Patent Application
20100053022
The present description is directed, generally, to multiple-element antennas and, more specifically, to systems and methods employing components to reduce the effects of mutual coupling between and among multiple antenna elements.
As antenna systems grow smaller, space between antenna elements in those systems becomes more scarce. Not only does the spacing between antenna elements have the potential to affect the radiation pattern of a system, but it can also affect the amount of mutual coupling between antenna elements. Mutual coupling is inductive/capacitive coupling between two or more antennas, and it can sometimes result in unwanted performance degradation by interfering with signals being transmitted or by causing an antenna element to radiate unwanted signals. Generally, the closer the placement of two antenna elements, the higher the potential for mutual coupling.
Accordingly, modern antenna designers generally look for ways to decrease coupling (i.e., increase isolation) between some antenna elements. This is especially true for multi-channel systems, as the signals on one channel should usually and ideally be unaffected by the signals on other channels. It is also particularly true for Multiple Input Multiple Output (MIMO) antenna systems which require several antennas to operate at the same frequency but work independently of each other.
Some antenna systems employ antenna elements placed above a ground plane. In such systems, the antenna elements can induce currents in the ground plane that travel to other antenna elements and increase undesired coupling. To decrease the coupling, various techniques have been devised. For example, one solution has been to split the ground plane so that two antennas that might interfere are not connected by a continuous ground plane. However, such systems generally produce an inadequate amount of isolation.
Other proposed systems include intricate fabrication processes to produce structures with cells shorted to the ground through vias in a Printed Circuit Board (PCB). Such structures are analogous to Photonic Band Gap (PBG, used in optics) structures and generally act as bandstop filters and can be designed to cancel specific, unwanted signals. However, such systems are expensive in terms of both space and money because of the complexity of the three-dimensional shapes of the structures. Currently, no prior art system provides adequate isolation with a minimum of complexity.
Various embodiments of the invention are directed to systems and methods that include a coupling element in a multiple-element antenna system. In one example, a coupling element is placed between two antenna elements. The shape of the coupling element is designed so that it cancels out the current that is due to direct coupling of the elements. In some embodiments, the coupling element can be quite small, thereby offering economy of space. Furthermore, various embodiments are much less complex than PBG-inspired designs and, thus, are cheaper to manufacture than prior art systems that use PBG-inspired isolation elements.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.
For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
There are three regions of interest in
In regions 110 and 120, there is direct coupling between antenna elements 101 and 102. The current due to direct coupling is referred to in this example as IDirect, and it is equal to α IExcited, wherein α is a constant that is affected by distance between antenna elements 101 and 102 as well as by the sizes of regions 110 and 120. IDirect is in a direction opposite (i.e., 180° out of phase) that of IExcited. In region 130, the coupling between antenna elements 101 and 102 is not direct. Instead, in region 130, antenna elements 101 and 102 each couple with coupling element 103, rather than with each other. Antenna element 101 couples with coupling element 103, thereby inducing a current in coupling element 103 that is in the opposite direction of IDirect. The current that is induced in coupling element 103 then induces a current (ICancel) in antenna element 103 that is shifted by approximately 180 degrees again. The phase of ICancel is in a direction opposite that of IDirect and ICancel can be expressed as β IExcited, where β is a constant that depends on the distances between antenna elements 101 and 102 and coupling element 103 as well as on the size of coupling element 103. In this example, β is approximately equal to α, so that ICoupled=IDirect+ICancel˜zero.
In the present example, antenna elements 101 and 102 are shown as dipole elements, which are generally λ/2 in length. The total length of coupling element 103, including both the vertical and horizontal components, is also λ/2 as well. The constant β is affected by the length of the vertical portion (i.e., parallel to antenna elements 101 and 102) of coupling element 103. The horizontal portion (i.e., perpendicular to antenna elements 101 and 102) of coupling element 103 has very little, if any, effect on β. Instead, the horizontal portion is present so that the total length of coupling element 103 is λ/2.
While the example above refers to horizontal and vertical portions, such terms are used for ease of illustration only. More generally, it can be said that the portion of a coupling element (e.g., 103) that is mutually coupled with its proximate antenna elements (e.g., 101 and 102) affects β, whereas the portion that is not mutually coupled with the proximate antenna elements is used to ensure that the total length is a resonant length.
Antenna system 200 is built on Printed Circuit Board (PCB) 205, and it includes antenna elements 201 and 202, coupling element 203, and ground plane 204. As is apparent from
While the example of
Graph 250 shows the simulated and measured performance of an antenna system similar to that of antenna system 200, but without coupling element 203. By contrast, graph 260 shows simulation and measurement results for system 200. In graph 250 at 2.45 GHz there is −8 dB of coupling. Graph 260 shows −30 dB of coupling at 2.45 GHz, indicating an improvement of over −20 dB of isolation. The improvement is impressive, considering that −30 dB means that for every one thousand units of energy only one unit is coupling. For real world systems, it is very difficult to achieve zero coupling; however, embodiments of the invention can improve isolation such that the effects of coupling is near zero (as in graph 260). In many systems, reducing the effects of mutual coupling by as much as −20 dB can bring the effects of coupling down to a level where it has a negligible effect on the performance of the system.
While dimensions are given in
System 200 has directional diversity, in that antenna elements 201 and 202 radiate in different directions. Because of the diversity in antenna system 200, antenna system 200 can be adapted for use in MIMO applications. Coupling element 203 between antenna elements 201 and 202 enhances the performance of antenna system 200 by reducing the effects of coupling between the diverse resonating elements.
System 300 includes dipole antenna elements 301 and 302 and coupling element 303. Antenna system 300 is deigned for performance in the band around 2.4 GHz. Graph 310 shows simulation results for antenna system 300 with and without coupling element 303. As can be seen, the presence of coupling element 303 increases isolation around the resonant frequency of system 300.
Some embodiments can be applied to multi-band applications.
The embodiment of system 400 can be built on a form factor that is roughly the size of a flash “memory stick” and included in a Universal Serial Bus (USB) dongle, such as exemplary dongle 700 of
Numbers of antenna elements and coupling elements can be scaled for use in particular applications.
Embodiments of the invention can be adapted for use in any of a variety of antenna systems. For example, embodiments can be adapted for use in systems employ dipoles, monopoles, PIFAs, and any other kind of grounded or ungrounded antenna element. Furthermore, various embodiments can be adapted for use in many different arrays, such as 2D, 2.5D, and 3D arrays.
Various embodiments of the invention include techniques using coupling elements to increase isolation.
In action 801, a first current is excited in the first antenna element. In one example, the first antenna element is driven by a Radio Frequency (RF) module. The current can be in any RF band, including bands used in WiFi (IEEE 802.11) applications, cellular telephone applications, and other RF applications that are too numerous to list herein.
In action 802, the first current directly induces a second current in the second antenna element. An example of the first current directly inducing a second current is explained above with respect to
In action 803, a third current is induced by the first current in the coupling element. In action 804, a fourth current is induced by the third current in the second antenna element. The fourth current is out of phase with the second current and reduces the effects of the mutual coupling between the first and second antenna elements by at least partially cancelling the second current.
While method 800 is shown as a series of discrete steps, various embodiments of the invention are not so limited. Some embodiments may add, modify, rearrange, and/or omit one or more actions. For instance, from a human's perspective, it will appear that actions 801-804 occur simultaneously and continuously during operation of the antenna system. Furthermore, other methods may include such features as canceling the effects of mutual coupling in two or more operating bands, canceling the effects of mutual coupling between more than one pair of antenna elements, and the like.
Various embodiments of the invention provide advantages over prior art solutions. For example, PBG-inspired solutions are complex, expensive, and large. By contrast, coupling elements, such as those shown above, are relatively simple structures when compared to PBG-inspired solutions. Furthermore, when implemented with metal on a PCB, coupling elements often add little or no additional manufacturing cost for a given antenna system.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
