Patent Publication Number: US-6992549-B2

Title: Shutter switch for millimeter wave beams and method for switching

Description:
This application is a divisional of patent application Ser. No. 09/675,696 filed on Sep. 29, 2000, and claims priority of that application. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to millimeter wave beams and more particularly to a switch that either reflects or is transparent to a millimeter beam. 
   2. Description of the Related Art 
   Electromagnetic signals are commonly guided from a radiating element to a destination via a coaxial cable or metal waveguide. As the frequency of the signal increases, the coaxial cable or metal waveguide used to guide the signals have smaller cross-sections. For example, a metal waveguide that is 58.420 cm wide and 29.210 high at its inside dimensions, transmits signals in the range of 0.32 to 0.49 GHz. A metal waveguide that is 0.711 cm wide and 0.356 cm high at its inside dimensions, transmits signals in the range of 26.40 to 40.00 GHz. [Dorf,  The Electrical Engineering Handbook, Second Edition,  Section 37.2, Page 946 (1997)]. As the signal frequencies continue to increase a point is reached where the coaxial cables and waveguides become impractical. They become too small and expensive and require precision machining to produce. In addition, their insertion can become too great. 
   High frequency signals in the range of approximately 1 to 50 GHz, can be guided through a microstrip transmission line. However, at frequencies above this range, the microstrip suffers from the same problems; the transmission line becomes too small and the insertion loss from transmission through the line becomes too great. 
   Frequencies exceeding approximately 100GHz (referred to as millimeter waves) should not be transmitted over a distance by a microstrip transmission line because of the insertion loss. Instead, the signal can be transmitted as a free-space beam. The signal from a radiating element is directed to a lens that focuses the signal into a millimeter wave beam having a diameter up to several centimeters. The beam is transmitted to a receiving lens that focuses the signal to a receiving element which often includes an amplifier. This form of transmission is referred to as “quasi-optic” when the lens diameter divided by the signal wavelength is in the range of approximately 1–10. In the optic regime, the lens diameter divided by the frequency wavelength is normally much greater than 10. [IEEE Press, Paul f. Goldsmith,  Quasi - optic Systems , Chapter 1, Gaussian Beam Propagation and Applications (1999)] 
   For quasi-optic or optic transmission in military or commercial applications, a safety mechanism is normally needed in the beams path in the form of a shutter that either blocks the beam from reaching the component that needs protection, or allows the beam to reach the component. The mechanism is primarily used to protect delicate amplifiers at the receiving end of the transmission line from power surges at the radiating element. Mechanical shutters have been used for this purpose, but they are generally too slow at blocking the beam and are too unreliable because of complex mechanical components. 
   Another important characteristic of transmission in metal waveguides is the transmission cut-off frequency. If the frequency of the transmitted signal is above the cut-off frequency, the electromagnetic energy can be transmitted through the guide with minimal attenuation. Electromagnetic energy with a frequency below the cut-off will be totally reflected at entry to the guide and will be attenuated to a negligible value in a relatively short distance through the waveguide. The physical dimensions of a metal waveguide not only determines the range of frequencies that it transmits, but also the cut-off frequency for the fundamental (first) mode. The two waveguides described above have cut-off frequencies of 0.257 GHz and 21.097 GHz, respectively. 
   A structure has been developed that presents as a high impedance to transverse E fields of electromagnetic signals. [M. Kim et al.,  A Rectangular TEM Waveguide with Photonic Crystal Walls for Excitation of Quasi - Optic Amplifiers,  (1999) IEEE MTT-S, Archived on CDROM]. The structure is particularly applicable to the sidewalls and/or top and bottom walls of metal rectangular waveguides. Either two or four of the waveguide&#39;s walls can have this structure, depending upon the polarizations of the signal being transmitted. The structure comprises a substrate of dielectric material with parallel strips of conductive material that are separated by small (capacitive) gaps It also includes inductive metal vias through the sheet to a conductive sheet on the substrate&#39;s surface opposite the strips. At a certain frequency the inductance of the vias and the capacitance of the gaps resonate. At this “resonant” frequency, the surface impedance of becomes very high. 
   When used on a rectangular waveguide&#39;s sidewalls, the structure provides a high impedance boundary condition for the E field component of a fundamental mode vertically polarized signal, the E field being transverse to the conductive strips. The high impedance prevents the E field from dropping off near the waveguide&#39;s sidewalls, maintaining an E field of uniform density across the waveguide&#39;s cross-section. Current can flow down the waveguide&#39;s conductive top and bottom walls to support the signal&#39;s H field with uniform density. Accordingly, the signal maintains near uniform power density across the waveguide aperture. 
   When the high impedance structure is used on all four of the waveguide&#39;s walls, the waveguide can transmit independent cross-polarized signals each one being similar to a free-space wave having a near-uniform power density. The structure on the waveguide&#39;s sidewalls presents a high impedance to the E field of the vertically polarized signal, while the structure on the waveguide&#39;s top and bottom walls presents a high impedance to the horizontally polarized signal. The structure also allows conduction through the strips to support the signal&#39;s H field component of both polarizations. Thus, a cross-polarized signal of uniform density can be transmitted. 
   Waveguides employing these high impedance structures are also able to transmit signals close to the resonant frequency that would otherwise be cut-off because of the waveguide&#39;s dimensions if all of the waveguide&#39;s walls were conductive. At resonant frequency, the waveguide essentially has no cut-off frequency and can support uniform density signals when its width is reduced well below the width for which the frequency being transmitted would be cut-off in a metal waveguide. 
   SUMMARY OF THE INVENTION 
   The present invention provides a new millimeter beam shutter switch that is placed in a millimeter beams path and is either opaque and blocks the beam, or is transparent and allows the beam to pass with minimal attenuation. The new switch can change states between opaque and transparent in microseconds or less without employing complicated or unreliable mechanical components. 
   The new shutter switch includes a plurality of waveguides adapted to receive at least part of the electromagnetic beam. The waveguides are adjacent to one another with their longitudinal axes aligned with the propagation of the beam. The waveguides switchable to either transmit or block the transmission of their respective portions of the beam. 
   The new shutter switch uses rectangular waveguides with high impedance structures on at least two opposing interior walls. The high impedance structures allow smaller waveguides to transmit signals that would otherwise be cut-off if all of the waveguide&#39;s walls were conductive. The cross-section of each individual waveguide can be smaller than the beam&#39;s cross-section, and the shutter switch includes a sufficient number of waveguides to intercept the entire beam. The waveguides are mounted adjacent to one another to form a wall, with each of the waveguide&#39;s longitudinal axes aligned with the millimeter beam&#39;s propagation axis. Each of the high impedance structures has shorting switches that, when closed, cause the structure to change from a high impedance surface to a conductive surface. 
   One embodiment of the shutter switch uses waveguides that have high impedance structures on their sidewalls, which allows each of the waveguides to transmit uniform density, vertically polarized signals at a particular design frequency. The preferred high impedance sidewalls comprise a sheet of dielectric material with a conductive layer on one side. The opposite side of the dielectric material has a series of parallel conductive strips that are oriented down the waveguide&#39;s longitudinal axis. Each of the strips has a uniform width, with uniform gaps between adjacent strips. Vias of conductive material are provided through the dielectric material between the conductive layer and the conductive strips. The actual dimensions of the surface structure depend on the materials used and the signal frequency. 
   During transmission of a vertically polarized signal, the waveguide carries an E field component transverse to the surface structure&#39;s conductive strips. At a design frequency, the vias which extend through the substrate present an inductive reactance (2πfL), while the gaps between the strips present an approximately equal capacitive reactance (1/(2πfC)). The surface presents parallel resonant L-C circuits to the transverse E field component; i.e. a high impedance. The L-C circuits present an open-circuit to the transverse E-field, allowing it to remain uniform across the waveguide. The low impedance on the top and bottom waveguide walls allows current to flow and maintains a uniform H field. Each of the waveguides transmits the signal with uniform density, and the shutter switch appears transparent to the vertically polarized beams at the design frequency. 
   When the shorting switches on the high impedance structure are closed, the high impedance sidewalls are switched to a conductive surface. All of the waveguide&#39;s walls become conductive and, because of the waveguide&#39;s dimensions, signal transmission is cut-off. If the shorting switches are closed in all of the shutter switch&#39;s waveguides, transmission is blocked in all the waveguides and the shutter switch becomes opaque to the beam Similarly, if the shutter switch has waveguides with the high impedance structure on the top and bottom walls, the shutter switch could be used to block or transmit horizontally polarized signals. 
   In another embodiment of the waveguide used to form a shutter switch, the high impedance structure is placed on all four of the waveguides walls. This allows the waveguide to transmit a cross-polarized signal (vertical and horizontal) at a particular resonant frequency. When the shorting switches are closed on the high impedance structure in all the waveguides, the shutter switch blocks transmission of the cross-polarized signal. The shorting switches can also be selectively closed to block transmission of only one polarization of the cross polarized signal. Closing the shorting switches on the waveguide&#39;s sidewalls blocks the vertically polarized signal, while closing the shorting switches on the top and bottom walls blocks the horizontally signal. 
   In still another embodiment, either two or all four of the waveguides sidewalls have a multi-layered high impedance structure which causes each of the layers to present a high impedance to a transverse E field at widely separated resonant frequencies. The number of frequencies that the waveguide can transmit with uniform density depends on the number of layers in the structure. When the multi-layered structure is on the sidewalls only, the waveguide transmits vertically polarized signals; when the multi-layered structure on the top and bottom walls, the waveguide transmits horizontally polarized signals. When the multi-layered structure is on all four of the waveguide&#39;s wall, the waveguide can transmit either a single polarized signal or both cross-polarized signals. Shorting switches on the multi-layered structures can be selectively closed to block transmission of one or both of the polarizations, at one of the different transmission frequencies. 
   Different shorting switches can be used to switch the high impedance surface structures to a conductive surface. The preferred switches consume a relatively small amount of power and employ varactor layer diode technology or micro electromechanical system (MEMS) technology. 
   These and other further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of one embodiment of the new waveguide wall shutter switch; 
       FIG. 2  is a perspective view of one of the waveguides in the shutter switch of  FIG.1 , the waveguide having a high impedance structure on its sidewalls; 
       FIG. 3  is a sectional view of the waveguide in  FIG.2 , taken along section lines  2 — 2 ; 
       FIG. 4  shows the sidewall&#39;s high impedance resonant L-C circuits to a transverse E-field; 
       FIG. 5  is a perspective view of a second embodiment of the waveguide with a high impedance structure on all its walls; 
       FIG. 6  is a sectional view of the waveguide in  FIG. 5  taken along section lines  6 — 6 ; 
       FIG. 7  is a perspective view of a third embodiment of the waveguides with a layered high impedance structure on all of its walls; 
       FIG. 8  is a sectional view of layered high impedance structure; 
       FIG. 9  is a diagram of L-C circuits formed by the layered wall structure in response to the E fields of three different frequencies; 
       FIGS. 10   a – 10   c  are sectional views of a three-layer embodiment or the invention, illustrating how three different frequencies interact with the different layers; 
       FIG. 11  is a sectional view of the high impedance structure with MEMC switches to short the gaps between the conductive strips; 
       FIG. 12  is a sectional view of the structure shown in  FIG. 11 , taken along section lines  12 — 12 ; 
       FIG. 13  is the sectional view of the structure shown in  FIG. 12  with the switches in the closed state; 
       FIG. 14  is a sectional view of the high impedance structure with semiconductor varactor layers to short the gaps between the conductive strips; and 
       FIG. 15  shows the new shutter switch used in millimeter beam transmission. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a new waveguide wall shutter switch  10  constructed in accordance with the present invention. It has individual waveguides  12  that are mounted adjacent to one another to form a rectangular wall resembling a honeycomb. The shutter switch  10  is placed in the path of a millimeter beam of a particular resonant frequency and depending on whether the shutter switch is “on” or “off” it either blocks the beam or to allow to pass through. The shutter switch can have different cross-sections depending on the beam&#39;s cross-section and whether the entire beam is to be intercepted. For instance, additional waveguides can be included on the top, bottom and sides, to give the shutter switch  10  more of a circular cross-section. 
   The cross-section of each waveguide  12  is small enough that if all the waveguide&#39;s walls were conductive, transmission of the beam at a design frequency would be cut-off. To allow transmission, the waveguides  12  have structures  14  on two of their interior sidewalls that present are aligned with the signal&#39;s E field and present as a high impedance to the E field. The high impedance structure also has shorting switches that change the structure&#39;s  14  characteristics such that it appears as a conductive surface. When the switches are closed in all the waveguides in the shutter switch  10 , the walls in each waveguide become conductive and because of the dimensions of each waveguide transmission of the signal is cut-off. The shutter switch  10  becomes opaque, blocking transmission of the beam. 
   A portion of the incoming beam can reflect off the front edges of the waveguides  12 , degrading the signal. To reduce this reflection, each waveguide  12  can include a launching region  15  on each waveguide wall that has the high impedance structure. The launching region begins at the entrance of each waveguide  12  and continues for a short distance down the waveguide. It is similar to the thumbtack high impedance structure described above, and comprises “patches” of conductive material mounted in a substrate of dielectric material. “Vias” of conducting material running from each patch to a continuous conductive sheet on the opposite side of the dielectric substrate. 
   The launching region resonates at the frequency of the beam entering the waveguides in the module. The vias which extend through the substrate present an inductive reactance (L), while the gaps between the patches present an approximately equal capacitive reactance (C). The surface presents parallel resonant high impedance L-C circuits to the beams E field component The L-C circuits present an open-circuit to the E-field, allowing it to remain uniform across the waveguide. The low impedance on the top and bottom waveguide walls allows current to flow and maintains a uniform H field. 
   The gaps between the patches block surface current flow in all directions, preventing surface waves in she high impedance structures. This blocks TM and TE modes from entering the waveguide  12 , only allowing TEM modes to enter. Blocking the TM and TE modes reduces the front edge reflection and the front edge of the waveguide appears nearly transparent to the beam at the resonant frequency. 
   In describing the various embodiments of the individual waveguides below, the launching region is not discussed or shown. However, to reduce reflection in any module comprised of the waveguides below, each waveguide should include a launching region. 
   Single Polarization Beams 
     FIGS. 2 and 3  show one embodiment of the waveguide  12  used to construct the shutter switch  10 . Its top and bottom walls  22  and  24  are conductive, and the inside of its sidewalls  23 ,  25  have high impedance structure  26 . The structure  26  includes a sheet of dielectric material  28  with conductive strips  30  of uniform width on one side, the conductive strips  30  having a uniform gap  32  between adjacent strips  30 . A layer of conductive material  34  is included on the side of the dielectric material  28  opposite the conductive strips  30 . Vias  36  of conductive material are provided between the conductive strips  30  and the conductive layer  34 , through the dielectric material  28 . The conductive strips  32  are oriented longitudinally down the waveguide  12 . 
   The wall structure  26  is manufactured using known methods and known materials. Numerous materials can be used as the dielectric material  28  including but not limited to plastics, poly-vinyl carbonate (PVC), ceramics, or high resistance semiconductor material such as Gallium Arsenide (GaAs), all of which are commercially available. Highly conductive material must be used for the conductive strips  30 , conductive layer  34 , and vias  36 , and in the preferred embodiment all are gold. 
   The wall structure  26  is manufactured by first vaporizing a layer of conductive material on one side of the dielectric material  28  using any one of various known methods such as vaporization plating. Parallel lines of the newly deposited conductive material are etched away using any number of etching processes, such as acid etching or ion mill etching. The etched lines (gaps) are of the same width and equidistant apart, resulting in parallel conductive strips  30  on the dielectric material  28 , the strips  30  having uniform width and a uniform gap  32  between adjacent strips. 
   Holes are created through the dielectric material  28  at uniform intervals, the holes continuing through the dielectric material  28  to the conductive strips  30  on the other side. The holes can be created by various methods, such as conventional wet or dry etching. They are then filled or covered with the conductive material and the uncovered side of the dielectric material is covered with a conductive material, both accomplished using sputtered vaporization plating. The holes do not need to be completely filled but the walls of the holes must be covered with the conductive material. The covered or filled holes provide conductive vias  36  between the conductive layer  34  and the conductive strips  30 . The dimensions of the dielectric material  28 , the conductive strips  34  and the vias  39  depend on the particular design frequency for the waveguide  12 . 
   With the high impedance structure  26  on the waveguide&#39;s sidewalls such that the conductive strips run parallel to the waveguides longitudinal axis, the structure will present a high impedance to the E field component of a vertically polarized signal at the design frequency. As shown in  FIG. 4 , the gap  32  presents a capacitance  38  to the E field component that is transverse to the conductive strips. The capacitance  38  is primarily dependent upon the width of the gap  32  between the strips  30  but is also impacted by the dielectric constant of the dielectric material  28 . The structure  26  also presents an inductance  40  to a transverse E field, the inductance  40  being dependant primarily on the thickness of the dielectric material  28  and the diameter of the vias  36 . At resonant frequency, the structure presents parallel resonant L-C circuits  42  to the vertically polarized signal and, as a result, a high impedance to a transverse E field. The E field maintains uniform power density across the waveguide, during transmission through the waveguide. 
   Current can flow along the top and bottom waveguide walls in the direction of propagation and as a result, the design frequency signal also maintains a uniform H field during transmission. With a uniform density E and the H field, the signal maintains uniform power density through transmission, with minimal attenuation. 
   The wall structure  26  also has a shorting switch  39  at each of the gaps  32  that short their respective gap when closed, the, details of the switches are described below and shown in  FIGS. 11–14 . When the switches  39  are open, the structure functions as described above, presenting a high impedance to a transverse E field. The gaps  32  form the capacitive part of the resonant L-C circuits and by closing the switches  39 , the gaps  32  and their capacitance are shorted. The conductive strips  30  and closed switches  39  change the characteristics of the structure  26  such that it presents as a continuous conductive sheet. The waveguide  12  now has conductive sidewalls along with the conductive top and bottom walls. Because the waveguides physical dimension “A” in  FIG. 2  is less than the critical dimension required for the frequency, signal transmission is cut-off and blocked. In the preferred embodiment, the switches  39  in all the waveguides of the shutter switch  10  are closed simultaneously, causing all the waveguides to block transmission of the signal. 
   Cross-Polarized Beams 
     FIGS. 5 and 6  show a second embodiment of a waveguide  50  used to construct the shutter switch. It operates similarly to the waveguide in  FIGS. 1 and 2 , but can block one or both polarizations (horizontal and vertical) if they are simultaneously present. 
   The waveguide  50  has the high impedance structure  57  on all four walls  51 – 54 , with the corresponding shorting switches  56  at each gap between the conductive strips  55 . The conductive strips  55  are oriented longitudinally down the waveguide  50 . The structure on all four walls  51 – 54  allows the waveguide  50  to simultaneously transmit signals with horizontal and vertical polarizations while maintaining a uniform power density. The signal with vertical polarization will have an E field with uniform density as a result of the high impedance presented by the structure  57  on the sidewalls  51  and  53 . Current flows along the strips of the structure on the waveguide&#39;s top wall  54  and/or bottom wall  52  of the waveguide, maintaining a uniform H field. For the portion of the signal having horizontal polarization, the E field maintains uniform power density because of the wall structure at the top wall  54  and bottom wall  52 , and the H field remains uniform because of current flowing along the strips of the sidewalls  51  and  53 . Thus, when the waveguide is transmitting, the power density of the cross polarized signal is uniform across the waveguide. 
   Closing all the switches  56  on all of the waveguide&#39;s walls causes them to appear as conductive surfaces. The waveguide will appear as a metal waveguide to both polarizations and because of the waveguide&#39;s dimensions A and B, transmission will be cut-off and blocked. 
   However, closing the switches on the waveguide&#39;s sidewalls  51 ,  53  only causes the waveguide  50  to appear as a metal waveguide to the vertically polarized signal and blocks only that portion of the cross-polarized signal. The E field of the vertically polarized signal is transverse to the conductive strips  55  on the waveguide&#39;s sidewalls  51 ,  53 , and the sidewalls with present as a high impedance series of L-C circuits. However, closing the switches  56  on the sidewalls  51 ,  53  causes them to appear as a conductive surface to the signal&#39;s E field. For the H field component of the vertically polarized signal, current runs down the strips  55  on the top and bottom walls  52 ,  54 . As a result, the waveguide  50  appears as though all its wall are conductive and the transmission of the vertically polarized signal is cut-off. 
   Similarly, for the horizontally polarized signal, the top and bottom walls  52 ,  54  appear as a high impedance to the E field, maintaining its uniform density, and the strips  55  on the sidewalls  51 ,  53  allow current to flow, maintaining a uniform H field. When the switches are closed on the top and bottom walls  52 ,  54 , all of the waveguide&#39;s walls will appear conductive to the horizontally polarized signal, and transmission of that portion will be cut-off. 
   The structure  57  is manufactured using similar materials and processes described above for the embodiment shown in  FIGS. 2 and 3 , and the manufacturing of the shorting switches is described below. By selectively closing the switches on opposing walls of the waveguide  50 , the horizontal portion, vertical portion, or both, can be cut-off. A shutter switch constructed of these waveguides can selectively block portions of a cross-polarized beam, or the entire beam. 
   Multi-Frequency Single and Cross-Polarized Beams 
     FIG. 7  shows another embodiment of the waveguide  70  used to construct the shutter switch  10 . The waveguide has a three-layered high impedance  71  structure on its walls  72 – 75 . In an alternative embodiment the structure  71  can be on the waveguides sidewalls  72 ,  74  with its top and bottom walls  73 ,  75  being conductive, or the structure can be on the waveguides top and bottom walls  73 ,  75  with its sidewalls  72 ,  74  being conductive. The structure  71  can have different numbers of layers, depending on the number of frequencies to be transmitted by the waveguide. The structure  71  shown has three layers and presents a high impedance to transverse E fields at three different resonant frequencies. 
   Referring to  FIG. 8 , each of the layers  82 ,  84 ,  86  in the structure  71  include respective dielectric substrates  88 ,  90 ,  92  that are progressively thinner from the bottom layer  82  to the top  86 . Conductive strips  94 ,  96 ,  98  are provided respectively on each of the substrates  82 ,  84 ,  86  and their width is progressively smaller from the bottom layer to the top. The strips in each layer are parallel and aligned over the strips in the layers below and above, and preferably have uniform width and a uniform gap between adjacent strips. Because the width of the strips  94 ,  96 ,  98  progressively decreases for each successive layer, the gaps between adjacent strips progressively increases. The higher frequency strips with smaller dimensions are situated on the upper layers. In an alternative embodiment, (not shown) there may be as many as three to five higher frequency strips positioned on each lower frequency strip. 
   The structure  71  includes vias  100  that connect each vertically aligned set of strips to a ground plane conductive layer  102  located at the underside of the bottom layer  82 . The preferred vias  100  are equally spaced down the longitudinal centerlines of the strips  94 ,  96 ,  98 . Alternatively, the location of the vias  50  can be staggered for adjacent strips. 
   The structure  71  is formed by stacking the layers  82 ,  84 ,  86  after their dielectric substrates have been metalized. Numerous materials can be used for the dielectric substrates, including but not limited to plastics, poly-vinyl carbonate (PVC), ceramics, or high resistance semiconductor materials such as Gallium Arsenide (GaAs), all of which are commercially available. Each layer in the structure  71  can have a dielectric substrate of a different material and/or a different dielectric constant. A highly conductive material such as copper or gold (or a combination thereof) should be used for the conductive layer  102 , strips  94 ,  96 ,  98 , and vias  100 . 
   The strips  94 ,  96 ,  98  on each layer are formed prior to stacking by first depositing a layer of conductive material on one surfaces of each dielectric substrate  88 ,  90 ,  92 . Parallel gaps in the conductive material are then etched away using any of a number of etching processes such as acid etching or ion mill etching. Within each layer, the etched gaps are preferably of the same width and the same distance apart, resulting in parallel conductive strips on the dielectric substrate of uniform width and with uniform gaps between adjacent strips. 
   The different layers  82 ,  84 ,  86  are then stacked with the strips for each layer aligned with corresponding ones in the layers above and below, resulting in aligned strips  94 ,  96 ,  98 . The layers  82 ,  84 ,  86  are bonded together using any of the industry standard practices commonly used for electronic package and flip-chip assembly. Such techniques include solder bumps, thermos-sonic bonding, electrically conductive adhesives, and the like. 
   Once the layers  82 ,  84 ,  86  are stacked, holes are formed through the structure for the vias  100 . The holes can be created by various methods, such as conventional wet or dry etching. The holes are then filled or at least lined with the conductive material and preferably at the same time, the exposed surface of the bottom substrate is covered with a conductive material to form conductive layers  102 . A preferred processes for this is sputtered vaporization plating. The holes do not need to be completely filled, but the walls must be covered with the conductive material sufficiently to electrically connect the ground Diane to the radiating elements of each layer. 
   Each of the layers  82 ,  84 ,  86  presents a pattern of parallel resonant L-C circuits and a high impedance to an E field for different resonant frequencies. The bottom most layer  82  presents a high impedance to the lowest frequency and the top most layer  86  presents as a high impedance to the highest frequency. To present the high impedance, at least a component of, and preferably the entire E field, must be transverse to the strips  94 ,  96 ,  98 . A signal normally incident on this structure will ideally be reflected with a reflection coefficient of +1 at the resonant frequency, as opposed to a −1 for a conductive material. 
   Like the embodiments described above, the capacitance of each layer  82 ,  84 ,  86  is primarily dependant upon the widths of the gaps between adjacent strips or patches, but is also impacted by the dielectric constants of the respective dielectric substrates. The inductance is primarily dependent upon the thickness of the substrates  88 ,  90 ,  92  and the diameter of the vias  100 . 
   The dimensions and/or compositions of the various layers  82 ,  84 ,  86  are different to produce the desired high impedance to different frequencies. To resonate at higher frequencies, the thickness of the dielectric substrate can be decreased, or the gaps between the conductive strips can be increased. Conversely, to resonate at lower frequencies the thickness of the substrate can be increased or the gaps between the conductive strips or patches can be decreased. Another contributing factor is the dielectric constant of the substrate, with a higher dielectric constant increasing the gap capacitance. These parameters dictate the dimensions of the structure  71 . Accordingly, the layered high impedance ground plane structures described herein are not intended to limit the invention to any particular structure or composition. 
     FIG. 9  illustrates the network of capacitance and inductance presented by a new three layer structure which produces an array of resonant L-C circuits to three progressively higher frequencies f 1 , f 2  and f 3 . The bottommost layer appears as a high impedance surface to signal f 1  as a result of a series of resonant L-C circuits, with L 1 /C 1  representing the equivalent inductance and capacitance presented by the bottommost,layer to its design frequency bandwidth. The second and third layers also for respective series of resonant L-C circuits L 2 /C 2  and L 3 /C 3 , at their frequency bandwidths. 
     FIGS. 10   a – 10   c  illustrate how the three signals interact with layers of the new structure  71 . An important characteristic of the structure&#39;s layers  104 ,  106 , and  108  is that each appears transparent to E fields at frequencies below its design frequency, and the strips appear as a conductive surface to E fields at frequencies above its design frequency. For the highest frequency signal f 1 , the top layer  108  presents as high impedance resonant L-C circuits to the signal&#39;s transverse E field. The strips  110  on second layer  106  appear as a conductive layer and become a “virtual ground” for the top layer  108 . Signal f 2  is lower in frequency than f 1  and, as a result, the first layer  104  is transparent to f 2 &#39;s E field, while the second layer  106  appears as high impedance resonant L-C circuits. The strips  112  on the third layer appear as a conductive layer, becoming the second layer&#39;s virtual ground. Similarly, at f 3  the top and second layers  108  and  106  are transparent, but the third layer  104  appears as high impedance resonant L-C circuits, with the conductive layer  114  being ground for the third layer  104 . 
   Referring again to  FIG. 7 , the new layered structure  71  is mounted on the interior of all four walls  72 – 75 , with the conductive strips  76  oriented inward and longitudinally down the waveguide. The layered structure  71  allows the waveguide  70  to transmit signals at multiple frequencies, with uniform density at both horizontal and vertical polarizations. For a three layered structure, the waveguide can transmit three different frequencies, with each of the layers responding to a respective frequency. 
   The vertically polarized signal maintains a uniform density as a result of the high impedance presented by the wall structure on the sidewalls  72 ,  74  and current flowing along the strips  76  on the top wall  75  and/or bottom wall  76 . The horizontally polarized signal maintains uniform power density because of the layered structure at the top and bottom wall  75 ,  76 , and current flowing down the conductive strips  76  of the sidewalls  72  and  74 . Thus, the cross-polarized signal has a generally uniform power density across the waveguide. If the waveguide is transmitting a signal in one polarization (vertical or horizontal), it only needs the new layered structure on only two opposing walls to maintain the signals uniform power density. 
   Shorting switches  116  are shown as symbols on the top layer of the structure  71  walls  72 – 75 , and the details of the switches are described below and shown in  FIGS. 11–14 . If the switches are closed on the top layer on all four of the waveguide&#39;s walls, the waveguide  70  is changed from transparent to opaque at all three frequencies. For instance, at the lowest frequency, when the first two layers of the structure appear transparent and closing the switches on the top layer shorts the gap capacitance and causes the signal to see only the conductive surface presented by the top layer&#39;s conductive strips and closed switches. The same is true for the next higher frequencies. Closing the switches causes them to see only a conductive surface, cutting off transmission. 
   Closing the shorting switches  116  on the sidewalls  72 ,  74  blocks transmission of vertically polarized signals at all three frequencies. The structure on the top and bottom presents as a high impedance to the E field of horizontally polarized signals and the waveguide still transmits the horizontal signals at all three design frequencies. The shorting switches  116  are closed on the top and bottom walls  73 ,  75  to block transmission of the horizontally polarized signals, while still transmitting the vertically polarized signals at all three frequencies. 
   If switches  116  are included at each of the layers (not shown) then different frequencies at different polarizations can be selectively blocked. For example, f 3  could be blocked in both polarizations if the switches  116  are closed on the bottom layer  82  (shown in  FIG. 8 ) on all four walls. Only for f 3  will all the layers appear as conductive layers, cutting off transmission at f 3 . If the shorting switches  116  are closed on the bottom layer  82  on the top and bottom walls  73 ,  75  only, transmission of the horizontally polarized signal at f 3  is blocked, while still transmitting the vertically polarized signals at f 3 . If the switches  116  are closed on the bottom layer  82  on the sidewalls, transmission of the vertically polarized signal at f 3  is blocked. By selectively closing the switches  116  at the other layers  84 ,  86 , the different frequencies in different polarizations can be blocked. 
   Switching Mechanisms 
   The shorting switches used to short the conductive strips can employ many different known switches, with the preferred switches using micro electromechanical system (MEMS) technology or varactor layer diode technology. MEMS switches are generally described in Yao and Chang, “A Surface Micromachined Miniature Switch For Telecommunication Applications with Signal Frequencies from DC up to 4 Ghz,” In Tech. Digest (1995), pp. 384–387 and in U.S. Pat. No. 5,578,976 to Yao, which is assigned to the same assignee as the present application. U.S. Pat. No. 5,578,976 to Yao, also discloses and discusses the design trade-offs in utilizing MEMS technology and the fabrication process for MEMS switches. 
     FIGS. 11 ,  12  and  13 , show one embodiment of the MEMS shorting switches  132  constructed in accordance with the present invention to short the conductive strips  134  in the high impedance structure  130 . The switches  132  are fabricated using generally known micro fabrication techniques, such as masking, etching, deposition, and lift-off.  FIG. 11  is a sectional view of the high impedance structure  130  taken transverse to the conductive strips  134 .  FIG. 12  is a sectional view taken along sectional lines of one of the shorting switches  132 . Both show high impedance structure&#39;s dielectric material  136 , vias  138  and conductive layer  140 . 
   The switches  132  are manufactured by depositing semiconductor layer  141  the conductive strips  134  and over the exposed surface of the dielectric material  136 , the preferred semiconductor material being Si 3 N 4 . Stand-off isolators  142  are deposited at intervals down the gap between the conductive strips  134  and are preferably formed of an insulator material such as silicon dioxide. A respective strip of metallic material  144  is mounted over each of the gaps by affixing it on the top of the stand-offs  142  along one of the gaps. 
   In operation, each metallic strip  144  has either 0 volts or voltage potential applied, with the preferred potential being 50 volts. With 0 volts applied, the strips  144  remain suspended above their respective gap between the stand off isolators  142  as shown in  FIG. 12 . The switches are in the “Off” state and the structure  130  presents as a high impedance to the design frequency E field transverse to the conductive strips  134 . The gaps between the strips  134  presents a capacitance and the vias  138  present an inductance, with the structure presenting as a series of resonant L-C circuits to the transverse E field. 
   Referring now to  FIG. 13 , to close the switch  132  and short the gap between conductive strips  134  a 50 volt potential is applied to the metallic strips  144 . This causes an electrostatic tension between the metallic strips  144  and the respective conductive strips  134  below, pulling the switch strip down such that it makes capacitive contact with the strip  134  on each side of the gap. This provides a conductive bridge across the, gap, shorting the gap. With all the metallic strips  144  pulled to the strips  114  below the high impedance structure appears as a conductive surface to the signal&#39;s E field. This switching network consumes very little and has a very fast closure time on the order of 30 μs. 
     FIG. 14  shows a high impedance structure  150  with a second embodiment of the shorting switches  152  that utilize varactor diode technology to short the gaps. The varactor diode is an ordinary junction diode that relies on its voltage dependent capacitance. Each varactor switch includes a N+ (highly conducting) layer  154  grown or deposited in the each gap between the conductive strips  156 . An N− (moderately conducting) layer  158  is grown on top of top of a portion of the N+ layer  154 . 
   In fabricating the switches  152 , the N+ and N− layers  154  and  158  are etched into mesas that will provide a strip of varactor material along the length of the gaps between the conductive strips  156 . The switching of the varactor is controlled by a second conductive strip  160  sitting on an insulator layer  162  that is sandwiched between the second strip  160  and each conductive strip  156 . The insulator layer  162  provides a capacitive coupling to conductive strip  156  and the ground plane. Voltage applied to the second strip  160  controls the capacitance of the varactor layer and thus the shorting of the gap. 
   The presence of zero voltage on the varactor layer creates a high capacitance at the gap, virtually shorting (closing) the gap. This causes the high impedance structure to appear as a conductive surface, cutting off transmission of the signal and making the shutter switch appear opaque. When a high voltage is applied to the varactor the capacitance at the gap is reduced. The high impedance structure is then resonant at the operating frequency and the waveguide will transmit the beam. With all its waveguides transmitting, the shutter switch appears transparent to the incident beam. 
     FIG. 15  shows millimeter beam transmission system  170  used in various high frequency applications such as munitions guidance systems (e.g. seeker radar). A transmitter  172  generates a millimeter signal  174  that spreads as it moves from the transmitter. Most of the signal is directed toward a lens  176  that collimates the signal into a beam  177  with little diffraction. The collimated beam travels to a second lens  178  that focuses the beam to a receiver  180 . The shutter switch  182  is positioned between a millimeter wave transmitter  172  and receiver  180  such that it intercepts the transmission beam  177 . When the shorting switches on the shutter switch&#39;s waveguides are open, the shutter switch  182  is transparent to the beam and the signal passes from the transmitter  172  to the receiver  180 . When the shorting switches are closed, transmission of the signal through each of the waveguides is cut-off, making the shutter switch  182  opaque to the beam  177  and blocking transmission from the transmitter to the receiver. 
   As described above, when the waveguides in the shutter switch  182  have the high impedance structure on the sidewalls and the top and bottom walls, the beam can have horizontal and vertical polarization and the shutter switch  182  can block one or both of the polarizations. When the high impedance structure has multiple layers, the shutter switch can be transparent or block signals at multiple frequencies and at one or both polarizations. 
   Although the present invention has been described in considerable detail with reference to certain preferred configurations thereof, other versions are possible. The waveguides in the shutter switch can have different high impedance structures and the new shutter switch can be used in other applications. Therefore, the spirit and scope of the appended claims should not be limited to their preferred versions describes therein or to the embodiments in the above detailed description.