WO2023060018A1 - Antenna systems having radio nodes with vertical monopole antennas and interleaved circular arrays of horizontal dipole antennas therein - Google Patents

Abstract

A MIMO antenna system includes a pair of vertical monopole radiating elements on a forward-facing surface of a substrate, and first and second arrays of horizontal dipole radiating elements, which are arranged as a closed-loop of radiating elements on a rear-facing surface of the substrate. Each radiating arm of each of the horizontal dipole radiating elements in the first and second arrays is electrically coupled by a respective horizontal feed stalk to edges of a ground plane, which is provided on the rear-facing surface of the substrate. First and second feed signal routing networks are provided on the forward-facing surface of the substrate, and are electrically coupled via the substrate to the first and second arrays of radiating elements, respectively.

Description

ANTENNA SYSTEMS HAVING RADIO NODES WITH VERTICAL MONOPOLE ANTENNAS AND INTERLEAVED CIRCULAR ARRAYS OF HORIZONTAL DIPOLE ANTENNAS THEREIN

FIELD

[0001] The present invention relates to cellular communications systems and, more particularly to distributed antenna systems and methods of operating same.

BACKGROUND

[0002] Small cells, distributed antenna systems (DAS), and Wi-Fi can be used to provide in-building and outdoor wireless service with lower cost and lower power consumption, as compared to other antenna systems, such as macro-cells. As will be understood by those skilled in the art, an indoor radio point (a/k/a radio node) can be configured to include multiple antennas that are optimized to provide the widest possible coverage across an indoor space when mounted therein (e.g., on a ceiling(s)).

[0003] As shown by FIGS. 1-3, an indoor radio point/node 10 may be configured to include a pair of spaced-apart and forward-facing vertical monopole antennas (va, vb) and a pair of spaced-apart horizontal monopole antennas (ha, hb) on an underlying printed circuit board 12 (with underside “rear-facing” ground plane 14) and chassis 16, which can collectively support a maximum 4x MIMO system. As illustrated by the directivity patterns of FIG. 2 (at 3.95 GHz), each vertical monopole antenna of FIG. 1 may support an omni coverage pattern in an azimuth plane (e.g., xy-plane), where the co-polarization directivity (proportional to gain dB of a vertical polarization as a function of angular direction in the azimuth plane) is shown as Dir_0, and the cross-polarization directivity is shown as Di r_cp (proportional to gain dB of a horizontal polarization as a function of angular direction in the azimuth plane), where gain = directivity*efficiency, and efficiency = 3D radiated power/input power.

[0004] In FIG.1 , it is assumed that the radio point/node is mounted on a ceiling of an indoor space so that the z-axis points downward towards the floor. As shown by FIG. 3, the current I0 running on the vertical monopole va is in parallel with the z-axis. Thus, at an observation point P(r, q>, 0) within the indoor space, the total electric field radiated from 10 can be decomposed into two orthogonal field vectors, E0 and Eq), where: (i) Eq) is defined as cross-polarization because it’s always perpendicular to 10, and (ii) E0 is defined as co-polarization. Moreover, in FIG. 2, the average directivity of the copolarization radiation over the xy-plane (<p = 0°-360°) equals: Mean(Dir_0) = -0.18 dB, and the standard deviation of the directivity of the co-polarization over the xy-plane equals: Std(Dir_0) = 1.04dB, whereas the average directivity of the cross-polarization equals: Mean(Dir_cp) = -15.63 dB.

[0005] Finally, as illustrated by the directivity patterns of FIG. 4 (at 3.95 GHz), each horizontal monopole antenna (ha, hb) of FIG. 1 may support radiation currents along the y-axis because the horizontal monopoles are perpendicular to the vertical monopoles along the z-axis. In FIG. 4, E0 is defined as the cross-polarization radiation, and Eq) is defined as the co-polarization radiation. The average directivity of the co-polarization over the xy-plane (<p = 0°-360°) equals: Mean(Dir_cp) = -4.65 dB, and the standard deviation of the directivity of the co-polarization over the xy-plane equals: Std( D ir_ cp) = 6.44 dB, whereas the average directivity of the cross-polarization equals: Mean(Dir_ 0) = -2.8 dB. As compared to FIG. 2, the co-polarization strength of the horizontal monopole ha is much weaker than the vertical monopole va, whereas the variation of the co-polarization of the horizontal monopole ha is much greater than the vertical monopole va.

[0006] Moreover, as will be understood by those skilled in the art, Eq), Dir_ q>, and Gain_cp are always in parallel with the xy-plane (i.e. , the horizon), and Eq), E0 and r (propagation direction) are always perpendicular to one another. In addition, for a vertical monopole in the z-axis, cross-polarization is defined as Eq>; thus, E0 is the co- polarization (since Eq) is always perpendicular to Io and, theoretically, Eq) = 0). In contrast, for a horizontal monopole along the y-axis (or an arbitrary direction in the xy- plane), E0 + 0 and Eq) + 0; however, the 3D integration of Dir_q> is always larger than that of Dir_ 0. Thus, Eq) is treated as the co-polarization. SUMMARY

[0007] A multi-input and multi-output (MIMO) antenna according to some embodiments of the invention includes a pair of vertical monopole radiating elements on a forwardfacing surface of a substrate, along with first and second arrays of horizontal dipole radiating elements, which are arranged as a closed-loop plurality of horizontal dipole radiating elements on the substrate. In some of these embodiments, the plurality of horizontal dipole radiating elements within the first array are interleaved with the plurality of horizontal dipole radiating elements within the second array, around a periphery of the substrate.

[0008] According to further embodiments of the invention, the first array of horizontal dipole radiating elements are arranged as a first plurality of spaced-apart pairs of horizontal dipole radiating elements, and the second array of horizontal dipole radiating elements are arranged as a second plurality of spaced-apart pairs of horizontal dipole radiating elements. In particular, the first and second arrays of horizontal dipole radiating elements may be interleaved so that each pair of horizontal dipole radiating elements in the first array is bordered on opposing sides by first and second pairs of horizontal dipole radiating elements in the second array, and vice versa. Moreover, each of the first plurality of spaced apart pairs of horizontal dipole radiating elements may be generally aligned along respective first arcs, which are concentric, and each of the second plurality of spaced apart pairs of horizontal dipole radiating elements may be generally aligned along respective second arcs, which are concentric. The first and second arcs may also be concentric, in some embodiments of the invention. In addition, the first arcs may have an equivalent first radius of curvature and the second arcs may have an equivalent second radius of curvature. The first radius of curvature may be equivalent to the second radius of curvature in some embodiments of the invention.

[0009] According to additional embodiments of the invention, the dipole arms of the plurality of horizontal dipole radiating elements within the first array may be generally aligned along respective first arcs, which are concentric, and the dipole arms of the plurality of horizontal dipole radiating elements within the second array may be generally aligned along respective second arcs, which are concentric. In some of these embodiments, a radius of curvature of the first arcs may be less than a radius of curvature of the second arcs.

[0010] According to further embodiments of the invention, at least some of the horizontal dipole radiating elements in the first array may be arranged closer to a center of the forward-facing surface of the substrate relative to other ones of the horizontal dipole radiating elements in the first array. For example, a first half of the horizontal dipole radiating elements in the first array may be arranged closer to the center of the forward-facing surface of the substrate relative to a second half of the horizontal dipole radiating elements in the first array, and a first half of the horizontal dipole radiating elements in the second array may be arranged closer to the center of the forward-facing surface of the substrate relative to a second half of the horizontal dipole radiating elements in the second array. In addition, each of the first half of horizontal dipole radiating elements in the first array may extend immediately adjacent a corresponding one of the first half of horizontal dipole radiating elements in the second array.

[0011] According to other embodiments of the invention, each radiating arm of each of the horizontal dipole radiating elements in the first and second arrays is electrically coupled by a respective horizontal feed stalk to edges of a ground plane on a rearfacing surface of the substrate. This ground plane may be circular-shaped or generally rectangular-shaped in some embodiments of the invention, however, other shapes may also be feasible.

[0012] In addition, a feed signal routing network may be provided on the forward-facing surface of the substrate, which may be a double-sided printed circuit board (PCB). For example, a first feed signal routing network may be provided, which is configured to distribute a first input feed signal to each of the horizontal feed stalks associated with the first array of horizontal dipole radiating elements. This first feed signal routing network may use power splitters and/or power dividers (e.g., Wilkinson power dividers) and surface mount jumpers to evenly distribute a first input feed signal to a plurality of horizontal dipole radiating elements within the first array. Likewise, a second feed signal routing network may be provided, which is configured to distribute a second input feed signal to each of the horizontal feed stalks associated with the second array of horizontal dipole radiating elements. The first feed signal routing network and the second feed signal routing network may also terminate at hook-shaped feed lines associated with each of the horizontal dipole radiating elements within the first array and each of the horizontal dipole radiating elements within the second array, respectively. These hook-shaped feed lines may extend opposite a corresponding pair of horizontal feed stalks associated with each of the horizontal dipole radiating elements within the first and second arrays.

[0013] According to still further embodiments of the invention, a multi-input and multioutput (MIMO) antenna includes a substrate having multiple interleaved arrays of horizontal dipole radiating elements thereon, which are arranged as a closed-loop plurality of horizontal dipole radiating elements around a periphery of the substrate. In addition, multiple feed signal routing networks are provided on a first surface of the substrate and a ground plane is provided on a second surface of the substrate. This ground plane may be electrically coupled to the closed-loop plurality of horizontal dipole radiating elements. For example, if each of the plurality of horizontal dipole radiating elements includes a pair of horizontal feed stalks, then the ground plane may be electrically coupled to a base of each of the horizontal feed stalks. In some embodiments, the first and second surfaces of the substrate may be forward and rearfacing surfaces of the substrate, respectively, and the substrate may be a double-sided printed circuit board (PCB). A pair of vertical monopole radiating elements may also be provided, at spaced apart locations on the first surface of the substrate.

[0014] According to some of these embodiments of the invention, the multiple feed signal routing networks include: (i) a first feed signal routing network, which is configured to distribute a first input feed signal to a first plurality of horizontal dipole radiating elements within the closed-loop, and (ii) a second feed signal routing network, which is configured to distribute a second input feed signal to a second plurality of horizontal dipole radiating elements within the closed-loop. In some embodiments, a feed signal routing network may use power dividers and surface mount jumpers to evenly distribute an input feed signal to a plurality of horizontal dipole radiating elements within the closed-loop (e.g., circular loop). In other embodiments, at least some of the horizontal dipole radiating elements within the closed-loop are arranged closer to a center of the second surface of the substrate relative to other ones of the horizontal dipole radiating elements within the closed-loop. For example, half of the horizontal dipole radiating elements within the closed-loop may be arranged closer to a center of the second surface of the substrate relative to another half of the horizontal dipole radiating elements within the closed-loop.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 is a perspective view of an indoor radio point/node having two vertical monopole antennas (va, vb) and two horizontal monopole antennas (ha, hb), according to the prior art.

[0016] FIG. 2 illustrates co-polarization and cross-polarization directivity patterns in the azimuth plane, for a vertical monopole antenna within the radio point/node of FIG. 1.

[0017] FIG. 3 illustrates a pair of orthogonal electrical field vectors (E0 and Ecp) within a spherical coordinate system, which correspond to the radiation generated by the radio point/node of FIG. 1.

[0018] FIG. 4 illustrates co-polarization and cross-polarization directivity patterns in the azimuth plane, for a horizontal monopole antenna within the radio point/node of FIG. 1 .

[0019] FIG. 5A is a perspective view of a circular multi-input and multi-output (MIMO) antenna, according to an embodiment of the invention.

[0020] FIG. 5B is a plan view of the MIMO antenna of FIG. 5A.

[0021] FIG. 5C is an enlarged plan view of a portion of the MIMO antenna of FIG. 5B, which illustrates feed signal routing to a pair of horizontal dipole radiating elements, according to an embodiment of the invention.

[0022] FIG. 5D is an enlarged perspective view of a portion of the MIMO antenna of FIG. 5A, which illustrates configuration of a surface mount jumper, according to an embodiment of the invention.

[0023] FIG. 5E illustrates co-polarization and cross-polarization directivity patterns in the azimuth plane, for the MIMO antenna of FIG. 5A.

[0024] FIG. 6A is a plan view of a generally rectangular MIMO antenna, according to an embodiment of the invention. [0025] FIG. 6B is a plan view of a beam pattern having an outline that shows copolarization directivity in the azimuth plane for the MIMO antenna of FIG. 6A, according to an embodiment of the invention.

[0026] FIG. 6C is a perspective view of the MIMO antenna of FIG. 6A.

[0027] FIG. 6D illustrates co-polarization and cross-polarization directivity patterns in the azimuth plane, for the MIMO antenna of FIG. 6A.

[0028] FIG. 6E is a plan view of a generally rectangular MIMO antenna, according to an embodiment of the invention.

[0029] FIG. 6F is a plan view of a beam pattern having an outline that shows copolarization directivity in the azimuth plane for the MIMO antenna of FIG. 6E, according to an embodiment of the invention.

[0030] FIG. 6G is a plan view of a generally rectangular MIMO antenna, according to an embodiment of the invention.

[0031] FIG. 6H is a plan view of a beam pattern having an outline that shows copolarization directivity in the azimuth plane for the MIMO antenna of FIG. 6E, according to an embodiment of the invention.

[0032] FIG. 7A is a plan view of a circular 4x MIMO antenna having a first array of twelve horizontal dipole radiating elements interleaved with a second array of twelve horizontal dipole radiating elements, according to an embodiment of the invention. [0033] FIG. 7B is a plan view of a beam pattern having an outline that shows copolarization directivity in the azimuth plane for the MIMO antenna of FIG. 7A, according to a first activation of a first subset of the radiating elements in FIG. 7A.

[0034] FIG. 7C is a plan view of a circular 4x MIMO antenna having a first array of twelve horizontal dipole radiating elements interleaved with a second array of twelve horizontal dipole radiating elements, according to an embodiment of the invention. [0035] FIG. 7D is a plan view of a beam pattern having an outline that shows copolarization directivity in the azimuth plane for the MIMO antenna of FIG. 7C, according to a second activation of a second subset of the radiating elements in FIG. 7C.

[0036] FIG. 7E is a plan view of a circular 4x MIMO antenna having a first array of twelve horizontal dipole radiating elements interleaved with a second array of twelve horizontal dipole radiating elements, according to an embodiment of the invention. [0037] FIG. 7F is a plan view of a beam pattern having an outline that shows copolarization directivity in the azimuth plane for the MIMO antenna of FIG. 7E, according to a third activation of a third subset of the radiating elements in FIG. 7E.

[0038] FIG. 8A is a schematic plan view of a MIMO antenna with co-located horizontal radiating elements, according to an embodiment of the invention.

[0039] FIG. 8B is a schematic plan view of a MIMO antenna with co-located horizontal radiating elements, according to an embodiment of the invention.

[0040] FIG. 8C is a schematic plan view of a MIMO antenna with co-located horizontal radiating elements, according to an embodiment of the invention.

[0041] FIG. 8D is a graph of Envelope Correlation Coefficient (ECC) for the MIMO antenna embodiments of FIGS. 8A-8C.

[0042] FIG. 9A is a schematic plan view of a MIMO antenna with interleaved and coplanar horizontal radiating elements, according to an embodiment of the invention. [0043] FIG. 9B is a schematic plan view of a MIMO antenna with interleaved and coplanar horizontal radiating elements, according to an embodiment of the invention. [0044] FIG. 9C is a schematic plan view of a MIMO antenna with interleaved and coplanar horizontal radiating elements, according to an embodiment of the invention. [0045] FIG. 9D is a graph of Envelope Correlation Coefficient (ECC) for the MIMO antenna embodiments of FIGS. 9A-9C.

[0046] FIG. 10A is a schematic plan view of a MIMO antenna with offset arrays of horizontal radiating elements, according to an embodiment of the invention.

[0047] FIG. 10B is a perspective view of a MIMO antenna (e.g., 4x4 MIMO) with offset arrays of horizontal radiating elements, and a pair of offset vertically polarized radiating elements, according to an embodiment of the invention.

[0048] FIG. 11 A is a schematic plan view of a MIMO antenna with concentric arrays of horizontal radiating elements, according to an embodiment of the invention.

[0049] FIG. 11 B is a schematic plan view of a MIMO antenna with concentric arrays of horizontal radiating elements, according to an embodiment of the invention.

[0050] FIG. 11 C is a graph of Envelope Correlation Coefficient (ECC) for the MIMO antenna embodiments of FIGS. 11 A-11 B. [0051] FIG. 12A is a perspective view of a circular multi-input and multi-output (MIMO) antenna with artificial perfect magnetic conductor (PMC), according to an embodiment of the invention.

[0052] FIG. 12B is a plan view of the MIMO antenna of FIG. 12A.

[0053] FIG. 13A is a perspective view of a 4-element horizontally-polarized and circular antenna array, according to an embodiment of the invention.

[0054] FIG. 13B is a side view of the antenna array of FIG. 13A in combination with an artificial perfect magnetic conductor (PMC) and metal chassis, according to an embodiment of the invention.

DETAILED DESCRIPTION

[0055] The present invention now will be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being 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 reference numerals refer to like elements throughout.

[0056] It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention.

[0057] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. As used herein, the singular forms "a," "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprising", "including", "having" and variants thereof, when used in this specification, specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. In contrast, the term "consisting of" when used in this specification, specifies the stated features, steps, operations, elements, and/or components, and precludes additional features, steps, operations, elements and/or components.

[0058] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0059] Referring now to the perspective and plan views of FIGS. 5A-5B, a multi-input and multi-output (MIMO) antenna 100 (e.g., 4x MIMO) according to an embodiment of the invention is illustrated as including a pair of spaced-apart vertical monopole radiating elements/antennas 106 (va, vb) on a forward-facing surface 102a of a substrate 102, along with a first array 110a (ha) of horizontal dipole radiating elements 110, and a second array 110b (hb) of horizontal dipole radiating elements 110. As shown, the horizontal dipole radiating elements 110 are arranged as a closed-loop plurality of horizontal dipole radiating elements 110 on a rear-facing surface 102b of the substrate 102, which is attached to a supporting chassis 104, such as a ceiling-mounted chassis.

This chassis 104 may include horizontal and vertical feed signal generating circuitry (not shown) therein. Although not shown, alternative embodiments of the invention may also use dipole elements, which are fed by a corresponding tween line, and/or monopole radiating elements, such as monopole radiating elements located adjacent comers of a substrate (see, e.g., FIG. 6A).

[0060] In particular, the plurality of horizontal dipole radiating elements 110 within the first array 110a (i.e. , ha) are interleaved with the plurality of horizontal dipole radiating elements 110 within the second array 110b (i.e., hb), around a periphery of the substrate 102. More specifically, the horizontal dipole radiating elements 110 within the first array 110a are arranged as a first plurality of spaced-apart pairs of horizontal dipole radiating elements 110, and the horizontal dipole radiating elements 110 within the second array 110b are arranged as a second plurality of spaced-apart pairs of horizontal dipole radiating elements 110. Thus, as shown, the first and second arrays of horizontal dipole radiating elements 110a, 110b are interleaved so that each pair of horizontal dipole radiating elements 110 within the first array 110a is bordered on opposing sides by first and second pairs of horizontal dipole radiating elements 110 within the second array 110b, and vice versa. Advantageously, the number of radiating elements within each array is sufficient to preclude significant nulls in the azimuth plane, while still maintaining sufficient decoupling and spatial diversity between the first and second arrays 110a, 110b (i.e. , sustain sufficiently high MIMO decorrelation and MIMO throughput).

[0061] In addition, as shown by the embodiment of FIGS. 5A-5B, the closed-loop plurality of horizontal dipole radiating elements 110 is illustrated as a circular loop, which extends along an outermost periphery/rim 102c of the “circular” substrate 102. Based on this circular loop arrangement, each of the first plurality of spaced apart pairs of horizontal dipole radiating elements 110 in the first array 110a is generally aligned along a respective first arc (A) that traverses a corresponding first portion of the rim 102c, and each of the second plurality of spaced apart pairs of horizontal dipole radiating elements 110 in the second array 110b is generally aligned along a respective second arc (B) that traverses a corresponding second portion of the rim 102c.

Moreover, because the first and second arcs are concentric, and have the same radii of curvature in the illustrated embodiment, the first and second arcs lie along respective portions of a circle defined by the rim 102c.

[0062] Referring now FIGS. 5C-5D, which are enlarged views of portions of the antenna of FIGS. 5A-5B, each of the pair of radiating arms 112a associated with each of the horizontal dipole radiating elements 110 in the first and second arrays 110a, 110b is electrically coupled by a respective horizontal feed stalk 112b to edges (e.g., a periphery) of a circular-shaped ground plane 114 on a rear-facing surface 102b of the circular substrate 102. This substrate 102 may by a double-sided printed circuit board (PCB) in some embodiments of the invention; however, other embodiments of the invention (not shown), a multi-layer PCB may be provided with sufficient layers such that antenna elements may be printed on different layers and be separated both horizontally and vertically for better decoupling. In addition, the forward-facing surface 102a of the substrate 102 includes first and second feed signal routing networks 120a, 120b thereon. As shown in FIG. 5B, the first feed signal routing network 120a is configured to distribute a first input feed signal IN1 to each of the horizontal feed stalks 112b associated with each of the horizontal dipole radiating elements 110 within the first array 110a. Likewise, the second feed signal routing network 120b is configured to distribute a second input feed signal IN2 to each of the horizontal feed stalks 112b associated with each of the horizontal dipole radiating 110 elements within the second array 110b.

[0063] Moreover, in some embodiments of the invention, the first and second feed signal routing networks 120a, 120b may utilize power splitters 122 (or power dividers (e.g., Wilkinson power dividers)) and surface mount cross-over jumpers 124 to evenly distribute their respective input feed signals IN1 , IN2, with same magnitude and phase, to corresponding hook-shaped feed lines 116 on the forward-facing surface 102a, which extend opposite corresponding pairs of horizontal feed stalks 112b within the first and second arrays 110a, 110b of radiating elements 110 on the rear-facing surface 102b, as shown by FIGS. 5C-5D. Advantageously, because power may be equally split (and combined) for both TX and RX, a cross resistor may not be required; nonetheless, a Wilkinson power divider having equal power division may utilize a cross-resistor between its two branches. Each surface mount jumper 124 may also extend over (and be spaced apart from) an underlying pair of parallel metal strips 126, which are located directly on the forward-facing surface 102a as metal traces. As shown, these parallel metal strips 126 are electrically connected to corresponding rows of metallized through- substrate vias 128, which are electrically connected to the underlying ground plane 114 on the rear-facing surface 102b.

[0064] Next, as illustrated by FIG. 5E, the directivity patterns in the azimuth plane for the first array 110a of radiating elements of FIGS. 5A-5D, show: (i) a co-polarization radiation Ecp (at 3.95 GHz) having an average directivity over the xy-plane (<p = 0°-360°) equal to: Mean(Dir_cp) = -1.70 dB, and with a standard deviation of: Std(Dir_ cp) = 3.03 dB, and (ii) a cross-polarization radiation E0 having an average directivity equal to: Mean(Dir_ 0) = -20.73 dB. Thus, as compared to horizontal monopole radiation of FIG. 4, the first array 110a has a significantly higher mean and lower standard deviation associated with the co-polarization radiation Ecp; here, Ecp is the electric field strength whereas the corresponding directivity Dir_cp = F(Ecp)², where “F” is a linear function. [0065] Referring now to FIGS. 6A-6D, a multi-input and multi-output (MIMO) antenna 200 according to another embodiment of the invention is illustrated as including a planar substrate 202, such as a double-sided PCB having a forward-facing surface 202a and a rear facing surface 202b thereon, which is mounted on an underlying chassis 204. The substrate 202 is illustrated as a generally rectangular-shaped substrate having eight (8) sides, including four straight sides 202d, which are joined together by four arcuateshaped corner segments 202e. These arcuate-shaped corner segments 202e may be concentric in some embodiments of the invention, or the centers of the arcuate-shaped corner segments 202e may be spaced-apart from each other (i.e. , at four corners of a square).

[0066] The rear facing surface 202b of the substrate 202 includes a generally rectangular-shaped ground plane 214 thereon with rounded comers, which may align with a perimeter of the underlying chassis 204, as shown. In addition, sixteen (16) horizontal dipole radiating elements 210 having tilted feed stalks 212b may extend outwardly (at acute/obtuse angles) from the straight sides of the ground plane 214. These 16 dipole radiating elements 210 are illustrated as being grouped into four (4) arrays: ha1-ha4, hb1-hb4, hc1-hc4, and hd1-hd4. In particular, a first plurality of radiating elements hd4, ha1 , hb1 and hc1 are shown as extending outwardly from a first side of the ground plane 214, a second plurality of radiating elements hd1 , ha2, hb2 and hc2 are shown as extending outwardly from a second side of the ground plane 214, a third plurality of radiating elements hd2, ha3, hb3 and hc3 are shown as extending outwardly from a third side of the ground plane 214, and a fourth plurality of radiating elements hd3, ha4, hb4 and hc4 are shown as extending outwardly from a fourth side of the ground plane 214. Moreover, because the ground plane 214 is rectangular shaped, the radiating arms 212a of the “side” radiating elements (ha1 , hb1 ), (ha2, hb2), (ha3, hb3), and (ha4, hb4) extend closer to a center of the planar substrate 202 relative to the radiating arms 212a of the “corner” radiating elements (hc4, hd4), (hc1 , hd1 ), (hc2, hd2) and (hc3, hd3).

[0067] In addition, the outermost surfaces of the radiating arms 212a of side-by-side radiating elements hc4, hd4 are shown as being generally aligned with a first arcuateshaped corner segment 202e, the outermost surfaces of the radiating arms 212a of side-by-side radiating elements hc1 , hd1 are shown as being generally aligned with a second arcuate-shaped corner segment 202e, the outermost surfaces of the radiating arms 212a of side-by-side radiating elements hc2, hd2 are shown as being generally aligned with a third arcuate-shaped corner segment 202e, and the outermost surfaces of the radiating arms 212a of side-by-side radiating elements hc3, hd3 are shown as being generally aligned with a fourth arcuate-shaped corner segment 202e.

[0068] In contrast, the outermost surfaces of the radiating arms 212a of side-by-side radiating elements ha1 , hb1 are shown as being generally aligned with a first arc A1 having a first center C1 at an intersection between: (i) a radially-extending line passing from the first center through a center of the tilted feed stalk 212b associated with radiating element ha1 , and (ii) a radially-extending line passing from the first center through a center of the tilted feed stalk 212b associated with radiating element hb1. Likewise, the outermost surfaces of the radiating arms 212a of side-by-side radiating elements ha2, hb2 are shown as being generally aligned with a second arc A2 having a second center C2 at an intersection between: (i) a radially-extending line passing from the second center through a center of the tilted feed stalk 212b associated with radiating element ha2, and (ii) a radially-extending line passing from the second center through a center of the tilted feed stalk 212b associated with radiating element hb2. Next, the outermost surfaces of the radiating arms 212a of side-by-side radiating elements ha3, hb3 are shown as being generally aligned with a third arc A3 having a third center C3 at an intersection between: (i) a radially-extending line passing from the third center through a center of the tilted feed stalk 212b associated with radiating element ha3, and (ii) a radially-extending line passing from the third center through a center of the tilted feed stalk 212b associated with radiating element hb3. Finally, the outermost surfaces of the radiating arms 212a of side-by-side radiating elements ha4, hb4 are shown as being generally aligned with a fourth arc A4 having a fourth center C4 at an intersection between: (i) a radially-extending line passing from the fourth center through a center of the tilted feed stalk 212b associated with radiating element ha4, and (ii) a radially- extending line passing from the fourth center through a center of the tilted feed stalk 212b associated with radiating element hb4. The radii of curvature of the first through fourth arcs A1 -A4 may be smaller than the radii of curvature of the first through fourth arcuate-shaped corner segments 202e, in some embodiments of the invention. And, the first through fourth centers C1 , C2, C3 and C4 may overlap at a center of the substrate 202, in some embodiments of the invention.

[0069] As further shown by FIG. 6A, a pair of 180° spaced-apart vertical monopole radiating elements 206 (va, vb) may be provided, which extend forwardly of the forwardfacing surface 202a. Although not shown, these vertical monopole radiating elements 206 are electrically coupled through a pair of apertures in the ground plane 214 to underlying feed signal generating circuitry within the chassis 204. This pair of vertical monopole radiating elements 206 may be optional in some embodiments of the invention. Alternatively, a quad arrangement of vertical radiating elements (e.g., va, vb, vc, vd, not shown) may be provided in other embodiments of the invention to support larger Nx MIMO applications, such as a 8x MIMO; however, such embodiment may result in some degradation of omni-coverage for the H-pol antennas, which results from deep nulls within the azimuth patterns.

[0070] Referring now to FIGS. 6A-6H, a three-way comparison of azimuth (Az) performance will be made for the multi-input and multi-output (MIMO) antenna 200 having a chassis 204 with length (L) and width (W) dimensions of 180x180 mm², a height (H) of 60 mm, and a PCB substrate 202 with length (L) and width (W) dimensions of 224x224 mm², as shown by FIG. 6C. In a first example, as shown by FIGS. 6A-6B and 6D, the eight (8) radiating elements 210 in arrays ha1-ha4 and hc1-hc4 are combined, and a feed signal routing network (not shown) is provided to evenly distribute an input feed signal, with same magnitude and phase, to each of the hook-shaped feed lines 216 associated with arrays ha1-ha4 and hc1-hc4, which have different orientation (i.e. , tilting) and different spacing relative to a center of the substrate 202. Based on this configuration, the corresponding first beam pattern Az_BP1 and directivity patterns of FIGS. 6B and 6D, respectively, reveal: (i) a co-polarization radiation Ecp (at 3.95 GHz) having an average directivity over the xy-plane (<p = 0°-360°) equal to: Mean(Dir_cp) = - 2.41 dB, and with a standard deviation of: Std(Dir_ cp) = 2.51 dB, and (ii) a cross- polarization radiation E0 having an average directivity equal to: Mean(Dir_ 0) = -15.93 dB.

[0071] Next, in a second example, as shown by FIGS. 6E-6F, the eight (8) radiating elements 210 in arrays ha1-ha4 and hb1-hb4 are combined, and a feed signal routing network (not shown) is provided to evenly distribute an input feed signal, with same magnitude and phase, to each of the hook-shaped feed lines 216 associated with arrays ha1-ha4 and hb1-hb4, which have different orientation (i.e., tilting) but otherwise equivalent spacing relative to a center of the substrate 202. Based on this configuration, the corresponding second beam pattern Az_BP2 of FIG. 6F demonstrates an Az coverage that is essentially equivalent to the coverage demonstrated in FIG. 5E for the MIMO antenna 100 of FIGS. 5A-5D.

[0072] Finally, in a third example, as shown by FIGS. 6G-6H, the eight (8) radiating elements 210 in arrays hc1-hc4 and hd1-hd4 are combined, and a feed signal routing network (not shown) is provided to evenly distribute an input feed signal, with same magnitude and phase, to each of the hook-shaped feed lines 216 associated with arrays hc1-hc4 and hd1-hd4, which are located at the arcuate-shaped corner segments 202e of the substrate 202. Based on this configuration, the corresponding third beam pattern Az_BP3 of FIG. 6H demonstrates an Az coverage with somewhat more pronounced “ripples” relative to Az coverage demonstrated in the first beam pattern Az_BP1 of FIG. 6B and the second beam pattern Az_BP2 of FIG. 6F.

[0073] Referring now to FIGS. 7A-7B, a circular 4x MIMO antenna 300 is illustrated as including: (i) a pair of vertical monopole radiating elements va, vb, as described herein, (ii) a first array ha0-ha11 of twelve (12) horizontal dipole radiating elements 310a, which are evenly distributed around a circular ground plane 314, and (iii) a second array hbO- hb11 of 12 horizontal dipole radiating elements 310b, which are interleaved with the radiating elements 310a within the first array ha0-ha11 . As shown best by the plan view of FIG. 7A, both the first array ha0-ha11 of radiating elements 310a and the second array hb0-hb11 of radiating elements 310b are patterned in respective circular loops, which extend adjacent a periphery of a rear facing surface of a PCB substrate 302, which is mounted on an underlying cylindrically-shaped chassis. However, the radiating elements 310a in the first array ha0-ha11 have shorter feed stalks relative to the radiating elements 310b in the second array hbO-hb11 . Accordingly, the dipole radiating arms 312a of the radiating elements 310a within the first array ha0-ha11 are generally aligned to a circumference of a first circle, and the dipole radiating arms 312a of the radiating elements 310b within the second array hb0-hb11 are generally aligned to a circumference of a second circle having a larger diameter relative to the first circle. [0074] In addition, as shown by FIG. 7B, when the smaller first array ha0-ha11 of radiating elements 310a is uniformly activated in response to an evenly distributed input feed signal (not shown), a beam pattern Az_BP1’ having a generally circular outline including a repeating pattern of relative peaks (12) and nulls (12) demonstrates a substantially uniform co-polarization directivity in the azimuth plane for the MIMO antenna of FIG. 7A, which includes a PCB substrate having a diameter of 238 mm on an underlying cylindrical chassis having a height of 40 mm and a diameter of 203 mm. [0075] Referring now to FIGS. 7C-7D, a circular 4x MIMO antenna 300’ is illustrated as equivalent to the circular 4x MIMO antenna 300’ of FIG. 7A; however, the larger second array hb0-hb11 of radiating elements 310b is uniformly activated in response to an evenly distributed input feed signal (not shown). And, in FIG. 7D, a corresponding beam pattern Az_BP2’ having a generally circular outline including a repeating pattern of relative peaks (12) and nulls (12) is shown, which demonstrates a somewhat less uniform co-polarization directivity in the azimuth plane relative to the beam pattern of FIG. 7B.

[0076] Next, referring to FIGS. 7E-7F, a circular 4x MIMO antenna 300” is illustrated as equivalent to the circular 4x MIMO antenna 300’ of FIG. 7A; however, four sets of three radiating elements are uniformly activated in response to an evenly distributed input feed signal (not shown). These four sets include: (hbO, haO, hb1 ), (hb3, ha3, hb4), (hb6, ha6, hb7), and (hb9, ha9, hb10). And, in FIG. 7F, a corresponding beam pattern Az_BP3’ having a generally square outline with four comers demonstrates a significantly non uniform co-polarization directivity in the azimuth plane relative to the beam patterns of FIGS. 7B and 7D. [0077] Referring now to FIG. 8A, a schematic plan view of a circular MIMO antenna 400 is illustrated as including: (i) a first array of eight (8) horizontal radiating elements A1-A8 (e.g., monopole, dipole) of a first polarization, and (ii) a second array of eight (8) horizontal radiating elements B1-B8 (e.g., monopole, dipole) of a second polarization, which may be different from the first polarization. As shown, the radiating elements A1- A8 in the first array are arranged around a circle having a radius ra, and the radiating elements B1-B8 in the second array are arranged around a circle having a radius rb, where ra = rb. In addition, as shown schematically, each of the radiating elements A1- A8 in the first array is co-located with a corresponding radiating element B1-B8 in the second array. Thus, the pairs of co-located radiating elements (A1 , B1 ), (A2, B2), ... , (A8, B8) are spaced apart from each other along circular arcs spanning 45°.

[0078] In some embodiments, the radiating elements A1-A8 and B1-B8 may be patterned on a rear-facing surface of a PCB substrate, along with a generally circular ground plane. Vertical monopole radiating elements and feed signal routing, which may be similar to the corresponding elements in FIG. 5A, may also be provided on a forward-facing surface of the substrate. In addition to being co-located, the radiating elements A1-A8 and B1-B8 may be configured as coplanar elements using a single layer of patterned metallization and jumpers, if necessary, or may be independently patterned on slightly spaced-apart metallization layers within a multi-layer PCB substrate.

[0079] Referring now to FIG. 8B, a schematic plan view of another circular MIMO antenna 400’ is illustrated as including: (i) a first array of four (4) horizontal radiating elements A1 , A3, A5 and A7 of a first polarization, and (ii) a second array of four (4) horizontal radiating elements B1 , B3, B5 and B7 of a second polarization. As shown, the radiating elements in the first array are arranged around a circle having a radius ra, and the radiating elements in the second array are arranged around a circle having a radius rb, where ra = rb. In addition, as shown schematically, each of the radiating elements A1 , A3, A5 and A7 in the first array is co-located with a corresponding radiating element B1 , B3, B5 and B7 in the second array. Thus, the four pairs of radiating elements (A1 , B1), (A3, B3), ... , (A7, B7) are spaced apart from each other along circular arcs spanning 90°. [0080] Next, as shown by FIG. 8C, a schematic plan view of another circular MIMO antenna 400” is illustrated as including two horizontal radiating elements A1 and A5, and two horizontal radiating elements B1 and B5 of a second polarization. As with the embodiments of FIGS. 8A-8B, the radiating elements A1 , A5 are arranged around a circle having a radius ra, and the radiating elements B1 , B5 are arranged around a circle having a radius rb, where ra = rb. In addition, because the radiating elements A1 , A5 are co-located with the radiating elements B1 , B5, respectively, the co-located pairs of radiating elements (A1 , B1 ) and (A5, B5) are spaced apart from each other along circular arcs spanning 180°.

[0081] Referring now to FIG. 8D, a comparative graph of Envelope Correlation Coefficient (ECC) for the MIMO antenna embodiments of FIGS. 8A-8C, is shown. As will be understood by those skilled in the art, ECC specifies a degree of independence between two antennas’ radiating patterns, and takes into account radiation pattern shape and polarization, and even takes into account the relative phase of fields between the two antennas. Thus, if one antenna is completely horizontally polarized, and the other antenna is completely vertically polarized, the two antennas would have a correlation coefficient of zero.

[0082] Moreover, in FIG. 8D, ECC is plotted as a function of

where corresponds to an angle between the polarization of the radiating elements An and a tangent line of the array circle, where ra = rb = 0.5A and A corresponds to a wavelength of a center frequency associated with a frequency band of the radiating elements; likewise, - corresponds to an angle between the polarization of the radiating elements Bn and the tangent line. As shown by FIG. 8D, each curve has a different ECC=0 point in a range from about = 40° to about = 60°. In addition, based on simulations of directivity patterns in the elevation and azimuth planes for the MIMO antenna 400 of FIG. 8A, it has been demonstrated that relatively high ECC and high pattern circularity may occur simultaneously, along with high directivity (elevation plane), but higher MIMO benefit occurs with lower ECC.

[0083] Referring now to FIG. 9A, a schematic plan view of a circular MIMO antenna 500 is illustrated as including: (i) a first array of eight (8) horizontal radiating elements A1-A8 of a first polarization, and (ii) a second array of eight (8) horizontal radiating elements B1-B8 of a second polarization. As shown, the radiating elements A1-A8 in the first array are arranged around a circle having a radius ra, and the radiating elements B1-B8 in the second array are arranged around a “concentric” circle having a radius rb, where ra = rb. In addition, as shown schematically, the radiating elements A1-A8 in the first array are interleaved with the radiating element B1-B8 in the second array. Thus, each of the sixteen radiating elements A1-A8 and B1-B8 are equally spaced from each other along circular arcs spanning 22.5°. Similarly, as shown by FIG. 9B, a circular MIMO antenna 500’ is illustrated as including a first array of four horizontal radiating elements A1-A4 of a first polarization, and a second “concentric” array of four horizontal radiating elements B1-B4 of the second polarization, such that the eight radiating elements A1-A4 and B1-B4 are equally spaced from each other along circular arcs spanning 45°. Finally, as shown by FIG. 9C, a circular MIMO antenna 500” is illustrated as including a first array of two horizontal radiating elements A1-A2, and a second “concentric” array of two horizontal radiating elements B1-B2, such that the four radiating elements A1-A2 and B1-B2 are equally spaced from each other along circular arcs spanning 90°.

[0084] Next, in FIG. 9D, ECC for the embodiments of FIGS. 9A-9C is plotted as a function where corresponds to an angle between the polarization of the radiating elements An and a tangent line of the array circle, where ra = rb = 0.5A, and - corresponds to an angle between the polarization of the radiating elements Bn and the tangent line. As shown by FIG. 9D, ECOO for the embodiments of FIGS. 9A-9B when is in a range from about = 55° to about = 70°, whereas a relatively low ECC of about 0.1 is present in a range from = 0° to = 90° for the embodiment of FIG. 9C. [0085] Referring now to FIGS. 10A-10B, a MIMO antenna 600 is illustrated that achieves higher MIMO benefit (e.g., lower ECC) while maintaining high pattern circularity (e.g., broader and smoother azimuth plane coverage), by using a pair of 8- element circular arrays that are “offset” relative to each other by an offset distance “d”, as shown by FIG. 10A. Although not wishing to be bound by any theory, a typical specification for a MIMO may include having an ECC of less than about 0.1 , which based on simulation may suggest an offset distance of d > 0.3A. Moreover, by combining horizontal polarized arrays with vertical antennas (va, vb), as shown by FIG. 10B, higher order MIMO systems may be achieved.

[0086] Finally, in addition to offset distance “d”, circular size differences between the two horizontally polarized arrays may also support lower ECC. Thus, as shown by FIG.

11 A, a circular MIMO antenna 700 may include a first array of eight (8) horizontal radiating elements A1-A8 of a first polarization, and a second array of eight (8) horizontal radiating elements B1-B8 of a second polarization. As shown, the radiating elements A1-A8 in the first array are arranged around a circle having a radius ra, and the radiating elements B1-B8 in the second array are arranged around a “concentric” circle having a radius rb, where ra < rb. Likewise, with respect to FIG. 11 B, an alternative circular MIMO antenna 700’ may include a first array of only four (4) horizontal radiating elements A1-A4 of the first polarization, which is concentric within a second array of eight (8) horizontal radiating elements B1-B8 of the second polarization. And, as shown by FIG. 11 C, ECC is plotted for the MIMO antennas of FIGS. 11 A-11 B, for the case where = 0°, d=0, rb = 0.5A and ra varies from 0.1 A to 0.5A.

[0087] Referring now to FIGS. 12A-12B, a circular 4x MIMO antenna 1200 is illustrated as including: (i) a pair of vertical monopole radiating elements va, vb, which are supported on an underlying chassis 1210 (ii) a first array of eight (8) horizontal dipole radiating elements ha, which are evenly distributed around a rear-facing surface of a substrate 1210, such as a circular printed circuit board (PCB), and (iii) a second array of eight (8) horizontal dipole radiating elements hb, which are interleaved with the radiating elements ha in the first array. Alternatively, the radiating elements ha may be paired and the radiating elements hb may be paired in a manner similar to the pairing shown in FIG. 5A. Furthermore, variations on the dipole radiating elements ha, hb may be configured to utilize a rear-facing ground plane (e.g., circular shaped), as shown by reference number 114 in FIGS. 5A-5C.

[0088] As described hereinabove with respect to FIGS. 5A-5D, a forward-facing surface of the circular PCB 1210 may include first and second feed signal routing networks (not shown) for the interleaved radiating elements ha, hb. In some embodiments of the invention, these feed signal routing networks may be fed by coaxial cables (not shown) provided between the chassis and a center of the circular dipole array, and may utilize microstrip power dividers on the forward-facing surface. Alternatively, the coaxial cables may be replaced by board-to-board connectors, such as MMBX Female - bullet - Female connectors.

[0089] Also included is an artificial perfect magnetic conductor (PMC) 1220, which extends in parallel with, and at a first distance from, a rear-facing surface of the circular PCB 1210. As shown, the artificial PMC 1220 is configured as a two-dimensional array of rectangular-shaped (and ungrounded) metal patches 1222 on a forward-facing surface 1224a of a substrate 1224 that is spaced at the first distance from the rearfacing surface of the circular PCB 1210, and at a second distance from a forward-facing surface of an underlying chassis 1230. However, according to alternative embodiments of the invention, the substrate 1224 may include a ground plane on a rear-facing surface thereof, which is electrically coupled (e.g., through conductive vias) to the metal patches 1222. The metal patches 1222 may also shapes different than rectangular shapes.

[0090] As further shown by FIGS. 12A-12B, the pair of vertical monopole radiating elements va, vb extend forwardly from the chassis 1230 and through openings 1226 within the artificial PMC 1220. As will be understood by those skilled in the art, the artificial PMC 1220 operates as a textured ground plane that presents a high impedance to incident waves and nearby horizontal antennas over a prescribed frequency range. In addition, the artificial PMC 1220 generally suppresses the propagation of both transverse electric (TE) and transverse magnetic (TM) surface waves, thus increasing the radiation from a horizontal antenna in the azimuth plane on the horizon.

[0091] Referring now to FIGS. 13A-13B, an antenna array 1300 is illustrated as including: (i) a circular array of four (4) horizontal dipole radiating elements 1310 having arcuate-shaped radiating arms 1312 on a rear-facing surface 1320b of a substrate 1320, such as a circular dual-sided PCB, and (ii) a plurality of hook-shaped baluns 1322 on a forward-facing surface 1320a of the substrate 1320. As shown, the plurality of hook-shaped baluns 1322 may be fed by a 1 :4 radial power divider 1324 at a center of the forward-facing surface 1320a. In addition, an MMBX connector 1326 may be provided, such that an output port thereof is electrically coupled to the 1 :4 radial power divider 1324. As shown best by FIG. 13B (and also shown similarly by FIG. 12B), an artificial PMC 1330 may be provided as a two-dimensional array of rectangular-shaped metal patches 1332 (e.g., grounded or ungrounded patches) on a forward-facing surface of an intermediate substrate 1334, which is spaced at the first distance “d1” (e.g., 11.5 mm) from the rear-facing surface of the PCB substrate 1320, and supported vertically at a second distance “d2” (e.g., 6 mm) from an underlying chassis 1340, and support substrate 1342, which supports the pair of vertical monopole radiating elements va, vb. Although not wishing to be bound by any theory, variations in d1 and d2 are possible for overall height reduction with tradeoffs in impedance and radiation pattern frequency bandwidth.

[0092] In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.

Claims

THAT WHICH IS CLAIMED IS:

1 . A multi-input and multi-output (MIMO) antenna, comprising: a pair of vertical monopole radiating elements on a forward-facing surface of a substrate; and first and second arrays of horizontal dipole radiating elements arranged as a closed-loop plurality of horizontal dipole radiating elements on the substrate.

2. The MIMO antenna of Claim 1 , wherein the plurality of horizontal dipole radiating elements within the first array are interleaved with the plurality of horizontal dipole radiating elements within the second array, around a periphery of the substrate.

3. The MIMO antenna of Claim 2, wherein the first array of horizontal dipole radiating elements are arranged as a first plurality of spaced-apart pairs of horizontal dipole radiating elements, and the second array of horizontal dipole radiating elements are arranged as a second plurality of spaced-apart pairs of horizontal dipole radiating elements; and wherein first and second arrays of horizontal dipole radiating elements are interleaved so that each pair of horizontal dipole radiating elements in the first array is bordered on opposing sides by first and second pairs of horizontal dipole radiating elements in the second array, and vice versa.

4. The antenna of Claim 3, wherein each of the first plurality of spaced apart pairs of horizontal dipole radiating elements are generally aligned along respective first arcs, which are concentric; and wherein each of the second plurality of spaced apart pairs of horizontal dipole radiating elements are generally aligned along respective second arcs, which are concentric.

5. The antenna of Claim 4, wherein the first and second arcs are concentric.

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6. The antenna of Claim 5, wherein the first arcs have an equivalent first radius of curvature and the second arcs have an equivalent second radius of curvature, which is equivalent to the first radius of curvature.

7. The antenna of Claim 2, wherein radiating arms of the plurality of horizontal dipole radiating elements within the first array are generally aligned along respective first arcs, which are concentric; wherein radiating arms of the plurality of horizontal dipole radiating elements within the second array are generally aligned along respective second arcs, which are concentric; and wherein a radius of curvature of the first arcs is less than a radius of curvature of the second arcs.

8. The antenna of Claim 2, wherein at least some of the horizontal dipole radiating elements in the first array are arranged closer to a center of the forward-facing surface of the substrate relative to other ones of the horizontal dipole radiating elements in the first array.

9. The antenna of Claim 8, wherein a first half of the horizontal dipole radiating elements in the first array are arranged closer to the center of the forward-facing surface of the substrate relative to a second half of the horizontal dipole radiating elements in the first array; and wherein a first half of the horizontal dipole radiating elements in the second array are arranged closer to the center of the forward-facing surface of the substrate relative to a second half of the horizontal dipole radiating elements in the second array.

10. The antenna of Claim 9, wherein each of the first half of horizontal dipole radiating elements in the first array extends immediately adjacent a corresponding one of the first half of horizontal dipole radiating elements in the second array.

11 . The antenna of Claim 1 , wherein each radiating arm of each of the horizontal dipole radiating elements in the first and second arrays is electrically coupled by a respective horizontal feed stalk to edges of a ground plane on a rear-facing surface of the substrate.

12. The antenna of Claim 11 , wherein the ground plane is circular-shaped or generally rectangular-shaped.

13. The antenna of Claim 11 , further comprising a first feed signal routing network on the forward-facing surface of the substrate.

14. The antenna of Claim 13, wherein the first feed signal routing network is configured to distribute a first input feed signal to each of the horizontal feed stalks associated with the first array of horizontal dipole radiating elements.

15. The antenna of Claim 1 , further comprising a first feed signal routing network on the forward-facing surface of the substrate, said first feed signal routing network configured to distribute a first input feed signal to a plurality of horizontal dipole radiating elements within the first array.

16. The antenna of claim 15, wherein the substrate is a double-sided printed circuit board (PCB).

17. The antenna of Claim 15, wherein the first feed signal routing network uses power splitters and/or power dividers, and surface mount jumpers to evenly distribute a first input feed signal to a plurality of horizontal dipole radiating elements within the first array.

18. The antenna of Claim 17, wherein each radiating arm of each of the horizontal dipole radiating elements in the first and second arrays is electrically coupled by a respective horizontal feed stalk to edges of a ground plane on a rear-facing surface of the substrate.

19. The antenna of Claim 18, wherein the first feed signal routing network terminates at respective hook-shaped feed lines associated with each of the horizontal dipole radiating elements within the first array; and wherein each hook-shaped feed line extends opposite a pair of horizontal feed stalks associated with each of the horizontal dipole radiating elements within the first array.

20. The antenna of Claim 18, wherein each horizontal feed stalk associated with the radiating elements in the first and second arrays intersects a corresponding edge of the ground plane at an acute angle or an obtuse angle.

21. A multi-input and multi-output (MIMO) antenna, comprising: a substrate having multiple interleaved arrays of horizontal radiating elements thereon, which are arranged as a closed-loop plurality of horizontal radiating elements around a periphery of the substrate.

22. The antenna of Claim 21 , further comprising multiple feed signal routing networks on a first surface of the substrate, and a ground plane on a second surface of the substrate.

23. The antenna of Claim 22, wherein the ground plane is electrically coupled to the closed-loop plurality of horizontal radiating elements.

24. The antenna of Claim 23, wherein each of the plurality of horizontal radiating elements comprises a pair of horizontal feed stalks therein; and wherein the ground plane is electrically coupled to a base of each of the horizontal feed stalks.

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25. The antenna of Claim 22, wherein the first and second surfaces of the substrate are forward and rear-facing surfaces of the substrate, respectively.

26. The antenna of Claim 25, wherein the substrate is a double-sided printed circuit board (PCB).

27. The antenna of Claim 25, further comprising a pair of vertical monopole radiating elements, which are spaced apart on the first surface of the substrate.

28. The antenna of Claim 27, wherein the multiple feed signal routing networks include a first feed signal routing network, which is configured to distribute a first input feed signal to a first plurality of horizontal radiating elements within the closed-loop.

29. The antenna of Claim 28, wherein the multiple feed signal routing networks include a second feed signal routing network, which is configured to distribute a second input feed signal to a second plurality of horizontal radiating elements within the closed- loop.

30. The antenna of Clam 28, wherein the first feed signal routing network uses power splitters and/or dividers, and surface mount jumpers to evenly distribute the first input feed signal to the first plurality of horizontal radiating elements within the closed- loop.

31 . The antenna of Claim 21 , wherein the closed-loop plurality of horizontal radiating elements are arranged in a circle around the periphery of the substrate.

32. The antenna of Claim 22, wherein at least some of the horizontal radiating elements within the closed-loop are arranged closer to a center of the second surface of the substrate relative to other ones of the horizontal radiating elements within the closed-loop.

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33. The antenna of Claim 22, wherein half of the horizontal radiating elements within the closed-loop are arranged closer to a center of the second surface of the substrate relative to another half of the horizontal radiating elements within the closed- loop.

34. A MIMO antenna system, comprising: a substrate having at least two interleaved arrays of horizontal radiating elements thereon, which are arranged as a closed-loop plurality of horizontal radiating elements around a periphery of the substrate having a ground plane on a surface thereof, said ground plane having at least one straight edge, and said closed-loop plurality of horizontal radiating elements comprising at least one radiating element within the first array and at least one radiating element within the second array that are electrically connected to the straight edge.

35. The antenna system of Claim 34, wherein a feed stalk associated with the at least one radiating element within the first array intersects the straight edge at an acute/obtuse angle.

36. The antenna system of Claim 35, wherein the horizontal radiating elements are dipole radiating elements.

37. The antenna system of Claim 34, wherein at least some of the radiating elements within the first array extend closer to a center of the substrate relative to other radiating elements within the first array.

38. The antenna system of Claim 34, further comprising at least one pair of vertical radiating elements on a forward-facing surface of the substrate.

39. The antenna system of Claim 38, wherein the at least two arrays of horizontal radiating elements are patterned on a rear-facing surface of the substrate.

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40. A multi-input and multi-output (MIMO) antenna, comprising: first and second arrays of horizontal radiating elements, which are each arranged as a closed-loop plurality of horizontal radiating elements on the substrate.

41 . The MIMO antenna of Claim 40, wherein each of the horizontal radiating elements in the first array is co-located with a corresponding horizontal radiating element in the second array.

42. The MIMO antenna of Claim 40, wherein the first and second arrays of horizontal radiating elements are respective first and second circular arrays of horizontal radiating elements.

43. The MIMO antenna of Claim 42, wherein a radius of curvature of the first circular array of horizontal radiating elements is less than a radius of curvature of the second circular array of horizontal radiating elements.

44. The MIMO antenna of Claim 41 , wherein each of the horizontal radiating elements in the first array is substantially coplanar with a corresponding horizontal radiating element in the second array.

45. The MIMO antenna of Claim 40, wherein the horizontal radiating elements are dipole radiating elements.

46. The MIMO antenna of Claim 40, wherein the horizontal radiating elements in the first array have different polarizations relative to the horizontal radiating elements in the second array.

47. The MIMO antenna of Claim 40, wherein the horizontal radiating elements in the first array are interleaved with the horizontal radiating elements in the second array.

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48. The MIMO antenna of Claim 42, wherein a center of the first circular array of horizontal radiating elements is laterally offset relative to a center of the second circular array of horizontal radiating elements.

49. The MIMO antenna of Claim 48, further comprising a first vertical monopole radiating element aligned with a center of the first circular array of horizontal radiating elements, and a second vertical monopole radiating element aligned with a center of the second circular array of horizontal radiating elements.

50. An antenna array, comprising: an array of horizontal dipole radiating elements on a substrate; and an artificial perfect magnetic conductor (PMC) extending at a first distance from a rear-facing surface of the substrate.

51 . The antenna array of Claim 50, wherein the substrate comprises a doublesided printed circuit board (PCB); and wherein dipole arms of the horizontal dipole radiating elements are patterned on the rear-facing surface of the PCB.

52. The antenna array of Claim 51 , further comprising a plurality of hook-shaped baluns on a forward-facing surface of the PCB.

53. The antenna array of Claim 52, wherein the plurality of hook-shaped baluns are fed by a 1 :4 radial power divider provided on the forward-facing surface of the PCB.

54. The antenna array of Claim 53, further comprising an MMBX connector having an output port electrically coupled to the 1 :4 radial power divider.

55. The antenna array of Claim 50, wherein the artificial PMC comprises a two- dimensional array of rectangular-shaped metal patches.

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56. The antenna array of Claim 51 , wherein the artificial PMC comprises a two- dimensional array of rectangular-shaped metal patches on a forward-facing surface of a substrate that is spaced at the first distance from the rear-facing surface of the PCB.

57. A multi-input and multi-output (MIMO) antenna, comprising: first and second arrays of horizontal dipole radiating elements arranged as a closed-loop plurality of horizontal dipole radiating elements on a substrate; and an artificial perfect magnetic conductor (PMC) extending at a first distance from a rear-facing surface of the substrate.

58. The antenna array of Claim 57, wherein the substrate comprises a doublesided printed circuit board (PCB); and wherein dipole arms of the first and second arrays of horizontal dipole radiating elements are patterned on the rear-facing surface of the PCB.

59. The antenna array of Claim 58, wherein the artificial PMC comprises a two- dimensional array of rectangular-shaped metal patches on a forward-facing surface of a substrate that is spaced at the first distance from the rear-facing surface of the PCB.

60. The antenna array of Claim 59, further comprising a pair of vertical monopole radiating elements extending through openings within the artificial PMC.

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