Multiple-input, multiple-output (MIMO) technology has attracted attention in wireless communications, because it offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. It achieves this goal by spreading the same total transmit power over the antennas. Because of these properties, MIMO is an important part of many modern wireless communication standards.
In accordance with an aspect of the invention, a multiple input, multiple output (MIMO) antenna system includes an electrically conductive, substantially planar, ground reference and a plurality of antenna assemblies electrically connected to respective antenna feeds at respective connection points. The antenna assemblies extend on a first side of the ground reference and are formed from a conductive material. Each antenna assembly is rotationally asymmetric around an axis perpendicular to the ground reference and passing through the connection point.
In accordance with another aspect of the invention, a multiple input, multiple output (MIMO) antenna system includes an electrically conductive, substantially planar, ground reference and a plurality of antenna assemblies electrically connected to respective antenna feeds at respective connection points. Each antenna assembly extends on a first side of the ground reference and is formed from multiple conductive elements joined at a common apex and extending at an oblique angle relative to the ground plane.
In accordance with yet another aspect of the present invention, a MIMO antenna system includes a substantially circular planar ground reference and three antenna assemblies extending on a first side of the ground reference. The three antenna assemblies are electrically connected to respective antenna feeds at respective connection points evenly spaced around a circle having its center at the centroid of the ground plane. Each of the plurality of antenna assemblies is formed from a conductive material and includes multiple conductive elements joined at a common apex and extending at an oblique angle relative to the ground plane.
The foregoing and other features of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, in which:
The MO antenna system 10 comprises an electrically conductive, substantially planar, ground reference 12 and a plurality of antenna feeds 14 and 16. The ground reference 12 may be comprised of any appropriate electrically conductive material such as, for example, copper or stainless steel. Each antenna feed 14 and 16 is operatively connected to an antenna assembly 20 and 30 at associated connection points, with the antenna assemblies 20 and 30 each formed from a conductive material and extending on a same side of the planar ground reference 12. The antenna feeds 14 and 16 can each include, for example, a SubMiniature, version A (SMA) coaxial connector, or a similar connector, and a transmitter/receiver circuit board (not shown). The connector and circuit board can be electrically connected together by a length of coaxial cable. An SMA connector allows a center conductor of the coaxial cable to electrically connect to the associated antenna assembly 20 and 30 and allows a ground braid of the coaxial cable to electrically connect to the ground reference 12. A dielectric material can be used to electrically insulate the center conductor and the antenna assembly 20 and 30 from the ground reference 12.
Each antenna assembly is tuned to operate at or around the characteristic frequency of the antenna system 10. As will be described in more detail below, these antenna assemblies may exhibit significant wideband sensitivity, so it will be appreciated, for example, that an assembly tuned “at or around” a characteristic frequency of 2.4 GHz can be receptive to signals over a band of several gigahertz that includes the characteristic frequency. In accordance with an aspect of the present invention, the antenna assemblies 20 and 30 can be positioned in close proximity to decrease the necessary size of the planar ground reference 12. Specifically, the antenna assemblies 20 and 30 can be separated by less than one-quarter of the characteristic wavelength of the antenna system 10 while still maintaining substantial independence among the antenna assemblies (e.g., a correlation coefficient of less than 0.5).
To facilitate independent operation of the antenna assemblies, each antenna assembly 20 and 30 is shaped to be rotationally asymmetric around an axis perpendicular to the planar ground reference 12 and passing through the connection point with the antenna feed 14 and 16. By “rotationally asymmetric,” it is meant that, when viewed at a position distal from the antenna assembly along the axis, a full three-hundred sixty degree rotation would be necessary for the antenna assembly to return to shape and configuration substantially identical to that of an initial orientation. The inventors have found that the substantial polarization diversity provided by the illustrated antenna assemblies 20 and 30 allow the spacing between the various antenna assemblies to be substantially reduced. In one implementation, the antenna assemblies 20 and 30 can have overlapping footprints in a plane parallel to the planar ground reference 12. In the illustrated example, each antenna assembly 20 and 30 is formed from a plurality of radiative elements 22, 24, 32, and 34 joined at a common apex and extending at an oblique angle from the planar ground reference. It will be appreciated, as illustrated in
The illustrated antenna system 50 comprises two substantially identical antenna assemblies 52 and 54 arranged along a diameter of an associated circular ground plane 56. Each antenna assembly 52 and 54 includes a plurality of linear elements of differing lengths joined at a common apex at respective first ends. In accordance with an aspect of the present invention, each linear element of each antenna assembly 52 and 54 extends at an oblique angle relative to the ground plane 56. In the illustrated implementation, the oblique angle is substantially equal to sixty degrees for each of the linear elements, although it will be appreciated that, in some implementations, the angle can be varied from: element to element. The linear elements can be evenly spaced around the connection point, such that, in the illustrated six element system, the elements positioned at intervals of approximately sixty degrees. While the specific geometry of the antenna system 50 can vary, in the illustrated implementation, the connection points of each antenna assembly is positioned as to be spaced evenly around a circle having its center at the centroid of the ground plane.
The illustrated antenna system 50 is configured to operate in a frequency band around 2.4 gigahertz. Accordingly, the lengths of the linear elements can lie in a range including a quarter of a wavelength associated this frequency, specifically around 1.23 inches. In the illustrated implementation, none of the linear elements have a same length. A first element has a length of approximately seven-eights of an inch, a second element has a length of approximately fifteen-sixteenths of an inch, a third element has a length of approximately an inch, a fourth element has a length of approximately one and one-eighth inches, a fifth element has a length of approximately one and one-quarter inches, and a sixth element has a length of approximately one and three-eights inches. It will be appreciated that these lengths are merely provided for the purpose of example, and other lengths within a range around one quarter of the characteristic wavelength could be used. Further, it will be appreciated that the range itself, and its associated lengths, will vary essentially linearly with the characteristic wavelength of the system 50.
In accordance with an aspect of the present invention, the antenna assemblies 52 and 54 can be spaced closer together to one another than existing MIMO antennas. For example, the antenna assemblies 52 and 54 of the illustrated assembly can be separated by a distance just over a quarter of the characteristic wavelength of the antenna system 50 while still maintaining a correlation coefficient greater than 0.5. This allows for a significant reduction in the size of the ground plane, such that a maximum distance between any two points on the ground plane can be less than one wavelength associated with an operating frequency of the antenna. In the illustrated implementation, it will be noted that an associated footprint 60 of each of the plurality of antenna assemblies has a reasonably large diameter. Due to the reduced size of the ground plane, this footprint can actually have a diameter that is not less than one-quarter of a diameter of the ground plane.
The illustrated antenna system 100 comprises two antenna assemblies 102 and 104 arranged along a diameter of an associated circular ground plane 106. A first antenna assembly 102 includes a plurality of linear elements of differing lengths joined at a common apex at respective first ends in a configuration similar to the antenna assemblies 52 and 54 of
A second antenna assembly 104 comprises a plurality of radiative elements formed as oblique, elliptical cones, with each of the cones having an open base and being connected to the connection point and to one another at their respective apexes. The sides of the conical radiative elements can be either solid or formed from a mesh of appropriate size for the operating frequency of the antenna. It will be appreciated that the radiative elements in the second antenna assembly may show some deviation from a perfect conical shape, particular in that the open base of the cone may be curved in a manner departing from a plane defined by the base. In the illustrated implementation, each conical radiative element of the second antenna assembly 104 spans an angle relative to the ground plane 106 from approximately a fifty-five degrees to eighty degrees, such that the conical element extends at an oblique angle relative to the ground plane. It will be appreciated, however, that these upper and lower angles can be varied from element to element. The conical elements can be evenly spaced around the connection point, such that, in the illustrated three element system, the elements positioned at intervals of approximately one hundred twenty degrees.
The illustrated antenna system 150 comprises three antenna assemblies 152, 154, and 156 connected to associated antenna feed connectors 162, 164, and 166 through an associated circular ground plane 158. Each antenna assembly 152, 154, and 156 includes a plurality of linear elements of differing lengths joined at a common apex at respective first ends in a configuration similar to the antenna assemblies 52 and 54 of
It will be appreciated that the three antenna assemblies 152, 154, and 156 share a common three-dimensional shape. In the illustrated implementation, however, each antenna assembly 152, 154, and 156 is rotated one hundred and twenty degrees relative to the neighboring antenna assemblies, such that a same location on the common three-dimensional shape faces a centroid of the ground plane for each antenna assembly.
The illustrated antenna system 200 comprises three antenna assemblies 202, 204, and 206 connected to associated antenna feed connectors through an associated circular ground plane 208. First and second antenna assemblies 202 and 204 each include a plurality of linear elements of differing lengths joined at a common apex at respective first ends in a configuration similar to the antenna assemblies 52 and 54 of
While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.