Patent Application
20050012677
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.
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.
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
Further, it may be noted that the geometry of elements 102 in
Referring now to
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
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
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
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
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
In
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'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.
