Abstract:
A tuneable impedance surface for steering and/or focusing a radio frequency beam. The tunable surface comprises a ground plane; a plurality of elements disposed a distance from the ground plane, the distance being less than a wavelength of the radio frequency beam; and a capacitor arrangement for controllably varying the capacitance of at least selected ones of adjacent elements. A method of tuning the high impedance surface allows the surface to mimic, for example, a parabolic reflector or a diffraction grating.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with government support under Contract No. N6601-99-C-8635. The government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     This invention relates to a surface having a tunable electromagnetic impedance, and includes a conductive sheet of metal or other conductor, covered with an array of resonant elements, which determine the surface impedance as a function of resonance frequency. The surface impedance governs the reflection phase of the conductive sheet. Each resonant element is individually tunable by adjusting a variable capacitor, thereby controlling the electromagnetic impedance of the surface. By having a tunable, position-dependent impedance, this surface can be used to focus a reflected Radio Frequency (RF) beam by forming an effective Fresnel or parabolic reflector or to steer a reflected wave by forming an effective prism or grating. The tunable impedance surface can be used to steer or focus an RF beam, which is important in such fields as satellite communications, radar, and the like. 
     BACKGROUND OF THE INVENTION 
     Prior art approaches for RF beam steering generally involve using phase shifters or mechanical gimbals. With the tunable surface disclosed herein, beam steering is accomplished by variable capacitors, thus eliminating expensive phase shifters and unreliable mechanical gimbals. The variable capacitors can be controlled electronically using variable dielectrics, or tuned using devices to impart relatively small mechanical motion such as microelectromechanical (MEM) switches. 
     Focusing an RF beam by a flat surface has been accomplished in the prior art by using an array of nearly resonant half-wave dipoles, which are designed to have a particular reflection phase. However, if such a structure is to include a ground plane, this prior art structure must be one-quarter wavelength thick. In the present invention, the thickness of the tunable surface is much less than one-quarter wavelength. The available bandwidth is partly determined by the tunability of the small resonant elements on the surface, which are tuned by variable capacitors. 
     The present application is related to U.S. patent application Ser. No. 09/537,921 entitled “An End-Fire Antenna or Array on Surface with Tunable Impedance” filed Mar. 29, 2000 and to U.S. patent application Ser. No. 09/537,722 entitled “An Electronically Tunable Reflector” filed Mar. 29, 2000 the disclosures of which are hereby incorporated herein by this reference. 
     The prior art includes U.S. Pat. No. 4,905,014 to Daniel G. Gonzalez, Gerald E. Pollen, and Joel F. Walker, “Microwave phasing structure for electromagnetically emulating reflective surfaces and focusing elements of selected geometry.” This patent describes placing antenna elements above a planar metallic reflector for phasing a reflected wave into a desired beam shape and location. It is a flat array that emulates differently shaped reflective surfaces (such as a dish antenna). 
     The prior art includes U.S. Pat. No. 5,541,614 to Juan F. Lam, Gregory L. Tangonan, and Richard L. Abrams, “Smart antenna system using microelectromechanically tunable dipole antennas and photonic bandgap materials”. This patent shows how to use RF MEMS switches and photonic bandgap surfaces for reconfigurable dipoles. 
     The prior art includes RF MEMS tunable dipoles ¼ wavelength above a metallic ground plane, but this approach results in limited bandwidth and limited tunability. We improve on this approach by replacing the reconfigurable dipole array with a tunable impedance surface, resulting in a thinner structure, with broader bandwidth. 
     The prior art further includes a pending applications of D. Sievenpiper, E. Yablonovitch, “Circuit and Method for Eliminating Surface Currents on Metals”, U.S. provisional patent application, Ser. No. 60/079,953, filed on Mar. 30, 1998. 
     A conventional high-impedance surface, shown in FIG. 1, consists of an array of metal top plates or elements  10  on a flat metal sheet  12 . It can be fabricated using printed circuit board technology with the metal plates or elements  10  formed on a top or first surface of a printed circuit board and a solid conducting ground or back plane  12  formed on a bottom or second surface of the printed circuit board. Vertical connections are formed as metal plated vias  14  in the printed circuit board, which connect the elements  10  with the underlying ground plane  12 . The metal members, comprising the top plates  10  and the vias  14 , are arranged in a two-dimensional lattice of cells, and can be visualized as mushroom-shaped or thumbtack-shaped members protruding from the flat metal surface  12 . The thickness of the structure, which is controlled by the thickness of the printed circuit board, is much less than one wavelength for the frequencies of interest. The sizes of the elements  10  are also kept less than one wavelength for the frequencies of interest. The printed circuit board is not shown for ease of illustration. 
     Turning to FIG. 2, the properties of this surface can be explained using an effective circuit model or cell which is assigned a surface impedance equal to that of a parallel resonant LC circuit. The use of lumped cells to describe electromagnetic structures is valid when the wavelength is much longer than the size of the individual features, as is the case here. When an electromagnetic wave interacts with the surface of FIG. 1, it causes charges to build up on the ends of the top metal plates  10 . This process can be described as governed by an effective capacitance C. As the charges slosh back and forth, in response to a radio-frequency field, they flow around a long path P through the vias  14  and the bottom metal surface  12 . Associated with these currents is a magnetic field, and thus an inductance L. The capacitance C is controlled by the proximity of the adjacent metal plates  10  while the inductance L is controlled by the thickness of the structure. 
     The structure is inductive below the resonance and capacitive above resonance. Near the resonance frequency,          ω   =     1     LC         ,                          
     the structure exhibits high electromagnetic surface impedance. 
     The tangential electric field at the surface is finite, while the tangential magnetic field is zero. Thus, electromagnetic waves are reflected without the phase reversal that occurs on a flat metal sheet. In general, the reflection phase can be 0, π, or anything in between, depending on the relationship between the test frequency and the resonance frequency of the structure. The reflection phase as a function of frequency, calculated using the effective medium model, is shown in FIG.  3 . Far below resonance, it behaves like an ordinary metal surface, and reflects with a π phase shift. Near resonance, where the surface impedance is high, the reflection phase crosses through zero. At higher frequencies, the phase approaches −π. The calculated model of FIG. 3 is supported by the measured reflection phase, shown for an example structure in FIG.  4 . 
     A large number of structures of the type shown in FIG. 1 have been fabricated with a wide range of resonance frequencies, including various geometries and substrate materials. Some of the structure were designed with overlapping capacitor plates, to increase the capacitance and lower the frequency. The measured and calculated resonance frequencies for twenty three structures with various capacitance values are compared in FIG.  5 . Clearly, the resonance frequency is a predictable function of the capacitance. The dotted line in FIG. 5 has a slope of unity, and indicates perfect agreement. The bars indicate the instantaneous bandwidth of the surface, defined by the frequencies where the phase is between π/2 and −π/2. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Features of the present invention include: 
     1. A device with tunable surface impedance; 
     2. A method for focusing an electromagnetic wave using the tunable surface; and 
     3. A method for steering an electromagnetic wave using the tunable surface. 
     This invention provides a reconfigurable electromagnetic surface which is capable of performing a variety of functions, such as focusing or steering a beam. It improves upon the high-impedance surface, which is the subject of U.S. Provisional Patent Ser. No. 60/079,953, to include the important aspect of tunability, as well as several applications. The tunable structure can have any desired impedance, and thus any desired reflection phase. Therefore, by programming the surface impedance as a function of position, it can mimic such devices as a Fresnel reflector or a grating, and these properties can be reprogrammed electronically. 
     The present invention provides, in one aspect, a tuneable impedance surface for steering and/or focusing a radio frequency beam, the tunable surface comprising: a ground plane; a plurality of top plates disposed a distance from the ground plane, the distance being less than a wavelength of the radio frequency beam; and a capacitor arrangement for controllably varying the capacitance of adjacent top plates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a conventional high-impedance surface fabricated using printed circuit board technology of the type disclosed in U.S. Provisional Patent Ser. No. 60/079,953 and having metal plates on the top side connect through metal plated vias to a solid metal ground plan on the bottom side; 
     FIG. 2 is a circuit equivalent of a pair of adjacent metal top plates and associated vias; 
     FIG. 3 depicts the calculated reflection phase of the high-impedance surface, obtained from the effective medium model and shows that the phase crosses through zero at the resonance frequency of the structure; 
     FIG. 4 shows that the measured reflection phase agrees well with the calculated reflection phase; 
     FIG. 5 depicts the measured resonance frequency compared to the calculated resonance frequency, using the effective circuit model of FIG. 2, for twenty three examples of the surface shown in FIG. 1; 
     FIG. 6 depicts a high impedance surface with an array of variable capacitors placed between neighboring top plates; 
     FIG. 7 depicts a circuit equivalent of the surface shown by FIG. 6, modified so that the addressing of each variable capacitor occurs by applying a voltage through an associated conducting via; 
     FIG. 8 depicts a top view of one embodiment of the present invention 
     FIG. 9 depicts a top view of another embodiment of the present invention; 
     FIG. 9 a  depicts a top view of one embodiment of the present invention similar to that of FIG. 9, but with all elements being controllable; 
     FIG. 10 depicts a top view of yet another embodiment of the present invention; 
     FIG. 10 a  depicts a top view of one embodiment of the present invention similar to that of FIG. 10, but with all elements being controllable; 
     FIG. 11 depicts another technique for tuning the capacitance by using heaters arranged below the surface, which heaters causing bimetallic strips on the top surface to bend; 
     FIG. 12 demonstrates how beam can be steered by impressing a linear reflection phase function on the tunable impedance surface—phase discontinuities of 2π are used to steer to large angles, making the surface resemble a grating; and 
     FIG. 13 demonstrates how a parabolic reflection phase function can be used to focus a beam. 
    
    
     DETAILED DESCRIPTION 
     In accordance with the present invention, a high-impedance surface is modified by adding variable capacitors  18  as illustrated in FIG.  6 . These variable capacitors  18  can take a variety of forms, including microelectromechanical capacitors, plunger-type actuators, thermally activated bimetallic plates, or any other device for effectively varying the capacitance between a pair of capacitor plates  10 . The variable capacitors  18  can alternatively be solid state devices, in which a ferroelectric or semiconductor material provides a variable capacitance controlled by an externally applied voltage. An example is shown in FIG. 6, where individual variable capacitors  18  are disposed between each neighboring pair of hexagonal metallic top plate elements  10 . By changing the capacitance, the curves in FIGS. 3 and 4 are shifted according to the resonance frequency given by the relation:        ω   =     1     LC                              
     as verified by the data depicted in FIG.  5 . This has the effect of changing the impedance at a single frequency. By varying the capacitance as a function of distance along (or location on) the surface, a position-dependent or location-dependent impedance can be generated on the surface  30  (FIGS.  6  and  7 ), and thus a position-dependent or location-dependent reflection phase occurs. A tunable high-impedance surface  30  is thus provided. 
     The variable capacitors  18  can be provided by microelectromechanical capacitors, thermally activated bimetallic strips, plungers, or any other device for moving a capacitor plate. Alternatively, elements  18  could be semiconductor or ferroelectric variacs. 
     The capacitance C of a cell of the high impedance surface can be less than 1 pF. As such the amount of capacitance to be added to each cell to change the impedance can also be quite small and therefor the physical size of elements  18  can likewise be small. Indeed, elements  18  adding capacitance in the range of 0.1 to 1.0 pF per cell will often be quite suitable. 
     The tunable surface of FIG. 6 is preferably built or disposed on a substrate  24  (FIG. 7) such as a printed circuit board. The thickness of the printed circuit board is kept preferably much less than the wavelength associated with the frequency or frequency band of interest. For high frequency applications, that means than the printed circuit board is rather thin. Thin printed circuit boards having a thickness of only 0.1 mm are readily available For example, polyimide printed circuit boards are commercially available as thin as 1 mil (0.025 mm) and therefore the disclosed structure with printed circuit board technology can be used in very high frequency applications, if desired. The elements  10  are electrically conductive and typically made of a metal conveniently used in printed circuit board fabrication processes and are disposed on one surface of the substrate  24 . The back plane  12  is disposed on the opposite surface of substrate  24 . Vias are typically provided and plated to form conductors  14 . Conductors  14  are connected to the elements  10  at one end thereof and are coupled, either capacitively or directly, as will be discussed later, at or near another end thereof to the back plane  12 . 
     Elements  10  should be sized to be less than one half the wavelength associated with the frequency of interest. However, to minimize sidelobes, the performance of the high-impedance surface will improve as the cell size is reduced, i.e. as the physical size of the elements  10  is reduced. Preferably, the size of the elements  10  is kept to less than one tenth the wavelength associated with the frequency of interest, since that yields good results while keeping the high impedance surface reasonably manufacturable. 
     If elements  18  are provided by microelectromechanical capacitors, or by solid state variacs, the capacitance can be changed by changing an applied voltage, which can be routed through the conductive vias  14 . This can be accomplished by dividing the array of elements  10  into two subsets:  10   a  and  10   b.  One subset  10   a  is electrically grounded, while the second subset  10   b  would have an applied control voltage that may be different for each element in subset  10   b.  The control voltage is applied through a via  14   b,  which in this case would not be connected to the ground plane  12 , but instead to an external data bus  20 . This embodiment is illustrated by FIG.  7 . The data lines  20  are fed to an external control unit (not shown) for generating the desired control voltages for various beam steering or focusing operations. In this embodiment, the data lines  20  each preferably include an RF choke (not shown) wired in series to prevent radiation to the back side. 
     Additionally, the vias  14   b  are capacitively coupled to the ground plane  12  so that they appear to be connected to the ground plane  12  at the RF frequencies of interest, but not at the much lower frequencies of the control voltages (which would typically be considered to be comparatively slowly changing DC voltages). Since the vias  14   b  conveniently pass through the ground plane  12 , they are conveniently capacitively coupled to the ground plane  12  where they penetrate the ground plane  12  and that capacitance at that point  14   c  can be conveniently controlled using techniques well known in the art. Preferably, the capacitance at the penetration point  14   c  is much larger than the capacitance of elements  18 . 
     FIG. 8 shows one embodiment of an hexagonal array of elements  10   a  and  10   b.  Recall that elements  10   a  are directly connected to the ground plane while elements  10   b  are connected to control voltages (but are capacitively or effectively coupled to the ground plane for the frequencies of the impinging RF waves of interest). The capacitances added by elements  18  are controlled by the control voltages on bus  20 . Considering some particular elements  10  identified by the letters A, B, and C in FIG. 8, it will be noted that element A is directly coupled to ground since it is a member of subset  10   a,  while elements B and C have control voltages applied thereto as they both belong to subset  10   b.  The element  18  between elements A and B is controlled by the control voltage applied to element B through its associated via  14   b.  The capacitance between elements A and B is controlled by (i) their physical relationship and (ii) the capacitance contributed by the aforementioned element  18 . Likewise, the element  18  between elements A and C is controlled by the control voltage applied to element C through its associated via  14   b.  However, the capacitance between elements B and C is fixed in this embodiment by their physical relationship. Of course, an element  18  could be provided between elements B and C in which case the capacitance contributed by that added element  18  would be based on the difference of the control voltages applied to elements B and C. Those skilled in the art will appreciate that such control based on voltage differences adds additional complication, since the added capacitances provided by at least some of the elements  18  are then a function of the differences in the control voltages. But if that added complication is warranted in order to provide greater control of the impedance of the surface, then even more (or perhaps all) of the elements  10  could be controlled by control voltages (in which case less or none of the elements would be directly grounded as in the case of subset  10   a ). As can be seen, the ratio of controlled (subset  10   b ) to uncontrolled (subset  10   a ) elements  10  can vary greatly. 
     Alternatively, all of the elements  10  can be directly connected to ground plane  12  and the control voltages from bus  20  can be connected directly to the various variable capacitors  18  through other vias (not shown), in which case no element  10  would be a controlled element of subset  10   b.    
     FIG. 9 shows one embodiment of a rectangular arrangement of the elements  10   a  and  10   b.  The ratio of controlled (subset  10   b ) to uncontrolled (subset  10   a ) elements in this figure is shown as being 1:1 and an element  18  is disposed between each element  10 . However, if all of the elements  18  are controlled and therefore all belong to subset  10   b  (no  10   a  elements), then the embodiment shown in FIG. 9 a  is arrived at. Again, the ratio of controlled (subset  10   b ) to uncontrolled (subset  10   a ) elements  10  can vary greatly. 
     FIG. 10 shows one embodiment of a triangular arrangement of the elements  10   a  and  10   b.  The ratio of controlled (subset  10   b ) to uncontrolled (subset  10   a ) elements in this figure is shown as being 1:1 and an element  18  is disposed by between each element  10 . However, if all of the elements  18  were controlled by making them subset  10   b  elements (in which case subset  10   a  is of a zero size), then the embodiment shown in FIG. 10 a  is arrived at. As previously mentioned, the ratio of controlled (subset  10   b ) to uncontrolled (subset  10   a ) elements  10  can vary greatly. 
     The ratio of controlled (subset  10   b ) to uncontrolled (subset  10   a ) elements  10  can be less than 1:1, if desired, which will also have the effect of reducing the number of capacitor elements  18  utilized, but, of course, with less control of the impedance of the surface. However, that could be quite suitable in certain embodiments. 
     As an alternative method of tuning the capacitance, heaters  26  (FIG. 11) can be arranged below the surface, which would actuate an array of bimetallic strips  18 , which would bend according to the local temperature. This embodiment is shown by FIG. 11 where heaters  26  are provided to control the position of the adjacent bimetallic strips  18 . As the metallic strips  18  move to a close position, the capacitance increases. Another method of tuning the capacitance involves mechanical plungers, which could be moved by hydraulic pressure or by a series of magnetic coils. The examples given here are not meant to limit how additional capacitance can be added. Any available technique for tuning the capacitance may be utilized. 
     The operations that can be performed depend on the surface impedance, and thus the reflection phase, as a function of position. If the reflection phase assumes a linear slope  44 , the surface can be used to steer an RF beam  32 , as illustrated in FIG.  12 . FIG. 12 demonstrates how incident beam  32  can be steered to produce a reflected beam  34  by impressing a linear reflection phase function  44  on the tunable impedance surface  30 . To steer to large angles, phase discontinuities of 2π can be included, so the surface acts like a diffraction grating. 
     Alternatively, a parabolic function  46  can be used to focus a reflected beam  36 , as shown in FIG.  13 . FIG. 13 demonstrates how an incident RF beam  32  can be steered by impressing a parabolic reflection phase function  46  on the tunable impedance surface  30 . To steer to large angles, phase discontinuities of 2π are included, so the surface acts like a Fresnel or parabolic reflector to focus an incident wave  32 . 
     Of course, the tunable impedance surface  30  can be easily tuned by adjusting the capacitors  18  so that the impedance of the surface  30  varies as a function of location across the surface. As can be seen by reference to FIGS. 12 and 13, changing the impedance profile on the tunable impedance surface  30  has a profound effect on how an incident RF wave  32  interacts with the surface  30 . 
     Indeed, surface  30  can be planar and yet act as if it were a prior art parabolic dish reflector or a diffraction grating. Even more remarkable is the fact that surface  30  can be effectively programmed to mimic not only parabolic reflectors of different sizes, but also flat, angled reflectors or any other shape of reflector or diffraction grating by simply changing the impedance of the surface as a function of location on the surface. 
     In the embodiments shown by the drawings the tunable impedance surface  30  is depicted as being planar. However, the invention is not limited to planar tunable impedance surfaces. Indeed, those skilled in the art will appreciate the fact that the printed circuit board technology preferably used to provide a substrate  24  for the tunable impedance surface  30  can provide a very flexible substrate  24 . Thus the tunable impedance surface  30  can be mounted on any convenient surface and conform to the shape of that surface. The tuning of the impedance function would then be adjusted to account for the shape of that surface. Thus, surface  30  can be planar, non-planar, convex, concave or have any other shape and still act as if it were a prior art parabolic dish reflector or as a diffraction grating by appropriately tuning its surface impedance. 
     The top plate elements  10  and the ground or back plane element  12  are preferably formed from a metal such as copper or a copper alloy conveniently used in printed circuit board technologies. However, non-metallic, conductive materials may be used instead of metals for the top plate elements  10  and/or the ground or back plane element  12 , if desired. 
     Having described the invention in connection with certain embodiments thereof, modification will now certainly suggest itself to those skilled in the art. As such, the invention is not to be limited to the disclosed embodiments except as required by the appended claims.