Abstract:
An antenna system comprises a first antenna element mutually coupled with a second antenna element, the mutual coupling between the first and second antenna elements causing a first current in the second antenna element, and a coupling element disposed at least partially between the first and second antenna elements, wherein the coupling element is mutually coupled to each of the first and second antenna elements, and wherein the coupling element is configured to induce a second current in the second antenna element that at least partially cancels the first current.

Description:
TECHNICAL FIELD 
     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. 
     BACKGROUND 
     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. 
     BRIEF SUMMARY 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is an illustration of an exemplary antenna system, adapted according to one embodiment of the invention; 
         FIG. 2  is an illustration of an exemplary antenna system, adapted according to one embodiment of the invention; 
         FIG. 3  is an illustration of an exemplary system, adapted according to one embodiment of the invention; 
         FIG. 4  is an illustration of an exemplary system, adapted according to one embodiment of the invention; 
         FIG. 5  is an illustration of an exemplary system adapted according to one embodiment of the invention; 
         FIG. 6  shows exemplary antenna arrays, adapted according to embodiments of the invention; 
         FIG. 7  is an illustration of an exemplary USB dongle, adapted according to one embodiment of the invention; and 
         FIG. 8  is an illustration of an exemplary method adapted according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is an illustration of exemplary antenna system  100 , adapted according to one embodiment of the invention. System  100  includes antenna elements  101  and  102 , as well as coupling element  103 . In this example, antenna element  101  is driven by a Radio Frequency (RF) feed, and the current in antenna element  101  is I Excited . The total current in antenna element  102  that is due to mutual coupling with antenna element  101  is I Coupled . 
     There are three regions of interest in  FIG. 1 . Region  110  is where coupling element  103  does not lie between antenna elements  101  and  102 . In other words, in region  110 , each antenna element  101  and  102  is in the other&#39;s line of sight. Region  120  is similar to region  110 . In region  130 , coupling element  103  is positioned between antenna elements  101  and  102 . 
     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 I Direct , and it is equal to α I Excited , 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 . I Direct  is in a direction opposite (i.e., 180° out of phase) that of I Excited . 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 I Direct . The current that is induced in coupling element  103  then induces a current (I Cancel ) in antenna element  103  that is shifted by approximately 180 degrees again. The phase of I Cancel  is in a direction opposite that of I Direct  and I Cancel  can be expressed as β I Excited , 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 I Coupled =I Direct +I Cancel ˜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. 
       FIG. 2  is an illustration of exemplary antenna system  200 , adapted according to one embodiment of the invention.  FIG. 2  shows an antenna system design with dimensions (in mm) thereon and also provides graphs  250  and  260  to explain the performance of antenna system  200 . 
     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  FIG. 2 , antenna elements  201  and  202  are Planar Inverted F Antenna (PIFA) elements. Due to their proximity to each other, antenna elements  201  and  202  experience mutual coupling. Coupling element  203  reduces or eliminates the effects of mutual coupling, thereby improving the performance of antenna system  200 . 
     While the example of  FIG. 1  shows a coupling element of total length λ/2, not all embodiments are so limited. In embodiments that use antenna elements of a resonant length λ/4, the total length of the coupling element is also λ/4. Examples of antenna elements that have resonant lengths of λ/4 include, e.g., monopoles and PIFAs. In the case of antenna system  200 , which uses PIFAs as antenna elements  201  and  202 , coupling element  203  has a length of λ/4. 
     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. 
       FIG. 2  shows that the coupling length (i.e., not the total length) of coupling element  203  is two millimeters. In designing an antenna system the coupling length can be adjusted to tune the performance of the system by affecting β. In fact, differing lengths can be simulated and/or tested to arrive at an optimal length. 
     While dimensions are given in  FIG. 2 , the invention is not so limited. Any of a variety of designs and structures can be used, and each system can be adapted to perform in specific bands and employ different dimensions. In fact, any dimensions given in this description are illustrative and exemplary but not limiting. 
     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. 
       FIG. 3  is an illustration of exemplary system  300 , adapted according to one embodiment of the invention. Various embodiments of the invention include Three-Dimensional (3D) structures, such as the embodiment shown as system  300 . 
     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.  FIG. 4  is an illustration of exemplary system  400 , adapted according to one embodiment of the invention. System  400  is a MIMO antenna that provides performance at 2.4 GHz and 5 GHz. System  400  is built on PCB  405  and includes PIFA elements  401  and  402 , coupling element  403 , and ground plane  404 . Coupling element  403  includes two coupling portions: The portion including  403   a  and  403   c  and the portion including  403   b  and  403   c . Each coupling portion  403   a  plus  403   c  and  403   b  plus  403   c  has a different coupling length (i.e., a different β) as well as a different effective total length, thereby giving each coupling portion  403   a  plus  403   c  and  403   b  plus  403   c  a different operating band. In this example, coupling element  403  provides isolation to antenna system  400  at 2.4 GHz and 5 GHz. 
     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  FIG. 7 . In fact, system  400  can be connected to a computer through a USB interface to provide wireless Local Area Network (LAN) connectivity. 
     Numbers of antenna elements and coupling elements can be scaled for use in particular applications.  FIG. 5  is an illustration of exemplary system  500  adapted according to one embodiment of the invention. System  500  includes antenna elements  501 - 504  and coupling elements  511 - 514 . Coupling element  511  provides isolation between antenna elements  501  and  502 ; similarly, coupling element  513  provides isolation between antenna elements  503  and  504 . Coupling elements  512  and  514  provide isolation between antenna elements  502  and  503 , as well as  501  and  504 , respectively. 
     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.  FIG. 6  shows exemplary antenna arrays  610 ,  620 ,  630 ,  640 , and  650 , adapted according to embodiments of the invention. Coupling elements, such as those shown above in  FIGS. 1-5 , can be used to increase isolation between antenna elements in the arrays of  FIG. 6 . 
     Various embodiments of the invention include techniques using coupling elements to increase isolation.  FIG. 8  is an illustration of exemplary method  800  adapted according to one embodiment of the invention. Method  800  can be performed on embodiments, such as those described above in  FIGS. 1-7 . 
     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  FIG. 1 , wherein I Excited  induces I Direct . 
     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&#39;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.