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The rapid and nondestructive detection of tea leaf moisture content (MC) is of great significance to processing tea with an automatic assembly line. This study proposes an MC detection method based on microwave scattering parameters (SPs). Through the established free-space electromagnetic measurement device, 901 different frequency points are taken between 2.45 and 6 GHz using a vector network analyzer (VNA). The SPs of tea leaves with different moisture contents (5.72–55.26%) at different bulk density and different sample thicknesses were measured. The relationship between frequency, S21 amplitude and moisture content, thickness, and bulk density of tea was analyzed using correlation coefficients, significance analysis, and model construction. Back propagation (BP) neural network, decision tree (DT), and random forest (RF) MC prediction models were established with the frequency, amplitude, and phase of the SPs, thickness, and bulk density of the samples as inputs. The results showed that the RF-based model had the best performance, with determination coefficient (R2) = 0.998, mean absolute error (MAE) = 0.242, and root mean square error (RMSE) = 0.614. Compared to other nondestructive testing processes for tea, this method is simpler and more accurate. This study provides a new method for the detection of tea MC, which may have potential applications in tea processing. [ABSTRACT FROM AUTHOR]

As 5G technology advances, so does the prevalence of harmful electromagnetic waves, and as a result, researchers have been progressively interested in developing materials that can absorb microwaves. So, it's become desirable to fabricate unique microwave absorbent materials (MAMs) to mitigate the negative effects of electromagnetic radiation and offer protection to biological beings and electronic data. Herein, we fabricated a novel HoFeO 3 @WSe 2 @SiO 2 composite via the co-precipitation method. The microwave absorption properties of the HoFeO 3 ,WSe 2 , SiO 2 and HoFeO 3 @WSe 2 @SiO 2 (with 25, 35 and 45 wt% loading) were examined in the X-band frequency range. 35 wt% filler loading shows reasonable reflection loss values with the broadest effective absorption bandwidth (EAB). The optimum impedance matching is responsible for the enhanced microwave absorption performance of the HoFeO 3 @WSe 2 @SiO 2 with 35 wt% filler loading. In addition, the best sample is measured using both waveguide and free-space techniques and the results have been compared. The free-space method has been demonstrated to be an effective alternative testing strategy due to its contact-free and non-destructive features. Furthermore, the technique described in this study to design innovative HoFeO 3 @WSe 2 @SiO 2 microwave absorbers can offer significant assistance with a wider absorption band. In this context, HoFeO 3 @WSe 2 @SiO 2 is expected to be used as a highly efficient microwave absorbent. [ABSTRACT FROM AUTHOR]

Civilization can be shielded from the dangerous electromagnetic spectrum by using microwave absorption materials, however, absorbing electromagnetic radiation with thin thickness and high bandwidth remains a challenge, especially at scales that are significant. Herein, we propose a novel architecture where worm-like Cu 2 S particles are decorating CuCo 2 S 4 micro-spheres were decorated, and this method is thought to be a successful one for enhancing the created nanocomposite's ability to dissipate electromagnetic radiation. Changing the filler loading percentage allows the nanohybrids' electromagnetic characteristics and microwave dissipation effectiveness to be efficiently changed. This leads to the creation of ultra-bandwidth absorbers with thin thickness, which are then tested using waveguide and free-space techniques. The sample with a thickness of 1.4 mm has a maximum reflection loss of −18 dB and a maximum bandwidth of 3.6 GHz. The hetero-structures, multi-interfaces, and multiple relaxations phenomena, as well as the combined effects of the two components, are credited with the superior microwave absorption performance compared with the state-of-the-art. This finding demonstrates that CuCo 2 S 4 /Cu 2 S nanohybrids pave the way for the development of future high-performance microwave absorption materials. [ABSTRACT FROM AUTHOR]

This paper describes a prototype of a free-space transmission setup for dielectric parameters estimation. The transmitter and receiver, both configurable by software and working in the range 0.85–1.55 GHz, are not synchronized, as they use different clocks. An estimation of the dielectric permittivity of a planar sample was obtained by comparing our measurements with a numerical model. A parametric study with different variables was conceived in order to find the best fit between measurements and simulations. Customized techniques were applied to deal with noise and inconsistencies found in the measurements. The genetic algorithm was used to adjust the constants that minimized the error between simulated and experimental data. Results for a reference sample of polymethylmethacrylate are presented and discussed. Although the accuracy of the proposed approach in recovering the dielectric parameters of the sample was relatively low, the simplicity and cost-effectiveness of this setup make it interesting for scenarios where a rough characterization of the material is sufficient. [ABSTRACT FROM AUTHOR]

Temperature-dependent electromagnetic properties are not only closely related to the manufacturing process of carbon fiber reinforced polymer (CFRP) components, e.g., curing of CFRP composites using microwaves, but also directly determines some key performance indicators of CFRP structures like electromagnetic stealth and anti-electromagnetic interference (EMI). In this paper, we proposed a resistance heating assisted free space method to measure the temperature-dependent electromagnetic properties of both CFRP materials and structures. On this basis, the temperature-dependent electromagnetic properties of a unidirectional carbon fiber/PEEK composite material were measured in the temperature range from room temperature to 400 °C. Moreover, the temperature-dependent electromagnetic properties of a CFRP structure, i.e., a multidirectional carbon fiber/PEEK laminate covered by subwavelength resonance structures, were also measured. The results indicated that temperature has a significant effect on the electromagnetic properties of both CFRP materials and structures. Our work provides a simple and practical solution to measure the temperature-dependent electromagnetic properties of CFRP materials and structures.

Accurate acquisition of the complex refractive index of materials is of great significance to understand their physical properties. Based on a single time-domain transmission model in $Y$ -band, here, we develop a free-space nondestructive testing method combined with parameter extraction of flat plate materials. This method overcomes the limitation of the narrow frequency band of S-parameters of vector network analyzer (VNA), truncates multiple echo interferences in time-domain signals inside sample after the inverse fast Fourier transform (IFFT), and eliminates the crimping effect of the calculation results. In order to ensure the good signal integrity of the model, an empirical formula is proposed to calculate the minimum thickness of the sample corresponding to unit bandwidth. An $8f$ quasi-optical system is designed and optimized to provide a measuring condition of terahertz (THz) plane wave with high energy density. Compared with the values calculated by the conventional methods, the refractive index deviation is less than 0.004 (0.2%), which verifies the effectiveness and applicability of our proposed method. The adaptability of different processes to the corresponding physical transmission model is further analyzed, and the reasons for the calculation deviation are explained. This work reduces the complexity and difficulty of the material measurement in the $Y$ -band, as well as the requirements of system calibration. [ABSTRACT FROM AUTHOR]

The rapid and nondestructive detection of tea leaf moisture content (MC) is of great significance to processing tea with an automatic assembly line. This study proposes an MC detection method based on microwave scattering parameters (SPs). Through the established free-space electromagnetic measurement device, 901 different frequency points are taken between 2.45 and 6 GHz using a vector network analyzer (VNA). The SPs of tea leaves with different moisture contents (5.72–55.26%) at different bulk density and different sample thicknesses were measured. The relationship between frequency, S21 amplitude and moisture content, thickness, and bulk density of tea was analyzed using correlation coefficients, significance analysis, and model construction. Back propagation (BP) neural network, decision tree (DT), and random forest (RF) MC prediction models were established with the frequency, amplitude, and phase of the SPs, thickness, and bulk density of the samples as inputs. The results showed that the RF-based model had the best performance, with determination coefficient (R2) = 0.998, mean absolute error (MAE) = 0.242, and root mean square error (RMSE) = 0.614. Compared to other nondestructive testing processes for tea, this method is simpler and more accurate. This study provides a new method for the detection of tea MC, which may have potential applications in tea processing.

This paper describes a prototype of a free-space transmission setup for dielectric parameters estimation. The transmitter and receiver, both configurable by software and working in the range 0.85–1.55 GHz, are not synchronized, as they use different clocks. An estimation of the dielectric permittivity of a planar sample was obtained by comparing our measurements with a numerical model. A parametric study with different variables was conceived in order to find the best fit between measurements and simulations. Customized techniques were applied to deal with noise and inconsistencies found in the measurements. The genetic algorithm was used to adjust the constants that minimized the error between simulated and experimental data. Results for a reference sample of polymethylmethacrylate are presented and discussed. Although the accuracy of the proposed approach in recovering the dielectric parameters of the sample was relatively low, the simplicity and cost-effectiveness of this setup make it interesting for scenarios where a rough characterization of the material is sufficient.

In order to meet the requirements of "wide, thin, strong and light" for military stealth materials, it is of great practical value to study the absorbing characteristics of multi-layer MWCNTs/Fe3O4/NBR absorbing materials in space. First, we use the large-scale software COMSOL Multiphysics to simulate the absorbing characteristics of the composite thin plate in space. Then the four-port network matrix is used to calculate the absorbing characteristics of the composite plate in space. Finally, the Free-Space method is used to measure the reflection attenuation loss, and the results of the three methods are compared and analyzed. The results show that when the frequency is 10 GHz, the reflection loss of multi-layer MWCNTs/Fe3O4/NBR reaches the maximum value of −27.91, −27.01 and −22.56 dB by COMOSL numerical simulation, four-port network the matrix and Free-Space experimental measurement. The results of the three methods show that the reflection loss is less than −10 dB in the frequency band of 6–14 GHz. [ABSTRACT FROM AUTHOR]

A new double-reflection free-space method is presented in this paper for measuring the actual wall material permittivity in indoor scenes, especially for the narrow wall corners which cannot accommodate test settings used in the traditional free-space method. The method is nondestructive, accurate, and easy to implement. In addition, it does not require a priori knowledge of the thickness of the walls under test. Two experimental setups, namely, double-polarization and double-parameter free-space measurements, were conducted according to the proposed method. The presented technique acquires the accurate material permittivity on site, which is the key parameter in the modeling and simulation of the radio wave propagation (RWP) prediction. The permittivity of the walls in the buildings measured by the proposed method, which considers the influences from the aging, machining, and practical usage of the wall materials, is more accurate than that by sample test in the laboratory. The method was examined by measuring the permittivity of the designed wall corner with two perpendicular plates and validated by the waveguide measurement of the sample chosen from the tested plates. Finally, the explicit uncertainty and error analyses were derived. The proposed method is applicable for accurate modeling of RWP prediction in practice. [ABSTRACT FROM AUTHOR]

The most pressing need for the majority of applications is the development of microwave absorption composites with thin thickness to make them practical to use and capable of high absorption with adjustable effective absorbing bandwidth. In this research, an innovative nanocomposite made of a LaFeO 3 compound decorated with CuCo 2 O 4 particles was chemically developed, and its electromagnetic properties were extracted to study microwave absorbing capability using both waveguide and free space methods, as well as related mechanisms. The resin-based absorber sample with 40% LaFeO 3 /CuCo 2 O 4 nanocomposite possesses the best wave dissipation feature, with a reflection loss of − 20 dB and an absorption bandwidth of 3.3 GHz in an absorber sample with just 1.8 mm thickness. This is due to the best loss capacity matching factor as well as improvements in impedance matching and attenuation coefficient capability. The sample was made on a 20 × 20 cm scale in accordance with the waveguide mode's results, and the findings were completely supported after carrying out the assessments that are required. This study presents a unique approach to developing EMW-absorbing materials with a broad effective absorption bandwidth and an appropriate absorption capacity by assessing the absorption properties using two different testing methodologies. [Display omitted] • CuCo 2 O 4 -wrapped LaFeO 3 nanocomposite was prepared to study microwave absorption properties. • Measurement was done using free-space and waveguide techniques; both of these methods show the same results. • Impedance matching, along with the best loss capacity matching factor is achieved. [ABSTRACT FROM AUTHOR]

To investigate the reinforcement architectures effect on the electromagnetic wave properties of carbon fiber reinforced polymer composites, three-dimensional (3D) interlock woven fabric/epoxy composites, 3D interlock woven fabric with stuffer warp/epoxy composites, and 3D orthogonal woven fabric/epoxy composites were studied by the free-space measurement system. The results showed that the three types of 3D woven carbon fiber fabric/epoxy composites had a slight difference in electromagnetic wave properties and the absorption was their dominant radar absorption mechanism. The electromagnetic wave absorption properties of the three types of composites were more than 90% (below −10 dB) over the 11.2–18 GHz bandwidth, and more than 60% (below −4 dB) over the 8–12 GHz bandwidth. Compared with unidirectional carbon fiber reinforced plastics, the three kinds of 3D woven carbon fiber fabric/epoxy composites exhibited better electromagnetic wave absorption properties over a broadband frequency range of 8–18 GHz. Therefore, the three kinds of 3D woven composite are expected to be used as radar absorption structures due to their excellent mechanical properties and outstanding absorption capacity. The total electromagnetic interference shielding effectiveness of the three types of 3D carbon fiber woven composites are all larger than 46 dB over the 8–12 GHz bandwidth, which is evidence that the three types of 3D carbon fiber woven composites can be used as excellent shielding materials for electromagnetic interference. [ABSTRACT FROM AUTHOR]

As a new broadband-wave absorber material in millimeter waves, the thin rubber sheet containing a porous carbon material “PHYTOPOROUS” made from plant-based ingredients were prepared. We investigated the complex permittivity εr*and the reflection loss Γ measured by free-space method using dielectric lenses and incident angle dependence of the reflection loss measured by free-space arch method. The rubber sheet containing PHYTOPOROUS made from soybean hulls showed the reflection loss of more than −20 dB with a bandwidth of 18 GHz at centered frequency of 90 GHz. [ABSTRACT FROM AUTHOR]

This paper describes a microwave free-space method using flat artificial lens antennas with anti-reflection layer. The lens antenna is made of an artificial material of metal particle. Comparing with our previous study, Anti-reflection (AR) layers are supplemented to the lens in this study to obtain a wave matching on the air-lens interface. The improved lens is in a disk shape of 50 mm diameter and 5.9 mm thickness. The lens is applied in a free-space setup, in which it is set in front of a patch antenna resonating at 15 GHz to get a high gain. The free-space setup is used to measure microwave attenuation and phase shift through a sawdust sample. The experimental results show that the multiple-reflection in the free-space method becomes small, because the reflection on air-lens interface has been reduced. The proposed AR lens antenna is flat and very small in the size. It is possible to construct a very compact and low cost free-space setup using the improved lens. [ABSTRACT FROM AUTHOR]

Temperature-dependent electromagnetic properties are not only closely related to the manufacturing process of carbon fiber reinforced polymer (CFRP) components, e.g., curing of CFRP composites using microwaves, but also directly determines some key performance indicators of CFRP structures like electromagnetic stealth and anti-electromagnetic interference (EMI). In this paper, we proposed a resistance heating assisted free space method to measure the temperature-dependent electromagnetic properties of both CFRP materials and structures. On this basis, the temperature-dependent electromagnetic properties of a unidirectional carbon fiber/PEEK composite material were measured in the temperature range from room temperature to 400 °C. Moreover, the temperature-dependent electromagnetic properties of a CFRP structure, i.e., a multidirectional carbon fiber/PEEK laminate covered by subwavelength resonance structures, were also measured. The results indicated that temperature has a significant effect on the electromagnetic properties of both CFRP materials and structures. Our work provides a simple and practical solution to measure the temperature-dependent electromagnetic properties of CFRP materials and structures. • A resistance heating assisted free space method was proposed. • The effectiveness of the resistance heating assisted free space method was validated. • The temperature-dependent electromagnetic properties of a carbon fiber/PEEK composite material were measured. • The temperature-dependent electromagnetic properties of a carbon fiber/PEEK structure were determined. [ABSTRACT FROM AUTHOR]

A free-space method for the measurement of biaxial material is proposed in this paper. Four complex transverse constitutive parameters are directly computed from the reflection and transmission coefficients of a planar sample in free space for the normally incident plane wave with two polarizations. However, because of the extreme insensibility of the reflection and transmission coefficients versus the longitudinal constitutive parameters for thin planar biaxial materials, the correct reconstruction of longitudinal parameters cannot be accomplished by the free-space method. Considering that the principal coordinate system (PCS) to characterize biaxial material possibly does not coincide with the measurement coordinate system (MCS), an approach to determine PCS is also presented to ensure practicability of this method. A measurement system has been set up to perform measurements of transverse parameters of biaxial material and the validity of this method is experimentally verified. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 46: 72–78, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20905 [ABSTRACT FROM AUTHOR]

For in situ measurement of the complex permittivity of planar materials, a free-space system based on reflection or transmission ellipsometry has been developed and extended to microwave frequencies. Different angles of incidence were studied in the range [35–50°]. Original numerical methods based on contour-line charts and least-square optimization have been developed. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 33: 443–448, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10346 [ABSTRACT FROM AUTHOR]

Free-space method approaches for obtaining the complex permittivity of dielectric layers embedded in a multilayer dielectric material are investigated. Solids, liquids, and granular materials have been tested; a description of the de-embedding process is presented in detail, as are the measured permittivity values. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 33: 422–426, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10340 [ABSTRACT FROM AUTHOR]

A simple and efficient method for determining the complex permittivity of dielectric materials from both reflected and transmitted signals is presented. It is also novel because the technique is implemented using two pyramidal horns without any focusing mechanisms. The dielectric constant of a noninteractive and distributive (NID) mixture of dielectrics is also determined. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 26: 117–119, 2000. [ABSTRACT FROM AUTHOR]

The knowledge of the electromagnetic constitutive properties of materials is crucial in many applications. Free-space methods are widely used for this purpose, despite their inherent practical difficulties. This paper describes an affordable free-space experimental setup for the characterization of flat samples in 1–6 GHz in a non-anechoic environment. The extracted properties are obtained from the calibrated Scattering Parameters, using a frequency-by-frequency solution or a multi-frequency reconstruction. For the first, we describe how the Time-Domain Gating can be implemented and used for filtering the signals. For the latter, a weighting factor is introduced to balance the reflection and transmission data, allowing one to have a more favorable configuration. The different role of transmission and reflection measurements on the achievable results is analyzed with regard to experimental uncertainties and different noise scenarios. Results from the two strategies are analyzed and compared. Good agreement between simulation, measurement and literature is obtained. According to the reported results for dielectric materials, there is no need of filtering the data by a Time-Domain Gating in case of the multi-frequency approach. Experimental results for Polymethylmethacrylate (PMMA) and Polytetrafluorethylene (PTFE) samples validate both the setup and the processing. [ABSTRACT FROM AUTHOR]

The complex permittivity of low-loss dielectric materials (ABS and graft polymer) in the 94 GHz band is measured using the free-space method. The validity of the free-space method at 94 GHz is confirmed, and the data of the complex permittivity of these materials are presented. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 22: 291–292, 1999. [ABSTRACT FROM AUTHOR]