Stripline parallel-series-fed proximity-coupled cavity backed patch antenna array

[0001] This invention generally relates to antennas, and more particularly to planar antenna arrays.

[0002] In the provision of wireless communication services within a cellular network, individual geographic areas or “cells” are defined and serviced by base stations. A base station typically has a cellular tower and utilizes RF antennas that communicate with wireless devices, such as cellular phones and pagers. The base stations are linked with other facilities of the service provider, such as a switching or central office, for handling and processing the wireless communication traffic.

[0003] A base station may be coupled to a processing facility through cables or wires, referred to as land lines, or alternatively, the signals may be transmitted or backhauled through microwave backhaul antennas, also located on the cellular tower and at the facility. Backhauls may be used in situations where land lines are unavailable or where a service provider faces an uncooperative local carrier and wants to ensure independent control of the circuit. In such a scenario, the backhaul may be referred to as a point-to-point backhaul, referencing the base station and the processing facility as points.

[0004] Point-to-point backhauls, are currently being deployed in the unlicensed spread spectrum bands, (e.g. Industrial, Scientific, and Medical (ISM) band covering 902-928 MHz, Unlicensed National Information Infrastructure band (U-NII) at 5.15-5.25 GHz, 5.25-5.35 GHz, and 5.725-5.825 GHz, etc.), to avoid the cost and time delays associated with installation in licensed frequency bands. One type of antenna that may be used for point-to-point backhauls utilizes a parabolic dish that is mounted to a tower, a wall, a building or in another location, and aimed at the other point in the backhaul. Parabolic dishes are sometimes unsightly and spoil the aesthetic appearance of the location where they are mounted.

[0005] Another type of antenna that may be used for point-to-point backhauls is a planar antenna array. Planar antenna arrays may also be mounted to a tower, a wall or a building, with the antenna being electrically pointed, i.e., via beamsteering, at the other point in the backhaul. Planar antenna arrays are generally thought of as more aesthetically appealing than parabolic dishes. Moreover, beamsteering makes planar antenna arrays more desirable in reconfiguring a cellular network. However, planar antenna arrays generally suffer from a variety of limitations.

[0006] For instance, planar antennas arrays tend to be constructed using arrays of patch radiating elements. In order to form these elements and ease manufacturing, planar antennas may be constructed using printed circuit boards. However, these boards often utilize multiple layer construction techniques in order to form the elements and the feed networks used therewith. Such construction increases the cost of such boards.

[0007] Moreover, planar antennas constructed using arrays of patch radiating elements formed using multiple layer circuit boards typically use corporate feed networks for coupling the elements in the arrays. Such corporate feed networks are often in the form of microstrip or twin-lead feed lines deposited on one or more layers of a circuit board. Such corporate feed networks typically have high losses, while such microstrip or twin-lead feed lines typically result in poor cross-polarized performance of an antenna.

[0008] In addition, the use of multiple layer circuit boards may economically and/or practically limit the size of the antenna. For example, current production capabilities of circuit board suppliers, along with the production costs associated with constructing a circuit board larger than currently available, limit the size of multiple layer circuit boards. Further, techniques of coupling two or more circuit boards together, thereby realizing a larger circuit board, are largely thwarted as interconnection of multiple conductive layers in each board tends to be impractical. Due to these economic and practical limitations in the size of circuit boards available, planar antennas constructed using such circuit boards may be limited in aperture size, i.e., the distance between the outer two most arrays of elements in an antenna, which determines in part the ability to electrically point the antenna.

[0009] Thus, these limitations typically associated with planar antennas may reduce antenna performance, efficiency and increase amplification requirements, and may limit the ability to electrically point such an antenna.

[0010] Therefore, a need exists for a low cost, low loss, large aperture planar antenna having an improved front-to-back ratio and cross-polarized performance with reduced susceptibility to other sources of radiation for applications such as a point-to-point microwave backhaul.

[0017] The present invention provides a stripline parallel-series fed proximity-coupled cavity backed patch antenna array. By using a two dimensional stripline feed for improved isolation and cross-polarization for coupling proximity-coupled cavity backed microstrip patch elements, a large aperture antenna is provided using one or more multi-layer substrates. Such an antenna allows the use of adaptive beamforming for beamsteering and/or null forming thereby reducing susceptibility to other sources of radiation for applications such as a point-to-point microwave backhaul.

[0018] Referring initially to FIG. 1, there is shown an exemplary stripline parallel-series fed proximity coupled cavity backed patch antenna array 10 for purposes of explaining the present invention. Antenna array 10 may be configured to provide a point-to-point backhaul in one of the unlicensed spread spectrum bands referred to hereinbefore. As will be appreciated by those skilled in the art, other embodiments of the present invention may be configured for other applications besides a point-to-point backhaul. Moreover, embodiments of the present invention may be configured for operation in either other unlicensed or licensed frequency bands.

[0019] Antenna array 10 comprises a plurality of multi-layer substrates 12a-d and a plurality of antenna elements 14 formed by the multi-layer substrates 12a-d. The antenna elements 14 may be proximity coupled cavity backed patch elements as illustrated.

[0020] The antenna elements 14 may be formed in a series of columns 16, to allow beamsteering and/or null forming, and rows 18. Each multi-layer substrate 12a-d in FIG. 1 includes twenty-one columns 16 containing twenty-one rows 18; thus, antenna array 10 comprises 42 columns and 42 rows. However, those skilled in the art will readily appreciate that any number of columns and rows may be used without departing from the spirit of the present invention. Moreover, an antenna array consistent with the present invention need not constitute rows per se.

[0021] Each multi-layer substrate 12a-d is advantageously within current production capabilities of circuit board manufactures. The use of multi-layer substrates 12a-d facilitates an antenna of larger physical dimensions without incurring the costs associated with the production of a larger circuit board. However, it will be appreciated that as larger circuit boards become more economically viable in the future, the principles of the present invention apply equally to those larger circuit boards.

[0022] Thus, those skilled in the art will appreciate that embodiments of the present invention may use any number of multi-layer substrates as desired for economical and/or practical or other reasons. Further, the present invention need not constitute multiple substrates. Rather, embodiments of the present invention may use a single substrate should such a single substrate be desirable. Antenna array 10 merely uses four substrates 12a-d by way of example.

[0023] The larger dimensions of array 10, facilitates a larger aperture size 20, defined by the distance across the series of columns 16. As will be readily appreciated by those skilled in the art, a larger aperture 20 increases beamsteering ability, thereby increasing the flexibility in mounting the antenna array 10.

[0024] Each multi-layer substrate 12a-d is homogenous and mirrored in construction about the inner most edges of the substrates 12a-d, both horizontally and vertically, with respect to the other substrates 12a-d. Thus, for ease of explanation, FIGS. 2 and 3 refer to a cross section 22 and a portion 44 of multi-layer substrate 12a, respectively, whereas FIG. 4 illustrates an inner conductive layer 28 of multi-layer substrate 12b. In certain circumstances where differences in the multi-layer substrates further illustrate the principles of the present invention, those differences will be described in more detail, such as in FIG. 5.

[0025] Referring now to FIG. 2, a cross-section 22 through line 2—2 of multi-layer substrate 12a in antenna array 10 is illustrated. Cross-section 22 of multi-layer substrate 12a typifies the construction of multi-layer substrates 12a-d as, again, the multi-layer substrates 12a-d are homogeneous. Cross-section 22 is taken through an antenna element 14 for purposes of further illustrating the formation of an antenna element 14.

[0026] Multi-layer substrate 12a comprises a top and bottom ground plane 24, 26 and an inner conductive layer 28, spaced by dielectric materials 30, 30′ using techniques well know to those skilled in the art. Cut, etched or otherwise formed out of the top ground plane 24 is a radiating patch or patch 34. Multi-layer substrate 12a forms antenna element 14 by the element 14 including vias or plated through holes 32 connecting the top and bottom ground planes 24, 26 around a perimeter 36 (shown in FIG. 3). The plated through holes 32 are spaced relative to one another so that they electromagnetically form a cavity 38, below radiating patch 34, at the operating frequency of the antenna element 14. Those skilled in the art will appreciate that the width of the wall of plated through holes 30 may be made less than half a guide or stub 42 wavelength thereby eliminating propagation of real power from the cavity 38 due to waveguide modes.

[0027] The inner conductive layer 28 includes waveguide or stub 42 (shown in more detail in FIG. 3) and a distribution trace 40 (shown in more detail in FIG. 4). Stub 42 is located under patch 34 so that radiation from the stub 42 is contained within the cavity 38 and reradiated by the patch 34. Such an arrangement improves the front-to-back ratio performance of antenna array 10.

[0028] Referring now to FIG. 3, a top view 44 of a portion of multi-layer substrate 12a forming a proximity coupled cavity backed patch element 14 used in the antenna array 10 of FIG. 1 is shown. Element 14 includes plated through holes 32 connecting the top and ground planes 24, 26 around the perimeter 36 of the element 14 forming a cavity 38, as described in conjunction with FIG. 2. In FIG. 3, the patch 34 and top layer of dielectric material 30, both of which were shown in FIG. 2, have been removed to further illustrate stub 42. Stub 42 may advantageously be a dual three-quarter wavelength stub to achieve greater frequency variation. A more thorough description of such an antenna element may be found in “An Enhanced Bandwidth Design Technique for Electromagnetically Coupled Microstrip Antennas” by Sean M. Duffy, IEEE Transactions on Antennas and Propagation, Vol. 48, No. 2, February 2000, which is incorporated herein by reference in its entirety.

[0029] Referring to FIG. 4, a diagram of an exemplary distribution trace 40 including a coupler 56 extending along the inner conductive layer 28 of the multi-layer substrate 12b shown in FIG. 1 is illustrated. Portions of antenna elements 14, such as patches 34 have been included for additional reference thereby covering stubs 42 (shown in FIGS. 2 and 3). Distribution trace 40 is a tapered trace, the width of which is readily varied by those skilled in the art to effectuate parameters such as impedance, power, phase, etc. of an electrical signal carried by the trace 40. Distribution trace 40 also includes a feed connection 52. Distribution trace 40 may be referred to as a “stripline” by virtue of being located between two ground planes 24, 26 (shown in FIG. 2).

[0030] As illustrated, distribution trace 40 includes a uniform power distribution portion 48 and a tapered power distribution portion 50 for coupling radiating elements 14 within a column 16. Uniform and tapered power distribution to radiating elements 14 within the sections 48, 50 is accomplished through varying the width of the trace 40 as will be readily understood by those skilled in the art. Due to varying the width of the trace 40 in portions 48, 50, the power received or transmitted by the elements 14 in those sections 48, 50 is apportioned as desired. As such, those elements 14 in the uniform power distribution portion 48 may be referred to as connected in “parallel”, whereas those elements in the tapered power distribution portion may be referred to as being connected in “series”. Thus, distribution trace 40 may be referred to as a stripline parallel-series network that feeds proximity coupled cavity backed patch elements 14 in antenna array 10.

[0031] Advantageously extending along the inner conductive layer 28 of the multi-layer substrate 12b is a coupler 46 in the form of a trace 56. Coupler 46 includes a coupling connection 54. Coupler 56 may be optionally terminated with a load formed in trace 56, as indicated at reference numeral 58. Coupler 46 is formed by locating trace 56 proximate distribution trace 40 and adjacent a column 16. Coupling connection 54 allows a signal applied to the coupler 46 to vary, e.g. amplitude and/or phase, a signal applied through distribution trace 40 to a respective column 16. Thus, coupler 46 may be configured for beamforming, beamsteering and/or null forming antenna array 10. Those skilled in the art will readily appreciate that beamforming, beamsteering and/or null forming may be applied to any number or all of the columns 16 in antenna array 10, as desired.

[0032] Referring to FIG. 5, a diagram showing the assembly of the antenna array 10 of FIG. 1 is illustrated. In FIG. 5, multi-layer substrates 12a-d are shown from the side opposite that shown in FIG. 1, viewing bottom ground plane 26 as seen in FIG. 2. Areas in the bottom ground plane 26 have been etched away to facilitate feed connections 52 and coupling connections 54 formed in the inner conductive layer 28 shown in FIG. 4. For purposes of explanation feed connections 52 for all four multi-layer substrates 12a-d are shown, whereas coupling connections for only the outer most four columns 16 of multi-layer substrates 12a and 12d are shown.

[0033] As illustrated in FIG. 5, circuit boards 64, 66 are used for connections 52, 54, respectively. The circuit boards function to gather connections 52, 54 to reduce the number of cables that are needed for connection to antenna array 10.

[0034] Circuit board 64 comprises a feed combiner 68 that connects to the feed connections 52 of each distribution trace 40 of each multi-layer substrate 12a-d and includes a main feed 60 for the antenna array 10. Circuit board 66 comprises coupling combiners 70 that connect couplers, within a respectively column 16, on multi-layer substrates 12a, 12d and provides column connections 70 for beamforming, beamsteering and/or null forming. Those skilled in the art will appreciate that other manners of gathering connections 52, 54 to reduce the number of cables that are needed for connection to antenna array may be used as desired.

[0035] By virtue of the foregoing, there is thus provided a low cost, low loss, large aperture planar antenna having an improved front-to-back ratio and cross-polarized performance with reduced susceptibility to other sources of radiation for applications such as a point-to-point microwave backhaul.

[0036] While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of applicant's general inventive concept.