating element that is part of a first array of radiating elements that is configured to operate in a first frequency band and a second radiating element that is part of a second array of radiating elements that is configured to operate in a second frequency band. In these antennas, the first radiating element comprises a feed stalk that includes an RF feed line and a radiator that is coupled to the RF feed transmission line. The feed stalk includes a frequency selective surface that is configured to, on average, be at least 25%transparent to RF energy in the second frequency band.

In some embodiments, the RF feed line comprises a signal line and a ground line, and wherein at least part of the frequency selective surface forms the ground line.

In some embodiments, each first metal pattern comprises a respective cross-shaped pattern, and each second metal pattern comprises a respective polygonal pad.

In some embodiments, the frequency selective surface comprises a metal pattern that includes longitudinally and transversely extending metal traces, and wherein the signal line overlaps one of the longitudinally extending metal traces.

In some embodiments, the base station antenna may further comprise a second frequency selective surface, where the first radiating element is mounted forwardly of the second frequency selective surface and the second radiating element is mounted rearwardly of the second frequency selective surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic perspective view of a conventional low-band cross-dipole radiating element.

FIG. 1B is a schematic side view of the conventional low-band cross-dipole radiating element of FIG. 1A.

FIG. 2A is a schematic perspective view of a passive/active antenna system that includes a passive base station antenna that may be implemented using low-band radiating elements according to embodiments of the present invention.

FIG. 2B is a schematic front view of the passive/active antenna system of FIG. 2A with the radomes thereof omitted.

FIG. 3A is a schematic perspective view of a low-band radiating element according to embodiments of the present invention.

FIG. 3B is a side view of the first feed stalk printed circuit board that is included in the feed stalk of the low-band radiating element of FIG. 3A.

FIG. 3C is a schematic "top" plan view of the first feed stalk printed circuit board that is included in the feed stalk of the low-band radiating element of FIG. 3A..

FIG. 3D is a schematic plan view of the first feed stalk printed circuit board that is included in the feed stalk of the low-band radiating element of FIG. 3A with the first (top) metal layer and the first dielectric substrate of the first feed stalk printed circuit board removed in order to show the second (middle) metal layer.

FIG. 3E is a schematic "bottom" plan view of the first feed stalk printed circuit board that is included in the feed stalk of the low-band radiating element of FIG. 3A..

FIG. 3F is a perspective view of three of the unit cells of one of the frequency selective surfaces included in each feed stalk printed circuit board of the radiating element of FIG. 3A.

FIG. 4 is a graph of the S1: 1 and S1: 2 transmission characteristics of the ground lines of the low-band radiating element of FIGS. 3A-3F.

DETAILED DESCRIPTION

The above-described passive/active antenna systems allow a cellular operator to support both legacy 2G/3G/4G cellular service and 5G cellular service using a single base station antenna system. Unfortunately, however, in practice the radiating elements of the passive 2G/3G/4G arrays that are mounted in front of the 5G beamforming array can cause "scattering" of the antenna beams generated by the beamforming array. Scattering is undesirable as it may reduce the gain of the 5G antenna beams by changing the shape thereof in both the azimuth and elevation planes. For example, scattering tends to negatively impact the beamwidth, beam shape, pointing angle, gain and front-to-back ratio of the 5Gantenna beams.

Two different types of scattering can occur. First, conductive structures of the radiating elements of the lower frequency (passive) arrays that are mounted in front of the 5G beamforming array can reflect RF energy transmitted by the radiating elements of the beamforming array. Some of this reflected RF energy may then exit the base station antenna in undesired directions (potentially after further reflecting off of other metal structures in the base station antenna such as the reflector, etc. ) or may exit the base station antenna in a desired direction but with a phase that causes the reflected RF energy to destructively combine with non-reflected RF energy. The net result is that when RF energy emitted by the beamforming array reflects off the radiating elements of the passive 2G/3G/4G linear arrays, these reflections generally act to distort the radiation pattern generated by the beamforming array in undesirable ways.

The second type of scattering occurs when a conductive structure of the radiating elements of the passive 2G/3G/4G linear arrays has an electrical length that makes the structure resonant in the operating frequency band of the 5G beamforming array. A conductive structure of a radiating element of one of the passive (lower frequency band) arrays may be resonant in the operating frequency band of the 5G beamforming array if, for example, the conductive structure has an electrical length that is about 1/2 a wavelength or about a full wavelength of a frequency within the operating frequency band of the 5G beamforming array. In many cases, the operating frequency band of the beamforming array may be about four times frequencies within the operating frequency band of the passive low-band linear arrays and about twice frequencies within the operating frequency band of the passive mid-band linear arrays. Since the dipole arms of the radiating elements of the low-band linear arrays typically have an electrical length of about 1/4 of a center wavelength of the low-band operating frequency range, they may have a resonant length with respect to RF energy emitted by the 5G beamforming array. As such, RF energy transmitted by the 5G beamforming array may couple to, for example, the dipole arms of nearby low-band radiating elements, and the higher-band currents formed on these dipole arms generates additional high-band radiation that distorts the antenna beams generated by the linear arrays of low-band radiating elements.

So-called "cloaking" radiating elements are known in the art that have dipole arms that are designed so that high-band currents will largely not form thereon in response to RF radiation in pre-selected frequency ranges. These radiating elements can reduce or eliminate the second of the above-described types of scattering of higher frequency band radiation by the dipole arms of nearby lower frequency band radiating elements. The present invention is based, in part, on the realization that the feed stalks of the lower frequency band radiating elements may also cause both of the above-describe types of scattering. A feed stalk of a cross-dipole radiating element refers to a structure that feeds RF signals to and from the dipole arms of the radiating element. In most case, the dipole arms are mounted on the distal (forward) end of the feed stalk, and the base (rear) end of the feed stalk is mounted on the reflector of the base station antenna or on a feed board printed circuit board that is mounted on the reflector

The feed stalks of the low-band radiating elements may include a number of metal patterns that can reflect high-band RF radiation emitted by a high-band beamforming array that is mounted behind the low-band radiating elements (i.e., the feed stalks of the low-band radiating elements can cause the first type of scattering discussed above) . Such reflections can degrade the shape and characteristics of the antenna beams formed by the high-band beamforming array. In addition, the feed stalks of the low-band band radiating elements also typically have metal structures that have lengths that are about 1/4 of the center wavelength of the low-band operating frequency range, and hence the feed stalks of the low-band radiating elements may also cause the second type of scattering discussed above with respect to RF radiation emitted by the 5G beamforming array. While the amount of scattering caused by the feed stalks tends to be much lower than the scattering caused by non-cloaked low-band dipole arms, the amount of scattering may still be significant enough to distort the antenna beams formed by the 5G beamforming array.

Pursuant to embodiments of the present invention, base station antennas are provided that include low-band radiating elements that have feed stalks that comprise one or more frequency selective surfaces. These frequency selective surfaces may be configured to be at least partially transparent to RF radiation in an operating frequency band of high-band radiating elements that are included in the base station antenna. As such, RF radiation emitted by the high-band radiating elements that is directed in the direction of the low-band radiating elements may experience less scattering by the low-band radiating elements.

In some embodiments, each frequency selective surface may comprise part of an RF feed line that is provided on a feed stalk of the low-band radiating element. The RF feed line may comprise a signal line and a ground line. The signal line may be configured to be coupled to a first conductor of an RF transmission line that feeds that first radiating element, and the ground line may be configured to be coupled to a second conductor of the RF transmission line that feeds that low-band radiating element. In some embodiments, the ground line may be implemented as the frequency selective surface. As such, the metal forming the ground line may cause less scattering of RF signals emitted by the high-band radiating elements, as the ground line may be partially transparent to RF radiation in the high-band operating frequency band. In some embodiments, the frequency selective surfaces may have bandpass frequency responses. In such embodiments, the frequency selective surface may, for example, be configured to pass at least 25% (on average) of incident RF energy in the high-band operating frequency band and, more preferably, may be configured to pass at least 40% (on average) of incident RF energy in the high-band operating frequency band.

In some embodiments, the RF feed line may comprise three stacked metal layers, namely a first metal layer that includes the signal line and second and third metal layers that form the ground line having the frequency selective surface. The feed stalk may be conveniently formed using a multi-layer feed stalk printed circuit board that includes three metallization layers that are separated from each other by first and second dielectric substrates, but it will be appreciated that embodiments of the present invention are not limited to feed stalk printed circuit board implementations.

The frequency selective surface may comprise a plurality of unit cells that are electrically connected to each other. Each unit cell may comprise a first metal pattern that is implemented in the first metal layer and a second metal pattern that is implemented in the second metal layer so that the second metal pattern overlaps the first metal pattern. Each first metal pattern may comprise, for example, a metal pad (e.g., a polygonal metal pad) . The metal pads of adjacent unit cells may be spaced apart from each other so that adjacent unit cells are electrically coupled via edge capacitances between the metal pads of the adjacent unit cells. Each second metal pattern may comprise, for example, two intersecting metal lines that form a cross-shaped. The cross-shaped metal lines of adjacent unit cells may be galvanically coupled to each other so that the second metal layer comprises a grid of intersecting longitudinally-extending and transversely-extending metal traces.

The radiating elements according to embodiments of the present invention may be included in multi-band base station antennas, and may reduce the amount of interaction between the arrays in the different frequency bands. Base station antennas that include the radiating elements according to embodiments of the present invention may be used, for example, as sector antennas in the above-described cellular communications systems.

Before discussing the radiating elements according to embodiments of the present invention it is helpful to discuss the design and operation of a representative conventional low-band radiating element for a base station antenna.

FIG. 1A is a perspective view of a conventional low-band cross-dipole radiating element 1. FIG. 1B is a shadow side view of cross-dipole radiating element 1 that illustrates the metallization patterns on a first feed stalk printed circuit board 20-1 of radiating element 1. In FIG. 1B, the solid lines are the metallization patterns on a first side of feed stalk printed circuit board 20-1 and the dashed lines are the metallization patterns on a second (opposed) side of feed stalk printed circuit board 20-1. In FIG. 1B, only a side surface of a second feed stalk printed circuit board 20-2 is visible as the major surfaces of feed stalk printed circuit board 20-2 are perpendicular to the viewing angle. It should be noted that herein like elements may be referred to individually by their full reference numeral (e.g., feed stalk printed circuit board 20-2) and may be referred to collectively by the first part of their reference numeral (e.g., the feed stalk printed circuit boards 20) .

As shown in FIG. 1A, the conventional cross-dipole radiating element 1 includes a feed stalk 10 and a pair of dipole radiators 70-1, 70-2. The feed stalk 10 comprises first and second feed stalk printed circuit boards 20-1, 20-2. Each feed stalk printed circuit board 20-1, 20-2 includes a respective RF feed line 16-1, 16-2 that carry RF signals between first and second RF transmission lines (not shown) that connect to the radiating element 1 to pass RF signals to and from the radiating element 1. Each such RF transmission line may comprise, for example, a coaxial cable or a microstrip transmission line on a feed board printed circuit board.

Referring to both FIGS. 1A and 1B, each feed stalk printed circuit board 20 has a base 22 and a distal end 24 that is positioned forwardly of the base 22. The first feed stalk printed circuit board 20-1 includes a slit 26 that extends forwardly from the base 22 thereof, and the second feed stalk printed circuit board 20-2 includes a slit 26 that extends rearwardly from the distal end 24 thereof. Feed stalk printed circuit boards 20-1 and 20-2 are arranged perpendicular to each other with the slits 26 thereof engaged so that the two mated feed stalk printed circuit boards 20-1, 20-2 have a cross-shape when viewed from the front.

Rear portions of each feed stalk printed circuit board 20 may include projections that are inserted through slits in a feed board printed circuit board (not shown) . Metallized pads on the projections may be soldered to metallized pads on the feed board printed circuit board to mechanically mount the radiating element 1 on the feed board printed circuit board and to electrically connect the RF feed lines 16-1, 16-2 on the feed stalk 10 to the RF transmission lines on the feed board printed circuit board.

The dipole radiators 70-1, 70-2 are positioned at the distal ends 24 of the feed stalk printed circuit boards 20 and may be (and typically are) physically mounted on the feed stalk printed circuit boards 20. The first dipole radiator 70-1 extends along a first axis and the second dipole radiator 70-2 extends along a second axis that is generally perpendicular to the first axis. The first dipole radiator 70-1 includes first and second dipole arms 80-1, 80-2, and the second dipole radiator 70-2 includes third and fourth dipole arms 80-3, 80-4. The dipole radiators 70-1, 70-2 may be formed in a dipole radiator printed circuit board 82. The dipole arms 80 are cloaking dipole arms that are formed as a series of widened metal segments 84 that are interconnected by narrow metal traces 86 (see FIG. 1A) . The dipole radiators 70-1, 70-2 are shown as having an elongated "figure 8" shape where each dipole arm 80 is formed as a loop. A wide variety of dipole arms are known in the art, including dipole arms that have many different shapes or that are formed in different ways (e.g., using sheet metal) . It will be appreciated that the radiating elements according to embodiments of the present invention that have the feed stalk designs disclosed herein may have any appropriate dipole arm design, including dipole arms having any shape, that are formed, for example, in any of the ways discussed above.

Dipole arms 80-1 and 80-2 of first dipole radiator 70-1 are center fed by the first RF feed line 16-1 on the first feed stalk printed circuit board 20-1 and radiate together at a first polarization. In the depicted embodiment, the first dipole radiator 70-1 is designed to transmit and receive signals having a slant +45° linear polarization. Dipole arms 80-3 and 80-4 of second dipole radiator 70-2 are center fed by the second RF feed line 16-2 on the second feed stalk printed circuit board 20-2 and radiate together at a second polarization that is orthogonal to the first polarization. The second dipole radiator 70-2 is designed to transmit and receive signals having a slant -45° linear polarization.

As shown in FIG. 1B, a twin line transmission line structure is formed on the second side of feed stalk printed circuit board 20-1. The twin line transmission line structure comprises first and second ground lines 30-1, 30-2 that are implemented as first and second metallized regions that extend from the base 22 of the first feed stalk printed circuit board 20-1 to the distal end 24 thereof. Each ground line 30-1, 30-2 is coupled to the ground conductor of the first RF transmission line that feeds radiating element 1 (not shown) . The connections between the first and second ground lines 30-1, 30-2 and the ground conductor of the first RF transmission line may be at the base 22 of the first feed stalk printed circuit board 20-1. The first and second ground lines 30-1, 30-2 may each have an electrical length of about 1/4 the center wavelength of radiating element 1.

A signal line 40 is formed on the first side of feed stalk printed circuit board 20-1. The signal line 40 is coupled to the signal conductor of the RF transmission line that feeds the first feed stalk printed circuit board 20-1. The signal line 40 extends forwardly from the base 22 of the first feed stalk printed circuit board 20-1 and travels about two-thirds of the way toward the distal end 24 thereof. The signal line 40 then goes through a first 90° turn to extend transversely across the first side of feed stalk printed circuit board 20-1. Finally, the signal line 40 goes through a second 90° turn to extend rearwardly toward the base 22 of the first feed stalk printed circuit board 20-1.

The signal line 40 includes a forwardly extending segment 42-1, a transversely extending segment 42-2, and a rearwardly extending segment 42-3. The forwardly extending segment 42-1 overlaps the first ground line 30-1. Herein, two elements on a printed circuit board (or an equivalent structure) "overlap" if an axis that is perpendicular to a major surface of the printed circuit board intersects both elements. The transversely extending segment 42-2 extends from the end of the forwardly extending segment 42-1, to cross over a gap 36 (i.e., an unmetallized region) that is provided between the first and second ground lines 30-1, 30-2. The transversely extending segment 42-2 overlaps portions of both the first ground line 30-1 and the second ground line 30-2. The rearwardly extending segment 42-3 extends at a right angle from the end of the transversely extending segment 42-2 back toward the base 22 of the first feed stalk printed circuit board 20-1. The rearwardly extending segment 42-3 overlaps the second ground line 30-2.

As discussed above, pursuant to embodiments of the present invention, cross-dipole radiating elements are provided that have feed stalks that may be at least partially transparent to RF energy in the operating frequency bands of one or more nearby higher frequency band radiating elements. In particular, the radiating elements according to embodiments of the present invention may have first and second ground lines that are implemented as frequency selective surfaces that are at least partially transparent in the operating frequency bands of the nearby higher frequency band radiating elements. As such, the feed stalks of the radiating elements according to embodiments of the present invention may cause less scattering of the RF energy emitted by the nearby higher frequency band radiating elements, which may improve the peak directivity and antenna beam shape for the nearby higher-band radiating elements.

The discussion of the cross-dipole radiating elements according to embodiments of the present invention below will focus on low-band radiating elements that have feed stalks that are partially transparent with respect to RF radiation emitted by nearby high-band radiating elements as an example. However, it will be appreciated that the techniques disclosed herein may be used, for example, to provide mid-band radiating elements that are partially transparent with respect to RF radiation emitted by nearby high-band radiating elements or in any other appropriate application. Thus, while the radiating elements according to embodiments of the present invention are described below as being low-band radiating elements, it will be appreciated that they may alternatively be reduced in size to operate as, for example, mid-band radiating elements.

Before describing example embodiments of the radiating elements of the present invention, an example base station antenna in which the radiating elements according to embodiments of the present invention may be used will first be described.

FIGS. 2A-2B illustrate a conventional passive/active antenna system 100 that includes both a passive base station antenna 110 and an active antenna module 150. In particular, FIG. 2A is a schematic rear perspective view of the passive/active antenna system 100, while FIG. 2B is a schematic perspective view of the passive/active antenna system 100 of FIG. 2A with radomes of both the passive base station antenna 110 and the active antenna module omitted. In FIGS. 2A and 2B, the axes illustrate the longitudinal (L) , transverse (T) and forward (F) directions of the base station antenna system 100. In the description that follows, the antenna 100 and the radiating elements included therein will be described using terms that assume that the antenna 100 is mounted for normal use on a tower with a longitudinal axis of the antenna 100 extending along a vertical axis and the front surface of the antenna 100 mounted opposite the tower pointing toward the coverage area for the antenna 100.

Referring to FIG. 2A, the passive/active antenna system 100 may be mounted, for example, on an antenna tower 102 using mounting hardware 104. The active antenna module 150 may be mounted directly on a rear surface of the passive base station antenna 110, or may be held in place behind the passive base station antenna 110 by the mounting hardware 104. The front surface of the passive/active antenna system 100 may be opposite the antenna tower 102 facing toward a coverage area of the passive/active antenna system 100. The passive base station antenna 110 includes a tubular radome 112 that surrounds and protects an antenna assembly that is mounted inside the radome 112. A top end cap 114 covers a top opening in the radome 112 and a bottom end cap 116 covers a bottom opening in the radome 112. A plurality of RF ports 118 extend through the bottom end cap 116 and are used to connect the passive base station antenna 110 to one or more external radios (not shown) . The active antenna module 150 may be removably mounted behind the passive base station antenna 110 so that the active antenna module 150 may later be replaced with a different active antenna module.

Referring to FIG. 2B, the passive base station antenna 110 includes a reflector assembly 120. The reflector assembly 120 may be referred to herein as a "passive reflector assembly" since it is part of the passive base station antenna 110. The passive reflector assembly 120 includes a main reflector 122 and spaced-apart first and second reflector strips 124-1, 124-2 that extend longitudinally from respective first and second opposed sides of the main reflector 122. The passive reflector assembly 120 may further include a third reflector strip 124-3 that extends in a transverse direction between top ends of the first and second reflector strips 124-1, 124-2. An opening 126 is defined between the first and second reflector strips 124-1, 124-2. For example, the opening 126 may be bounded by a top portion of the main reflector 122, the first and second reflector strips 124-1, 124-2, and the third reflector strip 124-3. At least the main reflector 122 may comprise or include a metallic surface (e.g., a sheet of aluminium) that serves as a reflector and ground plane for the radiating elements of the antenna 100. Various mechanical and electronic components of the antenna (not shown) may be mounted behind the passive reflector assembly 120 such as, for example, phase shifters, remote electronic tilt units, mechanical linkages, controllers, diplexers, and the like.

The passive base station antenna 110 further includes a plurality of passive linear arrays of radiating elements that extend forwardly from the passive reflector assembly 120. The linear arrays may support, for example, 2G, 3G and/or 4G cellular service. In the example passive base station antenna 110 shown in FIGS. 2A-2B, the linear arrays include first and second low-band linear arrays 130-1, 130-2 that are configured to operate in all or part of the 617-960 MHz frequency band. Each low-band linear array 130 comprises a vertically-extending column of low-band radiating elements 132. The passive base station antenna 110 further includes first through fourth mid-band linear arrays 140-1 through 140-4 that are configured to operate in all or part of the 1427-2690 MHz frequency band. Each mid-band linear array 140 comprises a vertically-extending column of mid-band radiating elements 142. Each of the low-band and mid-band linear arrays 130, 140 may generate relatively static antenna beams that provide coverage to a predefined coverage area (e.g., antenna beams that are each configured to cover a sector of a base station) , with the only change to the coverage area occurring when the electronic downtilt angles of the generated antenna beams are adjusted (e.g., to change the size of the cell) .

Each of the low-band and

mid-band radiating elements

132, 142 may be implemented as dual-polarized radiating elements that include first and second radiators that transmit and receive RF energy at orthogonal polarizations. When such dual-polarized radiating elements are used, each of the low-band and mid-band linear arrays 130, 140 may be connected to a pair of the RF ports 118. The first RF port 118 is connected between a first port of a radio (e.g., a remote radio head mounted on the antenna tower 102 near the passive base station antenna 110) and the first polarization radiators of the radiating elements in one of the linear arrays, and the second RF port 118 is connected between a second port of a radio and the second polarization radiators of the radiating elements in the linear array. RF signals that are to be transmitted by a selected one of the linear arrays 130, 140 are passed from the radio (s) to one of the RF ports 118, and passed from the RF port 118 to a power divider (or, alternatively, a phase shifter assembly that includes a power divider) that divides the RF signal into a plurality of sub-components that are fed to the respective first or second radiators of the radiating elements in the linear array, where the sub-components of the RF signal are radiated into free space.

The low-band and/or

mid-band radiating elements

132, 142 may be mounted on feed board printed circuit boards that couple RF signals to and from the

individual radiating elements

132, 142. In FIG. 2B, the mid-band radiating elements 142 are shown as being mounted in pairs on a plurality of mid-band feed board printed circuit boards 148 (the low-band radiating elements are likewise mounted on feed board printed circuit boards but they are not visible in the figure) . Cables may be used to connect each feed board printed circuit board 148 to other components of the antenna such as diplexers, phase shifters or the like.

Most of the low-band and

mid-band radiating elements

132, 142 are mounted to extend forwardly from the main reflector 122. However, low-band linear arrays 130-1, 130-2 extend substantially the full length of the passive/active antenna system 100 and hence extend beyond the main reflector 122. The first and second reflector strips 124-1, 124-2 may provide mounting locations for low-band radiating elements 132 that are positioned above the main reflector 122. The first and second reflector strips 124-1, 124-2 may be integral with the main reflector 122 so that the first and second reflector strips 124-1, 124-2 and the main reflector 122 will be maintained at a common ground voltage, which may improve the performance of the low-band linear arrays 130-1, 130-2.

Each low-band radiating element 132 may comprise a slant -45°/+45° cross-dipole radiating element that includes a slant -45° polarization dipole radiator 134-1 and a slant +45° polarization dipole radiator 134-2. The dipole radiators 134-1, 134-2 may be mounted on a feed stalk (not shown) . In some cases, the three uppermost low-band radiating elements 132 that are above the main reflector 122 may be mounted on a frequency selective surface that covers the opening 126. This frequency selective surface is described in further detail below. In other cases, the low-band radiating elements 132 may include tilted feed stalks that allow these radiating elements to be mounted on the first and second reflector strips 124-1, 124-2 while the dipole radiators 134 of these radiating elements 132 are in front of the opening 126. Each low-band radiating element 132 has dipole radiators 134 that are designed to be substantially transparent to RF energy emitted by the mid-band radiating elements 142.

The active antenna module 150 includes a multi-column beamforming array 160 of radiating elements 162 and a beamforming radio (not visible in the figures) . The multi-column beamforming array 160 may be mounted in a forward portion of the active antenna module 150, and the beamforming radio may be mounted behind the multi-column beamforming array 160. The beamforming array 160 may, for example, comprise a plurality of vertically-extending columns of high-band radiating elements 162 that are configured to operate in all or part of the 3.1-4.2 GHz frequency band (e.g., in the 3.1-3.6 GHz frequency band) . The high-band radiating elements 162 are mounted to extend forwardly from a reflector 154 of the active antenna module 150 (herein the "active reflector" ) . The beamforming radio is capable of electronically adjusting the amplitude and/or phase of the subcomponents of an RF signal that are output to different radiating elements 162 of the multi-column beamforming array 160. For example, each port of the beamforming radio may be coupled to a column of radiating elements of the beamforming array 160, and the amplitudes and phases of the sub-components of the RF signals that are fed to each column may be adjusted so that the generated antenna beams are narrowed in the azimuth plane and pointed in a desired direction in the azimuth plane.

The beamforming array 160 of active antenna module 150 is mounted behind the opening 126 in the passive reflector assembly 114. The beamforming array 160 is visible in FIG. 2B as the radomes of the passive base station antenna 110 and the active antenna module 150 are omitted in FIG. 2B. The opening 126 in the passive reflector assembly 120 allows the antenna beams generated by the beamforming array 160 to pass through the passive base station antenna 110 to provide service to the coverage area of the passive/active antenna system 100.

As discussed above, a frequency selective surface (not shown) may cover the opening 126. The frequency selective surface may be configured to allow RF energy emitted by the high band radiating elements 162 in the beamforming array 160 to pass therethrough, while the frequency selective surface reflects RF energy in lower frequency bands (and specifically, low-band RF signals that are emitted by the low-band radiating elements 132. The frequency selective surface may be coplanar with the opening 126, in front of the opening 126 or behind the opening 126. The frequency selective surface can have a grid pattern such as a grid of metal pads and/or other metal structures. The grid pattern can be arranged in any suitable manner and may be symmetric or asymmetric across a width and/or length of the frequency selective surface. The grid pattern may comprise sub-wavelength periodic microstructures. The metal pads/structures may be arranged in one or more layers. The frequency selective surface may be formed on a substrate such as, for example, a printed circuit board or of sheet metal. In some embodiments, the frequency selective surface may comprise a portion of the passive reflector assembly 120 that is stamped to form the metal grid structure therein. In such cases, the "opening 126" comprises a large number of small openings that act as a large opening with respect to RF energy in the operating frequency band of the beamforming array 160.

One difficulty with the passive/active base station antenna system 100 of FIGS. 2A-2B is that some of the low-band radiating elements 132 are mounted directly in front of the high-band beamforming array 160. As such, metal elements of the low-band radiating elements 132 may partially block/reflect the RF radiation emitted by the high-band beamforming array 160 and/or the high-band RF radiation may induce current on metal elements of the low-band radiating elements 132 that then reradiate the high-band radiation in ways that act to distort the shape of the antenna beams generated by the high-band beamforming array 160.

FIG. 3A is a perspective view of a low-band radiating element 200 according to embodiments of the present invention. FIG. 3B is a side view of the first feed stalk printed circuit board 220-1 that is included in the feed stalk 210 of the low-band radiating element 200. FIGS. 3C-3E are schematic plan views of the three metallization layers included on the first feed stalk printed circuit board 220-1 included in the radiating element 200 of FIG. 3A. Finally, FIG. 3F is a perspective view of three of the unit cells of one of the frequency selective surfaces included in each feed stalk printed circuit board 220 of the radiating element 200 of FIG. 3A. Low-band radiating element 200 may, for example, be used to implement the low-band radiating elements 132 of base station antenna 100.

Referring to FIG. 3A, low-band radiating element 200 includes a feed stalk 210, a first dipole radiator 270-1, and a second dipole radiator 270-2. The feed stalk 210 comprises first and second feed stalk printed circuit boards 220-1, 220-2. Each feed stalk printed circuit board 220 has a base 222 and a distal (forward) end 224 that is positioned forwardly of the base 222. The first feed stalk printed circuit board 220-1 includes a slit 226 (see FIG. 3B) that extends rearwardly from the distal end 224 thereof, and the second feed stalk printed circuit board 220-2 includes a slit 226 that extends forwardly from the base 222 thereof. Feed stalk printed circuit boards 220-1 and 220-2 are arranged perpendicular to each other with the slits 226 in the two feed stalk printed circuit boards 220 received within each other so that the two mated printed circuit boards 220-1, 220-2 have a cross-shape when viewed from the front.

Each feed stalk printed circuit board 220-1, 220-2 includes a respective RF feed line 240-1, 240-2 that carries RF signals between first and second RF transmission lines (not shown) for the radiating element 200 and the respective cross-dipole radiators 270-1, 270-2. Each RF feed line 240 comprises a signal line 242 and a pair of ground lines 246-1, 246-2, which are discussed in more detail below. Each RF transmission line (which are not shown) may comprise, for example, a coaxial cable or a microstrip transmission line on a feed board printed circuit board. Each RF transmission line carries RF signals between the radiating element 200 and other components of a base station antenna that includes radiating element 200.

The dipole radiators 270-1, 270-2 are mounted adjacent (and typically on) the distal end 224 of the feed stalk printed circuit boards 220. The first dipole radiator 270-1 includes first and second dipole arms 280-1, 280-2, and the second dipole radiator 270-2 includes third and fourth dipole arms 280-3, 280-4. The dipole radiators 270 and dipole arms 280 may be identical to the dipole radiators 70 and dipole arms 80 described above with reference to FIGS. 1A-1B, and hence further description thereof will be omitted here.

As mentioned above, respective RF feed lines 240-1, 240-2 are formed in the first and second feed stalk printed circuit boards 220-1, 220-2. Each RF transmission line structure 240 includes a signal line 242 and a pair of ground lines 246-1, 246-2. FIGS. 3B-3F illustrate the design of one of the RF feed lines 240 in greater detail.

Referring first to FIG. 3B, it can be seen that each feed stalk printed circuit board 220 may be implemented using a multilayer printed circuit board 230 that includes first and second dielectric substrates 232-1, 232-2 and first through third metal layers 234-1 through 234-3. The first metal layer 234-1 is formed on the outer surface of the first dielectric substrate 232-1, and the third metal layer 234-3 is formed on the outer surface of the second dielectric substrate 232-2. The second metal layer 234-2 is formed on the inner surface of one the first dielectric substrate 232-1 and the second dielectric substrate 232-2. For convenience of description, herein the first metal layer 234-1 may also be referred to as the "top" metal layer, the third metal layer 234-3 may also be referred to as the "bottom" metal layer, and the second metal layer 234-2 may be referred to as the "middle" metal layer.

FIG. 3C is a schematic "top" plan view of the first feed stalk printed circuit board 220-1 that illustrates the first (top) metal layer 234-1 that is formed on the outer (top) surface of the first dielectric substrate 232-1. As shown in FIG. 3C, the top metal layer 234-1 may comprise the signal line 242 of the RF feed line 240. The signal line 242 may be galvanically coupled to the signal conductor of the RF transmission line (not shown) that feeds the first feed stalk printed circuit board 220-1, typically at the base 222 of the first feed stalk printed circuit board 220-1. The signal line 242 extends forwardly from the base 222 of the first feed stalk printed circuit board 220-1 and travels about two-thirds of the way toward the distal end 224 of the first feed stalk printed circuit board 220-1. The signal line 242 then goes through a first 90° turn to extend transversely across the first side of the first feed stalk printed circuit board 220-1. Finally, the signal line 242 goes through a second 90° turn to extend rearwardly toward the base 222 of the first feed stalk printed circuit board 220-1. The signal line 242 thus includes a forwardly extending segment 244-1, a transversely extending segment 244-2, and a rearwardly extending segment 244-3. The forwardly extending segment 244-1 comprises a widened pad region near the base of the first feed stalk printed circuit board 220-1 and a narrower trace that extends forwardly from the widened pad region. The transversely extending segment 244-2 extends from the end of the forwardly extending segment 244-1, to cross over a gap 236 (i.e., an unmetallized region) that is provided between the first and second ground lines 246-1, 246-2. The rearwardly extending segment 244-3 comprises a narrow trace that extends at a right angle from the end of the transversely extending segment 244-2 toward the base 222 of the first feed stalk printed circuit board 220-1.

FIG. 3D is a schematic plan view of the first feed stalk printed circuit board 220-1 with the first (top) metal layer 234-1 and the first dielectric substrate 232-1 removed in order to show the second (middle) metal layer 234-2 and the second dielectric substrate 232-1. FIG. 3E is a schematic "bottom" plan view of the first feed stalk printed circuit board 220-1 that illustrates the third (bottom) metal layer 234-3 that is formed on the outer (bottom) surface of the second dielectric substrate 232-2. FIGS. 3D and 3E illustrate the structure of the first and second ground lines 246-1, 246-2 of the RF transmission line structure 240. The first and second ground lines 246-1, 246-2 are implemented as a frequency selective surface that is formed on the middle and bottom layers 234-2, 234-3 of the first feed stalk printed circuit board 220-1.

As shown in FIG. 3D, the middle metal layer 234-2 comprises two grids of longitudinally-extending metal traces 248 and transversely-extending metal traces 250. In the depicted embodiment, each grid includes a total of three longitudinally-extending metal traces 248 and eight transversely-extending metal traces 250. The longitudinally-extending metal traces 248 may extend substantially from the base 222 of the first feed stalk printed circuit board 220-1 to the distal end 224 thereof. It will be appreciated, however, that embodiments of the present invention are not limited thereto. The grids of longitudinally-extending metal traces 248 and transversely-extending metal traces 250 may function similarly to the twin ground lines 30-1, 30-2 on the feed stalk 10 of the conventional low-band radiating element 1 of FIGS. 1A-1B.

The forwardly extending segment 244-1 of the signal line 242 may overlap the middle one of the longitudinally-extending metal traces 248 of the first ground line 246-1. The rearwardly extending segment 244-3 of the signal line 242 may overlap the middle one of the longitudinally-extending metal traces 248 of the second ground line 246-2.

As shown in FIG. 3E, the bottom metal layer 234-3 comprises two sets of metal pads 252. Each set of metal pads 252 comprises three columns of metal pads 252, where each column extends in the longitudinal direction of the feed stalk printed circuit board 220-1. In the depicted embodiment, each column includes eight metal pads 252, but it will be appreciated that embodiments of the present invention are not limited thereto, and that more or less of three columns of metal pads 252 may be provided. Each longitudinally-extending metal trace 248 in the middle metal layer 234-2 may overlap a respective one of the columns of metal pads 252 included in the bottom metal layer 234-3.

Each ground line 246-1, 246-2 is coupled to the ground conductor of the first feed transmission line (not shown) for radiating element 200. In particular, one or more of the longitudinally-extending metal traces 248 in each grid of the second metal layer 234-2 may be galvanically connected to the ground conductor of the first feed transmission line (not shown) for radiating element 200 at the base 222 of the first feed stalk printed circuit board 220-1. The metal pads 252 in the third metal layer 234-3 may be capacitively coupled to the second metal layer 234-2. The second metal layer 234-2 may also be galvanically connected to one or more of the dipole arms 280.

The second feed stalk printed circuit board 220-2 may have substantially the same design as the first feed stalk printed circuit board 220-1 except that the slit 226 in the second feed stalk printed circuit board 220-2 extends forwardly from the base 222 of the second feed stalk printed circuit board 220-2 instead of extending rearwardly from the distal end 224 thereof as is the case with the slit 226 in the first feed stalk printed circuit board 220-1, and a signal line 242 on the second feed stalk printed circuit board 220-2 extends farther forwardly before bending to form a U-shape so that the signal line 242 may extend past the slit 226. Thus, further description of the second feed stalk printed circuit board 220-2 will be omitted here.

Referring again to FIG. 1B, it can be seen that the first and second ground lines 20-1, 20-2 that are part of the feed stalk 10 of the conventional low-band radiating element 1 each comprise a large metallized region. As such, when the passive/active antenna system 100 of FIGS. 2A-2B includes linear arrays of low-band radiating elements 1, the ground lines 20 may reflect RF radiation emitted by the high-band beamforming array 160 (i.e., may cause the first type of scattering) , particularly when the high-band beamforming array 160 is electronically scanned in the azimuth plane. In addition, the first and second ground lines 20-1, 20-2 may each have a length of about 1/4 of the center wavelength of radiating element 1, and hence may have a length that is about one wavelength of a frequency within the operating frequency range of the high-band beamforming array 160. As such, high-band currents may form on the ground lines 20 in response to RF radiation emitted by the high-band beamforming array 160, and the ground lines 20 may then emit high-band radiation in response to these currents, meaning that the conventional feed stalk 10 may also cause the second type of scattering discussed above.

The first and second ground lines 246-1, 246-2 that are provided on each feed stalk printed circuit board 220 of the low-band radiating element 200 according to embodiments of the present invention are implemented using respective frequency selective surfaces. These frequency selective surfaces may be configured to be at least 25%transparent and, more preferably, at least 40%transparent, to RF radiation in the operating frequency band of the high-band beamforming array 160. As such, the ground lines 246-1, 246-2 may reflect less of the RF radiation emitted by the high-band beamforming array 160, and hence will have less of an impact on the antenna beams formed by the high-band beamforming array 160. In addition, implementing the ground lines 246-1, 246-2 as respective frequency selective surfaces may change the electrical length of the ground lines 246 (for a fixed physical length) , which change may be used to make the ground lines 246 less resonant in the operating frequency band of the high-band beamforming array 160.

Frequency selective surfaces refer to metal structures that are designed to have frequency selective responses with respect to RF radiation that is incident thereon. For example, a frequency selective surface may be designed to partially or substantially pass RF energy in a first frequency band while substantially reflecting RF energy in a second different frequency band, thereby acting as a spatial filter. The frequency selective surface may comprise a plurality of periodically arranged unit cells, where each unit cell may be implemented in a single layer or in multiple fully overlapping metal layers. The unit cells may include inductive and/or capacitive structures that couple with each other or with the inductive and/or capacitive structures of adjacent unit cells so that the frequency selective surface is an LC resonant circuit. The LC resonant circuit may be designed to be more transparent to RF energy in a first frequency range than in a second frequency range. A general discussion of frequency selective surfaces may be found in Ben A. Munk, Frequency Selective Surfaces: Theory and Design, ISBN: 978-0-471-37047-5; DOI: 10. 1002/0471723770; April 2000, Copyright

2000 John Wiley &Sons, Inc. the contents of which are hereby incorporated by reference as if recited in full herein.

FIG. 3F is a schematic perspective view of one of a small portion of the first feed stalk printed circuit board 220-1 that corresponds to three unit cells 262 of the frequency selective surface used to implement one of the ground lines 246. As shown, the top metal layer 234-1 includes the signal line 242. The top metal layer 234-1 is not part of the frequency selective surface that forms the ground lines 246. The middle metal layer 234-2 includes the longitudinally-extending metal traces 248 and the transversely-extending metal traces 250 (only one of which is visible in the enlarged view of FIG. 3F) . The bottom metal layer 234-3 includes the metal pads 252. The signal line 242 may fully overlap the middle one of the longitudinally-extending metal traces 248 that form the upper layer of the two-layer ground line 246. The longitudinally-extending metal trace 248 and the transversely-extending metal trace 250 that are provided in each unit cell 260 form a cross-shaped metal pattern 262. The center of the cross-shaped metal pattern 262 of each unit cell 260 may overlap the middle of the metal pad 252 of the respective unit cell 260. As shown, the cross-shaped metal patterns 262 that comprise the upper layer of each unit cell 260 of the frequency selective surface ground line 246 galvanically connect to the cross-shaped metal patterns 262 of the adjacent unit cells 260 (and this occurs in both the longitudinal and transverse directions, even though the connections are only visible in the transverse direction in FIG. 3F) . In contrast, the metal pads 252 that comprise the lower layer of each unit cell 260 of the frequency selective surface ground line 246 are spaced apart from each other and hence each metal pad 252 is only capacitively edge coupled with the metal pads 252 of adjacent unit cells 260. Each metal pad 252 is also capacitively coupled to the corresponding cross-shaped metal pattern 262 of the unit cell 260.

In some embodiments, the frequency selective surfaces may have bandpass frequency responses. In some embodiments, the frequency selective surfaces may have both bandpass and bandstop frequency responses.

By implementing the first and second ground lines 246-1, 246-2 as frequency selective surfaces, the negative impact that the feed stalk 210 of the low-band radiating element 200 may have on the antenna beams generated by the high-band beamforming array 160 may be substantially reduced. As can be seen in FIG. 1B, the twin ground lines 30-1, 30-2 that are provided on the feed stalk of a conventional low-band radiating element comprise the vast majority of the metallization on the feed stalk 10, and are quarter wavelength structures, and hence it is these ground lines 30 that primarily act to distort the antenna beams generated by a nearby high-band beamforming array 160. Since the low-band radiating element 200 according to embodiments of the present invention has feed stalk printed circuit boards 220 with twin ground lines 246 that are partially transparent to RF energy in the high-band frequency range, the impact of the feed stalk 210 on the high-band antenna beams may be dramatically reduced. Moreover, the electrical length of the first and second ground lines 246-1, 246-2 may be different than the electrical length of the twin ground lines 30-1, 30-2 included on each feed stalk printed circuit board 20 of the conventional low-band radiating element 1, even though the physical lengths of the ground lines 30, 246 may be the same. Consequently, the ground lines 246 of low-band radiating element 200 may potentially be designed to not be resonant in the operating frequency band of the high-band beamforming array 160, which may also reduce or eliminate the second type of scattering.

FIG. 4 is a graph of the S1: 1 and S1: 2 transmission characteristics of the ground lines of the low-band radiating element of FIGS. 3A-3F. Here, the beamforming array 160 was designed to operate in the 3.1-3.6 GHz frequency band. As shown in FIG. 4, the frequency selective surface used to implement each ground line 246 passes at least -4 dB (40%) of incident RF energy in the 3.1-3.6 GHz frequency band, and as much as -2.4 dB (58%) of incident RF energy in the middle of the 3.1-3.6 GHz frequency band. On average (i.e., an average taken at 10 MHz intervals across the frequency band) , these frequency selective surfaces will pass about 50%of incident RF energy in the 3.1-3.6 GHz frequency band. As such, the extent to which the ground lines on the feed stalks of the radiating elements according to embodiments of the present invention impact the high-band antenna beams can be significantly reduced.

As discussed above, by implementing the feed stalk ground lines as frequency selective surfaces, the low-band radiating elements according to embodiments of the present invention may exhibit less scattering with respect to RF energy emitted by the high-band beamforming array 160. Moreover, these frequency selective surface ground lines may fully pass low-band RF energy exhibiting, for example, return loss values that are below -45 dB for the entire low-band frequency range. Thus, the radiating elements according to embodiments of the present invention may improve performance of the high-band beamforming array 160 without any degradation of the performance of the low-band arrays.

It will be appreciated that many modifications may be made to the radiating elements discussed above without departing from the scope of the present invention. For example, the above-described radiating elements are formed using feed stalk printed circuit boards. In other embodiments, other types of feed stalk implementations may be used such as, for example, sheet metal feed stalks. As another example, a wide variety of different frequency selective surface designs may be used to implement the feed stalk ground lines including, for example, single layer and triple layer different frequency selective surface designs and different frequency selective surfaces having different unit cell structures than the example structure discussed above.

As another example, the signal traces on the feed stalks may alternatively or additionally be implemented as a frequency selective surface to reduce scatting by the signal trace metallization on the antenna beams generated by the high-band beamforming array. As still another example, the same techniques may be used on radiating elements that operate in frequency bands other than the low-band, such as on mid-band radiating elements.

While the dipole arms of the low-band radiating elements described above are implemented in dipole radiator printed circuit boards, it will be appreciated that embodiments of the present invention are not limited thereto. For example, in other embodiments, the dipole arms may be implemented as sheet metal dipole arms or using other metal structures.

Embodiments of the present invention have been described above with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present invention. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

It will be understood that when an element is referred to as being "on" another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being "directly on" another element, there are no intervening elements present. It will also be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (i.e., "between" versus "directly between" , "adjacent" versus "directly adjacent" , etc. ) .

Relative terms such as "below" or "above" or "upper" or "lower" or "horizontal" or "vertical" may be used herein to describe a relationship of one element, layer or region to another element, layer or region as illustrated in the figures. It will be understood that these terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures.

Herein, the term "substantially" means within +/-10%.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a" , "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" "comprising, " "includes" and/or "including" when used herein, 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.

Aspects and elements of all of the embodiments disclosed above can be combined in any way and/or combination with aspects or elements of other embodiments to provide a plurality of additional embodiments.

Claims

A radiating element, comprising:

a feed stalk having a signal line and a ground line, the ground line comprising a frequency selective surface; and

a radiator mounted on the feed stalk.
The radiating element of Claim 1, wherein the signal line overlaps the ground line.
The radiating element of Claim 1, wherein the frequency selective surface comprises at least two spaced-apart metal layers.
The radiating element of Claim 1, wherein the signal line and the ground line together comprise a radio frequency ( "RF" ) feed line that is configured to feed RF signals to the radiator.
The radiating element of Claim 1, wherein the frequency selective surface comprises a bandpass filter.
The radiating element of Claim 1, wherein the frequency selective surface comprises a plurality of unit cells, each unit cell comprising a first metal pattern and a second metal pattern that is positioned behind the first metal pattern.
The radiating element of Claim 6, wherein the first metal pattern comprises a cross-shaped pattern.
The radiating element of Claim 7, wherein the second metal pattern comprises a polygonal pad.
The radiating element of Claim 8, wherein the first metal pattern of each unit cell is galvanically coupled to the first metal patterns of respective adjacent unit cells.
The radiating element of Claim 9, wherein the second metal pattern of each unit cell is capacitively edge coupled to the second metal patterns of respective adjacent unit cells.
A base station antenna, comprising:

a first radiating element that is configured as the radiating element of Claim 1; and

a second radiating element that is configured to operate in a higher operating frequency band than the first radiating element,

wherein the frequency selective surface comprises a bandpass filter having a passband that encompasses at least a portion of the higher operating frequency band.
The base station antenna of Claim 12, wherein the first radiating element is mounted forwardly of the second radiating element.
A radiating element, comprising:

a dipole radiator; and

a feed stalk printed circuit board having a first metal layer that includes a signal line that is configured to be coupled to a first conductor of a radio frequency ( "RF" ) transmission line and a second metal layer that is configured to be coupled to a second conductor of the RF transmission line, and a third metal layer.
The radiating element of Claim 13, wherein the feed stalk printed circuit board further includes a first dielectric substrate that is between the first metal layer and the second metal layer and a second dielectric substrate that is between the second metal layer and the third metal layer.
The radiating element of Claim 14, wherein the second metal layer and the third metal layer together form a frequency selective surface.
The radiating element of Claim 15, wherein the second metal layer comprises a ground line that is configured to be coupled to a second conductor of the RF transmission line.
The radiating element of Claim 16, wherein the signal line and the ground line together comprise an RF feed line.
The radiating element of Claim 15, wherein the frequency selective surface comprises a bandpass filter.
The radiating element of Claim 15, wherein the frequency selective surface comprises a plurality of unit cells, each unit cell comprising a first metal pattern and a second metal pattern that is positioned behind the first metal pattern.
The radiating element of Claim 19, wherein the first metal pattern comprises a cross-shaped pattern and the second metal pattern comprises a polygonal pad.
The radiating element of Claim 20, wherein the first metal pattern of each unit cell is galvanically coupled to the respective first metal patterns of unit cells that are adjacent thereto, and the second metal pattern of each unit cell is capacitively edge coupled to the respective second metal patterns of the unit cells that are adjacent thereto.
A base station antenna, comprising:

a first radiating element that is configured as the radiating element of Claim 15; and

a second radiating element that is configured to operate in a higher operating frequency band than the first radiating element,

wherein the frequency selective surface comprises a bandpass filter having a passband that encompasses at least a portion of the higher operating frequency band.
A base station antenna, comprising:

a first radiating element that is part of a first array of radiating elements that is configured to operate in a first frequency band; and

a second radiating element that is part of a second array of radiating elements that is configured to operate in a second frequency band;

wherein the first radiating element comprises:

a feed stalk that includes a radio frequency ( "RF" ) feed line; and

a radiator that is coupled to the RF feed transmission line,

wherein the feed stalk includes a frequency selective surface that is configured to, on average, be at least 25%transparent to RF energy in the second frequency band.
The base station antenna of Claim 23, wherein the RF feed line comprises a signal line and a ground line, and wherein at least part of the frequency selective surface forms the ground line.
The base station antenna of Claim 24, wherein the frequency selective surface comprises a bandpass filter.
The base station antenna of Claim 25, wherein the frequency selective surface comprises a plurality of unit cells, each unit cell comprising a first metal pattern and a second metal pattern that is positioned behind the first metal pattern.
The base station antenna of Claim 26, wherein each first metal pattern comprises a respective cross-shaped pattern, and each second metal pattern comprises a respective polygonal pad.
The base station antenna of Claim 24, wherein the frequency selective surface comprises a metal pattern that includes longitudinally and transversely extending metal traces, and wherein the signal line overlaps one of the longitudinally extending metal traces.
The base station antenna of Claim 23, further comprising a second frequency selective surface, wherein the first radiating element is mounted forwardly of the second frequency selective surface and the second radiating element is mounted rearwardly of the second frequency selective surface.
The radiating element of Claim 11, wherein the frequency selective surface is configured to, on average, be at least 25%transparent to RF energy in the higher operating frequency band
The radiating element of Claim 11, wherein the frequency selective surface is configured to, on average, be at least 40%transparent to RF energy in the higher operating frequency band.
The radiating element of Claim 1, wherein the ground line comprises a plurality of longitudinally-extending traces and a plurality of transversely-extending traces that are interconnected to form a grid.
The radiating element of Claim 22, wherein the frequency selective surface is configured to, on average, be at least 25%transparent to RF energy in the higher operating frequency band.
The radiating element of Claim 16, wherein the ground line comprises a plurality of longitudinally-extending traces and a plurality of transversely-extending traces that are interconnected to form a grid.