Patent Publication Number: US-7898481-B2

Title: Radio frequency system component with configurable anisotropic element

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to radio frequency system components. 
     BACKGROUND 
     Radio frequency technology is used in a variety of applications, two broad categories of which are sensing and communication. The former category includes such diverse applications as Magnetic Resonance Imaging (MRI) and Radio Detection and Ranging (Radar). The latter category includes wireless communication using a myriad of different frequency bands and protocols including cellular telephony. Cellular telephony has revolutionized communication and continues to grow in importance. For cellular telephony in particular distinct frequency bands are often used in the same geographic area because there are competing standards and in order to support legacy devices. Moreover, more frequency bands are being allocated for higher bandwidth services that are being introduced. A particular wireless device may support more than one protocol for more than one application. Examples of protocols are, RFID, WLAN, WiMAX, UWB, 3G and 4G. Examples of applications are multimedia, mobile internet, connected home solutions, and sensor-networks. In this situation it is desirable to provide increasing physical channel diversity (e.g., frequencies, polarizations) in a single wireless communication device. Diversity can also be a means to improved Quality of Service (QoS) in challenging Radio Frequency (RF) environments (e.g., urban settings). Moreover, reconfigurable, multimode antennas are needed to be able to adapt to multiple user positions, restrictive data mode grips, and other environmental variables. As a result, there is a strong demand for antennas that are resonant at multiple frequencies or can be tuned to multiple frequencies and/or different polarizations and that have thin and flexible form factors. Consumer expectations call for small wireless handsets (e.g., cellular telephones, smart phones, etc.), which have limited space for their antenna systems. Thus, there is a strong need for antenna systems that provide more frequency bands and agile polarization diversity without requiring much more space. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention. 
         FIG. 1  is a fragmentary sectional elevation view a planar antenna according to an embodiment of the invention; 
         FIGS. 2-3  are cross sectional views of a cell including an electrically configurable anisotropic medium that is used in the antenna shown in  FIG. 1  according to an embodiment of the invention; 
         FIG. 4  is a plan view of a cross-shaped slot used in the antenna shown in  FIG. 1  according to an embodiment of the invention; 
         FIG. 5  is a plan view of an H-shaped slot used in the antenna shown in  FIG. 1  according to an alternative embodiment of the invention; 
         FIG. 6  is a plan view of “dog bone” shaped slot used in the antenna shown in  FIG. 1  according to yet another alternative embodiment of the invention; 
         FIG. 7  shows a plan view of the cell shown in  FIGS. 2-3  along with an arrangement of control electrodes in a first state according to an embodiment of the invention; 
         FIG. 8-9  show alternative states of the electrodes and cell shown in  FIG. 7 ; 
         FIG. 10  is a fragmentary sectional elevation view of a planar antenna according to an alternative embodiment of the invention; 
         FIG. 11  shows a plan view of a cell including an electrically configurable electromagnetically anisotropic medium along with an arrangement of control electrodes used in the planar antenna shown in  FIG. 10 ; 
         FIG. 12  shows an approximate pattern of alignment of elongated conductors when suspended in a liquid crystal having a positive anisotropy and subjected to an electric field established in the cell; 
         FIG. 13  is similar to  FIG. 12  but with a liquid crystal having a negative anisotropy; 
         FIG. 14  shows a plan views of a cell holding an electrically configurable electromagnetically anisotropic media along with an arrangement of an outer control electrode and via pins according to another alternative embodiment of the invention; 
         FIG. 15  is similar to  FIG. 11  but with an alternative outer electrode shape; 
         FIGS. 16-17  are plan views of a planar antenna that has a 2-D array of drive electrodes and cells holding an electrically configurable electromagnetically anisotropic media; 
         FIG. 18  is a plan view of a planar antenna element that has a plurality of linear drive electrodes alternating in position with cells holding an electrically configurable electromagnetically anisotropic media; 
         FIG. 19  is a planar inverted “F” antenna that includes multiple tuning cells for frequency tuning; and 
         FIG. 20  is schematic of a biasing circuit for the antenna shown in  FIG. 19 . 
     
    
    
     Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. 
     DETAILED DESCRIPTION 
     Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method steps and apparatus components related to radio frequency technology. Accordingly, the apparatus components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
     In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. 
     Nanostructures such as nanotubes and nano-wires show promise for the development of radiation elements of antennas. Preparation of these nanostructures by chemical vapor deposition (CVD) has shown a clear advantage over other approaches. The CVD approach allows for the growth of high quality nanotubes by controlling their length, diameter, location, and pattern using catalytic nano-particles. In particular, carbon nanotubes are typically a helical tubular structure grown with a single wall or multi-wall, and commonly referred to as single-walled nanotubes (SWNTs), or multi-walled nanotubes (MWNTs), respectively. Single wall carbon nanotubes typically have a diameter in the range from a fraction of a nanometer to a few nanometers. Multiwall carbon nanotubes typically have an outer diameter in the range from a few nanometers to several hundreds of nanometers, depending on inner diameters and numbers of layers. Each layer of a MWNT is a single wall tube. Carbon nanotubes can function as either a conductor, like metal, or a semiconductor, according to the rolled shape (chirality) and the diameter of the helical tubes. With metallic nanotubes, a carbon-based structure can conduct a current in one direction at room temperature with essentially ballistic conductance so that metallic nanotubes can be used as ideal radiation elements. 
     Liquid crystals (LCs) with several basic phases are widely used for various display devices. Recent publications have shown that a liquid crystal, for instance, nematic phase, can be utilized to host carbon nanotubes (CNTs) and effectively disperse the CNTs in the LC host matrix. CNTs are thus uniformly distributed in a LC host matrix. The LC host is made up of elongated molecules and has anisotropic dielectric properties. The so-called Freedericksz transition is a fundamental aspect of liquid crystals. In the transition a collective reorientation of the LC director along the direction of an applied electric field for, e.g., positive dielectric anisotropy and the molecules align with each other in a process of self-organization. It has been shown that the LC order can be transferred to carbon nanotubes dispersed in the LC by elastic interactions. Therefore, well-aligned nanotubes with their tube axes aligned in the direction of the LC director can be formed and controlled by an applied electric or magnetic field. Very large increases of electrical conductivity (e.g., several orders of magnitude) have been observed. The increases are theorized to be due to the formation of multiple conducting paths through tube-to-tube conducting and super conductivity of metallic CNTs. Moreover, small quantities of conductive ions existing in a LC host have been shown to be trapped by the CNTs in tube-to-tube conducting areas through charging. Dipole moments due to ion trapping by CNTs can serve to further enhance long-range elastic interactions for the realignment of the CNTs under an applied electric or magnetic field. Combining the low loss, high anisotropic conductivity of metallic CNTs and the proper utilization of electric or magnetic field for alignment control and switching, and the choice of various LC phases, the LC-CNT media can be uniquely used for antenna designs with agile polarization diversity and multi-bands in a limited design space. The aforementioned properties are exploited in the present innovation. 
       FIG. 1  is a fragmentary sectional elevation view a planar patch antenna  100  according to an embodiment of the invention. The planar patch antenna  100  comprises a number of patterned conductor layers separated by dielectric layers as will be described. A DC grounding layer  102  is located on the bottom of the planar antenna  100 . The DC grounding layer  102  is spaced by a first dielectric layer  104  from a stripline feed  106 . The stripline feed  106  is connected to a transceiver (not shown) which receives and/or transmits using the planar patch antenna  100 . The stripline feed  106  is spaced by a second dielectric layer  108  from a slot  110  which is formed in an antenna ground plane  109 . Various possible alternative slot shapes with different excitation methods and bandwidth enhancement are shown in  FIGS. 4-6 .  FIG. 4  shows a crossed slot  402 ,  FIG. 5  shows an H-slot  502 ,  FIG. 6  shows a “dog bone” slot  602 . The stripline feed  106  is an active element of the antenna  100 . A cell  112  holding an electrically configurable anisotropic material  202  is located above the slot  110  and spaced from the slot  110  by a third dielectric layer  114 . Several electrodes  116  are positioned around the cell  112  and make electric contact with the cell  112  on their edge surfaces. The cell  112  holding the electrically configurable anisotropy material in combination with the electrodes  116  act as a parasitic (passive) radiating element of the antenna  100 . The specifically-shaped slots shown in  FIGS. 4-6  are able to excite the passive radiating element in different ways via electromagnetic coupling. As shown in  FIG. 2  the cell  112  includes the anisotropic material  202  enclosed between a lower dielectric film  204  and an upper dielectric film  206  that can be called a superstrate. The lower dielectric film  204  is not necessary if a cavity for the cell  112  is formed on the surface of the third dielectric layer  114 . According to embodiments of the invention the electrically configurable anisotropic material  202  includes elongated conductive bodies  208  dispersed in a medium  210 . According to certain embodiments the elongated bodies  208  are Carbon Nanotubes (CNT) and the medium  208  is a Liquid Crystal (LC). The latter combination is referred to herein below by the abbreviation LC-CNT. According to certain embodiments the CNTs are Multi-Walled Carbon Nanotubes (MWCNTs). Metallic single-walled carbon nanotubes (SWCNTS) can also be used as the CNTs. Other types of metallic nano-wires can also be used as the elongated bodies. Pre-alignment of the LC-CNT can be achieved by mechanical means such as rubbing technique on the inner surfaces of dielectric films  204  and  206 . However, pre-alignment is not required. 
       FIG. 2  shows a random arrangement of the elongated bodies  208  that prevails when no voltage is applied to the electrodes  116 . On the other hand  FIG. 3  shows a parallel alignment and tube-tube conducting paths of the elongated bodies that are established when an electric field is applied to two or more of the electrodes  116 . 
     The overall size of the cell  112  and electrodes  116  depends on the frequency (wavelength) of the antenna  100  which may be varied for different applications. The cell size  112  can range from nanometers for optical antennas, sub-micron for terahertz, to micron for sub-millimeter wave, and to millimeter for millimeter wave and microwave antennas. The volume fraction of the elongated bodies  208  such as CNTs needs to be sufficiently high so that multiple conducting paths can be established after the LC-CNT alignment. Started from a certain percentage, e.g., the so-called percolation percentage where at least a conducting path is established, the CNT volume fraction can be ranged from 0.01 percent to 50 percent and even higher if needed. The volume fraction depends on the choice of the average CNT length ranging from nanometers to micrometers and millimeters, the CNT length distribution and aspect ratio (length to diameter) distribution. Millimeter long CNTs can be used in larger sized cells  112  for microwave antennas. Moreover, the LC-CNT media  202  can be doped with the small amount of conducting ions. In some cases, the ions are present as impurities. Furthermore, strong charge transfer from the adjacent LC molecules to CNTs and consequently ion trapping by the CNTs can be used for enhancing electric conductivity and alignment by creating CNT&#39;s with a long-range permanent dipole moment. Ions trapped between CNTs after alignment by electrical and/or mechanical means can significantly increase the CNT tube-to-tube conductivity. Different kinds of liquid crystals (LCs) can be selected as the media  210 . Nematic, cholesteric, semectic phases and their mixtures can be chosen although the nematic LC is preferred. 
       FIGS. 7-9  are plan views of the planar antenna  100  showing the cell  112  and the electrodes  116 . In  FIGS. 7-9  the electrodes  116  are identified by unique reference numerals. As shown in  FIGS. 7-9  the electrodes  116  include an upper electrode  702 , a right electrode  704 , a bottom electrode  706  and a left electrode  708 . The electrodes  702 - 708  are used to apply different electric fields to the material  202  in order to change the electric current directionality and pattern of the anisotropy of the material  202 . As shown in  FIG. 7  a positive potential is applied to the upper electrode  702  and a negative potential is applied to the lower electrode  706  while the right electrode  704  and left electrode  708  are grounded. With the potential as shown in  FIG. 7 , in a first case that the LC exhibits positive dielectric anisotropy the directors of the LC will align vertically parallel to the electric field extending from the upper electrode  702  to the lower electrode  706 , leading to a radiated field having a first polarization state. Alternatively, if the LC has a negative dielectric anisotropy the LC directors will align perpendicular to the electric field. Moreover, charge transfer from LC molecule to CNT and the ion trapping by CNTs result in permanent dipole moments. The long-range moments strongly assist alignment under the applied electric field. In either case the alignment results in the formation of tube-to-tube electric contacts for creating multiple long-range conducting paths crossing the cell  112  length scale and reaching to electrodes  116 . Therefore, an anisotropic polarization is formed by the anisotropic polarization media. The polarization pattern or the distribution of electrical current directions can be controlled by an applied electric (or alternatively magnetic) field. 
     In  FIG. 8  positive and negative potentials are applied to the right electrode  704  and the left electrode  708  respectively while the upper electrode  702  and the lower electrode  706  are grounded. With the potentials applied as shown in  FIG. 8 , if the LC exhibits a positive anisotropy a second polarization state of the radiated field that is different from the first polarization state will be produced. As shown in  FIG. 9  the positive potential is applied to the upper electrode  702  and the left electrode  708  and negative potential is applied to the right electrode  704  and the lower electrode  706 . Each different set of electrode potentials will lead to a different electric field, a different pattern of the alignment of the directors of the LC and CNTs, and therefore, a different polarization pattern by controlled distributions of electrical currents&#39; directions in the radiation element. Because the CNTs exhibit anisotropic conductivity and are properly dispersed inside the dielectric LC media, aligning the CNTs in different patterns will alter the radiation pattern of the planar antenna  100 . By using flexible materials for the dielectric layers  104 ,  108 , and  114 , the antenna structure  100  with the cell  112  and electrodes  116  can also be made conformal so that the antenna can be mounted on a curved surface such as a device housing. The antenna  100  could also be molded onto a housing of a wireless device by different molding techniques such as insert, injection, and two-shot moldings. 
     According to certain embodiments of the invention the slot  110  is shaped and oriented relative to the stripline feed  106 , so that the stripline feed will excite an elliptical (e.g., circularly) polarized mode. Alternatively, the slot  110  is shaped and oriented to produce a linearly polarized mode that is aligned at an angle (e.g., 45 degrees) relative to the cardinal alignment (e.g., up, down, left, right) of the electrodes  702 - 708 . In either case, by altering the pattern of alignment of the CNTs in the cell  112  the radiation pattern of the planar antenna  100  will be altered. In particular, the polarization of waves emitted by the antenna  100  can be varied and tuned by the antenna designs with different combinations of anisotropic polarization elements composed of cell  112  and electrode  116  from  FIG. 7-9  with slot shapes of  110  from  FIG. 4-6 . Thus, the antenna  100  is capable of increasing the physical channel diversity and frequency agility. 
       FIG. 10  is a fragmentary sectional elevation view of a second planar antenna  1000  according to an alternative embodiment of the invention. The second planar antenna  1000  differs from the planar antenna  100  shown in  FIG. 1  in that the second planar antenna  1000  includes conductive trace  1002  that extends along a bottom surface  1004  of the first dielectric layer  104  to a conductive via  1006  that extends through the first dielectric layer  104 , through an aperture  1008  in the stripline feed  106 , through the second dielectric layer  108 , through the slot  110  and the third dielectric layer  114  to the cell  112 . For microwave frequencies the via can have a diameter of several microns. For sub-millimeter, terahertz or optical communications a smaller diameter via may be appropriate. In the latter case, a single MWCNT or the bundle of MWCNTs or SWCNTs can be used for constructing the via  1006  by proper metallization of the end of the CNTs and connection with the conductive trace  1002 . The conductive via  1006  works in conjunction with a peripheral electrode  1010  that surrounds the cell  112 , allowing radial electric fields to be established for the purpose of aligning an electrically configurable anisotropic material (e.g., LC-CNT) in the cell  112 .  FIG. 11  shows a plan view of the cell  112  with the peripheral electrode  1010  and the top of the conductive via  1006 .  FIG. 12  shows an approximate two-dimensional pattern of alignment of elongated conductors when suspended in a liquid crystal having a positive dielectric anisotropy and subjected to an electric field established in the cell  112  as shown in  FIG. 11 .  FIG. 13  is similar to  FIG. 12  but with a liquid crystal having a negative dielectric anisotropy. Different patterns of electric current distributions can be established by aligning CNTs in LC having different anisotropy properties. By combining one of the slot shapes shown in  FIGS. 4-6  with an electric current distribution pattern supported by the LC-CNT patterns shown in  FIG. 12-13 , multiple resonant frequencies and an agile polarization pattern can be obtained in a single patch antenna construction, thereby achieving increased physical channel diversity. 
       FIGS. 14-15  show plan views of cells holding electrically configurable electromagnetically anisotropic media along with arrangements of control electrodes according to other alternative embodiments of the invention. In  FIG. 14  in addition to the single central conductive via  1006  there are four additional conductive vias  1402  arranged in a specific pattern. Locations of the vias  1402  are dependent on the shape of the slot  110  and can be determined by routine experiment. The via location can be tuned to match desired frequency bands. Via numbers can be increased or decreased as needed to achieve specific frequency bands and/or polarization patterns. Vias can also be switched on simultaneously or sequentially for applying different electric fields for CNT alignment and pattern formation. This capability further increases the antenna design robustness and tunability for both frequency and polarization patterns. Alternatively, the vias  1402  can also be used as shorting pins by connecting them with the antenna grounding plane while the central via  1006  is used for applying a voltage to establish a field for CNT alignment. Similar to via  1006 , the additional vias  1402  can be constructed by using a single MWCNT or CNT bundles. 
     In  FIG. 15  a round cell  1502  is used instead of the square cell  112  with a round peripheral electrode  1504 . In the round cell  1502 , radial or circumferential (azimuthal) conductivity can be obtained by using a LC host that exhibits positive or negative dielectric anisotropy respectively after an electrical (or magnetic) field is applied for CNT alignment. In combination with the feeding slots ( FIGS. 4-6 ), the round cell can also create different frequency bands with polarization agility. 
     After aligning the CNTs&#39; with an applied electric (or magnetic) field adjusting the LC-CNT alignment pattern in order to achieve operation in predetermined frequency bands with predetermined polarization patterns for particular RF applications, the LC-CNT mixture material  202  inside the cell  112  can be polymerized. In this way, well-dispersed CNTs with multiple conducting paths and electrical polarization patterns are locked-in and embedded inside a liquid crystal polymer matrix. In this case of off-line alignment and tuning, high voltage can be applied to generate a very strong field for better CNT alignment and tube-to-tube conducting. The field can be removed after the pattern is locked-in by polymerization. 
       FIGS. 16-17  show a planar antenna  1600  according to another embodiment of the invention. The planar antenna  1600  has a rectangular array  1602  of rectangular electrodes  1604  (only a few of which are indicated by reference numeral to avoid crowding the drawing), supported on a dielectric substrate  1606 . (Alternatively the shape of the array  1602  and/or the shapes of the electrodes  1604  may be other than rectangular, for example, oval or circular.) An array of cells  1608  (only a few of which are indicated by reference numeral) holding the configurable anisotropic material  202  including the elongated bodies  208  dispersed in a medium  210  (e.g., the LC-CNT material) are located in interstices between the electrodes  1604 . Thus, the electrodes  1604  are positioned around the cells  1608  and by applying different combinations of voltages to the electrodes  1604 , different electric field patterns can be established in the cells  1608  in order to configure the configurable anisotropic material  202 . In  FIGS. 16-17  ‘+’ and ‘−’ signs and zero marked on the electrodes  1604  indicated applied voltages. Additionally, the alignment of the elongated bodies (e.g., CNT) is indicated by cross hatching and diamond shapes in the cells  1608 . 
     More patterns than are represented in  FIGS. 16-17  can be produced by applying different combinations of voltages to the electrodes  1604 . The sizes of the cells  1608  and electrodes  1604  is scaleable to accommodate operation at different frequencies ranging from microwave frequencies to millimeter, and sub-millimeter wave frequencies. For higher frequency bands up to Terahertz and beyond, the cells  1608  and electrodes  1604  can be fabricated at micro and nano scales if needed. At such scales shorter CNTs with nanometer lengths can be used. Even if the voltage that can be applied to the electrodes  1604  in order to align the LC-CNT material is limited, the cell  1608  size can be reduced and numbers of the cells can be increased in order to achieve high electric field stength. Therefore, the robustness of the design shown in  FIGS. 16-17  with the scalable capability provides device solutions for antennas for a wide range of frequency bands. The slots  402 ,  502 ,  602  shown in  FIGS. 4-6  can be used to drive the planar antenna  1600  which would be arranged overlying but spaced from the slots  402 ,  502 ,  602 . Alternatively, an in-plane antenna feed  1610  can be coupled directly (e.g., at a corner) to the antenna  1600 . Alternatively, the antenna  1600  can be made into a phased array antenna by spacing the cells  1608  by about one-half the operating wavelength. Such a phased array antenna will be active with the capability of polarization diversity. 
       FIG. 18  is a plan view of a planar antenna element  1800  that has a plurality of linear drive electrodes alternating in position with cells holding an electrically configurable electromagnetically anisotropic media. The antenna element  1800  can be located over a slot antenna such as shown in  FIGS. 4-6  and function as a radiation modifier, or can be fed microwave energy directly using a stripline  1802  and act as an active antenna element. The planar antenna element  1800  has a set of elongated horizontally extending (in the perspective of  FIG. 18 ) electrodes  1804  that are spaced apart from each other. Located between the horizontally extending electrodes  1804  are a plurality of cells  1806  that hold the aforementioned LC-CNT material. A plurality of vertical spacer bars  1808  extend between each pair of adjacent horizontally extending electrodes  1804 . At the left and right sides of the antenna element  1800  there are vertically extending electrodes  1810  located between the horizontal electrodes  1804 . 
     In the configuration shown in  FIG. 18  successive horizontal electrodes in the set  1804  alternate between positive and a negative applied voltages, and the vertically extending electrodes  1810  have zero voltage. With the foregoing set of voltages, assuming a positive anisotropy of the LC, the LC-CNT material will be vertically polarized effectively providing microwave conductance in the vertical direction. Conductance in the horizontal direction will be provided by the horizontally extending electrodes  1804 . In the case that the antenna element  1800  is directly driven using the stripline  1802 , the antenna element  1800  will be able to radiate two orthogonal polarization components. When the voltages on the horizontally extending electrodes  1804  is removed, the vertical conductance of the LC-CNT will diminish and the vertical polarization radiation component will diminish. This capability provides a de-tuning solution. 
     In the case that the antenna element  1800  is used over a slot antenna, varying the voltages on the horizontally extending electrodes  1804  will vary the relative magnitude of the two orthogonal polarization components. 
       FIG. 19  is a planar inverted “F” antenna  1900  that includes multiple tuning cells for frequency tuning. The antenna  1900  has a ground leg  1902 , a first feed leg  1904  and a second feed leg  1906 , a common ground plane  1908 , and a main radiating element  1910  that is arranged parallel to and spaced from the ground plane  1908 . The ground leg  1902  extends from the ground plane  1902  to the main radiating element  1910 . The feed legs extend from a location proximate the ground plane  1902  to the main radiating element  1910 . 
     The feed legs  1904 ,  1906  include the LC-CNT  1912  (or other configurable anisotropic medium) held between two dielectric substrates  1914 . A microwave signal can be coupled through either of the feed legs  1904 ,  1906 . One of the feed legs  1904   1906  is selectively activated by a DC biasing signal through the electrodes  1915 ,  1927  in order to apply a DC field to the LC-CNT  1912 . End electrodes  1915  are provided for coupling the microwave signal to the LC-CNT  1912  and applying the DC biasing signal to the LC-CNT. The DC biasing signal sets up a longitudinal electric field that orients the LC-CNT material  1912  to switch on the feed legs  1904  and  1906 . Selecting between the feed legs  1904 ,  1906  enables the antenna  1900  to be tuned to different frequency ranges as needed. 
     The main radiating element  1910  comprises conducting portion  1916  to which the ground leg  1902  and the feed legs  1904 ,  1906  attach, as well as a first extension  1918 , a second extension  1920  and a third extension  1922  which are connected in series to the conducting portion  1916 . The conducting portion  1916  is an active element of the antenna. With reference to the first extension  1918  in  FIG. 19 , each extension includes a layer of LC-CNT material  1924  held between two dielectric strips or substrates  1926 . Electrodes  1927  located at ends of the extensions  1918 ,  1920 ,  1922  and the feed legs  1904 ,  1906  are used to apply DC biasing fields to the LC-CNT  1924 ,  1912 . There is a gap between the electrodes  1927  of the different extensions  1918 , 1920 , 1922 , and between the first extension  1918  and the conducting portion  1916  which isolates DC bias current but passes microwave currents by capacitive coupling. The gap can be filled with air or other dielectric materials. Different combinations of the extensions  1918 ,  1920 ,  1922  can be activated by applying DC biasing signals in order to establish longitudinal electric fields in the extensions  1918 ,  1920 ,  1922 . Actuating different combinations of activated extensions  1918 ,  1920 ,  1920  will cause the antenna  1900  to operate at different frequencies by changing its physical length, the impedance, and/or by parasitic tuning elements. In the case that there is an active extension (e.g.,  1920 ,  1922 ) separated from the conducting portion  1916  of the main radiating element  1910  by an inactive extension (e.g.,  1918 ), the active extension will act as a parasitic antenna element. Thus, frequency diversity is achieved by activating different combinations of the feed legs  1904 ,  1906  and the extensions  1918 ,  1920 ,  1922 . Although three extensions  1918 ,  1920 ,  1922  are show, alternatively more or less than three extensions can be provided. Alternatively the antenna  1900  is a non-planar (wire) inverted F antenna. 
       FIG. 20  is schematic of a biasing circuit  2000  for the antenna shown in  FIG. 19 . The circuit  2000  is for biasing the extensions  1918 ,  1920 ,  1922 . A similar circuit can be used for biasing the feed legs  1904 ,  1906 . Referring to  FIG. 20  a series of capacitances  2002  provide DC isolation between the conducting portion  1916  and the first extension  1918  and between successive extensions  1918 ,  1920 ,  1922 . The capacitances  2002  may be realized by discrete capacitors or a gap filled with air or other dielectric materials. Microwave signals can pass through the capacitances  2002 . 
     A biasing DC voltage source  2004  is selectively applied through the circuit in order to establish a longitudinal biasing E-field in one or more of the extensions  1918 ,  1920 ,  1922 . The biasing voltage source  2004  may be variable. The biasing source  2004  is connected to the left side of the first extension  1918  through a first switch  2006  and a first inductor  2008 . A first capacitor  2010  is connected between the junction of the first switch  2006  and the first inductor  2008  and an RF ground. The first inductor  2008  and the first capacitor  2010  as well as other similar arrangements of capacitors and inductors described below serve to isolate the biasing voltage source  2004  from microwave currents flowing in the antenna  1900 . 
     The right side of the first extension  1918  is connected to a second inductor  2012  which is connected to a first resistor  2014  and a second capacitor  2016 . The first resistor  2014  is connected to a biasing signal ground and the second capacitor  2016  is connected to the RF ground. The left side of the second extension  1920  is connected through a third inductor  2018  to the first resistor  2014  and the second capacitor  2016 . 
     The biasing voltage source  2004  is connected through a second switch  2020  and a fourth inductor  2022  to the right side of the second extension  1920 . A third capacitor  2024  is connected between the junction of the fourth inductor  2022  and the second switch  2020  and the RF ground. 
     Similarly, the biasing voltage source  2004  is connected through a third switch  2026  and a fifth inductor  2028  to the left side of the third extension  1922 , and a fourth capacitor  2030  is connected between the junction of the fifth inductor  2028  and the third switch  2026  and the RF ground. 
     Additionally, the right side of the third extension  1922  is connected through a series of a sixth inductor  2032  and a second resistor  2034  to ground; and a fifth capacitor  2036  is coupled between the junction of the sixth inductor  2032  and the second resistor  2034  and the RF ground. 
     By selectively closing one or a combination of the switches  2006 ,  2020 ,  2026  the voltage from the biasing source  2004  can be applied to one or a combination of the extensions  1918 ,  1920 ,  1922 . Components of the biasing circuit can be located both on the planar inverted “F” antenna  1900  itself and on a circuit board that includes the ground plane  1908 . 
     The inductors  2008 ,  2012 ,  2018 ,  2022 ,  2028 ,  2032  are RF chokes to isolate the DC power supply from the RF signal. In addition, capacitors  2010 ,  2016 ,  2018 ,  2030 ,  2036  are RF bypass capacitors to further protect the DC circuit and are connected to a common RF ground. Switches  2006 ,  2020 ,  2026  are used to turn on or off the DC voltage source  2004 . If AC grounding is to be separated from DC grounding by shielded lines or other means known in the art, a simplified circuit can be utilized for the circuit  2000 . A similar circuit can also be used for biasing the feed legs  1904 ,  1906 . and active 
     It will be apparent to persons of ordinary skill in the art that the embodiments shown in  FIG. 1-20  are merely examples of wide variety of antennas that can be variably loaded using a cell with a configurable anisotropic medium in order to achieve polarization and/or frequency agility. 
     In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.