Free-space Method Research Articles

A new method for measuring complex relative permittivity of dielectric material by reflection method

In this paper, a new lossless free space method for measuring the complex relative permittivity of dielectric material by reflection method is presented. When this method is used to measure the complex relative permittivity of dielectric material, there is no multi-value problem. The measurement system used in this method consists of a horn lens antenna and a vector network analyzer(VNA). In this paper, HFSS microwave simulation software is used to simulate the complex relative permittivity measurement method proposed in this paper. The simulation results show that this method can accurately measure the complex relative permittivity of dielectric material.

Numerical simulation study of free space method for measuring sample permittivity

The composition and performance of the material can be expressed by the permittivity. The paper introduces the measurement principle of free space method and TRL calibration principle, the calculation formula of the sample permittivity is theoretically analysed, a horn lens antenna is designed to simulate the emission of plane waves, and numerical simulation software is used for simulation and optimization. In the working frequency range, the voltage standing wave ratio of the antenna is less than 1.49, and the maximum gain is 21.8dB. A free space measurement system is established in the software to measure the sample permittivity with different thicknesses, and compare the calculated value with the actual value. The results show that the root mean square error of the real part is less than 5.3%, and the root mean square error of the imaginary part is less than 1.3%. The correctness and applicability of the formula for calculating the permittivity are also proved.

Study on space absorbing of multilayer MWCNTs/Fe3O4/NBR composites

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.

A procedure and device for determining complex material permittivity using the free-space resonance method.

The essential technologies of the complex permittivity of microwave dielectric materials are systematically designed, and the complex permittivity of materials is tested nondestructively by the free-space resonance method. A testing system was built by using a mobile surveying platform, and the complex dielectric constant of the material in the X band was nondestructively tested by using the algorithm of variable physical cavity length and constant physical cavity length. Focusing on the impact of variable physical cavity length on the test results, the cavity calibration technology is proposed to reduce the influence on the complex dielectric constant test of materials. The free-space resonance method was used to test the complex permittivity of polytetrafluoroethylene, glass steel plate (fiber reinforced plastics), and corundum plate. The results show that the test results of complex permittivity obtained by the two algorithms are consistent, and the error of complex permittivity is less than 5%.

Исследование потерь пропускания в поликристаллическом CVD-алмазе в миллиметровом диапазоне длин волн методом свободного пространства

Radio transparency of polycrystalline CVD-diamond disks with diameter up to 75 mm in millimeter-wave range was measured by free-space method. The structure of the disks was characterized by Raman spectroscopy and scanning electron microscopy. Dielectric loss tangent tan δ of the samples in the frequency range of 50 – 67 GHz was found to be in the range of 7.5·10−3 − 2.8·10−2, increasing with frequency. The transmission loss due to the radiation absorption is about 1%.

Accurate Characterization of Electromagnetic Band-Gap Structures

In this paper, electromagnetic band-gap (EBG) structures are accurately characterized using a free-space method. The field transmission and reflection parameters are exploited to determine the effective material parameters both inside and outside the band-gap regions of the EBG structures. The electromagnetic (EM) properties of the effective refractive index, effective wave impedance, effective permittivity, and effective permeability at multiple band-gap regions are derived from a single scattering computation. The derived phase and attenuation constants correctly define the boundaries of the band-gap region and the level of attenuation within the band gap. Deviations in the incidence angle from the normal direction alter the EM properties; hence, the operating bandwidth of the band-gap region is as expected due to the inherent structure anisotropy of the unit cells.

Assessing the Impact of Data Filtering Techniques on Material Characterization at Millimeter-Wave Frequencies

This article reports on an investigation into data filtering techniques for material characterization using a commercially available WR-15 (50-75 GHz) material characterization kit (MCK). The MCK uses a guided free-space method (operating like the conventional quasi-optical focused beam system) that enables measurement of S-parameters of dielectrics using a vector network analyzer (VNA). Multiple reflections and resonances exist in the MCK and they cannot be readily eliminated by calibration. To minimize these effects on extracting dielectric constant and loss tangent from the measured S-parameters, data filtering techniques (e.g., Savitzky-Golay filter and time-gating) can be adopted. A comparison of the measurement results using these two techniques was carried out on five common dielectric materials. Generally, the results obtained using these two filtering techniques are consistent with each other, and the Savitzky-Golay filter offers better performance in the scenario where the specimen quality is not ideal. Three specimens were also measured using an open resonator (operating at 36 GHz). There is generally good agreement between the results from the MCK and the open resonator.

Ultra-porous and lightweight microwave absorber based on epoxy foam loaded with long carbon fibers

A simple methodology to fabricate ultra-porous and lightweight epoxy foam composites loaded with carbon fibers is presented. These materials are proposed for microwave absorbing application. The elaborated composite density can be controlled via proper proportioning of carbon fiber/solvent/surfactant/epoxy components constituting the absorber composites. The influence of the weight percentage of these different components, on the composite structure, is studied. The density of the composites can be reduced to 0.05g‧cm−3 while the density of the neat epoxy foam is about 0.12g‧cm−3. Dielectric characterization of composites elaborated with low weight percentages of carbon fibers (from 0.0125 wt.% to 0.075 wt.%) is conducted using free space method. The simulation of a multilayer absorber is conducted and predicts a promising absorption performance (reflection coefficient <-10 dB) for the wide studied frequency band; this result was confirmed by the measurement of a multilayer absorber prototype. Finally, these performances are compared to commercial absorber materials having same dimensions.

A New Approach to Estimate the Complex Relative Permittivity of Materials Based on Free-Space Reflection Method

A microwave free-space reflection method for determining the complex permittivity of planar materials is demonstrated. Improved the MUltiple SIgnal Classification (MUSIC) algorithm to estimate the complex relative permittivity of material based on the reflection coefficients measurement of a metal-backed material sample placed in a free-space with the thickness of the samples is the much greater time the wavelength. The measurement system consists of a pyramidal horn antenna operating at X-band and a metal-backed sample placed in a free-space. From the measured values of the reflection coefficients through a known thickness of a planar slab of the material sample, the complex relative permittivity of the material samples is estimated by the proposed algorithm without determining an integer time the wavelength in the sample under test. The proposed algorithm is verified with different thickness material samples at X-band. The estimation results show that the complex relative permittivity of material samples is accurately when thickness changes.

Graphene Epoxy-Based Composites as Efficient Electromagnetic Absorbers in the Extremely High-Frequency Band.

We report on the synthesis of the epoxy-based composites with graphene fillers and test their electromagnetic shielding efficiency by the quasi-optic free-space method in the extremely high-frequency (EHF) band (220-325 GHz). The curing adhesive composites were produced by a scalable technique with a mixture of single-layer and few-layer graphene layers of few-micrometer lateral dimensions. It was found that the electromagnetic transmission, T, is low even at small concentrations of graphene fillers: T<1% at a frequency of 300 GHz for a composite with only ϕ = 1 wt% graphene. The main shielding mechanism in composites with the low graphene loading is absorption. The composites of 1 mm in thickness and a graphene loading of 8 wt% provide an excellent electromagnetic shielding of 70 dB in the sub-terahertz EHF frequency band with negligible energy reflection to the environment. The developed lightweight adhesive composites with graphene fillers can be used as electromagnetic absorbers in the high-frequency microwave radio relays, microwave remote sensors, millimeter wave scanners, and wireless local area networks.

Picosecond-precision optical two-way time transfer in free space using flexible binary offset carrier modulation

Free-space optical time transfer that features high precision will act as a crucial role in near-future outdoor timing service and ground-to-satellite/inter-satellite clock networks. Here we propose a free-space optical two-way time transfer method using flexible binary offset carrier modulation. The alternative method could yield a comparative precision compared to optical binary phase-shift keyed modulation. For verification, a time transfer experiment with our home-built system between two sites separated by a 30-m free-space path outside the laboratory was conducted. Over a 15 h period, the time deviation is 2.3 ps at 1-s averaging time, and averages down to 1.1 ps until ∼30 s. The fractional frequency instability exhibits 4.0×10−12 at a gate time of 1 s, and approaches to 1.3×10−15 at 10000 s.

A 3-D Lumped-Components-Free Absorptive Frequency-Selective Transmission Structure Featuring Very Wide Two-Sided Absorption Bandwidths

A 3-D absorptive frequency-selective transmission (AFST) structure that does not require any lumped components and featuring very wide two-sided absorption bandwidths is proposed in this letter. The fundamental design concept is illustrated and the possibility to design AFST structure without lumped components is discussed. The proposed AFST structure exhibits a passband at 6.3 GHz with two-sided absorption bands from 1.15 to 5.45 GHz (130.3% bandwidth) for the lower absorption band and from 7.1 to 12.9 GHz (58% bandwidth) for the upper absorption band. The proposed structure is modeled and simulated using two different full-wave simulation tools. An equivalent circuit model is constructed based on an approximated unit cell. To verify the simulated results, a prototype is fabricated and measured with the free-space method. A comparison with related designs presented in recent literature is also performed to ascertain the implications of the proposed design regarding the lumped component-free feature and wide absorption bandwidth.

Portability improvement for free-space reflectivity measurement

The free-space method is a standard technique for the reflectivity measurement of the absorbing material, where the horn antenna is usually used as the transmitting antenna and receiving antenna. In order to improve the ability to perform reflectivity in-situ measurement using a small sample volume and readily available instrumentation, it is necessary to study the portability of the free-space reflectivity test system. This paper describes a versatile low-cost solution for the reflectivity measurement. By designing a radiation-enhanced Vivaldi antenna and choosing the transceiver configuration reasonably, a portable and broadband reflectivity test system is proposed. As a demonstration, reflectivity measurement of the absorbing sheets is experimentally demonstrated in the band of 2–18 GHz, good consistency with the test result from the standard arch system shows that the proposed system is a very adapted candidate for the industrial application.

Investigation on microwave absorption characteristics of conductive-coated honeycomb absorber

Because microwave absorbers are limited in their absorption performance and bandwidth by their thickness, research on lightweight microwave absorbers having a wide bandwidth has been increasingly conducted. Lightweight structures with periodicity, such as honeycomb structures, have been increasingly attracting attention as they have the potential for application as lightweight broadband absorbers. Aramid or glass fiber-based honeycomb cores are dielectric materials that are often used in industrial applications. If a honeycomb absorber is manufactured by coating a conductive material on a honeycomb core, microwave absorption performance can be expected in a wide frequency range. However, structures with a certain periodicity, such as honeycombs, may bring about unexpected effects due to the shape with respect to the propagation direction of the microwave. In this study, the characteristics of absorbing electromagnetic energy in honeycomb absorbers were investigated, and the possibility of analyzing honeycomb absorbers by the transmission line analysis method was examined. Based on this investigation, a honeycomb absorber was fabricated, and the simulation results were verified by evaluating the performance by the free-space measurement method. Because of unpredictable scattering of the microwave polarized in the in-plane direction, for honeycomb absorbers, it is necessary to keep the coating resistance low or carefully consider the influences through simulation.

A Method for Predicting Radiated Acoustic Field in Shallow Sea Based on Wave Superposition and Ray

The traditional free-space and half-space analysis method ignore the reflection of the upper and lower boundaries of shallow sea and are not suitable for analyzing shallow sea problems especially at high frequency. Hence, a method combining ray theory and wave superposition theory is proposed in this paper to predict the high-frequency radiated acoustic field in shallow water. The proposed method takes into account the effect of channel boundaries on the acoustic field and has good adaptability to complex channel environments and accuracy of the calculated acoustic field.

Synthesis, characterization of Fe3O4/polymer composites with stealth capabilities

We have successfully fabricated the magnetite (Fe3O4) nanoparticles by using the simple method and dispersed with epoxy resin and polyethylene. The resulting composites have potential for application in microwave and thermal image absorption. The products developed were identified by the FT-IR spectroscopy, X-ray diffractometry, and electron microscopy techniques. The absorption of microwaves of the Fe3O4 composite powders is determined from the magnetic (permeability) and dielectric (permittivity) properties, by fitting the samples in free space method, for reflectivity levels in the frequency range from 2 to 18 to 18–40 GHz. The absorption characteristics of Fe3O4 composites, mixed with epoxy resin, for different thickness of the sample have also been investigated. The results demonstrate that the best reflectivity properties and microwave absorbing frequency bands shifting were observed in the ranges from 2 to 18 to 18–40 GHz. Infrared thermal image screen experiments at 3–5 μm (mid-wave infrared MWIR) and 8–12 μm (long-wave infrared LWIR) were taken to analyse the infrared attenuating efficacy of the composites. The values of these measurements indicated that thermal infrared reduction can be achieved using a composite of Fe3O4 and polyethylene.

Enhanced microwave absorption of rice husk‐based pyramidal microwave absorber with different lossy base layer

The size of pyramidal microwave absorbers (PMA) is one of the main considerations for their selection for use in anechoic chambers. In this work, a new type of PMA is introduced, with rice husk (RH) as its filler. Meanwhile, a lossy base layer is also introduced in the PMA using a combined RH and carbon nanotube (CNT) composite. To do so, the dielectric properties of the RH-CNT composite are first investigated from 2–18 GHz by using a dielectric probe. Next, the microwave absorption properties of the PMA were investigated in terms of different height of the pyramidal structure and base layer. The free-space method was used to measure the microwave absorption of PMA at oblique incident angles of 0° to 60°, respectively. Results indicated good microwave absorption characteristics in the 2–6 GHz with a height of 11.5 cm the PMA structure, and 1 cm thickness of the base layer. The lossy base layer is found to increase the bandwidth of the absorber and improve the microwave absorption of PMA in the lower GHz frequencies. Further investigations concluded that the microwave absorption performance of the PMA is dependent on its dielectric properties, size and base layer.

Remarkable microwave absorption efficiency of low loading ratio of Ni0.25Co0.25Ti0.5Fe2O4/SrCoTiFe10O19/Cu composite coated with polyprrole within polyurethane matrix

NiCoTi spinel ferrite, SrCoTi hexagonal ferrite and copper microparticles were coated with polypyrrole (PPY) using in situ chemical oxidative method. The resulted microwave absorbing material was introduced into polyurethane matrix. XRD patterns indicated the successful preparation of spinel and hexagonal ferrites. SEM micrographs showed the semi-spherical morphology of the prepared composite. The magnetic properties have been investigated using VSM. The microwave absorber with 20%w/w loading percentage exhibits the best microwave absorption performance (bandwidth under −10 dB of 3 GHz and reflection loss of −24 dB at the matching frequency 9.75 GHz and thickness of 1.8 mm). The measurements of permittivity and permeability reveal that the dielectric loss mechanism is the predominant in the current absorber. The measured reflection loss using free space method also indicates the superior performance of the sample with 20%w/w loading percentage.

An Accurate Method for Designing, Characterizing, and Testing a Multi-Layer Radome for mm-Wave Applications

This paper presents an accurate procedure to design, characterize and test a radome for millimeter-wave applications. In this procedure, an accurate analytical model is presented to design a radome structure that takes into consideration the bandwidth, losses, reflections and depolarization factor as a function of frequency and incident angle. To guarantee a good operational electrical performance, the proposed procedure includes a material characterization and accurate test procedure based on the free-space Gaussian beam method and time gating filtering technique. Four radomes (two-monolithic and two A-sandwich) at 82 GHz and 94 GHz are designed, characterized and tested to validate the proposed technique. Results obtained with this procedure are compared with numerical simulations and experimental results. This method is shown to be accurate and versatile for radomes in mm-Wave applications.

A Study of Electromagnetic Absorption Performance of Modern Biomass Wall Tile

Electromagnetic elimination was used to eliminate unwanted radiation that may interface with machinery or affect human health. The use of microwave absorbers has become necessary for an environment with a safe electromagnetic wave level. The objectives of this project are to design a microwave absorber as a modern biomass wall tile and to investigate the absorbing performance of the modern wall tile absorbers with different materials. The absorber was designed with similar shapes and different biomass materials. The concept of modern wall tile had been applied to design the absorbers in terms of their shape and dimension by using biomass materials. Biomass materials such as kenaf and coconut coir were used in the study due to their lightweight and environmental-friendly material behavior. The simulation was done using CST Suite Studio software to predict the preliminary result of the absorbers. The proposed designs of the modern wall tile from the CST simulation are then fabricated while the mixtures of the materials are moulded into a microwave absorber. NRL Arch free space method was used to determine the absorption performance of the modern wall tile absorbers at a frequency range from 1GHz to 12GHz. Both results of the simulation and free space measurement are analyzed and discussed. This study showed that absorber KCA has the best performance among all the absorbers with the absorption of more than -10 dB. The mixture of biomass material with carbon has a great absorption performance compared to the mixture of biomass material without carbon.