Patent Publication Number: US-6703982-B2

Title: Conformal two dimensional electronic scan antenna with butler matrix and lens ESA

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to antennas. More specifically, the present invention relates to electronically scanned antennas. 
     2. Description of the Related Art 
     Seekers are used to sense electromagnetic radiation. For certain applications, there is a requirement for at least two seekers. For example, in the missile art, there is a need for an infrared (IR) seeker and a radio frequency (RF) seeker. As both seekers must be mounted in the nose of the missile, one typically at least partially obscures the field of view of the other. The IR seeker not only creates a blind spot for the RF seeker, but also, degrades the field radiation pattern of the antenna thereof. 
     The situation is exacerbated by the fact that there is a trend toward the use of higher frequency seekers to achieve higher levels of performance in target detection and discrimination. While current RF seekers operate in the X band (8 to 12 GHz), these newer seekers are planned to operate in the Ka band or the W band (27 to 40 GHz). However, a need would remain for the X band capability. Hence, two antennas are required giving rise to the aforementioned problem of occlusion. 
     Accordingly, there is a need in the art for a system or method for integrating two or more seekers into a single housing in such a manner that neither seeker interferes with the operation of the other. 
     SUMMARY OF THE INVENTION 
     The need in the art is addressed by the antenna and antenna excitation method of the present invention. The inventive antenna includes an array of radiating elements, each of the elements being mounted at a predetermined substantially transverse angle relative to a longitudinal axis and a circuit for providing an electrical potential between at least two of the elements effective to scan a transmit or a receive beam of electromagnetic energy along an elevation axis at least substantially transverse to the longitudinal axis. 
     In the illustrative embodiment, the array includes a stack of the planar, parallel, conductive, ring-shaped radiating elements, each of which is filled with ferroelectric bulk material. Space matching material is disposed on the inner and outer periphery of each element. 
     A second circuit is included in the specific implementation for exciting at least some of the elements to cause the elements to generate a transmit or a receive beam of electromagnetic energy off-axis relative to the longitudinal axis. In the preferred embodiment, the second circuit is a Butler matrix and is effective to cause the beam to scan in azimuth about the longitudinal axis, the azimuthal axis being at least substantially transverse to the longitudinal axis and the elevational axis. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified sectional view of a nose cone of multi-mode missile constructed in accordance with conventional teachings. 
     FIG. 2 is a block diagram of a multi-mode antenna constructed in accordance with the teachings of the present invention. 
     FIG. 3 is a simplified disassembled perspective side view of the lens array of FIG.  2 . 
     FIG. 4 is a top view of a single radiating element of the array depicted in FIG.  3 . 
     FIG. 5 is a sectional side view of a portion of the plate depicted in FIG.  4 . 
     FIG. 6 is a diagram showing a portion of the binary feed of depicted in FIG.  2 . 
     FIG. 7 is a diagram which shows how the Butler matrix is connected to a single radiating element in accordance with the present teachings. 
     FIG. 8 is a simplified diagram which illustrates an arrangement by which the outputs of the Butler matrix are connected to each of the radiating elements of the array of the antenna of the present invention. 
     FIG. 9 is a diagram showing a monopulse arrangement with a Butler matrix and a cylindrical lens electronic scan array in accordance with the present teachings. 
    
    
     DESCRIPTION OF THE INVENTION 
     Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention. 
     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
     FIG. 1 is a simplified sectional view of a nose cone of multi-mode missile constructed in accordance with conventional teachings. As shown in FIG. 1, the missile  10 ′ has a nose cone  12 ′ within which an RF seeker  14 ′ is mounted. Electromagnetic energy  16 ′ radiated (or received) by the seeker  14 ′ is at least partially blocked by an IR seeker  18 ′ disposed at the distal end of the nose cone  12 ′. Hence, FIG. 1 illustrates the need in the art for a system or method for integrating two or more seekers into a single housing in such a manner that neither seeker interferes with the operation of the other. 
     As mentioned above, the need in the art is addressed by the antenna and antenna excitation method of the present invention. As discussed more fully below, the inventive antenna includes an array of radiating elements, each of the elements being mounted at a predetermined, substantially transverse, angle relative to a longitudinal axis and a circuit for providing an electrical potential between at least two of the elements effective to scan a transmit or a receive beam of electromagnetic energy along an elevation axis at least substantially transverse to the longitudinal axis. In the illustrative embodiment, the array includes a stack of the planar, parallel, conductive, ring-shaped radiating elements, each of which is filled with ferroelectric bulk material. Space matching material is disposed on the inner and outer periphery of each element. A second circuit is included in the specific implementation for exciting at least some of the elements to cause the elements to generate a transmit or a receive beam of electromagnetic energy off-axis relative to the longitudinal axis. In the preferred embodiment, the second circuit is a Butler matrix and is effective to cause the beam to scan in azimuth about the longitudinal axis, the azimuthal axis being at least substantially transverse to the longitudinal axis and the elevational axis. 
     FIG. 2 is a block diagram of a multi-mode antenna constructed in accordance with the teachings of the present invention. The antenna  10  includes a conformal (body-fixed) phased array of radiating elements  20 . 
     FIG. 3 is a simplified disassembled perspective side view of the lens array of FIG.  2 . The principal element of the lens array  20  is a TEM mode transmission line that has a parallel plates filled with ferroelectric bulk material. For a conformal array, the lens array  20  is a cylindrical shape. As shown in FIG. 3, the array  20  includes a stack of planar, parallel, ring-shaped plates of conductive material of which n are shown in FIG. 3 ( 22 ,  24 ,  26 ,  28  and  29 ). In the illustrative embodiment, the plates are made of gold or other suitable conductor. 
     FIG. 4 is a top view of a single radiating element of the array depicted in FIG.  3 . As illustrated in FIGS. 3 and 4, the plates are filled with ferroelectric material  23  and include an inner ring  25  and an outer ring  27  which provide space matching transformers. The dielectric constant of a ferroelectric material changes with the applied DC bias voltage and the phase of RF wave passing through the lens array changes as a function of the applied DC bias voltage. Hence, the stacked cylindrical lens elements will scan in elevation by setting proper DC biases to the cylindrical lens elements. 
     FIG. 5 is a sectional side view of a portion of the plate depicted in FIG.  4 . The space matching transformers may be made of high dielectric material or parallel plates. The function of the space matching elements is to radiate all the RF energy to the space. Those skilled in the art will appreciate that the invention is not limited to the size, shape, number or construction of the radiating elements  22 ,  24 ,  26 ,  28  and  29 . Numerous other designs may be used for various applications. 
     As will be appreciated by one of ordinary skill in the art, the use of ferroelectric material is advantageous in that on the application of an applied DC voltage, the dielectric constant of the material changes and effects a change in the elevation of the output beam radiated from the element as illustrated in FIG.  3 . That is, the microwave propagation velocity in the parallel plates varies as a function of the DC voltage bias between plates, as the dielectric constant of the ferroelectric material varies accordingly. As a result, the phase of an incoming RF signal is changed by the lens element according to its DC bias. When a stacked array of lens elements are biased with a proper set of DC bias voltages and are fed by a planar array, the output of the array will be scanned in one dimension. 
     Typical ferroelectric materials include BST (beryllium, strontium tetanate composit, liquid crystals, etc.). Those skilled in the art will appreciate that the present invention is not limited to the use of ferroelectric material in the radiating elements. Any arrangement that provides a change in the elevational angle of an output beam, in response to an applied voltage may be used without departing from the scope of the present teachings. 
     Returning to FIG. 2, the voltage differential V n  between the plates is supplied by a source  30 . In practice, the source  30  may be a power divider circuit, a digitally controlled power supply or other suitable arrangement. The source is controlled by a system controller  40  in response to inputs received via an input/output circuit  50 . 
     Scanning of the output beam in azimuth is effected through the use of a multi-beam (e.g. Butler matrix) circuit as discussed more fully below. 
     As shown in FIG. 2, a transmit signal from an RF transmitter (e.g. traveling wave tube)  60  is directed by a circulator  62  to a 1:m power divider  64 . Each of the ‘m’ outputs of the power divider is connected to an associated input of a Butler matrix via a phase shifter arrangement including a fixed phase shifter  66  and a variable phase shifter  68 . Each output of the power divider thus provides an input to a mode input to the Butler matrix  70 . In the first mode, the signal applied to the first input is provided at each of ‘x’ outputs of the Butler matrix  70 . The outputs of the Butler matrix circuit are applied to the radiating elements of the cylindrical array  20  via a feed arrangement  80 . The feed arrangement  80  is shown more fully in FIG.  6 . 
     FIG. 6 is a diagram showing a portion of the binary feed of depicted in FIG.  2 . In FIG. 6, the binary feed  80  is rotated to show the section of the radiating plates or lens in perspective. The binary feed, may be a corporate feed, simple power divider, series feed or other suitable arrangement. As is evident from FIG. 6, the plates  22 ,  24 , etc. need not be circular or ring-shaped disks. Small, piece-wise rectangular radiating elements could be used around the periphery of a body or housing without departing from the scope of the present teachings. 
     FIG. 7 is a diagram which shows how the Butler matrix is connected to a single radiating element in accordance with the present teachings. In FIG. 7, only nine connections are shown between the Butler matrix  70  and the element  22 . In practice, for 360° azimuthal coverage, each of the outputs of the Butler matrix  80  is connected to a corresponding location on the plate  22 . Moreover, in the best mode, each output of the Butler matrix  80  is connected to the same location on each of the other radiating elements in the array  20 . This is depicted in FIG.  8 . 
     FIG. 8 is a simplified diagram which illustrates an arrangement by which the outputs of the Butler matrix are connected to each of the radiating elements of the array of the antenna of the present invention. As shown in FIG. 8, the Butler matrix converts a two-dimensional (2D) aperture distribution into a three-dimensional (3D) aperture distribution. 
     With the distribution depicted in FIGS. 7 and 8, a first beam  82 , with an associated aperture distribution  83 , is generated at a first angle of φ 1  in azimuth by using all the circular mode generated by Butler matrix with proper phase shifter arrangement for each mode and a second beam  84 , with an associated aperture distribution  85 , is generated at a second angle of φ 2  in azimuth in a second excitation condition. Thus, scanning in azimuth is effected by proper selection of the fixed and variable phase shifters and by applying a signal sequentially to each of the inputs to the Butler matrix. 
     Hence, azimuth scan is accomplished with the Butler matrix  70  and the variable phase shifters and elevation scan is accomplished with the cylindrical lens electronic scan array (ESA)  20  via a set of variable DC voltage biases. Each input port of the Butler matrix represents a different circular mode on a cylinder. The input and output of the Butler matrix are a discrete Fourier transform pair. Simple superposition of these circular modes provides a desired aperture distribution for an azimuth scan position. The aperture distribution in FIG. 7 indicates that all the energy is distributed only in the desired radiation direction including proper low side lobe taper. By assigning a new set of phases with the variable phase shifters, the same aperture distribution may be freely rotated around the array  20 . Each binary feed output spatially or contiguously feeds the input port (inner circle of the cylinder) of lens array  20 . 
     The system controller  40  provides azimuth and elevation scan control signals. Thus, in the illustrative application, the system of FIG. 2 accommodates a seeker  18  located at the nose cone  12  of a missile, without blocking the view of the conical/cylindrical conformal antenna  10 . 
     In short, the system depicted in FIG. 2 can be used for dual mode (IR &amp; RF or RF &amp; RF) seeker. In this embodiment the RF seeker can be either a sequential lobbing or a monopulse approach for target detection. 
     FIG. 9 is a diagram showing a monopulse arrangement with a Butler matrix and a cylindrical lens electronic scan array in accordance with the present teachings. The monopulse RF seeker can be realized with four Butler matrices with four extra phase shifter sets. The present teachings can be used for a dual mode seeker in an airborne missile, aircraft or stationary tracking system. 
     Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof. 
     It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention. 
     Accordingly,