Patent Publication Number: US-11041936-B1

Title: Autonomously reconfigurable surface for adaptive antenna nulling

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 15/706,100 filed on Sep. 15, 2017 and entitled “ADAPTIVE NULLING METASURFACE RETROFIT” the disclosure of which is hereby incorporated herein by reference. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     None. 
     TECHNICAL FIELD 
     The present disclosure is directed in general to the field of antennas and in particular to adaptive antenna nulling. 
     BACKGROUND OF THE DISCLOSURE 
     A variety of technologies for mitigating high power interference for antennas are known. Some of these approaches include conventional diode limiters which prevent damage to receivers when illuminated by high power radiation sources by shorting the input of the receiver. However, that approach also prevents the receiver from receiving incoming signals while exposed to the external high power source. See, for example, R. Henry et al “Passive Limiter for High-Frequency Waves” U.S. Pat. No. 3,768,044. 
     There are other approaches that use an array of electronically controlled parasitic scatters coupled to a central feed antenna to selectively null different sectors in the radiation pattern of the antenna by switching binary loads on or off at the parasitic scatters, but they do so by a trial and error method and are failure prone as they do not try to locate the bearing of the high-power incident waves. These approaches also do not provide for appropriate switching mechanisms to handle high power incident waves without failure. 
     Other techniques designed to mitigate interferers do so by notching a portion of the frequency spectrum on which the interferer is operating, and do not provide for means to mitigate interferers which are in-band or operating at the same frequency as the receiver. 
     To overcome the deficiencies of these other technologies, antenna technology could well utilize a radome for existing antennas which can autonomously identify the bearing of a high-power threat and subsequently null that threat without replacing the existing antenna and/or modifying existing hardware or software. Of course, a radome which can autonomously identify the bearing of a high-power threat and subsequently null that threat may also be utilized with new design antenna as well. 
     SUMMARY OF THE DISCLOSURE 
     One embodiment described in this disclosure provides for an autonomously reconfigurable null providing surface comprising an array of electrically conductive elements, such as crossed dipoles, mounted on a substrate, reactive tuning elements sparsely distributed and located between selected pairs of electrically conductive elements and electrically connected to the selected pairs of electrically conductive elements on both ends of each reactive tuning element and electrically connected to one or more microcontroller(s) mounted on the substrate, wherein the one or more microcontroller(s) control the tuning elements to provide nulls in a direction pointing to interferers, while allowing normal operation at other directions. 
     Another embodiment discloses an antenna system comprising a radiating aperture and a reconfigurable surface radome placed on top of the radiating aperture that comprises of an array of elements, such as crossed dipoles, mounted on a substrate, tuning elements sparsely distributed and located between the crossed dipoles and electrically connected to the crossed dipoles on both ends, and one or more microcontroller(s) mounted on the substrate, wherein the one or more microcontroller(s) control the tuning elements to provide nulls in a direction pointing to interferers while allowing normal operation at other angles. 
     Another embodiment discloses a method of adaptive nulling for an antenna system comprising mounting a surface radome on top of a radiating aperture, mounting an array of elements, such as crossed dipoles, on a substrate forming the radiating aperture, distributing tuning elements sparsely between some of the crossed dipoles and electrically connecting the tuning elements to the crossed dipoles and mounting one or more microcontroller(s) on the substrate, wherein the one or more microcontroller(s) control the tuning elements to provide nulls in a direction pointing to interferers while allowing normal operation at other angles. 
     Certain embodiments may provide various technical advantages depending on the implementation. For example, a technical advantage of some embodiments may include the capability to provide adaptive antenna nulling Other embodiments may include autonomous reconfiguration of the surface to adapt to moving interferers. 
     The terms “sparse”, “sparsely” or “sparseness” and “aperiodic” and “autonomous” appear in this disclosure. In the context of the present application, the terms “sparse”, “sparsely” and “sparseness” refer to the degree to which a lattice or an array is fully populated or underpopulated. The locations in the disclosed arrays (lattices) are occupied (or not) by RF sensors and/or reactance tuning elements. “Sparseness” applies to both the RF sensors and the reactance tuning elements in the disclosed arrays (lattices). The reactance tuning elements are preferably tuned using circuit elements which exhibit reactance. “Aperiodic” refers to the fact that the different values of reactance may be developed by the reactance tuning elements and to the fact that the reactance tuning elements are non-uniformly distributed in the lattice or array. “Autonomous” refers to the fact that the reactance tuning elements do not necessarily require input or data from an external source to set the reactances of their reactive circuit elements, rather the the reactance tuning elements are preferably controlled by nearby or adjacently disposed microcontroller(s). 
     Although specific advantages have been enumerated above, various embodiments may include some, none, or all of the enumerated advantages. Additionally, other technical advantages may become readily apparent to one of ordinary skill in the art after review of the following figures and description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like or similar parts: 
         FIG. 1  illustrates a system overlay of an autonomous reconfigurable null providing surface according to an embodiment of the present disclosure; 
         FIG. 1 a    is similar to  FIG. 1  but unneeded conductive elements have been omitted; 
         FIGS. 2-1 and 2-2  provide an overall view of an embodiment of the disclosed system.  FIG. 2-1  illustrates an application where an autonomous reconfigurable surface is mounted as a radome on top of an existing antenna to protect the antenna from RF energy emitted by one or more interfering RF sources; portion  2   a  of  FIG. 2-2  illustrates an aperiodic grid of subwavelength elements sparsely loaded with a combination high power sensing and tuning circuits, according to an embodiment of the present disclosure; portion  2   b  of  FIG. 2-2  is similar to  FIG. 2-1  and illustrates an application where an autonomous reconfigurable surface is mounted as a radome on top of an existing antenna, according to an embodiment of the present disclosure; portion  2   c  of  FIG. 2-2  illustrates a simplified high power tuning circuit used in the autonomous reconfigurable surface, according to an embodiment of the present disclosure; and Portion  2   d  of  FIG. 2-2  illustrates a simplified high power sensing circuit used in the autonomous reconfigurable surface, according to an embodiment of the present disclosure; 
         FIG. 3 a    illustrates simulation results of antenna gain as a function of azimuth angle of the radiating aperture, according to an embodiment of the present disclosure;  FIG. 3 b    illustrates a sparse arrangement of small electrically sensing and tuning elements to achieve a low loss conformal surface, according to an embodiment of the present disclosure 
         FIG. 4  illustrates a system design used for simulation of the antenna performance in  FIG. 3 a    using a 6×6 array of discretely tuned crossed dipoles over an L1 Patch antenna, according to an embodiment of the present disclosure; 
         FIG. 5  illustrates simulation results of the effect of bias network on insertion loss as a function of number of active elements in the array, according to an embodiment of the present disclosure; 
         FIG. 6 a    illustrates simulated performance in terms of direction finding error, according to an embodiment of the present disclosure;  FIG. 6 b    illustrates a simple autonomously reconfigurable surface (ARS) placed over a 1.5 GHz patch antenna that is used for the simulation in  FIG. 6 a   , according to an embodiment of the present disclosure; 
         FIG. 7  depicts an embodiment of the surface for adaptive antenna nulling which comprises two distinct layers or substrates; 
         FIGS. 8 a  and 8 b    depict the two substrates of the embodiment of  FIG. 7  in greater detail; 
         FIG. 9  depicts an embodiment of the surface for adaptive antenna nulling which comprises three distinct layers or substrates with two layers as in the embodiment of  FIGS. 7, 8   a  and  8   b , but with an additional FSS layer; 
         FIG. 10  depicts an embodiment of the surface for adaptive antenna nulling similar to the embodiment of  FIG. 9 , but in this embodiment two layers are formed on a single substrate; 
         FIG. 11  depicts alternative shapes of the metallic elements; 
         FIG. 12  represents a microcontroller with an on board ADC and an on board DAC, an on board ALU (Arithmetic Logic Unit) and an on board memory for storing, for example, the firmware program of  FIG. 13 ; and 
         FIG. 13  is a flow chart of an embodiment of how the mircocontroller(s) may be programmed so that the null providing surface is autonomously reconfigurable. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood at the outset that, although exemplary embodiments are illustrated below, the present technology may be implemented using any number of techniques, whether currently known or not. The present technology should in no way be limited to the example implementations, drawings, and techniques illustrated below. Additionally, the drawings are not necessarily drawn to scale. 
     To overcome the deficiencies of the prior art, inventive concepts are described herein to provide for autonomously reconfigurable surfaces for adaptive antenna nulling. The disclosed technology offers the possibility of a standalone retrofit solution for existing microwave systems to enable normal operation in the presence of in-band high power microwave attacks. Of course, the present invention is not limited to using in a retrofit application and thus some practicing the present invention may choose to utilize it in connection with newly designed radar and/or communication equipment and facilities. 
     Many receivers intended for navigation, communications, and sensing, are designed to operate with maximum sensitivity to allow for increased standoff range and improved fidelity. However, this increased sensitivity also makes these receivers particularly vulnerable to jamming and electronic attack. In the case of High Power Microwave (HPM) weapons, this vulnerability is even more significant as many tactical functions of these receivers can be denied at substantial ranges with modest power levels. While out-of-band HPM attacks can be mitigated using filters, in-band microwave attacks cannot be mitigated without compromising the performance of the aperture. For these in-band attacks, conventional antennas can be replaced with adaptive nulling arrays as described here or reconfigurable antennas. These systems determine the angle of arrival (AOA) of an incident jammer and adaptively null the pattern of an antenna in the direction of the HPM attack. However, in most cases, achieving this functionality requires substantial modifications to existing infrastructure (i.e. replacement of existing antennas and receivers), which for many legacy platforms is undesirable. 
     An important feature of this invention is to provide a conformal surface which can be retrofit or overlaid on or over existing antennas or existing radomes to autonomously blank or null a sector or sectors in their radiation patterns to deny high power sources radiating towards these antennas or radomes from damaging or desensitizing sensitive electronics in the receiver and thus allow the receiver to continue operating without damage and preferably without desensitization. Compared to previous adaptive nulling systems which require modification or replacement of existing RF hardware, the disclosed technology enables adaptive nulling functionality to be retrofitted on existing platforms without external control inputs or modifications to the platform, thus minimizing recertification costs. 
     This document includes numerous embodiments of autonomously reconfigurable surfaces for adaptive antenna nulling Elements which either are identical or are very similar in function often share the reference numeral between these embodiments to avoid an unnecessary repetition of their descriptions. For example, an initial embodiment is described with reference to  FIG. 1  and an application of an autonomously reconfigurable surface for adaptive antenna nulling is described with reference to  FIG. 2-1 . Additional embodiments are described thereafter with their descriptions focusing more on the differences between the embodiments than the similarities. Elements which are either identical or very similar in terms of their function in the additional embodiments (compared to the embodiment of  FIG. 1  and also  FIG. 2-1 ) share the same reference numerals with little or no additional description needed to understand the additional embodiments. 
     An embodiment of the null providing surface  100  for adaptive antenna nulling is shown in  FIG. 1  and comprises a lattice or array of electrically conductive elements  101 . In this embodiment the electrically conductive elements  101  (which are preferably metallic) appear in the shape of cross-shaped metallic dipoles, but other shapes for elements  101  may be adopted as is described below with reference to  FIG. 11 . The elements  101  may made of electrically conductive traces  105  (mentioned below) disposed on a thin, and preferably conformal, dielectric surface comprising a substrate  102  such as such as, but not limited, to Rogers RO4003, Rogers RO3003, Rogers RO5880, Kapton, or Polyethylene Terephthalate (known commonly as PET). These substrates are often available with an integral copper layer and that copper layer may be patterned to provide the aforementioned conductive traces which form, among other things, lattice or array of elements  101 . 
     The substrate  102  is aperiodically loaded with reactance tuning elements  104  and with RF power sensors  103 . Moreover, the numbers of reactance tuning elements  104  and numbers of RF power sensors  103  in this 5×5 array are sparse as will be explained in greater detail below. One or more analog to digital convertors (ADCs)  106 , one or more digital to analog convertors (DACs)  108 , and one or more microcontroller(s)  107  are also preferably mounted on this substrate  102  and all interconnected as needed via the electrically conductive traces  105 . The traces  105  (including the array of elements  101 ) may be defined from a copper clad substrate  102  using printed circuit fabrication techniques to define those traces and elements in the aforementioned copper. So the electrically conductive traces  105  and the lattice or array of elements  101  as well as the various interconnections between the ADC(s)  106 , DAC(s)  108  and the microcontroller(s)  107  may be defined by patterning (etching) the copper of a copper clad substrate  102 . Of course, the material which is electrically conductive and defining the electrically conductive traces  105  and the lattice or array of elements  101  as well as the various interconnections may be some material other than copper, but copper is convenient and inexpensive material to utilize for these purposes. The lattice or array of electrically conductive elements  101  could be quite small in some embodiments, in which case semiconductor integrated circuit fabrication techniques could be used instead of the printed circuit board fabrication techniques previously alluded to. 
     The microcontroller(s)  107  may be implemented, for example, by a small computing device such as the ATmega1280 microcontroller made by Microchip Technology Inc. of Chandler, Ariz. The ADC(s)  106  and DAC(s)  108  may be embodied in the microcontroller(s)  107  and therefor they need not be separate elements (see  FIG. 12  which depicts an integrated microcontroller with an on board DAC, an on board ADC, an on board ALU and an on board memory for storing, for example, the firmware program of  FIG. 13 ). The ADC(s)  106  and DAC(s)  108  are illustrated as separate elements from microcontroller(s)  107  in other figures merely for ease of illustration and explanation. These elements are bonded to the metal on substrate  102  after the it is patterned preferably using conventional printed circuit fabrication techniques unless they are all fabricated as a unit using semiconductor integrated circuit fabrication techniques. 
     The embodiment of null providing surface  100  depicted by  FIG. 1  is a 5×5 array of unit cells (two of which are defined by dashed lines) with an electrically conductive element  101  possibly occupying each unit cell. Generally speaking, surfaces  100  may have many thousands of unit cells.  FIG. 1  only shows a 5×5 array for ease of illustration. If the array of unit cells has M rows and N columns, then if the number of reactance tuning elements  104  is less than (M−1)×(N−1), then some possible places where a reactance tuning element  104  might occur has no reactance tuning element  104 . Indeed, many of the possible places where a reactance tuning element  104  might occur will have no reactance tuning element  104  and the total number of reactance tuning elements  104  may be less than or equal to √{square root over (M×N)} (the square root of M times N). Likewise, if the number of RF power sensors  103  is also less (M−1)×(N−1), then some possible places where a RF power sensor  103  might occur has no RF power sensor  103 . Indeed, many of the possible places where a RF power sensor  103  might occur will have no RF power sensor  103  and the total number of RF power sensors  103  may be less than or equal to √{square root over (M×N)} (the square root of M times N). If there are fewer reactance tuning elements  104  in the array than the array could otherwise accommodate, then the array of reactance tuning elements  104  is considered to be sparse. Likewise, if there are fewer RF power sensors  103  in the array than the array could otherwise accommodate, then the array of RF power sensors  103  is considered to be sparse. 
     In the embodiment of  FIG. 1 , the four reactance tuning elements  104  are depicted while four RF power sensors  103  are also depicted. Since each of those numbers is less than (M−1)×(N−1), where M and N both equal five, the number of reactance tuning elements  104  and the number of RF power sensors  103  are sparse according to either the (M−1)×(N−1) or the √{square root over (M×N)} definitions given above. Indeed, the total number of RF power sensors  103  plus the total number of reactance tuning elements  104  is also less than (M−1)×(N−1). 
     The RF power sensors  103  and the reactance tuning elements  104  are preferably arranged aperiodically in the array, that is they preferably are not periodic. The may fall in an arrangement which may be called random or pseudo-random. 
     As will be seen, however, embodiments of null providing surface  100  are not necessarily flat and therefor the array of unit cells may not lay out in a nice rectangular grid as depicted by  FIG. 1 . Whether the number of reactance tuning elements  104  and the number of high power sensors  103  are sparse is then defined in terms of the total number of unit cells. If the number of reactance tuning elements  104  is less than the total number of unit cells and preferably less than √{square root over (Total Number of Unit Cells)} then they are sparse. Likewise, if the number of RF power sensors  103  is less than the total number of unit cells and preferably less than √{square root over (Total Number of Unit Cells)}, then they are also sparse. 
     If the embodiment of null providing surface  100  depicted by  FIG. 1  is compared with the embodiment of null providing surface  100  depicted by  FIG. 1 a   , in  FIG. 1 a    electrically conductive elements  101  have been omitted from those units cells where, if they were otherwise present, are not coupled with either a reactance tuning element  104  or a RF power sensor  103 . But omitting the electrically conductive elements  101  which are not connected to either a reactance tuning element  104  or a RF power sensor  103  thereby renders the array of electrically conductive elements  101  sparse. 
     The embodiments of  FIGS. 1 and 1   a  utilizes a single substrate  102 , but as will be described, other embodiments may utilize multiple substrates. 
     The analog outputs of the DACs  108  are networked to reactance tuning elements  104  via a first network of electrically conductive traces  105 . The analog inputs of the ADCs  106  are networked to RF power sensors  103  via a second network of electrically conductive traces  105 . The digital outputs of ADCs  106  and the digital inputs of DACs  108  are networked to microcontroller(s)  107  via a third network of electrically conductive traces  105 . Of course, if either or both ADCs  106  and DACs  108  are embodied in the microcontroller(s)  107 , then the traces  105  between them may be omitted as corresponding connections occur inside microcontroller  107 . 
     The null providing surface  100  for adaptive antenna nulling may be disposed over or on existing antennas and apertures as a retrofit antenna cover or as a covering applied to, on or over existing radomes or the conformal null providing null providing surface  100  may be integrated into new design domes and radomes disposed over antennas and apertures. The null providing surface  100  is preferably conformal, meaning that it preferably preserves the correct angles between directions of impinging radio waves within small areas, but while this is a desirable feature, some utilizing the present invention may choose to forego it and utilize, instead, a planar embodiment of null providing surface  100 . Once exposed to a high-power radio frequency radiation (which may be an interfering signal or signals, either intentional or unintentional), the microcontroller(s)  107  disposed on or adjacent the null providing surface  100  determine the direction of one or more of the incident high power sources of such signals (by sensing that power with RF sensors  103 ) and adaptively adjust the reactance of reactance tuning elements  104  in the surface to reconfigure the radiation or gain pattern of the antenna which it is covering, to place a null preferably in the direction(s) of each of the interfering signals while allowing normal operation at other angles. The interfering signals are called “high-power” herein because, if there are intentionally produced (to jam the existing antenna or aperture), then they are very apt to be of a higher power compared to signals of interest. But the interfering signal my also arise from a source which just happens to interfere, but intentionally so with the existing antenna or aperture, and such signals may be of a lower power, but still be objectionable, and the null providing surface  100  may be employed to null them out as well. 
     An important feature of this technology is illustrated in  FIG. 2-1  of an embodiment of a null providing surface  100  for adaptive antenna nulling. The null providing surface  100  may be referred to from time to time as a “system” since it includes both a substrate  102  various components mounted thereon in addition to conductive traces  105  for interconnecting those components and forming elements  101  as is shown in greater detail by  FIG. 1 a   . Null providing surface  100  may serve as a conformal element  210  which can be retrofitted (or back fit) over an existing antenna element  212  mounted on an existing surface  211 . Alternatively, null providing surface  100  may be designed to function with (cooperate with) an antenna element  212  of new design (thus null providing surface  100  is not limited to use in retrofitting applications). 
     The null providing surface  100  preferably autonomously and adaptively nulls high power interferers  213  in the radiation environment of a receiver (not shown) preferably without modification to the antenna element  212  or the receiver connected to antenna element  212  and without the need for input (data) from external sensors or systems. Although the word element is used in the singular with respect to antenna element  212 , those skilled in the art will appreciate that a receiving antenna element  212  which is to be protected from undesirable and possibly jamming RF sources may be comprises many different elements, some active and possibly some passive (such as an array of dipoles or patches. for example), or just a single element (such as a disk or horn type antenna, also for example). 
       FIG. 2-2  depicts various elements and embodiments of circuits preferably used in providing null providing surface  100  in greater detail. 
     The embodiment of the adaptive null providing surface  100  shown by  FIGS. 1 a    and  2 - 2  comprises an array or lattice of conductive elements  101  (preferably embodied as cross-shaped metallic dipoles, but elements  101  my take other shapes as mentioned above and below) mounted on a thin conformal substrate  102  such as, but not limited to, Rogers RO4003, Rogers RO3003, Rogers RO5880, Kapton, or PET, and aperiodically loaded with reactance tuning elements  104  and/or RF power sensors  103 . The conformal surface  102  may also have static capacitors  109 , one or more ADC(s)  106 , one or more DAC(s)  108 , and one or more microcontroller(s)  107  mounted on it. As illustrated in  FIG. 1 , the analog outputs of the DAC(s)  108  are connected to reactance tuning elements  104  via a network of thin copper traces  105 . The analog inputs of the ADC(s)  106  are connected to RF power sensors  103  via a network of thin copper traces  105 . The digital outputs of ADC(s)  106  and the digital inputs of DAC(s)  108  are either connected to one or more microcontroller(s)  107  via a network of thin copper traces  105  and/or embodied in the microcontroller(s)  107  as previously discussed. Portion  2   b  of  FIG. 2-2  illustrates how this conformal element  210  would preferably be mounted over an existing microstrip patch antenna  212 . 
     Portion  2   c  of  FIG. 2-2  illustrates an embodiment of a ractance tuning circuit  104 C for embodying a reactance tuning element  104 . This embodiment has three transistor switches  104   a  which may be selectively triggered on (or off) to switch in one or more of three possible reactances Z 1 -Z 3  into parallel with each other and connecting the ends or arms of two adjacent elements  101 . 
     Null steering is integrated into the disclosed structures by loading a sparse subset of unit cells or elements  101  with reactance tuning elements  104 . These loadings can be implemented using either traditional varactor diodes or switched reactance banks, controlled by resistively or reactively loaded bias lines  104   b , which preferably minimize RF loss through the surface. In the case of the switched reactance banks such as that shown at portion  2   c  of  FIG. 2-2 , these tuning elements Z 1 -Z 3  can be implemented with high power COTS (Commercial Off the Shelf) GaN transistor switches  104   a . Unlike Electronically Steerable Passive Array Radiator (ESPAR) and adaptive nulling arrays which utilize Si or GaAs components to achieve null steering, COTS GaN switches can withstand voltages up to 200 v. 
     This embodiment of a reactance tuning circuit  104 C comprises of a set of parallel transistor switches  104   a  with their biasing network  104   b , each transistor switch  104   a  being arranged in series with an impedance element Z 1  through Z 3 . There may be, of course, more than three transistor switches  104   a  and more than three associated impedances Z in each reactance tuning circuit  104 C. By controlling the switches, one can control how Z 1  through Z 3  are connected in parallel to yield a varying effective impedance between neighboring elements  101  required to tune the null providing surface  100 . Several such reactance tuning circuits  104 C can be placed in the radome surface as shown to provide the necessary notches to null the one or more high power interferers. With just three impedance element Z 1  through Z 3 , the parallel connected impedance element Z 1  through Z 3  can be set to eight different impedances, which are preferably primary reactive, but some resistance will be inherently present. 
     Circuit  2   d  of  FIG. 2-2  illustrates an embodiment of a RF power sensing circuit  103 C for embodying a RF power sensor  103 . The direction or bearing of high power interferers (and also even low power interferers) is determined using RF power sensing circuits  103 C to implement the RF power sensors  103  integrated with a small subset of unit cells or elements  101  as shown in  FIG. 1 a   . These RF power sensing circuits  103 C are preferably implemented using phased locked voltage controlled oscillators (PLL VCO)  103   a  and mixers  103   d , which return relative phase and amplitude measurements (I&amp;Q) from distinct points in the system  100  to an integrated microcontroller  107  as illustrated in  FIG. 1 a   . These vector measurements representing phase and amplitude of an interfering source can then either be correlated with a known array manifold to determine the angle of arrival (AOA) or can be used with onboard optimization codes, such as those employed for ESPAR antennas, to iteratively null the interfering source in the environment. Furthermore, these RF power sensing circuits  103 C are preferably implemented with integrated thresholding, allowing them to discriminate between incident high power threats and normal received signals. To ensure that these RF power sensing circuits  103 C are capable of measuring an incident HPRF (High Power Radio Frequency) attack as well as surviving the large incident fields associated with an HPRF attack (&gt;400V/m), multiple matching circuits  103   b  may be implemented in series with each sensing element  103   e . These matching circuits  103   b  are used to ensure that enough voltage is dropped across the mixer  103   d  to perform direction finding for even low power attacks. For large attacks these circuits are preferably tuned to act as an attenuator, preventing an over-voltage condition at the mixer  103   d . The circuit  2   d  of  FIG. 2-2  illustrates an embodiment of a RF power sensing circuit  103 C that can be used to sense the direction of the interferers. RF power sensing circuits  103 C are preferably scattered at various locations in the array of elements  101  and each RF power sensing circuit  103 C reports the amplitude and phase detected at each location, which is then fed via an ADC  106  to (or embodied in) microcontroller  107  to identify the AOA(s) of the incident interfering signal(s). 
     To ensure that this autonomously reconfigurable surface of the system  100  can be readily conformed to existing radomes and apertures, the elements  101  in this array should be significantly smaller than the aperture they are intended to protect. In many cases, this requirement means that these elements  101  need to be significantly smaller than the operating wavelength (&lt;λ/4) of the communication or radar system being protected. Unlike previous approaches of conformal reconfigurable surfaces, such as active frequency selective surfaces (FSS) and metasurfaces, which utilize dense and fully populated grids of tuning elements to achieve full control over the radiation pattern of an antenna or an incident wave, the disclosed technology is implemented with as a sparse array of tuning elements since it needs only steer one or hopefully only a small number of nulls in the radiation pattern of an antenna. As such, only a small subset of the elements  101  are associated with or connected to reactance tuning elements  104 . This sparse arrangement of tuning elements  104  allows the disclosed system  100  to achieve significantly lower insertion loss when inactive, compared to traditional active FSSs and metasurfaces where the tunable elements (often varactors) interconnect each and every neighboring metallic element. 
     Since most HPRF systems are linearly polarized, sensing and tuning elements may only need to operate in a single polarization, although this technology can also be made to respond to circular and elliptical polarizations (if need be) by providing RF power sensors  103  and reactance tuning elements  104  in both the vertical and horizontal gaps between elements  101 . As will be noted by going back and again reviewing  FIG. 1 , the reactance tuning elements  104  and the RF sensors  103  only occur in the vertically oriented connections to the elements  101  in this figure since that embodiment is for a single linear polarization. 
     Furthermore, while  FIG. 1  shows an element  101  disposed in every possible unit cell, the elements  101  may be omitted from unit cells where they do not directly couple to either a reactance tuning element  104  or a RF sensing element  103 . See the embodiment of  FIG. 1 a    where the unused elements  101  (that is, elements  101  of the embodiment of  FIG. 1  are unconnected to either elements  103  or  104 ) are omitted in the embodiment of  FIG. 1 a    from the array of elements  101 . Of course, some unconnected elements  101  may be retained while others may be omitted, if desired. 
     Since the null providing surface  100  may be conformal in some embodiments, it may well be that omitting unused elements  101  from the array of elements  101  will make is simpler to define an array of elements  101  that can be conveniently arranged on a non-planar, conformal surface. In some embodiments, the array of elements  101  may be disposed on a planar surface. 
     The term “null” has been used above and those skilled in art should appreciate that the term “null” refers to reducing the gain of the antenna in the direction of the interfering source(s). The higher the amount of reduction, the greater the “null” is. “Null” does not necessarily mean the interfering source(s) are completely eliminated, although that would be a very desirable result if it could be obtained. 
     To demonstrate the feasibility of null steering and direction finding with a sparse array of electrically small elements  101 , simulations of a 6×6 array of elements  101  (embodied as cross-shaped metallic dipoles) 36 mm away from an 1.5 GHz patch antenna (see  FIG. 4 ) were performed in HFSS (High Frequency Simulation Software). This scenario is similar to using the disclosed null providing surface  100  with a small navigation antenna, for example, as opposed to a relatively larger radome for a radar receive antenna. Of course, the same concept can be scaled up to larger horn and parabolic reflector antennas commonly used in radar or other applications. 
     In the first part of this simulation, steerable nulls (&gt;20 dB) were generated from zenith to horizon using only seven possible load values placed between four of the 36 elements  101  or unit cells in the array (as depicted by  FIG. 3 b   , at locations  204 ). Using a network analysis technique similar to that developed by J. R. Mautz and R. F. Harrington (see “Modal Analysis of Loaded N-Port Scatterers”  IEEE Transactions on Antennas and Propagation , Vol. 21, No. 2, 1973, incorporated herein by reference), load values for each of these four unit cells were determined to steer deep nulls (&gt;20 dB) to five desired angles in the radiation pattern of the patch antenna. In addition to null steering, this simulation can also be used to demonstrate the integrated direction finding capability of the disclosed null providing surface  100 . Voltage measurements from five different elements in the simulated surface were generated for an incident plane wave. Using a simulated array manifold, a correlation direction finding algorithm was implemented and used to demonstrate an average direction finding accuracy of +/−1.86° over a 100° field of view (FOV) (see  FIG. 3 a   ).  FIG. 3 a    illustrates simulated performance characteristics of system  100  as embodied in  FIG. 3 b   . The results of the simulated null steering can be seen in  FIG. 3 a    for a simple autonomously reconfigurable surface (ARS)  300  that has been placed over a 1.5 GHz patch antenna  312  as illustrated in  FIG. 3 b   . The surface  300  comprises of a six by six array of dipoles  301  with four integrated active tuning elements  304  as illustrated in  FIG. 3 b   . Deep nulls are steered throughout the FOV (Field of View) of the ARS using a sparse array of tuning elements (four). When the radome is turned off, ARS generates a broad radiation pattern similar to the patch antenna in isolation (&lt;1 dB insertion loss) as seen in  FIG. 3   a.    
       FIG. 4  illustrates an autonomously reconfigurable null providing surface  100  as simulated in more detail. To demonstrate the feasibility of null steering and direction finding with a sparse array of electrically tuned elements  104 , in this illustrative embodiment, a 6×6 array of crossed metallic dipoles  101  are placed 36 mm away from a 1.5 GHz patch antenna  212  and the performance was simulated. The null providing surface  100  further comprised of a substrate  101  assumed to be made of RO04003 and the dipoles  101  and the patch antenna  212  were assumed to be etched in PEC (Perfect Electric Conductor) such as copper for the purposes of this simulation. The design dimensions are illustrated in  FIG. 4 . A perfect conductor or perfect electric conductor (PEC) is an idealized material exhibiting infinite electrical conductivity or, equivalently, zero resistivity. This scenario illustrates application of the disclosed technology to a small navigation antenna, although the same concept could readily be applied to larger horn and parabolic reflector antennas commonly used in radar applications. 
     As noted above, since most HPRF systems are linearly polarized, sensing and tuning elements are usually only required to operate in a single polarization, although this technology can also be made to respond to circular and elliptical polarizations by adding sensing and tuning elements in both the vertical and horizontal gaps between crosses. In the first part of this simulation, steerable nulls (&gt;20 dB) were generated from zenith to horizon using only seven possible load values placed in four of the 36 unit cells in the array. Using a network analysis technique, load values for each of these four unit cells were determined to steer deep nulls (&gt;20 dB) to five desired angles in the radiation pattern of the patch antenna ( FIG. 3 a   ). In addition to null steering, this simulation can also be used to demonstrate the integrated direction finding capability of the disclosed technology. Voltage measurements from five different elements in the disclosed technology were generated in simulation for an incident plane wave. Using a simulated array manifold, a correlation direction finding algorithm was implemented and used to demonstrate an average direction finding accuracy of +/−1.86° over a 100° field of view (FOV) ( FIG. 3 a   ). 
       FIG. 5  is based on a simulation and illustrates the number of active elements typically used in traditional approaches such as tunable FSS or tunable metamaterial and the resulting insertion loss. The length of board used in the simulation for  FIG. 5  was 286 mm as shown in  FIG. 4 . The simulation assumes that for each element added to the board one metallic line of 1 mm width is added to the back side of the board. The simulation also assumes that all lines are uniformly spaced. The aforementioned active elements are the sensing and/or tuning elements but since the simulation only considers the metallic line associated with the element the exact type of element used is not relevant for this simulation. Assuming each unit cell has one active element connecting to each of its neighbors, as the unit cell size shrinks (i.e. corresponding to a more densely packed board) so too does the number of metallic lines on the backside of the board in turn increasing the insertion loss through the board. Traditional conformal reconfigurable periodic surfaces, such as active frequency selective surfaces (FSS) and metamaterials, rely on a dense (fully populated) grid of tuning elements leading to high insertion losses of −6 dB or even much greater when the surface is not actively biased or tuned. To avoid this issue, the present invention utilizes a sparse distributed arrangement of electrically small RF sensors and reactance tuning elements (typically less than 25 active elements) to achieve a low loss (typically much less than −3 dB) conformal surface. The results illustrated in  FIG. 5  assumes that all bias lines are oriented perpendicular to the polarization of the incident HPRF attack. 
       FIG. 6 a    illustrates simulated performance measured in terms of error in direction finding of the incident high power interferer.  FIG. 6 b    illustrates the model used for this simulation with four simulated sensing elements  203  on a surface comprising a 6×6 array of dipoles  101  placed over a 1.5 GHz patch antenna  212 . Using a simulated array manifold, a correlation direction finding algorithm was implemented and used to demonstrate an average direction finding accuracy of +/−1.86° over a 100° FOV as can be seen in  FIG. 6   a.    
     Control and monitoring of the tuning and sensing elements on the null providing surface  100  is preferably accomplished by implementing a microcontroller  107 , digital-to-analog converter (DAC)  108 , and analog-to-digital converter (ADC)  106  on the surface  101  as illustrated in  FIG. 1 . Phase and amplitude measurements from the high-power sensing elements are processed by the ADC  106  and supplied to the microcontroller  107  as digital inputs. This information is then utilized with direction finding algorithms to determine the required bias voltages (supplied by the DAC  108 ) for each tuning element  104  to effectively null the source of any high-power interference. These control elements (ADCs, DACs and microcontroller(s)) are preferably placed on the periphery of the substrate  102  to minimize coupling and to easily interface with the tuning and sensing elements through a network of thin copper traces  105 . To ensure that the DACs, ADCs, and microcontroller are capable of surviving the large incident fields associated with an HPRF attack (up to 800V/m), these elements are preferably hardened using EMI shields for control components as well as reactive and/or resistive loadings on all control and monitoring lines. 
     In another embodiment (see  FIG. 7 ) of the surface  100  for adaptive antenna nulling, the null providing surface  100  comprises two distinct layers or substrates  102   1  and  102   2  which define the (possibly conformal) surface  210  for protecting antenna element  212 . Each layer or substrate  102   1  and  102   2  is provided with a lattice or array of metallic elements  101  (preferably embodied as dipoles in this embodiment) mounted on a thin conformal substrate such as but not limited to Rogers RO4003, Rogers RO3003, Rogers RO5880, kapton, or PET. The metallic elements  101  may be omitted (as discussed with reference to  FIG. 1 a   ) from unit cells where they do not connect with reactance tuning elements  104  on layer or substrate  102   1  or do not connect with RF sensors  103  on layer or substrate  102   2 . 
       FIGS. 8 a  and 8 b    show the layers or substrates  102   1  and  102   2  of  FIG. 7  in greater detail. In this embodiment, reactance tuning elements  104  are aperiodically mounted on one of these two layers (on layer  102   1  in this embodiment) and are connected to a digital to analog convertor (DAC) preferably mounted on the same layer (see  FIG. 8 a   ). Also preferably mounted on this layer is a microcontroller  107  which converts digital control signals from a microcontroller to analog voltages which in turn modify the reactance values of the aforementioned tuning elements through a network of thin copper traces. See also  FIGS. 8 a  and 8 b    showing the traces in greater detail. High power sensing circuits are aperiodically mounted on the second layer preferably along with an analog to digital convertor (ADC). The analog inputs of this ADC are networked to these high power sensing circuits via a network of thin copper traces (again, see  FIGS. 8 a  and 8 b   ). The digital outputs of this ADC are routed to the microcontroller located on the top layer through a via connecting the two layers. The direction or bearing of high power interferers is determined using the high power sensing circuits integrated into the top layer. These high power sensing circuits are implemented using phased locked voltage controlled oscillators (PLL VCO) and mixers, which return relative phase and amplitude measurements (I&amp;Q) from distinct points in the disclosed surface for adaptive antenna nulling to an integrated microcontroller. These vector measurements can then either be correlated with a known array manifold to determine the angle of arrival (AOA). Once this AOA is known, the microcontroller found on the top layer adjusts the reactance tuning elements on the bottom layer to place a null in the direction of the incoming radiation. 
     In a yet another embodiment (see  FIG. 9 ), the surface for adaptive antenna nulling has three distinct layers or substrates  102   1 ,  102   2  and  111  which define the (possibly conformal) surface  210  for protecting antenna element  212 . The top layer  111  in this embodiment is comprised of a periodic and subwavelength (less than one quarter wavelength in size and spacing) lattice of metallic scattering elements  101   s  such as but not limited to crossed metallic dipoles, mounted on a thin conformal substrate such as but not limited to Rogers RO4003, Rogers RO3003, Rogers RO5880, kapton, or PET. The scattering elements  101   s  may be identical (or different) in shape and size as the elements  101  utilized in layers in  102   1  and  102   2 . This top layer  111  acts as a Frequency Selective Surface (FSS) to block out-of-band interference and radiation to provide further protection for the antenna (and its receiver) to be protected by this technology. The middle layer  102   2  and bottom layer  102   1  are each comprised of an array or lattice of metallic elements  101  (here depicted as crossed dipoles) disposed on a thin conformal substrate such as but not limited to Rogers RO4003, Rogers RO3003, Rogers RO5880, kapton, or PET. Reactance tuning elements  104  are aperiodically mounted on the bottom one of these three layers (layer  102   1 ) and are connected to a digital to analog convertor (DAC) preferably mounted on the same layer or embodied in the microcontroller. The DAC  108  converts digital control signals from or inside a microcontroller  107  to analog voltages which in turn modify the reactance values of the aforementioned tuning elements through a network of thin copper traces (see, for example, the prior embodiment of  FIGS. 8 a  and 8 b   ). RF sensing circuits  103  are aperiodically mounted on the middle layer  102   2  and are connected to an analog to digital convertor (ADC  106 ) which may be a stand alone element or may be embodied with the microcontroller  107 . The analog inputs of the ADC  106  are networked to these RF sensing circuits  103  via a network of thin copper traces (see, for example, the embodiments  FIGS. 8 a  and 8 b   ). The digital outputs of this ADC  106  are routed to the microcontroller  107  preferably located on the middle layer through a via connecting the two layers if the ADC  106  is a stand alone element. The direction or bearing of high power interferers is determined using the high power sensing circuits integrated into the middle layer. These RF sensing circuits  103  are implemented using phased locked voltage controlled oscillators (PLL VCO) and mixers, which return relative phase and amplitude measurements (I&amp;Q) from distinct points to an integrated microcontroller  107 . These vector measurements can then either be correlated with a known array manifold to determine the angle of arrival (AOA). Once this AOA is known, the microcontroller found on the middle layer adjusts the reactance tuning elements on the bottom layer to place a null in the direction of the incoming radiation. 
     The scattering elements  101   s  of the FFS layer  111  are preferably non-sparsely (i.e., uniformly) located in an array and layer  111  periodically loaded with reactive elements between neighboring scattering elements  101   s . The elements  101  of the middle layer  102   2  are preferably sparsely located in an array and the sensing elements  103  are connected to neighboring elements  101  of the middle layer  102   2 . The depicted sensing elements  103  are aperiodic as depicted. The elements  101  of the middle layer  102   2  are depicted at each possible position of an element  101 , but those elements  101  which are not directly coupled to a RF sensing element  103  may be omitted (as in embodiment of  FIG. 1 a   ) thereby rendering the array of elements  101  also sparse. So the in the middle layer  102   2  all but ten of the elements  101  may be omitted thereby reducing the number of remaining elements  101  to ten in number (those being the elements  101  directly coupled to a RF sensing element  103 ). The elements  101  of the bottom layer  102   1  are also preferably sparsely located in an array and they are aperiodically loaded with a different sensing element at each sensing element  104  location in the depicted array. The elements  101  of the middle layer  102   2  are depicted at each possible position of an element  101 , but those elements  101  which are not directly coupled to a sensing element  104  may be omitted thereby rendering the array of elements  101  sparse in that layer as well. So, the in the bottom layer  102   1  all but eight of the elements  101  may be thereby reducing the number of remaining elements to eight (those being the elements  101  directly coupled to a RF sensing element  103 ). 
     In a still yet another embodiment (see  FIG. 10 ), the present invention is embodied with two distinct layers  100  and  111  which define the (possibly conformal) surface  210  for protecting antenna element  212 . The top layer  111  in this embodiment is comprised of a periodic and subwavelength (less than one quarter wavelength in size and spacing) lattice of scattering elements  101  such as but not limited to crossed metallic dipoles, mounted on a thin conformal substrate such as but not limited to Rogers RO4003, Rogers RO3003, Rogers RO5880, kapton, or PET. This top layer  111  acts as a frequency selective surface (FSS) to block out-of-band interference and radiation to provide further protection for the antenna (and its receiver) to be protected by this technology. The bottom layer  100  is comprised of a lattice of cross-shaped metallic dipoles  101  mounted on a thin conformal substrate such as but not limited to Rogers RO4003, Rogers RO3003, Rogers RO5880, kapton, or PET and aperiodically loaded with reactance tuning elements and RF sensing circuits as discussed with reference to  FIGS. 1 and 1   a . Also preferably mounted on this surface are analog to digital convertors (ADCs), digital to analog convertors (DACs), and microcontroller(s) (not shown). The analog outputs of the DACs are networked to reactance tuning elements  104  via a network of thin copper traces (not shown). The analog inputs of the ADCs are networked to high power sensing circuits via a network of thin copper traces. The digital outputs of ADCs and the digital inputs of DACs are networked to microcontroller(s)  107  (if need be) via a network of thin copper traces (see  FIGS. 1 and 1   a ). The direction or bearing of high power interferers is determined using the high power sensing circuits integrated into the bottom layer. These high power sensing circuits are implemented using phased locked voltage controlled oscillators (PLL VCO) and mixers, which return relative phase and amplitude measurements (I&amp;Q) from distinct points in the disclosed technology to an integrated microcontroller. These vector measurements can then either be correlated with a known array manifold to determine the angle of arrival (AOA). Once this AOA is known, the microcontroller adjusts the reactance tuning elements on the bottom layer to place a null in the direction of the incoming radiation. 
       FIG. 13  is a flow chart of an embodiment of how the microcontroller(s)  107  may be programmed so that the null providing surface  100  is autonomously reconfigurable. When the magnitude of the voltages received at any of the RF sensing elements  103  exceeds a pre-determined voltage threshold the control sequence loop shown in  FIG. 13  is initiated. First, voltages (both amplitude and phase) are measured at each of the sensing elements (see block  200 ) and tested as meeting some predetermined threshold (see block  202 ). These voltage measurements are then correlated against the previously measured array manifold (see block  206 ). The argmax of the output of this correlation is taken to be the AoA of the incident high power wave (see block  208 ). This angle is then used with a lookup table to determine the appropriate bias condition to null the incident wave (see block  210 ). This bias condition is then implemented (see block  204 ). This process is then repeated (via “yes” output of block  202 ) until the magnitude of the voltages received at each of the sensing elements is less than the predetermined voltage threshold (via “no” output of block  202 ). 
     In the foregoing embodiments the metallic elements  101  are depicted in the formed of crosses or crossed dipoles. However, the metallic elements  101  need not necessarily assume the form of crosses or crossed dipoles; rather while the crossed dipole shape for the metallic elements  101  may be preferred, the metallic elements  101  may assume any convenient shape such as those depicted by  FIG. 11  and others known in the art. The metallic elements  101  of  FIG. 11  each have the same maximum width (w) and maximum height (h), but in some embodiments the maximum width (w) and maximum height (h) may be different and also may vary in size throughout an array of same. 
     The disclosed technology offers significant benefits to various airborne and maritime platforms containing sensitive navigation, communication, and sensing platforms. Furthermore, this technology has value to communications and sensor system programs and product lines which need protection against intentional and unintentional jamming. 
     In particular configurations, it may be desirable to have a smaller number of tuning elements  104  without any RF sensing elements  103 , if direction finding is not critical. In other configurations, direction finding is critical and will require several RF sensing elements  103  to pin point the interferer(s). Based on the accuracy needed for the direction finding, size of the radome and number of interferers that must be dealt with, number of reactance tuning elements  104  and number of RF power sensing elements  103  will vary. But nevertheless it is desirable that they and the number of elements  101  be sparse (and this not occupy every possible unit cell) to reduce 
     The disclosed technology can be applied to retrofit any existing antenna or a radiating aperture by adding this autonomously reconfigurable surface that serves as a conformal radome to provide for the needed adaptive antenna nulling to suppress high power interferers. This approach does not require any antenna or receiver redesign to protect from high energy jammers. Number of ADCs, DACs and microcontroller(s) needed to be integrated on the radome is a function of application and mission requirements and will vary from application to application. Alternatively, the disclosed technology can be applied to an antenna or a radiating aperture of new design by adding this autonomously reconfigurable surface into that design. 
     Modifications, additions, or omissions may be made to the systems, apparatuses, and methods described herein without departing from the scope of the inventive concepts. The components of the systems and apparatuses may be integrated or separated. Moreover, the operations of the systems and apparatuses may be performed by more, fewer, or other components. The methods may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order. As used in this document, “each” refers to each member of a set or each member of a subset of a set. 
     To aid the Patent Office, and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants wish to note that they do not intend any of the appended claims or claim elements to invoke paragraph 6 of 35 U.S.C. Section 112 as it exists on the date of filing hereof unless the words “means for” or “step for” are explicitly used in the particular claim.