Dual-mode stealth base station antenna system using frequency selective surface

Abstract

A multi-band antenna may include a first antenna array for operating in a first frequency range and a second antenna array for operating in a second frequency range higher than the first frequency range. The first antenna array may include at least one antenna element having a plurality of components, where at least one of the plurality of components is configured with a frequency selective surface (FSS) including a plurality of FSS elements. The FSS may be configured to cause the at least one of the plurality of components to provide suppression of coupling of energy from a second frequency range, and to cause the at least one of the plurality of components to provide transmission of signals in a first frequency range suitable for use in a cellular communication system.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application Serial No. 63 / 388,008, filed on July 11, 2022, the entire contents of which are incorporated herein by reference. Technical Field

[0002] The present disclosure relates generally to cellular base station antennas, and more particularly to antenna systems having antenna elements with frequency selective surfaces. Background Art

[0003] The demand for mobile telecommunication services with higher data rates and greater connectivity has driven the recent rapid progress and development of fifth generation (5G) mobile networks. In recent years, more and more spectrum bands have been released and allocated to mobile telecommunication services, including the increasingly popular spectrum band of 3300-4200MHz, or the C-band range for 5G services. Operators are not only committed to enabling new technologies for 5G infrastructure, but also working in parallel to ensure the continued operation and support of existing or other traditional communication systems. A base station antenna (BSA) may include an array of antenna elements to form a directional radiation pattern to provide the desired radio frequency (RF) coverage. Since multiple spectrum bands may be supported, new BSA solutions that support new spectrum bands may also be required. Due to zoning issues, installation tower or structure dead load and wind load capabilities, and site rental costs, it may not be feasible to simply add new antennas to base station sites to support new spectrum bands.

[0004] Prior to 5G, most BSAs may support a low-band frequency range (such as 698-960 MHz) using at least one cross-polarized array of antenna elements and support a mid-band frequency range (such as 1695-2690 MHz) using at least a second cross-polarized array of antenna elements. For example, an arrangement may include two low-band arrays and two or more mid-band arrays co-located or sharing the same physical multi-antenna array or BSA package. Multiple arrays in each band allow for multiple antennas, multiple-input multiple-output (MIMO) processing, or support for different adjacent spectrum bands within the bandwidth of the array. The low-band antenna arrays and mid-band antenna arrays may be positioned and arranged in a number of ways, including interleaving the low-band elements with the mid-band elements to optimize the density of the BSA and minimize the risk of shadowing and mutual shielding of the low-band elements on the mid-band elements. Summary of the invention

[0005] In one example, the present disclosure describes a multi-band antenna that may include a first antenna array for operating in a first frequency range and a second antenna array for operating in a second frequency range higher than the first frequency range. The first antenna array may include at least one antenna element having a plurality of components, wherein at least one of the plurality of components is configured with a frequency selective surface (FSS) comprising a plurality of FSS elements. The FSS may be configured to cause the at least one of the plurality of components to provide suppression of coupling of energy from the second frequency range, and to cause the at least one of the plurality of components to provide transmission of signals in the first frequency range suitable for use in a cellular communication system.

[0006] In another example, the present disclosure describes a multi-band antenna that may include a first antenna array for operating in a first frequency range and a second antenna array for operating in a second frequency range higher than the first frequency range. The first antenna array may include at least one antenna element having a plurality of components, wherein at least one of the plurality of components is configured with a frequency selective surface (FSS) comprising a plurality of FSS elements. The FSS may be configured to cause the at least one of the plurality of components to provide a resonant wavelength in the second frequency range and to cause the at least one of the plurality of components to provide transmission of signals in the first frequency range suitable for use in a cellular communication system.

[0007] In yet another example, the present disclosure describes a multi-band antenna that may include a first antenna array for operating in a first frequency range, a second antenna array for operating in a second frequency range higher than the first frequency range, and a third antenna array for operating in a third frequency range higher than the second frequency range. The first antenna array may include at least one antenna element having a plurality of components, wherein at least one of the plurality of components is configured with a frequency selective surface (FSS) comprising a plurality of FSS elements. The FSS may be configured to cause the at least one of the plurality of components to provide suppression of coupling of energy from the second frequency range, to cause the at least one of the plurality of components to provide a resonant wavelength in the third frequency range, and to cause the at least one of the plurality of components to provide transmission of signals in the first frequency range suitable for use in a cellular communication system. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The teachings of the present disclosure may be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:

[0010] Figure 1 depicts an example base station antenna having multiple cross-polarized antenna arrays serving three different frequency bands;

[0011] FIG. 2( a ) depicts an example unit cell configured with a low-band cross-polarized antenna element and four mid-band cross-polarized antenna elements;

[0012] FIG2( b ) depicts an example azimuth radiation pattern of the mid-band antenna element of FIG2( a ) in the absence of a low-band antenna element;

[0013] FIG2( c ) depicts an example azimuth radiation pattern of the mid-band antenna element of FIG2( a ) in the presence of a low-band antenna element;

[0014] FIG3( a ) depicts the composition of an example single FSS element;

[0015] FIG3( b ) depicts an example graph of the transmittance response of a plane wave incident on an FSS element for two different tuned FSS elements;

[0016] FIG4( a ) depicts the composition of an example low-band dipole arm using an array of embedded FSS elements;

[0017] FIG. 4( b ) depicts an example graph of the transmittance response of a dipole arm represented as a transmission line using a FSS tuned for efficient transmission in a low frequency band;

[0018] FIG5( a ) depicts an example of a low-band dipole arm shielding a C-band antenna element;

[0019] FIG5( b ) depicts an example azimuth radiation pattern of the C-band antenna element of FIG5( a ) in the presence of a low-band dipole arm;

[0020] FIG. 6( a ) depicts an example of a low-band dipole arm constructed with a FSS tuned to approximately 4 GHz and shielding a C-band antenna element;

[0021] FIG6( b ) depicts an example azimuth radiation pattern of the C-band antenna element of FIG6( a ) in the presence of a low-band dipole arm including an FSS;

[0022] FIG. 7( a ) depicts an example of a low-band dipole arm shielding a mid-band antenna element;

[0023] FIG7( b ) depicts an example azimuth radiation pattern of the mid-band antenna element of FIG7( a ) in the presence of a low-band dipole arm;

[0024] FIG8( a ) depicts an example low-band dipole arm constructed with FSS tuned to suppress coupling above approximately 1.5 GHz and shielding a C-band antenna element;

[0025] FIG8( b ) depicts an example azimuth radiation pattern of the mid-band antenna element of FIG8( a ) in the presence of a low-band dipole arm including an FSS;

[0026] FIG. 9( a ) depicts an example of a mid-band antenna element adjacent to a low-band antenna element balun network with a continuous ground plane;

[0027] FIG9( b ) depicts an example azimuth radiation pattern of the mid-band antenna element of FIG9( a ) in the presence of the low-band antenna element balun network (but without its dipole arm(s);

[0028] FIG. 10( a ) depicts an example of a mid-band antenna element adjacent to a low-band antenna element balun network with a defective ground plane including a tuned FSS / array; and

[0029] Figure 10(b) depicts an example azimuth radiation pattern of the mid-band antenna element of Figure 10(a) in the presence of a low-band antenna element balun network having a defective ground plane including a tuned FSS / array (but without its dipole arm(s)).

[0030] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. DETAILED DESCRIPTION

[0031] Examples of the present disclosure describe devices and methods for minimizing unexpected induced currents in cellular antenna arrays using frequency selective structures (FSS). For example, in one example, the present disclosure provides a first stealth mode in which a larger antenna element operating at a lower frequency shields a smaller antenna element operating at a specific higher frequency band to appear electromagnetically transparent. This construction minimizes interference with the desired radiation pattern of the higher frequency antenna element caused by the shadowing effect from the lower frequency antenna element. In one example, the FSS structure is implemented on radiating components such as dipole arms, parasitic radiators, and any balanced-unbalanced transformer networks to provide the desired RF transparency. In one example, the present disclosure provides a second filtering mode in which the inlay of FSS elements on lower frequency antenna elements enables efficient low-pass functions for transmission and radiation at lower frequencies while also attenuating induced currents from higher frequencies. It is worth noting that many typical FSS structures are designed to resonate at their stealth mode frequencies using FSS elements having closed-loop or open-loop resonator characteristics. However, in one example, the present disclosure may provide a combination of closed-loop and open-loop features that enables a reduction in FSS element size for practical implementation onto larger antenna elements. This FSS element size reduction provides further design freedom for the arrangement of FSS elements or periods to enable a second filtering mode. Thus, in one example, the FSS element thus provides a dual-mode stealth technique, where one mode is a result of the FSS and the other mode is a derivative of the overall finite FSS structure.

[0032] As 5G has emerged in the C-band frequency range (3300-4200 MHz), it is desirable to include additional antenna arrays co-located with low-band antenna arrays and mid-band antenna arrays in the same or similar BSA form factor. This allows BSA antennas to be swapped out and upgraded to support C-band spectrum at existing cellular base station sites with minimal impact on the zoning, tower loading, and performance of existing cellular services. C-band arrays are designed for time division duplex (TDD) operation and can utilize beamforming features. Such beamforming for cellular systems may require antenna elements of at least four cross-polarized column arrays, with each column array spaced approximately half a wavelength apart for optimized operation. Adding four cross-polarized C-band arrays to a BSA with multiple low-band array structures and mid-band array structures while maintaining the same overall BSA form factor and minimizing performance degradation becomes challenging. Co-located antenna arrays operating at multiple frequency ratios are more closely spaced, which increases mutual coupling and increases array-to-array blocking or shadowing. This blocking or shadowing effect can cause interference to the radiation pattern, thereby affecting the coverage of the network.

[0033] Figure 1Illustration of one of the challenges of co-locating arrays for different frequency ranges in a limited space. Figure 1 The present invention depicts a dual-polarized antenna element (1011-1012) for operating in a first frequency range (eg, a low-band frequency range). l ) is a multi-band BSA (100) having a first planar array. There is a second planar array including two column arrays, each column array having m dual-polarized antenna elements (1021-102 m and 1031–103 m ), wherein the second array is designed for operation in a second spectral range (e.g., a mid-band spectral range) and is positioned below the plane of the first planar array of antenna elements. A third array of dual-polarized antenna elements (104) is shown for operation in a third frequency range, such as the C-band frequency range, and is positioned below the plane of the second planar array. The third array may have antenna element spacing optimized for beamforming operation in the horizontal or azimuth plane.

[0034] Most base station cellular antennas utilize dual-polarized antenna arrays that include dual-polarized antenna elements. Such dual-polarized antenna elements may be composed of two single-polarized or single-polarized antenna elements with orthogonal polarizations and are typically co-located with each other, thereby providing an additional antenna array with no or negligible increase in physical size. It should be noted that the present disclosure may use the term "antenna element" to refer to both single-polarized antenna elements and dual-polarized antenna elements.

[0035] refer to Figure 1 As an example, a first problem may be the presence of significant shadowing, shielding, or blocking of a C-band antenna array element (104) caused by the presence of the first two antenna elements (1011 and 1012) of the first array (and to a lesser extent blocking of the C-band antenna array by the second array). The far-field radiation pattern produced by the C-band array may be non-ideal and may have pattern distortion caused by the blocking effect.

[0036] Disturbances to the radiation pattern may be caused by scattering and reflections caused by the presence of blocking structures such as low-band elements. It is also worth noting that these blocking structures may typically be located at distances that are considered to be in the radiating far field of the C-band element. The scattered component waves may interact with each other and may also interact with the source radiated waves. This interaction of the component waves may result in different vector and phase combinations at different azimuths and elevations in the far field, resulting in undesirable peaks and troughs in the far field radiation pattern of each element, which also means a potentially undesirable radiation pattern for the entire array of C-band elements when driven by beamforming. C-band frequency radiated waves incident on the low-band element dipoles may also be incident on other components of the low-band antenna element, such as the feed structure (which will be perpendicular to the dipole and at the bottom of the beam). Figure 1 The low-band antenna element component structure may also induce C-band currents and induce higher harmonic resonant modes, and may result in some re-radiation of the C-band component RF energy.

[0037] Depend on Figure 1 The second problem illustrated is that the elements of the second array (1021-102 m ,1031-103 m ) is also represented by the first array element (1012-101 l ) is partially shielded or blocked. This blocking is a more general problem associated with many multi-array BSA designs and is not dependent on, for example, a BSA with a C-band array. The second antenna array is designed to radiate signals in a frequency band that is approximately twice the frequency of the low-band array. This means that for most BSA designs, the distance between the low-band element structure and the mid-band element structure is typically within each other's reactive near-field, or at most within each other's radiating near-field. Instead of reflection as the dominant mechanism, blocking in the context between the low-band element and the mid-band element means that there may be more modified current distribution near the mid-band element due to the strong mutual coupling or induction field from the mid-band element to the low-band element. The induced mid-band currents onto the low-band dipole arms may result in a modified current distribution at the mid-band frequencies and may result in a disturbed far-field radiation pattern.

[0038] FIG2(a) shows a base unit 200 (having a length 201) comprising a dual polarized cross-dipole antenna element (1012) for operating in a first frequency band, such as a low-band frequency range, wherein each of the two dipoles cross orthogonally and are oriented at angles of +45° and -45°. The cross-dipole antenna element (1012) is centrally located in the base unit (200) and is surrounded by four cross-dipole antenna elements (1021, 1022, 1031, 1032) for operating in a second frequency band of the spectrum, such as a mid-band frequency range, wherein each of these dipoles cross orthogonally and are oriented at angles of +45° and -45°. Due to typical BSA operating frequencies, the cross-dipole antenna element (1012) for operating in the first frequency band is typically twice the size of each of the cross-dipole antenna elements (1021, 1022, 1031, 1032) for operating in the second frequency band. The cross-dipole antenna element (1012) has a dipole arm (203) having a defined width (202).

[0039] In FIG. 2( a ), the mid-band antenna elements ( 1021 , 1022 , 1031 , 1032 ) are illustrated as being directly below the low-band dipole arms ( 203 ) in order to visually emphasize the blocking effect. However, it should be noted that other arrangements of the mid-band element positions are possible, such as where the mid-band antenna array is moved upward by a distance equal to half the inter-element spacing. This would place the mid-band elements between the low-band dipole arms and would result in less visual shielding by the low-band dipole arms. However, the mid-band elements and the low-band elements would remain at a coupling distance within the reactive near field, such that induction remains the dominant coupling mechanism. Although not shown in FIG. 2 , the feed structures for the low-band elements and the mid-band elements (such as a balun) may be located directly below the center of the elements and may be arranged in a plane perpendicular to the plane of the elements. The feed structures for the low-band elements and the mid-band elements may also be coupled together and more strongly coupled in the case where the mid-band elements are placed between the low-band dipole arms. Thus, it should be noted that the mid-band element and the low-band element may be strongly coupled regardless of the precise positioning of the mid-band element relative to the low-band element.

[0040] Referring back to FIG. 2( a ), when any of the mid-band elements (1021 , 1022, 1031 , 1032) are radiating, current on the low-band dipole arm (203) may be induced because the low-band dipole arm (203) is within distance for strong inductive coupling. This coupling of energy between the mid-band dipole arm to the low-band dipole arm (203) may result in a modified current distribution on the low-band dipole arm (203), which in turn may result in interference with the far-field radiation pattern. As the width (202) of the low-band dipole arm increases, more physical shadowing may occur, but may result in stronger coupling with each of the mid-band elements (1021 , 1022, 1031 , 1032). The feed or balun structure for the low-band element is not shown in FIG. 2( a ), but will reside below the center of the low-band element disposed in a plane perpendicular to the plane of the element. The balun structure may also be a source of pattern interference for the far-field radiation pattern of mid-frequency band components.

[0041] FIG2(b) shows the far-field co-polarized radiation pattern (204, solid line) and cross-polarized radiation pattern (206, dashed line) for the mid-band frequency range for one of the mid-band elements in the absence of the low-band element dipole arm (203) (and therefore no associated coupling). The far-field radiation pattern has a smooth response as a function of azimuth and a low cross-polarized energy component over the range of angles covered by the cell sectors that will serve a three-sector cell site, which is from -60° to +60° in azimuth. FIG2(c) shows the corresponding degraded far-field co-polarized radiation pattern (205, solid line) and cross-polarized radiation pattern (207, dashed line) resulting from the partially shielded mid-band dipole that excites the mid-band antenna element (1031), which is a result of the coupling interaction between the low-band element and the mid-band element. Typically, pattern degradation worsens as antenna element coupling becomes stronger, which can occur if mid-band and low-band elements are placed closer together.

[0042] A frequency selective surface (FSS) is a two-dimensional thin periodic arrangement of an array of FSS elements. These FSS elements can be configured to reflect, reradiate, or absorb RF power when the frequency of a plane wave matches the resonant frequency of the FSS element. The periodic FSS array structure can be designed to be radiative or non-radiative, and thus act as a spatial filter. In practice, the nominal operation of the FSS may require several FSS element periods. Truncating the FSS after a finite number of periods may have an impact on the effectiveness of the FSS. When it is illuminated by an incident plane wave, the FSS element can be treated as a resonant circuit. The resonant frequency can be determined by the formula f=1 / (2π√LC), where L and C represent the equivalent inductance and capacitance of the element, respectively.

[0043] Etching of FSS structures has been previously implemented on the radiating portion of an antenna element and has been used to allow RF signals to be conducted and radiated at the operating frequency of the antenna element through the FSS configuration as a transmission line, while allowing the FSS to become resonant for reradiating a second, higher frequency; thus, the antenna element becomes electrically transparent at the second, higher frequency due to the spatial filtering effect of the FSS. Such an arrangement allows the second radiating source to radiate at a higher frequency with little or no attenuation of the first antenna element constructed using the FSS.

[0044] In one example, the present disclosure provides the use of an FSS structure having a split ring or hexagonal FSS element, which increases the number of FSS periods over a limited size of the radiating portion of the antenna element. Such a radiating portion of the antenna element may include a dipole arm, which may need to have a specific size for operating at the expected frequency range of the antenna element. FSS elements using split rings or split hexagonal slots utilize a closed loop or closed perimeter to support resonance, and thus achieve a significant reduction in the overall size of the FSS element. The closed loop geometry has a path for induced currents, which is doubled via the ring perimeter of the slot to enable lower resonant frequency modes. Using an array of split ring or split hexagonal slotted FSS elements for the conducting and radiating portions of the antenna element, the passband frequency of the FSS can be reduced relative to not using split ring or split hexagonal slotted FSS element geometries.

[0045] Example 1

[0046] In an illustrative example, multiple periods of FSS elements are used to support conduction and radiation of RF signals in at least a radiating portion of at least one antenna element of a first array of multiple antenna elements, wherein the antenna element is designed to operate in a first frequency range, such as a low-band frequency range, and wherein the FSS utilizes a split ring, split hexagonal slotted FSS element, or other closed loop resonant FSS element geometry. The radiating portion of the low-band antenna element may include at least a dipole arm, but may also include any feed arrangement or balun network connected to the dipole arm. The FSS utilizing split ring or split hexagonal slotted FSS elements is designed to be electrically transparent to incident plane wavefronts in a second frequency band, such as a C-band frequency range. Such use of compact FSS elements allows a maximum number of periods of FSS elements to be accommodated on a component of the low-band antenna element to optimize FSS bandpass performance.

[0047] FIG3(a) shows an example of a single FSS element (310) where the shaded hexagonal region (301) may include a conductive copper layer having a hexagonal slot (302) etched therein, the hexagonal slot having a slot gap of radius (303) and width (304). The area between the ends of the hexagonal slot represents the open portion of the hexagonal slot having a slot opening width (305). The FSS element (310) may be resonant when the length of the current path represented by the dashed line (306) is approximately half the wavelength of the frequency of the incident RF plane wave. The current path length (306) may be a function of the slot perimeter or ring length, which is a function of the slot gap width (304), slot length (307), slot opening width (305) and hexagonal element radius (303) of the FSS element (310). The induced current from the incident wavefront parallel to the same plane of the FSS element (310) can flow from one end to the other along the outer edge of the slot and then return via the inner edge of the slot. This circular current flow behavior maximizes the length of the current path, thereby reducing the resonant frequency of the FSS.

[0048] FIG3( b) depicts a graph (320) of the transmittance characteristics of a plane wave on a FSS as a function of frequency, the plane wave having a wavefront in the same plane as the plane of the FSS element, and the wavefront traveling in a direction perpendicular to the plane of the FSS element of FIG3( a). In FIG3( b), the resonant bandpass frequency is shown to be around 2 GHz, illustrated by the transmittance curve (308), which is achieved when the length (306) is approximately half the wavelength through tuning of the dimensions (303, 304, and 305). The exact dimensions of these parameters are not provided because FIG3( a) and FIG3( b) are merely illustrative in order to demonstrate that resonance can be achieved. This means that a wavefront having a frequency of 2 GHz can be transmitted through the FSS element (310) with little or no attenuation. If the path length (306) is halved through tuning of the dimensions (303, 304, 305, and / or 307), resonance can be achieved at approximately 4 GHz, as shown by the curve (309) in FIG3( b). The frequency at which the FSS becomes resonant such that the FSS reradiates any incident wave and appears essentially invisible to an incident wavefront of the same frequency may be referred to as a stealth frequency.

[0049] The size of the FSS element using slotted geometry is relatively small compared to the free space wavelength. Although FIG3(a) shows a single FSS element, in order to achieve stable resonance at different incident angles of a plane wave, the FSS structure may use several elements or periods constructed in an array of elements. To further reduce the resonant or stealth frequency of the FSS, a periodic arrangement of FSS elements placed in a closely packed formation may be used to increase the inter-element capacitance. In one example, a hexagonal slotted geometry for the FSS element may be used to increase the FSS element packing density and inter-element capacitance.

[0050] In one example, because the FSS elements / structures may be used as the radiating portion of the low-band antenna element, these radiating components may be constructed to be sufficiently conductive at the expected operating frequency of the low-band antenna element to ensure that current can flow efficiently through the FSS. For example, in the case where the low-band antenna element has a dipole arm as the radiating portion, then the dipole arm constructed using the FSS structure should perform similarly to a conventional dipole arm and ensure that there is appropriate antenna impedance matching for radiation efficiency at the expected operating frequency of the low-band antenna element.

[0051] An FSS configured to support conduction and radiation of RF signals on a dipole arm can be analyzed as a transmission line (e.g., having a width represented by (202)). FIG. 4(a) depicts an array (400) of inlaid FSS elements (401, 402, 403) for analysis as a transmission line, wherein each FSS element is composed of an open hexagonal ring FSS structure. The number of FSS element periods along and across the transmission line can be varied, as well as the selection between full hexagonal ring FSS elements (402) and different truncated hexagonal ring FSS elements (401, 403). In other words, the number of full elements and truncated elements can be conformed to the shape of the transmission line (e.g., a dipole arm). The transmission characteristics of an RF signal starting from a left position 404 of the transmission line and arriving at a right position 405 of the transmission line are analyzed and shown as a function of frequency in the graph 410 of FIG. 4(b). The graph demonstrates that signals below about 1.4 GHz can be supported by the FSS as a transmission line, and any frequency above 1.5 GHz will experience significant attenuation. In other words, the array (400) can support efficient transmission of signals below about 1.4 GHz as a transmission line (e.g., in one example, the loss is no greater than 1.5 dB, in another example, the loss is no greater than 3 dB, etc.), which is suitable for use in cellular communication systems. In general, having more FSS periods can also improve the overall conductivity and radiation efficiency of the dipole arms.

[0052] Referring again to FIG. 4( a ), it should be noted that in one example, it may be desirable to maximize the number of FSS element periods that span the width ( 202 ) of the transmission line, regardless of whether incomplete elements are caused by such a mosaic. In one example, the slot gap width, slot length, slot opening width, and / or element radius may be adjusted to minimize or avoid incomplete elements (such as ( 401 )) while maintaining a desired element path length, or to balance performance characteristics with non-ideal path lengths and performance characteristics with incomplete elements (such as ( 401 )). Similarly, the width ( 202 ) may be varied to minimize or avoid incomplete elements (such as ( 401 )). It should be noted that half elements (such as ( 403 )) may still exist in such an array. However, with small adjustments, only slightly incomplete elements (such as ( 401 )) may be avoided or minimized.

[0053] FIG. 5( a ) illustrates an antenna element (eg, FIG. 1 ) from a first array of antenna elements designed to operate in a first frequency range (such as a low-band frequency range of 698-894 MHz). Figure 1 The low-band dipole arm (203) is a low-band dipole arm (203) of one of the radiating portions of the antenna (1011)). The low-band dipole arm (203) can be constructed of a continuous copper plate having a dipole arm width represented by dimension (202), wherein the low-band dipole arm (203) is placed between at least one dual-polarized antenna element (104) designed to operate in the higher frequency range. i ). For example, in this example, the antenna element (104 i ) can be designed to operate in the C-band spectrum range of 3300-4200 MHz. Although not shown in FIG. 5( a ), the low-band dipole arm ( 203 ) and the C-band antenna element ( 104 i ) may share a common ground reflector plane, for example, parallel to the low-band dipole arm (203) and the C-band antenna element (104 i ) and in the plane below it. For example, the low-band dipole arm (203) and the C-band antenna element (104 i ) can each be positioned above a common reflective plane at a distance of approximately one quarter wavelength at each of their respective operating frequencies. It can be shown that this geometric arrangement means that the low-band dipole arm (203) can be considered to be well within the C-band antenna element (104). i )’s radiation far field.

[0054] FIG. 5( b ) depicts the C-band antenna element ( 104 ) as a function of azimuth angle for three frequencies within the C-band frequency range. i) of the C-band antenna element (104). i ) is a dual polarized antenna element where a dipole aligned with the same polarization as the low band dipole arm (203) is analyzed to obtain maximum shadowing effect. FIG5(b) reveals that there is some pattern variation or interference in the co-polarized pattern (605), resulting in some pattern asymmetry, which may affect the performance of cellular services delivered using an array of such C-band antenna elements.

[0055] FIG6( a) depicts an equivalent or similar arrangement according to FIG5( a), but wherein a low band dipole arm (213) of an antenna element from a first array of antenna elements (having a dipole arm width represented by dimension (202)) is constructed using an FSS comprising a number of periods of FSS elements along and across the low band dipole arm (213), the low band dipole arm (213) being tuned to a stealth frequency just below 4 GHz for frequencies in the C-band spectrum range. Similar to the example of FIG5( a), the low band dipole arm (213) is positioned between at least one dual polarized antenna element (104) designed to operate in the higher frequency range. i ) above. The arrangement of FSS elements may also be configured to support the transmission and radiation of low-band RF signals by the low-band dipole arm (213) using FSS, similar to that illustrated in FIG4(a). FIG6(b) depicts the C-band antenna element (104) as a function of azimuth angle for three frequencies within the C-band frequency range. i 5( b) reveals reduced pattern variation or interference in the co-polarization pattern (607) relative to FIG. 5( b). In particular, the preservation of pattern symmetry is improved, and there is much less attenuation over the angular range that would serve a typical sector of a three-sector cellular site (e.g., angles of -60° to +60° from the boresight or main beam center direction). Notably, these pattern attributes may be desirable characteristics for the C-band frequency range used for cellular networks.

[0056] For purposes of illustration, the dipole arms 203 and 213 of FIG. 5( a) and FIG. 6( a) are associated with a low-band antenna array. However, this example is equally applicable to other types of resonant radiating antenna elements, such as patch antennas. Additionally, an FSS element based on an open hexagonal slot structure is illustrated in FIG. 3( a). However, additional examples of the present disclosure may include a wider series of slotted FSS elements, such as split rings, and more generally a series of FSS elements designed to increase the resonant wavelength.

[0057] Example 2

[0058] As noted above, FIG. 4( a) and FIG. 4( b) illustrate that an array of FSS elements may be arranged to support efficient transmission of RF signals below 1.4 GHz. The transmission diagram of FIG. 4( b) also reveals that as a transmission line, the array will not support any current above 1.5 GHz (e.g., the array suppresses coupling above 1.5 GHz, e.g., at least 20 dB or greater, 30 dB or greater, etc., of loss). In one example, this characteristic of the FSS is used in a second mode of operation, e.g., for frequency or band suppression of any RF signal that is tightly coupled to a low-band antenna element utilizing the FSS structure. For example, this example may address the problem of antenna elements designed for different frequency bands that are close to each other and risk coupling to each other and causing interference in the current distribution across the antenna elements and the resulting far-field radiation pattern.

[0059] In one example, multiple periods of FSS elements in an FSS structure / array are used to support conduction of RF signals in at least a radiating portion of at least one antenna element of a first array of multiple antenna elements, wherein the antenna element is designed to operate in a first frequency range, such as a low-band frequency range, and wherein the FSS utilizes a split ring or split hexagon slotted FSS element geometry. As in the above examples, the radiating portion of the low-band antenna element may include a dipole arm, but may also include any feed arrangement or balun network connected to the dipole arm. The FSS utilizing split ring or split hexagon slotted FSS elements may provide band suppression at a second frequency by using a second mode of the FSS to significantly attenuate transmission and re-radiation of RF energy at the second frequency via the FSS (e.g., a loss of at least 20 dB or greater, 30 dB or greater, etc.). For example, RF energy at a second frequency may be generated by at least one antenna element of a second array of antenna elements that are designed to operate in a second frequency band, such as a mid-band frequency range (e.g., 1695-2690 MHz, etc.), and wherein the RF energy is incident upon an antenna element of a first array using an FSS structure / array. In this context, the incident RF energy may include RF energy from a plane wavefront, a near-field radiating region of an antenna element of the second array of antenna elements, or a near-field reactive region of an antenna element of the second array of antenna elements.

[0060] FIG. 7( a ) illustrates an antenna element (eg, Figure 17( a)). The low-band dipole arm (203) may be formed of a continuous copper plate having a low-band dipole arm width represented by dimension (202), wherein the low-band dipole arm (203) is placed above an antenna element (1021) of a second array of antenna elements for operating in a higher frequency range. For example, in this example, the antenna element (1021) may be designed for operation in a second frequency range such as a mid-band range of 1695-2690 MHz. Although not shown in FIG. 7( a), the low-band dipole arm (203) and the mid-band antenna element (1021) may share a common ground reflector plane, such as in a plane parallel to and below the low-band dipole arm (203) and the mid-band antenna element (1021). The low-band dipole arm (203) and the mid-band antenna element (1021) may each be positioned above a common reflective plane at a distance of approximately one quarter wavelength at each of their respective operating frequencies. In this geometric arrangement, the low-band dipole arm (203) and the mid-band antenna element (1021) may be considered to be within the radiating near field, or possibly within the reactive near field of the mid-band antenna element (1021).

[0061] FIG7( b) depicts a graph 710 of the resultant co-polarized far-field radiation pattern (601, solid curve) and the cross-polarized far-field radiation pattern (602, dashed curve) for the mid-band antenna element (1021). The radiation patterns are shown as a function of azimuth, spanning three frequencies within the mid-band frequency range. The low-band dipole arm (203) is aligned with the same polarization as the excited mid-band antenna element dipole to provide maximum shadowing effect. Although the radiation patterns are not shown for the case with an unshielded antenna element (1021), FIG7( b) does reveal that there is frequency-dependent pattern variation in the co-polarized pattern as well as some overall pattern asymmetry. FIG7( b) also indicates an increased level of cross-polarized power components, especially toward the sector edges of any three-sector cellular base station site using an array of such antenna elements, which can result in sub-optimal cellular network performance. The pattern distortion and increased cross-polarization power may be due to the presence of the low-band dipole arm (203), which induces a current in the incident RF energy from the mid-band antenna element (1021) onto the low-band dipole arm (203), and ultimately re-radiates the component RF energy at the mid-band frequency with a current profile that may have complexity and multiple current peaks and nulls because the low-band dipole arm is tunable for wavelengths at least twice as long as the mid-band wavelength. This re-radiation of the component RF energy may be combined with the RF energy vector radiated directly from the mid-band antenna element (1021), which may also result in fluctuations in the azimuth radiation pattern as a function of azimuth and frequency.

[0062] FIG8(a) depicts a similar or equivalent arrangement according to FIG7(a), but wherein a low-band dipole arm (213) (having a dipole arm width represented by dimension (202)) is constructed using FSS, the FSS including several periods of FSS elements along and across the dipole arm (213), the FSS elements being tuned to suppress coupling of RF energy at frequencies in the mid-band frequency range using a second mode of the FSS (e.g., a loss of at least 20 dB or greater, 30 dB or greater, etc.). Similar to the above examples, the low-band dipole arm (213) is placed above at least one dual-polarized antenna element (1021) designed to operate in a higher frequency range (e.g., a mid-band frequency range). The arrangement of the FSS elements may also be configured to support efficient transmission and radiation of low-band RF signals by the dipole arm (213) using the FSS, similar to the example of FIG4(a) (e.g., in one example, no more than 1.5 dB of loss, in another example, no more than 3 dB of loss, etc.). For example, a change in the number of element periods over the width (202) from 1.5 to 2 can reduce the cutoff frequency by approximately 100-200 MHz. Increasing the length of the array can also reduce the cutoff frequency (e.g., from 3 GHz to 2 GHz, to 1.5 GHz, etc.), while at the same time the frequency roll-off can become steeper as the number of elements along the length increases (for an array of the same width).

[0063] FIG8( b ) depicts a graph ( 810 ) of the resultant co-polarized far-field radiation pattern ( 603 , solid curve) and the cross-polarized far-field radiation pattern ( 604 , dashed curve) of the mid-band antenna element ( 1021 ). The radiation patterns are shown as a function of azimuth angle, spanning three frequencies within the mid-band frequency range. FIG8( b ) reveals much less pattern variation or interference in the co-polarized pattern relative to FIG7( b ). For example, the preservation of pattern symmetry is improved, there are fewer cross-polarized power components, and there are fewer variations in the pattern response across the three frequencies, which may be desirable properties of a cellular antenna. Additionally, there is minimal pattern distortion or increase in cross-polarized power, which may be due to negligible induced current from incident RF energy from the mid-band antenna element ( 1021 ) onto the low-band dipole arm ( 213 ), which is a result of utilizing the second mode of the FSS to minimize re-radiation of component RF energy at mid-band frequencies from the low-band dipole arm. In the worst case, there may be a small attenuation in the overall radiated RF power due to the physical blocking provided by the dipole arms (213) and absorption of some mid-band RF energy by the second mode of the FSS.

[0064] FIG. 7( a) and FIG. 8( a) depict geometric arrangements in which the low-band dipole arms (203, 213) are directly in front of the mid-band element (1021). It should be noted that the low-band dipole arm (203) omits the FSS. The low-band dipole arm (213) includes the FSS. It should also be noted that even if the mid-band antenna element (1021) is moved downward in these figures so that the low-band dipole arms (203, 213) are not directly in front of the corresponding mid-band element (1021), the mid-band element (1021) will remain close to the corresponding low-band antenna element (203, 213). As such, there is still a risk of strong coupling and risk of component RF re-radiation from the low-band antenna element dipole arm (203) (in the absence of FSS on the dipole arm (203)).

[0065] Example 3

[0066] In the above examples, the dipole is the primary radiating component of the low-band antenna element. In this regard, it is noted that antenna elements using dipoles typically use a feed arrangement to optimally connect the RF cable to the dipole arm. In one example, such a feed network performs the conversion of the RF signal from the unbalanced cable transmission line to the balanced dipole transmission line and is referred to as a balun (balanced-unbalanced) network. FIG. 9( a) illustrates a balun network (801) for one of two orthogonally polarized antenna elements. The balun network (801) may be about or just less than 1 / 4 wavelength high, wherein the top portion (802) of the balun network (801) is connected to two dipole arms of a pair of two mutually orthogonally polarized dipole arm pairs. For clarity, the arms of the dipole are not shown in FIG. 9( a). The balun (e.g., balun network (801)) may be formed of a microstrip line, which is also not shown in FIG. 9(a), but which would be on the opposite side of the visible portion of the balun network (801) in FIG. 9(a). The microstrip line may be parallel to the larger ground plane (803) visible in FIG. 9(a), and utilize the larger ground plane (803) to form a transmission line from the connecting cable to the dipole arm. The connecting cable is not shown in FIG. 9(a), but would be connected to the balun network (801) below the common reflector plane (800). The balun network (801) and in particular the balun ground plane (803) of the low-band antenna element may also be a structure coupled to the mid-band antenna element (1021). Depending on the proximity of the mid-band element (1021), such coupling may include inductive coupling when the low-band antenna element balun network (801) is, for example, within the reactive near field of the mid-band antenna element (1021), or more generally, when the low-band antenna element balun network (801) acts as a reflecting or scattering surface for RF radiated fields from the mid-band antenna element (1021).

[0067] FIG9( b) illustrates a graph 910 of the radiation pattern as a function of azimuth angle for a mid-band antenna element (1021) as depicted in FIG9( a) and thus in the vicinity of the low-band antenna element balun network (801) (without its dipole arms). In particular, FIG9( b) shows the co-polarization pattern (solid line 805) for three different frequencies spanning the range of 1695 MHz and 2690 MHz within the mid-band frequency range. FIG9( b) also shows the cross-polarization pattern (dashed line 806) for the same three different frequencies. It is observed that there is some frequency-dependent pattern variation, and there is also a relatively large cross-polarization power component towards the sector edges (-60° and +60°) in the case where the antenna is used in a three-sector base station implementation.

[0068] In one example, the present disclosure may use a tuned FSS to construct a balun network, and in particular, a balun ground plane for a low-band antenna element in a multi-array BSA. Advantageously, using a tuned FSS for a balun ground plane utilizes a second mode of the FSS such that there is negligible coupling from mid-band RF radiated energy to the low-band antenna element balun network ground plane. This in turn ensures that induction to, reradiation from, scattering, or reflection from the low-band antenna element balun network ground plane is minimized. The tuned FSS may be implemented onto the balun ground plane using an open hexagonal FSS slot geometry. A ground plane having intentional discontinuities in the plane may be referred to as a defective ground plane (where such a term does not imply a defect in performance, but simply refers to the fact that the ground plane is not uniform).

[0069] Similar to the above example, FIG10(a) depicts a configuration of a mid-band antenna element (1021) proximate to a low-band antenna element balun network (801) according to FIG9(a), but wherein the balun network ground plane (803) is constructed using a FSS having a mosaic array of open hexagonal slot FSS elements (804). The FSS is tuned to utilize a second mode of the FSS, wherein the FSS supports efficient transmission of low-band RF signals from the RF cable connection to the dipole arm, and wherein the FSS also ensures that induced RF energy from the nearby mid-band antenna element (203) is not significant, and therefore minimal mid-band RF component power is generated by reflection, scattering, or re-radiation from the balun ground plane (803).

[0070] FIG10( b) illustrates a graph 1010 of the radiation pattern as a function of azimuth angle for a mid-band antenna element (203) according to the arrangement depicted in FIG10( a), wherein the mid-band antenna element (1021) is in the vicinity of a low-band antenna element balun network (803) utilizing a tuned FSS utilizing a second mode of a FSS having a mosaic array of open hexagonal slot FSS elements (804). FIG10( b) shows a co-polarization pattern (solid line 807) for three different frequencies spanning 1695 MHz and 2690 MHz within the mid-band frequency range. Additionally, FIG10( b) also shows a cross-polarization pattern (dashed line 808) for the same three different frequencies. In FIG. 10( b ), there is less frequency-dependent pattern variation than in the case of the non-FSS balun grounded configuration in FIG. 9( b ), and there is also a small improvement or reduction in the cross-polarization power components toward the sector edges (-60° and +60°) in the case where the antenna is used in a three-sector base station implementation.

[0071] Example 4

[0072] In additional examples of the present disclosure, a tuned FSS may be used to construct a balun network, and more specifically used as a defective balun network ground plane for a low-band antenna element in a multi-band array (e.g., a BSA) utilizing a first (stealth) mode of the FSS. Advantageously, use of the tuned FSS for the balun ground plane utilizes the first mode of the FSS such that there is minimal scattering or reflection of C-band RF energy from the low-band antenna element balun network ground plane as a result of the C-band antenna element irradiating the low-band antenna element balun network. The tuned FSS may be configured to resonate and reradiate in the C-band frequency range, and may be implemented on the balun ground plane utilizing FSS elements using an open hexagonal FSS slot structure such that the balun ground plane appears transparent to any C-band radiation from an array of C-band antenna elements that are also proximate to the low-band antenna element.

[0073] Example 5

[0074] In an additional example of the present disclosure, the first mode and the second mode of the FSS can be used simultaneously in a tri-band antenna, where each band uses at least one cross-polarized array of antenna elements, and where the FSS structure can utilize slotted hexagonal slot elements, and when the FSS structure can be deployed on at least the radiating component of the low-band antenna element. In this example, the first mode of the FSS can be utilized by tuning the stealth characteristics at C-band frequencies, which means that the C-band radiation from the C-band antenna element is not blocked, reflected or scattered by the larger low-band antenna elements in the dense multi-array BSA environment. The second mode of the FSS can be utilized by ensuring efficient and low-loss transmission and radiation of low-band RF signals from the low-band antenna element while ensuring minimum induced current from the nearby mid-band antenna element to the low-band antenna element. In other words, in this example, open hexagonal slotted FSS elements can be used for the FSS structure, which is tuned to utilize both the first mode and the second mode, so that the mutual coupling and interaction of the antenna array are minimized, so as to optimize the antenna array performance and / or allow the minimum distance between antenna elements designed for different frequency bands.

[0075] In view of the foregoing, it should be noted that the tuned FSS structure can be used on other BSA components to optimize the radiation pattern, such as RF components, including but not limited to fences, resonators, parasitic resonators, directors, etc. While the foregoing describes various examples according to one or more aspects of the present disclosure, other and additional example(s) according to one or more aspects of the present disclosure may be devised without departing from the scope of the present disclosure as determined by the appended claims and their equivalents.

[0076] Aspects of the various embodiments are specified in the claims. Those and other aspects of the various embodiments are specified in the following numbered clauses.

Claims

1. A multi-band antenna, comprising: a first antenna array for operating in a first frequency range; as well as a second antenna array for operating in a second frequency range, wherein the second frequency range is higher than the first frequency range; wherein the first antenna array comprises at least one antenna element, wherein the at least one antenna element comprises a plurality of components, wherein at least one of the plurality of components is configured with a frequency selective surface (FSS), wherein the frequency selective surface (FSS) comprises a plurality of FSS elements; Wherein, the FSS is configured to cause the at least one of the plurality of components to provide suppression of coupling of energy from the second frequency range, and to cause the at least one of the plurality of components to provide transmission of signals in the first frequency range suitable for use in a cellular communication system.

2. The multi-band antenna according to claim 1, wherein: The suppression of the coupling of the energy from the second frequency range comprises a loss of at least 20 dB.

3. The multi-band antenna according to claim 1, wherein: The transmission of the signal in the first frequency range suitable for use in the cellular communication system comprises a loss of no more than 3 dB.

4. The multi-band antenna according to claim 1, wherein: At least a first subset of the plurality of FSS elements comprises complete FSS elements, wherein each of the complete FSS elements comprises a slotted aperture etched into a conductive surface around a perimeter of the complete FSS element, and wherein the slotted apertures form an incomplete hexagonal shape or an incomplete annular shape, wherein a total inner circumference of the slots is a wavelength of the resonant wavelength.

5. The multi-band antenna according to claim 4, wherein: The plurality of FSS elements are mosaicked into an array, wherein the array comprises at least the first subset of the plurality of FSS elements and at least a second subset of the plurality of FSS elements, wherein the at least the second subset of the plurality of FSS elements comprises truncated FSS elements to conform to a shape of the at least one of the plurality of components.

6. The multi-band antenna according to claim 1, wherein: The plurality of components includes at least one component for radiating radio frequency (RF) energy.

7. The multi-band antenna according to claim 6, wherein: The plurality of components also includes a balun feed network ground plane.

8. The multi-band antenna according to claim 7, wherein: The at least one of the plurality of components configured with the FSS comprises at least one of: the at least one component for radiating RF energy; or Balun feed network ground plane.

9. The multi-band antenna according to claim 1, wherein: The first antenna array includes a plurality of antenna elements including the at least one antenna element.

10. A multi-band antenna, comprising: a first antenna array for operating in a first frequency range; as well as a second antenna array for operating in a second frequency range, wherein the second frequency range is higher than the first frequency range; wherein the first antenna array comprises at least one antenna element, wherein the at least one antenna element comprises a plurality of components, wherein at least one of the plurality of components is configured with a frequency selective surface (FSS), wherein the frequency selective surface (FSS) comprises a plurality of FSS elements; Wherein, the plurality of FSS elements have features configured to enable the at least one of the plurality of components to provide a resonant wavelength in the second frequency range, and to enable the at least one of the plurality of components to provide transmission of signals in the first frequency range suitable for use in a cellular communication system.

11. The multi-band antenna according to claim 10, wherein: The transmission of the signal in the first frequency range suitable for use in the cellular communication system comprises a loss of no more than 3 dB.

12. The multi-band antenna according to claim 10, wherein: At least a first subset of the plurality of FSS elements comprises complete FSS elements, wherein each of the complete FSS elements comprises a slotted aperture etched into a conductive surface around a perimeter of the complete FSS element, and wherein the slotted apertures form an incomplete hexagonal shape or an incomplete annular shape, wherein a total inner circumference of the slots is a wavelength of the resonant wavelength.

13. The multi-band antenna according to claim 12, wherein: The plurality of FSS elements are mosaicked into an array, wherein the array comprises at least the first subset of the plurality of FSS elements and at least a second subset of the plurality of FSS elements, wherein the at least the second subset of the plurality of FSS elements comprises truncated FSS elements to conform to a shape of the at least one of the plurality of components.

14. The multi-band antenna according to claim 10, wherein: The plurality of components includes at least one component for radiating radio frequency (RF) energy.

15. The multi-band antenna according to claim 14, wherein: The plurality of components also includes a balun feed network ground plane.

16. The multi-band antenna according to claim 15, wherein: The at least one of the plurality of components configured with the FSS comprises at least one of: the at least one component for radiating RF energy; or Balun feed network ground plane.

17. The multi-band antenna according to claim 10, wherein: The first antenna array includes a plurality of antenna elements including the at least one antenna element.

18. A multi-band antenna, comprising: a first antenna array for operating in a first frequency range; a second antenna array for operating in a second frequency range, wherein the second frequency range is higher than the first frequency range; and a third antenna array configured to operate in a third frequency range, wherein the third frequency range is higher than the second frequency range; wherein the first antenna array comprises at least one antenna element, wherein the at least one antenna element comprises a plurality of components, wherein at least one of the plurality of components is configured with a frequency selective surface (FSS), wherein the frequency selective surface (FSS) comprises a plurality of FSS elements; Wherein, the FSS is configured to cause at least one of the plurality of components to provide suppression of coupling of energy from the second frequency range, to cause at least one of the plurality of components to provide a resonant wavelength in the third frequency range, and to cause at least one of the plurality of components to provide transmission of signals in the first frequency range suitable for use in a cellular communication system.

19. The multi-band antenna according to claim 18, wherein: The suppression of the coupling of the energy from the second frequency range comprises a loss of at least 20 dB.

20. The multi-band antenna according to claim 18, wherein: The transmission of the signal in the first frequency range suitable for use in the cellular communication system comprises a loss of no more than 3 dB.

21. The multi-band antenna according to claim 18, wherein: At least a first subset of the plurality of FSS elements comprises complete FSS elements, wherein each of the complete FSS elements comprises a slotted aperture etched into a conductive surface surrounding a perimeter of the complete FSS element, and wherein the slotted apertures form an incomplete hexagonal shape or an incomplete annular shape, wherein a total inner circumference of the slots is a wavelength of the resonant wavelength.

22. The multi-band antenna according to claim 21, wherein: The plurality of FSS elements are mosaicked into an array, wherein the array comprises at least the first subset of the plurality of FSS elements and at least a second subset of the plurality of FSS elements, wherein the at least the second subset of the plurality of FSS elements comprises truncated FSS elements to conform to a shape of the at least one of the plurality of components.

23. The multi-band antenna according to claim 18, wherein: The plurality of components includes at least one component for radiating radio frequency (RF) energy.

24. The multi-band antenna according to claim 23, wherein: The plurality of components also includes a balun feed network ground plane.

25. The multi-band antenna according to claim 24, wherein: The at least one of the plurality of components configured with the FSS comprises at least one of: the at least one component for radiating RF energy; or Balun feed network ground plane.

26. The multi-band antenna according to claim 18, wherein: The first antenna array includes a plurality of antenna elements including the at least one antenna element.

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