Abstract:
A structure for selectively transmitting electromagnetic energy over a selected frequency range during a first operating mode and for substantially preventing the transmission of electromagnetic energy at any frequency during a second operating mode either by absorption or by reflection of such energy. The structure includes at least one shutter means comprising continuous and discontinuous elements and diode means interconnecting the discontinuous portions of the discontinuous element. The diode means are biased in a non-conductive direction during the first operating mode and are biased in a conductive direction during the second operating mode. The structure can further include filter means which include continuous and discontinuous elements arranged so as to be resonant over the selective frequency range during both operating modes. The filter means and the shutter means can be positioned at distances from each other of approximately one or more quarter wave lengths of the center frequency of the selective frequency range.

Description:
This application is a continuation of application Ser. No. 415,260, filed Sep. 7, 1982, abandoned. 
    
    
     INTRODUCTION 
     This invention relates generally to structures for selectively transmitting electromagnetic energy and, more particularly, to electronic circuit structures arranged so that at selected times the transmission of electromagnetic energy therethrough is permitted only in a selected frequency range, energy outside such range being essentially rejected, and at other selected times the transmission therethrough of energy in such selected frequency range is substantially reduced, while energy outside such range is still rejected. Such structures can be used, for example, as special radomes shielding microwave antennas and auxilliary equipment from externally incident energy. 
     BACKGROUND OF THE INVENTION 
     Radome structures are conventionally used to protect microwave antennas from the physical environment. It is also desirable to shield such equipment from externally incident electromagnetic energy which can adversely affect the electrical operating characteristics thereof. Ideally, such a shield during operation of the antenna equipment should be transparent to the energy in a selected frequency range handled by the antenna (the &#34;in-band&#34; frequency range) but should reject all frequencies outside such frequency range (the &#34;out-of-band&#34; frequency range). Further, when the antenna equipment is not operating, such a shield should reject electromagnetic energy over all frequencies of concern. 
     Radome shields having such characteristics have often been referred to as &#34;shutter-type&#34; radomes, the frequency shutter in effect being effectively &#34;closed&#34; to all frequencies during non-operation and the frequency shutter being effectively &#34;opened&#34; only to the desired operating frequency band during operation. Shutter-type radomes presently used in the art have consisted primarily of electro-mechanical devices which are relatively bulky and cumbersome to fabricate and use and which have the added disadvantage of being relatively slow in changing from one mode of operation to the other. It is desirable to develop simpler structures for such purpose which structures can provide relatively fast operation shifting from the &#34;shutter open&#34; to the &#34;shutter closed&#34; states. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the invention, a shutter type structure includes electronic means for providing a resonant structure which utilizes suitable diodes. In its &#34;open&#34; state the diode means are biased in such a manner as to provide a selected band-pass characteristic for the structure which permits the transmission therethrough of electromagnetic energy having frequencies within the selected passband, energy having frequencies outside the pass band being effectively rejected. During the &#34;closed&#34; state the diode means are biased in such a manner to radically modify the behavior of the structure so as to substantially reduce the transmission of energy within the selected pass band, energy outside the pass band still being effectively rejected. 
     Thus in one exemplary embodiment of the invention, a shutter-type radome shield comprises a plurality of substrates, each of which is separated by one-quarter of the wavelength (λ/4) of the center frequency of the selected in-band frequency range. At least one substrate contains an array of diodes together with an array of continuous wires so as to provide the desired bandpass characteristics, the diode array being used to create the desired shutter effect. The remaining one or more substrates may contain an array of diodes and an array of continuous wires if the structure is to operate as an energy absorbing structure, or may contain an array of continuous and discontinuous wires (with no diodes present) if the structure is to be energy reflective. At least one substrate which contains diodes is designated as an &#34;in-board&#34; array, the remaining substrates being designated as &#34;out-board&#34; arrays. When the diodes are reversed biased and the dimensions of the wire array are suitably selected, energy in the in-band frequency band can be transmitted through the shield. When the in-board diodes are sufficiently forward biased, substantially all frequencies of concern are prevented from being transmitted through the shield. The out-board diodes, if present, in an energy absorbent structure are only slightly forward biased and act as resistances to effectively absorb some of the power from electromagnetic energy which is &#34;externally&#34; incident on the shield. The out-board arrays which do not contain diodes act to reflect out of band externally incident energy. 
     The fabrication of such a shield can be readily performed and the structure can be effectively used with a conventional radome structure. Moreover, the conversion of the shield from the open to closed shutter modes can be performed in a relatively rapid fashion. 
    
    
     DESCRIPTION OF THE INVENTION 
     The invention can be described in more detail with the help of the accompanying drawings wherein 
     FIG. 1 depicts a portion of an exemplary embodiment of the structure of the invention; 
     FIG. 2 depicts an equivalent circuit representing the operation of the embodiment of FIG. 1 in one mode of operation thereof; 
     FIG. 3 depicts an equivalent circuit representing the operation of the embodiment of FIG. 1 in the alternative mode of operation thereof; 
     FIG. 3A depicts an ideal equivalent circuit corresponding to FIG. 3; 
     FIG. 4 depicts in diagrammatic form a radome structure which shows one manner in which an embodiment of the invention can be used; 
     FIG. 5 depicts in diagrammatic form a radome structure which shows another way in which an embodiment of the invention can be used; 
     FIG. 6 depicts an alternative embodiment of the invention shown in FIG. 1; 
     FIGS. 7 and 7A depict a portion of a substrate which represents a further alternative embodiment of the invention; 
     FIG. 8 depicts a plan view of a still further embodiment of the invention as used in an exemplary waveguide structure; 
     FIG. 9 depicts a view in section along lines 9--9 of a portion of the embodiment of FIG. 8; 
     FIG. 10 depicts a view in section, along lines 10--10 of another portion of the embodiment of FIG. 8; and 
     FIG. 11 depicts curves of the operating characteristics of the embodiment of FIGS. 8-10. 
    
    
     As can be seen in the portion of a particular embodiment of the invention shown in FIG. 1 a plurality of relatively thin dielectric substrates 10A, 10B and 10C are separated by suitable low density foam or non-metallic honeycomb structures 11. Each substrate panel carries a plurality of parallel continuous metallic wires 12. In addition each panel carries a diode array 13, formed as conductive paths each of which include a plurality of series connected diodes, the paths being interconnected and positioned between and in parallel with the continuous wires as shown. Panels 10A and 10B can be referred to as &#34;out-board&#34; panels, while panel 10C can be referred to as an &#34;in-board&#34; panel. 
     The series connected diodes on out-board panels 10A and 10B are commonly connected to a first DC bias power supply 14, while the series connected diodes on in-board panel 10C are commonly connected to a second DC bias power supply 15. The power supplies can be rapidly switched from one polarity to the other in a conventional manner so as to reverse bias or to forward bias the diodes as desired. 
     During the operating mode, i.e., when it is desired that electromagnetic energy which is incident upon the panels and which lies in a selected frequency range, be transmitted through the panel and all other frequencies be prevented from such transmission, all of the diodes are reverse biased. In such case, the wire grids containing the diodes effectively operate as discontinuous wire grids. Such operation can be best understood from an examination of the electrical equivalent circuit shown in FIG. 2. 
     The reversed diode arrays (equivalent to capacitors) and continuous wire grids (equivalent to inductors) of both the out-board and in-board panels each effectively form a tuned circuit, as shown by tuned circuits 16 and 17 corresponding to out-board panels 10A and 10B and tuned circuit 18 corresponding to in-board panel 10C. The tuned circuits each act as parallel resonant circuits the center frequency of which is equal to the center frequency of the in-band frequency range and the bandwidth of which is made to correspond to that of the in-band frequency range. The selection of the dimensions of the wire grids, formed by continuous wires 12 in each panel determine the pass bands of the tuned circuits 16-18. The dimensions of the discontinuous wire grids (i.e. containing the diodes) are selected to resonate the continuous wires, taking into account the diode capacitances. Each of the tuned circuits is separated, as in a transmission line, by one quarter wave length (λ/4) as depicted in FIG. 2. Thus, during the operating mode all frequencies in the pass band of the tuned circuits are transmitted through the structure shown in FIG. 1 (as depicted diagrammatically by the arrows in FIG. 2). The use of multiple panels permits the band pass characteristics of the overall structure to be suitably shaped. 
     Although the diodes are reversed biased in the embodiment discussed above, the diodes may be of the zero-bias type so that instead of reverse biasing them the power supply may be simplified to provide no bias voltage during the operating mode so as to effectively achieve the same operation. 
     FIG. 3 depicts the equivalent circuits of FIG. 2 during the non-operating mode, i.e., when the transmitting and/or receiving antenna is non-operative. In such mode the diodes in the in-board panel 10C are biased so as to provide a first forward bias current while the diodes of the out-board panels 10A and 10B are biased so as to provide a second forward bias current. The in-board forward bias current is selected to be sufficiently large to provide full conduction in the forward direction so that the diodes effectively appear as short-circuits (a residual wire inductance tends to remain as shown by the relatively small inductance 20 shown in FIG. 3). The out-board forward bias current is selected to be much lower than that of the in-board diodes, the slight forward biasing causing the diodes to behave predominantly as resistances. Such resistances thereby tend to absorb some of the power incident on the outboard panels. FIG. 3A represents an ideal condition desired during the forward biasing mode, wherein the out-board circuits are pure resistances and the in-board circuit is a short circuit. In practice, however, such ideal conditions do not occur and the equivalent circuit tends to appear as shown in FIG. 3. Thus, while the incident energy is not completely prevented from being transmitted through the structure, the amount transmitted is substantially reduced. The amount of power absorbed can be controlled by the number of the out-board panels used. Normally, only a single in-board panel is required for reflective type shielding, although more than one may be used in some applications. 
     Panels of the type shown in FIG. 1 can be shaped in such a manner as to conform to the shape of a radome structure as shown in FIG. 4 wherein the shield structure 21 of the invention conforms to the substantially conical (ogive) shape of the radome structure 22 which encloses antenna structure 23. Alternatively, the radome and shield structures can be integrally formed during manufacture. 
     As a further alternative the shield structure can be shaped independently of the shape of the radome enclosure and formed separately therefrom as depicted in FIG. 5 wherein the shield forms an individual hemispherical cover 24 for antenna 25 within the conical radome enclosure 22. 
     An alternative embodiment of the structure depicted in FIG. 1 is shown in FIG. 6 in which an in-board panel 10C&#39; of the type used in FIG. 1 (using continuous wires 12&#39;, diodes 13&#39; and bias supply 15&#39;) is also utilized. In the alternative embodiment out-board panels 10A and 10B each use a plurality of continuous wires 12&#39; and a plurality of dis-continuous wires 19&#39;, the diodes shown in panels 10A and 10B of FIG. 1 being eliminated to provide the dis-continuities. During the operating mode, the dimensions of the dis-continuous wires are selected to resonate with the continuous wires, as in FIG. 1, to provide appropriate resonant circuits, as before. During the non-operating mode the continuous and discontinuous wire grid arrays of FIG. 6 continue to resonate and, consequently, do not absorb power as in FIG. 1. The overall structure then effectively operates as a reflective energy structure wherein a substantial amount of incident energy impinging thereon is effectively reflected back from the structure so that the amount transmitted therethrough is substantially reduced. 
     The embodiments of FIGS. 1 and 6 are effective for electromagnetic energy which has a polarization substantially parallel to both the continuous wire paths and the diode array paths or the dis-continuous wire paths shown therein. If it is desired that the performance characteristic of the shield be effectively independent of polarization, each panel can be arranged to contain orthogonal grids of continuous wire and diode array paths, as shown in FIG. 7, or of continuous and dis-continuous wire paths, as shown in FIG. 7A. The orthogonal continuous wires 26 and the orthogonal diode array 27 or the orthogonal continuous wires 26&#39; and orthogonal dis-continuous wires 27&#39; can be suitably positioned, for example, on the surfaces of the substrates 28 and 28&#39;, respectively, which are opposite to the surface on which wires 12 and diodes 13 or wires 12&#39; and wires 19&#39; are positioned in FIGS. 1 and 6. 
     The dimensions and spacings of the wires will depend upon the application in which the above configurations are to be used, i.e. on the frequencies and band widths of interest. The bias currents required will depend on the diodes which are selected for use. Such values can generally be empirically determined by those in the art using conventional techniques so that such structures can be readily fabricated for the applications desired. 
     While the invention is most effectively embodied in panel form as discussed above for use in radome structures, the invention can also be used in a different structural environment such as the waveguide structure depicted in FIGS. 8-10. 
     As can be seen therein, as waveguide 30 has first and second filter elements 31 and 32, respectively, and a shutter element 33 placed therein at selected regions thereof separated by a quarter-wavelength of the center frequency of a selected in-band frequency range. Each of the filter elements includes a pair of oppositely disposed upper and lower vertically adjustable metallic posts 34 and 34A, respectively, as best seen in FIG. 9, the spacing 35 therebetween being selected to provide a desired capacitive effect, as discussed in more detail below. A further pair of fixed posts 36 and 37 (see FIG. 9) extend between the upper and lower inner surfaces of the waveguide 30 and effectively act as inductive elements. The combination of adjustable posts 34, 34A and fixed posts 36 and 37 form an equivalent parallel LC circuit which is resonant over a selected frequency band at a selected center frequency. 
     The shutter element 33 includes a pair of outer rectangular metallic strips 40 and 41 extending between the upper and lower inner surface of waveguide 30, as best seen in FIG. 10. A further discontinuous metallic strip 42 is effectively formed of upper and lower portions 42A and 42B and center portion 42C which portions are interconnected as shown by diodes 43 and 44. Suitable leads 45 are used to connect the diodes to a d.c. bias supply (not shown). 
     When the adjustable posts 34 and 34A are suitably positioned relative to each other in each filter element, the filter elements are effectively resonant over a selected band width having a selected center frequency. When the diodes are reversed biased (operating in effect as open circuits) the dimensions and spacings of the metallic strips 40, 41 and 42 are arranged to resonate over the same selected bandwidth and at the same selected center frequency. Under such conditions electromagnetic energy over such pass band which enters waveguide 30 is transmitted through the waveguide with substantially little or no loss. 
     When the diodes are sufficiently forward biased (operating in effect as short circuits) the portions 42A, 42B and 42C are interconnected and the shutter element as a whole tends to substantially reduce the electromagnetic energy which can be transmitted through the waveguide. Such energy will accordingly be reflected back from shutter element 33. 
     In a specific exemplary structure based on the embodiment of FIGS. 8-10, the dimensions are as follows: 
     Waveguide 
     Inner Height=1.34&#34; 
     Inner Width=2.84&#34; 
     Filter Elements 
     Posts 36 and 37--Diameter=0.250&#34; 
     Posts 34 and 34A--Diameter=0.132&#34; 
     Post 34--Length from top of waveguide=0.65&#34; 
     Post 34A--Length from bottom of waveguide=0.3555&#34; 
     Spacing between=0.3555&#34; 
     Shutter Element 
     Strips 40 and 41--Width=8.9 mm. 
     Strips 42A, 41B, 42C--Width=12.7 mm. 
     Strip 42C--Height=12.7 mm. 
     Vertical spacing between strips 40 and 42 and strips 41 and 42=0.75 mm. 
     Horizontal spacing between strips 42A and 42C and strips 42B and 42C=2.0 mm. 
     Zero Bias Diodes (Typical) 
     &#34;Reverse&#34; capacitance=0.2 pfd. 
     Series resistance--1.0 ohm (at 100 mA) 
     Forward Bias Current=20 mA 
     Reverse Bias Voltage=0 volt 
     The above dimensions provide a center resonance frequency of 3.15 gigahertz (GHz). Using &#34;zero-bias&#34; diodes, the reverse bias voltage was 0 volts. The frequency response thereof in the &#34;open&#34; shutter mode (zero biased diodes) is shown by curve 45 in FIG. 11. A relatively small insertion loss of about 1.0 dB exists over the flat portion of the response, the response being down by about 3.0 dB over a pass band from about 2.9-3.4 GHz. 
     When the diodes are forward biased at a sufficient voltage to provide a forward biased current of about 20 milliamperes (mA), the insertion loss increases and the response drops by approximately 15 dB over a pass band from about 3.0 GHz to about 3.3 GHz and even more substantially outside such pass band as shown by curve 46 in FIG. 11. Thus the transmission of electromagnetic energy is reduced considerably in the &#34;closed&#34; shutter mode. 
     While specific embodiments of the invention are described above with reference to FIGS. 1-11, modifications to such embodiments will occur to those in the art within the spirit and scope of the invention. For example, while the embodiments of FIGS. 1 and 8 show the use of filter elements in the form of out-board panels 10A and 10B and in the form of elements 31 and 32, respectively, in some applications it may be sufficient to use only a single &#34;shutter&#34; element, such as the in-board panel 10C of FIG. 1 or the element 33 of FIGS. 8 and 10. The single shutter member operates so as to provide filtering action when the diodes thereof are non-conductive and to provide an effective &#34;closed&#34; shutter when the diodes are forward biased so as to provide full conduction. In a still further embodiment a plurality of shutter elements may be used, without the use of filter elements e.g. a plurality of in-board panels all operated in the manner of in-board panel 10C of FIG. 1 or a plurality of shutter elements in a waveguide all operating as shutter element 33 of FIGS. 8 and 10. Other combinations of, and embodiments of, such filter and shutter elements may also occur to those in the art in accordance with the invention. Hence, the invention is not to be construed as limited to the particular embodiments shown and described herein except as defined by the appended claims.