The present invention relates to antenna arrays, such as for example unit cell antennas disposed over a common substrate/ground plane such that energy propagation along that substrate/ground plane might cause the antennas to mutually couple in the transmit and/or receive modes absent design considerations. Such antenna arrays may be disposed in satellite or terrestrial network elements and handheld portable transceivers that communicate with those network elements.
Particularly in satellites and base transceiver stations of a terrestrial mobile communications network, but also increasingly in handheld portable devices themselves, multiple antenna radiator elements for communicating over different frequency bandwidths are used. These devices often communicate over disparate frequency bands simultaneously. To conserve space and weight, multiple antennas are sometimes deployed in an organized array of like antenna radiator elements.
Typically, base station antennas are re-configurable in order to adapt to different environments. Re-configurable antennas can save operators and manufacturers substantial amounts of money in smaller inventory requirements. Normally, a large set of antennas that have different beamwidths and gain values is required. A re-configurable antenna can be set either manually prior to mounting, or electrically while in the mast. Smart antennas or adaptive antennas have even more requirements, since they are required to generate complex radiation patterns that have maxima and minima in certain directions. These antennas use phased array techniques to synthesize the required beam.
That the radiating elements communicate simultaneously over different frequency bands raises the specter of mutual coupling between the antenna elements that can degrade the performance of each, which can become a serious problem in smart base station antennas using phased array techniques. Mutual interference among various antenna radiating elements degrades the array's directivity, can de-tune the elements, and creates blind spots (i.e., directions into which the main beam can not be steered). If the mutual coupling is not below a certain level, depending on the application, the array performance may be compromised.
It is well known that mutual coupling may be reduced by increasing physical spacing between the antenna radiating elements, resulting in increased antenna size for the array. See for example C. A. Balanis, “A
One approach to reduce mutual coupling among antenna elements is to select substrate materials so as to minimize surface waves. For example, a study done by F. Rostan, E. Heindrich, W. Wiesbeck, entitled “H
Another approach is to use interference effects to eliminate mutual coupling. H. Wong, K. L. Lau, K. M. Luk, “D
Structural modifications of an antenna array can be applied to reduce mutual coupling. These include individual shielding of the antenna elements as in the paper by H. Wong et al. above, ground plane corrugations, using gridded patches for orthogonality, cavity backing of antenna elements, and the use of cuts in the substrate or in the groundplane. The expected mutual coupling levels by using these techniques are between about −25 to about −30 dB.
The use of photonic bandgap (PBG) materials in the ground plane may also be used to reduce mutual coupling. The use of PBG patches in a common ground plane of an antenna array has been reported at higher frequencies (e.g., 5.8 GHz), but the inventors are unaware of work showing that this technique would be operative for typical mobile telephony/cellular communication frequencies (e.g., 2 GHz and lower, especially the UMTS range 1.92-2.17 GHz and the GSM ranges 0.824-0.960 GHz and 1.710-1.990 GHz.). The problem has typically been that the commonly known PBG structures, like mushroom-PBG and uniplanar UC-PBG, are too large in size at low microwave frequencies.
The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently described embodiments of these teachings.
In accordance with an exemplary embodiment of the invention, there is provided an antenna array that includes a plurality of antenna unit cells and at least one artificial magnetic layer (AML) unit cell. The antenna unit cells are disposed in an array and spaced from one another. Each antenna unit cell includes a radiating element and a ground plane element. The AML unit cell is disposed between at least two adjacent ones of the antenna unit cells. The AML unit cell includes at least one pair of split-ring resonators The AML unit cell is capacitively coupled to the ground plane elements of the adjacent antenna unit cells.
Further, in accordance with another exemplary embodiment of the invention, there is provided an apparatus that includes an array of unit cells disposed on a common substrate. Each unit cell includes a first layer of dielectric material having a first and an opposed second major surface, a second dielectric layer that is disposed adjacent to the first major surface, a pair of intersecting conductive traces disposed on the opposed major surface of the first layer of dielectric material, and at least four conductive vias that each penetrate the first but not the second layer of dielectric material. Each of the conductive vias are spaced from one another and coupled to a conductive trace.
In accordance with another embodiment is a method of making an antenna array. In this method, a substrate is provided that is particularly adapted to retain the antenna unit cells and the tile components described below in spaced relation to one another. A plurality of antenna unit cells is secured to the substrate, such that each antenna unit cell is spaced from each other antenna unit cell. Each antenna unit cell includes a ground plane element spaced from a radiating element. Between each pair of adjacent antenna unit cells, a tile is secured to the substrate. The tile includes an array of artificial magnetic layer AML unit cells. Each AML unit cell includes a ring dielectric layer having a first and a second surface, a capacitor dielectric layer coupled to the first surface, a pair of conductive traces disposed adjacent to the second surface, and a set of at least four conductive vias penetrating the ring dielectric layer but not the capacitor dielectric layer. Each of the conductive vias are spaced from one another and coupled to one of the conductive traces. The capacitor dielectric layer is then capacitively coupled to at least one of the ground plane elements of the antenna unit cells, such a by transmitting or receiving with one of the antenna unit cells to generate a surface wave in its ground plane element.
In accordance with another embodiment of the invention is an arrayed apparatus that includes a plurality of means for wirelessly communicating RF energy over a frequency, a plurality of means for inhibiting mutual coupling between the means for wirelessly communicating RF energy, and conductive means. The plurality of means for wirelessly communicating RF energy are arrayed in spaced relation to one another. Each of the means for inhibiting mutual coupling is disposed between adjacent ones of the plurality of means for wirelessly communicating RF energy, and each of the means for inhibiting mutual coupling includes at least one split ring resonator. The conductive means is for electrically coupling to one another each of the plurality of means for inhibiting mutual coupling. Further in the arrayed apparatus, the conductive means and each of the means for inhibiting mutual coupling are disposed in a common ground plane. In one embodiment, the means for wirelessly communicating RF energy over a frequency includes a radiating element of an antenna unit cell, and the means for inhibiting mutual coupling includes at least one AML unit cell.
Further details as to various embodiments and implementations are detailed below.
The foregoing and other aspects of these teachings are made more evident in the following Detailed Description, when read in conjunction with the attached Drawing Figures, wherein:
What is needed in the art is an apparatus to arrange an array of antenna elements or antenna unit cells to control mutual coupling among the antenna unit cells at frequencies that include particularly cellular communications frequencies, for example the UMTS band of 1920 to 2170 MHz. Preferably, such a solution would enable a compact design that does not rely on physical spacing between the antenna unit cells to control mutual coupling.
Amplifiers 18 apply a gain to the uplink or downlink signal and may be coupled to a transmit/receive switch or a diplex filter to enable bi-directional signal propagation. Those signals are transmitted and received over an antenna array 20 that includes a plurality of antenna unit cells 22 (two shown) and at least one artificial magnetic layer AML unit cell 24 (six AML unit cells shown) between the antenna unit cells 22. Each antenna unit cell 22 includes a radiating element 26 and a ground plane element 28 spaced from one another by spacers 30, which may be vertically oriented stanchions as shown or a layer of insulating material at a defined and engineered thickness. Each radiating element 26 is coupled to the transceiver 12 so as to enable beamforming or selectivity of the various antenna unit cells 22 for transmissions and receptions on different frequencies. The AML unit cells 24 are co-planar with the ground plane elements 28 and electrically coupled to them, so as to functionally form a unitary ground plane 32 for the entire antenna array 20. As will be described, the AML unit cells 24 operate to disrupt mutual coupling between adjacent antenna unit cells 22 which is present in the known designs due to TE- (transverse electric field) and TM-mode (transverse magnetic field) surface wave propagation in the ground plane.
Embodiments of the invention described herein offer several distinct advantages. Specifically, wideband mutual coupling between distinct unit cells 22 or radiating elements 26 is reduced, for example in the 2 GHz range, by use of the AML unit cells 24 when the disposition of the antenna unit cells 22/radiating elements 26 relative to the AML unit cells 24 is optimized for that or any desired frequency range.
While the known solutions used at microwave and millimeter wave frequencies to reduce mutual coupling without expanding spacing between antenna radiators use artificial high-impedance surfaces, embodiments of the invention disclosed herein employ AML unit cells 24 between adjacent ones of the antenna unit cells 22 to impede electromagnetic energy propagation along the ground plane 32 that would otherwise enable mutual coupling among adjacent radiating elements 26. In operation, a magnetic field is induced by the radiating elements 28 into the AML unit cell 24, which induces electrical currents in the metal components of the AML unit cell 24 and in the unitary ground plane 32. The geometry of the AML unit cell 24 is chosen so that all or substantially all of the magnetic field components induced in the AML unit cell 24 strongly interact with that AML unit cell(s) 24. In the known photonic bandgap (PBG) surface solutions, only the tangential fields can effectively excite the structure of those PBG structures.
The ring dielectric layer 38 is configured to form pairs of split ring resonators (two split ring resonators shown in
While
In effect, the structure 24 of
Engineering the dimensions of those rings and selecting the dielectric materials for the layers 38, 40 of the AML unit cell 24 enables one to engineer a desired magnetic response to an applied magnetic field, and that ‘artificial’ magnetic response can easily be made to be much larger than the magnetic field associated with natural magnets such as ferrous metals at low microwave frequencies (e.g. UMTS band). The range of magnetic response found in naturally magnetic materials is a small subset of that theoretically possible with artificial magnetic materials. For example, artificial electric response has been induced in metallic wire grids with spacing much smaller than the wavelength. Artificial magnetic materials, also known as metamaterials, may be engineered for magnetic fields well in excess of those found in naturally magnetic materials.
In the antenna arts, naturally magnetic materials lose their effective magnetic properties or become too lossy in the microwave regime. Desired magnetic properties are achieved in embodiments of this invention by engineering the AML unit cell 24 from non-magnetic constituents. By designing the AML unit cell 24 to generate a sufficient magnetic field from a desired radio frequency RF field (e.g., the UMTS band, about 1920-2170 MHz), the near field of one radiating element 26 may be re-distributed so as to avoid mutual coupling with lobes from nearby radiating elements 26. In nearly all cases, only the adjacent radiating element 26 is of concern for mutual coupling, as the increased spacing from non-adjacent radiating elements 26 mitigates coupling to a substantial degree. Because the magnetic field induced in the AML unit cell 24 for a given wavelength at the radiating element 26 is engineered for a much stronger magnetic field than is typically found in naturally magnetic materials, radiation efficiency of the antenna unit cell 22 is improved because the AML unit cells 24 reduce surface wave propagation along the ground plane 32, inhibiting mutual coupling among adjacent antenna unit cells 22 by a mechanism other than simple attenuation due to wavelength-dependent spacing.
An important aspect of the invention is that the AML unit cells 24 and the ground plane elements 28 form a coherent, unitary ground plane 32. The broader ground plane 32, and not only the ground plane element 28 of a particular antenna unit cell 22, operates in conduction with the operative radiating element 26 to launch RF energy. Were it otherwise and only the ground plane element 28 of an individual unit cell 22 operated in conjunction with the radiating element 26 to transmit RF waves, then there would be no mutual coupling due to surface waves among adjacent antenna unit cells 22 because the broader ground plane 32 would not propagate energy. But antenna arrays 20 are more effective with a common ground plane 32, whether or not the individual antenna unit cells 22 include their own ground plane element 28 that becomes a part of the common ground plane 32. Where a plurality of AML unit cells 24 are disposed between adjacent antenna unit cells 22, each AML unit cell 24 acts as a scatterer of RF energy from one radiating element 26 that would otherwise propagate and couple with other radiating elements 26.
In testing with the apparatus of
Multiple unit cells as in
Exemplary embodiments of this invention are seen as advantageously used in scanning antenna arrays that employ smart adaptive antennas. Smart adaptive antennas beamform with a feedback mechanism to adapt to the local RF environment. The tiles 36 of AML unit cells 24 can be inserted between the antenna unit cells 22 to form an antenna array 20 such as the one shown schematically in
Any antenna array 20 (e.g., a base station antenna) can be made smaller in size if AML tiles 36 are located between the array columns and/or rows. The reduced mutual coupling helps in retaining the antenna matching even if the elements 26 are physically closer to each other. Where the AML unit cell 24 is selected/engineered to have a permeability of more than unity as is preferred, each AML unit cell 24 may be smaller than the photonic bandgap unit cells of the prior art and thereby enable a smaller antenna array 20 than the prior art but with identical performance as to mutual coupling.
Although described in the context of particular embodiments, it will be apparent to those skilled in the art that a number of modifications and various changes to these teachings may occur. Thus, while the invention has been particularly shown and described with respect to one or more embodiments thereof, it will be understood by those skilled in the art that certain modifications or changes may be made therein without departing from the scope and spirit of the invention as set forth above, or from the scope of the ensuing claims.
Number | Name | Date | Kind |
---|---|---|---|
6512494 | Diaz et al. | Jan 2003 | B1 |
6670932 | Diaz et al. | Dec 2003 | B1 |
6774867 | Diaz et al. | Aug 2004 | B2 |
6954177 | Channabasappa et al. | Oct 2005 | B2 |
7042419 | Werner et al. | May 2006 | B2 |
7071889 | McKinzie et al. | Jul 2006 | B2 |
7330161 | Matsugatani et al. | Feb 2008 | B2 |
20040008149 | Killen et al. | Jan 2004 | A1 |
20070285318 | Sotoudeh et al. | Dec 2007 | A1 |
Number | Date | Country | |
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20070285316 A1 | Dec 2007 | US |