Abstract:
A reconfigurable adaptive wideband antenna includes a reconfigurable conductive substrate for dynamic reconfigurablility of the frequency, polarization, bandwidth, number of beams and their spatial directions, and the shape of the radiation pattern. The antenna is configured as a reflect array antenna having a single broadband feed. Reflective elements are electronically painted on the reconfigurable conductive surface using plasma injection of carriers in high-resistivity semiconductors.

Description:
This application claims benefit of U.S. provisional patent application Ser. No. 60/233,185, filed Sep. 15, 2000, which is herein incorporated by reference. 
    
    
     The invention generally relates antenna systems and, more particularly, the invention relates to a reconfigurable adaptive wideband antenna. 
     BACKGROUND OF THE INVENTION 
     The detection, location, identification, and characterization of electromagnetic (EM) signals of types that have a low probability of intercept is an increasingly challenging problem. In general, EM signals with a low probability of intercept are transmitted by adversarial sources and thus employ various methods to reduce their signature. Such methods include frequency hopping, multiple signal polarizations, and spread-spectrum encoding techniques. In addition, the locations of the sources of such signals are not fixed and may change quite rapidly. The number of sources or EM signals that need to be located and tracked may also change depending on the particular circumstances. 
     A broadband antenna is generally required in order to track such EM signals. Frequency independent antennas such as spirals and quasi-frequency independent antennas such as log-periodic antennas are quite large and their use in an antenna array is quite limited. Also, an adaptive array using such broadband elements would require a feed structure integrated to a true-time delay network in order to achieve multiple beams and beam scanning. Such feed networks are difficult to design and are expensive to implement. 
     Therefore, there exists a need in the art for an adaptive wideband antenna capable of dynamic reconfiguration of operating frequency, polarization, bandwidth, number of beams and their spatial directions, and radiation pattern shape without the need for a feed network. 
     SUMMARY OF THE INVENTION 
     The disadvantages associated with the prior art are overcome by a reconfigurable adaptive wideband antenna capable of dynamic reconfigurability of several antenna parameters. Specifically, the present invention is a reflect array antenna comprising a reconfigurable conductive substrate and a single broadband feed. The reconfigurable conductive substrate is capable of dynamically forming conductive surfaces that can be used as reflective elements in the array. The conductive surfaces are electronically painted on the substrate using plasma injection of carriers in high-resistivity semiconductors. The reflective elements can be configured in many formations, including frequency independent fractal formations, that allow for wideband operation of the antenna. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 depicts a perspective view of a reconfigurable adaptive wideband antenna; 
     FIG. 2 illustrates a fractal formation of reflective elements; 
     FIG. 3 depicts an alternative embodiment of a reconfigurable adaptive wideband antenna; and 
     FIG. 4 depicts a detailed view of an exemplary reconfigurable conductive substrate. 
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     FIG. 1 depicts a perspective view of a reconfigurable adaptive wideband antenna  100  embodying the present invention. The antenna  100  comprises a frame  102 , a reconfigurable conductive substrate  104 , a tripod  106 , and a feed horn  108 . The reconfigurable conductive substrate  104  is mounted within the frame  102 , which is integral with the tripod  106 . The tripod  106  supports the feed horn  108 , which is positioned at a predetermined location above the antenna  100 . The reconfigurable conductive substrate  104  is capable of electronically “painting” conductive surfaces in any shape, size, number, or location. Such conductive surfaces can be used as reflective elements for the antenna  100 . In the present embodiment of the invention, the reconfigurable conductive substrate  104  includes a plurality of reflective elements  110  disposed in a planar array formation. 
     The reconfigurable adaptive wideband antenna  100  operates as a reflect array antenna. The reflective elements  110 , therefore, do not require any type of feed network. In response to an excitation, electromagnetic energy radiates from the feed horn  108  to illuminate the plurality of reflecting elements  110 . The plurality of reflecting elements  110  reflect the energy radiated from the feed horn  108  as a collimated wave (also known as the main beam) in a particular direction. The main beam can be scanned by coupling phase shifters or true-time delay lines to the plurality of reflective elements  110 , as is well understood in the phased array art. With the proper phase design or phase-changing device incorporated into each reflecting element  110 , the main beam can be tilted or scanned through large angles (e.g., 50° from the planar aperture broadside direction). Although the antenna  100  has been described in transmission mode, it is understood by those skilled in the art that the present invention is useful for both transmitting and receiving modes of operation. 
     The extent to which the planar array formation of reflective elements  110  allows the antenna  100  to be adaptive in terms of frequency of operation, bandwidth, and number and location of beams and nulls is very limited. As indicated above, however, the present invention is capable of dynamically reconfiguring conductive patterns on the reconfigurable conductive substrate  104 . This capability provides for maximum flexibility and adaptivity in defining the antenna structure. A very broad class of planar antennas can be implemented by electronically painting various conductive surfaces to generate the reflective elements  110 , which include dipoles, patches, spirals, and general arbitrary shapes and sizes. In addition, the conductive surfaces can also be used to provide the phase delay structures required in order to scan the main beam in a particular direction. 
     For example, FIG. 2 shows a fractal formation of reflective elements  110 . Fractal formations of antenna elements are known to be frequency independent and are more particularly described in “Fractal Antenna Engineering: The Theory and Design of Fractal Antenna Arrays,” D. H. Werner et al., IEEE Antennas and Propagation Magizine, Vol. 41, No. 5, October 1999, at pages 37-59. FIG. 2 shows the fractal formation known as the Sierpinski carpet. An array of reflective elements in such a formation provides the antenna  100  with frequency-independent multiband characteristics and a scheme for realizing low sidelobe performance. 
     FIG. 3 depicts an alternative embodiment of a reconfigurable adaptive wideband antenna  300 . The antenna  300  comprises a control layer  302 , at least one ground plane  304  (3 are shown), and a reconfigurable conductive substrate  104 . In the present embodiment of the invention, the reconfigurable conductive substrate  104  is configured with a Sierpinski carpet formation of reflective elements  306 . The reflective elements  306  are excited by a single broadband feed  308 , such as, but not limited to, a ridge waveguide feed horn or a spiral antenna. Utilization of the single broadband feed  308  eliminates the need for a complex feed network, increasing the efficiency of the antenna  300 . 
     The fractal formation of reflective elements  306  allows for wideband operation of the antenna  300  by defining sub-arrays of elements at all operating bands. Each ground plane  304  is frequency selective and provides a ground plane for each sub-array of elements at a particular operating frequency. The control layer  302  provides biasing control for the reconfigurable conductive substrate  104  and also includes adaptive processing electronics. 
     FIG. 4 depicts a detailed view of an exemplary reconfigurable conductive substrate  104 . The reconfigurable conductive substrate  104  comprises a dielectric sheet  402  having an active semiconductor layer  404  planted on the backside. In the present embodiment, the semiconductor layer  404  is made of thin, high-resistivity silicon. An array of trenches  406  is etched into the semiconductor layer  404  (a 4×4 array is shown), leaving the semiconductor layer  404  in a mesh formation. A plurality of PIN diodes  408  are integrated in the remaining semiconductor layer  404 , each PIN diode being adjacent to each side of each trench  406 . Each of the PIN diodes  408  comprises a doped p +  region  410 , a doped n +  region  412 , and an intrinsic region  414 . 
     The reconfigurable conductive substrate  104  is capable of electronically painting conductive surfaces by utilizing junction carrier injection in high-resistivity silicon. It is known that carriers in semiconductors form a plasma, which at high enough levels, causes the semiconductor to behave as a metallic medium. Formation of plasma in semiconductors is more particularly described in “The Effects of Storage Time Variations on the Forward Resistance of Silicon p + -n-n +  Diodes at Microwave Frequencies,” R. U. Martinelli, IEEE Trans. Electron Devices, Vol. ED27, No. 9, September 1980. 
     Returning to FIG. 4, when one of the PIN diodes  408  is correctly biased, carriers are injected into the intrinsic region  414  of the diode  408  so as to form plasma-filled conductive regions. The plasma is confined to the intrinsic region  414  by the respective adjacent trenches  406 . By selectively biasing particular PIN diodes  408 , a pattern of conductive surfaces can be formed, limited only to the resolution of the mesh formation of the semiconductor layer  404 . If the cell dimensions of the mesh formation are smaller than about {fraction (1/10)} of a wavelength of the RF signal, then the mesh behaves as a solid conductor sheet to the RF signal. Thus, conducting planar regions of any desired shape or size can be formed on the backside of the dielectric sheet  402  utilizing this conductive mesh. 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.