Fragmented aperture antennas and broadband antenna ground planes

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.

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.

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.sub.c. When p.sub.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 20a, 20b, 20c, 20d is assumed so
 that linear polarization results in the direction broadside to the
 aperture. The conducting element size is chosen so that a 31.times.31
 array fits within each quadrant 20a, 20b, 20c, 20d. 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., 20a) of the antenna 20 is a lattice of 31.times.31 square
 patches wherein each patch on the dielectric substrate can either be
 metallic or non-metallic. Thus, each quadrant 20a-20d has 961 degrees of
 freedom (2.sup.961 =10.sup.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.times.31 aperture (which is one quadrant (i.e., 20a) 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.times.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 20a, 20b, 20c, 20d. 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., 20b (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.sup.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.times.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 .lambda./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 .lambda./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 131a, 131b, 131c (mutual coupling
 terms are small or manageable). In the connected array, the antenna
 elements 131a, 131b, 131c may physically touch, so the embedded array
 behavior does not resemble the isolated antenna behavior. By allowing the
 antenna elements 131a, 131b, 131c 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 131a, 131b, 131c
 to be connected; however, as evidenced in FIG. 19, the elements 131a,
 131b, 131c 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 1/20wave length) and connected by switches 147. The
 switches are opened 147a and closed 147b to configure the antenna 143. As
 a non-limiting example, conducting patch 145a is connected to neighboring
 connected patch 145b by switch 147b'. 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 147a, 147b 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 210a-210f are fed by feed patches 215. In the
 fragmented aperture array 210, the antenna elements 210a-210f in the array
 may physically touch; hence, the embedded array behavior does not resemble
 the isolated antenna behavior. By allowing the elements 210a-210f 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.