Abstract:
System and method for controlling an antenna beam. The system can include a slot array ( 100 ) having a plurality of slot elements ( 102 ) and a fluid control system ( 206, 207, 302 ) responsive to a control signal  301 . The fluid control system can independently vary a selected volume of a fluid dielectric ( 300 ) coupled to each of the slot elements ( 102 ). In so doing, the system can steer and shape an electromagnetic field incident on the slot array. The slot array ( 100 ) can optionally comprise at least one conductive ground plane ( 308 ) for reflecting the incident electromagnetic field.

Description:
BACKGROUND OF THE INVENTION 
   1. Statement of the Technical Field 
   The inventive arrangements relate generally to methods and apparatus for steerable beam antennas, and more particularly to array structures that can be used for steering antenna beams. 
   2. Description of the Related Art 
   Beam steering relates to changing the direction of a beam emitted from an antenna. In general, there are two ways to steer the beam of an antenna: mechanically and electrically. A mechanically scanned antenna uses a gimbal or other device to physically change the direction the antenna is pointing. An electrically steered antenna uses an array of elements onto which a progressive phase shift is applied. This progressive phase shift along the array determines the direction from which the energy will add in phase when summed by the array. It will be appreciated that these structures are generally reciprocal in their operation. If the system will steer a transmitted beam, it will also steer the received beam in a similar way. 
   There are several ways to make reflective structures perform a beam steering function. A simple flat conducting plate can be used to perform beam steering by moving the orientation of the reflector plate. The same effect can be achieved using a flat conductive plate covered by a uniform dielectric. However, it is often desirable to steer the beam without the need for gross mechanical movement of the reflector plate. Conventional reflectarrays can perform this function electronically. One example of a reflectarray is disclosed in U.S. Pat. No. 4,684,952 to Munson et al. However, alternative arrangements are also known in the art. 
   A frequency selective surface (FSS) is conventionally designed to either block or pass electromagnetic waves at a selected frequency, but is not generally used for beam steering. These types of surfaces are generally comprised of a conducting sheet periodically perforated with closely spaced apertures, or an array of periodic metallic patches. One or more layers formed with such structures can be used in the FSS. 
   FSS arrays that use metallic patches are sometimes called wire arrays. The patches can be formed in a dipole configuration or can have more than two-fold symmetry. For example, the patches can be tripoles or quadrupoles. FSS arrays that use openings in an otherwise continuous conductive sheet are commonly referred to as slot arrays. Slot and wire arrays can be combined in a single FSS having multiple layers. Optionally, a ground plane may be included as one of the layers of the FSS if the surface is to have reflective properties. 
   In general, an FSS comprised of wire arrays will be reflective in some frequency band (called the stopband) and transmissive at other frequencies. Conversely, slot arrays are generally transmissive in some frequency band (called the passband) and reflective at other frequencies. The phase shift caused by the FSS with respect to reflected or transmitted waves varies significantly in the transition region between passband and stopband. An FSS over a ground plane is of course entirely reflective, but the phase of the reflected wave will be different in the passband and stopband. 
   Many types of FSS element configurations are known, including circles, Jerusalem crosses, dipoles, tripoles, quadrupoles, 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. Notably, signals that are blocked by an FSS are typically reflected away from the FSS, but the reflected direction is often not a matter of concern for the designer. 
   Properties of the FSS, such as frequency response, are determined by element shape in arrays, element spacing in arrays, properties of dielectric materials comprising the FSS, and the presence or absence of a ground plane. With regard to the dielectric, it is known that FSS properties are significantly affected by the permittivity/permeability of the material in close proximity to the array layers. 
   SUMMARY OF THE INVENTION 
   The invention concerns a system and method for controlling an antenna beam. The system can include a slot array having a plurality of slot elements and a fluid control system responsive to a control signal. The fluid control system can independently vary a selected volume of a fluid dielectric coupled to each of the slot elements. In so doing, the system can steer and shape an electromagnetic field incident on the slot array. The slot array can optionally comprise at least one conductive ground plane for reflecting the incident electromagnetic field. Also, a fixed beam source for the electromagnetic field can be provided for generating the incident electromagnetic field. 
   The slot array can be formed of a conductive sheet having a plurality of slot elements formed as shaped perforations in the conductive sheet. The conductive sheet can be supported over an air dielectric or disposed over a dielectric substrate. An array of cavity structures can be provided for containing the fluid dielectric coupled to the slot elements. The cavity structures are preferably formed from a dielectric. For example, if a dielectric substrate is used, the cavity structures can be defined within the substrate. 
   The fluid control system can be comprised of one or more fluid control components such as a fluid reservoir, a pump, a valve and a controller. According to one aspect of the invention, one or more components of the fluid control system can be at least partially shielded from the electromagnetic field by the conductive sheet. 
   Further, the slot array can be configured as a frequency selective surface. The frequency selective surface can be transmissive or reflective at a predetermined operating frequency corresponding to the electromagnetic field. Alternatively, the frequency selective surface can have a transition region between a stopband and a passband, and the slot array can be configured for operation in the transition region at a predetermined frequency corresponding to the electromagnetic field. 
   According to yet another aspect of the invention, the slot elements can be configured as tripoles. In that case, the fluid dielectric can be advantageously constrained in an area corresponding to an intersection of each arm of the tripole. 
   The invention can also include a method for controlling an electromagnetic field. The method can include the steps of positioning a slot array in a path of an incident electromagnetic field and selectively varying a volume of a fluid dielectric coupled to each of the slot elements in the array. In this way, the direction of travel of the electromagnetic field and a shape of the electromagnetic field can be selectively modified. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial cutaway top view of an FSS comprised of a plurality of elements and which is useful for understanding the invention. 
       FIG. 2  is an enlarged view of an element of the FSS in FIG.  1 . 
       FIG. 3  is a cross-sectional view of the element in FIG.  2 . 
       FIG. 4  is a drawing that is useful for understanding the transmissive beam steering that can be performed using the present invention. 
       FIG. 5  is a drawing that is useful for understanding reflective beam steering. 
       FIG. 6  is a top view of an FSS comprised of a plurality of hexagonal loop slots. 
       FIG. 7  is a flowchart that is useful for understanding a method for steering an antenna beam using the FSS. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention controls the electrical characteristics of a dielectric layer associated with a frequency selective surface to perform beam steering.  FIG. 1  is a partial cutaway top view of a slot array  100  that can comprise a frequency selective surface. The slot array  100  is comprised of a conductive sheet  104  which can be disposed over a dielectric substrate  106 . In  FIG. 1 , conductive sheet  104  is shown partially cutaway to reveal the dielectric substrate  106 . The elements  102  of the slot array are formed as shaped perforations in the conductive sheet  104 . In  FIG. 1 , the shaped perforations defining elements  102  are tripoles. However, it should be understood that the invention is not limited to any particular slot shape. Instead, the invention described herein can preferably be used with any type of slot array, including but not limited to arrays formed of slots in the shape of circles, Jerusalem crosses, dipoles, tripoles, quadrupoles, squares and so on. 
     FIG. 2  is an enlarged view of a typical element  102  of the slot array  100 . According to a preferred embodiment, the structure of the dielectric substrate  106  can be formed to define at least one cavity structure  202  within the dielectric substrate  106  in an area generally aligned with element  102 . The cavity structure can be co-extensive with the area defined by element  102 , less than the total area defined by element  102  or can extend somewhat beyond the limits of element  102 . If a tripole element is used as shown in  FIG. 2 , the cavity structure  202  is preferably positioned at the intersection  201  of element arms  200  as shown for maximum effect. 
   Notably, the cavity structure  202  shown in  FIG. 2  is less than the total area defined by element  102  and extends slightly beyond the area defined by element  102 . However, the invention is not so limited. For example,  FIG. 6  shows a hexagonal slot array in which it can be desirable to allow the cavity structure to be co-extensive with the area defined by the slot. 
     FIG. 3  is a cross-sectional view taken along line  3 — 3  in FIG.  2 . As shown in  FIGS. 2 and 3 , a reservoir  204  can be provided in fluid communication with the cavity structure  202 . A fluid control system can be provided for moving a volume of fluid dielectric  300  between reservoir  204  and cavity structure  202 . The fluid control system can comprise any combination of pumps, valves, sensors and electronic controls suitable for selectively varying a volume of fluid in the cavity structure  202  in response to a control signal. For example, in  FIG. 3  there is provided a pump  206 , a valve  207 , fluid sensors  209  and an electronic controller  302 . However, the invention is not limited to the precise fluid control arrangements shown, and those skilled in the art will readily appreciate that numerous alternative arrangements are also possible. 
   The pump  206  and valve  207  can be of the conventional miniature variety or can be formed as micro electromechanical devices, either or both of which can optionally be integrated into the dielectric substrate  106 . Similarly, reservoir  204  can be external to the dielectric substrate  106  or can be formed integral therewith as shown in  FIGS. 2 and 3 . According to a preferred embodiment, the reservoir  204 , the pump  206 , and valve  207  can be located beneath the conductive sheet  104  so as not to interfere with the electrical operation of the slot array. In fact, this is an advantage of a slot array in this application to the extent that the conductive sheet  104  can be used to effectively shield the fluid control system. More particularly, the shielding provided by the conductive sheet can be used to limit or prevent any direct beam steering interaction which might otherwise be associated with the fluid dielectric contained within the control system. Instead, such beam steering interaction will primarily occur with the fluid dielectric  300  contained in the unshielded area associated with cavity structure  202 . 
   The pump  206  and valve  207  associated with each element  102  is preferably operable independently from corresponding pumps and valves associated with other elements  102 . According to a preferred embodiment, the pump  206  and valve  207  can each be controlled in response to an element control signal  303  from the controller  302 . 
   According to one embodiment, the portion  304  of cavity structure  202  and reservoir  204  not occupied by fluid dielectric  300  can be occupied by an inert gas. Vent tube  306  allows displacement of any of the inert gas contained within the cavity structure  202 . If the relative permeability or permittivity of the fluid dielectric is selected to be different as compared to the inert gas, then increasing or decreasing the amount of fluid dielectric  300  contained within the cavity structure  202  will vary the phase shift of signals traversing through the element  102 . 
   According to an alternative embodiment, the portion  304  of the cavity structure and reservoir  204  not occupied by the fluid dielectric  300  can be occupied by a second fluid dielectric with electrical properties different as compared to fluid dielectric  300 . In that case, the second fluid dielectric can be selected to be immiscible with the first fluid dielectric so as to define an immiscible fluid interface between the two fluids. An example of immiscible fluids would include oil and water. 
   The volume of fluid in each cavity structure  202  can be selectively varied to control the amount of phase shift that occurs in each element. For example, if this variance is linear across the surface of the slot array  100 , then the transmitted beam which passes through the slot array  100  will be steered in proportion to the phase shift from element to element. The foregoing concept is illustrated in FIG.  4 . As shown therein, each incident ray path  402  and transmitted ray path  404  ray is represented for equal time. Paths with less delay are illustrated by longer rays that cover more distance in the same amount of time. By creating a linear variance in the volume of fluid contained in the cells along a direction from left to right in  FIG. 4 , the wave front is tilted as shown. 
   Further, by varying the volume of fluid dielectric in accordance with other patterns, it is possible to vary the shape of the transmitted beam. For example, as show in  FIG. 1 , increasing or decreasing the amount of fluid dielectric contained in each cell in a radial direction  110  away from a center  108  of the surface defined by the slot array can selectively widen or narrow the transmitted beam. 
   Those skilled in the art will appreciate that the foregoing techniques can also be adapted for use in a reflective-type slot array. For example, in  FIG. 3 , an optional ground plane  308  can be provided so that the slot array  100  can be used in the manner of a conventional reflect-array. Referring to  FIG. 5 , the theory of operation of such a reflectarray is illustrated. As shown therein, a plurality of incident rays  502  which have some angle of arrival relative to the surface of substrate slot array  100 , can be redirected at a second angle to form a redirected rays  504 . The precise mechanism by which the beam is redirected will be determined by the relative phase shift introduced to the incident signal by each element  102  of the array. Additional detail regarding such beam steering techniques are described in U.S. Pat. No. 4,684,592, the disclosure of which is expressly incorporated herein by reference. 
   For convenience, the slot array structures shown in the figures are flat. However, it should be appreciated that the invention is not so limited. For example, the invention can also be used in connection with curved surface slot arrays. A curved surface will modify beam shape as well as direction, and curved reflectors are more often used for beam shaping as opposed to beam steering. However, those skilled in the art will readily appreciate that the concepts disclosed herein have applicability to both types of reflector surfaces. 
   Also, it should be noted that the phase shift caused by each element  102  of the array with respect to reflected or transmitted waves varies significantly in the transition region between passband and stopband. Accordingly, it can be desirable in certain circumstances to configure the slot array to operate in this region. In particular, operation in this transition region can potentially provide substantial phase variation with only a small amount of variation in fluid volume. 
   Composition of the Fluidic Dielectric 
   The fluidic dielectric as described herein can be comprised of any fluid composition having the required characteristics of permittivity and permeability as may be necessary for achieving a selected range of phase shift and impedance matching. For example, those skilled in the art will recognize that one or more component parts can be mixed together to produce a desired permeability and permittivity required for achieving a particular phase shift and impedance match to free space for a particular element  102 . 
   The fluidic dielectric  300  also preferably has a relatively low loss tangent to minimize the amount of RF energy lost in each element. However, devices with higher loss may be acceptable in some instances so this may not be a critical factor. Many applications also require a broadband response. Accordingly, it may be desirable in many instances to select fluidic dielectrics that have a relatively constant response over a broad range of frequencies. 
   Aside from the foregoing constraints, there are relatively few limits on the range of materials that can be used to form the fluidic dielectric. Accordingly, those skilled in the art will recognize that the examples of suitable fluidic dielectrics as shall be disclosed herein are merely by way of example and are not intended to limit in any way the scope of the invention. Also, while component materials can be mixed in order to produce the fluidic dielectric as described herein, it should be noted that the invention is not so limited. Instead, the composition of the fluidic dielectric could be formed in other ways. All such techniques will be understood to be included within the scope of the invention. 
   Those skilled in the art will recognize that a nominal value of permittivity (ε 1 ) for fluids is approximately 2.0. However, the fluidic dielectric used herein can include fluids with higher values of permittivity. For example, the fluidic dielectric material could be selected to have a permittivity values of between 2.0 and about 58, depending upon the amount of phase shift required. 
   Similarly, the fluidic dielectric can have a wide range of permeability values. High levels of magnetic permeability are commonly observed in magnetic metals such as Fe and Co. For example, solid alloys of these materials can exhibit levels of μ, in excess of one thousand. By comparison, the permeability of fluids is nominally about 1.0 and they generally do not exhibit high levels of permeability. However, high permeability can be achieved in a fluid by introducing metal particles/elements to the fluid. For example typical magnetic fluids comprise suspensions of ferro-magnetic particles in a conventional industrial solvent such as water, toluene, mineral oil, silicone, and so on. Other types of magnetic particles include metallic salts, organo-metallic compounds, and other derivatives, although Fe and Co particles are most common. The size of the magnetic particles found in such systems is known to vary to some extent. However, particles sizes in the range of 1 nm to 20 μm are common. The composition of particles can be selected as necessary to achieve the required permeability in the final fluidic dielectric. Magnetic fluid compositions are typically between about 50% to 90% particles by weight. Increasing the number of particles will generally increase the permeability. 
   More particularly, a hydrocarbon dielectric oil such as Vacuum Pump Oil MSDS-12602 could be used to realize a low permittivity, low permeability fluid, low electrical loss fluid. A low permittivity, high permeability fluid may be realized by mixing same hydrocarbon fluid with magnetic particles such as magnetite manufactured by FerroTec Corporation of Nashua, N.H., or iron-nickel metal powders manufactured by Lord Corporation of Cary, N.C. for use in ferrofluids and magnetoresrictive (MR) fluids. Additional ingredients such as surfactants may be included to promote uniform dispersion of the particle. Fluids containing electrically conductive magnetic particles require a mix ratio low enough to ensure that no electrical path can be created in the mixture. Solvents such as formamide inherently posses a relatively high permittivity. 
   Similar techniques could be used to produce fluidic dielectrics with higher permittivity. For example, fluid permittivity could be increased by adding high permittivity powders such as barium titanate manufactured by Ferro Corporation of Cleveland, Ohio. For broadband applications, the fluids would not have significant resonances over the frequency band of interest. 
   Array Structure, Materials and Fabrication 
   According to one aspect of the invention, the dielectric substrate  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. 
   Beam Control Process 
   Referring now to  FIG. 7 , a process shall be described for controlling the angle of a redirected RF beam using the slot array  100 . In step  702  and  704 , controller  302  can wait for an antenna control signal  301  indicating a requested angle for a redirected beam. Once this information has been received, the controller  302  can determine in step  706  a required phase shift for each element  102  and/or a required amount of fluid dielectric  300  that is needed for each cavity structure  202  in order to produce the required phase shift. In step  708 , the controller  302  can selectively operate the control pumps  206  and valves  207  respectively associated with each element  100  to produce the required phase shift in each element of the slot array. 
   As an alternative to calculating the required configuration of the fluid dielectric, the controller  302  could also make use of a look-up-table (LUT). The LUT can contain cross-reference information for determining control data for each element  102  necessary to achieve various redirected beam angles. For example, a calibration process could be used to identify the specific sensor output data communicated to controller  302  necessary to achieve a specific angle for the redirected beam. These digital control signal values could then be stored in the LUT. Thereafter, when control signal  301  is updated, the controller  302  can immediately operate the pumps  206  and valves  207  for each element to produce the sensor output data for each cell that is required to produce the redirected beam angle or shape indicated by the control signal  301 . 
   As an alternative, or in addition to the foregoing methods, the controller  302  could make use of an empirical approach that applies a reference signal to each radiating element and then measures the phase shift that occurs at each element  100 . Specifically, the controller  302  can check to see whether the updated phase shift for each element has been achieved. A feedback loop could then be employed to control each pump  206  and valve  207  to produce the desired redirected beam angle or shape. 
   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.