Patent Publication Number: US-7710326-B2

Title: Antenna clusters for active device reduction in phased arrays with restricted scan

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is related by subject matter to U.S. Application for patent Ser. No. 10/997,422, entitled “A Device for Reflecting Electromagnetic Radiation,” U.S. Application for patent Ser. No. 10/997,583, entitled “Broadband Binary Phased Antenna,” both of which were filed on Nov. 24, 2004, and U.S. Pat. No. 6,965,340, entitled “System and Method for Security Inspection Using Microwave Imaging,” which issued on Nov. 15, 2005. 
   This application is further related by subject matter to U.S. Application for patent Ser. No. 11/088,536, entitled “System and Method for Efficient, High-Resolution Microwave Imaging Using Complementary Transmit and Receive Beam Patterns,” U.S. Application for patent Ser. No. 11/088,831, entitled “System and Method for Inspecting Transportable Items Using Microwave Imaging,” U.S. Application for patent Ser. No. 11/089,298, entitled “System and Method for Pattern Design in Microwave Programmable Arrays,” U.S. Application for patent Ser. No. 11/088,610, entitled “System and Method for Microwave Imaging Using an Interleaved Pattern in a Programmable Reflector Array,” and U.S. Application for patent Ser. No. 11/088,830, entitled “System and Method for Minimizing Background Noise in a Microwave Image Using a Programmable Reflector Array” all of which were filed on Mar. 24, 2005. 
   This application is further related by subject matter to U.S. Application for patent Ser. No. 11/181,111, entitled “System and Method for Microwave Imaging with Suppressed Sidelobes Using Sparse Antenna Array,” which was filed on Jul. 14, 2005, U.S. Application for patent Ser. No. 11/147,899, entitled “System and Method for Microwave Imaging Using Programmable Transmission Array,” which was filed on Jun. 8, 2005 and U.S. Application for patent Ser. Nos. 11/303,581, entitled “Handheld Microwave Imaging Device” and 11/303,294, entitled “System and Method for Standoff Microwave Imaging,” both of which were filed on Dec. 16, 2005. 
   This application is further related by subject matter to U.S. Application for patent Ser. No. 11/552,193, entitled “Convex Mount for Element Reduction in Phased Arrays with Restricted Scan” which was filed on Oct. 20, 2006, and U.S. Application for patent Ser. No. 11/551,382, entitled “Element Reduction in Phased Arrays with Cladding,” which was filed on Oct. 20, 2006. 
   BACKGROUND OF THE INVENTION 
   Various microwave imaging systems have been proposed to satisfy the demand for improved security inspection systems, such as those used in airports to screen passengers and baggage. At present, there are several microwave imaging techniques available. For example, one technique uses an array of microwave detectors (hereinafter referred to as “antenna elements”) to capture either passive microwave radiation emitted by a target associated with the person or other object or reflected microwave radiation reflected from the target in response to active microwave illumination of the target. A two-dimensional or three-dimensional image of the person or other object is constructed by scanning the array of antenna elements with respect to the target&#39;s position and/or adjusting the frequency (or wavelength) of the microwave radiation being transmitted or detected. 
   Microwave imaging systems typically include transmit, receive and/or reflect antenna arrays for transmitting, receiving and/or reflecting microwave radiation to/from the object. Microwave radiation is generally defined as electromagnetic radiation having wavelengths between radio waves and infrared waves. Such antenna arrays can be constructed using traditional analog phased arrays or binary reflector arrays. In either case, the antenna array typically directs a beam of microwave radiation containing a number of individual microwave rays towards a point or area/volume in 3D space corresponding to a voxel or a plurality of voxels in an image of the object, referred to herein as a target. This is accomplished by programming each of the antenna elements in the array with a respective phase shift that allows the antenna element to modify the phase of a respective one of the microwave rays. The phase shift of each antenna element is selected to cause all of the individual microwave rays from each of the antenna elements to arrive at the target substantially in-phase. The resulting microwave image of the object can be displayed as a two-dimensional (2D) or three-dimensional (3D) image to an operator. Examples of programmable antenna arrays are described in U.S. patent application Ser. Nos. 10/997,422, entitled “A Device for Reflecting Electromagnetic Radiation,” and 10/997,583, entitled “Broadband Binary Phased Antenna.” 
   In traditional phased arrays, the custom is to place the antenna elements apart by λ/2 in both directions to suppress sidelobes throughout a hemispherical scan. The number of antenna elements in a circular area array is about π(D/λ) 2  where D is the diameter of the circle and A is the wavelength of the radiation. The number of antenna elements, and therefore the cost of the array, is proportional to (D/λ) 2 . Each antenna element has traditionally been controlled by its own active device. However, the active devices used in controlling the antenna elements can be expensive, and in some cases may even require one or more stages of amplifiers. Even when the active devices are relatively inexpensive, the system may require a very deep digital memory to support a large set of focal areas or volumes. 
   One approach for reducing the number of antenna elements is to simply omit elements from the traditional “dense” phased array. The result is known as a “sparse array”. While using a sparse array does reduce the number of active devices required, a new problem is created. Sparse arrays are well-known in the ultrasound and microwave/millimeter-wave literature to be associated with grating sidelobes. Sidelobes produce unwanted ghosting phenomena in the scanning or imaging process. 
   Various remedies have been tried to remove or negate the effect of the sidelobes. For example, deconvolution algorithms can be applied but the most successful of these are nonlinear algorithms which are both scene-dependent and very time-consuming. Two of the most popular deconvolution algorithms are CLEAN and the Maximum Entropy Method or MEM. An older, linear (and hence faster and more general) algorithm is Wiener-Helstrom filtering, but it is well known that it produces inferior image reconstruction compared to nonlinear (slower, more specialized) techniques such as Maximum Likelihood (ML) iteration. Correlation imaging, involving different subsets of an already sparse array, is another nonlinear scheme which tends to be quite slow. In some cases, e.g., radioastronomy, one has prior knowledge about the scene (say, from visible telescopes) which can be used to weed out much of the ghost phenomena. However, this solution is inadequate whenever one is dealing with a highly dynamic environment. 
   U.S. Application for patent Ser. No. 11/552,193, entitled “Convex Mount for Element Reduction in Phased Arrays with Restricted Scan,” which was filed on Oct. 20, 2006, and U.S. Application for patent Ser. No. 11/551,382, entitled “Element Reduction in Phased Arrays with Cladding,” which was filed on Oct. 20, 2006, disclose that when the range of solid scan angle is less than 2π steradians (i.e., less than a hemisphere), it is theoretically possible to reduce the element count without sidelobe degradation. However, U.S. Application for patent Ser. No. 11/552,193 requires that the antenna elements be mounted on a curved surface, and U.S. Application for patent Ser. No. 11/551,382 requires a special material to be applied to the surface of the antenna elements. 
   Therefore, a need still remains for a reduced-device phased array on a flat surface that does not suffer from sidelobe degradation. 
   SUMMARY OF THE INVENTION 
   A plurality of antenna clusters form an antenna array used in microwave imaging. Each antenna cluster has at least two antenna elements and an active device. The active device controls the two antenna elements to direct microwave radiation to and from an object to capture a microwave image of the object. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic block diagram illustrating an exemplary microwave imaging system, in accordance with embodiments of the present invention. 
       FIG. 2  is a schematic diagram of a front view of an exemplary antenna array for reflecting microwave radiation, in accordance with embodiments of the present invention. 
       FIG. 3  shows a diagram of a top view of an exemplary antenna array  12  to illustrate exemplary radiation patterns, in accordance with embodiments of the present invention. 
       FIGS. 4-8  show various possible types of antenna clusters that may be used in an antenna array, in accordance with embodiments of the present invention. 
       FIG. 9  shows one embodiment of an antenna array, in accordance with embodiments of the present invention. 
   

   DETAILED DESCRIPTION 
   As used herein, the terms microwave radiation and microwave illumination each refer to the band of electromagnetic radiation having wavelengths between 0.3 mm and 30 cm, corresponding to frequencies of about 1 GHz to about 1,000 GHz. Thus, the terms microwave radiation and microwave illumination each include traditional microwave radiation, as well as what is commonly known as millimeter wave radiation. In addition, as used herein, the term “microwave imaging system” refers to an imaging system operating in the microwave frequency range, and the resulting images obtained by the microwave imaging system are referred to herein as “microwave images.” 
     FIG. 1  is a schematic block diagram of a top view of an exemplary microwave imaging system  10 , in accordance with embodiments of the present invention. The microwave imaging system  10  can be used, for example, to provide ongoing surveillance to control a point-of-entry into a structure, monitor passers-by in an area (e.g. a hallway, a room, or outside of a building) or to screen individual persons or other items of interest. 
   The microwave imaging system  10  includes an antenna array  12  for absorbing or reflecting microwave radiation to scan an object  14 . Antenna clusters  16  are formed on the surface of the antenna array. Each antenna cluster  16  is capable of transmitting, receiving, and/or reflecting microwave radiation to capture a microwave image of the object  14 . The maximum scan angle θ max  is defined as the maximum required angle of deflection away from the central spot  13  of the object  14  to be scanned. θ max  is limited to less than π/2 radians (90 degrees) to avoid grating sidelobes. This translates into a solid scan angle of less than a hemisphere (2π steradians), which is sufficient for many applications. For example, a security portal for scanning a person only needs a scan angle big enough to scan the person&#39;s body size—limiting θ max  to less than 90 degrees is not a problem in this situation. 
     FIG. 2  is a schematic diagram of a front view of an exemplary antenna array  12 , in accordance with embodiments of the present invention. Antenna clusters  16 A- 16 C, are symbolic representations of different types of antenna clusters arranged on the surface of the antenna array  12 . Each symbol  16 A-C could also represent a subarray filled with antenna clusters of that type. Each antenna cluster  16  includes at least two antenna elements  18 . All antenna elements  18  within a cluster are controlled by a single active device (not shown). The active device is any switchable device, such as a transistor, diode, micro-electro-mechanical system (MEMS), variable capacitor (such as a barium strontium titanate capacitor), etc. For the sake of simplicity, only 3 cluster types are shown in  FIG. 2 , but many different types of antenna clusters can be formed. 
   Each cluster type  16 A- 16 C has a different far-field radiation pattern. Each antenna cluster  16  is capable of transmitting, receiving, and/or reflecting microwave radiation to and from an object to capture a microwave image of the object. 
   The antenna clusters arranged on an antenna array  12  are chosen so that the resulting combination of radiation patterns provides the desired scan coverage of the object  14 . To explain further, each subsection of the antenna array  12  has a quiescent angle to the central spot  13  of the object to be scanned. The antenna array  80  is partitioned so that each local area contains the cluster type whose far-field radiation pattern is optimally matched to the local quiescent angle; that is, when all the active devices are programmed into the same state, the antenna array has a natural bias toward the central spot  13 . Although the object may not be in the far field of the entire antenna array, it may still be in the far field of an antenna cluster because the cluster is so much smaller than the entire array. The cumulative effect is that the radiation patterns are directed towards the object. The number and types of antenna clusters needed will depend on various factors such as the size of the object to be scanned, the shape and size of the radiation patterns, etc. 
   By carefully selecting the desired antenna cluster type(s), an antenna array can be constructed with radiation patterns that are biased towards the center of an object and allow scan coverage of the object. Furthermore, using antenna clusters provides a practical cost savings since a single active device is used to control multiple antenna elements. 
   In one embodiment, antenna array  12  is a reflectarray, and a feedhorn  21  is used to transmit and receive microwave radiation to and from the antenna clusters  16 . The location of the feedhorn  21  should not be in a null or node of any of the antenna clusters. Ideally, the feedhorn  21  should be near an antinode for all of the antenna clusters. Each antenna cluster  16  includes an active device that presents a variable impedance to the antenna elements  18  within each antenna cluster. The variable impedance of the active device in turn controls the reflection amplitude and phase of the antenna cluster  16 . 
   Other modalities may be used to implement antenna array  12 , including but not limited to: continuous-phase transmit/receive arrays, transmission (lens) arrays, binary phase arrays, etc. 
     FIG. 3  shows a diagram of a side view of an exemplary antenna array  12  to illustrate exemplary radiation patterns, in accordance with embodiments of the present invention. Several radiation patterns are shown relative to an antenna cluster plane  19 . A broadside radiation pattern  24 , an endfire radiation pattern  50 , and an off-axis radiation pattern  28  are illustrated relative to the antenna cluster plane  19 . The broadside radiation pattern  24  is a radiation pattern in which the direction of maximum radiation  25  is perpendicular to the antenna cluster plane  19 . The endfire radiation pattern  50  is a radiation pattern in which the direction of maximum radiation  25  is in the antenna cluster plane  19 . The off-axis radiation pattern is a radiation pattern in which the direction of maximum radiation  25  is at an intermediate angle between a broadside radiation pattern  24  and an endfire radiation pattern  50 . 
   A first antenna cluster  22  has a broadside radiation pattern  24 . A second antenna cluster  26  has an off-axis radiation pattern  28 . The off-axis radiation pattern  28  may be tilted in the E-plane but centered in the H-plane; tilted in the H-plane but centered in the E-plane, or tilted in both planes depending on the cluster type design. The arrangement and shape of antenna elements within the second antenna cluster  26  determines the off-axis radiation pattern  28  and the degree and direction of its tilt. 
     FIGS. 4-8  show various possible types of antenna clusters  16  that may be used in an antenna array.  FIGS. 4-8  illustrate just a few of the many arrangements of antenna elements and various radiation patterns that are possible. 
     FIG. 4  shows a schematic diagram of a front view of a broadside E cluster  30 , in accordance with embodiments of the present invention. The broadside E cluster  30  includes antenna elements  32 ,  34 , and an active device  36 . An arrow E indicates the direction of the electric field vector. 
   In one embodiment, antenna elements  32  and  34  are planar patch antennas that reflect microwave radiation to and from a microwave transmitter/receiver, such as a feedhorn. The impedance of the active device  36  is varied to control the reflection phase of the antenna element  32 . The antenna element  32  is connected in series to antenna element  34  by a delay line  38 . The length of the delay line  38  is chosen so that the antenna element  34  will be excited in-phase with the antenna element  32  when fed by the active device  36 . Taking into account the half-wave length of antenna element  32 , the delay line  38  is a 180° degree delay line. Since antenna elements  32  and  34  are excited in-phase, this antenna cluster has a broadside radiation pattern  24  in the E-plane. The size and shape of the radiation pattern can be adjusted by adjusting various parameters such as the size and shape of the antenna elements  32  and  34 , Additional antenna elements can be added to this cluster using additional 180° degree delay lines. 
     FIG. 5  shows a schematic diagram of a top view of an off-axis E cluster  40 , in accordance with embodiments of the present invention. The antenna cluster includes a master antenna element  42 , a slave antenna element  44 , and an active device  46 . An arrow E indicates the direction of the electric field vector. In one embodiment, master antenna element  42  and slave antenna element  44  are planar patch antennas that reflect microwave radiation to and from a microwave transmitter/receiver, such as a feedhorn. The impedance of the active device  46  is varied to control the reflection phase of the master antenna element  42 . The active device  46  directly feeds master antenna element  42 . Slave antenna element  44  is parasitically coupled (as indicated by arrow  45 ) to master antenna element  42  in the E-plane direction—no actual physical connection between the antenna elements exists. 
   Due to their parasitic coupling, master antenna element  42  and slave antenna element  44  are excited out-of-phase, and therefore have an off-axis radiation pattern  28 . The tilt degree and direction of the radiation pattern  28  are determined by the strength of the parasitic coupling  45 , the size and shape of the slave antenna element  44  relative to the master antenna element  42 , and the position of the slave antenna element  44  relative to the master antenna element  42 . Although only a single slave antenna element is shown, additional slave antenna elements can be included to couple parasitically with the master antenna element  42 . 
     FIG. 6  shows a schematic diagram of a front view of a broadside H cluster  50 , in accordance with embodiments of the present invention. The broadside H cluster  50  includes antenna elements  52 ,  54 , and an active device  56 . An arrow E indicates the direction of the electric field vector. In one embodiment, antenna elements  52  and  54  are planar patch antennas that reflect microwave radiation to and from a microwave transmitter/receiver, such as a feedhorn. The impedance of the active device  56  is varied to control the reflection phase of the antenna elements  52  and  54 . The active device  56  is connected to antenna element  52  by a transmission line  58 . The active device  56  is connected to antenna element  54  by a transmission line  59 . Both transmission lines  58  and  59  are of equal length, so the antenna elements  52  and  54  are excited in phase. As a result, this antenna cluster has a broadside radiation pattern  24 . 
     FIG. 7  shows a schematic diagram of a front view of an off-axis H cluster  60 , in accordance with embodiments of the present invention. The off-axis H cluster  60  includes antenna elements  62 ,  64 , and an active device  66 . An arrow E indicates the direction of the electric field vector. In one embodiment, antenna elements  62  and  64  are planar patch antennas that reflect microwave radiation to and from a microwave transmitter/receiver, such as a feedhorn. The impedance of the active device  66  is varied to control the reflection phase of the antenna element  62 . The active device  66  is connected to antenna element  62  by a transmission line  58 . The active device  66  is also connected to antenna element  64  by a transmission line  59 . Transmission line  58  is of a different length than transmission line  59 . As a result, the antenna elements are excited out-of-phase and produce an off-axis radiation pattern  28  that is tilted in the H-plane. The tilt degree and direction of the radiation pattern  28  are determined by the difference in lengths of transmission lines  58  and  59 . 
   Both the antenna impedance of the cluster and the antenna amplitude balance within the cluster are functions of the phase offset. This is not an issue for the antenna clusters that are excited in-phase. However, it is a concern with respect to antenna clusters having out-of-phase excitations, especially with the topology of off-axis H cluster  60  in  FIG. 7 . As a result, the lengths and widths of transmission lines  58  and  59 , as well as the characteristics of antenna elements  62  and  64 , must be determined carefully to achieve amplitude balance within the cluster and to present the optimal antenna impedance to the active device  66 . 
     FIG. 8  shows a schematic diagram of a front view of an off-axis H cluster  70 , in accordance with embodiments of the present invention. The off-axis H cluster  70  includes master antenna element  72 , slave antenna element  74 , and an active device  76 . An arrow E indicates the direction of the electric field vector. In one embodiment, master antenna element  72  and slave antenna element  74  are planar patch antennas that reflect microwave radiation to and from a microwave transmitter/receiver, such as a feedhorn. The impedance of the active device  76  is varied to control the reflection phase of the master antenna element  72 . The active device  76  directly feeds master antenna element  72 . Slave antenna element  74  is parasitically coupled (as indicated by arrow  75 ) to master antenna element  72  in the H-plane direction—no actual physical connection between the antenna elements exists. 
   As a result of the parasitic coupling, slave antenna element  74  is excited out-of-phase with master antenna element  72 . As a result, an off-axis radiation pattern  28  that is tilted in the H-plane is produced. The tilt degree and direction of the radiation pattern  28  are determined by the strength of the parasitic coupling, the size and shape of the slave antenna element  74  relative to the master antenna element  72 , and the position of the slave antenna element  74  relative to the master antenna element  72 . Although only a single slave antenna element is shown, additional slave antenna elements can be included to couple parasitically with the master antenna element  72 . 
   The off-axis H-cluster  70  is an alternative to the off-axis H-cluster of  FIG. 7 . Since the coupling of antenna elements  72  and  74  is achieved parasitically, it is unnecessary to worry about the impedances of the feed transmission lines. 
   In all of the above examples of antenna clusters in  FIGS. 4-8 , the antenna elements are represented as planar patch antennas, but other types of antennas can be used. Example antenna types that can be used as antenna elements in the antenna clusters include, but are not limited to: dipoles, monopoles, slot antennas, loop antennas, open waveguides, horns, etc. Furthermore,  FIGS. 4-8  represent the planar patch antennas as passive elements that reflect microwave radiation to and from a microwave transmitter/receiver (such as a feedhorn) —however, active antenna elements that actively transmit and receive microwave radiation may also be used. 
   In addition, although only 2 antenna elements are shown in the figures, it should be apparent to one of ordinary skill in the art that each antenna cluster can easily be modified to include more than 2 antenna elements. Furthermore, the active device in each of the clusters can be any switchable device, such as a transistor, diode, micro-electro-mechanical system (MEMS), variable capacitor (such as a barium strontium titanate capacitor), etc. Finally, the degree and direction of tilt for the radiation pattern of any antenna cluster can be changed by varying parameters such as the size, shape, and location of the antenna elements within the cluster. 
     FIG. 9  shows one embodiment of an antenna array  80  according to the present invention. Antenna array  80  is a reflectarray with two types of antenna clusters: antenna clusters  16 A and  16 B. The antenna clusters are arranged symmetrically across a vertical symmetry plane  90  that bisects the antenna array  80 . A feedhorn  88  transmits and receives microwave radiation to and from all the antenna clusters. The feedhorn  88  is situated in the symmetry plane  90 , either above or below the object to be scanned. 
   Each antenna cluster  16 A has a broadside radiation pattern. Suitable antenna clusters include the broadside E cluster  30  of  FIG. 4 , and the broadside H cluster  50  of  FIG. 6 . The broadside antenna clusters  16 A are installed close to the symmetry plane  90  and have the same function regardless of whether the physical layout is mirrored or not, since antenna clusters  16 A have a broadside radiation pattern that is symmetrical. 
   The antenna clusters  16 B are installed further from the symmetry plane  90 . Each antenna cluster  16 B has an off-axis radiation pattern in the horizontal direction. Suitable antenna clusters are the off-axis E cluster  40  of  FIG. 5 , and the off-axis H clusters  60  and  70  of  FIGS. 5 and 6 , respectively. Care must be taken to install these antenna clusters with the right orientation. Since antenna clusters  16 B have off-axis radiation patterns, the radiation patterns will point away from the object if the antenna clusters  16 B are installed incorrectly. Notice in  FIG. 9  that the antenna clusters  16 B on the left side of antenna array  80  are the mirror image (across the symmetry plane  90 ) of the antenna clusters  16 B on the right side of antenna array  80 . 
   Preferably, both antenna clusters  16 A and  16 B have neutral (quasi-isotropic) radiation patterns with respect to the vertical direction. The feedhorn  88  is rotated to match the polarization of the antenna clusters. For example, in  FIG. 9 , the feedhorn  88  would be rotated if all antenna clusters were rotated. 
   The antenna clusters  16 A have broadside radiation patterns and are located centrally, close to the symmetry plane  90 . The antenna clusters  16 B have off-axis radiation patterns and are located along the further edges of the antenna array  80 . However, the radiation patterns of the antenna clusters  16 B are selected to tilt back toward the symmetry plane  90 . As a result, a centrally located object can be scanned with high efficiency. For optical scan coverage, the object should straddle or be near the symmetry plane  90  such that its central spot lies on the symmetry plane. 
   More than two types of antenna clusters may be used in building an antenna array. For example, antenna clusters that have off-axis radiation patterns in the vertical direction may be added as top and bottom rows to the antenna array in  FIG. 9 . Another option is to use only a single type of antenna cluster to build the antenna array. 
   Although antenna array  80  is depicted in  FIG. 9  as a reflectarray, and the antenna clusters shown reflect microwave radiation to and from the feedhorn  88 , other kinds of antenna arrays may also be used. For example, the antenna clusters may also consist of active transmitting and receiving antenna elements, in which case a feedhorn  88  is unnecessary. 
   Although the present invention has been described in detail with reference to particular embodiments, persons possessing ordinary skill in the art to which this invention pertains will appreciate that various modifications and enhancements may be made without departing from the spirit and scope of the claims that follow.