Patent Publication Number: US-7589674-B2

Title: Reconfigurable multifrequency antenna with RF-MEMS switches

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/702,281 filed on Jul. 26, 2005, which is incorporated herein by reference in its entirety. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with Government support under Contract No. F29601-00-C-0244 awarded by the Air Force Materiel Command/Space Electronics Modeling, Development and Experimentation and under Contract No. ECS0218732 awarded by the National Science Foundation. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to a reconfigurable antenna, and, more particularly to a reconfigurable antenna incorporating a self-similar planar antenna and radio frequency micro-electromechanical (RF-MEMS) switches, the reconfigurable antenna radiating on demand at three frequencies. 
     BACKGROUND OF THE INVENTION 
     Modern communication systems demand multiband antenna performance. An apparatus to address this need is by using reconfigurable antennas. Reconfigurable antennas are known. However, an increasing demand for reconfigurable systems, which are also versatile, has not yet been satisfactorily addressed. In particular, there is a need to provide a reconfigurable antenna operable at multiple frequencies. At the present time, multiple frequencies are obtained by utilizing PIN diodes or many different antennas in order to have an antenna for each desired frequency. Another approach has been to reconfigure antennas, particularly the reconfigurable aperture (recap) antenna with micro-electromechanical (MEMS) switches, which has been unsuccessful, and microstrip antennas using PIN diodes, with some success. Still another approach includes the use of known “Sierpinski” type multiband antennas. However, the known Sierpinski type antennas only radiate at a number of frequencies, related to the number of iterations of the Sierpinski structure. Accordingly, even with these reconfigurable antennas, there is no provision for an antenna including on-demand selection of one of three predetermined frequencies. 
     An integration of RF-MEMS switches into known antenna systems has been attempted; however, an integration of RF-MEMS switches with the antenna has not been satisfactorily achieved. Moreover, no multiband antenna has been shown or reported to be RF-MEMS reconfigurable. In particular, there continue to be problems overcoming the effect of switch bias lines on the antenna performance. The bias lines of the RF-MEMS switches have been found to problematically affect the radiation pattern of the antenna, as well as its resonant frequencies. 
     Furthermore, recovery from these problems can be difficult. For example, the continued miniaturization of antennas and their parts prevents spacing of bias lines at intervals which will not interfere with the radiation patterns of the antenna. One reason for the desired use of MEMS switches resides in their lower insertion loss, lower power requirements, higher linearity, reliability, and better isolation effects than any other biasing method such as, for example, PIN/FET. However, incorporation of these switches into an antenna configuration has not previously been successful because of the inability to bias them and place them in a way to not affect antenna performance. Disadvantages include their long switching times (on the order of 1-20 μs), high actuation voltage and they are unable to handle high-power RF applications. 
     Thus, there is a need to overcome these and other problems of the prior art and to provide a reconfigurable multifrequency antenna with RF-MEMS switches. The present invention successfully integrates RF-MEMS switches with compatible antenna structures in a very efficient way that enhances the performance of the conventional antenna by adding an additional resonant frequency without altering its radiation pattern. 
     SUMMARY 
     Accordingly, embodiments of the present invention are generally directed to a reconfigurable multifrequency self-similar planar antenna incorporating MEMS switches. In other words, the antenna is reconfigurable while maintaining similar patterns at different frequencies and radiates on demand at selected widely spaced frequencies. 
     In accordance with one embodiment, this constitutes a great advancement considering that with conventional antenna structures, sidelobes cannot be avoided at their higher modes of operation. A reconfigurable antenna system includes a substrate, and an antenna patch on the surface of the substrate. The antenna patch includes symmetrically opposed fractal geometry metallic patches defining a Sierpinski configuration. Switches operatively connect adjacent antenna patches on each arm of the Sierpinski configuration, and a power source is provided for selectively actuating the switches. 
     In accordance with the present teachings, a method of fabricating an RF-MEMS-based self-similar reconfigurable antenna comprises forming a substrate of a high resistivity material, forming a bow-tie antenna on a surface of the substrate, the bow-tie antenna including the symmetrically opposed patches forms the Sierpinski gasket configuration of the first iteration, operatively connecting adjacent antenna patches on each arm of the Sierpinski configuration with an RF-MEMS switch, and selectively actuating the switches with a voltage source of 40 Volts. 
     Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is top schematic view depicting an exemplary reconfigurable antenna in accordance with embodiments of the present teachings. 
         FIG. 2  is a side schematic view of a switch used in the reconfigurable antenna of  FIG. 1  in accordance with embodiments of the present teachings. 
         FIG. 3  is top schematic view of the switch and associated bias lines in accordance with embodiments of the present teachings. 
         FIG. 4  is a diagrammatic view illustrating an antenna layout including a bias network in connection with the exemplary antenna. 
         FIG. 5  is graph illustrating an effect of a bow-angle with all switches OFF on an antennas first resonant frequency in connection with the exemplary antenna. 
         FIG. 6  is a graph illustrating an effect of a bow-angle with all switches ON for a first resonant frequency of an antenna in connection with the exemplary antenna. 
         FIG. 7  is a graph illustrating an effect of a bow-angle with all switches ON for a second resonant frequency of an antenna in connection with the exemplary antenna. 
         FIG. 8  illustrates an example of reconfigurable antenna performance. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     The following description of various exemplary embodiments including a self-similar fractal antenna configuration and a plurality of switches in a combination that yields a reconfigurable antenna that selectively radiates on demand at one of three different frequencies. The frequencies may be slightly varied according to a change in a bow-tie angle of the fractal antenna, but the length of each triangular patch is what affects the frequency the most. 
     The exemplary embodiments described herein are equally applicable to systems having more than one antenna iteration and various fractal self-similar configurations other than those described. In each instance, it will be appreciated that the outcome of a reconfigurable antenna operable on demand at a selected one of multiple frequencies will be obtained. 
     Various exemplary embodiments of the systems and methods according to this invention include a self-similar planar fractal antenna such as a modified Sierpinski gasket antenna and MEMS switches of the ohmic contact cantilever type as will be described. The feature of self-similarity of a fractal antenna provides the basis for the multiple frequency antenna herein. The antenna has the advantage of radiating similar patterns in a variety of frequency bands. 
     The following description is one possible implementation of the design but should not be considered the only possible implementation. 
     Referring first to  FIGS. 1 through 4 , an exemplary structure for a reconfigurable multifrequency antenna  100  is illustrated. In particular, the basis for the antenna  100  includes planar self-similar fractal antenna elements defining a Sierpinski configuration as shown. The reconfigurable antenna  100  is formed on a surface of substrate  300  and includes a DC voltage source  500  for selectively actuating a plurality of RF-MEMS switches  200 . The switches  200  and the reconfigurable antenna  100  are formed on the same substrate  300  in order to properly connect the switches  200  as will be described. 
     The fractal (or self-similar) antenna  100  includes a repeating triangular structure forming a Sierpinski gasket on each antenna arm. The antenna  100  may therefore be characterized as the described “self-similar” configuration with opposing arms  120  on the configuration  100 . Each arm  120  includes three triangular shaped antenna patches  130 . The antenna patches  130  each include a base end  132  and a vertex  134  opposing the base end  132 . The vertex  134  is joined to the base end  132  by sides  136  of the triangular antenna patch  130 . As will be apparent from the figures, base ends  132  of two antenna patches  130  define an outer end  122  of each antenna arm  120  and the vertex  134  of the remaining antenna patch  130  defines an inner angle  124  of the wing  120 . As such, the vertexes  134  of the outer end antenna patches  130  align with corners of the base end  132  of the remaining antenna patch  130 . Further, sides  136  of the triangular antenna patches  130  define common sides  126  of an overall antenna arm  120  as shown. The overall arm  120  defines a triangle as distinguished by a Sierpinski gasket antenna pattern. Opposing arms  120  are identical in structure and exhibit common characteristics as will be further described. 
     The individual antenna patches  130  are connected by the switch  200  at the vertexes  132  of the antenna patches aligned with the base end corners of the remaining triangular antenna patch  130 . Accordingly, two switches  200  are provided on each arm  120  of the antenna  100 . 
     It will be apparent that the radiation patterns of an antenna are inherently related to the distributions of the currents on its surface. By predetermining these current paths, the anitenna&#39;s radiation patterns can be defined at various frequencies of operation. By selectively actuating the individual switches  200 , a desired frequency may be obtained for the antenna  100 . In addition, the frequency will be further characterized based on the bow angle of the antenna configuration. 
     The switches  200  used herein are micro-electromagnetic switches (MEMS). The MEMS switches exhibit good radio frequency (RF) characteristics and can be used in both low and high frequency applications. 
     The switches  200  are arranged such that a single switch  200  is positioned at the vertex  134  of the two outermost antenna patches  130  to connect to base corners of the inner antenna patch  130  and thereby defining a Sierpinski gasket structure with connected triangular patches, as shown particularly in  FIG. 1 . The positioning of the four switches  200  permits a physical connection and disconnection of individual antenna patches  130  or sections of the antenna&#39;s conductive parts relative to each other. It will be apparent that the reconfigurable antenna  100  may be reconfigured in both symmetric and asymmetric designs. 
     The switches  200  enable either a bow-tie mode of operation in which all switches  200  are OFF, and a MEMS-enabled (or fractal) mode of operation in which all switches are ON. Since the fractal mode has an active (connected, interconnected or activated) structure consisting of the single-iteration Sierpinski gasket, two widely spaced resonant frequencies will result. 
     In an exemplary embodiment, when all switches  200  are OFF, the antenna  100  resonates at a first frequency of, for example 14 GHz, behaving as a bow-tie antenna. When all switches are ON, the antenna  100  resonates at two different frequencies of, for example 8 GHz and 23 GHz. These resonant frequencies are a result of the self-similar Sierpinski gasket fractal antenna configuration that is formed when all switches are ON. 
     It will be understood that two other non-symmetrical configurations may be obtained by setting one switch ON and one switch OFF on each arm  120  of the antenna  100 . The result is a total of four different paths for the currents to flow and therefore generates four possible antenna configurations. However, those switching connections generating non-similar radiating patterns with respect to the previously mentioned configurations, at their higher frequency resonances, are outside the scope of the present invention. Instead, it will be appreciated that the self-similarity between the two major modes of “bow-tie” and “fractal” results in similar radiation patterns which are of most importance to the present multiband invention. 
     Still further, an angle of the bow-tie antenna configuration contributes to the antenna radiating at a selected frequency. In an exemplary embodiment, a bow angle less than 90° gave satisfactory input impedance (close to 50Ω) and bandwidth for the OFF configuration. Also, a bow angle from about 35° to 60° gave satisfactory input impedance and bandwidth for both resonance frequencies of the switches ON configuration. By varying the bow angle, different input impedances can be obtained. An input impedance of about 50Ω is desired for all frequencies of interest. Also, considerable bandwidth is wanted to facilitate communications. The angle affects the bandwidth as well. Angles have been chosen where the impedance is about 50Ω and good bandwidth is observed. 
     According to an exemplary embodiment as shown in  FIGS. 2 and 3 , further details of the switches  200  are explained. The RF-MEMS switches  200  herein are formed on the substrate  300  such as, for example, a silicon substrate. The switch  200  includes an electrostatically actuated suspension membrane or cantilever  220  positioned above a biasing pull down electrode  230 . The pull down electrode  230  is overlaid with a dielectric material  240  such as silicon nitride. The input of the RF signal is denoted by RF IN  250  and the output of the RF signal is denoted with RF OUT  260  in  FIG. 2 , and are considered to be on the same metal layer with the antenna patches. High-resistive biasing lines  400 ,  410 , and  420  connect the switch  200  to corresponding DC biasing pads  402 ,  412 , and  422 , respectively. The biasing pads  402 ,  412 , and  422  can also be placed several wavelengths from the antenna  100  in order to mitigate any interference with the antenna&#39;s radiation. 
     The biasing voltage is a function of the area of the cantilevers  220  that is directly above the pull down (biasing) electrode  230 , the distance of the cantilever  220  from the electrode  230  when the cantilever  220  is up, the relative permittivity of the dielectric material  240  between the cantilever  220  and the electrode  230 , and the flexibility and thickness of the membrane material defining the cantilever  220 . Switching times of 5-30 μs have been achieved. The biasing voltage determines the minimum distance between the biasing lines  400 ,  410 , and  420  according to the breakdown voltage of the substrate material  300 . 
     In accordance with various embodiments, the biasing lines  400 ,  410 , and  420  are placed at a distance that withstands more than five times higher voltage than the actual voltage applied by DC voltage source  500 . 
     As indicated, each switch  200  is fabricated on the substrate  300 , such as a silicon wafer. The silicon substrate  300  may be, for example, a 400 μm thick, high-resistivity (p&gt;10 KΩ-sq) silicon wafer. The cantilevered flexible membrane  220  is suspended about 2 μm above the bottom pull down electrode  230 . The pull down electrode  230  is further connected to a DC probe pad (not shown) after its corresponding high-resistive line such that electrostatic biasing occurs on demand by applying a DC voltage of approximately 40 Volts to the DC probe pad. The switch  200  performs in the exemplary antenna applications for frequencies up to 40 GHz. 
     Accuracy of an applied potential difference to the switch  200  is ensured by grounding the other two biasing lines Bias  1  ( 410 ) and Bias  2  ( 420 ) in addition to the bias line Bias  0  ( 400 ) where the DC voltage to the switch  200  is applied. The bias lines  400 ,  410 ,  420  are connected to the switch  200  as shown in  FIG. 4 . The DC biasing pads  402 ,  412 ,  422  for each switch  200  are placed about 2500 μm away from the outermost conductive part of the antenna  100 , to minimize the deformation of the radiation pattern caused by the metallic surface of the probe chucks used for measurement (not shown). 
     The bias lines  400 ,  410 ,  420  are conductive and selection of the metal for the bias lines therefore affects the antenna&#39;s behavior. Accordingly, the present invention utilizes a high-resistive material for the metallic bias lines. For example, the conductive material of the bias lines can be Aluminum-deposited Zinc Oxide (AZO) deposited by a combustion chemical vapor deposition procedure. Even further, the DC bias lines may consist of two different materials including the highly resistive AZO and a thin layer of conductive metal in connection with the DC probe pads. The thin layer of conductive metal may be gold. The highly resistive bias lines are applied with a chemical etching process while the conductive thin layer of gold is applied with a lift off process. 
     The bias lines  400 ,  410 ,  420  are positioned to pass close to the antenna and parallel to its sides (edges) as shown in the biasing network of  FIG. 4 . In this manner, if any energy is radiated from the bias lines  400 ,  410 ,  420  or coupled to the bias lines, the energy will, most likely, constructively interfere with the antenna&#39;s radiation pattern and so it will not deteriorate the antenna&#39;s performance. The use of high-resistive materials for the metallic bias lines overcomes any potential increase of the currents surface density at the points where the bias lines  400 ,  410 , and  420  connect to the switch  200 . Thus, deformation of the antenna&#39;s radiation pattern is minimal and the slight extension of the currents&#39; path causes only a slight shift in the resonant frequencies. 
     Selective actuation of the switches  200  enables two different symmetric antenna configurations with each of the three resonant frequencies demonstrating a similar radiation pattern. In an exemplary embodiment of the antenna  100  and at a state defined by all switches OFF, a first band of 14 GHz is achieved. At a state defined by all switches ON, a second band of 8 GHz and a third band of 23 GHz can be achieved. 
     The DC pads are both of 150 μm and 400 μm pitch for measurement purposes. Further, the DC bias is applied from the top and bottom of the antenna, while the RF is applied from the side of the antenna. 
     In order to feed the antenna  100 , a balanced typed of feed that will set the voltage on its terminals to a 180° phase-difference is used. The antenna is fed with the RF probe through a coplanar waveguide (CPW) to coplanar stripline (CPS) transition. The transition maintains a 50Ω characteristic impedance and ends in the pads with 150 μm pitch. The RF feed line is fabricated on the same substrate as the antenna and enables the measurement of the antenna&#39;s performance using the available RF probes. Details of the transition are outside the scope of the present embodiments and will not be discussed further herein. 
     Another feature of the exemplary embodiments resides in the deposition and patterning of the thin layer of the silicon nitride dielectric material in connection with the switch. It will be appreciated that the thickness, smoothness, and uniformity of the layer should be well controlled to provide a good isolation layer between the cantilever membrane  220  and the pull-down electrode  230  of the MEMS switches  200 . 
     Referring now to  FIGS. 5 through 7 , graphs are provided to further illustrate an effect of the bow angle of the antenna when all switches  200  are OFF or ON. From  FIG. 5  (switches OFF), it can be seen that the resonant frequency diverges more and more for wider bow-angles from a predicted one when the antenna is placed on a dielectric half-space. This means that the capacitive coupling is greater for wider angles and thus increases the antenna&#39;s effective surface. 
     From  FIG. 6  and all switches ON, it can be seen that as the bow angle becomes larger, the self-similar antenna resonates at increasingly lower frequencies and thus its active area becomes slightly larger. This suggests that capacitive coupling between the triangles increases, and additional parts of the structure radiate causing the active area to enlarge. At the same time, the triangular gap in the structure defines different current paths on the antenna, and practically reduces its effective area and thus it increases the antenna&#39;s resonant frequency. 
     From  FIG. 7  and all switches ON, it can be seen that the antenna resonates at a frequency almost one and a half times higher than with all switches OFF. 
       FIG. 8  illustrates an example of reconfigurable antenna performance. The antenna is designed to resonate at three different frequencies, labeled as f 1 , f 2 , and f 3 . Two of the frequencies, f 1  and f 3  occur when all switches are ON, and the remaining frequency f 2  occurs when all switches are OFF. It will be apparent that the frequencies increase from f 1  to f 3  and are distinctly spaced. The representative visualization illustrates that the maximum effect of the bias lines on the antenna&#39;s performance occurs at the higher frequencies. 
     While the invention has been illustrated with respect to one or more exemplary embodiments, alterations and/or modifications can be made to illustrated examples without departing form the spirit and scope of the appended claims. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several embodiments such feature may be combined with one or more other features of the other embodiments as may be desired and advantageous for any given or particular function. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.” And as used herein, the term “one or more of” with respect to a listing of items such as, for example, “one or more of A and B,” means A alone, B alone, or A and B. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.