Abstract:
A method and apparatus for reconfiguring an antenna array by optical control of MEMS switches. A light source is provided to direct light to individual optically sensitive elements which control delivery of actuating bias voltage to the MEMS switches. The light source is preferably separated from the antenna array by a structure which conducts the controlling illumination but provides a high impedance electromagnetically reflective surface which reflects electromagnetic radiation over the antenna operating frequency range with small phase shift, and which is disposed very close to the antenna array. Optically sensitive elements preferably include photoresistive elements, which are best formed in the substrate upon which the MEM switches are formed, and may include photovoltaic elements.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present invention is related to the following commonly assigned and co-pending U.S. application, “Optically Controlled MEM Switches,” filed Oct. 28, 1999, invented by T. Y. Hsu, R. Loo, G. Tangonan, and J. F. Lam, and having U.S. Ser. No. 09/429,234, which is hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention pertains to remotely reconfigurable antennas, and particularly to reconfiguring antennas by optical control of mechanical switches. 
     BACKGROUND OF THE INVENTION 
     Reconfigurable antenna systems have applications in satellite and airborne communication node (ACN) systems where wide bandwidth is important and where the antenna aperture must be continually reconfigured for various functions. These antenna systems may comprise an array of individually reconfigurable antenna elements. Each antenna element may be individually reconfigurable to modify its resonant frequency, such as by varying the effective length of dipole elements. Varying the resonant frequency of individual elements may enable an antenna to operate at a variety of frequencies, and may also enable control of its directionality. 
     One means of varying the resonant length of a dipole antenna is to segment the antenna lengthwise on either side of its feed point. The resonant length of the antenna may then be varied by connecting or disconnecting successive pairs of adjacent dipole segments. Connection of a pair of adjacent dipole segments may be effected by coupling each segment to a switch. The adjacent segments are then joined by closing the switch. 
     Previous designs for reconfigurable antennas have been proposed which incorporate photoconductive switches as an integral part of an antenna element in an antenna array. See “Optoelectronically Reconfigurable Monopole Antenna,” J. L. Freeman, B. J. Lamberty, and G. S. Andrews,  Electronics Letters,  Vol. 28, No. 16, Jul. 30, 1992, pp. 1502-1503. Also, the possible use of photovoltaic activated switches in reconfigurable antennas has been explored. See C. K. Sun, R. Nguyen, C. T. Chang, and D. J. Albares, “Photovoltaic-FET For Optoelectronic RF/Microwave Switching,”  IEEE Trans. On Microwave Theory Tech.,  Vol. 44, No. 10, October 1996, pp. 1747-1750. One problem with these designs, however, is that the performance of ultra-broadband systems (i.e., systems having an operating frequency range of approximately 0-40 GHz) utilizing these types of switches suffer in terms of insertion loss and electrical isolation. 
     RF MEMS (micro-electromechanical) switches have been proven to operate over the 0-40 GHz frequency range. Representative examples of this type of switch are disclosed in Yao, U.S. Pat. No. 5,578,976; Larson, U.S. Pat. No. 5,121,089; and Loo et al., U.S. Pat. No. 6,046,659. Previous designs for reconfigurable antennas using RF MEMS switches incorporated metal feed structures to apply an actuation voltage from the edge of a substrate to the RF MEMS switch bias pads. A problem with the use of metal feed structures to apply an actuation voltage to the switches is that, in an antenna array, the number of switches can grow to thousands, requiring a complex network of bias lines routed all around the switches. These bias lines can couple to the antenna radiation field and degrade the radiation pattern of the antenna array. Even when the bias lines are hidden behind a metallic ground plane, radiation pattern and bandwidth degradation can occur unless the feed lines and substrate feedthrough via conductors are very carefully designed because each element in the antenna array may accommodate tens of switches. This problem is magnified enormously as the number of reconfigurable elements increases. 
     A conductive ground plane generally provides a phase shift of 180° upon reflection of electromagnetic waves. In practice, the conductive ground plane should be separated from the antenna elements by at least a quarter wavelength, to avoid destructive interference at the antenna elements between electromagnetic waves received directly at the antenna elements and waves received via reflection from the ground plane. Hence, if the switches are disposed above a conductive ground plane, the bias lines for the switches will extend at least one quarter wavelength above the ground plane. Bias lines of this length above the ground plane may provide the radiation pattern and bandwidth degradation described above. 
     Thus, there exists a need for a means to control selectable RF MEMS switches in an array to control antenna elements, while reducing interference from control lines. 
     SUMMARY OF THE INVENTION 
     The present invention solves the above-noted problem by providing a mechanism for optical control of an array of MEM switches which in turn modify antenna elements. 
     MEM switches are mounted on an antenna substrate so as to provide selectable connections between adjacent elements of an antenna structure. The switches are optically controlled, preferably by means of an active LED matrix or an LCD matrix. Control is preferably provided through a structure adjacent to the antenna array, which shields the optical control circuitry and preferably provides a reflective surface to aid the antenna. The low-power, voltage-controlled MEM switches are provided with an actuating bias voltage, either by means of direct connections, through the reflective surface if used, or by means of an illuminated series of photovoltaic (PV) cells. Optical control of each MEM switch is preferably provided by a photoresistive element that shunts the bias source to deactuate the switch. 
     The preferred reflective surface presents a high impedance to electromagnetic waves in the antenna operating frequency range, and accordingly reflects the waves with little or no phase shift (less than 90 degrees, and preferably near 0). This reduces array-to-reflector spacing distance and alleviates bandwidth constraints, which are imposed by that spacing. The preferred embodiment of the present invention includes a high impedance reflective surface fabricated on a multilayer printed circuit board as a matrix of conductive pads, each having controlled capacitance to adjacent pads and having a via with controlled inductance connecting from its center to a common plane on the opposite side of the board. The controlled inductance vias, or other vias through the reflective surface, may provide for light transmission from the active matrix optical panel to the photoelectric elements controlling the MEM switches, and may also conduct bias voltage for the switches. The antenna array elements are preferably disposed on a substrate positioned above the front side of the high-impedance surface of the circuit board and much less than ¼ wavelength from the front side of the high-impedance reflective surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an embodiment of the present invention showing an antenna substrate incorporating the reconfigurable antenna array, an optical transmission structure layer, and an optical source layer. 
     FIG. 2 shows a representative reconfigurable dipole antenna element. 
     FIG. 3 shows a cross-sectional view of a representative RF MEMS switch for use in the present invention. 
     FIG. 4 shows a top-down view of the RF MEMS switch depicted in FIG. 3 with a schematic representation of the elements providing control over the switch. 
     FIG. 5 shows the coupling of multiple antenna segments with RF MEMS switches. 
     FIG. 6 is a cross-sectional view of the antenna substrate, the optical transmission structure layer, and the optical source layer that illustrates the vias used to connect to the RF MEMS switches. 
     FIG. 7 shows the coupling of multiple antenna segments with RF MEMS switches having photo-voltaic cells providing bias voltages. 
     FIG. 8 is a cross-sectional view of the antenna substrate, the optical transmission structure layer, and the optical source layer that illustrates the optical vias used to control the RF MEMS switches depicted in FIG.  7 . 
     FIG. 9 is a perspective view of an embodiment of the present invention using slot antenna elements. 
     FIG. 10 shows a portion of a ground plane having slot antenna elements in which RF MEMS switches are used to reconfigure the slot antenna elements. 
     FIG. 11A shows a Cassegrain antenna using arrays of reconfigurable antenna subarrays according to the present invention. 
     FIG. 11B shows an enlarged view of a representative antenna subarray used in the Cassegrain antenna depicted in FIG.  11 A. 
    
    
     DETAILED DESCRIPTION 
     A ground plane comprising a conductive reflective surface lying below antenna elements is a common feature of most radio frequency antennas. The ground plane may be used to perform the useful function of directing most of the radiation into one hemisphere in which the antenna elements are located. As discussed above, the ground plane may also be used to electrically isolate antenna control functions from the antenna elements themselves, so as not to degrade antenna performance. A reflective surface for the present invention may be conductive, but that introduces restrictive wavelength-dependent constraints on the spacing between the reflective surface and the antenna array. Instead of a conductive reflective surface, it is preferable to use a non-conductive reflective surface. 
     Reflective surfaces are known in the art which reflect electromagnetic waves with a phase shift near zero, and are relevant to the preferred embodiment of the present invention. In particular, such “high impedance” surfaces may be formed on a printed circuit board, as described in publication WO 9950929 of international patent application PCT/US99/06884 by Yablonovitch and Sievenpiper. Yablonovitch and Sievenpiper disclose an array of separate conducting elements, each element comprising a resonant circuit that is capacitively coupled to adjacent elements and inductively coupled in common, and each element having an exposed surface. The conducting elements collectively act as a reflective surface that allows antenna elements to be disposed within much less than one quarter wavelength of the reflective surface. The reduced distance between the reflective surface and the antenna elements reduces the lengths of any connections that must be made to the antenna elements or switch elements used to connect or reconfigure the antenna elements. 
     For high frequencies, the wavelength of the electromagnetic waves is short; for example, at 30 GHz, the wavelength is about 1 cm. As discussed above, a conductive reflective surface for antenna elements operating at that frequency should be disposed one quarter wavelength below the elements, or 2.5 mm. This spacing increases the overall height of the resulting antenna array and also increases the likelihood of antenna control lines interfering with the performance of the antenna, since these lines will have lengths on the order of a quarter wavelength. With a high impedance surface, at 30 GHz, the spacing from the antenna elements to the high-impedance reflective surface is preferably substantially less than 2.5 mm, and is ideally not more than 250 μm. Essentially, the antenna elements are right on top of the reflective surface, so the lengths of any control lines above the surface are nearly negligible. 
     FIG. 1 shows a reconfigurable antenna array  100  according to an embodiment of the present invention. Reconfigurable antenna array  100  comprises a plurality of reconfigurable dipole antenna elements  200  formed on a surface of an antenna substrate  110 , an optical transmission structure layer  120  disposed below the antenna substrate  110 , and an optical source layer  130 . Preferably, the optical transmission structure layer  120  comprises a high-impedance electromagnetically reflective structure. The high-impedance electromagnetically reflective structure may be of the type disclosed in WO9950929 and briefly discussed above. 
     Reconfiguration of the antenna elements  200  is provided by RF MEMS switches (not shown in FIG. 1) on the antenna substrate  110  coupling individual segments of the elements  200 . The antenna elements  200  and the RF MEMS switches are formed on the underside of the antenna substrate  110  to allow the antenna elements  200  to be closely positioned to the optical transmission structure layer  120  and to allow the switches to be illuminated by optical energy provided by optical sources in the optical source layer  130 . While only two representative antenna elements  200  are illustrated in FIG. 1, it is to be understood that the number of elements actually used in a particular application will depend on the particular requirements of that application. Many applications will require large antenna arrays with hundreds or even thousands of antenna elements. Also, antenna configurations comprising antenna elements other than dipole elements, such as slot antenna elements or arrays of patch antennas, are provided by other embodiments of the present invention. 
     FIG. 2 shows, in greater detail, a representative reconfigurable dipole antenna element  200  of antenna array  100 . Antenna element  200  comprises a twin antenna feed structure  205 , a radiating structure comprising series of adjacent metal strip segments  240  formed on the substrate  110  (not shown in FIG. 2) and extending to either side of feed structure  205 , and RF MEMS switches  300  that electrically connect together each successive pair of adjacent metal strip segments  240 . Gaps  218  separate adjacent metal strip segments  240 . The gaps  218  between adjacent metal strip segments  240  are electrically bridged by the RF MEMS switches  300 , in a manner to be explained later. 
     FIG. 3 shows one form of an RF MEMS switch, which may be incorporated into the present invention. Embodiments of applicable RF MEMS switches are described in greater detail in pending U.S. patent application Ser. No. 09/429,234, incorporated herein by reference. The RF MEMS switch, generally designated  300 , is fabricated using generally known microfabrication techniques, such as masking, etching, deposition, and lift-off. In the preferred embodiment, RF MEMS switches  300  are directly formed on the antenna substrate  110  and monolithically integrated with the metal segments  240 . Alternatively, the RF MEMS switches  300  may be discreetly formed and then bonded to antenna substrate  110 . Referring once more to FIG. 2, one RF MEMS switch  300  is positioned proximate each gap  218  between pairs of adjacent metal segments  240  formed on the substrate  110 . 
     As seen in FIG. 3, the switch  300  comprises a substrate electrostatic plate  320  and an actuating portion  326 . The substrate electrostatic plate  320  (typically connected to ground) is formed on the MEMS substrate  310 . The substrate electrostatic plate  320  generally comprises a patch of a metal not easily oxidized, such as gold, for example, deposited on the MEMS substrate  310 . Actuation of the switch  300  electrically disconnects and connects the adjacent metal segments  240  to open and close the gap  218 , in a manner to be explained later. The MEMS substrate  310  preferably comprises semi-insulating material with photo-conductive properties. 
     The actuating portion  326  of the switch  300  comprises a cantilever anchor  328  affixed to the MEMS substrate  310 , and an actuator arm  330  extending from the cantilever anchor  328 . The actuator arm  330  forms a suspended micro-beam attached at one end to the cantilever anchor  328  and extending over and above the substrate electrostatic plate  320  and over and above electrical contacts  340 ,  341 . The cantilever anchor  328  may be formed directly on the MEMS substrate  310  by deposition buildup or by etching away surrounding material, for example. Alternatively, the cantilever anchor  328  may be formed with the actuator arm  330  as a discrete component and then affixed to the MEMS substrate  310 . The actuator arm  330  may have a bilaminar cantilever (or bimorph) structure. Due to its mechanical properties, the bimorph structure exhibits a very high ratio of displacement to actuation voltage. That is, a relatively large displacement (approximately 300 micrometers) can be produced in the bimorph cantilever in response to a relatively low switching voltage (approximately 20 V). 
     A first layer  336  of the actuator arm structure comprises a semi-insulating or insulating material, such as polycrystalline silicon. A second layer  332  of the actuator arm structure comprises a metal film (typically aluminum or gold) deposited atop first layer  336 . The second layer  332  typically acts as an electrostatic plate during operation of the switch. In the remainder of the description, the terms “second layer” and “arm electrostatic plate” will be used interchangeably. As shown in FIG. 3, the second layer  332  is coupled to the cantilever anchor  328  and extends from the cantilever anchor  328  toward the position on the actuator arm  330  at which electrical contact  334  is formed. Since the height of the cantilever anchor  328  above the MEMS substrate  310  can be tightly controlled using known fabrication methods, locating the second layer  332  proximate the cantilever anchor  328  enables a correspondingly high degree of control over the height of the second layer  332  above the MEMS substrate  310 . 
     The switch actuation voltage is dependent upon the distance between the substrate electrostatic plate  320  and the arm electrostatic plate  332 , so a high degree of control over the spacing between the electrostatic plates is necessary in order to repeatably achieve a desired actuation voltage. In addition, at least a portion of the second layer  332 , comprising the arm electrostatic plate, and a corresponding portion of the actuator arm  330 , on which second layer  332  is formed, are positioned above the substrate electrostatic plate  320  to form an electrostatically actuatable structure. An electrical contact  334 , typically comprising a metal that does not oxidize easily, such as gold, platinum, or gold palladium, for example, is formed on the actuator arm  330  and positioned on the arm so as to face the electrical contacts  340 ,  341  disposed on the MEMS substrate  310 . The electrical contacts  340 ,  341  are electrically coupled to the adjacent metal segments  240  so that the adjacent metal segments  240  are electrically connected when the switch  300  is closed, and are electrically isolated when the switch  300  is open. 
     FIG. 4 provides a top-down view of the RF MEMS switch shown in FIG.  3  and also illustrates schematically the operation of the switch. A voltage source V app  is coupled to the RF MEMS switch  300 . The voltage source V app  is coupled to a substrate plate contact  321  and an arm plate contact  333 . The arm plate contact  333  is connected to the electrostatic arm plate  332  through a resistive path  360  disposed on the substrate having a resistance vale of R se . The resistive path  360  may comprise sputtered CrSiO in a 6 micron line width, and conducts current from the arm plate contact  333  to the electrostatic arm plate  332  through an appropriate resistance of preferably about 1 megohm. The substrate plate contact  321  is electrically connected with the substrate electrostatic plate  20 . When voltage V app  is applied across the switch contacts  321 ,  333  and, correspondingly, across substrate and arm electrostatic plates  320  and  332 , the RF MEMS switch  300  is closed by means of this electrostatic attraction between the substrate electrostatic plate  320  located on the MEMS substrate  310  and the arm electrostatic plate  332  located on actuator arm  330 . 
     When the switch  300  is in the open state, the adjacent metal segments  240  constituting dipole antenna element  200  are electrically isolated from each other. When voltage V app  is applied across the electrostatic plates  320  and  332 , the arm electrostatic plate  332  is attracted electrostatically toward substrate electrostatic plate  320 , forcing actuator arm  330  to deflect toward the MEMS substrate  310 . Deflection of the actuator arm  330  toward the substrate electrostatic plate  320 , in the direction indicated by arrow  311  in FIG. 3, causes the electrical contact  334  to come into contact with the electrical contacts  340 ,  341 , thereby electrically bridging the gap  218  between the metal segments  240 . The voltage required close the RF MEMS switch  300  may be as low as 7 V or lower depending upon the sizes of the electrostatic plates  320 ,  332  and the materials used to fabricate the arm  330 . 
     The substrate electrostatic plate  320  and arm electrostatic plate  332  are insulated from the metal segments  240  constituting antenna element  200 , and the electrostatic plates  320 ,  332  are dielectrically isolated, even when the switch  300  is closed. Thus, only the application of a voltage difference between the plates  320 ,  332  actuates the switch  300  and no steady-state bias current is needed for the switch  300  to operate. Also, since no steady DC current flows from the applied voltage (only a transient current that builds up an electric field across the electrostatic plates), only a low current voltage source is required. 
     The opening of the RF MEMS switches  300  in order to reconfigure dipole antenna element  200  will now be discussed. When actuation voltage V app  is applied to RF MEMS switch  300 , the voltage V SA  appearing across substrate electrostatic plate  320  and arm electrostatic plate  332  is given by the relationship 
     
       
           V   SA   =V   app   R   st /( R   st   +R   se ) 
       
     
     where R St  is the resistance of semi-insulating substrate  110  between the substrate electrostatic plate  320  and arm electrostatic plate  332  (represented as the resistor  370  shown in FIG.  4 ), and R Se  is the resistive path  360 . When the RF MEMS switch  300  is not illuminated, R St  is much larger than the series resistance R Se , so that almost the entire voltage produced by the applied voltage V app  appears across the RF MEMS switch electrostatic plates  320 ,  332 . 
     However, a semi-insulating substrate, comprising a substance such as gallium arsenide or polycrystalline silicon, is photoconductive. Thus, when optical energy h v  illuminates the portion of the semi-insulating MEMS substrate  310  insulating the RF MEMS switch substrate electrostatic plate  320  from the RF MEMS switch arm electrostatic plate  332 , the optical energy h v  transferred to MEMS substrate  310  causes a proportion of the outer valence electrons of the substrate&#39;s constituent atoms to break free of their atomic bonds, thus creating free carriers. These free electrons are capable of carrying an electric current. Thus, when the RF MEMS switch  300  is illuminated, R St  is reduced by the photoconducting process and becomes much lower than RS Se . Consequently, the voltage drop across the electrostatic plates falls below the level required to close the RF MEMS switch  300 , causing the switch  300  to open, and interrupting the connection between adjacent metal segments  240  and changing the resonant length of dipole antenna element  200 . 
     FIG. 5 shows a view from below of two RF MEMS switches  300  disposed to electrically couple three metal segments  240 . The switches  300  electrically bridge the gaps between the segments  240  in the manner described above. In FIG. 5, the electrical contacts  340 ,  341  of the switches are shown to be electrically connected to the metal segments  240  by metal contacts  245 . The metal contacts  245  may comprise solder connections, deposited metal, or other electrically connecting means known in the art. Note also that microfabrication techniques may be used to integrally fabricate the electrical contacts  340 ,  341  of the RF MEMS switches  300  and the metal segments  240 , thus obviating the need for the separate electrical contacts  245  between the RF MEMS switch electrical contacts  340 ,  341  and the metal segments  240 . FIG. 5 also shows the bias lines  580 ,  590  used to provide the bias voltage for actuating the RF MEMS switches  300 . In FIG. 5, the bias lines  580 ,  590  are shown disposed to the side of the RF MEMS switches  300  for clarity purposes only. The bias lines  580 ,  590  are preferably disposed directly beneath the RF MEMS switches  300  to shorten the connections to the RF MEMS switches  300 . As described below, the majority of bias lines  580 ,  590  are preferably disposed beneath a shielding ground plane so as to minimize RF coupling effects between the bias lines  580 ,  590  and the antenna elements  200 . FIG. 5 shows a single pair of bias lines coupled to the RF MEMS switches  300 , wherein a single voltage source may be used to actuate all RF MEMS switches  300  in an array. Alternative embodiments of the present invention may each have individually controllable bias lines connected to each RF MEMS switch  300  in the antenna array. 
     FIG. 6 shows a cross-sectional view of the various layers of the preferred embodiment of the present invention. FIG. 6 shows the metal segments  240  and the RF MEMS switches  300  disposed on the bottom side of the antenna substrate  110 . The antenna substrate  110  preferably comprises a material that minimally affects the coupling of electro-magnetic energy to the metal segments  240 . The antenna substrate  110  may comprise either a semi-insulating material or a dielectric material, and may be fabricated from materials typically used to construct printed circuit boards (PCBs). Alternatively, the RF MEMS switches  300  may be integrated with the antenna substrate  110 , as previously discussed, so that the antenna substrate  110  and the MEMS substrate  310  comprise the same materials. 
     Beneath the antenna substrate  110  is the optical transmission structure layer  120 . If the optical transmission structure layer  120  comprises a high-impedance electromagnetically reflective surface, the optical transmission structure layer  120  will minimize the phase shift in electromagnetic waves, upon reflection, which allows the gap, with distance D, between the metal segments  240  and the high impedance surface layer  120  to be minimized. As discussed above, a high-impedance electromagnetically reflective surface allows the gap distance D to be much less than one quarter wavelength of the lowest operating frequency of the antenna. However, the metal segments  240  should not contact a high-impedance electromagnetically reflective surface, since this will effectively short all of the segments  240  together. The gap may simply be an air gap, where the antenna substrate  110  is supported above the high impedance surface by non-conductive structures distributed over the surface of the high impedance surface. Alternatively, the gap may comprise a layer of dielectric thin film material, such as a thin layer of polysilica or plastic, fabricated to support the antenna substrate and providing space for the RF MEMS switches to open and close, while electrically insulating the metal segments  240  from the high-impedance electromagnetically reflective surface. 
     The optical transmission structure layer  120  may contain bias line via holes  126 ,  128  that allow the bias voltage to be applied to each RF MEMS switch  300  by the bias lines  580 ,  590 , while ensuring that the lengths of the bias lines  580 ,  590  that protrude above the surface of the optical transmission structure layer  120  are minimized. FIG. 6 shows the bias lines  580 ,  590  horizontally disposed at the lower portion of the optical transmission structure layer  120  and vertically connecting through the optical transmission structure layer  120  to the RF MEMS switches  300 . Alternative embodiments of the present invention may dispose the bias lines  580 ,  590  in the optical source layer  130 , or the bias lines  580 ,  590  may be separately disposed in a bias line layer (not shown in FIG. 6) located beneath the optical transmission structure layer  120  or the optical source layer  130 , and vertically connecting through via holes  126 ,  128  to the RF MEMS switches  300 . Preferably, the bias lines  580 ,  590  are shielded from the metal segments  240  by a ground plane. As discussed earlier, a high-impedance electromagnetically reflective surface acts as a ground plane and, thus, may be used to shield the bias lines  580 ,  590  from the metal segments  240 . 
     The bias line via holes  126 ,  128  may be provided by fabricating the layer  120  with the requisite holes, drilling through the optical transmission structure layer  120 , or using any other means known in the art to create holes through the optical transmission structure layer  120 . If the optical transmission structure layer  120  comprises electrically conductive portions, insulating material may be used within the bias line via holes  126 ,  128  or as part of the via holes  126 ,  128  themselves to electrically isolate the bias lines  580 ,  590  from the optical transmission structure layer  120   
     The optical source layer  130  comprises a plurality of substrate illuminating optical energy sources  135  used to open the RF MEMS switches in the manner described above. Optical energy is coupled to the RF MEMS switches by optical via holes  125  contained within the optical transmission structure layer  120  (and any other layers between the optical sources and the RF MEMS switches). Note, in FIG. 6, the bias lines  580 ,  590  are shown disposed behind the optical via holes  125 . Alternative positions of the bias lines  580 ,  590  in relation to the optical via holes  125  may also be used. As discussed above, illumination of the semi-insulating substrate  310  by an optical energy source causes the RF MEMS switches  300  to open, thus providing control over the inter-segment coupling of the metal segments  240  disposed on the antenna substrate  110 . The optical source layer  130  may comprise an active matrix optical source, such as that provided by commercially available active matrix LED or LCD panels. The optical via holes  125  may be provided by fabricating the optical transmission structure layer  120  with the requisite holes, drilling through the optical transmission structure layer  120 , or using any other means known in the art to create holes through the optical transmission structure layer  120 . Each optical via hole  125  may simply comprise an opening in the optical transmission structure layer  120 , or a tube or other light directing means, such as optical lenses, optical fibers, etc., may be used to direct or focus light on the RF MEMS switch  300  that corresponds to each individual optical source  135 . 
     In operation, the bias lines  580 ,  590  preferably provide a bias voltage to every RF MEMS switch  300  in the antenna. Application of this bias voltage will cause every RF MEMS switch to initially be in the closed state. The optical energy sources  135  in the optical source layer  130  are then individually controlled to selectably provide optical energy to each corresponding RF MEMS switch  300 . The optical energy will be transmitted through the optical via hole  125  and directed onto the corresponding RF MEMS switch  300 . Transmission of the optical energy onto the MEMS substrate  310  will cause the switch to open, thus effectively reconfiguring the metal segments  240  coupled by the switches  300 . Commercial optical light matrix products built with random access brightness control, such as an active matrix LED panel, a liquid crystal display (LCD) panel used for notebook computers, may serve as the controllable matrixed light source for controlling the array of RF MEMS switches  300 . 
     An alternative embodiment of the present invention provides for the elimination of the DC bias lines and, instead, uses a photo-voltaic cell to provide the necessary voltage for closing the RF MEMS switch. FIG. 7 shows an RF MEMS switch  700  coupled to metal segments  240 , where the RF MEMS switch  700  comprises the same elements of the RF MEMS switch earlier described, except that a photo-voltaic cell  750  is coupled to the arm plate contact  333  and the substrate plate contact  321  is used to provide a bias voltage in place of the bias lines earlier described. As is known in the art, a photo-voltaic cell will produce a voltage when illuminated by optical energy. Hence, as shown in FIG. 7, the photo-voltaic cell  750  may act in place of bias lines to provide the actuating voltage required to close the RF MEMS switch  700 . When the photo-voltaic cell  750  is illuminated, a bias voltage providing electrostatic attraction between the arm electro-static plate and the substrate electro-static plate of the switch  700  is created, which causes the switch  700  to close. Illumination of the switch substrate will still cause the resistance between the arm electro-static plate and the substrate electro-static plate to lessen, and will cause the switch to open. 
     FIG. 8 shows a cross-sectional view of the various layers of the embodiment depicted in FIG.  7 . The antenna substrate  110  and the optical transmission structure layer  120  may comprise the same structure and materials as earlier discussed. As discussed above, this embodiment does not require DC bias lines and, therefore, no DC bias line vias are required. Instead, a second optical via hole  127  is provided to couple optical energy from a photo-voltaic cell optical source  137  to the photo-voltaic cell  750  located on the antenna substrate  110 . The optical source layer  130  may provide the substrate illuminating optical sources  125  and the photo-voltaic cell optical sources  137  using devices well-known in the art, such as the LED or LCD panels described above, or a second layer (not shown in FIG. 8) may be used to provide a separate source for the photo-voltaic cell optical sources  137 . Individually controllable photovoltaic cell optical sources  137  may be used, but are not required, since the substrate illuminating optical sources  125  provide control over the opening and closing of the RF MEMS switches. 
     Other embodiments of the present invention provide for the reconfiguration of antenna arrays comprising slot antenna elements. FIG. 9 shows an antenna array  900  comprising a plurality of slot antenna elements  920  with RF MEMS switches  300  disposed within the slot elements  920 . While only a few slot antenna elements  920  oriented in a parallel configuration are shown in FIG. 9, it is to be understood that the number of slot antenna elements used in a slot antenna array and the orientation of the slot elements will depend upon the particular requirements of the antenna array. Many slot antenna arrays may comprise hundreds or thousands of individual slot antenna elements. 
     In FIG. 9, the slot antenna elements  920  comprise slots fabricated within a ground plane layer  910 . Similar to previous described embodiments of the present invention, the antenna substrate layer  110  is disposed above the slot antenna elements  920 . The RF MEMS switches  300  may be formed as an integrated part of the antenna substrate  110  or may be disposed on the substrate  110  as discrete components. The optical transmission structure layer  120  is disposed beneath the ground plane layer  910  to provide a reflective surface for the slot antenna elements  920  and to shield RF and electrical connections to the slot antenna elements  920  and the RF MEMS switches  300 . The RF MEMS switches are illuminated from optical sources in the optical source layer  130  in the manner previously described. 
     FIG. 10 shows a view of a portion of the ground plane layer  910  on which four RF MEMS switches  300  are disposed to reconfigure two slot antenna elements  920 . The RF MEMS switches  300  electrically connect one side of an RF slot antenna element  920  to the other side of the slot element  920 , effectively shorting, and thus, shortening the element  920  at that point. Metal contacts  245  may be used to connect the electrical contacts  340 ,  341  of the RF MEMS switches  300  to opposite sides of the slot antenna element  920 , or the ground plane layer  910  and the RF MEMS switches  300  may be formed such that the electrical contacts  340 ,  341  are integral with the ground plane layer  910 . The bias lines  580 ,  590  are used to provide the bias voltages used for actuating the RF MEMS switches. The bias lines  580 ,  590  may be disposed directly beneath, but electrically isolated from, the ground plane layer  910 , or disposed in the manner previously described for other embodiments of the present invention. Alternative embodiments of the present invention actuate the RF MEMS switches  300  in the slot antenna elements  920  by using optical energy directed into a photo-voltaic cell, as previously discussed. 
     Thus, the reader will see that the present invention provides reliable actuation of switches in a reconfigurable antenna without the need for an intricate network of metallic bias lines proximate the antenna elements. 
     A larger antenna array may be created by combining smaller antenna subarrays according to the present invention. The smaller subarrays comprise modules with the the antenna substrate  110 , the optical transmission structure layer  120 , and the optical source layer  130  discussed above. The modules may then be connected and assembled together to form a larger array which has a common high-impedance backplane. A coarse reconfiguration of the resulting larger array can be achieved by using MEMS switches or hard-wire switch connections between the modules, and the individual modules can be controlled to change the final dimension of the antenna elements for the desired frequency band of operation. An individual module or a plurality of modules may be used to fabricate known reflective antenna topologies, such as a Cassegrain reflective antenna. 
     FIG. 11A shows the combination of multiple antenna subarrays  1130  to form a Cassegrain antenna  1100 . The Cassegrain antenna  1100  comprises a curved backplate  1150  on which a plurality of the antenna subarrays  1130  are disposed to form the primary reflector of the antenna. A secondary reflector  1110  is positioned in front of the antenna subarrays to direct radio frequency energy to and from a feed horn  1120 . The curved backplate  1150  may comprise the antenna substrate  110 , the optical transmission structure layer  120 , and the optical source layer  130  previously discussed, or the curved backplate  1150  may simply provide a structural foundation for those layers. The Cassegrain antenna  1100  may also use a flat backplate or other shapes for the backplate, in which additional elements are used to direct the radiation from the antenna elements on the backplate to and from the secondary reflector  1110 . 
     The antenna subarrays  1130  of the Cassegrain antenna shown in FIG. 11A comprise a matrix of nine patch antenna elements  1160  interconnected by RF MEMS switches  300 , as shown in FIG.  11 B. This configuration of patch antenna elements  1160  is provided for explanation purposes only. The antenna subarrays  1130  may comprise any number of antenna elements interconnected by RF MEMS switches in multiple configurations. The antenna elements may also be dipole antenna elements, slot antenna elements, or other antenna elements known in the art. 
     Although the present invention has been described with respect to specific embodiments thereof, various changes and modifications can be carried out by those skilled in the art without departing from the scope of the invention. For example, other configurations of reconfigurable antenna subarrays and antenna arrays beyond those described herein may be provided by other embodiments of the present invention. It is intended, therefore, that the present invention encompass such changes and modifications as fall within the scope of the appended claims.