Free-space Method Research Articles

An experimental investigation of the effects on microwave measurement in a free-space method from different properties of a radiating patch antenna

The microwave free-space technique is an important method to characterize material properties in the scientific fields and actual applications. A microstrip patch antenna is a good radiating element to construct a compact, low-cost free-space setup. It is necessary to clarify the effects of different properties of radiating patch antenna. In this study, microwave attenuation and phase shift are measured with patch antennas with different radiated microwave power and gain. Both the experimental results of attenuation and phase shift show that a high gain of the radiating elements is helpful to measure the microwave parameters in the free-space method accurately, while improving radiating power has less effect.

Optical Frequency Transfer on the Order of 10−19 Fractional Frequency Instability over a 64 m Free-Space Link

High-precision time–frequency is widely used in time measurement, satellite navigation, scientific research, and other fields. With the rapid development of optical clock technology, the fractional frequency instability and uncertainty of optical clock have reached 10−18 orders of magnitude, which is expected to contribute to generating the International Atomic Time and may even be used to redefine the “second” in the future. Therefore, the long-distance transfer of time–frequency signals between optical atomic clocks is of great significance. Free-space optical frequency transfer technology is one of the important technologies for solving the space-based optical clock comparison because of its high transfer precision and easy networking characteristics. In order to solve the long-distance space-based optical clock comparison, this paper investigates a free-space active phase noise compensation method using an Acousto-Optic Modulator (AOM), based on the traditional optical fiber phase noise compensation scheme. This new method is more flexible and scalable than the optical fiber time–frequency transfer technology. The optical frequency transfer over a 64 m free-space link is demonstrated. The fractional frequency transfer instability during free running is 9.50 × 10−16 at 1 s, and 4.44 × 10−16 at 2000 s, and the fractional frequency instability after compensation is 7.10 × 10−17 at 1 s, 3.07 × 10−19 at 2000 s, which is about 1–3 orders of magnitude better than that in free running, and provides a feasible scheme for space-based optical clock comparison.

Monostatic Free-Space Method for Relative Permittivity Determination Using Horn Antenna Near-Field Measurements

Monostatic Free-Space Method for Relative Permittivity Determination Using Horn Antenna Near-Field Measurements

Ultra-broadband transmission absorption of the all-dielectric water-based metamaterial

In this paper, a structurally simple transmissive all-dielectric metamaterial absorber is designed, fabricated and measured. The unit cell consists of photosensitive resin and water, which the water layers composed of rectangle loop, rectangular block and traffic circle. It achieves over 90% absorption of electromagnetic waves in the frequency range between 11.0 and 27.3 GHz. Meanwhile, three absorption peaks are located at 12.5, 18.4, and 26.0 GHz, which the absorptive are 97.4%, 98.3%, and 98.5%, respectively. In addition, the characteristics of the designed metamaterial reveal wide-incident angle absorption and polarization-insensitive properties. The experimental sample is measured by using the free-space method, and the experimental results are verified to be in well agreement with the simulation data. Observing the distribution of electric and magnetic fields at different resonance frequencies, the physical mechanism of the absorption is attributed to strong magnetic resonance. Therefore, the proposed metamaterial has potential for widespread use in the fields of civilian and military.

Estimation of Permittivity of Films Using the Free-Space Measurement Method at Ka-Band

Estimation of Permittivity of Films Using the Free-Space Measurement Method at Ka-Band

UAV 3D Trajectory Planning and Optimization Using A* Algorithm Based on Free Space Method

Due to the complexity of three-dimensional (3D) map modeling, the A* algorithm is inefficient in planning 3D trajectories for unmanned aerial vehicles (UAVs). This paper designs a method for planning 3D trajectories using A* algorithm based on free space method to address this issue. The method translates the 3D trajectory planning problem into a two-dimensional (2D) trajectory planning problem through flight surface extraction, thereby enhancing the efficiency of trajectory planning. In addition, a trajectory optimization method considering safety parameters and offset costs is designed for the waypoints that fail to meet UAV restrictions. The optimization method consists of longitudinal trajectory altitude adjustment and horizontal trajectory smoothing to ensure the trajectory aligns with the UAV climb rate constraint, normal overload constraint and minimum turning radius constraint. Simulation results show that the designed method delivers higher efficiency in 3D trajectory planning and optimization than the conventional method, and the optimized trajectory adheres to the above constraints.

Multi-Functional Reconfigurable Intelligent Surfaces for Enhanced Sensing and Communication.

In this paper, we propose a reconfigurable intelligent surface (RIS) that can dynamically switch the transmission and reflection phase of incident electromagnetic waves in real time to realize the dual-beam or quad-beam and convert the polarization of the transmitted beam. Such surfaces can redirect a wireless signal at will to establish robust connectivity when the designated line-of-sight channel is disturbed, thereby enhancing the performance of wireless communication systems by creating an intelligent radio environment. When integrated with a sensing element, they are integral to performing joint detection and communication functions in future wireless sensor networks. In this work, we first analyze the scattering performance of a reconfigurable unit element and then design a RIS. The dynamic field scattering manipulation capability of the RIS is validated by full-wave electromagnetic simulations to realize six different functions. The scattering characteristics of the proposed unit element, which incorporates two p-i-n diodes have been substantiated through practical implementation. This involved the construction of a simple prototype and the subsequent examination of its scattering properties via the free-space measurement method. The obtained transmission and reflection coefficients from the measurements are in agreement with the anticipated outcomes from simulations.

The Impact Study of Anti-Radiation Biomass Material on Partition Wall Absorption Performance

With the increasing public concern over radiation exposure and its potential health risks, exploring practical solutions for mitigating radiation in various environments is essential. This study aims to raise awareness about radiation-related issues by investigating the impact of anti-radiation biomass material on the reflectivity performance of partition walls. By incorporating this biomaterial into partition walls, we examine how radiation absorption capabilities can be improved, enhancing overall radiation safety. The process entails combining the biomass material, Palm Oil Fuel Ash (POFA), cement, aluminum, and water in the proper ratios to create a composite material. The combined material is subsequently shaped into partition walls. The anti-radiation biomass partition wall's reflectivity performance was measured using the Naval Research Laboratory (NRL) Arch free space method, with frequencies between 1 GHz and 12 GHz utilised in this project. The study showed that the anti-radiation biomass partition wall containing 35% POFA demonstrated the best reflectivity performance.

Achieving high-efficiency LaFeO3/CoS2 nanocomposite electromagnetic wave absorber for X-band stealth applications

Recently, the focus of international conflicts has shifted from military to scientific and technological with the rapid development of electronic devices, which also generate electromagnetic pollution. To overcome and/or mitigate this pollution, the fabrication of lightweight and high-efficiency microwave-absorbing materials (MAMs) has been an urgent task for the scientific community. MAMs possess significant potential across domains such as information security, 5 G communication, energy harvesting, diminished radar cross-section, and advancements in military stealth technology. Two-phase LaFeO3/CoS2 nanocomposite was fabricated to be expected as promising MAMs. Four different LaFeO3/CoS2-based MAMs showed different microwave absorption performances, where low filler loading samples showed poor features and high filler loading samples showed excellent features.MAMs with 30 and 40 wt% filler loading (named LFC-30 and LFC-40) could reach excellent values for both reflection loss (RL) and effective absorption bandwidth (EAB). The RL values were –34 and –46 dB, and the EAB values were 3.1 and 4.0 GHz for LFC-30 and LFC-40, respectively. The RL and EAB values of LFC-40 could absorb ∼99.99% of incident microwave and mostly cover the whole X band with a thin thickness of 1.6 mm. The excellent microwave absorption performance of LFC-40 could also be confirmed by the free-space method. Superior dielectric and magnetic loss tangents, rich relaxation polarization processes, good impedance matching, and a high attenuation constant could be the origins of high-efficiency MAMs in LFC-40.

Bidirectional planar absorber with polarization-selective absorption and reflection capabilities.

In this study, we developed a novel planar bidirectional perfect metamaterial absorber (PMA) with polarization-selective absorption and reflection capabilities. The proposed bidirectional PMA has near-perfect absorption for y- and x-polarized waves propagating in the -z and + z directions. It also reflects x- and y-polarized waves propagating in the -z and + z directions. We used full-wave simulations and Fabry-Perot cavity models to evaluate the performance of the proposed bidirectional PMA. We also used a free-space method to measure the fabricated sample. To demonstrate the potential of the proposed PMA in multiband systems, we extended our PMA design to a dual-band bidirectional absorber.

Bidirectional planar absorber with polarization-selective absorption and transmission capabilities.

In this study, we developed a novel planar bidirectional perfect metamaterial absorber (PMA) with polarization-selective absorption and transmission capabilities. The proposed structure can bidirectionally absorb x-polarized incident waves almost perfectly while functioning as a transparent surface for y-polarized incident waves at the same frequency. We discussed the performance and properties of the proposed PMA through simulation results and a theoretical model. We also used the free-space method in experimental tests of a fabricated sample. The results indicated fair consistency between the simulated and measured results, thereby validating the quality of our PMA design.

The Performance of Hollow Pyramidal Microwave Absorber Using Different Slot Size

Microwave absorber is commonly used in defense, electromagnetic compatibility (EMC)/electromagnetic interference (EMI) reduction and anechoic chamber application. Conventional electromagnetic (EM) absorbers have some constraints in practical handling due to its heavy weight. In this paper, the research focuses on the development of high performance and lightweight microwave absorber. A lightweight and simple design configuration of different rectangular slot size array implemented on hollow pyramidal microwave absorber are carried out. There are two different designs of different slot size array calculated based on 3GHz, 6GHz and 9GHz frequency slot size. The two designs have an opposite slots size array arrangement namely different slot size 3GHz, 6GHz, 9GHz order design array and different slot size 9GHz, 6GHz, 3GHz order design array. The absorption is measured using the Naval Research Laboratory (NRL) arch free space method in the frequency range of 1GHz to 12GHz covering L, S, C and X band. In the measurement result, the maximum absorption performance is obtained by the different slot size 9GHz, 6GHz, 3GHz order design array which is up to -44.23dB at X-band. The measurement results for both designs array show good absorption performance which exhibit below than -20dB especially at high frequency band. The proposed designs have been identified as the new approach to increase absorption improvement over a broad frequency range application.

Utilization of Carbon Biomass as an Absorbing Material in Anti-Microwave Brick Walls Manufacturing

The developments in communications technology bring changes in human life, and the positive aspects of these technological innovations make life more accessible. However, the use of communications technology at a certain frequency will produce radiation that could negatively affect human health. Thus, various studies have been performed to obtain a corresponding material to reduce the levels of exposure to the pollution of the electromagnetic wave signal. This study aims to build anti-microwave brick walls and analyze the absorption performance of the anti-microwave brick walls by using carbon biomass as an absorbing material. Carbon biomass is a physically defined product obtained in a highly controlled process to produce specifically engineered aggregates of carbon particles and in this project, carbon biomass was used as an absorbing material in brick production. The absorption performance of the anti-microwave brick walls was measured by using Naval Research Laboratory (NRL) arch free-space method, and the frequency range of 1GHz to 12GHz is used. The result shows that brick containing the highest percentage of carbon biomass produces the best absorption performance at a high-frequency band (4-8GHz), with maximum absorption of -29.53dB.

Structural engineering of double shells decoration for preparing a high-efficiency electromagnetic wave absorber

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 HoFeO3@WSe2@SiO2 composite via the co-precipitation method. The microwave absorption properties of the HoFeO3,WSe2, SiO2 and HoFeO3@WSe2@SiO2 (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 HoFeO3@WSe2@SiO2 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 HoFeO3@WSe2@SiO2 microwave absorbers can offer significant assistance with a wider absorption band. In this context, HoFeO3@WSe2@SiO2 is expected to be used as a highly efficient microwave absorbent.

Determining peanut moisture content by scattering coefficient

Moisture content is a fundamental parameter in peanut production and storage. Moisture content information is essential for crop analysis. Crop analysis is significant for production planning and improving yield. Thus, developing a moisture content measurement system with high precision and low cost can benefit peanut production, processing, and storage. To achieve accurate and rapid nondestructive testing of shelled peanuts and peanut pods, this study proposes a new method for measuring moisture content based on the microwave scattering coefficient. The microwave free-space method was used to measure the scattering parameters of peanut samples with different moisture contents (8.11–45.59% wet basis) at different temperatures and sample thicknesses. Moreover, a peanut moisture measurement model using S-parameters was developed. Using the amplitude, phase, and real part and imaginary part combinations of the S11 and S21 parameters of the peanut samples as the input and peanut moisture content as the output, moisture content and S-parameter prediction models based on the gradient boost regression trees (GBRT), extreme gradient boost (XGBoost), and fully connected deep neural network (FC-DNN) were established. Test results verified that the FC-DNN-based model had the best prediction performance, with a coefficient of determination (R2) = 0.9998, mean absolute error (MAE) = 0.0891, mean square error (MSE) = 0.0254, and root mean square error (RMSE) = 0.1593. This study provides a new method for the nondestructive measurement of moisture content during peanut production and storage. Moreover, the proposed model can be applied for moisture content measurement of other agricultural products in the food processing industry.

Orthogonal Adjacent-Order Filtering Element-Based Wideband Transmissive Linear-to-Circular Polarization Converter

This article proposes a unified design method for a highly efficient wideband transmissive linear-to-circular polarization converter composed of two orthogonal adjacent-order bandpass filtering elements for the first time. First, the linear-to-circular polarization converter is constructed to convert two orthogonal linearly polarized (LP) spatial waves split from a 45°-slant incident plane wave into a desired circularly polarized (CP) wave. Taking advantage of the wideband filter synthesis method and phase shifter theory, polarization converter elements with prescribed specifications [e.g., fractional bandwidth (FBW) and Chebyshev passband ripple] are then synthesized by the corresponding low-characteristic-impedance bandpass filtering networks with the loading of shunt short-circuited stubs on quarter-wavelength transmission line (TL) sections. The adjacent-order phase difference function with a natural ±90° value at a center frequency can be simultaneously manipulated by a filtering bandwidth. The theoretical axial ratio (AR) curve is calculated by the synthesized magnitude responses and phase deviation (PD) function. The design guideline is then summarized. Finally, based on the third- and fourth-order bandpass filtering networks, a prototype of the proposed linear-to-circular polarization converter operating at 5 GHz with a designable 70% 3 dB AR bandwidth is implemented, fabricated, and tested by the free-space measurement method. The transmitted CP wave with three in-band AR minima and relatively stable angular performances can be efficiently converted from the incident LP spatial wave as well verified in the experiment. The AR curves and design requirements of different adjacent-order bandpass filtering combinations are further explored according to the proposed design guideline.

Broadband, Stable, and Noniterative Dielectric Constant Measurement of Low-Loss Dielectric Slabs Using a Frequency-Domain Free-Space Method

A broadband, stable, and noniterative free-space method is proposed for dielectric constant <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon _{r}^{\prime }$ </tex-math></inline-formula> determination of low-loss dielectric slabs from reflection-only measurements through simple calibration standards (reflect and air). It is applicable for dispersive samples and does not require thickness information. Simulations of nondisperive and dispersive samples are performed to validate our method. Dielectric constant measurements of polyethylene and polyoxymethylene samples (9–11 GHz) are carried out to examine the accuracy of our method.

Free-Space Two-Tier One-Port Calibration Using a Planar Offset Short for Material Measurement

The scattering parameters of a material under test (MUT) are prerequisites for characterizing the material parameters of the MUT. This paper describes a free-space two-tier one-port calibration method using a planar offset short as a free-space calculable reflect standard for measuring the scattering parameters of an MUT from the two successive one-port calibrations of a free-space material measurement system without a precise positioning system in free space. The two-tier one-port calibration method is validated by comparing the measurement results with those of the thru-reflect-line (TRL) calibration method for two reciprocal MUTs (glass plates of 2.780 mm and 4.775 mm thickness) in the W-band (75–110 GHz). Good agreement between the measurement results from the two calibration methods demonstrates that the free-space two-tier one-port calibration method using a planar offset short can be a feasible and effective alternative to the conventional free-space two-port calibration methods.

Quasi-Optical Measurement and Complex Refractive Index Extraction of Flat Plate Materials Using Single Time-Domain Transmission Model in Y-Band

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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Y$ </tex-math></inline-formula> -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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8f$ </tex-math></inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Y$ </tex-math></inline-formula> -band, as well as the requirements of system calibration.

A resistance heating assisted free space method to measure temperature-dependent electromagnetic properties of carbon fiber reinforced polymer composites

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.