Patent Publication Number: US-6906668-B2

Title: Dynamically reconfigurable aperture coupled antenna

Description:
BACKGROUND OF THE INVENTION 
   1. Statement of the Technical Field 
   The invention concerns antennas and more particularly aperture coupled antennas that can be dynamically modified to operate over a relatively large bandwidth. 
   2. Description of the Related Art 
   Patch antennas are well known in the art and are used in a wide variety of applications. They can be manufactured in a nearly unlimited number of shapes and sizes, and can be made to conform to most surface profiles. Patch antennas also possess an omni-directional radiation pattern that is desirable for many uses. 
   One negative aspect of patch antennas is that they usually have a relatively narrow impedance bandwidth. For a typical classically fed patch antenna, bandwidth is usually about 2% to 3%. Patch antennas that are fed with an aperture or slot can have slightly higher bandwidths, in the range from about 4% to 6%, but this is still too narrow for many applications. The impedance of a patch antenna is also noteworthy as it can depart significantly from 50Ω. Consequently, most patch antennas need a matching network in order to ensure efficient power transfer, particularly if when fed with coaxial cables that can be lossy at high levels of VSWR. 
   Impedance matching for a patch antenna can be accomplished using several different approaches. For example, a quarter wave high impedance transmission line transformer can be used for this purpose. Alternatively since the impedance is at a minimum at the center of the patch and increases along the axis, a 50Ω microstrip line can be extended into the interior of the patch to achieve a suitable match. In yet another alternative, a center conductor of a coaxial line can be routed through a dielectric substrate on which the conductive patch is disposed to contact the underside of the patch at a selected impedance point. 
   Still, the performance of most conventional matching systems will be frequency dependent. Accordingly, the input impedance of the antenna system will tend to vary considerably over a relatively large bandwidth. Consequently, the usable bandwidth of the conventional patch antenna will remain relatively limited. 
   SUMMARY OF THE INVENTION 
   The invention concerns a method for controlling an input impedance of an antenna. The method can include the steps of coupling RF energy from an input RF transmission line to an antenna radiating element through an aperture defined in a ground plane. For example, the aperture can be a slot and the radiating element can be a conductive metal patch type element. The input impedance can be controlled by selectively varying one of both of a volume and a position of a fluid dielectric disposed in a predetermined region between the RF transmission line and the antenna radiating element. The volume and/or position of the fluid dielectric can be automatically varied in response to at least one control signal, which can include a feedback signal provided by a sensor. The fluid dielectric can be constrained in a dielectric cavity structure that can be formed in a substrate on which the RF transmission line or antenna radiating element is disposed. 
   According to one aspect of the invention the volume and/or the position of fluid dielectric can be controlled so as to maintain a relatively constant input impedance over a selected range of frequencies. As used herein, this should be understood to mean that the input impedance is maintained within a predetermined range of values that will ensure relatively low input VSWR over the range of frequencies, it being understood that slight variations in input impedance can occur. The permittivity and permeability of the fluid dielectric can be selected to produce a pre-determined value of input impedance, e.g. 50 ohms, over the selected range of frequencies. 
   According to another aspect, the invention can include an aperture coupled antenna comprised of an input RF transmission line, a antenna radiating element, and an aperture defined in a ground plane through which RF energy from the RF transmission line is coupled to the antenna radiating element. For example, the aperture can be a slot and the radiating element can be a conductive metal patch type element. A fluid control system can be provided for selectively varying the volume and/or position of a fluid dielectric disposed in a predetermined region between the RF transmission line and the antenna radiating element for controlling an input impedance of the antenna. The fluid dielectric can be constrained in a dielectric cavity structure which can, for example, be disposed between the aperture and the RF transmission line. the fluid control system further can comprise a controller, for automatically varying the volume and/or position in response to a control signal, and at least one or more of a valve, a pump and a fluid reservoir. 
   According to one aspect of the invention, the controller can vary at least one of the fluid volume and position to maintain a relatively constant input impedance over a selected range of frequencies. Also, the fluid dielectric is preferably selected to have a permeability for produce a pre-determined value of the input impedance over a selected range of frequencies. For example, the input impedance can be maintained at 50 ohms. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a patch antenna that is useful for understanding the present invention. 
       FIG. 2  is an exploded view of the patch antenna of FIG.  1 . 
       FIG. 3  is an enlarged cross-sectional view of the patch antenna of  FIG. 1  taken along line  3 - 3 . 
       FIG. 4  is an enlarged cross-sectional view of the patch antenna of  FIG. 1  taker along line  4 - 4 . 
       FIG. 5  is a flow chart illustrating a process for controlling an input impedance of the patch antenna. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a perspective view of an aperture-fed patch antenna  100  that is useful for understanding the invention. The antenna is-comprised of a radiating element  102  disposed on a dielectric antenna substrate  104 . The radiating element  102  in  FIG. 1  is shown as having a square geometry as is common for patch type antennas, but it should be understood that the invention is not so limited. Instead, the radiating element  102  can have any of a wide variety of geometric designs as would be known to those skilled in the art. 
   A feed line  106  can be disposed on a surface of the antenna  100  opposed from the radiating element  102 . According to a preferred embodiment, the feed line  106  can be a microstrip transmission line as shown. However, the invention is not limited in this regard and other arrangements are also possible. For example, feed line  106  could also be arranged in a buried microstrip or stripline configuration. 
   As illustrated in  FIGS. 1 and 2 , the feed line  106  can be disposed on a dielectric feed substrate  108 . The antenna substrate  104  can be separated from the feed substrate  108  by a conductive metal ground plane  110 . The antenna substrate and the feed substrate can be formed from any of a number of commercially available forms of dielectric materials. For example, low and high temperature cofired ceramics (LTCC, HTCC) can be used for this purpose. An example of an LTCC would include low temperature 951 cofire Green TapeTM from Dupont®. This material is Au and Ag compatible and has acceptable mechanical properties. It is available in thicknesses ranging from 114 μm to 254 μm and is designed for use as an insulating layer in hybrid circuits, multichip modules, single chip packages, and ceramic printed wire boards, including RF circuit boards. Alternatively, the dielectric substrates can be formed from other materials commonly used as RF substrates, including Teflon® PTFE (PolyTetraFluoroEthylene) composites of glass fiber, woven glass and ceramics. Such products are commercially available from a variety of manufacturers. For example, Rogers Corporation of Chandler, Arizona offers such products under the trade name RT/duroid including product numbers 5880, 6002, and 6010LM. Unlike LTCC materials, these types of substrates do not generally require a firing step before they can be used. 
   Aperture  112  is preferably provided in the ground plane  110  for coupling RF energy from the feed line  106  to the radiating element  102 . The aperture  112  is preferably a slot and can be approximately centered beneath the radiating element  102  in accordance with conventional aperture-fed patch antenna designs. However, other shapes and positions for the aperture  112  can also be acceptable. Further, the feed line  106  preferably traverses the area defined by the aperture  112  on a side of the feed substrate opposed from the ground plane  110  and can include a stub that terminates somewhat beyond the point of intersection as shown. 
   With the arrangement of the antenna  100  as described herein, RF energy communicated to the feed line  106  at feed port  114  can be effectively coupled to the radiating element  102 . In conventional aperture fed antenna systems, it is well known that there are several parameters that can be varied in order to control the input impedance of the antenna  100  as seen, for example, at feed port  114 . These parameters include the dimensions of the aperture  112 , the width of feed line  106 , the position of the aperture  112  relative to the radiating element  102  and the length of the feed line stub  116  extending past the aperture. Most commonly, the aperture length (transverse to the feed line  106 ) and the length of stub  116  are selected to control the input impedance observed at an antenna feed port  114 . The length of the aperture  112  determines the coupling level between the feed line  106  and the radiating element  102  and therefore can be used to vary the input impedance observed at antenna feed port  114 . Changing the length of the stub can compensate for the inductance of the aperture so as to create a real impedance for the radiating element. 
   One problem with impedance matching using the foregoing approaches is that they are static systems and cannot be varied once the design is selected. The present invention provides an approach by which dynamic control over the input impedance can be achieved using fluids to vary the coupling between the feed line  106  and the radiating element  102 . 
   According to one embodiment of the invention, coupling between the feed line  106  and the radiating element  102  can be controlled by selectively varying one or both of a volume and a position of dielectric fluid  128  in a region of the substrate near the aperture  112 . By choosing appropriate values of permittivity and permeability, variations in the volume and/or position of the fluid dielectric  128  communicated to this region can effectively vary the coupling between the feed line  106  and the radiating element  102 . In so doing, the input impedance of the antenna can be selectively controlled. For example, the matching system can change either or both of the volume and the position of fluid dielectric to dynamically compensate for impedance variations caused by changes in frequency. The changes in fluid volume can be performed on a continuously variable basis consistent with changes in frequency. Alternatively, the fluid can be varied in discrete steps to create two or more operating predetermined operating configurations that can correspond to particular operating conditions, e.g. two or more specific operational bands. According to one aspect of the invention, the impedance can be maintained at a relatively constant value over a range of frequencies. As used herein, the term “constant” should be generally understood to mean that the input impedance is maintained within a predetermined range of values that will ensure relatively low input VSWR over the range of frequencies, i.e. less than about 2:1. Slight variations in input impedance within this range are to be expected and are acceptable. 
   Referring now to  FIGS. 3 and 4 , the antenna  100  is shown in a cross-sectional view taken along line  3 - 3  and  4 - 4 , respectively. As illustrated therein, at least a portion of the feed substrate  108  aligned with aperture  112  can include a dielectric cavity structure  117  that defines at least one fluid cavity  118 . In  FIGS. 3 and 4 , the fluid cavity  118  is shown as a helical conduit that traverses at least a portion of the distance between the feed line  106  and the aperture  112 . However, the invention is not so limited. The fluid cavity  118  can be any other shape that provides the desired of variation in coupling as between the feed line  106  and the radiating element  102  when the volume of fluid dielectric  128  contained therein is varied in a predetermined way. Notably, varying the volume will also tend to affect the position of the fluid dielectric in this embodiment. In effect, the variation in the volume and position of the fluid dielectric can be used to make the aperture appear electrically smaller or larger. In this regard, it should be noted that while the fluid cavity  118  in  FIGS. 3 and 4  is shown only in the area between aperture  112  and feed line  106 , the invention is not limited in this regard. Instead, the fluid cavity  118  can extend above and below the aperture  112  and even through the area defined by the aperture  112  for the purpose of controlling the impedance match. Notably, increasing the volume of the fluid dielectric  128  will generally tend to also have some effect on the position of the fluid dielectric in the embodiments described herein. 
   A fluid control system can be provided to selectively vary at least one of the volume and the position of fluid dielectric  128  contained in fluid cavity  118 . The fluid control system can include any combination of fluid reservoirs, conduits, pumps, sensors, valves and controllers as may be appropriate for selectively varying the fluid volume communicated to the fluid cavity  118 . For example, as shown in  FIG. 4 , a quantity of fluid dielectric  128  can be stored in a reservoir  120 . The reservoir  120  can be defined within the feed substrate  108  as shown or can be provided externally. Fluid conduits  130 , pump  124 , sensor  132  and valves  126  can be provided for facilitating the transfer of dielectric fluid  128  to the fluid cavity  118 . Those skilled in the art will appreciate that the pumps, valves, and other components of the fluid control system can be conventional type designs or can be formed as micro-electromechanical systems (MEMS) which are also known in the art. A controller  122  can be provided which is responsive to an antenna control signal  123  and information received from sensor  132  for controlling the operation of the pump  124  and valves  126 . The controller can be comprised of a microprocessor, a look-up-table, or any other type of electronic control circuit that is responsive to a control signal  121  to perform the required impedance matching. 
   Composition of the Fluid Dielectric 
   The fluid dielectric  128  as described herein can be comprised of any fluid composition having the required characteristics of permittivity (εr) and permeability (μr) as may be necessary for achieving a selected range of 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 an impedance match for a particular aperture, radiating element and feed line configuration. 
   The fluid dielectric  128  also preferably has a relatively low loss tangent to minimize the amount of RF energy loss in the coupling. 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 fluid 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 fluid dielectric. Accordingly, those skilled in the art will recognize that the examples of suitable fluid 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 fluid dielectric as described herein, it should be noted that the invention is not so limited. Instead, the composition of the fluid 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 relative permittivity (εr) for fluids is approximately 2.0. However, the fluid dielectric used herein can include fluids with higher values of permittivity. For example, the fluid dielectric material could be selected to have permittivity values of between 2.0 and about 58, depending upon the range of impedance matching required required. 
   Similarly, the fluid 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 μr 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 fluid 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. 
   Antenna Structure, Materials and Fabrication 
   According to one aspect of the invention, the antenna substrate  104  and the feed substrate  108  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 preferred choice for use in the present invention. 
   Antenna Control Process 
   Referring now to  FIG. 5 , a process shall be described for controlling the matching system for the patch antenna as disclosed herein. In step  502  and  504 , controller  122  can wait for an antenna control signal  121  indicating a required impedance matching condition. This impedance matching condition can indicate a relatively small change in frequency or a switch to a different band of frequencies. Once this information has been received, the controller  122  can determine in step  506  a required amount of fluid dielectric  300  that must be injected into cavity  118  in order to produce the required impedance match. In step  508 , the controller  122  can selectively operate the pump  124  and valves  126  respectively associated with antenna  100  to produce the required impedance match. 
   As an alternative to calculating the required configuration of the fluid dielectric, the controller  122  could also make use of a look-up-table (LUT). The LUT can contain cross-reference information for determining control data antenna  100  necessary to achieve various impedance matches. For example, a calibration process could be used to identify the specific sensor output data communicated to controller  122  necessary to achieve a match at a particular frequency. These digital control signal values could then be stored in the LUT. Thereafter, when control signal  121  is updated, the controller  122  can immediately operate the pump  124  and valve  126  to produce the sensor output data that is required to produce the impedance match indicated by the control signal. 
   As an alternative, or in addition to the foregoing methods, the controller  122  could make use of an iterative approach that measures an VSWR at an antenna input  114  and then iteratively adjusts the volume of dielectric fluid  128  contained in cavity  118  in order to achieve the lowest possible value. A feedback loop could be employed to control pump  124  and valves  126  to minimize the measured VSWR. 
   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.