Free-space Measurement Research Articles

A study on the microwave absorbing honeycomb core embedded with conductive periodic patterned surfaces for the effective dielectric constant

Periodic patterned surfaces can provide effective dielectric constants to the microwave absorbing honeycomb partition wall depending on the size of the pattern. A characteristic study was conducted for a honeycomb-shaped absorber based on the geometric configurations and material properties. The range of the relative permittivity for increasing the absorption bandwidth of the absorber was determined through a parametric study. The imaginary term of the relative permittivity and depth of the wave propagation direction had a significant impact on the bandwidth of the absorber. Moreover, the materials patterned with carbon black- and graphite-based conductive inks using the screen-printing method were bonded to each other at intervals and stacked to form a honeycomb shape. Several absorber specimens were fabricated, and their absorption capabilities were measured using a free-space measurement system. Based on the results, the utility of a film-based honeycomb absorber that can continuously maintain broadband absorptivity was verified.

Miniaturized Frequency Selective Surface for 6G Communication.

A single-layer, quartz-supported frequency selective surface (FSS) with a gear-shaped metallic array is proposed for 6G communication. Full-wave simulation, along with the method of equivalent circuit, is applied to investigate the transmission characteristics, while the distributions of surface current distribution, as well as electric field and magnetic fields, are studied to further interpret the transmission mechanism. The simulation indicates that the resonant frequency of 131 GHz with an attenuation of −40 dB can be obtained and the relative bandwidth approximates to 12%. The transmission response of the fabricated FSS prototype is measured using the free space measurement setup. The measured results show a good agreement with the simulated ones, which demonstrates the reliability of the design and fabrication. The proposed FSS with the advantages of simple structure, low cost, easy fabrication, and integration can be applied in enhancing the communication performance and anti-interference ability in the future 6G communication system.

Surface Plasmon Coupled Directional Emission for Integrated Fluorescence‐Raman Biodetection: A Proof‐of‐Concept Study

AbstractThe development of multi‐functional nanoplatforms has attracted much attention. The realization of fluorescence and Raman dual‐mode bio‐detection typically needs complicated switching procedures or algorithm processes. In this work, we propose that a surface plasmon coupled emission (SPCE) technique could be used to break the bottleneck. Our approach enabled Raman spectra which would normally suffer interfered by strong fluorescence signal to be distinguished, simply by respective measurements at different angles. As a proof of concept, we prepared dual‐mode nano‐composites for measurement. Herein, in SPCE, fluorescence was sharply distributed at 46° and Raman at 50°. Attributed to the unique directional radiation feature, collection efficiency increased dramatically, with nearly 5‐fold and 14‐fold absolute intensity enhancement compared to conventional fluorescence and Raman method. Meanwhile, Raman detectability could be improved dramatically in SPCE detection in contrast to free space measurement in one single wavelength excitation device, for serious fluorescence interference to Raman tags could be eliminated.

Free-Space Measurement of Dielectric and Magnetic Properties by Double Planar Sample Method in Y-Band

In this work, to measure the complex permittivity and the complex permeability of materials, the double planar sample method is proposed, which is a variant of the classical free-space measurement method. The device under test is a double planar sample instead of a single piece of sample. The greatest advantage of this method is that it eliminates complicated calibration in the measurement of the reflection coefficient. In this novel method, the reflection coefficient is replaced by a second transmission coefficient. Two groups of transmission coefficients are obtained by changing the distance between these double planar samples; then, the complex permittivity and the complex permeability by the transmission coefficient are retrieved. The relative uncertainties of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\varepsilon ^{\prime}$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu ^{\prime}$</tex-math></inline-formula> are both 1%. Four materials (polytetrafluoroethylene, silicon, quartz, and leaf of banana) are measured by this method, the results of which are consistent with those reported in the literature. The relative uncertainty of this method and the conventional free-space method is consistent. However, the magnetic loss tangent is very small, and the retrieval method is not sensitive to it.

Rotation included 3-axis scanning free-space measurement and curvature compensation for electromagnetic evaluation of leading-edge and curved stealth structures

Since the last decade, stealth technology has been developing at a much higher speed with much research going on to evaluate the properties of radar absorbing structures (RASs) and scattering parameters of the developed stealth structures at different phases of practical use. A vector network analyzer plays an important role in RAS performance evaluation of stealth structures. Different scientists have used closed cell measurement techniques utilizing coaxial cables and waveguides to evaluate RAS materials at high accuracy, while others have used free-space measurement techniques utilizing radiating antennas to evaluate the RAS performance of larger stealth structures without damaging the specimen. All these conventional evaluation systems can scan a single point at a time. A scanning free-space measurement (SFM) system was an initial step toward automated scanning of large stealth structures. This system was capable of scanning 1000 mm × 1000 mm flat structures using raster scanning with two linear stages. However, the two-dimensional SFM was not applicable to leading-edge and cylindrical structures, and the signal-to-noise ratio (SNR) is highly affected by the specimen curvature. In this paper, a 3-axis scanning free-space measurement (3-axis SFM) system was developed. The system has two linear stages and a rotational stage for raster scanning of the specimen. The system performance was checked by scanning cylindrical RAS specimens with a radius of 100 mm. The results were then compared to the SFM system, which showed that the SNR of the 3-axis SFM is quite high compared to the SFM system. A coordinate-based scanning algorithm was also developed for 3-axis SFM to ensure a fixed interval scan of complex geometrically shaped specimens, such as wing leading edges, at lower incident angles. The algorithm achieved a zero-degree incident angle scan with ±1 degree of accuracy for the NACA-M3 symmetrical leading-edge specimen. Different scan parameters and results are visualized in the developed graphical user interface software. Finally, the inspected results were curvature-compensated to obtain the actual RAS performance of the specimen. The accuracy of the geometry-based perfect electric conductor (PEC) reflection-loss prediction algorithm was further improved by adding a stand-off distance effect and more specimen curvature data to the algorithm. The accuracy of the algorithm was verified by comparing the predicted PEC results to the measured PEC results of the corresponding specimen.

Ultra-thin, wide-angle and bandwidth-enhanced linear and circular metasurface-based reflection-type polarization converter at X-band microwave frequency

In this study, a low-profile ultra-thin metasurface-based reflection-type polarization converter with enhanced bandwidth for X-band (8.2–12.4 GHz) applications is proposed. The designed converter, which has a cell topology above a substrate (FR-4) material terminated in a metal conductor with overall thickness of 1.2 mm corresponding to a thickness of in terms of wavelength (ultra-thin), indicates the cross polarization performance with a polarization converter ratio (PCR) value of 90% in a wide (9–12 GHz) frequency range (normal incidence) and linear to circular polarization performance with left-handed circular polarization between 8.44 and 8.58 GHz and between 12.74 and 13.14 GHz. It is observed that the converter has a PCR value of 80% for incidence angles up to . Simulations of the designed converter performed by a commercial electromagnetic simulation program (CST Microwave Studio) were validated by free-space measurements.

Window tinting films for microwave absorption and terahertz applications

We demonstrate that commercially available window tinting films could be used multifunctionally, i.e., for electromagnetic shielding and absorption at frequencies below 1 THz along with visible light and ultraviolet protection. The fine control of the film optical properties by their structural composition can also be used to extend their performance to the lower frequency ranges, i.e., terahertz and microwave. The electromagnetic properties of two types of thin protective films loaded with either carbon or iron oxide micro- and nano-inclusions were studied in microwave (12–18 GHz) and terahertz (0.2–1.0 THz) frequency ranges vs their inner structure. The reflection and transmission coefficients of studied tint films were investigated using waveguide and free space measurements and compared with theoretical modeling results. The effective sheet resistance was estimated from the experimental data.

Optimization of electromagnetic shielding of three-dimensional orthogonal woven hybrid fabrics in ku band frequency region by response surface methodology

Electromagnetic shielding (EMS) has become the necessity of the present era due to enormous expansion in electronic devices accountable to emit electromagnetic radiation. The principal target of this paper is to originate three-dimensional (3D) orthogonal fabrics with conductive hybrid weft yarn and to determine their electromagnetic shielding. DREF-III core-spun yarn using copper filament in the core and polyphenylene sulfide (PPS) fiber on the sheath and fabric constructed of such yarn has a promising electromagnetic shielding characteristic. Box–Behnken experimental design has been employed to prepare various samples to investigate the electromagnetic shielding efficiency of 3D orthogonal woven structures. The orthogonal fabric samples were tested in an electromagnetic Ku frequency band using free space measurement system (FSMS) to estimate absorbance, reflectance, transmittance, and electromagnetic shielding. The increase in copper core filament diameter and hybrid yarn linear density enhances the EMS of orthogonal fabric. Statistical analysis has been done to bring out the effect and interaction of various yarn and fabric variables on EMS. Metal filament diameter, orientation, sheath fibers percentage, and fabric constructional parameters significantly affected electromagnetic shielding efficiency. The inferences of this study can be applied in other 3D structures like angle interlock, spacer fabrics for curtains, and coverings for civilians and military applications.

Determination of Flowing Grain Moisture Contents by Machine Learning Algorithms Using Free Space Measurement Data

The measurement of the moisture content of the stored grain in the silos provides the opportunity to take the necessary precautions to store the grain without spoiling. Since it is not possible to obtain all the moisture information of the stored grain with the current methods, in this study, a new method is proposed to determine the moisture content of the grain in real time during the loading processes. For this purpose, popular machine learning (ML) algorithms, i.e., KNN, SVR, and ANN, are used to predict the moisture content of the flowing grain. In order to measure the moisture content of the grain, a free-space electromagnetic measurement setup is constructed. Reflection and transmission coefficients are measured at 103 different frequency points between 1 and 2.48 GHz using a vector network analyzer (VNA) for three different grain types (Bulgur wheat, durum wheat, and corn silage kernel) with moisture content varying between 8% and 25%. In this way, three datasets are constituted as datasets 1–3 corresponding to each grain type. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k$ </tex-math></inline-formula> -fold cross-validation ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k$ </tex-math></inline-formula> -CV) technique is used to train and test the ML algorithms and the performance of the algorithms is evaluated with five different metrics. In addition, for each grain type, the error rates corresponding to each moisture content are evaluated separately and the relationship between moisture content and performance of algorithms is revealed. While the best results are obtained with KNN for durum wheat and corn silage kernel, SVR method gives the best results for bulgur wheat. This study reveals that the moisture content of flowing grain can be determined, thanks to proper modeling of ML algorithms and measurement setup.

Improved Method for Permittivity Determination of Dielectric Samples by Free-Space Measurements

A new deembedding technique is proposed for relative complex permittivity <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon _{r}$ </tex-math></inline-formula> determination of dielectric materials using gated free-space measurements. Its three main features are 1) it does not require any formal calibration procedure (calibration-free); 2) it is position-insensitive; and 3) it extracts ripple-free <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon _{r}$ </tex-math></inline-formula> due to gating process. The objective function derived to determine <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon _{r}$ </tex-math></inline-formula> by the proposed method is validated by free-space measurements of three dielectric samples (polypropylene, polyethylene, and polyoxymethylene). Besides, the accuracy of our method is compared with the accuracy of other calibration-free and calibration-dependent methods in the literature.

Wearable Pattern-Diversity Dual-Polarized Button Antenna for Versatile On-/Off-Body Communications

A wearable dual-port button antenna that excites pattern-diversity dual-polarized waves is proposed for on-/off-body applications. Its simple structure comprises of two radiators and a common ground plane designed into two substrates. The crossed-dipole radiator is made by two symmetric bowtie dipoles printed on a circular-shaped semi-rigid substrate, which covers an ultra-wideband (UWB) application with circular polarization and directional radiation patterns, high gain, and high efficiency suitable for off-body communication. The annular-ring radiator and common ground plane are made by a conductive textile on the square-shaped flexible-felt substrate. The annular-ring radiator is shorted to the ground plane using four vias, which generates triple TM modes covering the 2.45/5.85 GHz wireless body area networks (WBAN) with omni-directional radiation patterns and a 3.8 GHz C-band with a directional radiation pattern suitable for on-/off-body communications. The fabricated antenna is verified by measurement in both free space and phantom body environment. Measurements agreed well with simulations. Simulated specific absorption rates (SARs) under US and EU standards are below the safe level, making the proposed antenna suitable for on-/off-body communications.

3-D Printed Plug and Play Prototyping for Low-Cost Sub-THz Subsystems

Polymer-based additive manufacturing using 3-D printing for upper-millimeter-wave ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ca.</i> 100 to 300 GHz) frequency applications is now emerging. Building on our previous work, with metal-pipe rectangular waveguides and free-space quasi-optical components, this paper brings the two media together at G-band (140 to 220 GHz), by demonstrating a compact multi-channel front-end subsystem. Here, the proof-of-concept demonstrator integrates eight different types of 3-D printed components (30 individual components in total). In addition, the housing for two test platforms and the subsystem are all 3-D printed as single pieces, to support plug and play development; offering effortless component assembly and alignment. We introduce bespoke free-space TRM calibration and measurement schemes with our quasi-optical test platforms. Equal power splitting plays a critical role in our multi-channel application. Here, we introduce a broadband 3-D printed quasi-optical beamsplitter for upper-millimeter-wave applications. Our quantitative and/or qualitative performance evaluations for individual components and the complete integrated subsystem, demonstrate the potential for using consumer-level desktop 3-D printing technologies at such high frequencies. This work opens-up new opportunities for low-cost, rapid prototyping and small-batch production of complete millimeter-wave front-end subsystems.

Complex Permittivity and Thickness Evaluation of Low-Loss Dielectrics From Uncalibrated Free-Space Time-Domain Measurements

A free-space time-domain method is proposed to retrieve dielectric constant (<inline-formula> <tex-math notation="LaTeX">$\varepsilon _{r}$ </tex-math></inline-formula>), conductivity (<inline-formula> <tex-math notation="LaTeX">$\sigma _{e}$ </tex-math></inline-formula>), and thickness (<inline-formula> <tex-math notation="LaTeX">$d$ </tex-math></inline-formula>) of metal-backed low-loss dielectric samples using calibration-independent reflected power peak measurements. Its algorithm is validated by numerical calculations and simulations (CST Microwave Studio) using a sine-modulated Gaussian window. A sensitivity analysis is followed to examine its performance considering the dependencies of reflected power peaks with respect to <inline-formula> <tex-math notation="LaTeX">$\varepsilon _{r}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$\sigma _{e}$ </tex-math></inline-formula>. Free-space time-domain measurements have been implemented after transforming frequency-domain measurements into time-domain ones to extract <inline-formula> <tex-math notation="LaTeX">$\varepsilon _{r}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$\sigma _{e}$ </tex-math></inline-formula>, and <inline-formula> <tex-math notation="LaTeX">$d$ </tex-math></inline-formula> of polypropylene, polyethylene, and polyoxymethylene samples.

Miniaturized High Gain Flexible Spiral Antenna Tested in Human-Like Tissues

A miniaturized helical antenna is presented in this work. The antenna is flexible, it is 6100 μm long and it has a diameter of 352 μm. This antenna has such a small cross-section, that permits to be implanted in the human body with fine syringes and minimally invasive surgeries. The antenna can be used to receive power and/or send information in medical devices. The antenna is made of biocompatible materials: polytetrafluoroethylene (PFTE) and copper. The fundamental parameters of the antenna have been simulated and experimentally measured in animal human-like tissues, showing good agreement. The resonant frequency of the antenna is 4.7 GHz, with a reflection coefficient of -25.1 dB, and a gain of -4.7 dBi. As expected, the resonant frequency decreases inside biological tissues comparing to the free-space open-air measurement. Reducing the resonant frequency is an advantage because power signals can penetrate deeper into body tissues.

Development of Resistive-Ink Based Planar and Conformal Metasurfaces for RCS Reduction

This work reports design, development and characterization of both planar and conformal Metasurfaces using resistive Ink based lossy pattern printed on top of FR-4 dielectric substrate for RADAR stealth application. Different geometrical dimensions of the proposed unit cell have been optimized numerically using CST-Microwave Studio, to achieve 20 dB reflection reduction with 60% fractional bandwidth in frequency range 10.5-19.5 GHz under normal incidence with 0.12&#x03BB;_L thickness (where &#x03BB;_L corresponds to lower operating frequency) and 10 dB reflection reduction up to 40&#x00B0; oblique angle of incidence in the given frequency spectrum for both TE and TM polarized wave. An equivalent circuit analytical modelling has also been performed to understand and verify the numerical findings which are nearly in agreement. Polarization insensitive characteristics of developed surfaces is achieved using fourfold rotation symmetry of the designed unit cell. The designed absorbers are fabricated in form of planar, cylindrically bent and 90&#x00B0; dihedral Metasurfaces and are characterized using ABmm Vector Network Analyzer (VNA) system and free space measurement method. A thorough different experimental measurements are performed and it is found that measured characteristics are in agreement with numerical-analytical findings.

First SIMO Harmonic Radar Based on the SFCW Concept and the HR Transfer Function

This paper presents a new SIMO radar system based on a harmonic radar (HR) stepped frequency continuous wave (SFCW) architecture. Simple tags that can be electronically individually activated and deactivated via a DC control voltage were developed and combined to form an MO array field. This HR operates in the entire 2.45 GHz ISM band for transmitting the illumination signal and receives at twice the stimulus frequency and bandwidth centered around 4.9 GHz. This paper presents the development, the basic theory of a HR system for the characterization of objects placed into the propagation path in-between the radar and the reflectors (similar to a free-space measurement with a network analyzer) as well as first measurements performed by the system. Further detailed measurement series will be made available later on to other researchers to develop AI and machine learning based signal processing routines or synthetic aperture radar algorithms for imaging, object recognition, and feature extraction. For this purpose, the necessary information is published in this paper. It is explained in detail why this SIMO-HR can be an attractive solution augmenting or replacing existing systems for radar measurements in production technology for material under test measurements and as a simplified MIMO system. The novel HR transfer function, which is a basis for researchers and developers for material characterization or imaging algorithms, is introduced and metrologically verified in a well traceable coaxial setup.

Applications of radar measurement technology using 24 GHz, 61 GHz, 80 GHz and 122 GHz FMCW radar sensors

Abstract This paper presents a review of radar applications in high-accuracy distance measurement of a target. The radars included in this review are frequency modulated continuous wave (FMCW) radar sensors operating in four different millimeter-wave frequency bands, namely 24 GHz, 61 GHz, 80 GHz and 122 GHz. The radar sensors are used to measure the distance of standard and complex targets in a short range of a few meters, thus indicating that the choice of target and the medium used for radar signal propagation also play a key role in determining the distance measurement accuracy of an FMCW radar. The standard target is a trihedral corner reflector in a laboratory-based free space measurement setup and the complex targets include a piston in an oil-filled hydraulic cylinder and a planar positioning stage used in micromachining. In each of these measurement scenarios, a distance measurement accuracy in micrometer range is achieved due to the use of a sophisticated signal processing algorithm that is based on a combined frequency and phase estimation method. The paper is concluded with a technical comparison of the accuracy achieved by the FMCW radars reviewed in this article with other related works.

Design of assembly‐type frequency selective surface structure using Lego‐type blocks

AbstractA frequency selective surface (FSS) structure was designed using Lego blocks, which can be stacked or assembled to produce various structures. The unit structure of the designed frequency selection surface structure is a patch‐type cross‐dipole and a dipole structure that can be easily fabricated using Lego‐type blocks among the basic structures. Since the unit structure is designed using Lego‐type dielectric blocks and metal blocks, the structure can be easily changed and the resonance frequency can be easily reconfigured. The designed unit structure was fabricated using actual Lego‐type blocks of 4 mm × 4 mm × 5 mm, and the transmission coefficient of the fabricated FSS structure using the free space measurement method was measured. When the measurement result according to the change of polarization and incident angle was compared with the simulation result, the error of the resonance frequency was 1.2% and 0.64% at the maximum for each structure, confirming that the measurement result is relatively similar to the simulation result.

Complex Permittivity Measurements of Steel Fiber-Reinforced Cementitious Composites Using a Free-Space Reflection Method with a Focused Beam Lens Horn Antenna.

To measure the electromagnetic properties of steel fiber-reinforced concrete (SFRC) in the X-band, 1-port measurements were performed using a lens horn antenna in a free-space measurement system. Free-space 1-port calibration with translations of the position of the reflector regarding the characteristics of the focused beam lens horn antenna was applied. The intrinsic impedance and complex permittivity of the SFRC were obtained from the measured reflection characteristics. The steel fiber content increased and the electromagnetic properties of the SFRC gradually changed from a dielectric to a conductor, even in very low frequencies compared to the plasma frequencies of general metal, which are optical frequencies. This is considered to be the plasmon effect of the metallic structure formed by the steel fiber. This result is applicable for analyses of the electromagnetic phenomenon of large structures with fiber content.

Experimental comparison of absorber and conductive floor automotive near field antenna measurement systems

AbstractLarge truncated spherical near-field systems with conductive or absorbing floors are typically used in the measurement of the performances of vehicle-installed antennas. The main advantage of conductive floor systems is the ease of accommodation of the vehicle under test, but their performances are affected by the interaction with the reflecting ground floor. Instead, absorbing-based systems emulating free-space conditions minimize the effect of the interaction with the floor, but generally require longer setup times, especially at lower frequencies (70–400 MHz), where bulky absorbers are typically used to improve reflectivity levels. Considering scaled measurements of a vehicle model, the performances of these two typical implementations are analyzed in the 84–1500 MHz range and compared to free-space measurements. Absorbers with different dimensions and reflectivity have been installed in the scaled measurement setup, and measured data have been investigated with proper post-processing to verify the applicability to realistic systems. Figures of merit of interest for automotive applications, like gain and partial radiated powers, have been compared to free-space to evaluate the impact of different scenarios.