Abstract:
Methods and systems for evaluating a material sample is provided. The system includes a material sample holder including a first flange having an aperture therethrough, a second flange having an aperture therethrough, the first and second flanges configured to frictionally hold a material sample sandwiched therebetween, a waveguide coupled to a first end of each of the first flange and the second flange, each waveguide configured to direct electromagnetic waves through respective apertures, a waveguide adapter communicatively coupled to a second end of each waveguide, and a control unit electrically coupled to the wave source, the control unit configured to control the waveguide adapter to transmit and receive electromagnetic wave signals.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to the testing of materials and, more particularly, to methods and apparatus for evaluating material properties of radio frequency (RF) absorbent materials.  
         [0002]     At least some known methods used for testing RF absorbent materials use samples that are formed precisely for placement inside a waveguide. Such testing methods are generally not reliable for evaluating highly conductive fillers used with low observable (LO) applications because of gaps that may exist between an outer periphery of the sample and an inner periphery of the waveguide. More specifically, the gaps may introduce unpredictable measurement errors into the test, thus, resulting in inaccurate measurements of RF reflection loss in the waveguide from highly conductive fillers.  
         [0003]     Other known free space methods have been used to attempt to characterize conductive fillers. However, such methods generally have the disadvantage of requiring a large sample size.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0004]     In one aspect, a system for evaluating a material sample is provided. The system includes a material sample holder including a first flange having an aperture therethrough, a second flange having an aperture therethrough, the first and second flanges configured to frictionally hold a material sample sandwiched therebetween, a waveguide coupled to a first end of each of the first flange and the second flange, each waveguide configured to direct electromagnetic waves through respective apertures, a waveguide adapter communicatively coupled to a second end of each waveguide, and a control unit electrically coupled to the wave source, the control unit configured to control the waveguide adapter to transmit and receive electromagnetic wave signals.  
         [0005]     In another aspect, a material sample holder for testing an electromagnetic energy absorbent material is provided. The material sample holder includes a first flange including a face and an aperture therethrough, the first flange is configured to mate to a first surface of a material sample. The material sample holder also includes a second flange including a face and an aperture therethrough, the second flange is configured to mate to a second surface of a material sample, wherein the first and the second flanges are configured to sandwich the material sample such that the face of the first flange engages the first surface and the face of the second flange engages the second surface.  
         [0006]     In yet another aspect, a method of evaluating a material sample is provided. The method includes sandwiching a material sample between a transmitting waveguide flange having an aperture therethrough and in communication with a transmitting waveguide, and a receiving waveguide flange having an aperture therethrough and in communication with a receiving waveguide, the apertures are configured to be completely covered by the material sample when the sample is installed between the flanges, emitting an electromagnetic wave through the transmitting waveguide to the material sample, receiving electromagnetic energy from the electromagnetic wave through the sample, and determining a material property of the material sample using the emitted wave and the received energy. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a schematic illustration of an exemplary radio frequency material testing system that includes a sample holder, a first waveguide assembly, and a second waveguide assembly;  
         [0008]      FIG. 2  is an enlarged perspective front view of the sample holder that may be used with radio frequency material testing system shown in  FIG. 1 ;  
         [0009]      FIG. 3  is an enlarged perspective side view of the sample holder that may be used with radio frequency material testing system shown in  FIG. 1  taken along a view line shown in  FIG. 2 ;  
         [0010]      FIG. 4  is a graph of exemplary traces of a finite element comparison between a perfectly filled sample in a waveguide and a simulation of a measurement in accordance with one embodiment of the present invention; and  
         [0011]      FIG. 5  is a simplified block diagram of an exemplary architecture for radio frequency material testing system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]      FIG. 1  is a schematic illustration of an exemplary radio frequency material testing system  100  that includes a sample holder  102 , a first waveguide assembly  104 , and a second waveguide assembly  106 . Testing system  100  also includes a control unit  112 , for example, a wave analyzer. In the exemplary embodiment, sample holder  102  is configured to sandwich a sample  113  of RF absorbent material between a first flange  114  and a second flange  116  that are maintained in position with respect to each other using a clamping device (not shown), for example, but not limited to threaded fasteners, extending from flange  114  to flange  116 . Waveguides assemblies  104  and  106  each include an elongate waveguide  118  and  120  respectively, having a longitudinal bore (not shown) therethrough. Waveguide assemblies  104  and  106  are coupled to flanges  114  and  116  respectively such that an aperture through each flange  114  and  116  is oriented in substantial alignment with the longitudinal bore of respective waveguide assemblies  104  and  106 . A waveguide adapter  122  is coupled to a source end  124  of waveguide  118  and a waveguide adapter  126  is coupled to a source end  128  of waveguide  120 . An first analyzer test lead  130  is electrically coupled between waveguide adapter  122  and a first port  132  of control unit  112 . An second analyzer test lead  134  is electrically coupled between waveguide adapter  126  and a second port  136  of control unit  112 . In one embodiment, control unit  112  is a 8510C Vector Network Analyzer commercially available from Agilent Technologies, Inc., Palo Alto, Calif. In other embodiments, control unit  112  may be embodied on a computer, such as a stand-alone PC-based computer and/or a workstation in a client server relationship with a server through a network.  
         [0013]     In operation, RF absorbent material sample  113  is sandwiched between flanges  114  and  116  and waveguide assemblies  104  and  106  are assembled such that waveguide adapter  122  is in RF communication with waveguide  118  and waveguide adapter  126  is in RF communication with waveguide  120 . Each waveguide adapter  122  and  126  is coupled to respective ports  132  and  136  of control unit  112 . As RF energy is transmitted through waveguide  118  toward sample  113 , port  136  receives a signal from waveguide adapter  126  proportional to the RF energy that may leak through sample  113 . Similarly, RF energy is transmitted through waveguide  120  toward sample  113 , port  132  receives a signal from waveguide adapter  122  proportional to the RF energy that may leak through sample  113 . Using the received signals, reflection loss measurements may be obtained and when combined with finite element model (FEM) waveguide code, S parameter measurements obtained, may be converted into Rf material properties using a transfer function derived from the FEM analysis.  
         [0014]      FIGS. 2 and 3  are enlarged perspective views of sample holder  102  that may be used with radio frequency material testing system  100  (shown in  FIG. 1 ).  FIG. 2  is a front view of sample holder  102  and  FIG. 3  is a side view taken along a line  200  (shown in  FIG. 2 ). Sample holder  102  includes flange  114  and flange  116 . Each flange  114  and  116  includes an aperture  202  and  204  respectively. Apertures  202  and  204  are sized to couple to waveguide  118  and  120  respectively. In the exemplary embodiment, flanges  114  and  116  are substantially complementary such that apertures  202  and  204  are substantially aligned with respect to each other when sample holder  102  is assembled. Flanges  114  and  116  may each include complementary fastener holes  206  such as, apertures and/or slots. Fastener holes  206  facilitate clamping flanges  114  and  116  together with sample  113  between. Flanges  114  and  116  may also be clamped together using a separate clamping device (not shown). Sample holder  102  is configured to maintain sample  113  in a fixed position between flanges  114  and  116  using a friction force. Additionally, sample holder  102  may apply a sufficient clamping force to sample  113  such that a portion in contact with flanges  114  and  116  is compressed and a portion not in contact with flanges  114  and  116  is expanded. In such a case, the expanded portion may facilitate providing an interference fit between sample  113  and flanges  114  and  116 . One or more of flanges  114  and  116  may include a compression stop  208  configured to prevent excessive compression of sample  113 .  
         [0015]     In the exemplary embodiment, aperture  202  is illustrated as having a rectangular cross-section. It should be understood that this illustration is exemplary only and aperture  202  may be any shape capable of permitting radio frequency material testing system  100  to perform the functions described herein.  
         [0016]      FIG. 3  is a screen shot  300  of an exemplary output of a finite element model that may be used with control unit  112  (shown in  FIG. 1 ). Screen shot  300  includes a legend  302  and an output area  304  where an output  306  of the FEM calculation is displayed. Using received signals from control unit  112 , reflection loss measurements may be determined and when the reflection loss measurements are combined with a FEM waveguide code, S parameter measurements may be determined and converted into RF material properties using a transfer function derived from the FEM analysis. In the exemplary embodiment, output  306  is programmed to model radio frequency material testing system  100  and includes a transmit portion  308 , a receive portion  310  and a sample portion  312 . Transmit portion  308  models one of waveguide  118  or  120  during a test when the associated waveguide adapter is emitting RF energy into waveguide  118  or  120  toward sample  113 . Receive portion  310  models the other of waveguide  118  or  120  during a test when the associated waveguide adapter is receiving RF energy leaking through sample  113 . RF energy is emitted into transmit portion  308  from an entry portion  314  corresponding to waveguide adapter  122 . A standing wave in transmit portion  308  is illustrated by gradient areas  316  that correlate the RF energy at locations within transmit portion  308  to legend  302 . Similarly, RF energy received by receive portion  310  passing through sample  113  is displayed using legend  302 .  
         [0017]      FIG. 4  is a graph  400  of an exemplary trace  402  and an exemplary trace  404  of a finite element comparison between a perfectly filled sample in a waveguide and a simulation of a measurement in accordance with one embodiment of the present invention. Graph  400  includes an x-axis  406  that indicates a reflection loss magnitude for the sample in units of dB. A y-axis  408  indicates a magnitude of conductivity of the sample corresponding to each unit of reflection loss. As illustrated, traces  402  and  404  are substantially coincident in a region of interest  410  defined between approximately 0.04 dB and approximately 0.3 of reflection loss. The close correspondence between traces  402  and  404  indicate the measurement method in accordance with one embodiment of the present invention is substantially equivalent to a simulation using a perfectly filled sample in a waveguide for highly reflective materials.  
         [0018]      FIG. 5  is a simplified block diagram of an exemplary architecture for radio frequency material testing system  100  including a server system  502 , and a plurality of client sub-systems, also referred to as client systems  504 , connected to server system  502 . In one embodiment, client systems  504  are computers including a web browser, such that server system  502  is accessible to client systems  504  via the Internet. Client systems  504  are interconnected to the Internet through many interfaces including a network, such as a local area network (LAN) or a wide area network (WAN), dial-in-connections, cable modems and special high-speed ISDN lines. Client systems  504  could be any device capable of interconnecting to the Internet including a web-based phone, personal digital assistant (PDA), or other web-based connectable equipment. A database server  506  is connected to a database  520  containing information on a variety of matters, as described herein. In one embodiment, centralized database  520  is stored on server system  502  and can be accessed by potential users at one of client systems  504  by logging onto server system  502  through one of client systems  504 . In an alternative embodiment, database  520  is stored remotely from server system  502  and may be non-centralized.  
         [0019]     A technical effect of the various embodiments of the invention is to automatically determine a reflection loss of a highly conductive sample using a method that facilitates reducing leakage of RF energy past the sample that would otherwise affect the accuracy of the reflection loss evaluation.  
         [0020]     The various embodiments or components thereof may be implemented as part of a computer system. The computer system may include a computer, an input device, a display unit and an interface, for example, for accessing the Internet. The computer may include a microprocessor. The microprocessor may be connected to a communication bus. The computer may also include a memory. The memory may include Random Access Memory (RAM) and Read Only Memory (ROM). The computer system further may include a storage device, which may be, but not limited to, a hard disk drive, a solid state drive, and/or a removable storage drive such as a floppy disk drive, or optical disk drive. The storage device can also be other similar means for loading computer programs or other instructions into the computer system.  
         [0021]     As used herein, the term “computer” may include any processor-based or microprocessor-based system including systems using microcontrollers, reduced instruction set circuits (RISC), application specific integrated circuits (ASICs), logic circuits, and any other circuit or processor capable of executing the functions described herein. The above examples are exemplary only, and are thus not intended to limit in any way the definition and/or meaning of the term “computer.” 
         [0022]     The computer system executes a set of instructions that are stored in one or more storage elements, in order to process input data. The storage elements may also hold data or other information as desired or needed. The storage element may be in the form of an information source or a physical memory element within the processing machine.  
         [0023]     The set of instructions may include various commands that instruct the processing machine to perform specific operations such as the processes of the various embodiments of the invention. The set of instructions may be in the form of a software program. The software may be in various forms such as system software or application software. Further, the software may be in the form of a collection of separate programs, a program module within a larger program or a portion of a program module. The software also may include modular programming in the form of object-oriented programming. The processing of input data by the processing machine may be in response to user commands, or in response to results of previous processing, or in response to a request made by another processing machine.  
         [0024]     As used herein, the terms “software” and “firmware” are interchangeable, and include any computer program stored in memory for execution by a computer, including RAM memory, ROM memory, EPROM memory, EEPROM memory, and non-volatile RAM (NVRAM) memory. The above memory types are exemplary only, and are thus not limiting as to the types of memory usable for storage of a computer program.  
         [0025]     While the present invention is described with reference to RF energy absorbent conductive fillers, numerous other applications are contemplated. It is contemplated that the present invention may be applied to any material evaluation where leakage of a measurement medium past the sample may adversely affect the accuracy of the measurement and subsequent evaluation.  
         [0026]     The above-described radio frequency material testing system is a cost-effective and highly reliable means for determining material properties of a sample. The system is configured to receive a sample sandwiched between flanges of a sample holder such that the sample completely covers the flange aperture substantially eliminating the ability of the measurement medium to bypass the sample. Accordingly, the radio frequency material testing system facilitates measuring the material properties of a sample, and in particular conductive filler material, in a cost-effective and reliable manner.  
         [0027]     Exemplary embodiments of radio frequency material testing system components are described above in detail. The components are not limited to the specific embodiments described herein, but rather, components of each system may be utilized independently and separately from other components described herein. Each radio frequency material testing system component can also be used in combination with other radio frequency material testing system components.  
         [0028]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.