Patent Publication Number: US-6911957-B2

Title: Dynamically variable frequency selective surface

Description:
BACKGROUND OF THE INVENTION 
   1. Statement of the Technical Field 
   The inventive arrangements relate generally to methods and apparatus for frequency selective surfaces, and more particularly to frequency selective surfaces in which the element geometry can be dynamically modified. 
   2. Description of the Related Art 
   A frequency selective surface (FSS) is conventionally designed to either block or pass electromagnetic waves at a selected frequency. These types of surfaces are essentially periodic resonance structures that are comprised of a conducting sheet periodically perforated with closely spaced apertures, or may be comprised of an array of periodic metallic patches. FSS structures can generally be separated into two broad categories, namely inductive and capacitive type geometries. An inductive FSS, operates in a manner similar to a high-pass filter. A capacitive FSS, behaves in a manner that is similar to a low-pass filter. When the periodic elements comprising an inductive FSS are at resonance, the FSS will pass RF signals that are at or near the resonant frequency. In contrast, the capacitive FSS will reflect signals at or near the resonant frequency of the elements. 
   A typical capacitive FSS is constructed out of periodic rectangular metal patches disposed on a planar substrate. By comparison, an inductive type FSS is typically constructed using periodic rectangular apertures which are formed by perforating a metal sheet that has been deposited on a substrate. Many other types of FSS element configurations are known, including tripoles, circles, Jerusalem crosses, concentric rings, mesh-patch arrays or double squares supported by a dielectric substrate. Depending upon the geometry selected, these can combine features of inductive and capacitive elements and can be used to provide low-pass, high-pass, or band-pass responses. U.S. Pat. No. 3,231,892 describes some basic FSS geometries and one potential application for an FSS type periodic resonance structure. 
   SUMMARY OF THE INVENTION 
   The invention concerns a method for dynamically varying a frequency response of a frequency selective surface. The method can include controlling transmission of electromagnetic energy through a frequency selective surface by passing selected frequencies in a pass-band and blocking selected frequencies in a stop-band. The stop-band and the pass-band can be dynamically modified by controlling at least one of a position and a volume of a conductive fluid that forms a portion of the frequency selective surface. According to one aspect of the method, the conductive fluid can be selected to include gallium and indium alloyed with a material selected from the group consisting of tin, copper, zinc and bismuth. 
   The method can also include the step of selecting a geometry for the elements forming the frequency selective surface. For example, the geometry can be chosen so that the elements define tripoles, circles, crosses, Jerusalem crosses, rings, rectangles and squares. The conductive fluid can be used to change at least one dimension of the elements. The conductive fluid can also be used to change a shape of the elements. 
   The method can also include the step of forming a plurality of elements of the frequency selective surface as periodic perforations in the form of the selected geometry in a conductive ground plane. In that case, the step of modifying the stop-band and the pass-band can further include injecting the conductive fluid into a fluid channel formed adjacent to a portion of the conductive ground plane. Further, the conductive fluid contained in the channel can be electrically coupled to the conductive ground plane so that the ground plane and the conductive fluid are at the same electrical potential. The position and the volume of the conductive fluid contained in the channel can be varied in response to a control signal for modifying the pass-band and the stop-band of the frequency selective surface. 
   The method further include the step of disposing the conductive ground plane on a dielectric substrate. In that case, the conductive fluid can advantageously be stored in a cavity structure defined within the dielectric substrate. For example, the invention can include the step of forming the cavity structure within a portion of the dielectric substrate entirely within a boundary or perimeter defined by the conductive ground plane so as to shield the cavity structure from interfering with the operation of the frequency selective surface. 
   The invention can also include a dynamically variable frequency selective surface. The frequency selective surface can be formed of a periodic resonance structure having a plurality of elements periodically spaced over a surface. A fluid control system is provided for dynamically varying one or more of a position and a volume of the conductive fluid within the periodic resonance structure. In this way, the conductive fluid can be used to change at least one dimension of each of the elements. This modification of the element dimensions allows the fluid control system to dynamically modify the resonant frequency of each element. 
   According to one aspect of the invention, the plurality of elements can be comprised of periodic perforations of a selected geometry in a conductive ground plane. The fluid control system can selectively add and remove the conductive fluid from a fluid channel that can be formed adjacent to a portion of the conductive ground plane. The conductive fluid contained in the channel is advantageously electrically coupled to the conductive ground plane so that the conductive fluid is at the same relative potential as the ground plane. Consequently, the conductive fluid appears to be an extension of the ground plane which can effectively modify a dimension or shape of the perforation defining the element. 
   According to another aspect of the invention, the conductive ground plane can be disposed on a dielectric substrate and a cavity structure can be defined within the dielectric substrate for storing a predetermined volume of the conductive fluid. For example, the cavity structure can be disposed within a portion of the dielectric substrate entirely within a boundary or perimeter defined by the conductive ground plane. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top view of a frequency selective surface that is useful for understanding the invention. 
       FIG. 2  is a top view of the frequency selective surface in  FIG. 1  with a top dielectric layer shown partially cut away. 
       FIG. 3  is a cross-sectional view of the frequency selective surface in  FIG. 1  taken along line  3 — 3 . 
       FIG. 4  is an enlarged top view of a single element  102  taken along line  4 — 4  in FIG.  3 . 
       FIG. 5  is an enlarged cross-sectional view of a single element in  FIG. 3  identified by line  5 — 5  in FIG.  3 . 
       FIG. 6  is a flow chart that is useful for understanding a process for dynamically modifying a frequency selective surface. 
       FIGS. 7A-7F  are a series of drawings showing alternative element embodiments. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is top view of a dynamically variable frequency selective surface  100  covered by a dielectric radome layer  106  .  FIG. 2 , is a top view of the frequency selective surface  100  with the dielectric radome layer  106  shown partially cut away to reveal the underlying periodic resonance structure. The periodic resonance structure is comprised of a plurality of periodically spaced elements  102 . 
   The plurality of elements  102  can be comprised of periodic perforations of a selected geometry in a surface defined by conductive ground plane  104 . According to one embodiment illustrated in  FIG. 3 , the ground plane  104  can be disposed on a dielectric substrate  108 . However, those skilled in the art will appreciate that the invention is not limited to elements that are formed as perforations in a ground plane. For example, the elements can also be formed as metallic patches of selected geometries that are disposed on a dielectric substrate. 
   Further, it may be noted that the geometry of elements  102  in  FIG. 2  is define common tripoles. However, the invention is not limited to any such specific element geometry. Instead, the inventive arrangements as disclosed herein can be applied to elements of any geometry. For example, the elements  102  could also be formed without limitation so as to define, hexagons, circles, crosses, Jerusalem crosses, rings, rectangles and squares. 
   Referring now to  FIGS. 4 and 5 , a channel  404  can be provided extending along at least a portion of the perimeter  400  defining element  102 . According to one embodiment shown in  FIG. 4 , the channel  404  can extend entirely around the perimeter  400 , but the invention is not so limited. The channel can be defined on an opposing side by a dielectric form  402  that has an outer perimeter  406 . Top and bottom portions of the channel  404  can be provided by dielectric radome  106  and dielectric substrate  108 . 
   The channel  404  is preferably formed adjacent to the conductive ground plane  104  as shown so that at least a portion of the interior surface of the channel  404  is electrically coupled to the surrounding conductive ground plane  102 . More particularly, the channel is preferably formed so that when it is filled with a conductive fluid  500 , the conductive fluid will form a direct electrical connection to the ground plane  104  along substantially the entire perimeter  400 , thereby causing the conductive fluid  500  and the ground plane  104  to be at the same relative potential. Consequently, conductive fluid added to the channel  404  will appear to extend the perimeter  400  of the ground plane to include the portion of the channel  102  that is filled with conductive fluid  500 . 
   According to one embodiment of the invention shown in  FIG. 5 , channel  404  can have a tapered portion  510  so that variations in the volume of conductive fluid  500  contained in the channel  404  will also vary the degree to which perimeter  400  appears to be extended. Adding more fluid will further extend the ground plane  104  when a volume of the conductive fluid is within this tapered portion. The tapered portion  510  also permits the conductive fluid  500  to efficiently drain from the channel  404  as will be hereinafter described. 
   According to a preferred embodiment a fluid control system can be provided for controlling the position and volume of conductive fluid in the frequency selective surface  100 . According to one embodiment shown in  FIGS. 3 and 5 , the fluid control system can include at least one cavity structure  300  formed in the dielectric substrate  108  for constraining the conductive fluid  500  when it is not in use. A single cavity structure  300  can be provided for each element  102 . However, the invention is not limited in this regard and a single cavity structure  300  can be used to store conductive fluid for two or more elements  102 . 
   Similarly, the fluid control system can include suitable components for transferring a volume of conductive fluid  500  to a selected position and maintaining the conductive fluid in that position for a period of time. For example, as shown in  FIG. 5 , a pump  504  and valve  502  can be used to control the flow of conductive fluid  500  between the channel  404  and the cavity structure  300 . Each channel  404  can be provided with its own set of pumps and valves as shown in  FIGS. 3 and 5 . However, the invention is not limited in this regard and other arrangements are also possible. For example, a single set of pumps and valves can be used to communicate conductive fluid to a plurality of elements  102  with a fluid distribution network (not shown) disposed beneath the conductive ground plane  104 . 
   A fluid conduit  508  can be provided for transferring conductive fluid between the cavity structure  300  and the channel  404 . A pressure relief conduit  506  can also be provided for equalizing the pressure as between the cavity structure  300  and the channel  404 . A check valve (not shown) can be provided in the pressure relief conduit to prevent conductive fluid from unintentionally returning to the cavity structure  300  through the pressure relief conduit. 
   Numerous other arrangements will be apparent to those skilled in the art for controlling the volume and position of conductive fluid contained within the channel  404 , and all such embodiments are intended within the scope of the present invention. 
   Advantageously, the fluid control system can also include a controller  512 . Controller  512  can be any device capable of receiving an input control signal  514  for the frequency selective surface  100  and selectively controlling the appropriate pumps and valves to produce a desired frequency response. For example, the controller  512  can be an electronic circuit, a microprocessor, a software routine or any combination thereof. 
   According to one embodiment, the various pumps and valves can be disposed within the dielectric substrate  108  with suitable control circuitry provided. However, the invention is not limited in this regard, and the various pumps and valves can also be disposed external to the substrate. The pumps and valves can be of a conventional miniature variety or, in a preferred embodiment, they can be micro-electromechanical systems (MEMS). If MEMS type devices are used, they can be integrated directly into the dielectric substrate  108 . 
   The fluid control system described herein can be used to dynamically vary a position and/or a volume of the conductive fluid  500  within the periodic resonance structure defined by the elements  102 . In this way, the conductive fluid  102  can be used to change at least one dimension or a shape of each of the elements  102 . This modification of the element dimension and/or shape allows the fluid control system to dynamically modify the resonant frequency or other electrical characteristic of each element. 
   Control Process 
   Referring now to  FIG. 6 , a process shall be described for controlling the frequency selective surface  100  as disclosed herein. In step  602  and  604 , controller  512  can wait for an antenna control signal  514  indicating a selected pass-band or stop-band operating condition. This selected operating condition can indicate a relatively small change in frequency response or a switch to a different band of frequencies. Once this information has been received, the controller  512  can determine in step  606  a required position and or volume of the conductive fluid  500  that is necessary for the frequency selective surface to perform as requested. In step  508 , the controller  512  can selectively operate one or more of the pumps  504  and valves  502  respectively associated with the frequency selective surface  100  to move the proper amount of conductive fluid into or out of the channel  404 . Thereafter, the controller  512  can return to a waiting mode for a command indicating the next updated operating condition. 
   Alternative Geometries 
   Using the foregoing techniques, a variety of different types of element geometries can be modified in a variety of different ways. As illustrated in  FIGS. 7A-7D , elements  102 A- 102 D can be modified by selectively varying a volume of conductive fluid added to channels  404 A- 404 D, respectively. Dielectric forms can be used to define an inner perimeter of the channel. 
   In  FIGS. 7E and 7F , instead of perforations in a metal ground plane  104 , the elements  102 E and  102 F can be comprised of conductive patches  104 E,  104 F formed respectively on the dielectric substrate  108 E,  108 F. The size or shape of elements  102 E and  102 F can be modified by controlling a volume of conductive fluid added to channels  404 E and  404 F, respectively. Notably, in the embodiments illustrated in  FIGS. 7E and 7F , the conductive fluid  500  can be stored in a cavity structure directly beneath the element. In each case, the conductive fluid contained in the channels can be electrically coupled to the element. Of course, the various geometries in  FIG. 7A-7F  are merely examples of some possible geometries, and the invention is not limited to any particular element shape or arrangement. 
   The Conductive Fluid 
   According to one aspect of the invention, the conductive fluid used in the invention can be selected from the group consisting of a metal or metal alloy that is liquid at room temperature. The most common example of such a metal would be mercury. However, other electrically-conductive, liquid metal alloy alternatives to mercury are commercially available, including alloys based on gallium and indium alloyed with tin, copper, and zinc or bismuth. Conductive fluids which are electrically conductive and non-toxic, are described in greater detail in U.S. Pat. No. 5,792,236 to Taylor et al, the disclosure of which is incorporated herein by reference. Other conductive fluids include a variety of solvent-electrolyte mixtures that are well known in the art. 
   A system which relies on the presence or absence of a conductive fluid can also include some means to ensure that no conductive residue remains in/on the walls of the channel  404  when the channel is purged of conductive fluid. In this regard, the channels containing conductive fluid can be flushed with a suitable solvent after the conductive fluid has been otherwise purged. This flushing can be performed manually or by an automated system. For example, in the case of conductive fluids which may consist of particles in solution or suspension, an active purging system (not shown) may be employed which uses a non-conductive fluid to flush the cavities of any remaining conductive particles. Still, the use of such an active purging system is merely a matter of convenience and the invention is not so limited. 
   Structure, Materials and Fabrication 
   According to one aspect of the invention, the dielectric substrate  108  and the readome  106  can be formed from a ceramic material. For example, the dielectric structure can be formed from a low temperature co-fired ceramic (LTCC). Processing and fabrication of RF circuits on LTCC is well known to those skilled in the art. LTCC is particularly well suited for the present application because of its compatibility and resistance to attack from a wide range of fluids. The material also has superior properties of wetability and absorption as compared to other types of solid dielectric material. These factors, plus LTCC&#39;s proven suitability for manufacturing miniaturized RF circuits, make it a natural choice for use in the present invention. 
   While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as described in the claims.