Absorbing materials are widely applied in applications of wireless communication. It is of great importance to measure and characterize these materials efficiently and precisely. In this paper, the author first utilizes transmission line methods to acquire scattering parameters, which are used as input in the parameter retrieval process for EM properties. After successful characterization of these materials, EM properties are imported in commercialized software for simulation of reflectivity. For comparison, NRL arch reflectivity test is conducted and good comparison is found. Discrepancy between the two is analyzed in this paper.
In the measurement and characterization of absorbing materials, common methods include transmission line method and free space method. Transmission line methods involve placing the material under test (MUT) inside a portion of enclosed transmission line. In some sense, the open-ended coaxial probe is a cut off section of transmission line. For the enclosed transmission line, it is usually a section of rectangular waveguide or coaxial airline . It is reported that the major error in transmission line testing is due to MUT preparation, especially for thin, flexible absorbing materials. Free space method comes often in two set-ups: transmission configuration and reflection configuration. For the NRL arch reflectivity test, specific standards have been published, such as ASTM standards in the US and GJB 2038A-2011 in China. In the MUT preparation for NRL arch reflectivity test, size of the MUT is recommended to be larger than 5 λ × 5 λ. It becomes inconvenient for tests conducted at low frequencies such as UHF, both for its relatively large size and surface flatness. In this article, we first measure the S parameters of absorbing materials with transmission line methods. Then the electromagnetic parameters are extracted from S parameters, which served as input for simulation. The simulation model is set up for best match to real scenario of NRL arch reflectivity test, least model complexity and least computational cost. Actual NRL arch reflectivity test is conducted for comparison. For some suppliers in China, the only test conducted is NRL arch reflectivity test. However, if one wants to fully characterize absorbing materials and improve their EM performances, it is recommended that the dispersion properties of the materials can be fully characterize. Correct acquisition of these EM properties is crucial for impedance matching design, mixture ingredients design and material production.
The article is arranged in the following manner: Section II describes the measurement and characterization methods of absorbing materials in detail, including transmission line test with waveguides, S parameter retrieval, EM software simulation and NRL arch reflectivity test. Section III describes the results of the above mentioned methods, compares results from both simulation and measurements and analyzes discrepancies between the two. Section IV discusses the purpose of the article and proposes some suggestions in characterization of thin, magnetic absorbing materials.
Measurements of absorbing material have been performed for a long history. With new simulation modules and parameters extracted from experiments, one can perform EM simulation and compare to actual testing. After going through the whole process illustrated in Figure 1, one can acquire better understanding of the EM performance of absorbing materials. Through the comparison between simulation and measurement, one can appreciate the whole cycle of measurement and characterization thoroughly.
Before the transmission line test is conducted, samples of the MUT are prepared according to the inner boundary of the corresponding waveguides, as shown in Table 1. Size of the sample is of critical importance so that a tight fit with the waveguide is possible. Oversized sample may cause buckling, which can contribute substantially to incorrect characterization. For applications in 3G and 4G networks in China, we select the frequency from 1.72 to 2.61 GHz. The size of the MUT is 109.2 mm by 54.6 mm, whose pictures are shown in Figure 2. The model of the vector network analyzer is Agilent N5222A (now Keysight, frequencies up to 26.5 GHz). The version of the materials measurement software used is Agilent 85071E. And the type of straight waveguides in the system set up is HD-22WAL44 (HengDa Microwave Inc.), as shown in Figure 3(a). S parameters acquired from the transmission line method are imported to Agilent 85071E materials measurement software for parameter retrieval. Readers who are interested in the retrieval process can refer to [5,6]. Thickness of the MUT is recorded as 2.06 mm, which satisfied the requirements according to ref . It points out that thickness of the MUT is irrelevant to the retrieved material properties, and kL«1is recommended.
Transmission line method for acquisition of absorbing materials is robust and widely applied. S parameters from 1.72 to 2.61 GHz from the waveguide testing are shown in Figure 5. S11 and S22 parameters are equivalent for homogenous materials, which can be different for inhomogeneous or multi-layer materials, especially for asymmetric Metamaterial . EM properties acquired by Agilent 85071E material measurement software is shown in Figure 6. The properties are retrieved from S parameters with close form algorithms such as Nicolson-Ross . NIST model cannot apply since the MUT in this case is magnetic .
The purpose of this paper is to provide a systematic way of measurement and characterization of absorbing materials. And to provide a simulation method which is comparable to real NRL arch reflectivity test. The simulation is by no means trying to substitute or avoid real measurement, but to provide an option for early evaluation and fast characterization. Measurements mentioned in this article are mature and should be conducted by corresponding standards. The simulation method is enlightened by full wave simulation of Metamaterial. Discrepancies between the two are analyzed in last section.
More work can be done based on the research described in this article. For example, comparison of reflectivity from different incident angles between simulation and measurement can be conducted. Multiple layer absorbing materials can be investigated. Reflection and transmission properties of Metamaterial can be investigated with transmission line method and free space method in both simulation and measurement.
Abstract:The interest in composite materials has increased in the last decades since they have the advantages of combining intrinsic properties of each component and offer better performance with respect to the base constituents. In particular, these kinds of materials can have different electrical characteristics by varying the filling percentage and, therefore, they can be used in diverse applications. Thus, a detailed study of the microwave response of these composite systems is of great practical importance. In fact, the dielectric constant and loss tangent are key factors in the design of microwave components. In this frame, the outstanding properties of graphene-like fillers may be exploited to develop new very interesting materials to study and characterize. In this paper, microwave characterization of compounds, based on nylon 6 containing different percentages of graphene nanoplatelets, is carried out taking the neat matrix sample processed under the same conditions as benchmark. The measurements were carried out using two microwave systems, operating at two different frequency bands, appropriate to characterize solid and compact material samples. The achieved results, in line with limited data from the literature and from material data sheets, highlight the possibility to use the present polymers as an excellent electromagnetic interference shielding, as confirmed by full wave electromagnetic numerical simulations that were conducted with a numerical electromagnetic software.Keywords: graphene nanoplatelets; polyamide 6; composites; microwave characterization; complex permittivity; modeling
Tensile strength measurements were performed on eight specimens for each sample group according to ASTM D3039 [ASTM D3039/D3039M-00, American Society for Testing Materials standards] . A MTS test machine (Model QTEST 150) equipped with a 150 KN load cell as shown in Figure SI-1 was used. This floor instrument consists of two parts: the load frame and the Elite control system. Computer workstation and Testworks software are coupled with this instrument. The composite laminate was inserted into the grip jaws of the load frame section of the equipment while the Elite control system was used to control the movement of the load cells in upward and downward direction when needed. The results and data from the testing machine were displayed on the computer workstation using the Test works 4 software, which is a versatile software and simplifies test setup and increases test data reliability and repeatability. From each composite laminate, rectangular shaped tensile specimen (dimensions: 177.8 mm in length, 25.4 mm in width, and 2 mm in thickness) were
fabricated. The tabs, which are glass fibers/epoxy laminate, were bonded at the ends of the specimens for the tensile strength measurement. Figure SI-2 shows the images of the eight specimens used for tensile testing. The tensile tests were performed at a constant head-speed of 2.0 mm/min until the specimen fails. The tensile measurement was done at room temperature. Cross-sectional area 50.8 mm2 of the sample was available for tensile test for each specimen. The dimension of the specimens, materials used for bonded tabs and constant head speed of the test machine followed the recommendations from the standard method of ASTM D3039. 2b1af7f3a8