Abstract:
The present invention provides a low-cost, steerable antenna formed with a dielectric medium separating a pair of conductive plates and a centrally located signal feed. Switches selectively interconnect the conductive plates through the dielectric medium in patterns, which determine the direction of operation of the antenna. The directionality of the antenna may be fixed or rapidly changed, depending upon the application.

Description:
RELATED APPLICATION 
     This application claims priority from U.S. Provisional Patent Application Serial No. 60/200,781 filed Apr. 28, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to parallel plate antennas and, more particularly, to steerable, circular parallel plate antennas. 
     BACKGROUND OF THE INVENTION 
     Modern communications applications at millimeter band frequencies often require the use of high gain, directional antennas. Typically, directional antennas have narrow beamwidths which requires that the antenna be pointed directly at the communicating device or apparatus. When communicating in another direction, the antenna must be physically rotated to point in the new direction. In some dynamic situations, the antenna might require turning (i.e., rotating) at a faster rate than can be achieved mechanically. One antenna that has been used for these millimeter wave applications is the “pillbox” antenna, which derives its name from its size and shape, with the addition of a horn protruding on one side. Such antennas typically have parallel upper and lower conductive plates between which an electrode is positioned orthogonally with respect to the parallel plates. An arcuate rear reflector extends between the parallel plates and surrounds a significant part of the electrode, giving the antenna its “pillbox” shape. Opposite the rear reflector, the sides of a horn also extend between the parallel plates to collect and feed energy to and from the electrode. 
     Alternatively, phased arrays can position beams rapidly by adjusting the phase of the arrayed elements. However, many wireless communications applications today do not need any more gain than can be provided by a single antenna element. Consequently, relatively expensive, phased array systems are not necessary for these kinds of applications. The inventive antenna provides a means for rapidly steering the beam of a single element antenna electronically and/or optically. 
     It is therefore an object of the invention to provide a low-cost, compact steerable antenna for operation in k-band to w-band applications. 
     It is a further object of the invention to provide a low-cost, compact, steerable antenna that is steered electronically or optically. 
     It is another object of the invention to provide a low-cost, steerable antenna which may be co-located to provide both transmit and receive functions (i.e., full-duplex operation). 
     It is a still further object of the invention to provide a low-cost, steerable antenna which may be used to provide simultaneous multipoint communications. 
     It is yet another object of the invention to provide a low-cost, steerable antenna which may be fed either passively with a probe or actively with an embedded resonator. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided a low-cost, steerable antenna formed with a semiconductor dielectric medium located between two substantially parallel conductive plates. The plates may be selectively interconnected through the dielectric medium in different patterns defining different directions of operation for the antenna. In one form, photonic energy is used to activate the semiconductor medium to interconnect the plates and a pattern of openings in one or more of the plates act as optical ports for the application of that photonic energy. Activation of the exposed semiconductor with light causes a conductive region to be formed in the semiconductor, thereby connecting the plates with the shape and directionality of the desired antenna. By controlling the activation pattern, the directionality is controlled. The directionality may be fixed or rapidly changed depending upon the application. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent detailed description, in which: 
     FIG. 1 is a perspective view of an antenna constructed in accordance with one embodiment of the present invention; 
     FIG. 2 is a cross-sectional view of the antenna shown in FIG. 1; and 
     FIG. 3 is a schematic view showing a stacked pair of the antennas shown in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present embodiment features a steerable, parallel plate antenna shown in FIGS. 1 and 2. Antenna  100  includes a pair of substantially parallel conductive plates  104  and  106  separated by a dielectric medium  102 . Antenna  100  is nominally circular in shape and receives radio frequency (RF) energy through a central feed  114 . The directionality or steering of the antenna  100  is controlled through a multiplicity of switching means located between the two conductive plates  104  and  106 . These switching means are located along the pattern of openings or ports  108 ,  110  shown in the upper plate  104 . Activation of selected groups of switch means creates conductive barriers  118  within the dielectric medium  102 , which confines RF energy between the barriers to and from the feed  114 . 
     In one embodiment the switching means are formed by using a dielectric semiconductor for the dielectric medium  102  and by coupling photonic energy into the semiconductor dielectric medium  102  through the openings  108 ,  110 . This photonic energy causes the creation of conductive barriers  118  between the upper and lower parallel plates  104 ,  106 , which conductive barriers  118  cause the channeling and reflection of RF energy located within the dielectric medium  102 . 
     In one embodiment, a cylindrical section of semiconductor wafer forms dielectric medium  102 . Semiconductor materials found satisfactory for this application are typically monolithic intrinsic silicon, gallium arsenide, indium phosphide, etc. High resistivity silicon (˜5000 ohm-cm) is preferred with minority carrier lifetimes on the order of one millisecond. By doping the silicon, the lifetime can be shortened, thereby allowing for faster switching but with more signal loss in the substrate. A range of other materials are known to those skilled in the semiconductor arts which are suitable for use in this application. 
     The thickness of dielectric medium  102  is approximately one-fourth of the wavelength of the signal at which the antenna  100  is intended to operate. This thickness may also be used to adjust the impedance of the dielectric material to help match the impedance of feed  114  with the impedance of the transmission medium surrounding antenna  100  (typically air). As long as this distance remains less than one half of the wavelength for the intended functional bandwidth of the antenna  100 , proper operation of antenna  100  will be enabled. Although the plates  104 ,  106  are shown as parallel some variation in their separation may occur in radial directions from the feed  114 , to further gradually adjust the impedance of dielectric medium  102  and better match it to the surrounding transmission medium. Additional impedance matching material may also be used around antenna  100  depending upon the dielectric medium  102  and the surrounding transmission medium. Impedance matching is helpful in reducing reflection of RF energy back into a transmitting antenna and/or signal loss for received signals. 
     Conductive plates  104 ,  106  may take the form of thin metallized layers on the top and bottom surfaces of a semiconductor dielectric medium  102 . Plates  104 ,  106  may be vacuum deposited, sputtered, plated or produced using any other method or technology known to those skilled in the semiconductor arts. 
     A pattern of holes or optical ports  108 ,  110  is etched in top metallized plate  104 , exposing the dielectric medium  102 . These ports  108 ,  110  are typically etched, but may also be formed in any manner known to those skilled in the semiconductor manufacturing arts. The surface of the exposed semiconductor is then passivated to maintain the lifetime of the material in the vicinity of the opening. 
     To complement conductive plates  104 ,  106  the pattern, spacing, size and shape of the optical ports  108 ,  110  define the remaining antenna reflectors and some of the antenna&#39;s electrical characteristics. Conductive plate  104  shows the optical ports  108 ,  110  arranged in a patter defining an antenna shape which may be pointed in different directions. The ports  108 ,  110  include an inner circle  109  of ports and a multiplicity of radial spokes  111 . The basic antenna pattern produced by this embodiment is a pillbox with a round reflector, formed by most of inner circle  109 , located around most of the feed  114  and a horn, formed by two adjacent radial spokes, extending from an open, or inactivated portion of the inner circle  109 . This shape is exemplified by the unshaded ports  108 , of which all but one of the ports in the inner circle would be illuminated and only two of the radial spokes would be illuminated. 
     Spacing or location of ports  108 ,  110  is dependent upon the intended frequency of operation for the antenna  100 . As shown in FIG. 2, the conductive barriers  118  take the form of conductive columns and do not necessarily form a complete conductive wall across the plates  104 ,  106  between adjacent ports  108 ,  110 . This limited application of photonic energy helps to save power consumption in the operation of antenna  100  but does not affect the performance of the antenna. So long as adjacent openings  108 ,  110  are located within one-half of a wavelength, the resulting conductive columns will be effective in forming the desired waveguide for RF energy. Preferably, openings  108 ,  110  are located approximately one-quarter wavelength apart at the intended frequency of operation for the antenna  100 . 
     Although each of the ports  108 ,  110  is representationally shown as a equal diameter circle, the shape and size of openings  108 ,  110  may be varied between different openings to further enhance performance of the antenna  100 . For example, openings located along the radial spokes  111  of the pattern may have varying sizes or shapes to further enhance impedance matching over the radial extent of the medium  102 . For this purpose, openings further away from the central feed  114  along the spokes may be made smaller. Note that ports  108 ,  110  are substantially identical, but have been shown in a contrasting manner for purposes of a functional example described below: spots  108  representing photonically-illuminated spots and spots  110  representing non-illuminated spots. 
     As mentioned, photonic energy is controllably provided to the openings or ports  108 ,  110  in order to activate excess minority conductors within the semiconductor dielectric medium  102  and thereby form conductive barriers  118  within the semiconductor medium between the parallel plates  104 ,  106 . This photonic energy may be delivered to the medium  102  by any suitable means. In one embodiment, the energy is delivered by optical fibers  112  to individual holes for openings  108 ,  110  from an optical source. Alternatively, individual laser diodes  113  may be located over each port  108 ,  110 . Any other suitable delivery medium for photonic energy may also be applied to the present antenna  100 . Further, LEDs might also be formed directly in the semiconductor dielectric medium  102  and receive activation energy through ports  108 ,  110 . 
     In one embodiment, optical fibers  112  are attached to the exposed silicon  102  at all ports  108 ,  110 . Activating light, typically laser illumination, may be supplied at a distal end on optical fibers  112  and conducted to dielectric medium  102  at etched ports  108 ,  110 . Laser light in approximately the 1 μm wavelength range has been found satisfactory. The activating light source can be light emitting diodes (LEDs) or laser diodes. Between 10 mW and 25 mW of optical power is required to activate the conductive regions. 
     The radio frequency (RF) signal feed  114  is disposed at or near the center of dielectric medium  102 . The shape and dimensions of signal feed  114  are dependent upon the impedances of the signal feed and the antenna  100  and may typically take the form of a probe, as shown, or a slot radiator, although any suitable element may be used. 
     In operation, antenna  100  has a signal of a predetermined radio frequency applied to feed  114 . Selective illumination of ports  108  causes the semiconductor dielectric medium (FIG. 2) beneath ports  108  to become conductive and form conductive barriers  118  between the plates  104 ,  106 . Conductive barriers  118  are reflective of RF energy so that barriers formed within the inner circle  109  of ports reflect RF energy to and from feed  114  while barriers  118  formed along spokes  111  of the pattern couple RF energy to and from the center circle. The predetermined directionality of the antenna  100  is dependent upon the spots  110  selected for illumination. By choosing different spots  110  for illumination, the directionality of antenna  100  may be changed. Moreover, by rapidly changing the selected spots  110 , antenna  100  may be easily redirected or even continuously swept. The speed of switching is limited by the minority carrier lifetime within the bulk material. For silicon, this is about 100-1000 microseconds. While a transmission operation has been described for purposes of disclosure, the inventive antenna  100  is equally suited for use as a directional receiving antenna. 
     Because the radiation pattern from antenna  100  is from the edge of silicon disk  102  at a region between illuminated spots  108 , two or more antennas  100  may be stacked for simultaneous transmission and reception (full-duplex communications) or for transmission and/or reception at multiple frequencies. Referring now to FIG. 3, there is shown a schematic representation of such an arrangement, generally at reference number  300 . A pair of the inventive antennas  100  is supported on a central support  302 . Fiber optic waveguides or strands  112  connect antennas  100  and a transmitter/receiver/controller  304  and the upper and lower antennas  100 , respectively. Support  302  could be configured to have a pedestal (not shown), a clamp (not shown), or even a pointed arrow  310  in which the antenna could be deployed in difficult to reach areas by a projectile launcher or even by dropping. 
     In alternate embodiments, more than two elements could be stacked to provide full duplex operation. This arrangement, however, would require a very complex central probe feed because one element is used for receive and the other for transmit. The probe would have to be that of a pipe within a pipe with the wider pipe penetrating only the first layer, and the next inside coax extending to the next level in the stack, etc. Isolation between the two antennas is important to minimize noise. 
     Another embodiment is an array. The feed probe just becomes a serial probe or wire with a connector below and above the wafer. The top connects to the bottom of the stacked element through an appropriate delay line. 
     In yet another embodiment as a transmitting antenna, the antenna could be fed by an active device such as an impatt diode resonator at the center of the antenna, instead of a probe. This would require that only a modulation signal and power be brought to the antenna. 
     Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention. 
     Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.