Abstract:
The present invention provides a fragmented aperture antenna. The antenna includes a planar layer having a plurality of conductive and substantially non-conductive areas. Each area has a periphery that extends along a grid of first and second sets of parallel lines so that each area comprises one or more contiguous elements defined by the parallel lines. The locations of the conducting materials in the fragmented aperture antenna are determined by a multi-stage optimization procedure that tailors the performance of the antenna to a particular application. The resulting configuration and arrangement of conductive and substantially non-conductive areas enable communication of electromagnetic energy wirelessly in a specific direction to the planar layer when an electrical connection is made to at least one of the conductive areas.

Description:
CLAIM OF PRIORITY 
     This application claims priority to copending U.S. Provisional Application entitled, “Fragmented Aperture Antennas and Broadband Antenna Ground Planes,” having Ser. No. 60/136,721, filed May 28, 1999, which is entirely incorporated herein by reference. 
    
    
     GOVERNMENT LICENSE RIGHTS 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. XXXXXX-97-C-1229 awarded by the Department of Defense of the United States of America. The prefix XXXXXX is classified confidential. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to the field of broadband antennas, and more particularly, to fragmented aperture antennas with tailored electromagnetic performances. 
     BACKGROUND OF THE INVENTION 
     An antenna is a device that can both transmit and receive electromagnetic waves of energy. Designing an antenna can be a complicated task because of the inherent properties of electromagnetics. Presently, antenna engineers physically scale or modify conventional antennas to best meet a particular application. However, in many instances, this procedure is suboptimal because a suitable conventional antenna may not exist or is not similar enough to meet a particular need. Antennas with broadband frequency coverage are desirable so the antenna can operate in a greater number of applications, but many conventional antennas with broadband coverage also include inadequacies that render them ultimately unacceptable. 
     For example, a multi-turn spiral antenna is a broadband antenna. However, the gain of the spiral antenna is essentially flat with frequency. The optimal use of the aperture area would yield a gain that increases with frequency, so the spiral antenna is suboptimal from because of its increases in gain over frequency. 
     Another example of a broadband antenna is the bow-tie antenna. A bow-tie antenna will radiate over a wide range of frequencies. Because the direction of radiation for the bow-tie antenna changes over the range of frequency, this feature renders the bowtie as suboptimal. 
     Thus, there is a need for an antenna that can overcome these limitations, deficiencies and inadequacies that is heretofore unaddressed. 
     SUMMARY OF THE INVENTION 
     Briefly described, the preferred embodiment of the present invention provides a new family of antennas—fragmented aperture antennas. The antenna includes a planar layer having a plurality of conductive and substantially non-conductive areas. Each area has a periphery that extends along a grid of first and second sets of parallel lines so that each area comprises one or more contiguous elements defined by the parallel lines. The locations of the conducting materials in the fragmented aperture antenna are determined by a multi-stage optimization procedure that tailors the performance of the antenna to a particular application. The resulting configuration and arrangement of conductive and substantially non-conductive areas enable communication of electromagnetic energy wirelessly in a specific direction to the planar layer when an electrical connection is made to at least one of the conductive areas. 
     The present invention can also be viewed as providing one or more methods. As an example, one such method is for making an antenna. The method includes a step of defining a planar grid defined by first and second sets of parallel lines so that the grid comprises a plurality of elements defined by the lines. The method additionally includes determining a first plurality of said elements that should be substantially conductive and a second plurality of said elements that should be substantially nonconductive so that a hypothetical antenna formed from said planar grid elements exhibits a desired frequency spectrum. 
     In an alternative embodiment, a broadband ground plane is created by using a similar optimization strategy as described above. The fragmented ground plane is a second patterned sheet placed behind the radiating layer to reflect the energy in the forward direction of the antenna. The fragmented ground plane is a patterned layer similar to the radiating antenna aperture. A feed is applied to the radiating aperture, and the ground plane layer is placed in parallel to the radiating aperture at a predetermined distance. 
     The single fragmented aperture antenna as described above may also be placed in an array of multiple antenna elements. In an alternative embodiment, the fragmented aperture antennas configured in the array environment are allowed, through the optimization process, to physically touch neighboring antenna elements, thereby creating a connected array. To create the connected antenna array, a suitable antenna element is selected and then the spacing and size are chosen such that no grating lobes exist and that the required array gain is met. In the connected array, the individual antenna array elements may physically touch, so the embedded array behavior does not resemble the isolated antenna behavior. By allowing the individual antenna array elements to touch, the low frequency limit of operation is not set by the size of the isolated elements, but rather, it is set by the size of the array antenna. 
     Another embodiment of the invention realizes a reconfigurable aperture and achieves multiple fragmented aperture designs from a single aperture. The reconfigurable aperture is comprised of conducting elements and configurable switches that may be opened or closed to create a fragmented antenna. The switches may be configured to steer the emitted energy at some predetermined angle from broadside. 
     In yet another alternative embodiment, the switched aperture antenna may be constructed in a connected array such that a large configurable aperture is comprised of an array of identically smaller, reconfigurable elements. The switched fragmented aperture array structure is a connected array similar to the connected non-switched arrays as discussed above. Metal patches are connected by closed switches to form the antenna array. A separate feed patch feeds each antenna element of the array. In the switched fragmented aperture array, the antenna elements in the array may physically touch; hence, the embedded array behavior does not resemble the isolated antenna behavior. Different configurations of a configurable array can operate broadband for a particular set of beam widths and steering angles, and the configuration of each array element can be changed from different beam widths and steering angles. 
     Many antennas, methods, features, and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. In the drawings, like reference numerals designate corresponding parts throughout the several views. 
     FIG. 1 is a diagram of an antenna having pattern structures of conducting and substantially non-conducting elements utilizing the notion of percolation physics. 
     FIG. 2 is a diagram depicting the phase of the plane wave transmission coefficient for the antenna of FIG. 1 compared to that of a homogenous dielectric sheet. 
     FIG. 3 is a diagram of a fragmented aperture antenna optimized to operate from 800 MHz to 2.5 GHz with flat 6 dB system gain. 
     FIG. 4 is a diagram of a set of trapezoidal conducting strips arranged in fixed locations to provide a coarse description of the antenna ultimately developed as shown in FIG.  3 . 
     FIGS. 5 and 6 are flowcharts of the two-step optimization process implemented by the computer of FIG. 5 to create, for example, the antenna shown in FIG.  3 . 
     FIG. 7 is a diagram of the predicted and measured performance of the antenna radiating structure in FIG.  3 . 
     FIG. 8 is a graph of the measured H-plane radiation pattern of the antenna in FIG. 3 as compared to the design prediction. 
     FIG. 9 is a diagram of a fragmented aperture antenna over a 0.4-2.04 GHz optimized frequency range to achieve a system gain that follows the uniform aperture limit and was designed by the two-step optimization process shown in FIGS. 5 and 6. 
     FIG. 10 is a graph of the predicted performance of the antenna in FIG. 9 showing the directive gain, system mismatch gain and uniform aperture gain. 
     FIG. 11 is a diagram of a fragmented aperture antenna designed by the two-step optimization process shown in FIGS. 5 and 6 and optimized over a 1.4-1.8 GHz frequency range to achieve a system gain that follows the uniform aperture limit. 
     FIG. 12 is a graph of the performance for the antenna displayed in FIG.  11 . 
     FIG. 13 is a fragmented aperture antenna designed by the two-step optimization process shown in FIGS. 5 and 6 and optimized for dual polarization over a 1.4-1.8 GHz frequency range. 
     FIG. 14 is a graph of the predicted performance of the antenna displayed in FIG.  13 . 
     FIG. 15 is a diagram of an antenna designed by the two-step optimization process shown in FIGS. 5 and 6 with a fragmented ground plane. 
     FIG. 16 is a diagram of two separate ground plane layers designed for the same radiating aperture. 
     FIG. 17 is a graph of the performance of the fragmented aperture with the ground plane layers shown in FIG. 16 as compared to the uniform aperture limit. 
     FIG. 18 is a graph of the measured performance of the fragmented aperture antenna in FIG. 16 with a ground plane to show performance improvement. 
     FIG. 19 is a diagram of three fragmented aperture antennas arranged in a connected antenna array similar to the antenna shown in FIG.  3 . 
     FIG. 20 is a graph of the performance of the antenna array shown in FIG.  19 . 
     FIG. 21 is a diagram of a switched aperture antenna element arranged to form a fragmented aperture antenna similar to the antenna shown in FIG.  3 . 
     FIG. 22 is a switched aperture antenna similar to the antenna shown in FIG. 21 with several switches closed to realize an antenna created by the optimization process shown in FIGS. 5 and 6. 
     FIG. 23 is a graph of the performance of the switched aperture antenna in FIG.  22 . 
     FIG. 24 is a graph of the H-plane radiation pattern of the switched aperture antenna in FIG.  22 . 
     FIG. 25 is a diagram of a switched aperture antenna as in FIG. 21 for a 1.4 to 1.8 GHz frequency range for 30 degree steering. 
     FIG. 26 is a graph of the measured system gain as a function of frequency for the antenna in FIG.  25 . 
     FIG. 27 is a graph of the H-plane radiation pattern for the antenna in FIG. 25 that is steered toward 30 degrees from broadside. 
     FIG. 28 is a graph of three potential system gains for the switched aperture antenna in FIG. 22, FIG. 25 and a third configuration not shown. 
     FIG. 29 is a diagram of a connected array of switched aperture antennas as shown in FIG. 22 to create a large configurable aperture. 
     FIG. 30 is a fragmented aperture antenna created by the optimization process described in FIGS. 5 and 6 realized through screen printing techniques. 
     FIG. 31 is a diagram of a computer that may implement the optimization process as shown in FIGS. 5 and 6. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a diagram of an antenna design structure  10  involving pattern structures of conducting and substantially non-conducting elements  11 ,  13  and utilizing the notion of percolation physics. Dark regions  11  represent conducting material while light regions  13  represent substantially non-conducting material. Conducting material may be any material that has a higher conductivity than the substantially non-conducting material. As a non-limiting example, the conducting material may be a material with semi-conducting qualities, and the substantially non-conducting material may be any type of dielectric material. 
     Each site that is occupied by conducting material  11  has probability, p c . When p c  approaches a critical value, the percolation threshold, long chains  14  are likely to be formed in the structure  10 . For occupation probabilities greater than this threshold, there will be a continuous chain across the structure enabling direct current (DC) conduction to occur. 
     Near the percolation threshold, conducting chains  14  are created having a variety of lengths. These chains resonate at a wide range of frequencies and cause the structure  10  to have a broadband response. FIG. 2 is a graphical diagram  16  depicting the phase of the plane wave transmission coefficient  18  for the antenna design  10  (FIG. 1) compared to that of a homogenous dielectric sheet  19 . The transmission phase response  18  of the percolating structure  10  is relatively flat across a wide frequency band. In contrast, the homogenous material  19  exhibits a linear phase variation with frequency. The flat transmission phase  18  of the percolating structure  10  is a result of the wide variety of length scales represented in the structure  10 . Thus, this structure  10  is useful as a broadband antenna and is hereinafter referred to as a fragmented aperture antenna. 
     FIG. 3 is a diagram of a fragmented aperture antenna  20 , as a non-limiting example, optimized to operate from 800 MHz to 2.5 GHz with flat 6 dB system gain. Antenna  20  is a square planar aperture of side length 10 inches and includes conducting structures arranged in isolation  22  and in patches  24 . The conducting structures are arranged in a grid wherein groups of the structures create the conducting patches  24 . 
     To use the pattern structure  20  as shown in FIG. 3 as an antenna, the energy gets out of the structure  20  through one or more feed points. It is desirable, however, to have as few feed points as possible. In one embodiment, feed points may be added to a fixed pattern after determining possible locations that would serve as good locations for the feed points. Criteria for placement of feed points may include places of high current flow. In the preferred embodiment, feed points may be placed in a fixed position and different antenna patterns may be configured to reach the desired performance. 
     One non-limiting example of the preferred embodiment for designing a fragmented aperture antenna is to place a feed  21  at the center of a 10 inch aperture and search for patterns that yield the desired antenna performance. In this embodiment, the antenna  20  is fed by a single, centrally located transmission line  21  of characteristic impedance 100 Ohms. Quadrant symmetry of the pattern  20   a ,  20   b ,  20   c ,  20   d  is assumed so that linear polarization results in the direction broadside to the aperture. The conducting element size is chosen so that a 31×31 array fits within each quadrant  20   a ,  20   b ,  20   c ,  20   d . Numerous random selected patterns are then evaluated for broadside gain as a function of frequency. This random search results in some suitable antennas; however, a more optimized search strategy is required. 
     This preferred embodiment of the invention implements a multi-stage optimization approach to design the fragmented aperture antennas, such as antenna  20  in FIG.  3 . In designing the antenna  20 , the objective is to obtain the maximum system gain in the broadside direction over a specified, relatively wide bandwidth. System gain includes any loss due to impedance mismatch. The antenna  20  is assumed to have reflection symmetry about two orthogonal planes. Additionally, the radiating structure of the antenna  20  is optimized using a modified genetic algorithm approach. Each quadrant (i.e.,  20   a ) of the antenna  20  is a lattice of 31×31 square patches wherein each patch on the dielectric substrate can either be metallic or non-metallic. Thus, each quadrant  20   a - 20   d  has 961 degrees of freedom (2 961 =10 289  possible antennas). A direct genetic optimization with  961  binary genes exhibits very poor convergence as it is impractical to use a genetic algorithm directly in this 961 bit space because of computational requirements. 
     Implementing a two-step process, however, improves the convergence rate. The first stage implements a direct genetic optimization using a large-scale characterization of the antenna aperture  20 —typically 40 genes. The second stage is a stochastic hill climb optimization using the fine scale characterization—961 degrees of freedom for a typical 31×31 aperture (which is one quadrant (i.e.,  20   a ) of the antenna aperture  20 ). Briefly described, a simple stochastic hill climb consists first selecting a location in the aperture $$$at random. The bit at this location is toggled—in effect changing this location from free space to metal or metal to free space. This candidate antenna is evaluated. If the antenna is better than the previous antenna, then this change is retained. Otherwise, the antenna is returned to its previous state. This process is repeated many times until the rate of improvements practically stops. Alternatively, a genetic algorithm method similar to that as disclosed in U.S. Pat. No. 5,719,794, which discloses a method for designing wire antenna configurations and is herein incorporated by reference, may be implemented for design of fragmented aperture antennas. Moreover, other advanced hill climb procedures may also be used. These advanced methods include selecting multiple locations in the aperture and/or changing more than a single bit. Nevertheless, the stochastic hill climb is a random walk toward a more optimal antenna. This two-step approach exhibits acceptable rates of convergence and is described in more detail hereinafter. 
     The first stage of optimization process, to obtain a uniform antenna system gain over a desired frequency range, requires a description of potential antennas with a smaller number of binary digits than the full 961 required for the 31×31 aperture. FIG. 4 is a diagram of a set of trapezoidal conducting strips  30  arranged in fixed locations to provide a coarse description of the antenna  20  ultimately developed as shown in FIG.  3 . The coarse description of the antenna composed of the conducting strips is comprised of four quadrants  20   a ,  20   b ,  20   c ,  20   d . With additional reference to the flowchart  40  in FIGS. 5 and 6, which show the two-step optimization process, a set of trapezoidal conducting strips are arranged in fixed locations in a quadrant (i.e.,  20   b  (FIG.  4 )) to provide a coarse description of the antenna  20  (FIG.  3 ), as in step  41 . Binary genes describe the length of two opposite sides of the trapezoids  30  (FIG.  4 ), so that the conducting strip could be, for example, a triangular region  31  (FIG. 4) (one side equal to zero), a rectangular region  32  (FIG. 4) (both sides equal), a general trapezoid  34  (FIG. 4) (unequal but non-zero sides) or non-present  36  (FIG. 4) (both sides equal to zero). The term binary genes represents genes that consist of a series of bits; for example, a gene that consists of 5 bits has 2 5 = 32possible states. In the non-limiting example, the length of a side  38  may be represented as 32 possible lengths (between 0 to 31); therefore, five bits are needed in this non-limiting example to prescribe a given strip, as described in step  43 . In this embodiment, a typical antenna may contain 10 to 20 strips, so a total of 50 to 100 bits describes the antenna for the first stage of the optimization process, as shown in step  45 . 
     Once the genetic optimization is performed using the large-scale description of the aperture distribution as described above, a fine-scale optimization process is performed, as in step  47 . This process uses the full description of the antenna (961 bits for the 31×31 aperture). The fine-scale optimization process makes a minor modification to the antenna design and then compares the performance of the new antenna to that of the genetically optimized antenna. A random location in the antenna is selected, as in step  48 , and a determination is made of whether the site contains a conductor, as in step  49 . If the selected site contains a conductor  22  (FIG.  3 ), as in step  51 , the conductor is removed and the performance of the resulting antenna is computed, as in step  53 . If the site did not originally contain a conductor  22  (FIG.  3 ), as in step  54 , one is added and the performance is likewise computed, as in step  53 . If, as in step  56 , it is determined that the new antenna performs better than the initial antenna, it is kept, as in step  58 . Otherwise, as in step  59 , the initial antenna is retained if a determination is made in step  56  that the initial antenna outperforms the resulting antenna. The optimization process may be repeated as many times as desired or until no further improvements are found, as shown in step  60 . Ultimately, a final antenna design is rendered, as in step  62 . This procedure can dramatically change the appearance of the conductor distribution in the aperture and typically results in a 3 dB improvement in the antenna performance. 
     FIG. 7 is a diagram of the predicted and measured performance  64  of the antenna radiating structure  20  in FIG. 3 that was optimized using the two-stage process described above to yield the best broadside system gain over the frequency span of 800 MHz to 2.5 GHz. System gain  65  is defined as directive gain times mismatch. Directive gain is the ideal gain of the antenna that is in the direction of maximum radiation, and mismatch accounts for the difference between the load impedance and the generator impedance of the communicating system. Because the optimization includes the effect of mismatch, the Voltage Standing Wave Ratio (VSWR) of the designed antenna is directly constrained. Thus, the measured system gain  65  for the antenna  20  is compared with the design prediction  67  for the same antenna. Predicted results  67  are generated using a numerical code based on the Finite-Difference Time-Domain (FDTD) Method. 
     Additionally, the system gain  65  is seen to be relatively flat across the frequency region that extends beyond the design bandwidth at the high end. Line  68  represents the directivity of an aperture of the same area with a uniform distribution of current, and line  69  represents the gain of a spiral antenna (not shown). Since the optimization process attempts to achieve a flat gain, the result is limited by the lowest frequency in the band of operation as evidenced by the fact that the system gain  65  is fixed to be the same as the directivity of the uniform current  68  at the lower end of this specified frequency range. Thus, it is desirable to search for designs whose gain over frequency attempts to mimic the uniform aperture gain  68  instead of a flat gain as evidenced by the measured system gain  65 . 
     FIG. 8 depicts graph  70  which is the measured H-plane radiation pattern  71  of antenna  20  (FIG. 3) compared to the design prediction  72 . As corroboration to the graph of antenna  20  in FIG. 7, the radiation pattern  71  is directed in the broadside direction as designed. 
     FIG. 9 is a diagram of a fragmented aperture antenna  75  optimized over a 0.4-2.04 GHz frequency range to achieve a system gain that follows the uniform aperture limit. Antenna  75  is fed centrally by feed  76  and is quadrantly symmetrical similarly to antenna  20  in FIG.  3 . Antenna  75  is a result of the two-step optimization process as described above, and as shown in FIGS. 5 and 6. 
     FIG. 10 is a graph  80  of the predicted performance of antenna  75  in FIG. 9 showing the directive gain  77 , system mismatch gain  78  and uniform aperture gain  79 . The system mismatch gain  78  tracks the uniform aperture limit  79  within a few dB over the optimization range. As evidenced from the fragmented aperture antenna  75  in FIG. 9, the genetic algorithm placed metal conductors near the top and bottom edges of the aperture  75  in an attempt to use the full aperture to enhance the low-frequency performance. The directive gain  77  is shown in FIG. 10 which factors out mismatch loss of the antenna  75 . Over most of the frequency region, the directive  77  and system mismatch gains  78  are almost identical, indicating a good impedance match for the antenna. However, the peak in the directive gain  77  at 1 GHz shows how the optimization process allowed a larger mismatch loss at a point where it could achieve a higher directive gain  77 . 
     FIG. 11 is a diagram of a fragmented aperture antenna  81  optimized over a 1.4-1.8 GHz frequency range to achieve a system gain that follows the uniform aperture limit. Antenna  81  is fed centrally by feed  82  and is quadrantly symmetrical. The frequency design is 1.3:1 to cover the 1.4-1.8 GHz frequency range. 
     FIG. 12 is a graph  83  of the performance for the antenna  81  as displayed in FIG.  11 . For antenna  81 , the system gain  84  in the broadside direction is very close to the uniform aperture limit  86 . The antenna  81  (FIG. 11) is well matched over the design bandwidth as evidenced by the system and directive gains  84 ,  87  being essentially the same. As evidenced by graph  83 , the antenna performance falls off rapidly outside the optimization region. 
     The exemplar antenna designs  20 ,  75 ,  81  (FIGS. 3,  9  and  11 ) are all linearly polarized. The optimization process described above in FIGS. 5 and 6 may also be implemented to design a dual polarized antenna with a separate feed point for two linear polarizations. FIG. 13 is a non-limiting example of a fragmented aperture antenna  90  optimized for dual polarization over a 1.4-1.8 GHz frequency range. There are two sets of feed points (not shown) located in the center of the aperture. One set is oriented vertically and the other set is oriented horizontally. The two pairs form a cross shape. FIG. 14 is a graph  92  of the predicted performance of the antenna  90  displayed in FIG.  13 . The broadside system and directed gains  91 ,  93 , as shown in FIG. 14, both follow the uniform aperture limit  95 . 
     The planar antennas discussed above naturally radiate equally in both broadside directions. For some applications, the backward radiation can be detrimental to the performance of the antenna. Scattering from supporting hardware behind the antenna can significantly influence the antenna performance in an unpredictable manner. As a non-limiting example, an antenna near a human body incurs electromagnetic loss because the body reduces the efficiency. Thus, a ground plane can be used to reduce the radiation in the backward direction and help alleviate this problem. For a narrow band antenna, this can simply be accomplished by placing a metallic conductor at λ/4 behind the antenna. The energy reflected from the ground plane adds constructively with the direct radiation to enhance the gain by 3 dB (for the ideal case of a ground plane infinite in an extent). However, as the bandwidth of the antenna increases, this solution does not always apply. 
     In an alternative embodiment, a broadband ground plane is created by using a similar optimization strategy as described above in FIGS. 5 and 6 in regard to the design of a fragmented ground plane. The fragmented ground plane is a second patterned sheet placed behind the radiating layer to reflect the energy in the forward direction. FIG. 15 is a diagram of an antenna system  98  including antenna  100  with a fragmented ground plane  99 . The fragmented ground plane  99  is a patterned layer similar to the radiating aperture  100  and is designed to operate as a ground plane over the bandwidth of the radiating aperture  100 . Feed  101  is applied to the radiating aperture  100  and the ground plane layer  99  is placed in parallel to the radiating aperture  100  at a distance of λ/8 at the highest frequency. Ground plane  99  is designed after the radiating aperture  100  is created to simplify the optimization process. 
     FIG. 16 is a diagram of two separate ground plane layers  105 ,  106  designed for the same radiating aperture  108 . The ground plane  105  used the structure of the radiating aperture  108  as the starting point for the optimization process as described above which utilizes the stochastic hill climb method. The ground plane  106  was created through the optimization process described above and shown in FIGS. 5 and 6 based upon a solid metal sheet (not shown) as the starting point. While the ground plane layer structures  105 ,  106  are different, the results yielded by the ground planes are similar. 
     FIG. 17 is a graph diagram  104  of the performance of the fragmented aperture  108  with ground plane layers  105 ,  106  as compared to the uniform aperture limit  111 . The addition of either ground plane layer  105 ,  106  (FIG. 16) yields approximately 2.1-2.2 dB of improvement in the broadside gain  112 ,  114  of the antenna  108 . As a comparison, line  116  is based on the performance of antenna  108  with no ground plane layer at all. There is, however, a slight increase in mismatch when the ground plane is added since the directivity actually improves by approximately 3 dB. 
     FIG. 18 is a graph diagram  120  of the measured performance of the fragmented aperture antenna  108  with ground plane  105  (FIG. 16) to show performance improvement. Line  121  represents the performance measurement of the antenna with the ground plane  105 , and line  123  represents the performance measurement of the antenna without any ground plane. The measured results show the 2 dB of improvement with the fragmented ground plane  105 . The result of including the ground plane layer  105  is a significant reduction in the radiation in the backward direction as evidenced by the horizontal pattern  124  in FIG.  18 . In this diagram, the radiation pattern of the antenna with the ground plane layer  105  is represented by line  125 , and the radiation pattern of the antenna without any ground plane layer is represented by line  126 . 
     The single fragmented aperture antenna as described above may also be placed in an array of multiple antenna elements. In one embodiment, the fragmented aperture antennas configured in the array environment are allowed, through the optimization process, to physically touch neighboring antenna elements, thereby creating a connected array. FIG. 19 is a diagram of three fragmented aperture antennas similar to the antenna shown in FIG. 3 arranged in a connected antenna array  130 . To create the connected antenna array  130  in FIG. 19, a suitable antenna element is selected (based on bandwidth, gain, VWSR) and then the spacing and size are chosen such that no grating lobes exist and that the required array gain is met. The performance of the selected antenna array  130  is slightly modified by the presence of the neighboring antennas  131   a ,  131   b ,  131   c  (mutual coupling terms are small or manageable). In the connected array, the antenna elements  131   a ,  131   b ,  131   c  may physically touch, so the embedded array behavior does not resemble the isolated antenna behavior. By allowing the antenna elements  131   a ,  131   b ,  131   c  to touch, the low frequency limit of operation is not set by the size of the isolated elements, but rather, it is set by the size of the array antenna  130 . 
     In focusing on the antenna array  130  in FIG. 19, as a non-limiting example, an array spacing of ten inches allows broadside operation up to approximately 1.2 GHz before the potential appearance of grating lobes. A traditional wideband antenna, such as an 8-inch bow-tie, will operate down to 250 MHz. The connected array elements  130 , as in the non-limiting example in FIG. 19, are optimized to operate from 100 MHz to 1 GHz. The same two-step optimization approach discussed above and as shown in FIGS. 5 and 6 produces the antenna array  130  as shown in FIG.  19 . The genetic design approach does not necessarily force the elements  131   a ,  131   b ,  131   c  to be connected; however, as evidenced in FIG. 19, the elements  131   a ,  131   b ,  131   c  are, in fact, connected. 
     FIG. 20 is a graph  135  of the performance of the antenna array  130  shown in FIG.  19 . The system mismatch gain  137  for the antenna array  130  is acceptable over the 100 MHz to 1 GHz frequency span, i.e., the performance tracks the uniform stick directivity  138 . The performance of a comparable bow-tie antenna array is shown by line  139 , and the directive gain of the antenna array is line  140 . The performance of the connected array  130  is approximately 10 dB superior at the low frequency as compared to the performance of the bow-tie  139 . The bow-tie antenna frequency drops out at 0.6 GHz, but this drop out is not present in the results of the connected antenna array  130 , as shown by line  137 . Finally, because the system gain  137  tracks the uniform stick directivity  138  closely, the diffraction-limited performance is achieved to below 100 MHz. 
     The discussion above in regard to fragmented aperture antennas illustrates the construction of aperture patterns that yield optimized performance over selected frequency bands. Another embodiment of the invention is herein discussed which realizes a reconfigurable aperture and achieves multiple fragmented aperture designs from a single aperture. The reconfigurable aperture offers the potential for wideband antenna designs. 
     FIG. 21 is a diagram of a switched aperture antenna element  143 . The switched aperture antenna  143  includes a centrally located feed point  149  to transfer energy from the antenna. The antenna aperture  143  consists of a lattice of conducting patches  145  that are electrically small (approximately {fraction (1/20)}wave length) and connected by switches  147 . The switches are opened  147   a  and closed  147   b  to configure the antenna  143 . As a non-limiting example, conducting patch  145   a  is connected to neighboring connected patch  145   b  by switch  147   b ′. The configured antenna  143 , in this non-limiting example, is similar to a traditional bow-tie antenna as shown by dashed lines  146 . The switches  147  can be realized by using MEMS (Micro-Electromechanical Systems) devices, PIN diodes, latches, radio frequency (RF) transistors, or other similar devices known to those of ordinary skill in the art. 
     The switched aperture antenna  143  in FIG. 21 can be configured to realize optimized patterns arranged to operate over specific bands of frequency and directions of radiation. The expected performance of these designs should approach the levels achieved by the optimized fractured aperture antennas discussed above. In the preferred embodiment, the size of the aperture  143  is fixed to ten inches square, and the size of the individual metal patches  145  and switches  147   a ,  147   b  are four millimeters square. 
     FIG. 22 is a switched aperture antenna  150  with the several switches  152  closed to realize an antenna created by the optimization process described above. This non-limiting aperture design  150  is configured to radiate broadside to have the best system gain over 1.4 to 1.8 GHz frequency range. Metal patches  154  are connected by closed switches  152  while open switches  155  are shown as blank space. Feed point  156  is connected at the center of the array  150 . FIG. 23 is a graph  160  of the performance of the switched aperture antenna  150  as configured in FIG.  22 . The system gain  162  of the switched aperture antenna  150  is shown in FIG. 23 as a function of frequency. The system gain  162  tracks the uniform aperture gain  164  closely over the 1.4 to 1.8 GHz optimization range, and is within 1 dB of this limit  164 . The broadside gain is shown as line  166 . As a result, the performance of the antenna array  150  in FIG. 22 is nearly diffraction limited. The H-plane radiation pattern  170  is shown in FIG.  24 . The measured radiation pattern  172  is directed in the broadside direction as desired based upon the model pattern  174 . 
     A switched aperture antenna configuration may also be designed to radiate at, as a non-limiting example, 30 degrees from broadside with a system gain over the 1.4 to 1.8 GHz frequency range. FIG. 25 is a diagram of a switched aperture antenna  180  for over a 1.4 to 1.8 GHz frequency range for 30 degree steering. As compared to the switched aperture  150  in FIG. 22, switches  181  are configured in a non-symmetrical arrangement to achieve the beam steering in the configuration that connects the conducting patches  183 . The measured system gain  188  as a function of frequency is shown in the graph  185  of FIG.  26 . The measured system gain  188  closely follows the predicted gain  192 . The measured system gain  188  tracks the uniform aperture limit  190  over the 1.4 to 1.8 GHz optimization range. The H-plane radiation pattern  197  is shown in graph  195  in FIG.  27  and is clearly steered toward 30 degrees from broadside. As a result, the measured system gain  188  (FIG. 26) and H-plane radiation pattern  197  conform to the design predictions  198  based on the optimization procedure described above. 
     FIG. 28 is a graph diagram  200  of three system gains  202 ,  204 ,  206  for the switched aperture antenna  150  (FIG.  22 ),  180  (FIG. 25) and a third antenna optimized for a 2.4-3.0 GHz range (not shown). Thus, by arranging the switches of a switched aperture antenna in multiple configurations, the antenna can be modified to perform to different characteristics and still approach the uniform aperture gain limit  208  for different frequency ranges. 
     Switched aperture antennas may also be constructed in a connected array such that a large configurable aperture is comprised of an array of identically smaller, reconfigurable elements as shown in FIG.  29 . The fragmented aperture array structure  210  is a connected array similar to the connected non-switched arrays as discussed above. Metal patches  211  are connected by closed switches  213  to form the antenna array  210 . Each of the antenna elements  210   a - 210   f  are fed by feed patches  215 . In the fragmented aperture array  210 , the antenna elements  210   a - 210   f  in the array may physically touch; hence, the embedded array behavior does not resemble the isolated antenna behavior. By allowing the elements  210   a - 210   f  to touch, the lower frequency limit of operation is not set by the size of the isolated element, but rather it is set by the size of antenna array. Thus, one configuration of a configurable array can operate broadband for a particular set of beam widths and steering angles, and the configuration of each array element can be changed from different beam widths and steering angles. Such an architecture has a significant cost reduction savings due to the repeated fabrication of a small pattern of patches and switches. 
     Antennas that can be described as 2-dimensional structures can be considered planar antennas. These antennas, if flexible, can also be considered conformal antennas, that is, they can be molded around objects and made to conform to the surface of the underlying structure. The type of antennas designed and fabricated as part of the screen printing subtask are all planar, conformal antennas. 
     Screen printing (also known as silk screen printing) is a process whereby ink is forced through tiny holes in a screen onto a substrate. The areas of the screen where one does not want inks coming through are covered with a solid epoxy. The ink dries and an image is bonded to the surface of the substrate. To create an antenna by screen printing techniques, the process may implement, as a non-limiting example, conductive inks containing silver particles or, as another non-limiting example, resistive inks containing carbon particles. Antenna ground planes may also be fabricated using the same inks. 
     FIG. 30 is a fragmented aperture antenna  220  created by the optimization process described above and realized through screen printing techniques. Substrates such as Kapton, Tyvek, Polyester, and Mylar may be used as material receptive to the screen printing of the antenna. Feed  222  is centrally located similarly as described above. Antennas created by the optimization process described above in FIGS. 5 and 6 may be printed on these substrates for performances shown in the previous figures. 
     Any process descriptions or blocks in flow charts should be understood as representing modules, segments, or portions of code which include one or more executable instructions for implementing specific logical functions or steps in the process, and alternate implementations are included within the scope of the preferred embodiment of the present invention in which functions may be executed out of order from that shown or discussed, including substantially concurrently or in reverse order, depending on the functionality involved, as would be understood by those reasonably skilled in the art of the present invention. 
     The optimization process, as discussed above in relation to FIGS. 5 and 6, comprises an ordered listing of executable instructions for implementing logical functions, can be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “computer-readable medium” can be any means that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer readable medium can be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a nonexhaustive list) of the computer-readable medium would include the following: an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic), a random access memory (RAM) (electronic), a read-only memory (ROM) (electronic), an erasable programmable read-only memory (EPROM or Flash memory) (electronic), an optical fiber (optical), and a portable compact disc read-only memory (CDROM) (optical). Note that the computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via for instance optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in a computer memory. 
     The optimization process as discussed above and as shown in FIGS. 5 and 6 may be implemented on a computer. FIG. 31 is a diagram of a computer  230  that may be utilized to implement the optimized process as shown in FIGS. 5 and 6. Housing  232  contains a processor  234  that accesses memory  236  via local interface bus  238 . The memory  236  may store software  240  and other data  241 . A monitor  243  is coupled by a video interface  245  to the bus  238  for presenting a display to the user. One or more input interface cards  247  may be coupled between the bus  238  and a keyboard  249 , mouse  250 , a microphone  252  and/or a scanner  253 . The processor  234  may communicate with an external network  260  by a modem  261 . An output interface card  264  may also be coupled to the local interface bus  238  for outputting audio to a speaker  266  and for outputting other data to a printer  267 . A mobile data storage device  270  may be included in computer  230  and is coupled to the local interface bus  238 . 
     It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.