The disclosed invention will be described with reference to the accompanying drawings, which show important sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein:
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.”
Referring now to
As can be seen in
In one embodiment, the array 50 is a passive programmable reflector array composed of reflective or transmissive antenna elements 80 that reflect or transmit microwave radiation to and/or from one or more microwave antennas 60. For example, each of the reflective or transmissive antenna elements 80 can be programmed with a respective transmit direction coefficient to reflect or transmit microwave illumination emitted from one of the microwave antennas 60 towards the target. In addition, each of the reflective or transmissive antenna elements 80 can be programmed with an additional respective receive direction coefficient to reflect or transmit microwave illumination reflected from the target towards one of the microwave antennas 60. A single microwave antenna 60 can serve as both the source and receiver of microwave radiation, or separate microwave antennas 60 can be used for illuminating the array 50 and receiving reflected microwave illumination from the array 50, the latter being illustrated in
As an example, as shown in
In another embodiment, the array 50 is an active transmitter/receiver array composed of active antenna elements 80, each capable of producing and transmitting microwave radiation and each capable of receiving and capturing reflected microwave radiation. In this embodiment, microwave source/receive antennas 60 are not used, as the array 50 operates as the source of microwave radiation.
The microwave imaging system 10 further includes a processor 100, computer-readable medium 110 and a display 120. The processor 100 includes any hardware, software, firmware, or combination thereof for controlling the array 50 and processing the received microwave radiation reflected from the target 155 for use in constructing a microwave image of the object 150. For example, the processor 100 may include one or more microprocessors, microcontrollers, programmable logic devices, digital signal processors or other type of processing devices that are configured to execute instructions of a computer program, and one or more memories (e.g., cache memory) that store the instructions and other data used by the processor 100. The memory 110 includes any type of data storage device, including but not limited to, a hard drive, random access memory (RAM), read only memory (ROM), compact disc, floppy disc, ZIP® drive, tape drive, database or other type of storage device or storage medium.
The processor 100 operates to program the antenna array 50 to illuminate multiple targets 155 on the object 150. In exemplary embodiments, the processor 100 programs respective amplitude/phase delays or amplitude/phase shifts into each of the individual antenna elements 80 in the array 50 to illuminate each target 155 on the object 150. In addition, the processor 100 programs respective amplitude/phase delays or amplitude/phase shifts into each of the individual antenna elements 80 in the array 50 to receive reflected microwave illumination from each target 155 on the object 150. In embodiments using phase shifts, the programmed phase shifts can be either binary phase shifts, some other multiple number of phase shifts or continuous phase shifts.
The processor 100 is further capable of constructing a microwave image of the object 150 using the intensity of the reflected microwave radiation captured by the array 50 from each target 155 on the object 150. For example, in embodiments in which the array 50 is a reflector array, the microwave receiver 60b is capable of combining the reflected microwave radiation reflected from each antenna element 80 in the array 50 to produce a value of the effective intensity of the reflected microwave radiation at the target 155. The intensity value is passed to the processor 100, which uses the intensity value as the value of a pixel or voxel corresponding to the target 155 on the object 150. In other embodiments in which the reflected microwave radiation represents the intensity of an area/volume of voxels, for each microwave image of a target 155 (area/volume in 3D space), the processor 100 measures a Fourier transform component of the desired image of the object 150. The processor 100 performs an inverse Fourier transform using the measured Fourier transform components to produce the image of the object 150.
The resulting microwave image of the object 150 can be passed from the processor 100 to the display 120 to display the microwave image. In one embodiment, the display 120 is a two-dimensional display for displaying three-dimensional microwave images of the object 150 or one or more one-dimensional or two-dimensional microwave images of the object 150. In another embodiment, the display 120 is a three-dimensional display capable of displaying three-dimensional microwave images of the object 150.
In a similar manner, as shown in
In operation, microwave illumination 65 transmitted from horn 60 is received by various antenna elements 80 in the array 50. The antenna elements 80 in array 50 are each programmed with a respective transmission coefficient to direct transmitted microwave illumination 40 towards a target 155 on the object 150. The transmission coefficients are selected to create positive interference of the transmitted microwave illumination 40 from each of the antenna elements 80 at the target 155. Reflected microwave illumination 45 reflected from the target 155 is received by various antenna elements 80 in the array 50. The antenna elements 80 in array 50 are again each programmed with a respective transmission coefficient to direct transmitted microwave illumination 85 towards horn 60.
The horn 60 combines the transmitted microwave radiation 85 from each antenna element 80 in the array 50 to produce a value of the effective intensity of the reflected microwave radiation 45 at the target 155. The intensity value is passed to the processor 100, which uses the intensity value as the value of a pixel or voxel corresponding to the target 155 on the object 150. The processor 100 constructs a microwave image of the object 150 using the intensity of the reflected microwave radiation 45 captured by the array 50 from each target 155 on the object 150. The resulting microwave image of the object 150 can be passed from the processor 100 to the display 120 to display the microwave image. Although the microwave transmitter and receiver are shown in a confocal imaging configuration, it should be understood that in other embodiments, horn 60 can be implemented as two separate horns with different spatial locations.
The reflecting antenna element 200 is formed on and in a printed circuit board substrate 214 and includes the surface mounted FET 222, the patch antenna 220a, a drain via 232, a ground plane 236 and a source via 238. The surface mounted FET 222 is mounted on the opposite side of the printed circuit board substrate 214 as the planar patch antenna 220a and the ground plane 236 is located between the planar patch antenna 220a and the surface mounted FET 222. The drain via 232 connects the drain 228 of the surface mounted FET 222 to the planar patch antenna 220a and the source via 238 connects the source 226 of the surface mounted FET 222 to the ground plane 236.
In exemplary embodiments, the reflector antenna array is connected to a controller board 240 that includes driver electronics. The example controller board 240 depicted in
The patch antenna element 220a functions to reflect with more or less phase shift depending on the impedance level of the reflecting antenna element 200. The reflecting antenna element 200 has an impedance characteristic that is a function of the antenna design parameters. Design parameters of antennas include but are not limited to, physical attributes such as the dielectric material of construction, the thickness of the dielectric material, shape of the antenna, length and width of the antenna, feed location, and thickness of the antenna metal layer.
The FET 230 (non-ideal switching device) changes the impedance state of the reflecting antenna element 200 by changing its resistive state. A low resistive state (e.g., a closed or “short” circuit) translates to a low impedance. Conversely, a high resistive state (e.g., an open circuit) translates to a high impedance. A switching device with ideal performance characteristics (referred to herein as an “ideal” switching device) produces effectively zero impedance (Z=0) when its resistance is at its lowest state and effectively infinite impedance (Z=∞) when its resistance is at its highest state. As described herein, a switching device is “on” when its impedance is at its lowest state (e.g., Zon=0) and “off” when its impedance is at its highest state (e.g., Zoff=∞). Because the on and off impedance states of an ideal switching device are effectively Zon=0 and Zoff=∞, an ideal switching device is able to provide the maximum phase shift without absorption of electromagnetic radiation between the on and off states. That is, the ideal switching device is able to provide switching between 0 and 180 degree phase states. In the case of an ideal switching device, maximum phase-amplitude performance can be achieved with an antenna that exhibits any finite non-zero impedance.
In contrast to an ideal switching device, a “non-ideal” switching device is a switching device that does not exhibit on and off impedance states of Zon=0 and Zoff=∞, respectively. Rather, the on and off impedance states of a non-ideal switching device are typically, for example, somewhere between 0<|Zon|<|Zoff|<∞. However, in some applications, the on and off impedance states may even be |Zoff|<=|Zon|. A non-ideal switching device may exhibit ideal impedance characteristics within certain frequency ranges (e.g., <10 GHz) and highly non-ideal impedance characteristics at other frequency ranges (e.g., >20 GHz).
Because the on and off impedance states of a non-ideal switching device are somewhere between Zon=0 and Zoff=∞, the non-ideal switching device does not necessarily provide the maximum phase state performance regardless of the impedance of the corresponding antenna, where maximum phase state performance involves switching between 0 and 180 degree phase states. In accordance with one embodiment of the invention, the reflecting antenna element 200 of
Further, the antenna element 200 is configured as a function of the impedance of the non-ideal switching device (FET 230) in the on state, Zon, and the impedance of the non-ideal switching device 230 in the off state, Zoff. In a particular embodiment, the phase state performance of the reflecting antenna element 200 is optimized when the antenna element 200 is configured such that the impedance of the antenna element 200 is conjugate to the square root of the impedance of the non-ideal switching device 230 when in the on and off impedance states, Zon and Zoff. Specifically, the impedance of the antenna element 200 is the complex conjugate of the geometric mean of the on and off impedance states, Zon and Zoff, of the corresponding non-ideal switching device 230. This relationship is represented as:
Z
antenna*=√{square root over (ZonZoff)}, (1)
where ( )* denotes a complex conjugate. The above-described relationship is derived using the well-known formula for the complex reflection coefficient between a source impedance and a load impedance. Choosing the source to be the antenna element 200 and the load to be the non-ideal switching device 230, the on-state reflection coefficient is set to be equal to the opposite of the off-state reflection coefficient to arrive at equation (1).
Designing the antenna element 200 to exhibit optimal phase-amplitude performance involves determining the on and off impedances, Zon and Zoff of the particular non-ideal switching device that is used in the reflecting antenna element 200 (in this case, FET 230). Design parameters of the antenna element 200 are then manipulated to produce an antenna element 200 with an impedance that matches the relationship expressed in equation (1) above. An antenna element 200 that satisfies equation (1) can be designed as long as Zon and Zoff are determined to be distinct values.
Another type of switching device, other than the surface mounted FET 230 shown in
In a reflector antenna array that utilizes FETs as the non-ideal switching devices, the beam-scanning speed that can be achieved depends on a number of factors including signal-to-noise ratio, crosstalk, and switching time. In the case of a FET, the switching time depends on gate capacitance, drain-source capacitance, and channel resistance (i.e., drain-source resistance). The channel resistance is actually space-dependent as well as time-dependent. In order to minimize the switching time between impedance states, the drain of the FET is preferably DC-shorted at all times. The drain is preferably DC-shorted at all times because floating the drain presents a large off-state channel resistance as well as a large drain-source capacitance due to the huge parallel-plate area of the patch antenna. This implies that the antenna is preferably DC-shorted but one wishes the only “rf short” the antenna sees be at the source. Therefore, the additional antenna/drain short should be optimally located so as to minimally perturb the antenna.
It should be understood that other types of antennas can be used in the reflecting antenna element 200, instead of the patch antenna 220a. By way of example, but not limitation, other antenna types include dipole, monopole, loop, and dielectric resonator type antennas. In addition, in other embodiments, the reflecting antenna element 200 can be a continuous phase-shifted antenna element 200 by replacing the FETs 230 with variable capacitors (e.g., Barium Strontium Titanate (BST) capacitors). With the variable capacitor loaded patches, continuous phase shifting can be achieved for each antenna element 200, instead of the binary phase shifting produced by the FET loaded patches. Continuous phased arrays can be adjusted to provide any desired phase shift in order to steer a microwave beam towards any direction in a beam scanning pattern.
As used herein, the term symmetric antenna 310 refers to an antenna that can be tapped or fed at either of two feed points 311 or 313 to create one of two opposite symmetric field distributions or electric currents. As shown in
The symmetric antenna 310 is capable of producing two opposite symmetric field distributions, labeled A and B. The magnitude (e.g., power) of field distribution A is substantially identical to the magnitude of field distribution B, but the phase of field distribution A differs from the phase of field distribution B by 180 degrees. Thus, field distribution A resembles field distribution B at ±180° in the electrical cycle.
The symmetric antenna 310 is connected to the symmetric switch 315 via feed lines 316 and 317. Feed point 311 is connected to terminal 318 of the symmetric switch 315 via feed line 316, and feed point 313 is connected to terminal 319 of the symmetric switch 315 via feed line 317. As used herein, the term symmetric switch refers to either a SPDT or DPDT switch in which the two operating states of the switch are symmetric about the terminals 318 and 319.
For example, if in a first operating state of a SPDT switch, the impedance of a channel (termed channel α) is 10Ω and the impedance of another channel (termed channel β) is 1 kΩ, then in the second operating state of the SPDT switch, the impedance of channel α is 1 kΩ and the impedance of channel β is 10Ω. It should be understood that the channel impedances are not required to be perfect opens or shorts or even real. In addition, there may be crosstalk between the channels, as long as the crosstalk is state-symmetric. In general, a switch is symmetric if the S-parameter matrix of the switch is identical in the two operating states of the switch (e.g., between the two terminals 318 and 319).
Referring now to
Turning now to the details of
Regardless of the particular position of each focal point 600a and 600b on their respective arrays 50a and 50b, the focal points 600a and 600b are separated by a distance D, which is selected to create a parallax of Object A. As is understood in the art, a parallax is created by the apparent motion of an object against a background due to a change in position of an observer (i.e., a perspective shift). For example, when comparing an image of Object A taken from the first focal point 600a with an image of Object A taken from the second focal point 600a, Object A will appear to have moved, even though Object A has remained stationary. The distance D between the two focal points 600a and 600b in
Referring now to
Referring again to
For example, referring to
For example, referring again to
Although there is overlap between the sets of ON antenna elements in
At block 940, a stereoscopic microwave image of the object is formed from the first and second microwave images. For example, in one embodiment, a parallax of the object can be created from the different focal points, and using the parallax information, along with the first and second microwave images, the stereoscopic microwave image can be constructed. The stereoscopic microwave image may be used, for example, to detect anomalies in the imaged object.
As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a wide rage of applications. Accordingly, the scope of patents subject matter should not be limited to any of the specific exemplary teachings discussed, but is instead defined by the following claims.
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. ______ (Attorney Docket No. 10050857-1), 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. ______ (Attorney Docket No. 10051094-1), 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. No. ______ (Attorney Docket No. 10051409-1), entitled “Handheld Microwave Imaging Device” and Ser. No. ______ (Attorney Docket No. 10051410), entitled “System and Method for Standoff Microwave Imaging,” both of which were filed on Dec. 16, 2005.