BEAM STEERABLE ANTENNA

Abstract: A directional antenna system comprises an elongate dielectric waveguide (28) for transmitting and/or receiving electromagnetic energy of a frequency at which the waveguide propagates, adjustable means (32) having a plurality of conductive portions spaced apart along a lengthwise region adjacent and substantially parallel to the waveguide, and controlling means (34) to vary over a continuous range the spacing between the conductive portions and thus to control the angle of maximum radiation.

Description: BEAM STEERA8LE ANTENNA This invention was made with United States Government support under Contract F49620-82-C-0099 awarded by the Department of the Air Force. The United States Government has certain rights in this invention. FIELD This invention relates to directional antenna systems involving the interaction of dielectric waveguides and nearby periodic structures. It is especially useful for changing the angle of maximum radiation or reception of an antenna by changing over a continuous range the period of the nearby periodic conductive structure. BACKGROUND A beam-steerable antenna has applications such as in radar and seeker systems, communications systems, and satellite television reception. There is a need for,a simple, low-cost method of constructing beam- steerable antennas or phased arrays. In the prior art, microwave phased arrays achieve beam steering by high-speed phase shifters that comprise discrete PIN (positive intrinsic negative) diodes or ferrite devices. These phase shifters set the electrical phase on elements of the array. It is difficult and expensive to build discrete phase-shift devices for extremely high operating frequencies (e.g. in the millimeter-wave region of the spectrum) . An alternative approach is based on "diffraction electronics". A periodic structure in close proximity with a dielectric waveguide imposes periodic boundary conditions that provide radiation lobes from the combined waveguide and periodic structure. Experiments have been reported on combined structures of this type using fixed periodic structures made of grooved metallic blocks [S.D. Andrenko, V.G. Belyaev, N.E. Devyatkov, and V.P. Shestopalov, "Diffraction Input of Energy to a Dielectric Waveguide", Dokl. Akad. Nauk SSR, 247, pp 73-76 (July 1979).]. Such a grooved structure 5 is illustrated in Figure 2. Along the periodic structure of total length L, there is a repetitive spacing £, which is the period of the periodic structure. The direction (angle θ) of maximum radiation (or reception) from the dielectric waveguide is set by the period 10 I of the periodic structure. It has been shown that the periodic structure should be slightly tilted (angle a. in Figure 2) to produce less coupling on the feed end of the waveguide than on the load end of the waveguide [S.D. Andrenko, 15 .D. Devyatkov,' and V.P. Shestopalov, "Millimeter Field Band Antenna Arrays", Dokl. Akad. Nauk SSSR 240, pp 1340-1343 (June 1978).]. For maximum radiated power in a given direction, the tilt is optimum when the surface excitation is uniform along the length of the 20 periodic structure. For best performance, there is an optimum distance between the dielectric waveguide and the conductive surfaces of the periodic structure (e.g. the nominal distance ω in Figure 2). If the periodic structure 25is brought too close to the waveguide, non-uniform excitation and a poorly defined radiation lobe are obtained. If the structure is too far away from the waveguide, no radiation is produced. The optimum distance and tilt are best determined experimentally. A properly 30adjusted periodic structure can radiate about 90 percent of the total input power to the dielectric waveguide. The relationship between the fixed period I of the periodic structure in meters and the directi ^n of maximum wave propagation measured as the angle θ in radians relative to the load end of the waveguide can be derived from theoretical considerations. The interaction between the waveguide and the periodic structure, through evanescent mode coupling, causes a surface excitation on the periodic structure. The propagation constant on the fixed periodic structure is given by Floquet's Theorem [J.C. Slater, Microwave Electronics, D. Van Nostrand Co., p 170 (1950).]: B s = B 0 + m H (1) where B 0 = propagation constant of dominant waveguide mode, ~~- l = period of periodic structure, m m = mode number = 0, ±1, ±2,... The condition for radiation from the combined waveguide- periodic structure is that a wave in the radiating direction θ must satisfy the condition k 0 cos θ = B s = B 0 + m il ( 2 ) where k 0 = m ∑. - free space propagation constant , A λ = free space wavelength, m θ = angle from axis relative to load end of waveguide, radians Hence, the radiation condition is cosθ = Us. + π (3) k 0 I cosθ = £_ + mλ (4) v g £ where c = free-space speed of light, m/sec -~g m speed of propagation in the waveguide, m/sec The normal radiation mode of interest is associated with -t -1. There is one dominant radiation lobe for the condition ≤-- + 1 > A > ≤ 1 ( 5 ) v g ~ I v g ~ ' From equation (4) it is evident that the beam pointing angle θ is inversely related to the period I of the periodic structure. Thus, a combined waveguide and periodic structure with a desired fixed pointing angle θ can be constructed by use of a periodic structure with a fixed period I. Andrenko et al (1978) refer to a fixed periodic structure made of a grooved metallic block, such as illustrated in Figure 2, as a "diffraction - grating" with period I . They mention a possibility of stepwise adjustment of directivity in their mm band antenna array by mounting diffraction gratings of different periods on a cylindrical barrel which rotates around its axis. However, they suggest only discrete variation and do not mention any way of varying directionality over a continuous range. A more versatile and practical device would result if the period of the periodic structure were adjustable over a continuous range and if means were provided to rapidly adjust the period I so as to obtain any desired beam angle over a continous range as provided by the present invention. DISCLOSURE This invention relates to a directional antenna system in which the beam can be steered over a wide angle relative to the axis of the antenna. The beam-steerab antenna for transmitting and/or receiving electromagnetic energy of a frequency at which the waveguide propagates is based on the principles of "diffraction electronics". A typical beam-steerable antenna comprises a dielectric waveguide and an adjustable periodic structure in a lengthwise region adjacent and substantially parallel to the waveguide. A typical adjustable periodic structure can comprise a coiled helical spring in which the plurality of conductiv portions of the wire are equally spaced apart and means are provided to expand or compress the spring to vary the spacing between the coils of the spring. The spring typically contains a metallic core to guide the helical coils. The distance between the periodic structure and the dielectric waveguide and any small tilt of the axis relative to the dielectric waveguide are adjusted. for maximum power. Alternative adjustable periodic structures for use -in this invention are bellows-like flexible members, kinematic chains, and threaded polyhedrons. The means to adjust the spacing are chosen as appropriate for the type of periodic structure such as linear actuators, motors, solenoids, which would be obvious to those skilled in the art. For example, a periodic structure in the form of a hollow bellows could have the spacing adjusted by increasing or decreasing air pressure. In an alternative embodiment of this invention, the adjustable periodic structure is on a thin metallic - or metal-coated tape that is moved between a storage reel and a takeup reel to a tape position containing the desired spacing of conductive lines formed between intervening slots in the tape. Alternatively conductive elements with the desired spacing could be printed on a non-conductive tape by computer-controlled, conductive ink-jet printer on demand or for preformed tapes. In another embodiment of this invention, the adjustabl periodic structure comprises a photoconductive surface with periodic conductive bands produced by light diffracted by overlapping transparent ruled gratings forming moire fringes of frequency proportional to the adjustable angle between lines on the two gratings. In other embodiments of this invention, the adjustable periodic structure comprises a photoconductive surface with periodic conductive bands produced by light diffracted by an acousto-optical device or optically focused light produced by computer controlled selective activation of an array of diode lasers. In another embodiment of this invention, the adjustabl periodic structure comprises periodic conductive bands in an adjacent semiconductor surface adjustable by control of the electron beam in a cathode ray tube. DRAWINGS Figure 1 is a schematic front view of a test setup used for measuring the beam pointing angle resulting from the interaction of a dielectric waveguide with an adjacent periodic structure according to this invention. Figure 2 is an enlarged schematic front sectional view of a waveguide and periodic structure as in Figure 1, showing the important dimensions. Figure 3 is an enlarged schematic front view of a waveguide as in Figure 1 with an adjustable periodic structure comprising portions of a spring. Figure 4 is a graph of data showing the change in relative beam power and beam angle at several frequencie for a fixed period. Figure 5 is a graph of relative beam power versus beam angle for the waveguide and periodic structure of Figure 3 at one spacing of the coils of the spring. Figure 6 is a graph of data for the waveguide and periodic structure of Figure 3 showing the beam pointing angle as a function of period resulting from adjustment of the spacing of coils of the spring periodic structure in accordance with this invention. Figure 7 is a graph of data showing the linear correlation of half-power beamwidth as a function of wavelength, period, and beam angle. Figure 8 is a partial sectional axial view of a waveguide-spring adjustable periodic structure showing a modified spring coil. Figure 9 is a partial sectional axial view of one coil of a modified helical spring. Figure 10• is a partial sectional axial view of one coil of a rectangular spring. Figure 11 is a schematic perspective view of a bellows-like adjustable periodic structure. Figure 12 ' is a schematic view of an adjustable bellows periodic structure and waveguide. Figure 13 is a schematic view of a kinematic chain adjustable periodic structure and waveguide. Figure 14 is a schematic perspective view of a variable-pitch threaded polyhedron adjustable periodic structure. Figure 15 is a schematic view of a movable tape adjustable periodic structure and waveguide. Figure 16 is a schematic perspective view of a waveguide and the conductive ink lines on the tape of the adjustable periodic structure of Figure 15. Figure 17 is a schematic view of a preformed perforate tape type adjustable periodic structure. Figure 18 is a schematic sectional view of a waveguide and periodic structure comprising periodic conductive bands in an adjacent photoconductive surface adjustable by optical gratings. Figure 19 is a partial sectional view as indicated at 19-19 in Figure 18. Figure 20 is a schematic view of the moire fringe pattern indicated at 20-20 in Figure 18. Figure 21 is a schematic view of a waveguide and a periodic structure comprising periodic conductive bands in an adjacent photoconductive surface adjustable by an acousto-optical device producing a plurality of light beams. Figure 22 is a schematic view of a waveguide and a periodic structure comprising periodic conductive bands in an adjacent photoconductive surface adjustable by an acousto-optical light scanning device. Figure 23 is a schematic view of a waveguide , and a periodic structure comprising periodic conductive bands in an adjacent photoconductive surface adjustable by computer control of activation of an array of diode lasers and optically focused light. Figure 24 is a schematic view of a dielectric antenna showing the directions of radiation for surfaces of the periodic structure below the antenna. Figure 25 is a schematic view of a dielectric antenna showing the directions of radiation for surfaces of the periodic structure above the antenna. Figure 26 is a schematic view of a waveguide and a periodic structure comprising periodic conductive bands in an adjacent semiconductor surface adjustable by control of the electron beam in a cathode ray tube. Figure 27 is a sectional view as indicated at 27-27 in Figure 26. CARRYING OUT THE INVENTION The test setup, shown in Figure 1, was used to measure the beam pointing angle θ as a function of the period I for adjustable periodic structures according to this invention after first validating the test setup with a periodic structure of fixed period. A standard horn antenna 20 was used to measure the radiation pattern and a calibrated thermistor 21 and power meter 22 served as the detector. The arc radius of 0.6 ensured that the horn was in the far field of the radiation pattern. The test setup was designed for an operating frequenc of 94 GHz. A cw klystron 23 was used as the source of electromagnetic energy with a circulator (isolator) 24 and frequency meter 25. A standard W-band waveguide termination 26 was used at the load end with another thermistor 21 and power meter 22 connected by a 20 ' dB coupler 27. The dielectric waveguide antenna 28 was connected in the test setup through lengths of WR-10 waveguide 29. Polystyrene (ε r = 2.56) was used for the dielectric and was cut to have the same nominal cross-section as the slot in the metal waveguides (i.e. 0.127 x 0.254 cm) . The dielectric strip was about 10 cm long and, for a few millimeters back from either end, the top and bottom surfaces were tapered into a wedge shape. An electric field probe (not shown) was fabricated and was used with a crystal detector to probe the fields along the dielectric 28 in the absence of a periodic structure. The wavelength (λg) the dielectric strip 28 was found to be 2.6 mm at 94 H z . To verify the test setup, a periodic brass structure 30 was fabricated having a fixed period of I = 2.2 mm, a notch depth of 0.67 mm, and a length of 30 periods (L * 66 mm) at a 50 percent duty factor. The periodic structure 30 was placed on a tilt ' platform 31 to provide a small tilt angle (approximately 10-20 milliradians) , as shown in Figure 2, with a nominal distance ω of 1 mm between the periodic structure 30 and the dielectric waveguide 28. With the above periodic structure 30, the beam angle was 103.75° at λ ~ * 94 GHz and from equation (4) the effective propagation constant (c/Vg) was found to be 1.22. This value was consistent with the dielectric waveguide wavelength measured with the electric field probe. The generator frequency was varied over the range of 91 to 98 GHz to determine the variation of beam angle β with wavelength λ. The good agreement of the data shown in Figure 4 with equation ( ) was deemed sufficient to validate the test setup. To investigate the effect of an adjustable periodic structure, a spring 32, as shown in Figure 3, was used in place of the fixed periodic structure 30 shown in Figure 2. The spring 32 was selected which could be stretched and compressed to vary the period I over the range indicated in equation (5). Such a readily available spring had a diameter of 5.5 mm, and consisted of 32 spiral coils with a wire diameter of 0.5 mm. A brass rod of 5 mm diameter was used as the core 33. The spring 32 was initially adjusted by movement of actuator 34 against stop 35 to provide a spacing between coils of I = 2.2 mm (the same as the fixed period of the structure 30 used to validate the setup). The resultant radiation pattern is shown in Figure 5, normalized to the gain at beam maximum (the "gain" of the spring 32 was about 5 dB below that of the brass structure 30). No tilt was applied to the spring 32. The length, L, of the spring 32 was varied from " a minimum of 45.3 mm to a maximum of 99 mm, in steps of about 2.5 mm, to steer the beam from an angle θ » 166° to θ » 80°. The variation of pointing angle θ with spring coil spacing or period I is shown in Figure 6. The measured values in Figure 6 show good agreement with theory according to equation (4) for c/Vg = 1.22, m « -1, and λ = 3.19. The results shown in Figure 6, demonstrate that a directional antenna system comprising a dielectric waveguide and a spring as the adjustable periodic structure will perform in accordance with this invention and that beam steering can be accomplished by adjusting the period or spacing of the conductive coils of the spring. It was found that the best radiation efficiency was obtained if the brass rod 33 was of a length comparable to the length of the spring 32. Thus, three different rod lengths were used to accommodate the variation in the length of the spring of 66 mm ± 50 percent to achieve the resultant change in period I and beam angle θ. In essence, the "diffraction electronics" antenna is a self-excited array where each element of the periodic structure is driven in series. It would be expected that the total length of the array, L, as shown in Figure 3, would dictate the half-power beamwidth of the radiated pattern. The half-power beamwidth, Δθ in radians, should be approximately Δθ λ = A esc θ (6) L sin θ L In the aforementioned experiment with the spring 32 as in Figure 3, the 3dB beamwidth was measured at four spring positions or lengths L. The values are plotted in Figure 7 and the results are consistent with equation (6). • . This invention has been demonstrated and illustrated in the simplest embodiment in which the adjustable periodic structure can be a conventional helical spring 32 as shown in Figure 3. The term "spring" is used in the broadest sense that will accomplish the function intended and might have a slightly different configuration than a conventional spring in some applications. For example. Figure 8 shows an enlarged partial axial view of a spring 32* on a core 33 that has been flattened slightly on the surface 36 adjacent the waveguide 28 so that the distance ω is more uniform under the width of the waveguide compared to the variable distance of a conventional spring shown as a dashed line 38 in Figure 8. Figure 9 shows an alternative modification of a spring 32" in which a portion of the helical coil has an upraised flat surface 36' parallel to the waveguide which is advantageous if the spring diameter is small relative to the width of the waveguide. Figure 10 shows an alternative modification in which the coils of the spring 39 are rectangular in shape rather than circular and the wire cross section can be rectangular rather than circular and the cross section of the core 33' can be rectangular rather than circular which has the advantage of maintaining the flat surface 36" parallel to the waveguide 28 as the individual coils of the spring 39 slide along the core 33' during adjustment of the coil spacing. For the embodiments of this invention in which the adjustable periodic structure is a spring as shown in Figures 3,8,9, and 10, the controlling means comprises means for varying the length of the spring and thus the spacing between corresponding points on successive turns and thus the period I . The controlling means is typically an actuator 34 as indicated in Figure 3 that provides essentially axial movement of the spring in either direction when connected at one end of the spring with the other end fixed by a stop 35. Various types of linear actuators can be used such as pneumatic actuators, hydraulic actuators, motorized screw actuators, solenoid actuators, or other means known to those skilled in the art. Figure 11 shows an alternative embodiment of this invention wherein the adjustable periodic structure 40 is a bellows-like flexible member of which the outer edges 41 form the spaced-apart positions. Figure 12 shows an alternative embodiment in which the adjustable periodic structure is a bellows 42 and ' the controlling means can be an air supply 43 to affect by variation in air pressure the length of the bellows 42 and the spacing of the outer edges 44 of the bellows. Figure 13 shows an alternative embodiment of this invention in which the adjustable periodic structure is a kinematic chain 45 comprising a plurality of isosceles drag links 46 (lazy tongs) with the conductive portions 47 linked to the drag links at approximately equally spaced intervals. Figure 14 shows an alternative embodiment of the invention in which the adjustable periodic structure comprises approximately equally spaced identical polyhedron 48 which are internal threaded to fit on the external threads of a cylindrical member 49 having a plurality of portions threaded with threads of variable pitch. The first polyhedron 48 which need not move along the cylindrical member 49 can be on an unthreaded portion of the cylinder 49'. The second polyhedron 48' is mounted on an adjacent threaded portion 50 of the cylinder having x threads per unit length; the second polyhedron 48" is mounted on the next adjacent portion of the cylinder 50' having 2x threads per unit length and so on to a last polyhedron 48 n mounted on a last [(n-l)th] threaded portion 50 11 "" 1 of the cylinder having (n-l)x threads per unit length. The polyhedrons 48,48' ,48", 48 n are constrained by guiding means 51 for prevention of rotation of the polyhedrons while allowing axial movement of the polyhedrons along the cylindrical member 49. Controlling means 34 are provided for rotating the cylinder 49 and effecting a variation of spacing of the polyhedrons and the conductive surfaces 47' adjacent a dielectric waveguide not shown in Figure 14. Figures 15 and 16 show an alternative embodiment of the present invention wherein the adjustable periodic structure comprises spaced lines 52 of conductive ink on a non-conductive paper tape 53 as shown in Figure 16 that is moved at high speed between a supply spool 54 and a take-up spool 55 as shown in Figure 15. The lines of conductive ink can be formed practically instan¬ taneously on the tape by a high-speed jet printer 56 before the tape moves approximately parallel to the axis of the adjacent waveguide 28. The spacing of the lines 52 of conductive ink is controlled by controlling the speed of the tape 53. The embodiment of the invention shown in Figures 15 and 16 allows rapid changes of beam angle θ. For example, a high-speed jet printer 56 can produce approximately 10,000 lines per second and if 50 lines are used- to set the beam angle, approximate 200 changes in beam angle could be accomplished per second. For a period, £ = 2 mm, the tape speed would be on the order of 20 meters per second. An alternative modification of the aforementioned embodiment would use a preformed tape containing conductive spaced parallel lines on a non-conductive tape as might be produced by a conductive-ink jet printer or other means. Portions of the tape (e.g. approximately 100 mm in the prior example) approximately the length of the waveguide would contain equal spaced lines of period £ and an adjacent portion of the tape would contain equal spaced lines of slightly different period, £ + Δ£, and so on such that the total tape contained a plurality of portions with conductive lines of slightly different periods. Means for movement of the tape between the two spools to a position such that the portion of the tape with lines of the desired period, £, was adjacent the waveguide would control the structure period and beam-pointing angle θ. An alternative modification of the aforementioned preformed tape is shown in Figure 17 where the adjustable periodic structure consists of portions of tape 57 comprising parallel perforations 58 separating parallel conductive lines 59 which could be preformed on suitable tape (e.g. thin metal tape, metal-coated tape, or conducti coating on tape) by mechanical slotting, electroforming, or other means known to those skilled in the art. Figures 18,19, and 20 show an alternative embodiment of the invention wherein the adjustable periodic structure comprises a photoconductive surface 60 with periodic conductive bands produced by light from a source 61 through moire fringe bands 62 resulting from overlapping transparent ruled gratings 63,64. The gratings 63,64 are adjacent and parallel to the dielectric waveguide 28 over its length and fixed at one end by a bond for flexible gratings or by a pivot for rigid gratings and movable at the other end relative to each other in a plane parallel to the grating surfaces. The alternating light bands 62' and dark bands 62 with a period £ as shown in Figure 20 result from relative movement of the gratings through a distance x per unit length of grating L'. The light bands 62' which produce conductive bands in the photoconductive layer are oriented perpendicular to the waveguide axis 69 if the relative movement of the gratings is symetrical with respect to the waveguide axis 69 (i.e. x* = x" in Figure 20). The relative movement x/L', of the gratings required is small and can be accomplished by several' means such as the simple lever device 65 shown in Figure 19 connected to a suitable actuator to twist lever 65 at pivot 66. For example, if the parallel grating lines 67,68 have a period of 6 micrometers, a relative movement of x = 0.042 mm will produce 8 fringes as shown in Figure 20 with a period £ = 2.2 mm over a distance L' = 15.4 mm. Thus, for a length of waveguide and gratings of 100 mm, fringe periods £ of 1.40,2.20, and 3.20 mm require relative movements x of 0.428,0.273, and 0.187 mm respectively. Figures 21,22, and 23 show alternative embodiments of the invention wherein the beam steering of the antenna is accomplished by non-mechanical means. The adjustable periodic structure comprises a thin photoconductive surface 60 on a transparent substrate 70 with periodic conductive bands 62' produced in the photoconductive material by light rays 71,71',80 that are uniformly spaced an adjustable distance £ where they strike the surface of the photoconductor 60. There are several methods of controlling the light rays 71,71*, 80 in the respective embodiments. In Figure 21, a laser 90 or other light source is diffracted into a plurality of separate light beams 71 by an acousto-optical device 73. The latter device 73 is sometimes referred to as a Bragg cell and has been used for spectrum analysis. The Bragg cell is usually a block of glass or σyrstalline material such as lithium niobate approximately 1 cm x 1 cm in cross section and up to 10-20 centimeters long. A piezoelectric transducer 74 is bonded to the end. When the transducer 74 is excited with an electrical signal 75, a traveling acoustic wave 76 is set up in the material.. The acoustic - energy causes slight changes in the refractive index between the peaks 76 and valleys of the acoustic pressure. When light 72 is introduced at the correct angle, β, the reflections from the index change, add in phase, and Bragg diffraction takes place. A portion of the input light beam 72 is deflected and can be imaged on the photoconductive surface 60. The angle of deflection is proportional to the sound frequency generated by the acoustic transducer 74 in response to the drive signal 75. If there are multiple signals at different frequencies in the drive signal 75, there will be multiple defracted light beams 71. Thus, if the multiple signals in the drive signal 75 are a uniform series of frequencies f, f + Δω, f + 2Δω, f + 3Δω... f + nΔω, a plurality of diffracted light beams 71 of angles , a + Δx, α + 2Δx, α + 3Δx, ... " + nΔx will produce uniformly spaced conductive bands 62' in the photoconductive surface 60. Since the spacing of conductive bands £ is proportiona to the incremental frequency Δω, the value of £ can be adjusted by adjusting the incremental frequency Δω to accomplish beam steering by change in angle θ of the beam relative to the axis of the dielectric waveguide. An alternative modification, using a similar acousto- optic device 73 as in Figure 21, operates as a scanning device as shown in Figure 22. The laser beam 72 is rapidly diffracted through successive angles α, α + Δx, α + 2Δx, α + 3Δx, α + nΔx by successive changes in frequency f, f + Δω ' , f + 2Δω, f + 3Δω, f + nΔω of the drive signal 75' to the piezoelectric transducer 74. The sweep rate (complete cycle of discrete frequencies) of the diffracted beam 71* is made fast relative to the inherent decay rate of conductivity in the illuminated; photoconductive material. Figure 23 shows an embodiment in which the light source is a laser diode array 77 comprising a plurality of individual laser diodes 78 which are selectively activated by a computer 79 to produce individual light rays 80. A convex lens 81 is used to focus the light rays 80 on the photoconductive surface 60. A sufficient number of laser diodes 78 are used in the array 77 such that by computer-controlled electrical activation of selected laser diodes, periodic conductive bands 62' with adjustable period £ are produced in the photo¬ conductive surface 60 to.steer the beam 82 through an angle θ. In Figures 21,22, and 23, the beam 82 is shown schematically as emanating from the dielectric waveguide 28 in a direction 82 opposite from the periodic conductive structure 60.. It is to be understood that the antenna also radiates power in the opposite direction 82' (i.e. at an angle θ* = θ + 180 degrees) as shown in Figures 24 and 25. In Figures 24 and 25, the periodic conductive surface 83 is depicted as a segmented line in close proximity to the dielectric waveguide 28. Evanescent mode coupling causes a surface excitation on the periodic structure 83. when the periodic structure is very thin as with light activated conductive bands 62' in a thin photoconductor 60 (Figures 20-23) that provides little blockage for a beam, power is radiated in the direction 82* as well as direction 82 as shown in Figures 24 and 25. When the surfaces 83 of the periodic structure are part of massive metal 30 as in Figure 2 or similar beam blocking structures (e.g. wire 32 on metal core 33 in Figure 3), the principal radiated power is in the beam direction 82 at angle θ with little or no radiated power in beam direction 82' at angle θ'. As shown in Figures 24 and 25 beam angles are arbitrarily measured from the load end of the dielectric waveguide 28 opposite the feed end with power feed to the antenna indicated by the directional arrow 84. Usually, the adjustable periodic structure 83 is positioned on the opposite side of the dielectric waveguide 28 from the desired beam propagation direction 82 to avoid interference with the beam as shown in Figure 24. For adjustable periodic structures such as shown in Figures 21,22, and 23, the photoconductive surface 60 can be located on top of the dielectric waveguide 28 and, by the use of mirrors or other means to direct the light rays to the photoconductive surface 60, minimize interference with the beam 82 propagated in the direction of the adjustable periodic structure 83 as shown in Figure 25. For example, in experimental studies using apparatus as shown in Figure 1, a photoconductive surface 60 was positioned on top of the dielectric waveguide 28 and in close proximity (ω a 1 to 5 mm). The photo¬ conductive surface 60 comprised n-type amorphous silicon (dark resistivity of approximately 2000-3400 ohm-cm) in a film 1 micrometer thick on transparent substrate 70 of glass (microscope slide). The opposite surface of the substrate 70 (upper surface in this experiment) contained a mask of opaque grating lines alternating with transparent lines to provide a period £ ~~ 2.1 mm. The photoconductive surface was illuminated by a remote laser (Nd:YAG, 1.06 micrometer, 16 millijoules per pulse, 10 pulses per second, 3 nanoseconds per pulse). The laser beam was expanded so as to fill a reflecting mirror 6.3 cm in diameter. The reflected beam passed through a cylindrical lens (400 mm focal length) so that a rectangular area of 6 mm by 63 mm was illuminated near the polystyrene waveguide 28. The grating produced 30 light bands of 1 mm by 3 mm and a period of £ = 2.1 mm, each receiving approximately 20 microjoules per laser pulse. For this period, the measured beam angle in the apparatus of Figure 1 was θ = 76 degrees since the periodic conductive surface 83 was on top as in Figure 25. If the same periodic conductive surface 83 had been used under the antenna 28, as in Figure 24, the measured beam angle θ would have been 104 degrees which corresponds to data in Figure " 6 for similar conditions of the klystron source 23 operating at a frequency of 94 Ghz. The experiment demonstrated that a periodic structure comprising light induced conductive bands in a photoconductor surface 60 can be used for beam steering with results similar to those achieved with a periodic brass structure 30 as in Figure 2. The use of a photoconductive surface with equally spaced conductive bands adjustable by computer control of the light source, as in Figure 23, is advantageous for a beam steerable antenna with rapid adjustment of beam angle by non-mechanical means. Figures 26 and 27 show an alternative embodiment of the invention wherein beam steering of the antenna is accomplished by non-mechanical means using cathode- ray-tube (CRT) technology. The adjustable periodic structure comprises a thin semiconductor film 92 with periodic conductive bands 94 produced therein by an electron beam 90 where it strikes the surface of the semiconductor film 92. This embodiment of the invention uses an electron beam 90, rather than light rays 71,71' ,80, as in Figures 21,22, and 23, and thus the adjustable periodic structure 92 must be enclosed in a vacuum tube 86. CRT technoloy is used to control the location of the conductive bands 94 that are uniformly spaced an adjustable distance £. The cathode ray tube 85 comprises a vacuum tube 86 typically constructed of glass, an electron gun 88 typical of those used in laboratory oscilloscopes, and means 87,87' for deflecting the electron beam 90. Beam control means 87' is used to write a conductive band 94 in the semiconductor film 92 as shown in Figure 27. Beam control means 87 as shown in Figure 26 is used to write a plurality of spaced conductive bands 94 in the semiconductor film 92 during a single scan of the electron beam 90. The number of conducting bands 94 written across the semiconductor film 92 and hence their period £ determines the beam steering angle θ of the radiated beam 82. Methods of providing appropriate electrical signals to the beam control means 87,87' to obtain the desired written electron beam pattern of conductive bands 94 on the semiconductor film 92 are well known in CRT technology. For example, the width and spacing of the conductive bands 94 can be controlled electronically. The conductive bands 94 can be any width by writing a plurality of closely spaced conductive lines with the electron beam 90. Typically the width of the conductiv bands 94 is approximately one-half of the period £. The persistence of the conductive bands 94 of the electron beam pattern written on the semiconductor 92 would be one lifetime of the semiconductor excess carriers for a single scan of the beam 90, or indefinitely long for multiple writing of the same pattern. The persistence of the conductive bands should be comparable to the scan time so that the period £ can be changed quickly when desired (e.g. microseconds). The use of cathode-ray-- tube technology provides a method of achieving high beam steering rates with rapid adjustment of the period £ of the periodic structure by non-mechanical means. In a typical embodiment of the invention as shown in Figures 26 and 27, the dielectric waveguide 28' is an integral part of the cathode ray tube 85 forming the faceplate of the vacuum tube 86 with vacuum-tight seals 91. For this embodiment of the invention, the material of the dielectric waveguide 26' should be compatible with the remainder of the vacuum tube 86 to produce vacuum-tight seals 91. Preferred materials for the waveguide 28' are glass, quartz, a ceramic such as aluminum oxide or any other low-loss dielectric including polyethylene. In Figures 26 and 27, the dielectric waveguide 28' is the faceplate of the vacuum tube 86. Alternatively, the vacuum tube 86 could completely surround the waveguide 28*. For example, a typical faceplate (not shown in Figures 26 and 27) for a CRT such as glass could be positioned on the opposite side of the waveguide from 5 the adjustable periodic structure 92, spaced from, and essentially parallel to, the waveguide 28'. The radiated beam 82 would propagate through said faceplate. The semiconductor 92 is typically a thin film on the order of 1 micron thickness of a material such 0 as silicon or gallium arsenide and must be supported in some manner that positions it the optimum distance from the dielectric waveguide 28'. In one embodiment of the invention as shown in Figures 26 and 27, the support 93 can be a thin layer of glass on which the 15 film of semiconductor 92 is deposited on the side of the support opposite the waveguide 28'. The support 93 is shown attached to the walls of the vacuum tube 86. Alternatively, the support 93 could be attached to the waveguide 28'. The support 93 can be designed 20 with thickness for rigidity where it is attached to the walls of the vacuum tube and around the borders of the semiconductor 92 to form a picture frame. However, over the principal area of the semiconductor 92, the support 93 should be thin so that the conductive:.bands 25 94 in the semiconductor 92 can be effective in influencing beam steering of the waveguide, and so that the support 93 does.not act as a coupled waveguide and.divert microwave energy from the principal dielectric waveguide 28*. As discussed previously the nominal spacing (ω -30 i Figure 2) between, the waveguide 28* and the adjustable periodic structure or semiconductor 92 in Figure 26 is small and on the order of 1 mm for optimum antenna performance. Thus, the support 93 should be on the order of 1 mm in thickness or less. This precludes use of the glass faceplate of the typical vacuum tube 86 as the support 93 for the semiconductor 92. As mentioned previously, the dielectric waveguide 28' which serves as the faceplate of the vacuum tube 86 can be made of glass or other materials. An important design consideration is that the material of the support 93 in the area of the semiconductor 92 have a lower dielectric constant than the waveguide 28' for best performance. Thus, if the waveguide 28' is glass, the material of the support 93 over the area of the semiconductor 92 can be made of porous glass or air-foamed glass that has a low dielectric constant relative to solid glass. For thin adjustable periodic structures 83 as shown in Figure 24, there is a complementary beam 82' at angle θ'. In Figures 26 and 27, the outside walls of the vacuum tube 86 can be coated with microwave beam adsorber material or reflector material so as to maximize the power in the beam 82 at angle θ. An advantage of the embodiment of the invention based on CRT technology as shown in Figures 26 and 27 is that the amount of power in the beam 82 radiated by the waveguide 28' (or received) can be controlled by the amount of conductivity in the bands 94 in the semiconductor 92. For example, by control of the power supplied to the electron gun 88, the conductivity of the bands 94 can be varied, although the relationship between power and conductivity may not be linear. Alternatively, the conductivity of the bands 94 can be controlled by the number of closely spaced lines written by the electron beam 90 for a single conductive band 94. In addition, bands in the center of the periodic pattern could be written more strongly than the bands at the edge of the pattern so as to affect excitation amplitude taper and side-lobe control of the radiated pattern. The above concept of controlling beam power (radiating • or receiving) can also be applied to other embodiments of this invention which utilize photoconductors and light to form conductive bands in the adjustable periodic structure by control of the intensity of the light source (e.g. light source 61 in Figures 18 and 19; laser light source 90 in Figures 21 and 22; laser light diodes 78 in Figure 23) . While the forms of the invention herein disclosed constitute presently preferred embodiments, many others are possible. It is not intended herein to mention all of the possible equivalent forms or ramifications of the invention. It is to be understood that the terms used herein are merely descriptive rather than limiting, and that various changes may be made without departing from the spirit or scope of the invention.

Claims: CLAIMS 1. A directional antenna system comprising elongate dielectric waveguide means for transmitting and/or receiving electromagnetic energy of a frequency at which the waveguide propagates, adjustable means having a plurality of conductive portions spaced apart along a lengthwise region adjacent and substantially parallel to the waveguide means, and controlling means to vary over a continous range the spacing between the said conductive portions and thus to determine a characteristic of directionality in the system. 2. An antenna system as in Claim 1, wherein the conductive portions are substantially equally spaced apart. 3. An antenna system as in Claim 1, wherein the controlling means serves to determine a direction of maximum or minimum radiation or'receptive sensitivity. 4. An antenna system as in Claim 1, wherein the controlling means varies also the distance between at least some of the conductive portions and the waveguide means. 5. An antenna system as in Claim 4, wherein the controlling means serves to control the shape of the radiation pattern in the system. 6. An antenna system as in Claim 1, wherein the adjustable means comprises a conductive cylindrical helix of substantially round wire on a cylindrical core. 7. An antenna system as in Claim 6, wherein a portion of each turn of the conductive cylindrical helix comprises a substantially flat surface adjacent and approximately parallel to the dielectric waveguide. 8. An antenna system as in Claim 1, wherein the adjustable means comprises a conductive helix of rectangular turns of wire of rectangular cross section on a metal core of rectangular cross section. 9. An antenna system as in Claim 6, wherein the controlling means comprises means for varying the length of the helix and thus of the spacing between corresponding points on successive turns thereof. 10. An antenna system as in Claim 1, wherein the adjustable means comprises at least an outer folded portion of a bellows-like flexible member of which the outer edges form the conductive spaced-apart portions. 11. An antenna system as in Claim 10, wherein the controlling means comprises means for selectively expanding and contracting the flexible member and thus varying the spacing between the outer-edge conductive portions. 12. An antenna as in Claim 1, wherein the adjustable means comprises a bellows, the outer edges of which form the conductive spaced-apart portions. 13. An antenna system as in Claim 12, wherein the controlling means comprises means for selectively expanding and contracting the bellows and thus varying the spacing between the outer-edge conductive portions. 14. An antenna system as in Claim 13, wherein the means for selectively expanding and contracting the bellows comprises a fluid inside the bellows and means to change the pressure of said fluid. 15. An antenna system as in Claim 1, wherein the ajustable means comprises a kinematic chain. 16. An antenna system as in Claim 15, wherein the kinematic chain comprises a plurality of isosceles drag links (lazy tongs). 17. An antenna system as in Claim 16, wherein the conductive portions are linked to the drag links at approximately equally spaced intervals. 18. An antenna system as in Claim 17, wherein the controlling means comprises means for selectively expanding and contracting the drag links and thus varying the spacing between the conductive portions. 19. An antenna system as in Claim 1, wherein the adjustable means comprises a rotatable externally threaded cylindrical member. 20. An antenna system as in Claim 19, wherein the conductive portions comprise corresponding surfaces on a plurality of approximately identical polyhedrons internally threaded to fit on the external threads of the cylindrical member. 21. An antenna system as in Claim 20, wherein the polyhedrons are equally spaced along the cylindrical member in threaded engagement therewith and constrained by guiding means for preventing rotation of the polyhedrons while allowing movement of the polyhedrons along the cylindrical member. 22. An antenna system as in Claim 21, comprising also a first polyhedron that is not movable along the cylindrical member and wherein the next (second) polyhedro is mounted on the adjacent (first) threaded portion of the cylindrical member, having x threads per unit length; the next (third) polyhedron is mounted on the next adjacent (second) threaded portion of the cylindrical member, having 2x threads per unit length; and so on to a last (n th) polyhedron mounted on a last ((n-l)th) threaded portion of the cylindrical member, having (n-l)X threads per unit length. 23. An antenna system as in Claim 1, wherein the adjustable means comprises means to produce spaced lines of conductive ink on a non-conductive tape. 24. An antenna as in Claim 23, .wherein the means to produce spaced lines of conductive ink comprises a high-speed jet printer and means to move the non- conductive paper tape under the jet printer. 25. An antenna system as in Claim 24, wherein the means to move the non-conductive tape comprises a supply spool and a take-up spool and means to rotate the take-up spool. 26. An antenna system as in Claim 25, wherein the controlling means comprises means to control the rate of rotation of the take-up spool and thus the speed of the non-conductive tape and the spacing of the lines of conductive ink thereon. 27. An antenna system as in Claim 1, wherein the adjustable means comprises a tape with conductive lines of equal spacing over a portion of the tape greater in length than the dielectric waveguide and slightly different period on adjacent portions of said tape and the controlling means comprises means for moving said portions of tape between a supply spool and a take-up spool. 28. An antenna system as in Claim 27, wherein the controlling means comprises means for positioning selected portions of the tape with equally spaced conductiv lines adjacent the dielectric waveguide. 29. An antenna system as in Claim 27, wherein the conductive lines are conductive ink on a non-conductive tape. 30. An antenna system as in Claim 27, wherein the conductive lines are conductive material separated by slots in the tape. 31. An antenna system as in Claim 1, wherein the adjustable means comprises a photoconductive surface illuminated on one side by light directed through transparen closely-spaced, overlapping ruled gratings with lines substantially parallel to the axis of the dielectric waveguide, said gratings being fixed at one end and free to move relative to each other at the other end to form moire fringe bands oriented essentially normal to the axis of the waveguide. 32. An antenna system as in Claim 31, wherein the controlling means controls the angle between grating lines on the respective gratings and thus the spacing of moire fringe bands and bands of light striking the photoconductive surface to produce equally spaced conductive portions therein.- 33. An antenna system as in Claim 1, wherein the adjustable means. comprises a photoconductive surface, a source of light, and the controlling means comprises means for directing light from the source to provide and to control the spacing of conductive portions of said surface. 34. An antenna system as in Claim 33, wherein the means to control the spacing of the conductive portions comprises acousto-optical means including means for varying the frequency thereof. 35. An antenna system as in Claim 34, wherein the means for varying the frequency of the acousto-optical means comprises drive signal means for providing a plurality of discrete frequencies separated by adjustable differential frequencies. 36. An antenna system as in Claim 35, wherein the means for varying the frequency of the acousto-optical means comprises drive signal means for providing a sequence of a plurality of discrete frequencies separated by adjustable differential frequencies. 37. An antenna system as in Claim 33, wherein the source of light comprises an array of laser diode means and focusing optics means, and the means to control the spacing of the conductive portions comprises computer means for selectively activating appropriate diodes in said array. 38. An antenna system as in Claim 1, wherein the adjustable means comprises a semiconductor surface in a vacuum tube, a source of an electron beam, and the controlling means comprises means for deflecting the electron beam from the source to provide and to control the spacing of conductive portions of said surface.

Inventors: CLIFFORD JOHN E, SEILER MILTON R, WINSOR HARRY V, CLIFFORD, JOHN, E., SEILER, MILTON, R., WINSOR, HARRY, V.
Applicants: BATTELLE MEMORIAL INSTITUTE
Application year: 1986
Main classification: H01Q3
IPC codes: H01Q3, H01Q13