Patent Grant
7629930
The present description relates, in general, to systems with ground planes and, more specifically, to adjusting ground plane characteristics to optimize performance of antenna systems.
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 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.
The present invention is directed to systems and methods for attenuating unwanted signals in a ground plane through use of a filter configured as a pattern in the ground plane. An example system includes two elements (e.g, antenna elements) with the filter positioned therebetween. The elements cause unwanted signals in the ground plane, and the filter is adapted to reduce and/or eliminate the effects of the signals from the system.
In one example, the filter is a simple ground plane structure that can reduce mutual coupling between closely-packed antenna elements. In such an example, the structure can include a slotted pattern etched onto a single ground plane upon which the antenna elements are disposed. The slotted configuration creates a filter that acts as an inductive/capacitive (LC) component in the ground plane, and the size and shape of the slots can be designed so that the filter attenuates certain frequencies known to be most prevalent and/or cause most interference. Similarly, the structure can be applied to reduce mutual coupling between three, four, or more radiating elements. The slotted single ground plane structure can be simple and cost-effective to fabricate in some embodiments.
As mentioned above, embodiments of the invention are applicable for use in antenna systems, such as between two parallel individual planar inverted-F antennas (PIFAs) sharing a common ground plane. In another specific example, the mutual coupling between half-wavelength patches and monopoles can also be reduced with the aid of a filter disposed in the ground plane structure. One application for embodiments of the invention is in the design of compact antennas for MIMO wireless communication systems. Embodiments of the invention are further adaptable for use in attenuating unwanted signals caused by elements other than antenna elements. For example, any device including a populated Printed Circuit Board (PCB) with various components thereon causing unwanted signals may benefit from certain embodiments.
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 drawing, in which:
System 100 further includes active components 102 and 103 disposed proximate to ground plane 101. In one example, one or more of elements 102 and 103 are antenna elements, such as patch or Planar Inverted F Antenna (PIFA) type elements disposed on a substrate with some or all of the surface area thereof overlapping in the z-axis with ground plane 101. Such antenna elements are at least partially grounded. In another example, at least one of active components 102 and 103 is a Radio Frequency (RF) module sending/receiving RF signals in communication with one or more antennas. In fact, active components 102 and 103 can be any kind of component that is operable to cause signals in ground plane 101.
When active components 102 and 103 transmit data (e.g., for an antenna by resonating or for an RF module by sending/receiving signals through a port that is near or in a ground plane), each element 102 and 103 causes signals 105, 106 in ground plane 101. Signals 105 and 106 are induced currents that travel in ground plane 101 and can cause unwanted effects in the respective other active component 103 and 102. The phenomenon is referred to as “mutual coupling” or “cross coupling” between elements 102 and 103, and it is sometimes undesirable as it can create additional resonances.
In system 100, filter 104 is disposed as a pattern in the surface of ground plane 101. Filter 104 is adapted to receive and attenuate signals 105 and 106, thereby increasing isolation for each of active components 102, 103. It is not necessary in some embodiments for filter 104 to completely remove signals 105 and 106, as long as signals 105 and 106 are attenuated to some degree before reaching the respective other active component. For example, in one embodiment, attenuation of approximately twenty decibels is achieved.
In system 200, filter 204 is shown as a ground plane modification. Specifically, ground plane 201 includes eight slots (e.g., slot 204a). The slots in this example are orthogonal to a straight line path between elements 202 and 203, and the slots do not extend the whole distance across ground plane 201 such that solid conductive path 204b is formed thereon making the pattern appear similar to ribs and a backbone. The numbers, orientation, and sizes of the slots are merely exemplary, and other embodiments may include different configurations, as explained in more detail below.
When system 200 is viewed in a circuit context, it should be noted that the slots of filter 204 add reactance thereto. Specifically, the slots add a capacitive reactance component (“C”), and conductive path 204b adds an inductive reactance component (“L”). Thus, filter 204 is, in effect, an “LC” component.
The dimensions of system 200 determine, at least in part, the frequency response of filter 204. Generally, the lengths and widths of the individual slots define the sizes and spacing of the ribs, which can increase or decrease the capacitive component of filter 102. Specifically, as the ribs get closer together and wider, the capacitance thereof typically increases. Also, the inductance of backbone 204b tends to increase as it narrows. Further, the number of slots typically affects the amount of attenuation at a given frequency rather than affecting the frequency response of filter 204. For example, more slots usually provide greater attenuation, but also take up more surface area on ground plane 201. Thus, a typical design process involves shaping the slots to provide the correct frequency response while including enough slots to provide the desired amount of attenuation within the available surface area. Interelement spacing also generally affects the performance of system 200.
Table 2, below, is provided to describe some of the design constraints for a system, such as system 200, which takes the basic form shown in
The numbers of slot pairs in Table 1 are exemplary, as other numbers can be used. The values in Table 1 are optimized for performance in system 300 at the listed antenna band center frequencies. In optimized systems, center operating frequencies generally correspond to the centers of stop bands for the filter. For the example at 2.35 GHz center operating frequency, performance may be optimized by making each of the slots 21 mm by 1 mm. At different center frequencies, it may be desirable to use slots of different dimensions in order to create a filter with an appropriate stop band.
Systems according to the configuration of systems 100 (
A variety of arrangements are also possible for some embodiments.
In step 501, RF signals are transmitted with a first element. Transmitting can include wireless and conductor-based transmissions. Thus, in one example, the transmitting is wireless using an antenna element, and in another example, the transmitting is along a wire trace in a PCB or other kind of electrical signal transmission.
In step 502, RF signals are transmitted with a second element, wherein each of the first and second elements produce currents in the ground plane affecting the respective other of the first and second elements. As in step 501, the transmitting can be by conductor and/or by radiation of electromagnetic signals. Further, each of the first and second elements' transmitting produces currents in the ground plane that affect the other element. The effecting can include, e.g., causing unwanted signals to reach the other component, possibly causing unwanted operation. The undesired signals may include, for example, signals with different informational content, signals with different frequency components, out-of-phase signals, and the like.
In step 503, the currents in the ground plane are attenuated with a filter configured as a pattern in the surface of the ground plane. In one example, the filter is created from slots in the ground plane that produce an LC effect. Attenuating includes completely or partially cancelling, blocking, and/or removing the signals in the ground plane.
While method 500 is shown as a series of steps, various embodiments of the invention may add, delete, or rearrange the order of steps. In fact, some steps may be performed simultaneously. For example, steps 501, 502, and 503 may be performed at (or very nearly at) the same time. Further, various systems may include more than two elements and more than one filter, as shown in
Embodiments of the invention may provide one or more advantages over other solutions. For instance, in some PCB-based devices a ground plane filter can be manufactured by etching, or even sawing, such that no new components are added, and the size of the ground plane may not need to be increased. This may lead to ease and economy of manufacturing. Further, it is possible in some embodiments to construct a ground plane filter from a single layer of conductive material so that it is simple to design and manufacture.
One prior art solution simply constructs the ground plane out of separate, coplanar layers—one for each active component. While those solutions may provide cross-coupling attenuation in the range of eight decibels or less, various embodiments of the present invention employing similar systems can often provide up to and exceeding twenty decibels of attenuation. Still further, by providing increased isolation various embodiments can facilitate higher capacity input and output (as in Multiple Input Multiple Output systems), can improve antenna efficiency and power consumption, and can facilitate closer spacing between elements than lesser performing systems.
Systems and methods according to embodiments of the invention may be included in or performed by any of a variety of devices now known or later developed that include components proximate a ground plane that may produce interference.
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.
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